Variability of Western Indian Ocean Currents 83

Home , North Equatorial Current, Somali Current, South Equatorial Current

VariabilityofWesternIndianOceanCurrents

PeterN.Benny Department of Physical Oceanography, Cochin University of Science & Technology, Cochin, India –682016 E-mail: [email protected]

Key words: western Indian Ocean, currents, circulation, Somali Current, dynamic topography, variability

Abstract—In the study reported, an attempt was made to understand the intra-annual variability of the western Indian Ocean circulation by estimating the monthly dynamic topography with respect to 400db. The major currents in the western Indian Ocean are clearly depicted in the topography. Among the currents, the Somali Current exhibits strong annual variability. Eddy circulation is prominent in the northern part of the Somali Current during the southwest monsoon period. Seasonal variability is also noticed in the North Equatorial Current. Slight spatial and temporal changes are noticed in the South Equatorial Current and Equatorial Counter Current. The Equatorial Jet flow occurs in the monsoon transition periods of May and November between the equator and 3° South.

INTRODUCTION Equatorial Current, which was directed towards the west during the northeast monsoon, reverses The western Indian Ocean receives special its directions towards the east. This combines with attention from oceanographers and meteorologists, the eastward-flowing Equatorial Counter Current as it exhibits more dramatic seasonal variation than and the whole eastward flow from 7 to 15 °N is the rest of the Indian Ocean and is an important called the monsoon current. One component of the region of air–sea interaction. Intense upwelling South Equatorial Current turns to north and occurs during the south-west monsoon season in supplies the Somali Current up the east coast of the Somali area (Bruce, 1974; Packard, 1985). As Africa. The Somali Current is notable for its high a result of this upwelling, cold surface water is speeds of up to 200 cm/s and has a transport of brought to the nearshore surface layers, which about 65 Sv, most of it in the upper 200 m (Pickard spreads over extensive areas of the Arabian Sea. and Emery, 1964). However, the current speed in Thus, the local climate and biological productivity western tropical Indian Ocean is of the order of 75 are much controlled by the oceanic processes. to 100 cm/s (Bahulayan et al., 1997). Tomczak and The existence of a strong surface current during Godfrey (1994) have given a detailed account of the southwest monsoon flow parallel to the African currents and circulation. Coast in the northwestern Indian Ocean, close Several authors (Bruce, 1966; 1968; 1974; inshore and towards the northeast has been long 1979; 1985; Leetmaa, 1972; 1982; Reverdin & known from the ship reports of Findlay (1866) and Fieux, 1987; Schott et al., 1990; Fischer et al., Hoffmann (1886). Based on collection of ship logs, 1996; Benny & Mizuno, 2000) have studied the Schott’s (1935) chart gives a clear picture of the western Indian Ocean currents. Modelling efforts Somali Current. were made by Cox (1970; 1979), Anderson et al. During the southwest monsoon, the North (1976), Luther & O’Brien (1985), Das et al. (1987), 82 P. N. BENNY

