water masses and patterns of flow in the somali basin during the southwest monsoon of 1964

Deep-Sea Research, 1966, Vol. 13, pp. 825 to 860. Pergamon Press Ltd. Printed in Great Britain.

Water masses and patterns of flow in the Somali Basin during the

southwest monsoon of 1964"

BRUCE WARRENS', HENRY STOMMEL~ a n d J . C. SWALLOW§

(Received 25 July 1966)

Abstraet~Closely spaced hydrographic sections made during August-September 1964 in latitudes 3°S-12°N, and between the East African coast and longitude 56°E, define in detail a complex structure of water masses in the Somali Basin under the southwest monsoon. Reference to observations made elsewhere in the Indian Ocean permits clear identification of the source waters responsible for this structure. In the near-surface water the distributions of temperature and salinity show the course and lateral extent of the Somali Current, the offshore movement of cold water upwelled near the Somali coast, and two warm saline inflows from the Gulf of Aden and the Arabian Sea. At depths greater than 2000 m, small differences in temperature-salinity characteristics reveal a narrow northward flow adjacent to the continental slope, roughly paralleling the Somali Current. No definite inferences can be drawn concerning flow patterns at intermediate depths, both because of the apparent small scale of horizontal variation there, which is not resolved by the station spacing, and because of inherent ambiguity in core methods when applied to flows which may reverse seasonally.

I N T R O D U C T I O N

FROM early June through September, the southwest monsoon blows steadily over the northwestern Indian Ocean, and it is known from ship reports (e.g. FINDLAY, 1866; HOFFMANN, 1886) that during this period a strong surface current flows parallel to the African coast, close inshore, toward the northeast. There is some disagreement about the name of this current. The Times Atlas calls it the East African Coastal Current, and NEWELL (1957) also uses this term, but only for the portion of the current south of the equator (north of the equator he calls it the Monsoon Current). On the other hand, German and Russian hydrographers have called it the Somali Current; we find this name more descriptive because the current does in fact flow mainly along the coast of Somalia, and the term "East Africa" is being used less for this part of the world as time goes on. The name " Monsoon Current" seems confusing here, because it is more often applied to the eastward flow in the North Indian Ocean which is fed by this coastal current.

During the northeast monsoon, currents along the shore flow in the opposite direction, toward the southwest. They appear to be weaker then than in the northern summer, probably in correlation with the relative unsteadiness of the northeast monsoon as compared with the southwest.

An excellent portrayal of the Somali Current is given in ScI-IOTr's chart (1935) of

*Contribution No. 1748 from the Woods Hole Oceanographic Institution. This paper is also a contribution from the National Institute of Oceanography and from the Scripps Institution of oceanography.

tWoods Hole Oceanographic Institution, Woods Hole, Massachusetts. ~Massaehusetts Institute o f Technology, Cambridge, Massachusetts. §National Institute of Oceanography, Wormley, Godalming, Surrey.

825

826 BRUCE WARREN, HENRY STOMMEL and J. C. SWALLOW

surface currents, based upon collation of ship logs. This chart suggests that the Somali Current is a western boundary current similar to the Gulf Stream and Kuroshio with only the difference that it appears during the southwest monsoon, and is absent the rest of the year.

This interpretation appears to be in accord with the theory of wind-driven currents as given by MUNK (1950) and others (e.g. STOMMEL, 1965) because according to this theory intense northward-flowing currents occur along the western coasts of oceans over which there are large regions of negative curl of wind stress. Such a situation obtains in both the North Atlantic and North Pacific throughout the year--with exceptions of short duration--over extensive regions centered at 30°N approximately. It also occurs in the Indian Ocean during the southwest monsoon in a region extending from 15°S to 15°N, straddling the equator. Although the existence of the Somali Current is consistent with this application of the theory of wind-driven currents, the basis of the theory is not firmly established for very low latitudes.

The response of the ocean to variations in wind stress is not well understood either. Estimates of the time that would be required to establish the dynamic topography of the great subtropical gyres of the oceans from initially flat topography are of several decades (VERONIS and STOMMEL, 1956). The dynamic topography of equatorial regions is more gentle than in higher latitudes: therefore although it may respond more quickly to varying wind-stress, its response may be correspondingly difficult to deter- mine over the background noise inevitably present in oceanographic serial observations.

As theory of ocean currents develops, it will be useful to have some definite infor- mation about the Somali Current, because its dramatic changes can perhaps provide a rather special checkpoint for verification of theoretical ideas. For these reasons, and for sheer curiosity, an Anglo-Californian expedition to the Somali Current was organi- zed as part of the investigations of the R.R.S. Discovery and R.V. Argo during their assignment to the International Indian Ocean Expedition in the summer of 1964.

FOXTON (1965) has already reported on the surface temperature distribution as obtained from Discovery observations off the northeastern coast of Somalia, and on biological observations there. STOMMEL and WOOSTER (1965) have prepared a note on the Argo's bathythermograph observations, which show clearly the path of the Somali Current. SWALLOW and BRUCE (1966) have combined their current measurements by meters and comparisons of these with geostrophically determined currents observed more or less simultaneously.

The present paper concerns some of the serial observations of temperature, salinity and dissolved-oxygen concentration, made aboard the Argo and Discovery in the Somali Basin during the period 4 August-4 September 1964. Our attempt is to give a detailed description of the water-mass structure in the vicinity of the Somali Current and to infer (principally by core methods) the pattern of movement at various levels between the sea surface and the ocean bottom.

O B S E R V A T I O N S

The part of the Indian Ocean surveyed by the Argo and Discovery (Fig. 1) does not seem to have a specific name: no one, for example, calls it the" Somali Sea " in analogy with " Arabian Sea." Medieval Arabs spoke of the offshore waters of East Africa as

Water masses and patterns of flow in the Somali Basin during the southwest monsoon of 1964 8:27

the sea of the Zanj (MAS'UDI, 947), but although this name certainly delimits the desired portion of the ocean, it seems too esoterically archaic for modern usage. For lack of a satisfactory name we shall use " Somali Basin," as defined bathymetricaUy, even though this term is properly descriptive of a topographic feature, rather than of the water overlying it.

The pattern of hydrographic stations (Fig. 1) consists of a number of sections running across the Somali Current into the coast of Somaliland, a section along the 53rd meridian, a long section approximately on the 55th meridian, two sections across the passage connecting the Gulf of Aden to the Somali Basin, and several stations close to the coast in the well known area of summertime cold-upwelling near and south of Ras Hafun (PUFF, 1890; M]n'. COUNCIL, 1891). In all, 130 stations were made, somewhat more than two-thirds of which reach nearly to the ocean bottom. Salinities were measured on both ships by the conductivity method, and dissolved-oxygen concentra-

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by the R. R. S. Disc~ery.


tion by Winkler titration. Between stations hourly bathythermograms and surface salinity samples were taken, sometimes half-hourly. In addition, surface temperature was recorded continuously along the tracks of both ships.

PROFILES OF TEMPERATURE, SALINITY AND DISSOLVED-OXYGEN CONCENTRATION

TO illustrate the vertical distribution of properties ;.n the Somali Basin, profiles of temperature, salinity, and dissolved-oxygen concentration have been drawn for four Argo sections and two Discovery sections in the northern part of the region surveyed (Fig. 2). The profiles are shown in Figs. 3-8.

Profile B (Fig. 4) lies about 120 km downstream of Profile A (Fig. 3). Both sections intersect the Somali coast, and both were intended to be straight, nearly normal cross- ings of the Somali Current, but the ships were badly set by the swift surface current near the shore (estimated maximum speeds above 6 knots). The cross-stream thermo-

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Location and designation of profiles. Dots indicate station positions.

Water masses and patterns of flow in the Somali Basin during the southwest monsoon of 1964 829

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Fig. 3. Profile A. Argo Stas. 33-41 (15-17 August 1964). Temperature (°C), salinity (g.), and dissolved-oxygen concentration (ml/1.). Depths are in meters, and station numbers are given along the top of each profile. The bottom topography is interpolated from the soundings made on station. Vertical lines show the positions of the stations and the depths oftbe deepest observations. A latitude scale is included at the bottom of each profile, but the indicated station spacing is based on projections of the actual station positions on to the straight line joining Stas. 33-35.

cline slope characteristic of large-scale ocean currents is clearly apparent between Stas. 34 and 41 on Profile A and between Stas. 5547 and 5553 on Profile B. In these low latitudes the magnitude of the slope is only about 1 × 10 -3, contrasting markedly with values typical of the Gulf Stream such as 7 × 10 -3. Sta. 5571 on Profile B was occupied two weeks later than Stas. 5551 and 5552; consequently the dip inisohalines near 500 m on Sta. 5571 may be only an effect of the time lapse rather than an actual synoptic feature.

