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JANUARY 2002 75G A R R E A U D E T A L .

q 2002 American Meteorological Society

Coastal Lows along the Subtropical West Coast of South America: Mean Structureand Evolution

RENE D. GARREAUD, JOSE A. RUTLLANT, AND HUMBERTO FUENZALIDA

Department of Geophysics, Universidad de Chile, Santiago, Chile

(Manuscript received 22 November 2000, in final form 7 June 2001)

ABSTRACT

The typical conditions of the eastern boundary of the subtropical anticyclone [e.g., well-defined marineboundary layer (MBL), equatorward low-level flow] that prevail along the mountainous west coast of subtropicalSouth America are frequently disrupted by shallow, warm-core low pressure cells with alongshore and cross-shore scales of 1000 and 500 km, respectively. These so-called coastal lows (CLs) occur up to five times permonth in all seasons, although they are better defined from fall to spring. Marked weather changes along thecoast and farther inland are associated with the transition from pressure drop to pressure rise.

The mean structure and evolution of CLs is documented in this work, using a compositing analysis of 57episodes selected from hourly pressure observations at a coastal station at 308S during the austral winters of1991, 1993, and 1994, and concurrent measurements from a regional research network of nine automatic weatherstations, NCEP–NCAR reanalysis fields and high-resolution visible satellite imagery. Coastal lows tend to developas a migratory surface anticyclone approaches southern Chile at about 408S producing a poleward-orientedpressure gradient and geostrophically balanced offshore component in the low-level wind. At subtropical latitudesthe transition from negative to positive geopotential anomalies occurs around 850 hPa. Enhanced mid- and low-level subsidence near the coast and downslope flow over the coastal range and Andes Mountains leads to thereplacement of the cool, marine air by adiabatically warmed air, lowering the surface pressure at the coast andoffshore. As the midlatitude ridge moves to the east of the Andes, the alongshore pressure gradient reverts backand the easterly wind ceases to act. The recovery of the surface pressure toward mean values occurs as thecool, cloud-topped MBL returns to the subtropical coast, although the pressure rise can be attenuated by mid-latitude troughing. The return of the MBL resembles a Kelvin wave propagating along the coast from northernChile (where the MBL eventually thickened) into subtropical latitudes in about a day.

1. Introduction

The subtropical west coast of South America (north-central Chile; Fig. 1) is year-round under the influenceof the southeast Pacific anticyclone that results in asemiarid climate, with a very stable lower troposphere,relatively cold sea surface temperature, and predomi-nantly south-southwesterly wind along the coast. Thewell-defined marine atmospheric boundary layer(MBL), often topped by an extensive deck of coastalstratocumulus, is capped by a strong and persistent sub-sidence temperature inversion. At 308S the coastal MBLis about 800 m deep, and the inversion layer has a thick-ness of about 600 m and a temperature increment ofabout 68C (Rutllant 1994). This region is also charac-terized by its prominent topography: a coastal range thatin many places rises above 1000-m elevation, and theAndes Cordillera that rises sharply to its top above 4000m within 300 km of the coastline (Fig. 2). Under a wide

Corresponding author address: Dr. Rene D. Garreaud, Departmentof Geophysics, Universidad de Chile, Blanco Encalada 2085, San-tiago, Chile.E-mail: [email protected]

range of static stabilities, low- and midlevel westerlyflow is effectively blocked by these mountain ranges(Rutllant 1994).

Shallow, subsynoptic low pressure centers, not as-sociated with the polar front, are frequently observedin this region and referred to as coastal lows (CLs; Ru-tllant 1983). At coastal stations the lows are identifiedby a transient (;2 days) drop of surface pressure (;10hPa) at the same time that pressure is rising in the middletroposphere. The coastal pressure minimum generallyexhibits a southward displacement in the 278–378S lat-itude span, reaching occasionally as far as 428S in theaustral summer (Rutllant and Garreaud 1995). The im-portance of CLs in terms of their related weather chang-es, from shallow MBL and sunny conditions at theirleading edge (i.e., to the south of the pressure minimum)to overcast, cool, and moist conditions in connectionwith their trailing (northern) edge, has been the subjectof a number of applied studies [e.g., on air pollutionepisodes in Santiago (Rutllant 1981; Rutllant and Gar-reaud 1995); on the occurrence of water-collection ep-isodes from coastal stratocumulus (Fuenzalida et al.1990); and on ocean upwelling–favorable wind events

76 VOLUME 130M O N T H L Y W E A T H E R R E V I E W

FIG. 1. Long-term mean SLP (contoured every 2.5 hPa) and surfacewinds (reference vector at the bottom, in m s21) for the austral winter(May–Sep). Data from NCEP–NCAR reanalysis (1979–98). Blackarea indicates terrain elevation in excess of 2000 m. Dashed boxindicates area enlarged in Fig. 2.

FIG. 2. Topographic map of north-central Chile (shading scale in mMSL). Key meteorological stations are indicated by a solid circle.

along the north-central coast of Chile (Rutllant 1993,1997)].

Coastal lows or troughs have also been documentedaround the southern Africa coast (e.g., Gill 1982; Rea-son and Jury 1990). Furthermore, the recovery of theMBL along the coast at the demise stage of a CL (sur-face pressure rising) seems to share some of the typicalfeatures of coastally trapped disturbances (CTDs) alongthe mountainous western coast of North America (e.g.,Dorman 1985; Bond et al. 1996; Nuss et al. 2000). Thislatter phenomenon has received considerable attention,and debate still continues on its underlying mechanism(see Ralph et al. 2000 for a review). In the South Amer-ican case we are interested in the full evolution of thelow (including, but not restricted to, its demise), giventhe prominence of the surface pressure drop and therelevant weather changes during its development. Laterin this paper we further compare coastal lows in SouthAmerica with CTDs along the west coast of NorthAmerica, and relate their structural differences to dif-ferences in the synoptic-scale forcing and the regionaltopography.

