a biweekly mode in the equatorial indian ocean

A biweekly mode in the equatorial Indian Ocean

Debasis Sengupta and R. SenanCentre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India

V. S. N. Murty and V. FernandoPhysical Oceanography Division, National Institute of Oceanography, Goa, India

Received 16 February 2004; revised 31 May 2004; accepted 11 August 2004; published 8 October 2004.

[1] The National Institute of Oceanography, Goa, deployed moorings with severalsubsurface current meters at 0�, 93�E (in February 2000) and 0�, 83�E (in December 2000)in the eastern Indian Ocean. Observed meridional current at all depths has a 10- to 20-day(or biweekly) variability that is distinct from longer period (20- to 60-day) subseasonalvariability. Lags between different instruments suggest the presence of groups of westwardand vertically propagating biweekly waves with zonal wavelength in the range 2100 to6100 km. We use an ocean model forced by high-resolution scatterometer wind stress toshow that the observed biweekly variability is due to equatorially trapped mixed Rossby-gravity waves generated by subseasonal variability of winds. We demonstrate that quasi-biweekly fluctuations of surface meridional wind stress resonantly excite ocean waveswith westward and upward phase propagation, with a typical period of 14 days and zonalwavelength of 3000–4500 km. The biweekly wave is associated with fluctuatingupwelling/downwelling in the equatorial Indian Ocean, with amplitude of 2–3 m per daylocated 2–3 away from the equator. Possible reasons for eastward intensification ofbiweekly energy are discussed. INDEX TERMS: 4231 Oceanography: General: Equatorial

oceanography; 4255 Oceanography: General: Numerical modeling; 4512 Oceanography: Physical: Currents;

KEYWORDS: biweekly mode, equator, Indian Ocean

Citation: Sengupta, D., R. Senan, V. S. N. Murty, and V. Fernando (2004), A biweekly mode in the equatorial Indian Ocean,

J. Geophys. Res., 109, C10003, doi:10.1029/2004JC002329.

1. Introduction

[2] Observed currents in the equatorial Indian Ocean(EqIO) have consistently shown intraseasonal variabilitywith periods of a few days to 60 days [Schott andMcCreary, 2001]. A prominent 26-day oscillation of me-ridional current in the western EqIO [Luyten and Roemmich,1982] is due to mixed Rossby-gravity (MRG) waves withzonal wavelength of 1100–2000 km [Reverdin and Luyten,1986; Tsai et al., 1992]. Model studies suggest that the26-day MRG wave is generated by dynamic instability ofseasonal currents [Kindle and Thompson, 1989]. The 1973–1975 weekly record from Gan Island (�73�E) [Knox, 1976]shows dominant 30- to 60-day variability of equatorialzonal current, and suggests the presence of variability withperiods shorter than 20 days [McPhaden, 1982]. Theobservations of Schott et al. [1994] from the eastern EqIOin the 1990s gave the first clear demonstration of 10- to20-day (‘‘biweekly’’) variability. An array of three mooringscarrying acoustic Doppler current profilers (ADCP) androtor current meters (RCM) was deployed along 80.5�Esouth of Sri Lanka between December 1990 and March1992. Off-equator zonal current and upper ocean zonalvolume transport across this array has large fluctuations

with biweekly period. An ocean general circulation model(OGCM) forced by daily NCEP reanalysis [Kalnay et al.,1996] surface winds suggests that these fluctuations of off-equator zonal transport are mainly associated with westwardpropagating MRG waves with 12- to 15-day period forcedby intraseasonal fluctuations of wind stress [Sengupta et al.,2001]. Subsequent moored current meter observations (July1993 to September 1994) along 80.5�E from an arrayspanning the equator reveals a distinct 15-day spectral peak(Figure 1) in equatorial meridional current and off-equatorzonal current, consistent with the meridional structure ofMRG waves [Matsuno, 1966; Gill, 1982]. Apart from thesuggestion of Sengupta et al. [2001] that the observedbiweekly variability of zonal flow south of Sri Lanka ismainly due to wind-forced MRG waves, there has been nostudy of the nature of biweekly variability in the EqIO.[3] Atmospheric variability in the tropical Indian Ocean

has two dominant intraseasonal modes with a 30- to 60-dayand 10- to 20-day (or quasi-biweekly) period [Yasunari,1981; Krishnamurti and Ardunay, 1980; Krishnamurti etal., 1988; Goswami et al., 1998]. The 30- to 60-dayeastward propagating Madden-Julian oscillation is the dom-inant mode of intraseasonal variability [Madden and Julian,1972, 1994] in the equatorial region. The 30- to 60-daymode, which appears to propagate northward in summer[Sikka and Gadgil, 1980], and a westward propagatingquasi-biweekly mode (QBM) largely determine the active-

