Near-surface circulation and kinetic energy in the tropical Indian Ocean derived from Lagrangian drifters

Transcription

1 Journal of Marine Research, 57, , 1999 Near-surface circulation and kinetic energy in the tropical Indian Ocean derived from Lagrangian drifters by S. S. C. Shenoi 1, P. K. Saji 1 and A. M. Almeida 1 ABSTRACT Trajectories of 412 satellite-tracked drifting buoys deployed in the tropical Indian Ocean have been analyzed to document the surface circulation and kinetic energy eld. Only drifters drogued at 15 m depth and having drag area ratio greater than 35 are used to estimate current velocities. Unlike in earlier studies, the widening of the Equatorial Jet in the eastern equatorial Indian Ocean and the westward ow at the equator during July August are apparentin the present data set. The comparison of drifter data with the seasonal mean dynamic topography (0/1000 db) shows that the surface circulation pattern inferred from dynamic topography does not always represent the surface currents in the Indian Ocean. Both compare well for the South Equatorial Current, the Equatorial Counter Current, and the southwestwardcurrent along the IndonesianIslands; they differ in the Bay of Bengal during the southwest monsoon, but are similar during the northeast monsoon. Maps of mean and eddy kinetic energy show maxima in the regions of western boundary currents and equatorial currents and minima in the Arabian Sea, the Bay of Bengal, and south of 20 S. 1. Introduction Surface circulation in the Indian Ocean is unique because of its response to the annually reversing monsoon winds because of which the major currents in the Indian Ocean also undergo variations on semiannual and annual time scales. The seasonality is more prominent in the north than in the south. The major surface currents in the Indian Ocean are summarized in the schematic in Figure 1. During the northern winter (i.e., during the northeast monsoon), the surface current systems resemble the general circulation patterns in the Paci c and Atlantic oceans; the South Equatorial Current (SEC), the Equatorial Counter Current (ECC), the North Equatorial Current (Northeast Monsoon Current (NMC) hereafter), and the associated boundary currents are seen during this season. During the northern summer (i.e., during the southwest monsoon), the surface currents do not resemble the patterns seen in the other two oceans; the eastward Southwest Monsoon Current (SMC) replaces the westward NMC and a northward ow, the Somali Current (SC), replaces the southward ow along the coast of Somalia. At the equator, the scenario is further complicated with the appearance of an eastward Equatorial Jet (EJ) during the transition periods, April May and November December (Wyrtki, 1973). 1. Department of Physical Oceanography, National Institute of Oceanography, Dona Paula, Goa , India. shenoi@csnio.ren.nic.in 885

2 886 Journal of Marine Research [57, 6 Figure 1. Schematic of major surface currents in the Indian Ocean during (a) the northeast monsoon and (b) the southwest monsoon. The major currents depicted are: South Equatorial Current (SEC), Northeast Monsoon Current (NMC), Equatorial Counter Current (ECC), Equatorial Jet (EJ), East African Coastal Current (EACC), Somali Current (SC), Southwest Monsoon Current (SMC), West India Coastal Current (WICC), East India Coastal Current (EICC) and East Madagascar Current (EMC). The EJ, though depicted in the schematic for winter, does not appear either during summer or winter monsoon season; it appears during the transition period in April May and November December. The thickness of the curve represents the relative magnitude of the current.

3 1999] Shenoi et al.: Indian Ocean circulation 887 Just as the open ocean currents, the boundary currents in this region also undergo seasonal reversals (Shetye and Gouveia, 1998). The West India Coastal Current (WICC) ows poleward during winter (November February) and ows equatorward during summer (June September). The East India Coastal Current (EICC) ows poleward during February April and equatorward during November December; during the southwest monsoon, a weak poleward EICC appears in the south and an equatorward EICC appears in the north. The satellite-tracked drifting buoys are useful in tracking these space-time dependent surface currents. Earlier, Reverdin et al. (1983), Shetye and Michael (1988), and Molinari et al. (1990) used drifting buoy data to study the currents in the Indian Ocean. Molinari et al. concluded that, in general, the mean annual surface circulation determined from buoys is similar to that determined from the ship drift reports of Cutler and Swallow (1984) in the regions where buoy measurements are numerous. In this paper, we present a compilation of data on surface currents in the Indian Ocean from 412 buoys and discuss the distributions of kinetic energy due to the mean ow (E M ) and the temporally varying part (E E ). The temporally varying part, the eddy kinetic energy, represents the variance of the velocity eld due to uctuations spanning a broad range of time and space scales. The distribution of eddy kinetic energy (E E ) helps in identifying the regions with the most energetic temporally varying uctuations. Wyrtki et al. (1976) presented the kinetic energy distributions over the world oceans using ship-drift data; since then no similar estimates have been made for the tropical Indian Ocean. The kinetic energy distributions presented here, therefore, re ne those presented by Wyrtki et al. (1976). Traditionally, the ow elds inferred from the distribution of density elds are used to represent the surface circulation patterns. In this paper, we compare the surface ows derived from buoys and dynamic height elds. Not surprisingly, the comparison suggests that the surface circulation pattern inferred from the dynamic heights need not always represent the surface currents in the Indian Ocean. 2. The data set Satellite tracked Lagrangian drifting buoys have become an integral part of surface current measurement. Over the last two decades, several drifters have been deployed in the Indian Ocean for the measurement of surface currents. For this study, we have assembled a data set by accumulating data from 412 drifting buoys from three sources (archives of National Institute of Oceanography, NOAA/AOML/DAC, and R. Molinari). The data span a period of 22 years from 1976 to August 1998, with no data during The archived buoy data consist of an unevenly spaced time series of position xes (often 1 to 8 observations per day per buoy) determined by the CLS Argos system with an accuracy of approximately m (Anonymous, 1990). Following Hansen and Poulain (1996) the archived positions were rst edited to remove the dubious positions. The trajectories of the buoys (north of 25 S) plotted from all the buoys show the data availability and their distribution over the tropical Indian Ocean (Fig. 2). The number of observations in the