The Somali Current turned eastward between Lats. 7°-9°N, and Profile C (Fig. 5) represents an unsuccessful attempt to cross the current perpendicularly at the turn. Unfortunately, as the Argo worked away from the shore to make Stas. 42-47, it only


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became apparent when too late to make useful adjustments that the strong surface current was deflecting the vessel away from its intended southeastward course. Thus the Somali Current was approached very obliquely, and only a partial crossing was achieved.

Profile D (Fig. 6) was not intended to cross the Somali Current at all, but rather to close the Somali Basin to the northwest. It provides data concerning the exchange of water between the Gulf o f Aden and the Somali Basin.

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Fig. 5. Profile C. Argo Stas. 42-47 (18-19 August 1964), 59 (25 August 1964), and 63 (27 August I964). Temperature (°C), salinity (~), and dissolved-oxygen concentration (ml/l). See caption to Fig. 3. A scale of longitude is given along the bottom of each profile, and the indicated station spacing is based on projections of the actual station positions on to a parallel of latitude.

Profile E (Fig. 7) crosses the Somali Current at right angles after it has left the coast and turned eastward. The Current is indicated on the profile by the small slope in the thermocline between Stas. 56 and 47. Bathythermograms were used in addition to the station data for drawing the isotherms of the upper 150-200 m between Stas. 47 and 48: here the bow thermistor recorded a sharp surface-temperature minimum of 19.4°C.

The easternmost section, run approximately along the 55th meridian, is represented in Profile F (Figs. 8a-c). As on Profile E, bathythermograms have been used to draw


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Fig. 6. Profile D. Discovery Stas. 5554-5561 (18-20 August 1964). Temperature (°C), salinity (%0), and dissolved-oxygen concentration (ml/1.). See caption to Fig. 3. The indicated station spacing corresponds to the actual distances between stations, regardless of irregularities in the orientation of the section (Fig. 2); hence the longitude scale given at the bottom of each profile is

approximate.

near-surface isotherms in the central portion of the profile; and on the basis of the bow-thermistor record, the 20°-isotherm has been drawn so as to graze the sea surface between Stas. 63 and 64. It is shown below that the Somali Current lies between Stas. 64-66, but there is much less evidence in the slope of the thermocline here for its presence than on the other profiles.

These various property distributions are discussed in detail below in terms of the water-mass structure of the Somali Basin.

W A T E R MASSES

Surface water

Since surface water is in direct contact with the atmosphere, its temperature and salinity are not always sufficiently conservative to permit adequate definition of water types. In the surface layer of the Somali Basin, however, there are at least two distinctly different kinds of water: relatively warm, saline water derived from the Arabian Sea and the Gulf of Aden (exemplified at Stas. 62 and 63, Figs. 8a and b; and at Stas. 5556, 5560, and 5561, Fig. 6), and cooler, fresher water brought northward by the Somali Current. The fresh water (salinities under 35.3~oo) is found between Stas. 35 and 41 (Fig. 3), between stations 5547 and 5551 (Fig. 4), between Stas. 56 and 47 (Fig. 7), and Stas. 64 and 65 (Fig. 8b). This water seems to be carded into the Somali Basin by the South Equatorial Current (flowing westward in the latitude range 5°-15°S)from the general area of the Bay of Bengal. On MUROr, frsEv's (1959) surface salinity chart for the northern summer in the Indian Ocean, all water northeast of a line joining Ceylon and Sumatra is fresher than~34.0~o--a result of the large river discharge into the Bay of Bengal, and of the annual excess of rainfall over evaporation in the region.

Water masses and patterns o f flow in the Somali Basin during the southwest monsoon of 1964 833

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834 BRUCE WARREN, HENRY STOMMEL and J. C. Swm.,LOW

[According to the Morskoi Atlas, Vol. II (ISAKOV et al., 1953) this excess amounts to several hundred millimeters]. Such water appears to move southeastward and then southwestward into the South Equatorial Current, and finally westward until its last remnants are taken up by the Somali Current.

The high temperature of the Arabian Sea surface water (above 24°C at Stas. 62 and 63) is plainly due to the enormous radiation balance north of Lat. 10°N in the Arabian Sea: 1.2 × 105 to more than 1.4 × 105 cal/cm z annually, the largest values shown any- where on the earth's surface in the Morskoi Atlas. A correspondingly great annual excess of evaporation over precipitation in the northwest Arabian Sea--the Morskoi Atlas indicates 1000-1500 mm--accounts for the high salinity of the water (greater than 35.7%0 at Sta. 5556, greater than 35-8%0 at Stas. 62 and 63).

The even warmer surface water south of station 66 on Profile F (temperature > 26°C) does not seem to come from the Arabian Sea, however, because its surface salinity is too low ( < 35.4%°). MUROMTS~V'S (1959) charts of surface temperature and salinity suggest that it is simply a piece of the band of very warm, fresh, equatorial surface water which stretches across the entire Indian Ocean, increasing somewhat in temperature toward the east.

The interplay between these kinds of surface water is complicated during the southwest monsoon by upwelling off the Somali coast, which involves very cold, rather fresh water spreading away from the shore. The upwelling seems particularly intense at Sta. 42 (Fig. 5), where the surface temperature is below 14 ° and the dissolved oxygen concentration at the surface (3.46 ml/1.) is only about 60 % of the saturation value despite the strong steady wind (15-20 knots) of the southwest monsoon. Offshore movement of such water is suggested on Profile A (Fig. 3) by the folding over of the 35.2~/oo isohaline near the surface between Stas. 36 and 41; the fold also indicates that the upwelled water comes from depths less than 200-300 m.

Apparently it is this upwelled water which is responsible for the relatively low maximum surface temperatures ( < 22°C) on Profile D (Fig. 6) despite the association with fairly high surface salinities. Just to the north, nearer the Gulf of Aden, there is water both considerably warmer (temperature > 26°C) and very much more saline, its salinity exceeding 36.1 ~oo (see below, near-surface flow pattern) Profile D clearly lies in a zone of mixing between this water and the cool, fresh, upwelled water to the south: hence the surface characteristics here are not so extreme as on Profile F (Figs. 8a and 8b).

Intermediate depths

The salinity structure at intermediate depths in the Somali Basin is very disorderly. Its gross features consist of a low-salinity layer overlying a high-salinity layer, but sub- sidiary maxima and minima occur in both. The layer of minimum salinity can be identified on Profile A as being circled by the 35.1 ~o isohaline at a depth near 400 m. Below, at a depth of 500-1000 m, bounded on both top and bottom by 35.2~oo isohalines, is a rather ragged layer of high salinity, the most intense part being enclosed by a 35.3%0 isohaline. Similar values are found elsewhere, although salinities in the maximum layer reach 35.4%0 on Profiles B and C (Figs. 4 and 5), and exceed 35.5~o on Profiles E and F (Figs. 7 and 8). The shape of these layers is much more irregular than that of their counterparts in the South Atlantic: note, for example, the disjointed low-salinity layer at Sta. 5548 (Fig. 4) and the bubbly structure of the high-salinity


60 61 62 63 64 65 66 67 68 0 M

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F]l~.g. ga. l~mfil¢ F: .Arga ~ . 60--68 (26-29__ August 1964). Temperature (°C). See caption to • s. A scale ot !lautuae m given mong the bottom of the profile, and the indicated station

spacing is based on projections of the actual station positions on to a meridian of longitude.

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Fig. 8b. Profile F. Salinity (Too). See caption to Pig. 8a.

Water masses and patterns of flow in the Somali Basin during the southwest n'ronsoon of 1964 837

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838 BRUCE W~w,~t~, H~aw STOm~L and J. C. SWALLOW

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Fig. 9. Temperature-salinity relations pertaining to the formation of the intermediate-depth layers of high and low salinity in the Somali Basin. Inset shows station positions.

Water mas$¢'s and patterns o f flow in the Somali Basin during the southwest monsoon of 1964 839

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Fig. 10. Temperature-salinity relations pertaining to the formation of intermediate-depth salinity maxima in the Somali Basin. Inset shows station positions.

840 BRuc~ WARREN, HENRY STOMMEL and J. C. SWALLOW

water at Stas. 44-45 (Fig. 5), and at Stas. 50-51 and 57-58 (Fig. 7). Temperature- salinity curves for Stas. 50 and 57 (Fig. 9) and for Sta. 58 (Fig. 10) illustrate vertical aspects of this irregularity in more detail.