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, C10003, doi:10.1029/2004JC002329, 2004

Copyright 2004 by the American Geophysical Union.0148-0227/04/2004JC002329

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break cycle of the summer monsoon over the Bay ofBengal [Krishnamurti and Ardunay, 1980]. Although theexistence of the QBM has been known for some time,quasi-biweekly variability over the equatorial Indian Oceanhas not received much attention. The observational work ofChen and Chen [1993] on the QBM in boreal summer isbased on daily FGGE data and outgoing longwave radiation(OLR). It shows that the horizontal structure of this modeconsists of twin vortices, one centered near the IndianOcean equator, and one near 18�N. The vortices have zonallength scale of about 30�, and move west at 4–5 m s�1.Chatterji and Goswami [2004] have recently studied theatmospheric QBM using daily OLR and NCEP reanalysiswinds at 850 hPa and higher levels. They demonstrate thatquasi-biweekly activity is present throughout the year withvarying amplitude (see their Figures 2–4. Over the tropicalcentral and eastern Indian Ocean and the West Pacific, theQBM is associated with twin vortices of zonal scale 30� (i.e.,zonal wavelength of about 6000 km) and westward speedof 4.5 m s�1. Stability analysis suggests that the QBM isa convectively coupled first meridional mode equatorialRossby wave. Chatterji and Goswami [2004] suggest thatthe large-scale vorticity distribution causes the center of thesouthern (northern) vortex of the wave to be located close tothe equator during the summer (winter) monsoon, whereasthe other vortex is located in the summer hemisphere.[4] Recent observations of equatorial currents in the

eastern Indian Ocean show distinct biweekly variability.In February 2000, the National Institute of Oceanography(NIO), Goa, deployed a mooring with rotor current metersat several subsurface depths at 0�, 93�E [Murty et al.,2002] under the Ocean Observing Systems programme ofthe Department of Ocean Development, New Delhi. NIOhas since deployed two more current meter moorings at 0�,83�E and 0�, 76�E. The Japanese Marine Science andTechnology Centre (JAMSTEC) installed a mooring withan ADCP at 0�, 90�E in November 2000 (Y. Masumoto etal., Tropical Indian Ocean mooring array: Present statusand future plans, available at http://ocean-partners.org/POGO-4docs/IndianOceanArrays.eps). At the time of writ-

ing, both NIO and JAMSTEC observations are continuing.The NIO and JAMSTEC data sets show rich intraseasonalvariability of equatorial currents. The most energetic zonalcurrent (u) fluctuations in the upper ocean have a 30- to60-day period, whereas the meridional current (v) hasbiweekly variability [Murty et al., 2002; Y. Masumoto,personal communication, 2003] that is distinct from slowerintraseasonal oscillations.[5] Here we investigate the nature of the biweekly vari-

ability in the equatorial Indian Ocean using the NIO obser-vations at 0�, 93�E and 0�, 83�E and an OGCM forced byhigh-resolution surface winds from a satellite-borne scatter-ometer. The observations and model are described in the nextsection. The existence of distinct biweekly variability ofmeridional current in the observations is established insection 3. The model simulates the biweekly variability(but not the observed 30- to 60-day fluctuations of v) withreasonable accuracy at all depths up to 2000 m. We estimatethe zonal and vertical length scales of the biweekly modefrom the observations, and present the first results about itsspace-time structure with the help of the model simulation insection 4. We establish that the biweekly mode consists ofpackets of westward propagating wind forced MRG waves.Vertical phase propagation of these waves is generally, butnot always, upward. They are associated with deep, off-equator upwelling/downwelling. In section 5 we show thatthe variability of meridional wind stress associated with theatmospheric QBM has zonal wavelength comparable to thatof the oceanic biweekly mode. We propose that the atmo-spheric QBM resonantly forces biweekly MRG waves in theocean. In the upper ocean and at intermediate depths, theMRG waves have high amplitudes in the east (‘‘easternintensification’’). The possible reasons for eastern intensifi-cation are also discussed in section 5. Our main results aresummarized and their possible implications mentioned in theconcluding section.

2. Observations and Model

2.1. Observations

[6] A mooring with six subsurface current meters(Aanderaa RCM) was deployed at 0�, 93�E in February2000 by NIO. In December 2000, this mooring was rede-ployed with RCMs at nominal depths of 90, 245, 445, 950,1970, and 3960 m, and another mooring was installed at 0�,83�E. The shallowest current meter at 83�E was at a meandepth of 385 m, with similar instruments at 590, 1070, and2100 m; an ADCP mounted above the shallowest RCM didnot return any data. At all instruments, speed and directionare sampled 50 times in each recording interval, which iseither 1 or 2 hours, depending on depth. Each record is theaverage of these 50 values. Tidal frequencies are removedfrom the observed currents by taking a 49-hour runningmean. The instruments also measure temperature, salinity,and pressure. In this study we use daily detided observationsfrom December 2000 to March 2002 from the 93�E moor-ing, and December 2000 to December 2001 from the 83�Emooring.