4 888 Journal of Marine Research [57, 6 Figure 2. Trajectories of 412 satellite-tracked drifting buoys deployed from 1976 through August 1998 during various observationalprograms. No data are available for equatorial and southern Indian Ocean are higher than in the Arabian Sea and the Bay of Bengal because the buoys there have a greater chance of survival. In a smaller basin like the Arabian Sea or the Bay of Bengal, the operational period of the buoy is reduced drastically due to grounding. Most of the buoys used in this study had a similar design, a spherical surface otation unit, a tether, and a holey sock drogue as sea anchor centered at 15 m below the surface. The rest had a different design, an inverted cone attached with window shade drogue centered at 30 m or 10 m below the water level. The water-following capability of any mixed layer drifter depends critically upon the drag area ratio, R, which is the ratio of the drag of the drogue area to the drag of the submerged oat and tether. Niller and Paduan (1995) show that the FGGE type drifters, with window shade drogues of drag area ratios 10 12, have signi cant differences in water-following capabilities in comparison with that of the later designs. The most important design parameter that affects the water-following capability of a buoy is the drag area ratio and the most important environmental factor that affects the slip is the wind (Niiler et al., 1995). This implies that combining the buoys having different water-following capabilities will introduce errors in the estimation of surface currents. Hence, to construct a data set from drifters having similar water following capabilities, we avoided the drifters with R, 35. All buoys deployed prior to 1985 and a few buoys deployed after this period did not satisfy this condition; for the remaining 321 buoys, R ranged between 35 and 42. The removal of the buoys with R, 35 affected the coverage slightly; this effect was mainly in the western equatorial Indian Ocean.

5 1999] Shenoi et al.: Indian Ocean circulation 889 All buoys have a drogue attached when they are deployed. A drogue ON/OFF sensor (submergence sensor) tted to the buoy monitors its presence. Occasionally, the drogue gets detached from the buoy. Its loss affects the velocity measurement because it alters (i) the drag area ratio and hence the drift characteristics, (ii) the level at which the velocity is determined, and (iii) the effects of local winds and waves on the buoy. If concurrent wind data are available, empirical formulations (for example Poulain et al., 1996) can be used to correct for the effect of winds on undrogued data. The absence of concurrent wind data prevented us from applying this correction. Instead, to avoid using the affected data, we excluded the undrogued data by examining the submergence values reported by the buoy. The removal of undrogued data resulted in a further data loss of 13%. When the undrogued data were retained, the number of observations increased in some bins. Even though the monthly mean vectors estimated in a bin did not differ signi cantly from the results obtained using only drogued drifters, the standard errors on monthly means were generally higher. By attaching current meters to the drogue Niiler et al. (1995) measured the slippage of drogues through water caused by winds. Based on the measurements, they suggested the model U s 5 a R U w to correct for the wind slippage vector U s (cm s 2 1 ) in the direction of wind velocity vector U w (m s 2 1 ). The constant a was obtained from a least-squares t of measured slip and wind speed. R is the drag area ratio of the drifter involved. In the tropical Indian Ocean, generally, the wind speeds varied between 8 m s 2 1 and 15 m s 2 1 during the summer monsoon when the winds are strong. Assuming a wind speed of 15 m s 2 1, the wind slippage estimated for a buoy of R 5 42 is 2.5 cm s 2 1. Thus the error introduced by the wind-induced slippage in the individual surface velocity vector will not exceed 2.5 cm s 2 1. The buoy positions, retained after the above quality checks, were then interpolated to generate four equally-spaced values per day and to compute the drifter speed and direction. Finally, a ve-day Gaussian lter was applied to the data to reduce the effects of very high-frequency motions. 3. Drifter trajectories The rapidly changing surface currents in the Indian Ocean are best described by following individual buoy trajectories. Sketched in Figure 3, for example are, the currents a buoy encountered during its one-year lifetime. The buoy was released near the equator at 50 E on 15 October 1981 (Fig. 3a). On its release, the buoy drifted slowly toward the east and then started moving faster under the in uence of the winter EJ that developed in November. In November, under the in uence of the EJ, the buoy traveled about 4000 km over 30 days. By mid-december when the EJ collapsed, the drifter was at 82 E. At this point the drifter turned westward and drifted along the equator till 68 E. In late January,