While previous efforts have documented basic aspectsof CLs along the subtropical west coast of South Amer-ica, they were limited in their findings because the res-olution of the observational network is not sufficient toresolve key subsynoptic aspects. This paper addressessome of the remaining questions on this phenomenon,including the typical large-scale environment attendingthe life cycle of CLs, the mean amplitude and propa-

gation speed of the surface pressure perturbation, itsphase relationship with circulation and thermodynamicchanges in the lower troposphere, and the offshore ex-tent of the wind and pressure anomalies. To this effectwe have used high-resolution surface data from a re-gional network, routine synoptic observations, reanal-ysis fields, and high-resolution satellite imagery. Al-though CLs are a year-round phenomenon, we have con-centrated our analysis on wintertime episodes (May–Sep), when CLs are better defined and exert a morepronounced impact on regional weather (e.g., Rutllantand Garreaud 1995).

The paper is organized as follows. Datasets are pre-sented in section 2, and the analysis technique is de-scribed in section 3. The mean large-scale circulationand cloudiness features during CLs are described in sec-tion 4. In section 5 we present regional-scale aspects ofCLs based on surface station data. A conceptual modelfor the generation and propagation of CLs is proposedin section 6. In section 7 we include a discussion onthe role of free versus forced propagation during coastallows, along with a comparison of South American CLsand western North America CTDs. A summary of ourmain findings is presented in section 8.

2. Datasets

The primary data used in this study are surface ob-servations from a research network of nine automaticweather stations along the coast and farther inland, be-


TABLE 1. Surface stations. Variables measured include pressure ( p),air temperature (T ), relative humidity (RH), wind speed and direction(u), and solar radiation (SR). Measurement interval is 15 min in allstations.

StationLat(8S)

Lon(8W)

Elev(m MSL) Variables measured

CalderaHuascoCruz GrandeEl Tofo

27.0828.4729.4529.50

70.8771.2571.3271.00

113030

780

p,p,p,p,

T,T,T,T,

RH,RH,RH,RH,

u,u,u,u,

SRSR

Lengua de VacaLos MollesLas CrucesLo Prado

30.2532.2533.5033.45

71.6371.5171.6270.93

73726

1065

p,p,p,

T,T,

T,

RH,RH,

RH,

u,u,u,u,

SRSRSRSR

FIG. 3. (upper) The 30-min average of surface pressure, (middle)meridional wind, and (lower) solar radiation at station Lengua deVaca (30.18S, 71.58W, 7 m MSL) from 27 Aug to 26 Sep 1994. Inthe upper panel, thin line is the observed surface pressure and thickline is the surface pressure with the mean diurnal cycle removed(anomaly filter). Crosses indicate selected coastal lows in this period(see text for further details).

tween 278S and 348S at an average alongshore spacingof 100 km (Fuenzalida and Rutllant 1995). Their lo-cations are shown in Fig. 2 and their characteristics aresummarized in Table 1. The network operated betweenNovember 1993 and November 1994, recording 30-minaverages of pressure, air temperature, relative humidity,solar radiation at 2 m above ground, and wind vectorsat 4 m above ground. Station Lengua de Vaca (30.38S,71.68W, 7 m MSL) has a longer record, operating since1991 in a near-continuous fashion. Synoptic reports ev-ery 3 h at San Felix Island [27.28S, 808W, 12 m abovemean sea level (MSL) about 900 km off the coast] andCarriel Sur (36.88S, 73.18W, 21 m MSL) were used toextend our regional analysis to the open subtropicalocean and farther south.

The surface pressure at subtropical latitudes exhibitsa pronounced mean diurnal march (diurnal and semi-diurnal harmonics) produced by atmospheric tides.Over our target region, the amplitude of the mean di-urnal cycle can be as large as 3 hPa (e.g., Fuenzalida1995), masking pressure changes associated with CLsor other synoptic phenomena. In our subsequent anal-ysis we removed the seasonal mean diurnal cycle fromthe original series using an anomaly filter. This filterwas also applied to the series of air temperature, hu-midity, and wind components to remove the mean di-urnal cycle produced by the daytime heating/nighttimecooling at the coast.

The large-scale tropospheric circulation was charac-terized using the pressure-level National Centers for En-vironmental Prediction–National Center for Atmospher-ic Research (NCEP–NCAR) reanalysis fields, describedin detail in Kalnay et al. (1996). The original fields havea 6-h resolution on a 2.58 latitude–longitude grid andinclude all mandatory levels from 1000 to 100 hPa. Onemust keep in mind the coarse horizontal spacing of thereanalysis data compared with the relevant topographicfeatures (e.g., half-width of the Andes Cordillera). Wealso used NCEP–NCAR reanalysis profiles of temper-ature in the original sigma levels (same horizontal andtime resolution as the pressure-level fields) with 10points below 700 hPa. Hence, the sigma-level profilesare able to capture the essential features of the lower

troposphere over the coastal region (e.g., Garreaud etal. 2001). Preliminary analysis of low-level cloudinesswas also possible on the basis of Geostationary Oper-ational Environmental Satellite-7 reduced resolution ra-diance [the International Satellite Cloud ClimatologyProject (ISCCP) B3 product (see Schiffer and Rossow1985 for details)]. Here, we converted the original pixelmaps at 2100 UTC (1800 local time) of visible radiance(0.6 mm) to a regular 0.58 3 0.58 latitude–longitude gridof albedo following the procedure described in Garreaudand Wallace (1997).

3. Case selection and compositing technique

The mean structure of coastal lows is investigatedhere using a compositing analysis of events selected onthe basis of surface pressure minima at Lengua de Vaca(LdV), as described below. The station was selectedbecause of the temporal coverage of their observationsand its location on the coast of north-central Chile. Apressure minimum at LdV (in a temporal sense) is notnecessarily the lowest pressure along the coast (in aspatial sense), but a composite based on these eventsproduces a consistent signal of the coastal low evolution.Furthermore, as it will be shown later, most events se-lected in LdV were also recorded in the rest of thestations, so that our compositing analysis emphasizesthe typical features of the CLs that affect the entire


FIG. 4. Composite traces of surface pressure (solid line, hPa) atsix stations along the coast of Chile (station name and latitude isindicated at the top of the corresponding trace) based on 21 CLepisodes during 1994. Composite trace based on 57 episodes during1991, 1993, and 1994 is also shown for the observed pressure atstation Lengua de Vaca (thick line) and reanalyzed surface pressureat 30.08S, 72.58W (thick, dashed line). The mean diurnal cycle wasremoved at each station using an anomaly filter. The shaded areacorresponds to the mean value plus and minus one standard deviation.Vertical arrows indicate the onset of the troughing; horizontal barsindicate coastal low culmination.

subtropical west coast of South America. Structural dif-ferences in the CL due to the selection method (e.g.,key station for case identification) will be discussedelsewhere. Given the availability of other data we re-stricted our case selection to the austral winter (May–Sep) of 1991, 1993, and 1994. Figure 3 shows the timeseries of the surface pressure anomaly (diurnal cyclesubtracted, ) at LdV for June–August 1994. The pres-p*ssure is dominated by rapid fluctuations (2–3 days) withamplitudes of about 2.0 hPa, superimposed on synopticfluctuations (7–10 days) with amplitudes of about 5 hPa.