2.2. Model

[7] We use the Modular Ocean Model 2.2 [Pacanowski,1996] with Indian Ocean domain in this study. The model

Figure 1. Spectra of 25-m zonal current at 5�N (blackline) and meridional current at the equator (shaded line)along 80.5�E during July 1993 to September 1994 (adaptedfrom Reppin et al. [1999]). The dotted line corresponds to aperiod of 15 days.

C10003 SENGUPTA ET AL.: BIWEEKLY MODE IN THE INDIAN OCEAN

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domain is 30�S–30�N and 30�E–110�E, with a spongelayer at 30�S. Horizontal resolution is about 1/3� north of5�S. There are 19 levels in the vertical, six of which arein the top 100 m. Model bottom topography is based on the1/12� � 1/12� resolution ETOPO5 data from the U.S.National Geophysical Data Center. Horizontal eddy diffu-sivity and viscosity are 2000 m2 s�1. Vertical mixing isbased on the scheme of Pacanowski and Philander [1981].Surface temperature and salinity are relaxed to the observedseasonal cycle of Levitus [1982].[8] The model is forced by surface winds from the

scatterometer on board QuikSCAT [Liu, 2002], which areavailable since July 1999. The scatterometer estimates windspeed and direction with high accuracy [Chelton et al.,

2001]. Three-day running mean wind fields, created fromtwice-daily 1/4� gridded winds, have almost no gaps due tolimited swath or rain. Wind stress was obtained from the3-day running mean winds by using a constant dragcoefficient of 0.0013. The model run forced by QuikSCATwind stress is called the ‘‘QuikSCAT’’ simulation. A test runwith an objectively interpolated daily gridded wind stressfield created from QuikSCAT winds [Pegion et al., 2000]gives almost identical results. Initial conditions for theQuikSCAT simulation, in July 1999, are from a 15-yearrun of the model forced by daily wind stress obtained fromNCEP reanalysis daily surface winds (the ‘‘NCEP’’ simu-lation). In another run (the ‘‘seasonal’’ run) the model wasforced by the seasonally varying part of the QuikSCAT

Figure 2. Variance preserving power spectra of December 2000 to December 2001 observed and modelmeridional velocity (v). (a) Spectra of v at uppermost instrument depth 0�, 93�E from observations (thinline) and QuikSCAT simulation (thick line); the depth fluctuates between 90 m and 170 m due to mooringmotion, with a mean of 106 m. (b, c) Same as Figure 2a, but for the instruments at 465 m and 965 meandepth. (d, e, f ) Same as Figure 2a but for 0�, 83�E at mean depths of 385, 590, and 1070 m. The dashedlines are the 95% confidence levels based on a red-noise background. Units are arbitrary, but identicalacross all panels. The cutoffs (10 and 18 days) of the Lanczos filter used to extract the 10- to 20-daymode are shown.


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wind stress, obtained from the daily wind stress by remov-ing all variability with periods shorter than 90 days.

3. Biweekly Mode in Observations and Model

[9] Variance preserving Blackmann-Tukey spectra[Chatfield, 1975] from the observations and from theQuikSCAT simulation are compared in Figure 2; the modelv is interpolated to daily instrument depth. The spectrum ofobserved v from the shallowest current meter at 93�E,whose mean depth is 106 m, has a significant peak at20–30 days, in addition to biweekly variability (Figure 2a).The two period bands seem to be merged in the spectra of

model v from the QuikSCAT simulation. Observed v fromthe third and fourth instruments (mean depths 465 m and965 m) on the 93�E mooring shows a distinct biweekly peakin addition to variability on 20- to 60-day timescale(Figures 2b and 2c). Spectra at 83�E are qualitatively similar(Figures 2d, 2e, and 2f ). There are eight to ten powerspectral estimates in the period range 20–40 days, depend-ing on the length of the data. Some of the spectra thereforesuggest that the 20- to 30-day variability in the observationsis distinct from the 30- to 60-day variability (e.g., Figures 2eand 2f ). The QuikSCAT simulation captures the biweeklyvariance reasonably well, but does not reproduce the 30- to60-day variability. We do not understand the reason for this,but it may be related to inadequate model vertical resolutionbelow the thermocline. We do not investigate the 30- to60-day or 20- to 30-day variability in this paper. Thebiweekly mode is isolated using a 10- to 18-day Lanczosfilter [Duchon, 1979] with 121 weights for further study.[10] Daily unfiltered v for 2001 from the shallowest