The actual depths of the minimum and maximum salinities varied considerably through the series of stations. The lowest salinities observed in the fresh layer occurred at depths between 200 and 650 m, and on average at 450 m. The depths of the greatest observed salinities in the saline layer averaged 760 m, but ranged from 550 to 950 m.

The intermediate oxygen structure matches the salinity structure in complexity. In fact, the two variables are roughly correlated at these levels: the high-salinity layer is associated with a low-oxygen layer, and the low-salinity layer with a high-oxygen layer. Near the continental slope on Profiles A and B (Figs. 3 and 4) the oxygen maximum exceeds 3.0 ml/1. but lies some 150 m above the salinity minimum. Elsewhere, only at Sta. 45 (Fig. 5) and at Sta. 67 (Fig. 8) are comparably high values seen, and except at Sta. 67, the vertical separation between the oxygen maximum and the salinity minimum is smaller than on Profiles A and B, less than I00 m. The high- oxygen layer is least apparent on Profile F (Fig. 8); at Stas. 64 and 65, where there is a well-developed salinity minimum, there is only the faintest indication of an oxygen maximum in the serial observations (none in the isopleths). The low-oxygen layer is very pronounced on all the profiles, with minimum values always below 1.0 ml/l., and occasionally, as at Sta. 58 (Fig. 7) and Sta. 61 (Fig. 8c), less than 0.5 ml/1. The oxygen minimum generally occurs at the same depth as the salinity maximum, though at Sta. 67 (Fig. 8), Sta. 47 (Fig. 7), and Stas. 33 and 34 (Fig. 3), it is 100-200 m deeper. The ragged, bubbly character of the salinity distribution is carried over to the oxygen profiles, as is seen, for example, at Stas. 50 and 51 (Fig. 7).

This great irregularity seems due both to a complicated mingling of water masses on various levels and to lateral variations in the field of motion. Unfortunately, as is shown below, it is not possible to infer very much from these data about the pattern of flow at intermediate depths, nor, accordingly, to account satisfactorily for structural fluctuations in the horizontal; here, we shall consider only the mixing of water masses. Furthermore, because the oxygen concentration correlates roughly with the salinity, and because, as a non-conservative variable, oxygen is a somewhat ambiguous water- mass property, we shall concern ourselves principally with the salinity structure. Even the qualitative features of the process by which this structure is brought about, however, seem as elaborate as the structure itself. Consequently, to make our discussion of its origin more intelligible, we shall first consider in a rough fashion the fresh and saline source waters responsible for the low-salinity and high-salinity layers, and then describe in more detail the way in which we think the various water masses of the Indian Ocean interpenetrate to produce the particular structure found in the Somali Basin.

The reason for the existence of the low-salinity layer is not immediately apparent. On meridional profiles it is possible to trace the minimum back continuously to the salinity minimum defining Antarctic Intermediate Water, which lies at a depth of about 1000 m in Lats. 20°--40°S. This pronounced minimum is well defined as of salinity less than 34.7%o on the T-S curve for Discovery H Sta. 1758 (Fig. 9) which appears to be typical of the entire Indian Ocean at latitude 30°S. In fact, TCrn~RNIA, LACOMBE, and GUmOUT (1958) have suggested on this basis that the minimum in the North Indian Ocean is an effect of fresh Antarctic Intermediate Water, risen from depths of 900-1000 m up to depths as shallow as 200-300 m in order to override the


relatively saline water spreading southward from the Arabian Sea. On the other hand, the horizontal distribution of salinity in the depth range of the Somali-Basin minimum ----either on level surfaces (MOROMTSEV, 1959) or on isopycnic surfaces (RocHFORO, 1958; TAr'r, 1963)--shows a relatively fresh core within the South Equatorial Current emanating from the Eastern Archipelago with a 400-m salinity under 34.6~oo (Wille- brord Snellius Sta. 146, Fig. 9). The water appears to come southward from the Pacific Equatorial Currents by way of the Banda Sea (WYRTKI, 1961), and penetrates the more saline Indian water at least as far west as longitude 60°E, where its 400 m salinity is still as low as 34-9%0. Thus from salinity characteristics alone, the Somali Basin salinity minimum could be connected both with Equatorial Pacific water and with Antarctic Intermediate Water.

Closer examination, however, indicates that neither of these obvious source waters can be responsible for the minimum. Oxygen concentrations at appropriate depths in the fresh water coming from the Banda Sea seem to be about 2.3 ml/1. and not over 2.5 ml/1. (MuRoMISEV, 1959; TAFT, 1963) while as we saw above, oxygen values in the low-salinity layer near the Somali coast exceed 3.0 ml/l.; consequently the Archipelago cannot be the principal source of the relatively fresh water in question. Furthermore, the line of minimum salinity connecting the Antarctic Intermediate Water in the South Indian Ocean to the Somali-Basin minimum cannot also be a line of flow, because as TArT (1963) has pointed out, the specific volume anomaly changes by almost 30 cl/t (about 0.3 crt-units) along the line. (This argument does not, of course, preclude a southern origin for the fresh water at depths less than those of the core of Antarctic Intermediate Water).

The water-mass structure at intermediate depths in the subtropical South Indian Ocean is anomalous with respect to that in the South Atlantic and South Pacific. In the latter two oceans the core of Antarctic Intermediate Water, defined by the marked salinity minimum, is found at a depth of about one kilometer in association with a pronounced maximum in oxygen concentration (SVERDRUP, JOHNSON and FLEMING, 1942, chap. 15; NEKRASOVA and STEPANOV, 1963); Antarctic Intermediate Water there is both low in salinity and relatively high in oxygen. In the subtropical South Indian Ocean, however, the salinity minimum and oxygen maximum are distinctly separate, the former lying at about 1000 m, as in the South Atlantic and South Pacific, but the latter at 500 m (MUROMTSEV, 1959; IVANENKOV and GUBIN, 1960; NEKRASOVA and STEPANOV, 1963). In their classification of water masses, IVANENKOV and GUBIN (1960) divide the intermediate-depth water of the South Indian Ocean into two masses: Antarctic Intermediate Water (they actually say, " Subantarctic Intermediate" but we adhere to the more common, "Antarct ic Intermediate "), of temperature 3.5-7.0°C, and salinity less than 34.65~oo; and Subtropical Subsurface Water, overlying the former, of temperature 7-15°C, and salinity 34.7-35.5~o. Because the oxygen maximum is associated with the latter body of water, it seems appropriate thus to classify it as a distinct water mass. Furthermore, the oxygen maximum can be traced back to the sea surface in the vicinity of the Subtropical Convergence (near Lat. 40°S), thus suggesting that Subtropical Subsurface Water forms and sinks there. Although such water is therefore expected to be oxygen-rich, we do not understand why the mid-depth oxygen structure of the South Indian Ocean should be so very different from that of the South Atlantic and South Pacific. Whatever the explanation, we think of Subtropical Sub- surface Water in the Indian Ocean as a layer of high oxygen and of salinity transition,


and of Antarctic Intermediate Water as a layer of low salinity and of oxygen transition. Under this classification, northward-moving Antarctic Intermediate Water meets

high-salinity water moving southward from the North Indian Ocean in latitudes 5-15°S, mixes into it, and becomes fairly well obliterated. The shallower, northward- moving Subtropical Subsurface Water, however, lies above the high-salinity core of North Indian water and appears to penetrate well north of the equator. It is primarily this penetration to which we attribute the Somali Basin salinity minimum: if a layer in which salinity decreases with depth is also characterized by a relative maximum in oxygen concentration, and comes to overlie highly saline water below, the result will be formation of a layer having a salinity minimum as well as an oxygen maximum; and the salinity minimum will not in general coincide with the oxygen maximum. These are the properties of the intermediate fresh layer in the Somali Basin. This scheme of movement is a possible one, moreover, because the value of at at the salinity minimum is about 26.7-26.8, which is also the value of at near the center of the Subtropical Sub- surface Water in the South Indian Ocean.

The intermediate salinity maximum in the North Indian Ocean has usually been attributed to the very saline subsurface outflow from the Red Sea (e.g. SVERDRUP et al., 1942, chap. 15). TCHERNIA (1957), however, has examined unpublished Mabahiss data, and has shown that the Persian Gulf is also an important source of high-salinity water in the Arabian Sea; he suggests that these two sources combine to form the intermediate salinity maximum, and that it should be described not as " Red Sea Water," but as " Arabian Sea Water." On the basis of more recent observations, it seems to us that Persian Gulf Water can be distinguished from Red Sea Water, that it is the latter which is responsible for the salinity maximum in the Somali Basin, and that the more localized nomenclature is accordingly preferable to the general term "Arabian Sea Water."