A first pool of events was obtained by searching localminimum of within a moving 5-day time window,p*sand selecting those cases associated with a 24-h pressuredrop in excess of 5 hPa. As an example, several localminima are readily identified during September of 1994as shown in Fig. 3. Pronounced minima can be causedp*sby either midlatitude frontal disturbances approachingthe coast or coastal lows. To separate these two con-ditions we used the wind speed and wind direction atLdV: frontal disturbances lead to north-northwesterly

flow prior to the pressure minimum, while strong south-erly winds precede a CL culmination followed by analongshore wind relaxation. Thus, a minimum of thatp*ssatisfied the previous two conditions was classified asa coastal low culmination only if southerly winds (winddirection between 140 and 2358) in excess of 5 m s21

prevailed at least during the 24-h prior to the min-p*simum. This last criterion eliminates about one-third ofthe original minima, mostly prefrontal conditionsp*s(Rutllant and Garreaud 1995). For instance, the pressureminima on days 254 and 257 in Fig. 3 did not satisfythe wind criterion, and as such, they were not selectedas coastal lows.

Application of this procedure yields a total pool of57 CL culminations, nearly one event per week. Wethen took the traces of and other station-based var-p*siables on a 6-day window centered at the culminationtime (hour 0) of each case and averaged them to producecomposite time series. Negative (positive) times referto hours before (after) the culmination time. In the caseof the NCEP–NCAR reanalysis, available at 0000, 0600,1200, and 1800 UTC, culmination time was approxi-mated by the nearest of the four. For solar radiation andalbedo, day 0 was defined as the same day of the cul-mination if culmination time was before local noon.Otherwise, it would be the next day. Statistical signif-icance of the composite anomalies was locally testedusing a two-tailed Student’s t-test at the 95% confidencelevel.

Figure 4 shows the composite trace of at LdV usingp*sthe 57 episodes identified during the austral winters of1991, 1993, and 1994, as well as the composite basedin the 21 cases in the winter of 1994. The similaritybetween the two traces of at LdV is indicative thatp*sthe number of events incorporated in the second com-posite is enough to ensure that the signal stands wellabove the sampling variability. Later, in section 5a, thecomposite traces of at LdV and other coastal stationsp*sare described in detail. For the moment, it suffices topoint out the key features of a CL: weak initial ridging,a marked drop down to the pressure minimum (CL cul-mination), and a more gradual recovery to the winter-time average value. To assess the ability of the reanalysisto mimic sea level pressure (SLP) changes at the coast-line, Fig. 4 also shows the composite trace of reanalyzed

at the grid point (308S, 72.58W) closest to LdV. Thep*sreanalysis does capture the overall SLP evolution duringthe composite coastal low, but with an amplitude of justhalf of that observed in LdV.

4. Large-scale patterns

The composite mid- and upper-level circulation dur-ing a CL life cycle is characterized by a midlatituderidge, drifting westward from the southern Pacific (Fig.5). The ridge axis has a meridional orientation and itcrosses the Andes almost simultaneously with the CLculmination at LdV. At this time, the geopotential gra-


FIG. 5. Composite sea level pressure (contoured every 2 hPa) and 1000 hPa winds (arrows) at days (a) 22, (b)21, (c) 0, and (d) 11.5. Thick, solid line is the 5680-m geopotential height at 500 hPa. Reference wind vector (inm s21) is shown at the bottom of each panel. Black area indicates terrain elevation in excess of 2000 m. High andlow centers indicated by a letter H or L, respectively. H1 indicates the center of the migratory anticyclone. For day0 the light shading indicates SLP less than 1016.5 hPa.

dient is at a minimum over north-central Chile leadingto significantly weaker than normal (by a factor 0.7)westerly flow through the middle and upper troposphere.Further, weak easterly winds ( | u | # 5 m s21) over thesubtropical Andes (;500 hPa) were found prior to orat the CL culmination in 35% of the cases, in connectionwith a northwest–southeast-oriented ridge. Midtropos-pheric subsidence downstream of the ridge axis con-tributed to the warming (18C day21) and drying (1 gkg21 day21) observed below 700 hPa to the west of thesubtropical Andes from about 48 h before CL culmi-nation. The nearly out-of-phase geopotential heightanomalies between the middle and lower troposphereare better seen in the time–pressure section at 308S,72.58W (Fig. 6), emphasizing the nonmidlatitude char-acter of the coastal lows. Also notice that the 850-hPasurface remains level at about 1500 from day 21 to day12.

Composite maps of SLP and 1000-hPa winds are

shown for selected times in Fig. 5. Two days beforeCL culmination the surface conditions are close to cli-matology (cf. Fig. 1), but for a region of higher pres-sure around 408S (noted by H1 in Fig. 5) drifting justto the east of the midlevel ridge (thick solid line inFig. 5). By day 21, the migratory surface ridge (H1)has reached the coast of southern South America re-sulting in a stretching of the subtropical anticycloneeastward into the continent (recall that to the south of388S the elevation of the Andes drops to less than 1200m). Simultaneously, a narrow surface trough developsbetween the subtropical anticyclone and the AndesCordillera. Isallobaric analysis (not shown) indicatesthat at the time of the pressure minimum at LdV (day0) the coastal trough has deepened and the localizedregion of negative pressure tendency has progresseddown to 358S (Fig. 5c). In spite of the coarse resolutionof the reanalysis grid, the composite SLP field exhibitsa closed coastal low centered at 308S. Meanwhile the


FIG. 6. Time–pressure section of the geopotential height anomaly(gpm, reanalyzed values) over the grid box centered at 32.58S, 708W.The anomalies were calculated at each level as the difference betweenthe composite value and the long-term mean wintertime value. Timeis relative to the coastal low culmination (day 0).