Woodberry et al. (1989), Bhahulayen and Shaji possible to give an adequate description of annual (1996) and Shaji et al. (1997), to simulate the cycle of dynamic height. Hence, we tried to include western Indian Ocean circulation. Satellite the temperature/depth data also to estimate the observations of sea surface temperature, altimetery dynamic height employing the correlation between and scatterometer wind data have been used in heat content and dynamic height. studying the circulation and processes in the ∫z⌠ western Indian Ocean (Perigaud & Minister, 1988; The heat content (H)= 0 CpTdz Perigaud & Delecluse, 1989; 1992; 1993). where ⌠ is the density, Cp is the specific heat Considering the influence of the western Indian at constant pressure, T is the in situ temperature Ocean on the climate of the neighbouring region, and z is depth. efforts were made in the study reported here, to The correlation between the heat content and elucidate the intra-annual variability of the Ocean’s dynamic height with reference to 400 m was currents using available oceanographic information. determined using the hydrographic data. Significant correlation was obtained between DATASETSANDMETHODS dynamic height and heat content for the study region. The study was confined to the region between the The linear fit equation of correlation is eastern African coast and 60 °E, 20 °S and 15 °N. Historical data sets were used to produce the Y= – 0.251916 + 0.0751593 X dynamic topography with reference to 400db. The available hydrographic observations (BT, XBT, where Y is the dynamic height, X is the heat CTD and STD and Nansen casts) for the region content, correlation coefficient = 0.95 and standard from NODC (National Oceanographic Data deviation = 0.02. Centre, Washington, New York), INODC (Indian The temperature profiles aggregate to about National Oceanographic Data Centre, Goa) and 10,000 for the study region and only observations data collected by National Research Institute of extending to 400 m depths were considered for this Far Seas Fisheries, Japan were acquired and used study. Also, observations with no data for more for this analysis. than two consecutive standard depths (0, 10, 20, Ever since the existence of an intimate 30, 50, 75, 100, 125, 150, 200, 250, 300 and 400 relationship between the field of mass and field of m) were rejected. flow was established, dynamic topographies Thus, using the correlation dynamic height was (geopotential topographies) have been used to also estimated from the temperature/depth data and study the circulation of the oceans. the monthly dynamic topography maps were The anomaly of the dynamic depth (∆D) is prepared. Figure 1 shows the location of stations given by including temperature-salinity as well as the temperature profiles used for the estimation of ∆ =∫pδ dynamic height (steric height). DdP0

Where δ is the specific volume anomaly and P RESULTS is the pressure in decibars. The long-term mean monthly dynamic topography The first step involved is the use of in situ of the western Indian Ocean is presented in Figs temperature and salinity at observed depths to 2–13. compute the specific volume anomaly. For all integrations the depth in metres was taken to be January numerically equivalent to pressure in decibars. The temperature-salinity profiles number about The long term mean dynamic topography exhibits 3200 for the study region. Employing the strong gradient in steric height in the northern, temperature-salinity profiles alone, it is not southern and eastern parts of the study region VARIABILITY OF WESTERN INDIAN OCEAN CURRENTS 83

AFRICA

0 Latitude

-10

-20 35 40 45 50 55 60 Longitude

Fig. 1. Location of stations used for dynamic height estimation

(Fig. 2). The equatorial zone shows dull circulation latitudes, up to 18 °S. The Equatorial Counter where the steric height is less than 1 dynamic Current is much stronger and is between 2 and 7 metre. Along the Somali Coast a weak southward °S. Thus, the dynamic topography depicts that the flow is noticed. The Equatorial Counter Current is whole western Indian Ocean is dynamically active weak and is around the equator. The northern part in this month. is marked with counter-rotating eddies between 10 and 15 °N. The strong flow of the South Equatorial March Current is between 5 and 15 °S. Part of the South Equatorial Current turns south at the Madagascar The steric height distribution in March (Fig. 4) Coast. An anticyclonic eddy circulation is evident shows slight increase in height, but the gradient is between Africa and Madagascar. weakened in the northern Indian Ocean compared to February. An offshore current displaces the February southward flow along the Somali Coast. The conspicuous feature noticed in this month is the Slight changes in dynamic height distribution are presence of the North Equatorial Current in visible in this month (Fig. 3). The steric height is between 0 and 5 °N. Counter-rotating eddies are increased at the equatorial zone, 5 °N – 5 °S. The present in the northern part. To the south, the South flow along the Somali Coast is towards south itself. Equatorial Current and the Equatorial Counter The eddy circulation is weaker at the northern part Current keeps the same structure as in February. than it is in January. To the south, the South Strong anticyclonic circulation is present between Equatorial Current is more extended into the higher Africa and Madagascar. 84 P. N. BENNY

15 15 0.75 0.90 0.95 1.00 0.90 1.00 10 10 1.00 0.95

5 5 1.00 0.95 1.20 1.00 0 0 0.95 1.20 Latitude

Latitude 0.95 -5 -5 0.95

1.00 0.90 -10 -10 0.95 1.20

-15 1.20 -15 1.20 1.20 1.10 1.20 -20 -20 40 45 50 55 60 40 45 50 55 60 Longitude Longitude