The temperature-salinity curve for Discovery H Sta. 2901 (Fig. 10), which was occupied in the Gulf of Aden close to the strait of Bab-el-Mandeb, shows an extremely pronounced salinity maximum (36-78~,o), which is unquestionably due to Red Sea outflow alone. On Discovery Sta. 5016 (Fig. 10), at the mouth of the Gulf of Aden, the maximum is less pronounced (35.98~o), hut occurs at about the same density as the former (,rt = 27.1-27.3), and is certainly also an effect of Red Sea outflow. Off Oman, however, at Discovery Stas. 5052 and 5070 (Fig. 10), the large salinity maxima occur at a much lesser density, at about 26.6, but the T-S curves for both stations have small secondary maxima at a at of about 27-2-27.3. Clearly it is this secondary maximum which is related to the high-salinity layer in the Gulf of Aden, and the layer containing the primary maximum is distinctly different and must have a different origin. We know of no additional salt source in the Arabian Sea except the Persian Gulf; Stas. 5052 and 5070, moreover, are not far from its mouth. It seems very likely, therefore, that the low-density salinity maximum is produced by the Persian Gulf outflow, and that the major effects of the two sources can be distinguished by a simple criterion: a salinity maximum at at greater than 26.9 represents Red Sea Water, and a salinity maximum at at less than 26.9 (but greater than 26-0) represents Persian Gulf Water. In a somewhat different presentation, ROCHFORD (1964) has also distinguished these two maxima, and made similar suggestions concerning their origin.

Double maxima occur on several Argo stations, notably Sta. 58 (Fig. 7) and Stas. 61 and 62 (Fig. 8); as an example, see the T-S curve for the upper portion of Sta. 58


(Fig. 10). As on the Oman stations, we see two salinity maxima, one at at equal to 26-6, which we identify with Persian Gulf Water, and the other at at equal to 27-2, which we identify with Red Sea Water. In contrast to the Oman stations, however, the Red Sea maximum on Sta. 58 is much the more prominent. In fact, on nearly all the Somali Basin stations, the greatest intermediate-depth salinities occur at at equal to 27-2-27.3; hence we conclude that the pronounced, intermediate salinity maximum is of Red Sea origin, and that the high-salinity layer itself is principally Red Sea Water, with only a small component of Persian Gulf Water.

The foregoing discussion has been concerned basically with locating the source waters responsible for the existence of the low-salinity and high-salinity layers in the Somali Basin. To understand the actual intermediate-depth configuration of water masses and the shapes of the T - S curves, we must examine in more detail the temperature-salinity characteristics of the several regions from which intermediate-depth water moves into the Somali Basin. As representative examples of the vertical distribution requiring explanation, T - S curves for Argo Stas. 50 and 57 have been plotted in Fig. 9. Station 57 is qualitatively fairly typical of the 1964 Argo and Discovery data as a whole, while Sta. 50 illustrates the extreme sinuosity found at a few stations. In fact, inasmuch as nearly every observation on the latter station is a relative maximum or minimum at intermediate depths (at about 26.4-27.6), probably the curve connecting the points badly aliases the true structure; we include the station only to show the occasion- al occurrence of more extreme irregularity than seems typical. We believe that an adequate qualitative account of this structure can be given in terms of the five water masses described above: Antarctic Intermediate Water, Subtropical Subsurface Water, Red Sea Water, Persian Gulf Water, and Equatorial Pacific Water; T -S curves characteristic of these water masses are also shown in Fig. 9.

On Discovery H Sta. 1758, the salinity is fairly uniform and relatively high from the surface down to at equal to 26-4: this layer appears to be what IVANENKOV and GValN (1960) call Subtropical Surface Water. The high salinity is probably due to the fairly large excess of evaporation over precipitation in these latitudes; according to the Morskoi Atlas (ISAKOV et al., 1953) the annual excess is about 500 mm at 30°S, whereas at 10°S the difference is nearly zero, and at 50°S precipitation exceeds evaporation by some 300-4(10 mm annually.

In the at-interval 26.4-27.0, salinity decreases rapidly and linearly with decreasing temperature, and thus defines the layer of Subtropical Subsurface Water. The layer of minimum salinity, the Antarctic Intermediate Water, occurs below this, at at equal to 27-0--27-6.

Meridional profiles (e.g. MUROMTSEV, 1959) suggest that much of the water of at less than 27-6 spreads gradually northward, and encounters the South Equatorial Current in latitudes 5°-15°S. Norsel Sta. 8, located in the center of the Current (TcnERNIA et al., 1958), shows that in so doing salinities at all at less than 26.7 become much reduced. Most likely this reduction occurs through incorporation of water of such density into the very fresh water transported westward from the Eastern Archipel- ago by the South Equatorial Current. (The low surface salinity, however, is probably an effect of water from the Bay of Bengal, as described above). Snellius Sta. 146 demonstrates the low and remarkably uniform salinity (34.5-34.6~oo) of water from the Banda Sea. Between at equal to 26-7 and 27-0 on Norsel Sta. 8, we see Subtropical Sub- surface Water, little different from the lower part of that layer on Discovery H Sta.


1758. It is not clear why the Banda Sea effect should cut offso abruptly near at equal to 26.7 when there seems to be ample low-salinity water at greater densities in the extreme eastern Indian Ocean to freshen the lower as well as the upper part of the Subtropical Subsurface Water; perhaps the cut-off is related to large vertical shear in the South Equatorial Current, such that water much deeper than that of at equal to 26-7 simply is not carried westward in significant amounts. Salinities below at of 27.0 are about 0.3%o higher at Norsel Sta. 8 than at Discovery H Sta. 1758; this increase is plainly due to mixing of northward-moving Antarctic Intermediate Water with southward- spreading Red Sea Water (cf. Discovery Sta. 5016), for these water masses occupy about the same arintervat.

Argo Stas. 50 and 57 show that near-surface water in the northern Somali Basin (at of 26.4 occurs at a depth of 150-200 m) is 0-1-0.3~o more saline than that in the South Equatorial Current. This increase is probably due both to increased evaporation under the strong southwest monsoon, and to mixing of water brought northward into the Basin with the very saline surface water of the Arabian Sea. As pointed out above, the high salinity of this water (exceeding 36.1%o on Discovery Sta. 5070) is due to the extremely large excess of evaporation over rainfall in the northwest Arabian Sea.

Both Sta. 50 and Sta. 57 have salinity maxima in the at-interval 26.5-26.7; comparison with Discovery Sta. 5070 suggests that these are caused by Persian Gulf Water, as at Argo Sta. 58 (Fig. I0). Below the maximum on Sta. 57 there occurs a pronounced salinity minimum lying between the at-limits of Subtropical Subsurface Water at Norsel Sta. 8; as we stated in our preliminary discussion, we believe this minimum forms principally through northward transport of such water--as either a slow spreading, or a more rapid advection by the Somali Current. Since the minimum salinity occurs at a at only a little greater than that at the junction of Subtropical Subsurface Water and water showing Equatorial Pacific influence at Norsel Sta. 8, probably water from the Archipelago must be partly responsible for the low-salinity layer; although on account of both the difference pointed out above between the oxygen values of Banda Sea Water and Somali Basin low-salinity water, and the fact that in the at-interval 26.5-26-7 the Norsel Sta. 8 T-S curve lies much closer to the Discovery H Sta. 1758 curve than to the Snellius Sta. 146 curve, Banda Sea Water can only be of small import- ance in the formation of the layer. Argo Sta. 50 has a minimum salinity at 13°C, and another at 11 °C, both of which we interpret in the same way as the minimum at Sta. 57. Probably the maximum between them at 12.5°C is due to Persian Gulf Water, like the one above it.

At at greater than 27.0 there occur two strong salinity maxima at Argo Sta. 57, which are certainly effects of the Red Sea outflow (compare the T-S curve for Dis- covery Sta. 5016). The minimum between the two maxima we attribute to a slight penetration of Antarctic Intermediate Water, which, as noted above, occupies about the same ~t-interval as Red Sea Water. The shape of the T-S curve for Argo Sta. 50 in this ~t-range is similar to that for Argo Sta. 57, and we explain it in the same manner.