6 890 Journal of Marine Research [57, 6 Figure 3. Trajectories of buoys with ARGOS ID (a) 1885 and (b) The date of deployment is indicated at the beginning of the trajectory(hollow circle). Filled circles indicate the beginning of a month. when it reached near 70 E, the buoy started drifting northwestward because of the NMC, which developed north of the equator during January. In March, the buoy drifted along the coast of Africa with the northward Somali Current, which sets in in early March, much before the onset of the southwest monsoon. The buoy followed the northward SC till mid-may and later drifted toward the coast of Arabia. In mid-may, the buoy turned eastward under the in uence of the SMC that began to build up during May. After drifting eastward with the SMC, for two and half months, the buoy shifted into the equatorward WICC off the Indian west coast in early August. The WICC moved the buoy farther south (south of Sri Lanka) till it re-joined the eastward SMC. When the buoy reached near 87 E in September, it was pushed into the Bay of Bengal by a branch of the SMC that turns north near 87 E; later the buoy was caught in a clockwise eddy till it joined the EICC that owed equatorward in November. The EICC carried the buoy around Sri Lanka and the NMC brought it back into the Arabian Sea in December. Similarly, the trajectory shown in Figure 3b depicts the history of a buoy deployed in the Arabian Sea. This buoy, released on 12 May 1995 in the Arabian Sea at around 12 N, reached the SEC (at 15 S) within a year. Initially, the buoy moved southeast under the in uence of the SMC, joining the eastward EJ in November, and nally drifted into the SEC after traveling along the coast of the Indonesian island of Sumatra. After it entered the SEC, the buoy moved westward till it stopped transmitting. The life-history of these two buoys shows how variable the circulation in the Indian Ocean is; most current systems, especially those in the north and equatorial Indian Ocean, reverse with season. The above description captures this variation in a Lagrangian frame work. In the following section, we describe the variability in a Eulerian frame work. Before doing this, however, we examine the geographical distribution of surface current velocities by segregating the drifter speeds into four categories: low speeds (, 10 cm s 2 1 ), medium speeds (10 to 50 cm s 2 1 ), high speeds (50 to 100 cm s 2 1 ) and very high speeds (. 100 cm s 2 1 ). Of the 2,86,965 six-hourly velocity observations from 319 buoys, the low speeds accounted for 18% of the observations with no geographic preference. The medium velocities, which accounted for 74% of the observations, also showed no preference for

7 1999] Shenoi et al.: Indian Ocean circulation 891 speci c geographic location. However, the high speeds which accounted for about 7% of the total observations were concentrated in the equatorial and western boundary regions. Similarly, most of the observations with speeds greater than 100 cm s 2 1 (about 1%) were observed only in the equatorial region. From the trajectories shown in Figure 2 and from the above, it appears that majority (about 74%) of the mesoscale motions in the tropical Indian Ocean have speeds in the range of cm s Surface current vectors To describe the surface circulation pattern in a Eulerian frame work, the distribution of mean velocity vectors on a spatial grid and monthly time scale are prepared from 319 buoys. Due to the reasons discussed in Section 2, only the position xes from drogued buoys having R. 35 are used for this analysis. The grids containing less than 12 observations in a month are also excluded. The resulting mean eld was then spatially smoothed using a binomial smoother. The velocity variance within the bins include spatial variability at scales smaller than 200 km and temporal variability with time scales smaller than 30 days. From the trajectory plot (Fig. 2) it is obvious that the horizontal scales vary from a few tens of kilometers to basin-scale motions. Further, as a drawback of the nonuniform Lagrangian sampling, signi cant aliasing can occur between the spatial and temporal variability. Where eddies are strong and the mean is weak, the mean vectors may not be very reliable. Given the nonuniformly distributed observations in the bins we conclude that the present data set is not well suited for the study of variability in the Indian Ocean on time scales less than a month. However, as it will become apparent from the following, they can be used to describe the major currents in the Indian Ocean on seasonal time scales. Figure 4 shows the distribution of monthly mean velocity vectors; they include many more observations than those presented by Molinari et al. (1990). Figure 5 shows the monthly mean vectors in a grid and their standard deviations for two selected months, January and July; a majority of the vectors show low standard deviation. In general, the standard deviations are high for the grids having a smaller number of observations. In such cases, more observations per box would give a more stable vector. A general discussion on the surface currents in the Indian Ocean is available in Molinari et al. (1990). They characterize the circulation pattern in the tropical Indian Ocean as two primary gyres extending across the basin, one on either side of the equator, the southern gyre rotating clockwise and the northern gyre rotating anti-clockwise. Though the additional data provides a better picture of the surface circulation pattern in the Indian Ocean, in general, the circulation pattern that emerges from this analysis is not very different from that discussed by Molinari et al. a. Southern gyre The clockwise rotating southern tropical gyre appears as a permanent feature of the surface circulation in the Indian Ocean (Fig. 4). The SEC, observed as a broad westward

8 892 Journal of Marine Research [57, 6 Figure 4. Monthly mean surface current vectors based on a 2 -square analysis of surface drifting buoys. Missing vectors indicate data voids. ow between 8 S and 16 S, acts as the southern boundary of this gyre. Near the northeastern tip of Madagascar, a major portion of this current ows westward, while the other portion ows south along the east coast of Madagascar as the East Madagascar Current (EMC). The branch that ows past the northern tip of Madagascar turns northward and feeds the boundary current along east Africa, the EACC. Later, the EACC turns eastward and joins the ECC (axis around 3 to 5 S), which serves as the northern boundary of the southern tropical gyre. The surface currents in the region between the SEC and ECC are noisy and are often lled with recirculation cells. One such recirculation cell can be seen in June between 65 E 75 E and another can be seen in February centered at 10 S and 60 E. Woodbery et al. (1989) note that the shear zone between SEC and ECC is lled with westward propagating Rossby waves that are re ected by the Seychelles-Mauritius Ridge along 60 E. It is likely that the noisy nature of the currents and the recirculation cells noted above are a manifestation of the westward propagating Rossby waves and their re ections from the Seychelles-Mauritius Ridge. To the south of the SEC, only the northern portion of the southwestward ow due to the anticyclonic subtropical gyre is visible.