Fig. 2. Dynamic topography (January) Fig. 3. Dynamic topography (February)

15 15 0.95 0.90 1.00 0.95 1.00 1.05 1.00 10 10 1.00 1.05 1.05 5 5 1.10 1.05 1.05 0 1.00 0 1.00 1.05 Latitude -5 Latitude -5 0.95 0.95 0.90 0.90 1.00 0.90 -10 -10 1.10 1.15 1.10 1.15 -15 -15 1.10 1.20 1.10 1.00 0.90 1.20 -20 -20 40 45 50 55 60 40 45 50 55 60 Longitude Longitude

Fig. 4. Dynamic topography (March) Fig. 5. Dynamic topography (April) VARIABILITY OF WESTERN INDIAN OCEAN CURRENTS 85

April July

During April the steric height is further increased In July, tremendous changes have taken place in in the northern part of the study region and strong the northern part of the western Indian Ocean (Fig. meridional gradients are developed (Fig. 5). The 8). Along the Somali Coast a strong northward flow North Equatorial Current (0–5 °N) reaches up to has developed and branches at 10 °N, one branch the African Coast and turns southward. No coastal initiating an anticlockwise eddy and the other current is observed off Somali where the offshore continuing its northeastern flow towards the Gulf flow prevails. In the south, the Equatorial Counter region. Strong counter-rotating eddy circulation is Current is more shifted to the lower latitudes embedded in the northeastern part. Thus, very between 2 and 5 °S compared to the month of complex but strong dynamic conditions prevail in March. A slight shift in South Equatorial Current the western Indian Ocean between 3 and 15 °N. A towards the north is also between 40 and 45 °E. narrow band of eastward and westward flow is also The anticyclonic eddy is diffused and a clockwise evident around the equator in this month. The flow is present in the southwest region from 15– circulation pattern remains the same as in the 20 °S. month of June in the southern part except at the southwest end, where an anticyclonic circulation May is noticed instead of the cyclonic pattern of the earlier months. In May, slight changes are observed in the flow pattern (Fig. 6). The meridional steric gradients in August the northern part are diminished. A northward- tending coastal current starts in the northern end The conspicuous change observed during August of Somalia and it seems the coastal current is part is the weakening of the circulation in the northern of an anticyclonic eddy. The conspicuous feature region (Fig. 9). The northward Somali Current and noticed during this month is that the Equatorial the East Arabian Current are present but much Counter Current is modified by strong jet-like weakened. As in the previous month the Somali eastward flow just below the equator (0–3 °S). The Current branches at 10 °N. Counter-rotating eddies South Equatorial Current is more widened and are also noticed. The South Equatorial Current and takes a northward turn between 40 and 45 °E. An the Equatorial Counter Current are stronger anticyclonic eddy circulation is occurring between between 2 and 15 °S. A cyclonic circulation is Africa and Madagascar. observed at the southwest end. A strong anticyclonic eddy is noticed between Africa and June Madagascar (10–15 °S).

The dynamic topography for the month of June is September marked by the reversal of the Somali Current (Fig. 7). The eastward Equatorial Jet flow observed in A northward-flowing current is visible in the the earlier period is weakened. The South northern end off Somalia, but no coastal flow is Equatorial Current is more widened into the lower observed in the southern part (Fig. 10). Part of latitudes (between 7 and 17 °S) and turns northeast northward flow is turned southward and joins the at the African coast. Near the equator an opposing anticyclonic eddy circulation, whereas the other flow pattern is noticed, thus the eastward flow in component continues as the East Arabian Current. the southern part and westward flow in the northern The eddy circulation at the northeastern part is part. An anticlockwise circulation is strengthened weakened but the one between 5 and 10 °N is still between Africa and Madagascar. strong. Little change in circulation is observed in the south. The Equatorial Counter Current is more shifted towards the equator. The eddy circulation is strong between Africa and Madagascar. 86 P. N. BENNY