current meter at 93�E is shown in Figure 3a, together withdaily meridional velocity at the mooring location from theQuikSCAT simulation, interpolated to the daily instrumentdepth; Figures 3b–3f show model-data comparisons of 10-to 18-day filtered v at 106, 265, 465, 965, and 1970 m. Themodel simulates the biweekly variability reasonably well atall depths at this location, as well as at 83�E (not shown).The biweekly oscillation is absent in the seasonal run(Figures 3b–3f ), demonstrating that it is forced by intra-seasonal wind variability and not by dynamic instability.There is some intraseasonal variability of currents in theseasonal run, particularly at the uppermost level at 93�E(Figure 3b). However, the dominant periods of this vari-ability, which must be due to instability, are longer than20 days. The instability in the eastern EqIO has beendiscussed by Sengupta et al. [2001]. The NCEP simulationunderestimates the observed biweekly variance; it capturesthe phase of the oscillations with some success (not shown),but the QuikSCAT simulation is more realistic. The impor-tance of wind forcing is illustrated by means of a compar-ison of the QuikSCAT and NCEP simulations withtemperature observations. The current meter moorings haveno surface buoys. Therefore the depth of the instrumentscan change rapidly as the mooring sways due to currentdrag (Figure 4a). The uppermost current meter at 93�Eprovides vertical ‘‘profiles’’ of thermocline temperature,which are reasonably well reproduced in the QuikSCATsimulation, but are poor in the NCEP simulation, particu-larly from mid-January to August (Figure 4b). The superiorsimulation of vertical stratification in the QuikSCAT simu-lation can be attributed to the weak variability of NCEPreanalysis surface winds relative to QuikSCAT winds in theeastern EqIO [Goswami and Sengupta, 2003].[11] The intraseasonal variability of QuikSCAT wind

stress is mainly in 10- to 60-day periods (Figure 5). Bothty in the west (Figure 5a) and tx in the central and easternEqIO (Figures 5b and 5c) have a peak at a 10- to 20-dayperiod. The spectra of model equatorial v and off-equator uare narrow relative to the spectra of wind stress (Figures 5d,5e, and 5f ), with a distinct biweekly maximum. Off-equatorvertical velocity (w) has a biweekly peak, with additionalvariability at 5- to 10-day periods (not shown). The spectraof ocean currents are consistent with the spatial structure of

Figure 3. Comparison of observed and model v at 0�N,93�E. (a) Observed (thick line) and model (thin line)unfiltered v at uppermost instrument depth. (b–f ) Observed(thick line) and model (thin line) 10- to 18-day filtered v,and model v from the seasonal run (dashed line). Thecurrent meter at 106 m gave no data beyond October. Themean instrument depth, standard deviation, and correlationcoefficient of observed and model time series are mentionedabove each panel.


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MRG waves, which have symmetric v about the equator,and antisymmetric u and w with off-equator maxima.

4. Space-Time Structure of the Biweekly Wave

[12] Comparison of the time series of 10- to 18-dayfiltered v from the uppermost instrument at 83�E (depth385 m) with the observations at 465 m at 93�E shows that vat 83�E generally lags v at 93�E, suggesting westward phasepropagation. Oscillations of v generally occur in groups oftwo to four cycles (this is discussed later); the lag betweenthe two time series is not invariant in time over the fullrecord, indicating that a range of wavelengths is associatedwith the biweekly wave. Therefore, different segments ofthe time series with constant lag are identified, and the wavevector is estimated for each of these segments followingWeisberg et al. [1979]. The zonal wavelengths range from2100 to 6100 km (Table 1). The vertical wavelength isestimated in a similar manner from instrument pairs at thetwo locations (Table 2). The instruments pairs chosen arelocated at 385 m and 1070 m on the 83�E mooring, and465 m and 965 m on the 93�E mooring. In general, the v timeseries at the shallower instrument lags that at the deeperinstrument, showing upward phase propagation. However,there are occasions when the lag is unresolvable with dailydata (i.e., it is less than 1 day, or effectively zero); there arealso instances when the phase propagates downward, sug-gesting upward energy propagation due to bottom reflection.We have excluded such periods from the analysis.