Thus at intermediate depths in the northern Somali Basin we envisage an elaborate interpenetration of relatively fresh, high-oxygen water masses, and relatively saline, low-oxygen water masses. Between at equal to 26-4 and 27.0 we believe we see interfingering of Persian Gulf Water and Subtropical Subsurface Water (having a small component of Equatorial Pacific Water from the Banda Sea), and between at equal to 27-0 and 27.6, interfingering of Red Sea Water and Antarctic Intermediate Water. In


the lower err-layer the saline water mass is dominant, and in the upper layer the fresh water mass is the more prevalent. Naturally this interpretation of the intermediate depth complexity of the Somali Basin is provisional. In particular, our association of a high-salinity or low-salinity source with nearly every relative salinity maximum or minimum at mid-depths is not strictly a logical necessity, but is suggested by the apparent presence of such sources in the Indian Ocean. Our interpretation accords with the data presently available to us, but may require substantial modification as the other, more widespread observations of the International Indian Ocean Expedition become accessible. It will be most instructive, for example, to examine charts--prepared from these new data taken over the entire Indian Ocean--showing the distribution of salinity (and oxygen) on particularly significant surfaces of constant density (e.g. ~rt equal to 26-6 and 27.2).

Deep water

Below the salinity-maximum layer the distributions of temperature, salinity and dissolved oxygen are all very smooth and simple, quite in contrast to the striking irregularity of intermediate-depth water. Rather arbitrarily, we shall take the upper limit

I I

2.0

I.O

o.o

t~

tu

tu h.

I.--

Q_

I I l l l l l [ l l l | l

t

•

"k

l

• DISCOVERY ~,~ 2871 • ARGO (DODO'~2E) 48

ARGO (DODO "~JZI) 68

L I I I i I I J r ~ I I I I 34.6,5 .70 ,75 ~,4.80 SALINITY (%0)

Fig. 1 I. Relations between potential temperature and salinity demonstrating the source of deep water in the Somali Basin. Discovery H Sta. 2871 was made at 50 ° 09"S, 51 ° 23'E. The positions

of Argo Stas. 48 and 68 are shown in Fig. 1.


of this North Indian Deep Water to be the 27.6 crt-surface (temperature about 6.5°C, salinity about 35.1~o), which on all profiles lies at about 120(O1300 m. At greater depths temperature decreases monotonically to abyssal values of 1.3-1.4°C; on a few stations (e.g. Stas. 47 and 58, Fig. 7) the temperature increases with depth adiabatically by a few hundredths of a degree in the bottom 300-500 m. The lowest potential temperature on these sections is 0.90°C, at a depth of 4915 m at Sta. 68 (Profile F, Fig. 8). (The very lowest potential temperature recorded by the Argo or Discovery, however, was 0.85°C at 4988 m at Argo Sta. 14, which is not included on the profiles). Salinity also decreases monotonically, to abyssal values of 34.71-34-72%o. The dissolved oxygen concentration, however, increases with depth from about 1-0 ml/l. near et equal to 27-6 to 4.1-4-3 ml/l. in the very deep water.

At about 2°C the T-S curves in Fig. 9 for Discovery H Sta. 1758 and Argo Stas. 50 and 57 (Fig. 9) come together, approximately on to the T-S curve for Antarctic Cir- cumpolar Water (SVERDRUP et al., 1942, chap. 15). The deep-water relations between potential temperature (0) and salinity are illustrated on a finer scale in Fig. 1 I. Dis- covery H Sta. 2871 lies midway between the Antarctic and Subtropical Convergences, and illustrates the characteristics of Antarctic Circumpolar Water at depths greater than 1000 m near the 50th meridian. The salinity maximum is due to North Atlantic Deep Water carried eastward from the South Atlantic by the Circumpolar Current, and, as would be expected, it grows weaker to the east and stronger to the west of Dis- covery H Sta. 2871. This particular station was chosen for comparison with the two Argo stations in the Somali Basin because communication at depth between the circumpolar area and the Somali Basin is severely limited by the great, roughly meridional ridge systems of the Indian Ocean: the principal connecting passage lies approximately in the longitude range 50°-65°E. In the interval of potential temperature 0.9-1-8°C the Somali Basin stations may be slightly saltier than the Discovery H station, but it is not really possible to tell, because the salinities on Discovery H Sta. 2871 were titration measurements and hence not accurate to better than ~ 0,02~o. We conclude from the close coincidence of observations, however, that the water at depths greater than about 2500 m in the Somali Basin is essentially Circumpolar Water. Such an origin accounts for the high oxygen of the deep water, as the concentrations in Circumpolar Water commonly run about 4.0-5.0 ml/1. (At Discovery H Sta. 2871, the oxygen concentrations below water of potential temperature 1.8°C were all greater than 4-3 ml/l.).

Since the Argo salinities were measured by conductivity bridge--and are therefore accurate to a few thousands of a part per thousand--the difference in O-S relations between Stas. 48 and 68 must represent a real, though very small, difference in water characteristics. The difference seems to be geographically systematic, moreover, and is discussed at length below in connection with the pattern of deep flow.

In the deep water above 2°C, the salinity in the Somali Basin is greater than in the South Indian Ocean (Figs. 9 and 11). These high values seem to be an effect of the Red Sea and Persian Gulf outflows, whose effects are not confined, of course, to their immediate core layers, but must be felt through entrainment over a wide range of depths. The rather low oxygens above 2°C, and the difference in salinity at these depths between the Arabian Sea and the Somali Basin (cf. Discovery Stas. 5016 and 5070 with Argo Stas, 50 and 57) are consistent with this point of view,

Water masses and patterns of flow in the Somali Basin during the southwest monsoon o f 1964 847

F L O W P A T T E R N S

We have drawn horizontal charts of several kinds showing property distributions in an attempt to infer (mainly by core methods) the pattern of flow in and around the Somali Current. This approach often provides a fairly straightforward way to find qualitative flow features when the motion itself is more or less steady; but when, as in the present case, the near-surface circulation is known to reverse periodically, property distributions may not indicate the sense of motion unambiguously. This weakness in method did not prove particularly troublesome in dealing with the near-surface and deep water but it did contribute to precluding much discussion of flow at intermediate depths. With this general reservation, we have tried to construct in the simplest way possible a coherent, self-consistent picture of the flow pattern by tracing characteristic features in the distributions of temperature and salinity.

4 0 o 5 0 ° 5 5 ° 4 5 °

t 0°t~ R A S

I ~ A S M J

I 0 °

"~_ 24

~I • e • °

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26

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' ~ ~ - - [ ' ~ I 1 1 ~ I I " - - I I [ i ' 4 0 ° 45o 5 0 o 5 5 °

Fig. 12. Surface temperature (°C) in the Somali Basin as observed between 4 August and 4 September 1964. The isotherms are drawn at 2 ° intervals, with the 25 ° isothei'm (dashed line)

included to clarify the offshore structure. Dots indicate station positions.


Near-surface water

The course and lateral extent of the Somali Current are clearly indicated in the distributions of surface temperature and salinity (Figs. 12 and 13) and in the topography of the 20°C isothermal surface (Fig. 14). The surface charts are based on the Argo and Disco~ery station data and on the surface observations made while under way between stations. Occasionally it proved difficult to reconcile the latter surface observations with the station data, however, and such surface observations have been ignored in drawing the charts, as the station data are presumed to be more reliable. Furthermore, it was obvious that rapid changes were occurring north of the ninth parallel (especially west of 53°E), with the result that repeated observations inthe same area were not compatible. Thus in the triangular area between Ras Hafun, Cape Guardafui, and Socotra, the indicated distributions are based only on Discovery observations made in the period 18-21 August, even though the Argo passed through there a few days later, and the Discovery itself, after putting into Aden, returned to the

40 ° 4 5 * 5 0 ° 55"

ABD - A L - KUC~t , ~ O C . O ! T R ~,

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Surface salinity (~oo) in the Somali Basin as observed between 4 August and 4 September 1964. The isohalines are drawn at 0.1~o intervals. Dots indicate station positions.

Water masses and patterns of flow in the Somali Basin during the southwest monsoon o f 1964 849

40* 45* 50" ,55 °

i A B D - A L - KURI "M__--- ,'J C O T R A ,,,=,

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Fig. 14. Depths o f the 20°C isothermal surface in the Somali Basin, 4 A u g u s t a September 1964. The depth contours are drawn at 50-m intervals, with the 125-m contour included to

suggest the very slight structure near the equator. Dots indicate station positions.

area at the end of August. [The later temperature pattern observed by the Discovery is shown by FOXTON (1965)].

In both Figs. 12 and 13, the Somali Current appears as a banded zone 100-150 miles wide lying against the African shoreline; as one enters the zone from the coastal side, salinity and temperature both increase, then decrease, and finally, rise again to meet the warm, saline water on the seaward side of the Current. South of about latitude 3°N, the Current is defined more sharply by the salinity than by the temperature, while to the north its edges are shown more clearly in the temperature field. At the surface on the whole, the Somali Current carries fresh water northward, which, as remarked above, can be traced back to the South Equatorial Current, lying approximately between latitudes 5°-15°S. Apparently the banded structure described above is also present in the South Equatorial Current, for a similar progression of change was observed by the Argo between latitudes 7°-11°S on a line of stations (not depicted


here) made approximately along Long. 55 ° 30'E : from south to north, the surface temperature rose from 24.5°C to 25-9°C, then decreased to 24.5°C, and finally rose again above 25°C; while the surface salinity first increased from 34.7%o to 35-1%o, fell back to 35.0~o, and then increased to more than 35.2~o.