9 1999] Shenoi et al.: Indian Ocean circulation 893 Figure 4. (Continued) During April May and again during November, an eastward EJ (with speeds higher than 50 cm s 2 1 ) develops at the equator. At that time, the ECC becomes indistinguishablefrom the EJ; the EJ serves as the northern boundary of the southern tropical gyre. The eastward jet, forced by the near-equatorial westerlies is stronger in the east than in the west. As it accelerates from west to east, the width of the jet also increases from about 4 at 70 E to about 7 at 90 E. At around 90 E, the apparent width of the jet increases because of the veering off of the vectors; the vectors in the northern side of the jet veer toward the north and the vectors on the southern side of the jet veer toward the south. In July August, no well-organized eastward ow is observed along the equator to act as the northern boundary of the southern gyre (Fig. 4g h). The vectors during this period suggest a narrow westward ow, stronger in August to the west of 80 E. There is no indication of this westward ow near the equator in the ship-drifts of Cutler and Swallow (1984); the model runs of McCreary et al. (1993), however, report a westward ow in the eastern equatorial region during August. In the east, a southeastward current that ows along the Indonesian coast acts as the

10 894 Journal of Marine Research [57, 6 eastern boundary of the southern gyre; it is stronger when the EJ exists and is weaker during December January. b. Northern gyre During January to March a gyre is also seen on the northern side of the equator, but it is not as well de ned as the southern gyre (Fig. 4a c). The westward NMC (axis around 5 N), which forms during December and peaks in February (30 40 cm s 2 1 ), acts as the northern boundary of this gyre till April. The vectors in January clearly show the turning of this ow toward the south near the Somali coast. The southward ow (southward SC) meets the northward EACC at about 5 S and the two ow together into the ECC. In April, an anticyclonic eddy appears near the Somali coast at 5 N; it is similar to the Great Whirl described by Duing et al. (1980). Simultaneously, the northward SC also gets established along the coast of Somalia. In April, the ECC together with the eastward EJ acts as the southern boundary of the northern tropical gyre. On the eastern side, except during February, very few buoys are available to document the ows. With the arrival of southwest monsoon winds over the North Indian Ocean in June, the northern gyre disintegrates rapidly. The NMC vanishes and is replaced by the eastward SMC that persists till October. Associated with the SMC, southeastward ows develop all over the Arabian Sea, often extending to the equator. In September, a part of the SMC enters the Bay of Bengal while the other branch continues to ow eastward. On the western side, the SC strengthens further and continues to ow northward. The available observations along the west coast of India show a poleward current during January March and an equatorward current during April September. In the interior of the Arabian Sea the ows, which are similar to those reported in Cutler and Swallow (1984), are westward during December February and eastward during March April. During June September they intensify, ow toward the southeast and, feed the SMC. Along the coast of Arabia the ow is southward during December January and is northward during May September. Very few observations are available in the coastal regions of the Bay of Bengal. The available observations show a clockwise gyre during January May and an anticlockwise gyre during November December. The observations during the rest of the year do not suggest such gyres. In summary, the surface current vectors derived from a compilation of the surface drifting buoy data appear to be adequate to depict the major currents in the Indian Ocean on a seasonal time scale. It is obvious that the surface currents in the tropical Indian Ocean are highly seasonal and have strong annual and semi-annual periodicities. c. Annual and semi-annual harmonics To quantify the annual and semi-annual signals of surface circulation, the monthly mean data were subjected to harmonic analysis using a least-square t. The gaps in buoy data

11 1999] Shenoi et al.: Indian Ocean circulation 895 Figure 5. Monthly mean surface current vectors with standard deviation ellipses (a) for January and (b) for July. time series (mostly a gap of one month) were not lled as the scheme adopted can handle the data gaps. Figure 6 shows the amplitude and phase of the annual and semi-annual harmonic of the zonal component. The harmonics of the meridional component are not shown as the signals are low except near the boundaries. The annual signal in the zonal component is larger north of 10 S and is lower south of 10 S. The SEC exhibits a clear annual signal with a

12 896 Journal of Marine Research [57, 6 Figure 6. Amplitude and phase of the (a) annual and (b) semi-annual harmonic of zonal velocity component computed from drifting buoys. The phase, measured clockwise from north, is zero on 1 January (see inset). For the annual harmonic 360 covers one year. For semi-annual harmonic 360 covers six months (1 January to 30 June/1 July to 31 December). The length of the stick is proportionalto the amplitude. peak of about 20 cm s 2 1 in April. In the western Arabian Sea, the annual signal peaks during July August. At the equator, in the east, the annual signal peaks in July and in the west it peaks in August. In general, the patterns are similar to those presented by Molinari et al. (1990) except for the clear annual signal that emerges for the SEC from the present