[13] Like the observations, the biweekly wave in themodel has westward and generally upward phase propaga-tion (Figure 6). There is a range of zonal wavelengths andassociated phase speeds. Episodes of clear basinwide west-ward propagation are relatively infrequent. Group speed iseastward with a typical value of about 1 m s�1. The verticalscale of the wave below the thermocline is large (Figure 6b,Table 2) relative to the model vertical grid spacing, which isabout 125 m at a depth of 500 m. This might be the reasonwhy the model is able to resolve the biweekly wave atdeeper levels, although it has relatively coarse verticalresolution at these depths.[14] To determine the space-time characteristics of the

biweekly wave, wave number-frequency spectra of currentsat 50 m and 500 m from the QuikSCAT simulation arecomputed following the method of Wheeler and Kiladis[1999]. The symmetric component of v and antisymmetricu have large variance at 11- to 17-day period and wave-lengths between 2200 km and 6000 km (Figure 7), centeredat 14 days and 3000–4500 km. The biweekly signal at 500 mdepth has a similar range of zonal scales (Figure 8), in generalagreement with the wavelength range determined from thefiltered time series of observed v (Table 1). The region ofsignificant biweekly power lies on the dispersion curve of theMRG wave with a value of separation constant c [Gill, 1982]equal to 2.1 m s�1, suggesting a mix of first and higherbaroclinic modes. (The first baroclinic mode in the modelhas c = 2.6 m s�1.) There appear to be MRG waves witheastward phase propagation having a somewhat lower value

Figure 4. Time series of (a) daily mean depth of the uppermost current meter at 0�, 93�E;(b) temperature (�C) measured by the instrument (thick line), model temperature from the QuikSCATsimulation (thin line), and NCEP simulation (dashed line) interpolated to daily instrument depth.


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of c (�1.7 m s�1) at 8- to 10-day period (see Figures 2b and2c); however, the variance in this band is small relative to thatat 10–18 days. The spectra of symmetric v have significantpower at the�1000–1500 km wavelength and 20- to 30-dayperiod (Figures 7a and 8a), consistent with the secondarypeak in some model v frequency spectra.[15] The characteristic horizontal structure of biweekly

surface wind stress and ocean velocity fields is based on

composites of 15 biweekly events from June to September2000 and 2001, identified from 10- to 18-day filtered fields(Figure 9). Composites based on meridional wind stressshow that the atmospheric quasi-biweekly mode is associ-ated with symmetric ty with typical zonal scale (halfwavelength) of 15�–20� of longitude (Figures 9a, 9b, 9e,and 9f ). Composites based on wind stress curl (not shown)reveal that quasi-biweekly wind stress variability over the

Figure 5. Variance preserving power spectra of January 2000 to December 2001 QuikSCATwind stressand model upper ocean response. (a) Spectra of tx (thin line) and ty (bold line) averaged over 1�–1�N,60�E–65�E. (b) Same as Figure 5a, but for 75�E–80�E. (c) Same as Figure 5a, but for 90�E–95�E.(d) Model 50-m v (thick line) averaged over 1�S–1�N, 60�E–65�E, and model antisymmetric u (thinline), averaged over 3�N–5�N, 60�E–65�E. (e) Same as Figure 5d, but for 75�E–80�E. (f ) Same asFigure 5d, but for 90�E–95�E. Units are arbitrary but identical for both tx and ty in Figures 5a, 5b, and5c. Units of v and u are different but identical for each parameter across Figures 5d, 5e, and 5f.

Table 1. Zonal Wavelength Estimated From the Lag Between

Observed Time Series of 10- to 18-Day Filtered v at 465 m Depth,

93�E and at 385 m Depth, 83�E

Period Zonal Wavelength, km

10 Jan. to 7 Feb. 2001 34002 March to 22 March 2001 61001 June to 5 July 2001 210025 July to 30 Aug. 2001 31005 Sept. to 15 Oct. 2001 3100

Table 2. Vertical Wavelength Estimated From the Lag Between

Observed Time Series of 10- to 18-Day Filtered v at 465 m and

965 m at 93�E and 385 m and 1070 m at 83�E

93�E, 465 and 965 m 83�E, 385 and 1070 m

Period

VerticalWavelength,

m Period

VerticalWavelength,

m

30 Aug. to 27 Sept. 4700 25 Feb. to 20 March 32002 Nov. to 27 Nov. 1400 10 Oct. to 3 Nov. 3200


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eastern EqIO consists of vortices of alternating sign cen-tered on the equator, in agreement with Chen and Chen[1993] and Chatterji and Goswami [2004]. Periods ofintense quasi-biweekly atmospheric activity are marked bysymmetric ty that is strong on the western flank of thevortices, and antisymmetric tx. The composite biweeklymode in the eastern (Figures 9c and 9d) and western(Figures 9g and 9h) ocean at �100 m depth has the structureof an MRG wave with half wavelength of about 20�. Theaverage interval between the dates used for the eastern andwestern composites is about 9 days, corresponding towestward propagation at 4 m s�1. The ocean currentcomposites suggest that the zonal and meridional scale ofthe wave is somewhat larger in the east than in the west. Thedifference in meridional scale is consistent with the differentstratification of the upper ocean in the east and west. Thevertical velocity associated with the composite biweeklymode has maximum amplitude of 2–3 m per day. The w fieldis antisymmetric about the equator, with amplitude maximalocated at 2�–3� latitude, in keeping with theory. Thecomposite horizontal structure at 500 m depth (Figure 10)has qualitatively similar characteristics. The compositestructure in the latitude-depth plane (Figure 11) consists ofdeep upwelling/downwelling on either side of the equator,connected by cross-equatorial meridional flow.