The westward flow which feeds the Somali Current is severely obstructed by a series of islands, seamounts, and ridges lying between the northern tip of Madagascar (12°S, 49°E) and the Seychelles Islands (4°-5°S, 55°-56°E). There seem to be two passages, one between Madagascar and Farquhar Island (10°S, 51°E), and another between Providence Island (9 ° 30'S, 51°E) and Alphonse Island (7°S, 53°E). According to Discovery observations, the line of minimum salinity--and hence pre- sumably the axis of the South Equatorial Current--went through the southern passage during July, 1964. A year earlier, however, Atlantis H data indicated flow through the northern passage. Whichever route the flow takes, a substantial part subsequently turns northward as the Somali Current.

Both sets of surface data suggest some sort of irregularity in the path of the Somali Current near Eat. 4°N, but its nature is not clear: the data are too few for precise definition, and the situation was complicated by a large concurrent inflow from the east at this latitude (SWALLOW and BRUCE, 1966). The interpretation shown in Figs. 12 and 13 seems the simplest one, but should not otherwise be regarded as more than tentative. According to this interpretation, however, at least part of the Somali Current appears to have turned abruptly eastward near Lat. 4°N, moved some 100 km or more offshore, and then turned back to lie close against the coast again at Lat. 5°N. This meander seems especially noticeable in the indicated deformation of the low- salinity tongue (salinity <35.2~o but >35.1~o) embedded in the offshore part of the Current (Fig. 13). The band of low temperature described above as corresponding to this tongue appears to be similarly deformed near the fourth parallel, but the selection of isotherms in Fig. 12 somewhat obscures the existence of the band: the temperature minimum found at Discovery Sta. 5537 (temperature <24.0°C) is continuous with that at Argo Sta. 9, but the minimum temperatures between are all just greater than 24-0°C. Thus relative minima were actually observed at Argo Stas. 18, 19, 26, and 29, but all of value 24.1 °C. The current measurements made at the same time as these observations (SWALLOW and BRUCE, 1966), however, do not confirm meandering flow here, but on the other hand they are not really sufficiently numerous to preclude it.

Our reason for pointing out such an unclear feature in the flow pattern is that it overlies a pronounced feature in the bottom topography. AIthough isobaths of depth less than 1800 m parallel the coastline closely in this general area, the deeper isobaths (down to about 4000 m) bulge out very markedly from the coastline in latitudes 3°-5°N. This deep promontory (see below, Figs. 15-17) is by far the most striking irregularity on the East African continental slope between Lats. 2°S and 6°N. Thus if part of the Somali Current did indeed meander upon encountering this promontory, through a response to depth changes similar to that which apparently affects the Gulf Stream (WARREN, 1963), then the Somali Current must be a truly deep ocean current, penetra- ting into the deep water with non-vanishing (though probably small) bottom velocities. More direct evidence for deep flow is discussed below, though lack of conclusive evidence for vertical continuity between the deep and near-surface flows necessarily leaves the question of the depth of the Somali Current in this speculative state (see also SWALLOW and BRUCE, 1966).


North of the meander, the surface data (Figs. 12 and 13) show that between 7 ° and 9°N the Somali Current turned eastward and crossed the 55th meridian between latitudes 6°-8°N. The data near the southern edge of the Current are a little puzzling, however, in that there seems only very rough consistency between the temperature and salinity. The relatively high temperatures near 7 ° 30'N, 53°E, for example, do not correspond to any obvious feature in the salinity distribution, nor is the enclosed region of high salinity lying along Lats. 4°-5°N reflected very strongly in the temperature field.

4 0 * 45* 50*

ABD--AL- KURI

55 °

I 0 °

RA5 HAFUI

RAS MABegR I0"

!o

7 5 5 ~

5* •

*

750

1

755

0 • O* 7 5 0

5"

4 0 °

I I

i J

45* 50 ° 55"

Fig. 15. Salinity (minus 34.000~), on the surface of potential temperature 1"80°C, 4 August-4 September 1964. The surface ranged in depth from 2500 to 2800 m, having an average depth over all stations o f approximately 2600 m. Isobaths for the extreme depths (dashed lines) show the lateral boundaries o f the Somali Basin at this temperature. The isohalines are drawn at 0"005~o

intervals. Dots indicate station positions.

Near the coast north of latitude 7°N is found the very striking region of upwelling cold water which was mentioned in connection with Profile C (Fig. 5). It is most pronounced just off Ras Mabber, where the Argo bow thermistor registered the lowest surface temperature seen on the cruise, 12.8°C, This water apparently spreads away

852 BRUCE W A R R E N , HENRY STOMMEL a n d J . C . S W A L L O W

from the coast as a thin layer of fairly uniform salinity (35-1-35.2%o), whose temperature increases rapidly with distance from shore. Some of the cold water seems to be swept into the Somali Current as it leaves the coast, to form a remarkably narrow tongue of water colder than 20°C along the northern edge of the Current. This feature

~0 o 45 ° 50 ° 55 °

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Fig. 16. Salinity (minus 34"000~o) on the surface of potential temperature 1-40°C, 4 August--4 September 1964. The surface ranges in depths from 3200 to 3500 m, having an average depth over all stations of approximately 3350 m. lsobaths for the extreme depths (dashed lines) show the lateral boundaries of the Somali Basin at this temperature. The isohalines are drawn at 0-005~0~


was observed as far east as longitude 55°E, where, as noted above, just north of 8°N the bow thermistor on the Argo recorded a surface temperature of 20°C. The feature can be seen depicted near the surface both between Stas. 63 and 64 (Fig. 8a), and between Stas. 47 and 48 (Fig. 7). [The later Discovery observations made in the vicinity of Longs. 53°-54°E (FOXTON, 1965) show that over a period of about ten days both the cold tongue and the northern edge of the Somali Current moved about one degree of latitude north of the locations shown here].

The upwelled cold water is intruded upon from the north by an inflow along the western side of the channel between Cape Guardafui and Abd-al-Kuri of very warm,


saline water from the Gulf of Aden. The presence of this water at Stas. 5556, 5560, and 5561 (Profile D, Fig. 6) was pointed out earlier, and the profile shows that the intrusion is less than 100 m deep. Two days after the Discovery occupied this section, the Argo made surface observations along the same line, and found water 1-4°C warmer and about 0.1%o more saline than had the Discovery; hence an inflow did indeed seem to be occurring at the time of observation. It is not possible from these data to make a

4 0 * 4 5 ° 5 0 ° 5 5 *

~ ~ , - = v - ~ - - : ~ - ~ ~ ~ . . . . ~ 1 "k, v ~ J - ~ c " - ~ ~ ° ~ [J/

A B D - - A L - K URI

R A S

R A S M

. i

10 °

,120

5*

f , , ,

/

f

. 7 1 5 /

' t . " °

I

.720 ¢, I / ~

# - ; . . ,

.710

,~,5ooM.i

i /%/:;; i ( t ~ . . . . . . . . . I"

4 0 * 4 5 * 5 0 " 5 5 *

Fig. 17. Salinity (minus 34'000%o) on the surface of potential temperature 1-00°C, 4 August-4 September 1964. The surface ranges in depth from 4200 to 4500 m, having an average depth over all stations of approximately 4350 m. Isobaths for the extreme depths (dashed lines) show the lateral boundaries of the Somali Basin at this temperature. The isohalines are drawn at 0-005%0


reliable estimate of the rate of inflow, but surface speeds of the order of several miles per day seem required to account for the observed differences.

Another body of warm, saline water intrudes westward from the Arabian Sea upon the upwelled water. On Profile F (Fig. 8) this second body appears at Stas. 62 and 63, as also mentioned above, and extends to depths of about 200 m. Two weeks after Argo Stas. 50 and 51 were made (near l l °N , 53°E), the Discovery occupied Sta. 5575 at 11 ° 00'N, 53 ° 16'E (not shown on the charts), and found a surface temperature of


25.26°C and a surface salinity of 35.804%o. Hence this intrusion too was apparently taking place during the observational period, with speeds of the order of ten miles per day suggested. There also appears to be flow northward along Long. 52°E, and perhaps eastward between Socotra and the Arabian Sea intrusion, but cooler, and of lesser salinity; it may represent transport of upwelled water out of the Somali Basin. Thus what appears on Profile F as the western half of an eddy-like structure, with anticyclon- ic rotation, may be a transient feature of rather short period, although there are really no guidelines here to inform us clearly.