FIG. 7. Composite 925-hPa geopotential (contour) and wind (ar-rows) anomalies at culmination time (day 0). Contour interval is 10m, negative values in dashed line, and the zero contour is omitted.Reference wind vector at the bottom of the figure is 10 m s21. Lightshading indicates negative SLP anomalies larger than 21.5 hPa.Black area indicates terrain elevation in excess of 2000 m.

subtropical high has reached its maximum southeast-ward extension and the migratory anticyclone to theeast of the Andes has moved northeastward. This dis-tinctive high–low–high SLP pattern1 and associatedupper-air pattern at subtropical latitudes has been ear-lier identified in synoptic studies on coastal lows innorth-central Chile (e.g., Rutllant and Garreaud 1995).By day 11.5, the trough has vanished from the coastas an extratropical depression (noted by the L in Fig.5), often accompanied by a cold front, has moved closeto the coast of southern Chile, and the migratory an-ticyclone has slowly drifted into northern Argentinaand southern Brazil.

At the 925-hPa level (near the base of the subsidenceinversion), a wide area of positive height anomalies ex-tends from 308 to 508S off the western coast of SouthAmerica and protrudes to the east of the subtropicalAndes as far as 208S (Fig. 7) in connection with themigratory low-level ridge. The positive height anomalyexhibits a conspicuous trough just off the coast of north-central Chile, coincident with a region of negative SLPanomalies (258–358S) that signals the CL. The anomaly

1 This pattern is remarkably similar to the synoptic configurationduring cold air incursions to the east of the subtropical Andes (e.g.,Marengo et al. 1997; Vera and Vigliarolo 2000; Garreaud 2000),suggesting simultaneous occurrence of coastal lows in central Chileand cold surges in southern Brazil and Bolivia.

trough suggests cross-shore and alongshore scales of500 and 1000 km, respectively, for the CL at the maturestage. At the same level, wind anomalies are approxi-mately in geostrophic balance with the height anoma-lies. Of particular interest is a wide area of easterlyanomalies off the coast of central Chile, associated witha veering from the climatological onshore flow to off-shore (easterly) flow before and at CL culmination. Theveering is somehow difficult to notice in the compositesof actual winds (Fig. 5) since the coastal winds withinthe MBL are dominated by the alongshore component.

The composite maps of albedo for days 21, 0, and11, shown in Figs. 8a–c document changes in the cloudcover at the coast and over open ocean, providing in-dependent support for the synoptic picture previouslydescribed. The extensive and highly persistent deck ofstratocumulus off the subtropical west coast of SouthAmerica (e.g., Klein and Hartmann 1993) is disruptedby a region of clear skies (low albedo) that expandoffshore over time before the CL culmination (Figs. 8a,b). Similar marine stratocumulus clearing episodes areobserved along the coast of California, associated withoffshore flow effects (Kloesel 1992). At day 0 the regionof clear skies reaches its maximum extension, collocatedwith the coastal low (c.f. Figs 5c–8a). The clear skiesare likely indicative of a shrinking of the MBL, but thelack of upper-air data off the coast prevents a morethoughtful diagnosis of the clearing. The clear-sky re-gion begins to vanish by day 11, as a poleward-movingwedge of stratus returns along the coast, sometimes


FIG. 8. Composite maps of albedo at day 21, 0, and 11. Arrows indicate leading edge of the stratocumulus deck along the coast. Smallopen square indicates position of station Lengua de Vaca (308S). Dashed line indicates the approximate position of a frontal system atsurface.

merging with midlevel cloudiness associated with thefrontal system approaching southern Chile (Fig. 8c).

5. Regional aspects

a. Changes at sea level

Figure 4 shows traces of the surface pressure anomaly( ) in the six coastal stations at sea level between 278p*sand 338S averaged for the 21 CL episodes identified inLdV during 1994. It is useful to remember that the com-posite procedure based on the culmination time in LdVsmears out the signal at the other stations due to vari-ations in the episode’s propagation speed. The com-posite traces exhibit an asymmetric V shape, with alifespan of about 3 days and amplitude increasing pole-ward from 3.5 to 6.1 hPa. In all stations, the surfacepressure begins gradually to increase about 2.5 daysbefore the CL culmination at LdV, reaching a significantpositive anomaly during day 21, followed by a pro-nounced pressure drop down to the minimum. Afterculmination, the pressure recovers to its mean winter-time value in about a day without reaching significantpositive anomalies. The same data in Fig. 4 can be usedto construct alongshore composites of traces at dif-p*sferent times (not shown). At day 0, the composite CLappears as a closed low with a surface pressure mini-mum around LdV (308S) flanked at both sides by analongshore pressure gradient of about 0.3 hPa (100 km)21,as previously suggested by the reanalysis data.

A weaker, lagged evolution is found in the compositetraces of away from the coastal network (Fig. 9). Atp*sSan Felix Island, weaker SLP maximum and minimumoccur about 12 h later than their counterparts at LdV,while at Carriel Sur (inland mountains less than 1000m) the initial ridging continues ;18 h after the begin-

ning of the pressure drop at LdV. The nearly synchro-nous ridging–troughing sequence at these two stationsis consistent with the large-scale pattern found in theprevious section (i.e., ridge–trough couplet drifting east-ward at midlatitudes). Thus, the developing stage of theCL at subtropical latitudes occurs as pressure increasesfarther south and away from the coast (synoptic-scalesurface ridging), while the CL culmination occurs whenpressure is decreasing elsewhere (synoptic-scale surfacetroughing). The more rapid and pronounced pressurefall and rise along the coast farther north indicates thatthe basic large-scale changes are time shifted and am-plified along the subtropical coast. This presumably rep-resents the local impact of the coastal low.