13 1999] Shenoi et al.: Indian Ocean circulation 897 analysis. Woodberry et al. (1989) note that the seasonal cycle in the southern hemisphere trades generates an annual Rossby wave in the ocean, which manifests as an annual cycle in the SEC. The present analysis clearly shows this annual signal embedded in the SEC. It also shows an appreciable annual signal (10 to 30 cm s 2 1 ) for the surface currents in the Arabian Sea and the Bay of Bengal, which is not seen in Molinari et al. (1990) perhaps due to poor data coverage. The semi-annual harmonic shows a maximum at the equator (Fig. 6b), the peaks coinciding with the occurrence of an equatorial jet that appears in April May and in November. The semi-annual signals are also appreciable in the western Arabian Sea and southern Bay of Bengal, but they are weaker than the annual signals. For semi-annual harmonics there is general agreement with the results presented by Molinari et al. (1990). However, there are differences in the phases reported in the southern Bay of Bengal and in the regions south of Sri Lanka and the southern tip of India. For these regions, the present results compare well with harmonics estimated from ship drifts by Molinari et al. (1990). 5. Kinetic energy of the ow eld a. Kinetic energy of mean ow The kinetic energy of the ow eld was estimated from the surface current vectors described above. The distribution of the kinetic energy of the mean surface circulation is shown in Figure 7a. Assuming that there are a total of N velocity measurements from all drifters in a given box, the vector mean velocities were computed as u i j 5 v i j 5 1 N u N i j ok5 1 1 N N ok5 1 v i j. The mean kinetic energy (E M ), or the kinetic energy of the mean ow, is de ned as E Mi j 5 u i j2 1 v 2 ij. 2 The computed E M eld was smoothed using a three-point two-dimensional binomial smoother. Given the uneven distribution of velocity vectors (Fig. 4), the distribution of E M presented here is subject to large uncertainties. However, since the present data set is the most extensive of this type ever assembled, it provides one of the best estimates of these elds currently available. In general, the highest values of E M are associated with strong persistent ows, the meridional ows in the western basin, and the zonal SEC. A ridge of E M (. 200 cm 2 s 2 2 ) coincides with the region of the SEC. The maximum value of 1000 cm 2 s 2 2 observed in the Somali Basin is associated with the strong SC that exceeds 100 cm s 2 1 during May September. The values of E M are also high (greater than 100 cm 2 s 2 2 ) along the equator, where the EJ appears twice an year. To the west of the SEC

14 898 Journal of Marine Research [57, 6 Figure 7. Distribution of kinetic energy per unit mass based on a 2 -square analysis of surface velocity eld. (a) The kinetic energy of the mean ow and (b) the eddy kinetic energy. A ve point binomial smoothing was done prior to contouring. ridge, a bulge of high EM (400 cm2 s2 2 ) extends eastward (off East Africa) along 10 S; associated with the strengthening of the SEC when it ows past the northern tip of Madagascar, where the buoys moved with speeds. 50 cm s2 1. South of 20 S, the EM values did not exceed 100 cm2 s2 2. In the Arabian Sea and also in the Bay of Bengal, EM did not exceed 100 cm2 s2 2; the lowest values of EM are observed in mid-arabian Sea and mid-bay-of-bengal. The EM estimates have also been compared with the global maps of EM presented by Wyrtki et al. (1976) using ship drifts. There is qualitative agreement between the two distributions. Both report maxima in the western basin and minima in the Arabian Sea and

15 Shenoi et al.: Indian Ocean circulation 1999] 899 the Bay of Bengal. The ridges of high EM in the equatorial and the SEC regions and the drop in EM to the south of 20 S are well seen in both the analysis. There are, however, large quantitative differences between the two distributions. The buoy-derived EM are at least twice those derived from ship drifts. For example, the buoy-derived EM in the equatorial region are in the range cm2 s2 2 and those values derived from ship drifts are in the range cm2 s2 2. Similarly, in the western basin, the ship-drift-derived EM do not exceed 500 cm2 s2 2, whereas the drifter derived EM reach 1000 cm2 s2 2. In the Arabian Sea and the Bay of Bengal also, the drifting-buoy-derived EM are twice the ship-drift-derived EM. For the southern hemisphere, Patterson (1985) also reports a qualitative agreement between the EM derived from drifting buoys and those derived from the ship drifts by Wyrtki et al. (1976). Quantitatively, however, his buoy-derived values of EM too were 2 4 times larger than those of Wyrtki et al. Such differences in the magnitude of EM are reported to be due to the coarse spatial resolution (5 grid in Wyrtki et al.) and the errors inherent in the ship-drift measurements. The coarse resolution tends to smooth the curves in the path of the mean ow, which leads to systematically lower values of EM (Patterson, 1985). The way the ship drift observations are made, each ship-drift vector represents a value averaged over a period of 24 hours, equivalent to a distance of about 400 km. Hence, the ship-drift estimates assigned to a 5 box contain information from a larger box extending up to 200 km beyond the boundary of the 5 box. This makes the effective averaging box size as large as 9 instead 5 (Patterson, 1985). Conversely, due to the frequent observations of buoy movements, the buoy drifts contain no information beyond the boxes selected for averaging. Hence, it is likely that the EM values estimated from ship drifts are substantially lower. b. Eddy kinetic energy The distribution of eddy kinetic energy (EE ) due to the perturbations of the surface velocity about the mean ow is presented in Figure 7b. EE per unit mass was calculated as EEi j 5 1 2N 3o N k5 1 (uki j 2 ui j ) 1 2 o N k5 1 (vki j 2 vi j ) 2 4 where the sum is computed over all velocity measurements in a box. EE (Fig. 7b) is much higher than EM (Fig. 7a), indicating that most of the kinetic energy of the surface ows are in the eddy eld. EE contains a part of the low frequency oscillation as well as a part of the seasonal cycle. A harmonic analysis of the EE eld shows that the contribution of the average seasonal cycle is as high as 75%. The annual harmonic accounts for about 40%, the semi-annual cycle for about 25%, and the quarter annual cycle for about 10%; the balance is due to shorter time scales and interannual variations. EE is highest in the western boundary and equatorial currents. Qualitatively, both EM and EE show similar distributions with maxima in the western basin and minima in the Arabian Sea, in the Bay of Bengal, and south of 20 S. As with EM, a comparison of the EE estimated here and those estimated by Wyrtki et al.