The thermocline is roughly defined below by the 14 ° isotherm, and above by the 22 ° or 24 ° isotherm (Figs. 3-8). The layer itself is 50-150 m thick, and is very shallow, reaching the surface in the upwelling area near the coast (e.g., at Stas. 4246 , Fig. 5) and only extending as deep as 250 m in the north-central part of the area surveyed (Stas. 5544-5547, Fig. 4) and in the warm intrusion from the Arabian Sea (Sta. 61, Fig. 8). We have adopted 20°C as an isothermal surface representative of the thermocline as a whole, and we have mapped its depth (Fig. 14) as another indicator of near- surface flow.

South of Lat. 3°N the thermocline shows very little depth-variation--as is to be expected in quasi-geostrophic currents very near the equator. The slight variations which appear may have nothing at all to do with currents but may simply be reflections of internal waves or other short-period vertical displacements.

Between about 3 ° and 4°N the 20°C surface becomes abruptly shallower near the Somali coast, as both the 100-m and 50-m depth contours come out of the shore. Off- shore, the 150-m contour departs from its zonal orientation, and north of 3°N tends to lie on a northeast-southwest line. Thus from 3°N to 6°N the 50-m, 100-m and 150-m depth contours roughly parallel the coastline, and match fairly closely, with respect to both path and width, the Somali Current at the surface as inferred from the distributions of surface temperature and salinity (Figs. 12 and I3). Such correspondence is expected, of course, from the geostrophic-hydrostatic character of the flow. The northward increase in Coriolis parameter requires that the thermocline rise downstream along the inshore side of the Somali Current, but the change in depth that we actually see on the chart is puzzling, in that it occurs so abruptly, and not more gradually with latitude. In the offshore part of the Current, the data presented here are somewhat less clear with regard to pattern of flow than were the surface observations. Consequently the large perturbation of the 150-m contour shown in this area, though consistent with the observed depths of the 20°C isotherm, is not strictly required by them, but was included in Fig. 14 to represent the meandering flow suggested by the fields of temperature and salinity at the surface.

Farther offshore in Lats. 3°-5°N, the orientation of the 150-m and 200-m depth contours indicates considerable inflow to the Somali Current from the east. Inasmuch as the rather anomalous, elongate body of high salinity water pointed out above in connection with Fig. 13 (Discovery Stas. 5540-5546, Argo Stas. 33-34)lies directly above these contours, the indicated inflow may have been advecting this body into the Somali Current at the time of observation, having removed it from some " source" region earlier.

In the latitude interval 7°-9°N, the shape of the 20°C surface confirms the evidence in the surface distributions that the Somali Current turns eastward. The dense crowd- ing of depth contours on the 55th meridian (Sta. 64) suggests very large velocities in the


surface water. Closer to the Somali coast near the northern edge of the current, the depth contours should probably be interpreted with caution because of the vertical motions and large geostrophic departures associated with the upwelling.

The contour pattern between Socotra and the Somali Current is also consistent with the flow scheme inferred from the surface data. The large depth change between Stas. 62 and 63, implying westward flow nearer the surface, directly underlies the warm, saline intrusion from the Arabian Sea; while the indications in the surface temperature and salinity of flow northward along the 53rd meridian, and then eastward just south of Socotra, are also borne out by the underlying topography of the thermocline. It is worth noting, however, that although the pattern of streamlines suggested by this topography may have persisted for a moderate period of time, the property distributions could not have so endured, because the water moving westward across the 55th meridian between Lats. 8 ° and 10°N is of very different characteristics from that moving eastward between Lats. 10 ° and 12°N. This conclusion is corroborated by the evidence given above for westward advance of the high-salinity water during the period of the survey.

Deep water

Comparison of the temperature and salinity profiles for Argo Stas. 33-41 (Fig. 3) reveals a change in character of the Deep Water mass (below 2500 m) with approach to the continental slope. Between Stas. 34 and 35, for example, the deep isotherms and isohalines are inclined in opposite directions, such that at a given temperature the water just inshore is slightly fresher than that offshore. Similar opposed slopes are associated with the differing 0-S properties of Argo Stas. 48 and 68 (Fig. 11). The presence of the " f r e s h " water is not fortuitous, but seems characteristic of the vicinity of the continental slope. To show its occurrence, we have prepared charts giving the distribution of salinity on three surfaces of constant potential temperature: 0 ~ 1.80 °, 1-40 °, and 1-00°C (Figs. 15 and 17). The contour interval used (0.005%o) is close to the limit of accuracy of salinometers, but the data are of high quality and form a coherent geogra- phical pattern at this resolution; probably small details are not significant, but there can be little doubt about the validity of the general pattern. Very likely the observations at 0 : : 1.00°C (average depth about 4350 m, only a few hundred meters above the central abyssal plain) would be too few by themselves for one to accept contours with much confidence, but since the data available are entirely consistent with the more numerous observations at 0 -~ 1.40°C and 1.80°C, it seems virtually certain that the salinity pattern is basically the same from 2500 m to the bottom.

On all three charts we see a band of slightly fresh water, which is separated from the continental slope in the south, impinges on it at about 2°N, and then lies against it; between latitudes 9°-10°N the band turns and assumes a more nearly zonal orientation. We interpret this distribution as indicating deep flow northward and eastward near the continental slope (from the bottom up at least to 2500 m). It should be emphasized, however, that our main basis for supposing the flow to be northward rather than southward is the single very fresh station at 1 ° 58'S, 49 ° 20'E (Argo Sta. 14); without that station (and for 0 ----- 1.80 °, Argo Sta. 12 at I ° 54'S, 47 ° 25'E) there would be no significant salinity gradient along the axis of the fresh band, and hence no indication of the sense of motion.

A curious feature of these distributions is the pair of high-salinity pockets lying


between the continental slope and the fresh band, and located in indentations of the bottom contours at 5 ° 45'N, 50 ° 05'E and north of 10°N between Longs. 52°-55°E. They rather suggest saline backwaters trapped in coastal embayments.

Except south of Lat. 2°N the deep movement seems to follow the bottom topography much more closely than the surface current. The axis of the deep flow turns eastward about 1-2 ° of latitude farther north than the surface current, as do the very deep isobaths (Fig. 17). Possibly in the near-surface water the seaward movement of upwelled water and the intrusions from the Gulf of Aden and Arabian Sea combine in some way to displace the Somali Current to the south. On the northern part of the surface O = 1.00°C (Fig. 17) the 34.720~oo isohaline swings much farther to the south in crossing the 55th meridian than comparable isohalines on the other charts; this swing is required by only one observation (Argo Sta. 66 at 5 ° 45'N, 55 ° 26'E), which might easily be in error by a few parts per million, but it is suggestive of flow deflected southward to find a passage through the ridge running approximately northeastward from 6°N, 53°E. The line of minimum salinity ( < 34-715%o) on this potential temperature surface, however, runs into a barrier--the Carlsberg Ridge--between Longs. 55°-56°E. The Ridge runs southeastward from Socotra, approaching the Mid- Oceanic Ridge near 3°S, 70°E; the deepest passage across it seems to be at 9 ° 50'N, 55 ° 45'E (just visible in Fig. 17), where the sill depth is 410(0-4200 m, a little shallower than any observed water with a potential temperature of 1.00°C. Whether water so deep actually does climb over the sill into the Arabian Basin is not known.

As already demonstrated, all water in the Somali Basin of potential temperature less than about 1-8°C is essentially Circumpolar Water. The slightly low salinity of the deep flow adjacent to the continental slope indicates that it has arrived more recently from the circumpolar area than the bulk of the deep water in the Somali Basin, but the nature of its northward advance from high southern latitudes is not at all dear. We are unable to tell, for example, whether the deep flow is part of the reversing monsoon circulation, or instead a fairly steady movement. Obviously it would be of the greatest interest to find out to what extent such fresh water is present towards the end of the winter monsoon. In either case, the deep flow, unlike the surface current, cannot in any way be an extension of the South Equatorial Current. It appears to come directly from south of Lat. 3°S (Figs. 15-17) and deep fresh water of this sort was in fact observed south of the Seychelles on the line of stations mentioned above along Long. 55 ° 30'E. (Between Madagascar and the Seychelles the passage separating Providence Island from Alphonse Island is deeper than 4500 m, and is quite adequate to permit free flow between the Reunion Basin to the southeast and the Somali Basin to the northwest). But the Seychelles lie on the northern extremity of the Seychelles- Mauritius Ridge, which runs southeastward from the Seychelles to about 12°S, 62°E, then south-southwestward to Mauritius at 20°S, 57 ° 30'E; the Ridge is no- where deeper than 2000 m, and thus it completely blocks deep westward movement.