15 15 0.90 0.95 1.00 0.95 1.05 0.90 10 10 1.05

5 5 1.05 1.00 0 0 1.00 0.95 0.90 Latitude -5 Latitude 0.85 -5

0.95 -10 -10 0.85

1.15 1.15 1.05 -15 1.10 -15

1.05 1.15 1.20 0.95 -20 -20 40 45 50 55 60 40 45 50 55 60 Longitude Longitude

Fig. 6. Dynamic topography (May) Fig. 7. Dynamic topography (June)

15 0.90 15 0.80 0.75 0.90 0.95 1.05 1.05 10 10 0.85 1.00 1.10 5 5 1.00 1.05 0.90 1.05 0 0 0.95 1.00 1.00 0.95 Latitude -5 0.90 Latitude -5 0.85 0.85 -10 -10

1.15 1.15 1.05 -15 -15 1.10 1.10 1.10 1.15 1.15 1.20 1.00 -20 -20 40 45 50 55 60 40 45 50 55 60 Longitude Longitude

Fig. 8. Dynamic topography (July) Fig. 9. Dynamic topography (August) VARIABILITY OF WESTERN INDIAN OCEAN CURRENTS 87

15 15 0.70 0.95 0.80 0.75 0.80 0.85

10 1.00 10

1.10 1.10 5 1.05 5 1.05

0.95 1.00 0 0 1.00 0.95 Latitude -5 0.95 Latitude -5 0.90 0.90

0.90 -10 -10 1.15 1.00

1.20 1.10 -15 -15 1.15 1.15 1.10 1.00 -20 -20 40 45 50 55 60 40 45 50 55 60 Longitude Longitude

Fig. 10. Dynamic topography (September) Fig. 11. Dynamic topography (October)

15 1.05 15 0.80 0.95 0.85 1.00 0.80 1.00

10 10 0.95 0.95 1.00 5 5 1.00 0.90

0 0.95 0 0.95 0.90 0.95 Latitude -5 Latitude -5 0.85 0.95 0.85 0.90 -10 0.95 -10 0.95

1.20 1.15 1.15 -15 -15 1.20 1.20 1.10 1.15 1.15 1.20 1.20 1.10 1.05 -20 -20 40 45 50 55 60 40 45 50 55 60 Longitude Longitude

Fig. 12. Dynamic topography (November) Fig. 13. Dynamic topography (December) 88 P. N. BENNY