Figure 4 also reveals the southward propagation ofsome composite features at a mean speed that can beestimated from a least squares fit of the station data.The initial decrease of surface pressure (i.e., the firsttime in the composite trace at which ] /]t , 0, indi-p*scated by an arrow in Fig. 4) propagates at Ci 5 28.46 5.6 m s21, although inspection of individual CLsreveals a number of cases in which the onset of thetroughing occurs nearly simultaneously along the coast-al network (278–338S). The pressure minimum that sig-nals the local culmination time (indicated by a bar inFig. 4) propagates slower and more regularly, at a meanspeed Cc 5 16.1 6 4.9 m s21. Furthermore, a southwardpropagation of the minimum was found in 18 out ofp*s21 CLs, although with significant dispersion in the re-gional value of Cc (10–30 m s21). The mean value ofCc is consistent with previous estimates based on syn-optic data (Rutllant and Garreaud 1995).

A marked change in the coastal surface winds is ob-served during the CL lifespan, as shown in Fig. 10a bythe composite traces of wind speed and the dominant


FIG. 9. (a) Composite trace of surface pressure (solid line, hPa) atSan Felix Island (27.28S, 808W 12 m MSL). The shaded area cor-responds to the mean value plus and minus one standard deviation.The wintertime mean diurnal cycle has been removed from the ob-served time serie (3-h average) using an anomaly filter. For reference,the composite trace of surface pressure at Lengua de Vaca is shownby the dot–dashed line. Time is centered in the hour of minimumpressure at Lengua de Vaca. (b) As in (a) but for station Carriel Sur(36.88S, 71.68W, 21 m MSL).

FIG. 10. (a) Composite wind speed (thick line, m s21) and merid-ional wind (thin line) at Lengua de Vaca (30.38S, 71.68W, 7 m MSL).The shaded area corresponds to the mean speed plus and minus onestandard deviation. The wintertime mean diurnal cycle has been re-moved from the observed time series (3-h average) using an anomalyfilter. For reference, the composite trace of surface pressure at Lenguade Vaca is shown by the dot–dashed line. Time is centered in thehour of minimum pressure at Lengua de Vaca. (b) Composite solarradiation (W m22) at Lengua de Vaca. The shaded area correspondsto the mean speed plus and minus one standard deviation.

meridional wind component (roughly alongshore) atLdV. About 2 days before the minimum, the southerlyp*swind increases gradually to reach a maximum over 5m s21 just at the culmination time, followed by a sharpdecrease down to ;1 m s21. Isallobaric wind estimatesare in agreement with the observed southerly acceler-ation (Rutllant 1994, 1997). The weak southerlies persistfor about a day, and sometimes the wind reverses toweak northerlies along the coast. Pilot balloon obser-vations at LdV during intensive field experiments revealthat the strong surface southerlies before the CL cul-mination are associated with an alongshore low-leveljet at about 150–200 m above the surface, and lead toan almost simultaneous decrease of SST along the coastdue to enhanced coastal upwelling around 308S (Rutllant1993, 1997). A transition from enhanced to relaxedsoutherlies is also observed, at varying degrees, at therest of the coastal stations within 4 h from the localculmination (not shown).

A transition from sunny to overcast conditions,roughly centered at the culmination time, is also evidentin the composite solar radiation at LdV (Fig. 10b), aswell as the other coastal stations. These results are inagreement with the southward displacement of a wedgeof coastal stratus observed in the composite maps ofalbedo (Fig. 8). The leading edge of the stratus has across-shore scale of ;150 km, and it moves southwardat about 17 m s21, very close to the estimated propa-gation speed of the minimum. In spite of the solarp*s

radiation signal, there are no significant changes in rel-ative humidity and air temperature at sea level associ-ated with the passage of a CL. Probably, enhanced coldair advection and colder SST (in connection with thestrong southerlies) compensate for the increment of so-lar heating before the CL culmination.

b. Changes in the MBL and at the inversion level

The development of a CL along the subtropical coastis accompanied by a strengthening of the subsidenceinversion and a descent of its base, frequently down tothe surface, as documented in Rutllant (1981, 1983)using routine radiosonde data at Quintero (32.58S,71.38W, 8 m MSL). To illustrate this behavior, we haveused 1200 UTC (0800 local standard time) sigma-levelNCEP–NCAR reanalysis profiles of potential temper-ature at 32.58S, 72.58W as a surrogate of the Quinterosoundings. The composite profiles for the CL episodesidentified between 1991 and 1994 are shown in Fig. 11.Two days before the minimum pressure at LdV, theprofile is close to climatology (e.g., Rutllant 1994), witha well-mixed MBL below 950 hPa and the top of thetemperature inversion at about 900 hPa. By day 0, there


FIG. 11. Composite profiles of potential temperature (1200 UTCreanalysis values) over the grid box centered at 32.58S, 72.58W, fordays 22, 0, 11, and 12.

FIG. 12. (a) Composite trace of surface pressure (solid line, hPa)at El Tofo (298S, 718W, 780 m MSL). The wintertime mean diurnalcycle has been removed from the observed time series (30-min av-erage) using an anomaly filter. The shaded area corresponds to themean value plus and minus one standard deviation. For reference,the composite trace of surface pressure at Lengua de Vaca is shownby the dot–dashed line. (b) As in (a) but for the 2-m air temperature.(c) As in (a) but for 2-m specific humidity. In all panels time is indays, centered at the hour of minimum pressure at Lengua de Vaca.

is a generalized warming of the lower and middle tro-posphere. The largest warming (;68C) occurs between900 and 800 hPa. Of particular interest is the upwardand downward expansion of the temperature inversion,so that, even in this composite, the MBL vanishes byday 0. One day later, the warming continues at midlev-els, at the same time that the recovery of the MBL hasbegun. By day 12 the generalized cooling, in connec-tion with the incoming midlatitude trough, has broughtthe profile to near-normal conditions (Fig. 11).