5. Forcing Mechanism and ‘‘EasternIntensification’’ of the Biweekly Wave

[16] We have established that the biweekly MRG wavein the ocean is forced by intraseasonal variability of windstress, and that the dominant zonal wavelength is 3000–4500 km (Figures 7 and 8). The zonal scale of theatmospheric QBM, estimated in the literature from ana-lyzed winds at 850 hPa and higher levels, is reported to be6000 km. However, the zonal scale of the biweekly surfacewind-forcing is considerably smaller. Meridional wind

stress can effectively excite MRG waves, because thesewaves have symmetric v and antisymmetric u fields [Wunschand Gill, 1976]. We find that the zonal scale of meridionalwind stress at the surface associated with the atmosphericQBM is 3000–4500 km (Figure 12). Biweekly variability ofsymmetric ty over the central and eastern EqIO (but not inthe western EqIO) is associated with QBM vortices centerednear the equator both in summer and winter, as discussedbefore [Chatterji and Goswami, 2004]. The compositesshown in Figure 9 suggest that the zonal wavelength of tyis only 3000–4500 km, not 6000 km, because the mag-nitude of ty is significant only on the western flanks of thevortices. On the basis of these results we propose that theatmospheric QBM resonantly excites the biweekly wavein the Indian Ocean; that is, the period, zonal wavelength,and westward propagation speed of the ocean response

Figure 6. Propagation of the biweekly mode in summer2001. (a) Time-longitude plot of model 10- to 18-dayfiltered v at 50 m depth, averaged over 1�S–1�N; the rangeof phase speed is 2.5–5.0 m s�1 westward, and group speedis about 1 m s�1 eastward; (b) Time-depth plot of model 10-to 18-day filtered v at 0�, 85�E.

Figure 7. Zonal wave number–frequency power spectraof (a) symmetric component of 2�S–2�N v from QuikSCATsimulation at 50 m normalized by its background spectrumvbg. The background spectrum is obtained by averaging thepower of symmetric and antisymmetric v and smoothingseveral times with a 1–2–1 filter in frequency and wavenumber, (b) Same as Figure 7a but for the antisymmetriccomponent of 3�N–5�N u at 50 m. The dispersion curvesfor different classes of equatorial waves with separationconstant c = 2.1 m s�1 are superimposed. Biweeklyvariability falls on the dispersion curve of the MRG wave.The vertical line marks (westward) wavelength of 3500 km.


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matches that of the forcing, and corresponds approximatelyto the characteristics of free MRG waves. It is likely thatthe QBM excites biweekly waves in the west Pacific aswell. An MRG wave with westward phase propagation,4000-km zonal wavelength, and 14-day period has beenreported in the upper west Pacific from TOGA COAREobservations [Zhu et al., 1998]. According to the authors,this wave was generated by ty forcing associated with anatmospheric Rossby wave.[17] Biweekly variability of equatorial v in the upper

ocean is largest in the far eastern EqIO, although the 10- to18-day variability of ty is largest in the west (see Figure 5).Broadly speaking, some form of eastern intensificationmight be expected because MRG waves have eastwardgroup speed. The energy in biweekly MRG waves at agiven longitude is due to local forcing as well as energyarriving from the west. Note that the energy density ofMRG waves need not be highest in the east if there arereflected equatorially trapped waves carrying biweeklyenergy back to the west. However, there are no freewaves in the tropical ocean with periods between �1 weekand �1 month and westward group speed [Gill, 1982].Model results show that coastally trapped waves withbiweekly period are generated when biweekly MRGwaves are incident on an eastern boundary. In the deepocean, biweekly v variability at the equator is related to vvariability at higher latitudes within 200 km of the eastern

boundary (Figures 13 and 14b), as well as around theChagos ridge at 73�E. (The relation between equatorialMRG waves and coastally trapped biweekly waves is notas clear in the upper ocean, where coastal waves can beforced by surface wind stress variability.) Figure 14a showsthat biweekly ty is episodic. As mentioned earlier, theforcing generates episodic MRG waves in the ocean con-sisting of groups of two to four waves. There is no apparentseasonality in the amplitude of the forcing or the oceanresponse, although the record is too short to be conclusive.On the basis of these results, we propose that (1) a part ofthe energy of the MRG wave can escape to higher latitudesvia coastal Kelvin waves and (2) eastern intensificationarises essentially because the MRG wave has eastwardgroup speed and because there are no free equatoriallytrapped waves with westward group speed in the 10- to18-day period range. Therefore the biweekly MRG wavecannot ‘‘reflect’’ off an eastern boundary, and the biweeklycoastal Kelvin wave does not radiate Rossby waves.