16 900 Journal of Marine Research [57, 6 (1976) shows that they agree qualitatively, but not quantitatively. Both show highest eddy energy in the western basin and lowest eddy energy in the Arabian Sea and in the Bay of Bengal.Again, both estimates show high EE at the equator. One quantitative difference is in the magnitude of EE at the equator, where the drifting buoys show values greater than 1200 cm2 s2 2. Another difference is in the magnitude of EE in the Arabian Sea and the Bay of Bengal, where Wyrtki et al. (1976) report 600 cm2 s2 2 and the drifting buoy estimates are around 200 cm2 s2 2. On an average, the estimates from drifting buoys and ship-drifts made by Wyrtki et al. (1976) differ by about 200 cm2 s2 2. For example, the 1000 cm2 s2 2 contour that connects the tip of Africa and Sri Lanka in the ship-drift estimate corresponds to the 800 cm2 s2 2 contour in the drifting-buoy estimate; similarly, the 600 cm2 s2 2 contour in the ship-drift estimate corresponds to the 400 cm2 s2 2 contour in the drifting-buoy estimate. This difference of approximately 200 cm2 s2 2 was also noted by Richardson (1983) in the western North Atlantic. He attributed it to an error of about 20 cm s2 1 in the ship-drifts, which translates to an error of 200 cm2 s2 2 in eddy kinetic energy. 6. Comparison of drifter trajectories and dynamic topography Traditionally, the ow eld inferred from dynamic topography is used to represent the surface circulation. In this section we examine how far the dynamic topography elds are useful in representing the surface ows in the Indian Ocean. For want of monthly hydrographic data to compute dynamic topography in the Indian Ocean, the seasonal data compiled by Levitus et al. (1994) is used. Hence, it is important to note that the comparisons made are valid only for a season and lower periods are not resolved. A qualitative comparison between seasonal dynamic topography (0/1000 db) and seasonal mean surface drift vectors is shown in Figure 8. The computation of geostrophic velocities is avoided as they are sensitive to the selection of reference level. The comparisons show that the representation of surface ows with dynamic topography are not always correct. The role of the geostrophic ows in representing the surface ows varies both geographically and seasonally. For example, the drifter vectors and isolines of dynamic heights are in good agreement in the regions of SEC, ECC and the southward current along the coast of Indonesia during January March (Fig. 8a). In the NMC, they agree to the east of 70 E and disagree to the west, where the surface current vectors cross the dynamic height contours. The anticyclonic gyre in the dynamic height eld in the Bay of Bengal during January March is also seen in the buoy-derived vectors. The surface current vectors in the southeastern Arabian Sea also agree with the geostrophic ows, though they are somewhat different in the western regions. Similarly, the drifter vectors and isolines of dynamic topography for July September (Fig. 8b) agree well in the regions of SEC and the southeastward current along the Indonesian Islands. For the SMC, they agree well in the Arabian Sea, but cross the dynamic height contours east of 75 E. The drift vectors in the northeastward SC also do not appear to follow the geostrophic eld closely. In the Bay of Bengal also the surface current vectors do not follow the dynamic height contours during July September.

17 1999] Shenoi et al.: Indian Ocean circulation 901 Figure 8. Seasonal mean surface current vectors realized from satellite-tracked drifters superimposed on the seasonal mean dynamic topography (0/1000 db) from the climatology of Levitus et al. (1994). (a) January February March and (b) July August September. The vectors derived from the drifters represent the total near-surface ow comprising the geostrophic currents, the Ekman currents, and the Stoke drift. Hence, a good agreement between the drifter-vectors and dynamic heights is possible only when the geostrophic currents dominate the surface ows. In the absence of concurrent wind data, an estimate of the contribution from the Ekman drifts is not possible. However, for a qualitative assessment, an estimate of the Ekman drift was made based on the seasonal mean winds, in squares, available from the