Further details on the timing and amplitude of thechanges in the MBL/inversion layer can be obtainedfrom the observations at the two elevated AWS withinthe regional network. Station El Tofo is located at 780m MSL, close to the climatological base of the subsi-dence inversion over this region (Schemenauer et al.1988). The composite trace of at El Tofo parallelsp*sits counterpart at LdV (about 120 km to the south),although its amplitude is about 70% of that observed atsea level (Fig. 12a). Note that this signal in pressure isproduced by changes occurring above the top of theMBL, stressing the prominent role of the synoptic scalein setting the poleward pressure gradient. A gradualwarming–drying starting 30 h before the culmination ofthe CL is terminated by a sharp cooling–moisteningwithin 6 h after the pressure minimum (Figs. 12b,c).This marked asymmetry in the rate of warming andcooling is evident in nearly all individual episodes. Airtemperature continues a more gentle decrease duringday 11, down to a minimum about 28C lower than the

pre-CL value. The evolution of temperature and mois-ture, along with the composite soundings shown in Fig.11, is indicative of a gradual strengthening and descentof the subsidence inversion before the minimum,p*sfollowed by a rapid recovery of the MBL immediatelyafter culmination time.

Station Lo Prado is located on a saddle point of thecoastal range where the zonal component of the windis dominant, at 1000-m elevation and near the clima-tological top of the inversion layer at 338S (Rutllant andGarreaud 1995). The gradual warming–cooling se-quence is symmetrical with respect to the culminationtime at the closest coastal station, Las Cruces, (Figs.13b,c). The temperature increment at the inversion tophas a similar magnitude (;158C) to that observed atthe inversion base (El Tofo composite). Sustained east-erly winds begin simultaneously with the drop of surfacepressure at LdV, and thus they lead the beginning of thelocal warming by about 12 h (Fig. 13a). The easterlywind increases up to nearly 10 m s21, followed by asharp transition to weak westerlies around the culmi-nation time at LdV. The simultaneous changes of zonalwind at Lo Prado and surface pressure at LdV are note-worthy considering the 380-km spacing between the twostations. In some cases, the westerly flow and the in-


FIG. 13. (a) Composite trace of zonal wind (solid line, hPa) at LoPrado (33.58S, 70.98W, 1065 m MSL). The wintertime mean diurnalcycle has been removed from the observed time series (30-min av-erage) using an anomaly filter. The shaded area corresponds to themean value plus and minus one standard deviation. For reference,the composite trace of surface pressure at Lengua de Vaca is shownby the dot–dashed line. (b) As in (a) but for the 2-m air temperature.(c) As in (a) but for 2-m specific humidity. In all panels time is indays, centered at the hour of minimum pressure at Lengua de Vaca.

FIG. 14. Schematic representation of the surface pressure profilealong the west coast of South America during the evolution of acoastal low. At each stage, the actual profile along the coast is shownin a solid line and the large-scale synoptic profile (unaffected by thetopography) is shown in a dashed line. Dotted (crossed) circles in-dicate offshore (onshore) low-level winds, in geostrophic balancewith the coastal meridional pressure gradient. Vertical arrows indicatethe direction of the topographically induced vertical motion requiredto satisfy mass continuity close to the mountainous coastline.

version rise after the CL culmination are strong pro-ducing advection of cool, moist marine air into the in-land valleys to the east of the coastal range. Notice thatthe stronger westerly flow at Lo Prado lags the SLPminimum at LdV by about 6 h, consistent with the pre-viously estimated propagation speed of 17 m s21.

6. Conceptual model

By merging the results from the synoptic and regionalanalysis, a conceptual model of the composite CL evo-lution is now proposed. The climatological equatorwardsurface pressure gradient along the subtropical westcoast of South America can be reversed when a migra-tory anticyclone (connected with the midlevel ridge)approaches southern Chile at about 408S (Fig. 14a). Thelarge-scale low-level wind, dominated by the southerlycomponent, adjusts geostrophically to this change byveering from onshore to offshore (easterly) flow. Thecontinuity requirement after the onset of the low-leveleasterlies off the coast seems to be satisfied by down-slope flow over the western side of the Andes, so thatmarine, cool air begins to be replaced by adiabaticallywarmed air, as evidenced by the strengthening anddownward expansion of the inversion layer. Recalling

that the pressure remains nearly constant at ;1500 mMSL, the low-level warming leads to the hydrostaticfall in surface pressure. Downslope flow may be furtherenhanced when easterly flow exists in the middle tro-posphere, in connection with a northwest–southeast-ori-ented ridge, leading to foehn-like conditions.

The evolution of the alongshore surface pressure atthe onset of a CL is then determined by two oppositeeffects whose magnitude varies with latitude: synoptic-scale ridging and topographic troughing. To the southof 388S, the synoptic forcing is large and the height ofthe Andes is less than 1000 m MSL, so the surfacepressure is largely dominated by the synoptic effect(e.g., pressure trace at Carriel Sur; Fig. 9). Between 258


and 358S, the synoptic forcing is weaker and the Andestop exceeds 4000 m MSL, so that the initial pressurerise (synoptically forced) is eventually offset by the to-pographic troughing marking the onset of the coastallow at subtropical latitudes (Fig. 14b). The simultaneousoccurrence of sustained easterlies within the inversionlayer and the beginning of the surface pressure drop atsea level lends support to this hypothesis (cf. Figs. 4and 12). Farther to the north, the synoptic forcing isvery weak, so neither the synoptic nor the topographiceffects are important, and the stratus-topped MBL re-mains unperturbed.

The drop of surface pressure along the subtropicalcoast further increases the poleward pressure gradientbetween the developing CL and the midlatitude anti-cyclone, eventually decreasing the surface pressure(through adiabatic warming) to the south of the pressureminimum. At the same time, an equatorward pressuregradient and balanced westerly flow develop off thecoast to the north of the coastal pressure minimum. Con-sidering a representative height of the topographic bar-rier (coastal range and the Andes Cordillera) hm ; 2000m and U ; 3 m s21 as the upstream onshore velocityscale, the Froude number [Fr 5 U/( f 3 hm)] evaluatedat 278S is Fr ; 0.3. Thus, the onshore flow is likelyblocked by the coastal range, locally deepening theMBL and forming a weak surface mesoridge at thenorthern coast. Unfortunately, this later feature is notdocumented in our analysis because the northernmostcoastal station is at 278S.