6. Discussion

[18] Recent observations at 0�, 83�E and 0�, 93�E in theIndian Ocean suggest that meridional current has subseaso-nal variability on three distinct timescales: 10–20 days, 20–30 days, and 30–60 days (Figure 2). The biweekly vari-ability is episodic and present throughout the year with noclear seasonality. There are episodes lasting 1 to 2 monthswhen the phase difference between 10- to 18-day filtered vat �500 m depth on the two moorings is relatively constant,suggesting the presence of wave groups with zonal wave-length varying from 2000 to 6000 km; phase propagateswestward. Phase lags between v at different depths (�500 mand �1000 m) on each of the moorings occasionally implyupward phase propagation; the vertical wavelength duringthese episodes varies from 1400 m to 5700 m.[19] A model forced by QuikSCAT winds simulates the

biweekly variability well because the wind-forcing is accu-rate. The simulation does not suffer at deeper depths,possibly because the vertical scale of the biweekly modeis large relative to model vertical resolution. However, themodel fails to produce the observed 30- to 60-day variabil-ity. Space-time spectra of unfiltered model velocity fields,as well as summer composites based on filtered fields(Figure 9), show that the biweekly mode is a mixedRossby-gravity wave. The largest variance of model sym-metric v and antisymmetric u is at 14-day period and 3000-to 4500-km zonal wavelength (Figures 7 and 8). Biweeklyvariance lies approximately on the MRG dispersion curvewith c = 2.1 m s�1. This value of c suggests the presence offirst and higher baroclinic modes. The presence of highermodes is consistent with vertical propagation seen in theHovmoller plot (Figure 6b). Energy in the upper 2000 mpropagates eastward and generally downward, though thereare indications of upward energy propagation in both dataand model. Biweekly variability is enhanced in the fareastern region and just west of topographic features, suchas the ridge at 73�E (Figure 13), where the MRG wavegenerates biweekly waves trapped along the topography(Figure 14b). We propose that eastward intensificationarises partly because there are no allowed equatorial waveswith biweekly period and westward group speed.

Figure 8. Same as in Figure 7, but for v and u at 500 m.


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Figure 9. Composite horizontal structure of biweekly wind stress and model upper ocean velocity. (a, b)Meridional wind stress (dyn cm�2; contours with negative values shaded) and wind stress vector (dyncm�2) composites based on dates of maximum negative (left) and positive (right) ty averaged over 5�S–5�N, 85�E–90�E; (e, f ) Same as Figures 9a and 9b, but for 55�E–60�E. (c, d) Composite vertical velocity(m day�1; contours with negative values shaded) and current vectors at �100 m depth (cm s�1), based ondates of maximum southward (left) and northward (right) meridional volume transport across the equator inthe upper 100 m between 90�E and 95�E. (g, h) Same as Figures 9c and 9d, but for 60�E–65�E.

Figure 10. Composite horizontal structure of biweekly wave at �500 m. Composite vertical velocityat 470 m depth (m day�1; contours with negative values shaded) and current vectors at 530 m depth(cm s�1); the composites are based on dates of maximum (a) southward and (b) northward meridionalvolume transport across the equator, between 450 and 550 m and 60�E and 65�E.


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[20] The quasi-biweekly mode in the atmosphere at850 hPa and higher levels has a zonal wavelength of about6000 km [Chen and Chen, 1993; Chatterji and Goswami,2004]. These studies have shown that atmospheric QBMeast of 65�E is often associated with a vortex centeredaround the equator. This is crucial for the existence ofsubstantial symmetric meridional wind stress near theequator (Figure 9), which is the main forcing for oceanicMRG waves. The zonal wavelength of biweekly ty, how-ever, is 3000–4500 km (Figures 9 and 12), because merid-ional surface wind stress is strong only on the western flank

Figure 11. Composite vertical structure of the biweeklywave. Meridional velocity (0, 3, and 9 cm s�1 contours) andcurrent vectors in the latitude-depth plane, averaged over(a, b) 90–95�E and (c, d) 60�E–65�E, in the southward(left) and northward (right) phases of the wave. Verticalvelocity has been multiplied by 6000 to preserve vectororientation. Composites are based on the same dates as thecurrent composites in Figure 9.