18 902 Journal of Marine Research [57, 6 NCEP/NCAR reanalysis program (Kalnay et al., 1996). Using the NCEP/NCAR winds the Ekman surface ow, VE, was computed following Pond and Pickard (1986) as VE 5 t 0r 2 1 w (A * f * )2 0.5, where t 0 5 r acdww is the surface wind stress, r w the density of water, A the vertical eddy viscosity coefficient, f the Coriolis parameter and W the wind speed at 10 m. For computing VE, we used r a kg m2 3, r w kg m2 3, A m2 3 s2 1, and CD The direction of Ekman surface current vector was at a 45 angle to the right/left of the surface wind vector in the northern/southern hemisphere. The selection of the values for the constants is unimportant as the VE are used only for qualitative comparison. The seasonal mean Ekman drift (Fig. 9), not computed within 1.25 of the equator, is weaker where there is a good match between the observed surface drifts and the geostrophic currents envisaged from dynamic topography. In the Bay of Bengal, the Ekman surface drifts are weaker during January March (Fig. 9a); during this period the buoy-derived surface current vectors follow the dynamic height contours that depict the anticyclonic gyre. The Ekman drifts are stronger during July September when there is a mismatch between the buoy-derived surface currents and the dynamic height contours. During the southwest monsoon (June September), McCreary et al. (1996) also noted that the Ekman drifts dominate in the Bay of Bengal. Similarly, in the region of the NMC during January March, the Ekman drifts are weaker in the east, but they are stronger west of 60 E. In this case a mismatch between the buoy-derived surface current vectors and dynamic height contours was noticed to the west of 70 E. Using ship drifts as observed surface currents, Hastenrath and Greischar (1991) have discussed the individual contributions of geostrophic currents and Ekman drifts to the major surface currents in the Indian Ocean. They suggest a dominant geostrophic component for SEC, South Equatorial CounterCurrent (SCC), and Eastward Equatorial Jet (EEJ), and a dominant Ekman component for the westward NorthEast Monsoon current (NEM), the eastward SouthWest Monsoon current (SWM), the northward East African Current (EAC) and the northeastward Somali Current (SCN). Hastenrath and Greischar s SEC, SCC, EEJ, NEM, SWM, EAC and SCN correspond to our SEC, ECC, EJ, NMC, SMC, EACC and northward SC, respectively. Our analysis also shows the dominance of geostrophic ows for the SEC and ECC and the dominance of Ekman drift for the SC. Hastenrath and Greischar de ne a box bounded between 8 N and 10 N latitudes and 60 E and 80 E longitudes for the computation of representative mean velocity components for the NMC and SMC. In this box, the present analysis also suggests a lack of agreement between the geostrophic ow eld and surface vectors in the NMC and the SMC. However, outside the box de ned by Hastenrath and Greischar, the present analysis shows a good match between geostrophic ows and the drifting-buoy-derived vectors for the NMC and SMC; for example, the match is very good inside the Arabian Sea for SMC during June September. Similarly, the match is good for the NMC to the east of 75 E. In general,

19 1999] Shenoi et al.: Indian Ocean circulation 903 Figure 9. Seasonal mean Ekman surface currents estimated from the monthly mean climatology of NCEP/NCAR reanalysis surface winds. The computations were not done within 1.25 of the equator. (a) January February March and (b) July August September. the present analysis and the results from Hastenrath and Greischar (1991) agree well for all major currents in the Indian Ocean. 7. Summary and discussions The trajectories of 412 satellite-tracked, free-drifting surface buoys were analyzed. The analysis shows a good agreement between the monthly mean surface circulation and the features of large-scale circulation. Trajectory plots of individual buoys together with the

20 904 Journal of Marine Research [57, 6 monthly mean vectors shows that the surface circulation in the tropical Indian Ocean hosts a series of mesoscale variability. The typical speed of the surface current in the tropical Indian Ocean is below 50 cm s2 1 (92% of the individual estimates of surface drift speeds); a few vectors (1%) exceed 100 cm s2 1. The major currents revealed by the drifter data are summarized in Table 1. The maps of monthly mean vectors re ne the earlier maps presented by Molinari et al. (1990). Both data sets characterize the circulation pattern in the tropical Indian Ocean as two primary gyres extending across the basin, a southern gyre rotating clockwise and a northern gyre rotating anticlockwise. However, some features of large-scale circulation not depicted clearly in the earlier analysis are apparent in the present analysis. For example, the widening and branching of EJ in the east, the branching of SMC near 87 E, the westward ow at the equator during July August, the westward ow in the interior Arabian Sea during December February, the EMC, etc. From the buoy trajectories (see Fig. 3a) and monthly mean vectors (see Fig. 4), it is clear that the SC north of 5 N turns toward northeast in March well before the winds reverse; this is in agreement with hydrographic observations made during 1979 (Leetmaa et al., 1982) and model simulations (McCreary et al., 1993). In the equatorial region, in general, the surface ows are eastward, except during July August, when a narrow, weak westward ow appeared at the equator; a similar current is seen in the model simulation of McCreary et al. (1993). Two processes, local wind curl and equatorially trapped Rossby waves, are the likely contributors to these currents (McCreary et al., 1993). The eastward EJ that appeared during April May and again in November branched near 90 E; a major branch turned toward southward along the west coast of Sumatra and the other branch turned northward. The southward branch ultimately feeds the SEC and the northward branch contributes to the coastal current along the eastern rim of the Bay of Bengal; this is consistent with the model solutions of Yu et al. (1992) and McCreary et al. (1993), who attribute this to equatorial Kelvin waves propagating eastward in the equatorial waveguide. These waves re ect at the eastern boundary; a part of the re ected wave propagates northward along the eastern rim of the bay, setting up a northward (southward) current along the eastern rim if the Kelvin waves have a downwelling (upwelling) phase. The onset of the southwest monsoon winds over the north Indian Ocean (in May) replace the westward NMC with the eastward SMC. Associated with the SMC, everywhere in the Arabian Sea between 7 N and 22 N, the surface currents are southeastward. This implies that the water carried into the Arabian Sea during the southwest monsoon along its western boundary (by the northeastward SC) is removed immediately by the surface currents. The surface velocities were also used to infer the distribution of kinetic energy of the ow eld. Most of the kinetic energy of the surface circulation is in the eddy eld (ranging from cm2 s2 2 ). The mean kinetic energy (EM ) per unit mass ranges from cm2 s2 2. From the distribution of EE, it appears that the western boundary along the coast of Africa act as a source and the northern (Arabian Sea and Bay of Bengal) and the regions south of 20 S act as sinks. The proximity of the equatorial wave guide allows the