As the axis of the midlatitude anticyclone moves tothe east of the Andes (southern Argentina) pressure be-gins to drop in southern Chile. Eventually the pressuregradient between the subtropics and midlatitudes be-comes equatorward, and the large-scale coastal windbacks from offshore to onshore flow, marking the de-mise of the coastal low (Fig. 14d). At this time, thereis a marked alongshore gradient of surface pressure be-tween northern Chile (where the MBL has remainedunperturbed or slightly deepened) and the subtropicallatitudes (where the MBL is very low; see Fig. 11). Thecessation of the subsiding offshore flow, however, doesnot imply the recovery of the surface pressure. Rather,the rise of the surface pressure at a given point alongthe coast will occur in concert with the local recoveryof the cool MBL, as evidenced in the composite tracesat El Tofo and LdV (Fig. 12). Yet, such a positive pres-sure tendency caused by the low-level cooling might bepartially offset by the synoptic-scale approaching trough(mid- and upper-level divergence).

7. Discussion

The evidence presented in this paper suggests that theinitial and mature stage of a coastal low (surface pres-sure decreasing along the subtropical coast) is largelyproduced by the interaction between the large-scaleforcing (including changes above the MBL) and the

topography, in the form described above. A rough es-timate of the topographically induced subsidence overthe subtropical coast can be obtained from a scale anal-ysis of the continuity equation neglecting the alongshorevariation of the meridional wind (]u/]x 1 ]w/]z 5 0)and assuming a wall-like mountain. Let U ; 3 m s21

be the low-level, quasigeostrophic offshore velocityscale (obtained from the reanalysis composite; Fig. 5);X the cross-shore scale at which u approaches asymp-totically to U; W the vertical velocity scale; and H ;2 km the height scale of the layer where the most sig-nificant warming takes place. The Rossby radius of de-formation X is an appropiate cross-shore scale, so X 5NH/ f ; 200 km (where N 5 1.5 3 1022 s21 is thereference Brunt–Vaisala frequency, and f , the Coriolisparameter, is evaluated at 308S). The calculated scalefor the vertical velocity results in W ; 23 cm s21,nearly five times larger than the large-scale subsidence(reanalyzed value) ahead of the midlevel ridge. Withthis figure, we can estimate the adiabatic warming andthe associated pressure fall by integrating the hydrostaticequation over the lower troposphere:

T0 2 21DT 5 WN ø 6 K day (1)adb g

gp0 21Dp 5 2 DT H ø 5 hPa day , (2)h adb2RT0

where T0 ; 280 K is the reference temperature of thelower troposphere and p0 ; 1010 hPa is a referencesurface pressure. Thus, the topographic troughing be-comes comparable to the synoptic-scale ridging in abouta day. Such a drop in surface pressure would be moreor less uniform along the subtropical west coast (258–358S), leading to a quasi-stationary development of thecoastal low during the period of favorable synoptic-scaleconditions (typically 2–4 days).

Once the synoptic-scale forcing ceases to be favorablefor the development of the coastal low (low-level zonalflow backs from easterly to westerly), the pronouncedequatorward pressure and MBL depth gradients alongthe coast are favorable for free waves to develop,trapped by the Coriolis force against the coastal andAndean topography. Recall that in the composite (Fig.4), and in many individual cases, the recovery of thesurface pressure (i.e., the initial time at which ] /]t .p*s0 after the pressure minimum) occurs at increasing timesas one looks at the coastal stations from north to south,at a propagation speed Cc ; 16 m s21, and is typicallyassociated with a relaxation or reversal of the alongshorewinds. Further, the high-resolution visible imagery(Figs. 8b,c) suggests that, in most cases, the recoveryof the coastal stratocumulus occurs as a wedge (cross-shore scale ;200 km) moving from the north at a prop-agation speed close to Cc.

Topographic Rossby waves (e.g., Skamarock et al.1999) will indeed move the coastal pressure minimum


to the south.2 This displacement is, however, too slow(CR ; 6 m s21) compared with the observations, andhence, it is unlikely that topographic Rossby waves playa significant role during the demise of the coastal low.Bores and trapped gravity currents can produce pres-sure, wind, and cloudiness signatures similar to thoseobserved after the culmination of the coastal low, butthe northerly winds after the surface pressure minimumare too slow compared with the observed propagationspeed. A third candidate to explain the recovery of theMBL/surface pressure is a trapped Kelvin wave. Thepredicted phase speed for a Kelvin wave in which theMBL and the capping inversion behave as one layer(e.g., Fig. 11) is given by CK 5 [g*(H 1 H1)]1/2, whereg* is the reduced gravity, H the depth of the unperturbedMBL, and H1 the thickness of the unperturbed inversion(e.g., Ralph et al. 2000). Using typical values for sub-tropical South America (g* ; g/40, H ; 600 m, H1 ;600 m) results in CK 5 17 m s21. This rough agreement,together with the wind and cloudiness changes aroundthe time of surface pressure minimum, suggests that therecovery of the MBL after the culmination of the coastallow might occur as a Kelvin wave (see also Rutllant1994). Nevertheless, we do not have enough data toconclusively answer whether or not the propagation isa Kelvin wave. Key data missing include continuousprofiles of temperature and humidity at several pointsalong the coast in order to define the shape of the MBLleading edge. Mesoscale modeling efforts are currentlyunderway to clarify this point.

At this point it is interesting to compare the meanstructure of CTDs along the west coast of South andNorth America. The structure and dynamics of CTDsin this latter region have received considerable attention,as recently reviewed by Nuss et al. (2000). The existenceof similar CTDs in these regions is expected, given thebroad similarities in regional topography, oceanicboundary conditions, and mean atmospheric circulation.Further, low-level, localized offshore flow in the lowertroposphere also appears as a robust feature in the earlystages of warm-season CTDs along the west coast ofNorth America (Mass and Bond 1996), and idealizednumerical simulations over this region show that off-shore flow can lead to adiabatic warming and evacuationof the MBL near the coast causing the formation of asurface mesolow (Skamarock et al. 1999). The windshift that characterize CTDs along the coast of centraland northern California tends to occur 3–4 h after arelatively small surface pressure minimum (less than 1hPa with respect to undisturbed conditions), and it isfollowed by a gradual and more prominent pressure rise(;2–3 hPa) in the next 3–12 h. Thus, while the am-