October DISCUSSIONANDCONCLUSION

In October the northeastward-flowing Somali Current The annual cycle of currents in the western Indian beyond 7 °N turns eastward as it advances (Fig. 11). Ocean is clearly demonstrated in the long-term Thus, the flow is nearly zonal in the northern region. average monthly dynamic topography maps. The The anticyclonic eddy is still stronger and enlarged. major currents identified are the South Equatorial The South Equatorial Current is between 5 and 12 Current, North Equatorial Current, Equatorial °S, thus its latitudinal extent is reduced. The Counter Current, Somali Current and East Arabian Equatorial Counter Current occurs between 0 and 3 Current. The Equatorial Jet is noticed during the °S. The anticyclonic eddy circulation between Africa monsoon transition periods (Wyrtki, 1971). and Madagascar is weakened compared to the earlier Among the currents the Somali Current shows months. Southward flow prevails in the southwestern strong variability during the annual period. end. Southward flow prevails along the Somali coast during January and February. This is the general November flow pattern in the northwest Indian Ocean during the northeast monsoon. Numerical simulation of In November, the dynamic topography shows the Shaji et al. (1997) shows that even at 10 m depth characteristics of the post monsoon period (Fig. the flow fields are generally towards the south. 12). The northward flow of the Somali Current Field studies reveal that during winter the current ceases and a weak zonal flow prevails. The will turn offshore and merge with the eastward- anticyclonic eddy present in the earlier months has flowing South Equatorial Counter Current (Düing also disappeared. The South Equatorial Current is et al., 1978). more shifted to lower latitudes near the African The analysis indicates that the change in the Coast. Just below the equator a jet-like flow circulation pattern off Somali begins in March and appears. The anticyclonic eddy between Africa and that during this month the meridional gradient is Madagascar is again strengthened and counter- higher, the coastal flow is weaker and zonal rotating eddies have appeared in the southwestern circulation stronger. Schott et al. (1990) accounted end. the inflow into the coast during March–April in the Somali region. In mid-April the Somali Current December weakens and the East African Coastal Current advances northward. May is marked by the The dynamic topography of December is marked northward flow pattern of the Somali Current. with the northeast monsoon characteristics (Fig. Along the Somali coast the steric heights greatly 13). The flow along the Somali Coast is reversed increase indicating that the current is attaining towards the south and is strong in the northern part greater strength and the actual change in direction off Somali. Dull circulation prevails in the zone of the coastal flow is taking place in the form of between 0 and 10 °N. A complex circulation pattern an anticyclonic gyre in the Somali Basin in this that is embedded with eddies has been occurring month. in the northern region between 10 and 15 °N. The northward-flowing Somali Current is well Another conspicuous change noticed is the developed in June and is stronger in July. Two structure of South Equatorial Current, between 7 anticyclonic eddies are prominent in the Somali and 17 °S, which branches at about 45 °E, the major Basin. One between 4 and 8 °N and another part continuing towards south and the minor one between 9 and 12 °N. This is in agreement with flowing north. Bruce’s (1973) observation of two anticyclonic eddies along 6 and 10 °N. The northward-flowing Somali Current branches at about 10 °N; one part flows northeastward supplying to the East Arabian Current while the other branch joins the anticyclonic eddy circulation. The Somali Current VARIABILITY OF WESTERN INDIAN OCEAN CURRENTS 89 is still strong in August but its southern extent is evident in its position. The Equatorial Jet (Wyrtki, reduced. In September, the current is prominent 1973) occurs in the monsoon transition periods, only in the northern part beyond 5 °N. especially in May and November. The Equatorial One important peculiarity of the Somali Jet is very strong in May from 45 °E eastwards Current is the anticyclonic gyres. The maps clearly between 0 and 3 °S compared to the winter period. exhibit the generation, growth and decay of these Thus, western Indian Ocean currents show eddies. Surveys during INDEX program (1975– spatial and temporal variations. Among the 1978) indicated that the large ‘Prime Eddy’ currents, the Somali Current exhibits more described by Bruce (1968) was formed in the dramatic seasonal variation than any other current northern Somali basin between approximately 4 in the region. The strongest change in seasonal and 12 °N. After the onset of the southwest forcing cycle occurs during the onset of the summer monsoon in April–May, the current reverses monsoon. The Somali Current flows northeasterly northward and generally develops two gyres and strong during southwest monsoon whereas it separated by two tongues of cold upwelled water is weak and southward during northeast monsoon. (Swallow & Fieux, 1982). This system strengthens In the monsoon transitional period the flow is with time and reaches its maximum in summer mainly zonal in the Somali Basin. The directional (Leetmaa et al. 1982). It usually migrates change of the Somali Current takes place in a northward (Evans & Brown, 1981) before clockwise direction. Spatial and temporal changes weakening and collapsing at the end of the are also noticed in the North Equatorial Current, monsoon. Altimeter data show that in the Somali South Equatorial Current and in Equatorial Counter area the ‘Great Whirl’ migrates northeastward very Current. The Equatorial Jet is evident in May and rapidly (Perigaud & Minister, 1988). November. The Somali Current is further weakened in October and November, indicating clearly REFERENCES northeast monsoon features. The coastal flow is weakened and zonal flow is prominent. Onshore Anderson, D.L.T & Rowlands, P.B. (1976) The Somali and offshore flows occur in the southern and response to southwest monsoon, the relative importance of local remote forcing. J. Mar. Res. northern part of the Somali Basin. The Somali 34: 395–417. Basin has a southward flow during December. Bahulayen, N. & Shaji, C. (1996) Diagnostic model Seasonal variability is also seen for the North of 3-D circulation in the Arabian Sea and western Equatorial Current, which is visible in the late equatorial Indian Ocean. Proc. Indian Nat. Sci. northeast monsoon in March–April. 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