Figure 14. Time series of square of 10- to 18-day filtered(a) meridional wind stress averaged over 1�S–1�N and90�E–95�E and (b) meridional velocity at 0�, 93�E (black)and 7�N, 93�E (shading).

Figure 12. Zonal wave number-frequency power spectraof symmetric component of 5�S–5�N ty normalized by itsbackground spectrum; July 1999 to December 2002. Thedispersion curve of the first meridional mode (n = 1) Rossbywave with c = 15 m s�1 is superposed. The full vertical linemarks wavelength of 3500 km.

Figure 13. Standard deviation of 10- to 18-day filtered v(cm s�1) from the QuikSCAT simulation at (a) 50 m and(b) 500 m, July 1999 to December 2002.


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of the vortex. We conclude, therefore, that the quasi-biweekly oscillations of surface winds resonantly excitebiweekly MRG waves in the Indian Ocean. The biweeklywave is distinct from the 26-day wave reported in thewestern EqIO [Luyten and Roemmich, 1982] that is prob-ably generated by dynamic instability of ocean currents[Kindle and Thompson, 1989]. In this connection, it isinteresting that the observations (and the model) show someevidence of 20- to 30-day v variability in the eastern EqIO.[21] We have established that biweekly variability in the

equatorial ocean is due to wind-forced MRG waves. How-ever, our discussion of the generation mechanism and ener-getics of the biweekly waves is somewhat incomplete at thisstage. Although we find that the quality of the wind-forcing isimportant, we do not know to what extent our results aremodel dependent. For instance, a model with different mix-ing parameterization might distinguish the biweekly variabil-ity from 20- to 30-day variability in a more satisfactorymanner (Figure 2). We have not examined the possibility,suggested by symmetry considerations, that antisymmetric txmight generate MRG waves. Further work is needed on thegeneration mechanism, vertical energy propagation (includ-ing the possibility of bottom reflection), and energetics ofeastward intensification of the biweekly mode.[22] Finally, we note that the biweekly upwelling/down-

welling reported here might have implications for the bio-geochemistry of the Indian Ocean. Eastward equatorial jets inspring and fall, forced by westerly winds [Knox, 1976], leadto downwelling and a deep thermocline in the eastern IndianOcean [Wyrtki, 1973]. The appearance of sustained easterlywinds in the EqIO during strong ‘‘dipole’’ episodes such asthose of 1961, 1994, or 1997 can drive upwelling off the coastof Sumatra as well as at the equator [Murtugudde et al.,2000], associated with high chlorophyll concentrations in theeast [Murtugudde et al., 1999]. We have presented evidencethat upwelling in the EqIO is not confined to dipole events orto the eastern basin; episodic deep upwelling and down-welling associated with the biweekly mode is presentthroughout the year all along the EqIO, except in the farwest. The maximum upwelling speed in individual MRGwaves can be as large as 6–8 m day�1. Since upwellingfollowed by mixing is an irreversible process, the biweeklymode is likely to influence the vertical distribution oftemperature, nutrients, and chlorophyll in the equatorialIndian Ocean. The possibility that the biweekly mode hasan influence on meridional heat transport in the Indian Ocean[see Loschnigg and Webster, 2000] is also worth examining.

[23] Acknowledgments. We thank Jay McCreary, Toru Miyama, andBhupen Goswami for many useful discussions, and the members ofCLIVAR AAMP for comments. Reviewers of a previous short versionof this paper gave several useful suggestions. D. S. thanks Department ofScience and Technology, New Delhi, for funding. D. S. and V. S. N. M.would like to acknowledge long-term financial support from Department ofOcean Development, New Delhi, which made this work possible. V. S. N.M. thanks the director, NIO, for support, and all members of the mooringteam as well as officers and crew of ORV Sagar Kanya. The QuikSCATwind data were obtained from the ftp site ftp://podaac.jpl.nasa.gov/pub/ocean_wind/quikscat) of Physical Oceanography Distributed ActiveArchive Center (PO.DAAC), NASA Jet Propulsion Laboratory, Pasadena,USA. This is NIO contribution 3918.

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��R. Senan and D. Sengupta, Centre for Atmospheric and Oceanic

Sciences, Indian Institute of Science, Bangalore, 560012, India. ([email protected]; [email protected])V. Fernando and V. S. N. Murty, Physical Oceanography Division,

National Institute of Oceanography, Dona Paula, Goa, 430004, India.([email protected]; [email protected])


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a biweekly mode in the equatorial indian ocean

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