21 1999] Shenoi et al.: Indian Ocean circulation 905 Table 1. Major surface currents in the Tropical Indian Ocean. Name of Current South Equatorial Current (SEC) East Madagascar Current (EMC) East Africa Coastal Current (EACC) Equatorial Counter Current (ECC) Equatorial Jet (EJ) Northeast Monsoon Current (NMC) Southwest Monsoon Current (SMC) Somali Current (SC) West India Coastal Current (WICC) East India Coastal Current (EICC) Description A westward ow between 8 S and 16 S extending from 95 E to 50 E. Exists throughout the year. A southward ow, fed by the SEC, along the east coast of Madagascar. Exists throughout the year. A northward ow, fed by the SEC, along the east coast of Africa. Exists throughout the year. An eastward ow to the south (between 3 S to 5 S) of the equator. Absent during August. An intense eastward current that appears at the equator twice a year (during April May and November). A westward ow to the north of the equator, with its axis around 5 N; exists only during December April. An eastward ow to the north of the equator, with its axis around 5 N. Develops in June, associated with the southwest monsoon winds over the north Indian Ocean, and persists till October. Flows southward along the coast of Somalia during December February and ows northward during March September. The current along the west coast of India ows during November March poleward and equatorward during April September. The current along the east coast of India, ows during February April poleward and equatorward during November December. During the southwest monsoon a weak poleward ow develops in the south (south of 15 N) and an equatorward ow develops in the north. Remarks Also known as South Equatorial Counter Current (SECC) Also known as Wyrtki Jet Also known as North Equatorial Current (NEC) Also known as Indian Monsoon Current (IMC) energy to propagate away from the western boundary in the form of equatorial Kelvin waves and mixed Rossby waves. This is consistent with the model results of Yu et al. (1992) and McCreary et al. (1993), which suggest that the propagation of energy into the Bay of Bengal is in the form of Kelvin waves along its eastern rim.

22 906 Journal of Marine Research [57, 6 The near-surface circulation depicted by the drifters was compared with the geostrophic component of the circulation computed from the seasonal mean elds of dynamic topography. The comparisons corroborate the ndings of Hastenrath and Greischar (1991), who estimated the individual contributions due to geostrophic and Ekman components in the Indian Ocean; the surface circulation in the Indian Ocean realized from dynamic topography need not always represent the surface currents. To conclude, this analysis has provided a composite overview of the kinematics of the surface circulation of the tropical Indian Ocean. While re ning the earlier observations, the analysis also provides observational evidence for model solutions. There is a paucity of observations in the east along the eastern rim of the Bay of Bengal and in the coastal regions of the Arabian Sea. To complete the picture, more buoy deployments are needed in these areas. Acknowledgments. We are grateful to Robert Molinari, and NOAA/AOML/DAC, Miami, Florida for generously providing the drifter data from their archives. The Department of Ocean Development, New Delhi provided the nancial support through a project grant. We thank L. V. G. Rao, Project Coordinator, for support and encouragement. The softwares FERRET, GMT, and netcdf were used extensively. D. Shankar provided the computer code for harmonic analysis. This is NIO contribution no REFERENCES Anonymous User Manual, CLS ARGOS, Toulouse, Cedex, France. Cutler, A. N. and J. C. Swallow Surface currents of the Indian Ocean (To 25 S, 100 E). IOS Technical Report, Institute of Oceanographic Sciences, UK, 187 pp. Duing, W., R. L. Molinari and J. C. Swallow Somali Current: Evolution of surface ow. Science, 209, Hansen, D. V. and P.-M. Poulain Processing of WOCE/TOGA drifter data. J. Atmos. Oceanic Technol., 13, Hastenrath, S. and L. Greischar The monsoonal current regimes of the tropical Indian Ocean: Observed surface ow elds and their geostrophic and wind-driven components. J. Geophys. Res., 96, Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell, S. Saha, G. White, J. Woolen, Y. Zhu, M. Chelliah, W. Ebisuzaki, W. Higgins, J. Janowiak, K. C. Mo, C. Ropelewski, J. Wang, A. Leetmaa, R. Reynolds, R. Jenne and D. Joseph The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteo. Soc., 77, Leetmaa, A., D. R. Quadfasel and D. Wilson Development of the ow eld during the onset of the Somali Current J. Phy. Oceanogr., 12, Levitus, S., R. Burgett and T. P. Boyer World Ocean Atlas NOAA Atlas NESDIS, U.S. Govt. Print. Off., Washington, D.C. McCreary, J. P., P. K. Kundu and R. L. Molinari A numerical investigation of dynamics, thermodynamics and mixed-layer processes in the Indian Ocean. Prog. Oceanogr., 31, McCreary, J. P., W. Han, D. Shankar and S. R. Shetye Dynamics of the East India Coastal Current: 2. Numerical solutions. J. Geophys. Res., 101, Molinari, R. L., D. Olson and G. Reverdin Surface current distributions in the tropical Indian Ocean derived from compilations of surface buoy trajectories. J. Geophys. Res., 95, Niiler, P. P. and J. D. Paduan Wind-driven motions in the Northeast Paci c as measured by Lagrangian drifters. J. Phy. Oceanogr., 25,

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