In the western Indian Ocean south of Lat. 25°S, however, LE PICI-ION (1960) has found two cores of relatively fresh deep water moving northward from the Antarctic; one core lies along the 60th meridian, the other between longitudes 25°-40°E. As far north as 30°S, salinities less than 34.710~oo have been observed at depths greater than 4000 m. Apparently such water can enter the Reunion Basin from the Kerguelen Basin across a sill of probable depth about 4500 m near 26°S, 65°E, and possibly also from the Agulhas Basin across a sill some 4000 m deep near 35°S, 46°E. Perhaps,


then, the deep flow found by the Argo and Discovery is a continuation of this northward movement far to the south, but if the deep flow in the Somali Basin changes drastically with the monsoons, the continuation would be only seasonal.

Intermediate depths Unfortunately, at intermediate depths there is considerable structure of small

horizontal scale (tens of kilometers or less) which our station spacing is simply not fine enough to resolve. Our data can therefore be contoured in a variety of different ways, with the result that attempts to infer flow patterns from proFerty distributions (e.g. the minimum and maximum observed salinities and oxygens, or salinities on isopycnal surfaces) have proved abortive. Consequently we are unable to describe the mid-depth field of motion.

Although in both Figs. 3 and 4 the layers of high and low salinity seem most markedly developed under the surface current, as if the mid-depth flow were also most intense close against the shore, even such gross features are difficult to interpret in terms of flow because of the possible ambiguity of core methods when applied to periodically reversing currents. We do not know how deep the seasonal current- reversal penetrates, and it is quite conceivable that the core of Red Sea Water was not moving southwestward, away from its source, at the time of observation--as one would think at first glance--but had done so instead during the previous winter monsoon, and was now retreating back to the Arabian Sea in response to the summer wind-system. (One feels less perplexed, of course, about the Subtropical Subsurface Water, since its source is to the south, but the confusion might be inverted with winter- time observations).

It is clear therefore that much more numerous and more closely spaced observations, including observations during the northeast monsoon for comparison, are required to give a convincing description of the mid-depth flow during the southwest monsoon. Nor will it be possible without such a description to establish with certainty whether the deep flow in the Somali Basin is vertically continuous with the Somali Current at lesser depths, as is hinted by the irregularity in the path of the surface current at Lat. 4°N.

C O N C L U D I N G R E M A R K S

It may help, as a guide to thinking about use of core methods for determining current direction and flow pattern, to present some simple times of response or "renewal " figured for different water layers on the basis of plausible volumes and transports. We approximate the region surveyed by the Argo and Discovery (Fig. 1) as trapezoidal in plan, with zonal bases of length 4 ° (northern boundary) and 12 ° (southern boundary), and with the meridional eastern boundary at 55°E of length 14°; then with a uniform vertical dimension of 4 kin, the total volume of the region amounts to 4.5 z 1015 m 3. We consider a boundary current 1½ ° wide at the surface and at intermediate depths, but only I ° wide in the deep water; we allow it to turn eastward about 4 ° south of the northern boundary. The area of the current is then 2-6 x 10 it m 2 at the surface and at mid-depths, and 1.7 x 10 it m 2 in the deep water, in contrast to the total area of the region of I 1-2 x 1011 m z.

A current of volume transport 30 × 106 ma/sec (Table 1) would require 1736 days


Table 1. Renewal time ford(fferent wate~ layers Basic horizontal area is a trapezoid with bases 4 ° and 12 ° long, and altitude l'4°---or about

1-1 x 1012m 2 Boundary current is taken as 1 ° wide in the deep water, 1½° wide above it

Water layer Renewal thne (days) "Boundary

Layer Volume Boundary Whale current thickness transport current region width

(m) (10 s ma/sec) alone (km)

Surface layer 200 30 J 20 Subtropical Subsurface [

Water 300 5 120 Red Sea Water 300 5 120 Deep Water 2000 25 158

86_ 15o

~78 150 778 t50

1037 loo

Full depth 4000 30 332 1736 Enclosed times are less than half a year; the associated volumes can therefore be rene~ed seasonally.

to fill this entire volume, or t o " flush " i t of water previously in it. The figure 30 × 106 mS/sec is a rough estimate of the kind of Sverdrup t1:ansport which a wind-stress distribution like that prevailing during the southwest monsoon could produce. Obviously, then, the complete renewal of all the water in the volume under consideration is unlikely during such a short interval of time as a single monsoon period: there would be water from many seasons included in the volume.

On the other hand, we do not really expect all of the mass flux during the southwest monsoon to be distributed uniformly over the, entire volume: on the contrary, it will ,mostly be in the near-surface waters, With the same mass flux we see that the entire surface layer (200 m thick) could be replaced in only 86 days (Table 1). Thus renewal or replacement of all surface waters is possible during a single monsoon, although some time lag might be expected in the more sluggishly moving offshore areas away from the western boundary current. I f the replacement were confined to the boundary current itself, the renewal time could be as small as 20 days. It is evident that within the a~xis of the boundary current all memory of the previous season is swept away.

The ease at intermediate depths, with respect t0 the Red Sea Water and the Sub- ttopicat Subgurface Water, is marginal. We do not, of course, expect such large volume transports at ' these depths. The number given in Table 1 for each layer is 5 ' x :106 m3/" sec--and'frfinkly is a mere guess. Th~ renewal time of 120 days for thewestern boundary current (for layers 300 m thick), however, suggests that a seasonal variation bY properties within i t should be noticeable, and that core methods may have applicability to th~ boundary current, The distribution of properties outside the current; howevei', would be a mixture of many seasons' flow, and would probably appear nearly unintel- ligible.

The Deep Water is spread over a much larger depth interval than the other water masses(roughly 2000 m). It seems that a transport o f at least 25 x 106 ma]sed would be


requi red for renewal o f the western b o u n d a r y current in this water mass in o rder to reveal any seasonal reversal .in flow; whereas the long renewal t ime o f 1037 days for such a t r anspo r t in the entire body of deep water is sufficiently great to make seasonal var ia t ions negl ig ib le there. Of course, whether the deep flow actual ly does reverse seasonal ly is s imply no t known.

Figures such as these are only to be taken as order-of-magf i i tude est imates and a ids t o th inking; and they are presented here with no other intention.

Acknowledgements--The authors are grateful to the officers, crews, and scientific parties of the Dis- covery and Argo, in particular to R. I. CURmE, W. S. WOOSTEg (chief scientists on the Discovery and Argo respectively) and J. CREASE for their collaboration; to N. W. RAKESaXAW for performing oxygen titrations on the Argo; and to A. MANTYLA, S. FERREmA, and D. WIRTZ for executing the Argo's hydrographic stations. Detailed, unpublished bathymetric charts of the Somali Basin were kindly furnished us by A. S. LAUGHTON. The work of the Discovery and Argo formed part of the United Kingdom and United States contributions to the International Indian Ocean Expedition, that by the Argo having been sponsored by the National Science Foundation. Analysis of data by one of us (B.W.) was supported by the Office of Naval Research under contract NONR-2196 (01); and the Massa- chusetts Institute of Technology participation (H.S.), by the National Science Foundation, Grant GP-2564.

Data--Argo Stasl 1-79 from Cruise Dodo VI are on file at the National Oceanographic Data Center, Washington, D.C., and Discovery Stas. 5016, 5052, 5070 and 5523-5573 are at World Data Center A. Discovery H Sta. 1758 is included in Discovery Investigations Station List: 1935-t937 (1944), and Discovery H Stas. 2871 and 2901, in Discovery Investigations Station List: 1950-1951 (1957). Snellius Sta. 146 is listed by VAN RIEL, et al. (1950), and Norsel Sta. 8 is listed by TCrrERNIA, et al. (1958).

R E F E R E N C E S

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cheskii. Glavnii Shtab Voenno-Morskikh Sil, Ministerstvo Oboroni S.S.S.R., 76 charts. IVANENKOV V. N. and F. A. GtmrN (1960) Water masses and hydrochemistry of the western

and southern parts of the Indian Ocean. Trudy Morsk. Gidrofiz. Inst., Akad. Nauk S.S.S.R., 22, 33-115. Eng. transl: Trans. Mar. Hydrophys. Inst., Acad. Sci. U.S.S.R. 22, 27-99. A.G.U.

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ROCHFORD D. J. (1958) Characteristics and flow paths of the intermediate depth waters of the southeast Indian Ocean. J. mar. Res. 17, 483-5124.

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water masses and patterns of flow in the somali basin during the southwest monsoon of 1964

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