2 Using linear theory, Rynes (1970) provides the alongshore phasespeed of a topographic Rossby wave: CR 5 aN/(k2 1 l2)1/2, where ais the mountain slope, and k and l are the cross- and alongslopewavenumbers, respectively. Using a 5 H/X 5 2 km/200 km, and anominal wavelength Lx 5 Ly ; 500 km, yields CR ; 6 m s21.

plitude and qualitative evolution of the surface pressureanomalies during the full cycle of a North AmericanCTD are similar to their South American counterparts,the strength of the precursor coastal low and the sub-sequent coastal ridge are inverted (recall that positiveSLP anomalies after the culmination are not significantin the composite South American CL; Fig. 4). Thisasymmetry in the surface pressure anomalies during thelife cycle of a coastally trapped disturbance is the basisfor its generic name: coastal low along the west coastof South America and South Africa, and coastal ridgesalong the west coast of North America and Australia(Reason and Steyn 1990).

We hypothesize that the more prominent troughingin CTDs in the subtropical west coast of South Americais related with the sharper and higher near-coastal to-pography (including the Andes Cordillera) along thisregion, which implies stronger vertical velocities (and,hence, adiabatic warming) than in North America giventhe same low-level offshore flow. The topographicallyenhanced adiabatic warming during CLs might also ex-plain the differences in the vertical structure of Northand South American CTDs. In the former case, Ralphet al. (1998, 2000) document a significant upward ex-pansion of the MBL inversion after the surface windshift but rather minor changes in the MBL depth. In ourcomposite, in addition to the warming at and above thelevels of the trade inversion, there is a clear shrinkingof the MBL as the surface pressure drops, with the in-version base reaching the surface in many individualCLs (see Fig. 11).

Another common feature of CTDs in North and SouthAmerica is their poleward propagation. Along the west-ern coast of North America, the wind shift, the precursorcoastal low, and the subsequent coastal ridge tend tomove in concert at a ground-relative speed between 7and 12 m s21. In many cases, the wind shift stalls duringdaytime, leading to its decoupling with the coastal low.For a typical episode in North America, Ralph et al.(2000) found that its propagation speed and structure‘‘is best characterized as a mixed Kelvin wave–borepropagating within the inversion above the MBL, withthe MBL acting as a rigid lower boundary.’’ In ourcomposite along the west coast of South America theminimum pressure (coincident with the recovery of theMBL) moves poleward faster, at about 16 m s21 groundrelative speed (;18 m s21 intrinsic phase speed), withinthe range of speeds for Kelvin wave when both the MBLand the MBL inversion behave as one layer (Fig. 11).

The alongshore wind anomalies during the SouthAmerican coastal lows are similar but weaker (Dy ; 4m s21) than those observed in North America (Dy ; 8m s21). In some episodes we did observe a transitionfrom the climatological southerly winds to the fully dis-turbed northerly winds. Nevertheless, our wind obser-vations come from surface land stations (in contrast withcoastal buoys in North America), likely influenced bythe local topography, so we cannot confidently assess


the changes in coastal wind during CL episodes. Forthis reason we have also refrained from assessing anydiurnal cycle in the wind shift along the subtropical westcoast of South America (a prominent feature in NorthAmerican CTDs). Yet, the surface pressure observa-tions, after subtracting the diurnal cycle, do not indicateany preferential timing for the onset, culmination, ordemise of the CL.

8. Concluding remarks

Coastal lows on the west coast of subtropical SouthAmerica are observed to occur year-round at a rate ofabout one per week, causing significant day-to-dayweather changes along the coast and further inland, es-pecially during the austral winter. Their mean structureand evolution has been documented in this work on thebasis of atmospheric reanalysis, visible satellite imag-ery, and surface observations from a regional networkalong the coast of north-central Chile.

The large-scale synoptic environment during the CLdevelopment stage is characterized by a midlatitude sur-face anticyclone drifting from west to east connectedwith a midlevel ridge. The low-level anticyclone seemsinstrumental in the onset and deepening of the CL atsubtropical latitudes because it can revert the alongshorepressure gradient leading to offshore flow along thecoast. The easterly winds and enhanced subsidencealong the subtropical coast (258–358S) enhance the tradeinversion and depress the MBL. Since pressure remainsnearly constant at about 1500 m MSL, the replacementof cold, marine air by adiabatically warmed air producesa surface coastal low that extends about 500 km awayfrom the coast. Consistently, clear skies and warm, dryconditions prevail at the coast during the developingstage of the CL. This stage is also characterized by anintensification of the upwelling-favorable southerlywinds along the coast. The surface pressure at subtrop-ical latitudes reaches the local minimum (CL culmi-nation) as the axis of the midlatitude, midlevel ridgeaxis crosses the Andes Mountains.

The demise of the CL (surface pressure rising) tendsto occur as a midlatitude surface-low/upper-trough ap-proaches southern Chile. In this condition the coastalflow reverts from offshore to onshore and the adiabaticwarming ceases to act. The recovery of the surface pres-sure toward mean values occurs at increasing times asone looks at the coastal stations from north to south(mean propagation speed Cc ; 16 m s21) and is typicallyassociated with a relaxation or reversal of the alongshorewinds and the reestablishment of the cloud-toppedMBL. Thus, the demise stage of the coastal low resem-bles a trapped Kelvin wave propagating along the coastfrom northern Chile (where the MBL was not perturbedor even thickened) into subtropical latitudes in about aday or so.

Acknowledgments. This research was supported byFONDECYT (Chile) under Grant 1000913, and by theDepartment of Research and Development, Universidadde Chile, under Grant I99/002. The regional network innorth-central Chile was also supported by FONDECYTunder Grant 1930806. Rodrigo Sanchez assembled datafrom this network. NCAR–NCEP reanalysis was pro-vided by NOAA’s Climate Diagnostics Center. The B3ISCCP data were obtained from the NASA-Langley Re-search Center EOSDIS Distributed Active Archive Cen-ter. The authors gratefully acknowledge P. Aceituno forcomments and a careful reading of the manuscript. Con-structive criticisms and thoughtful comments of twoanonymous reviewers were highly appreciated.

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