South Equatorial Countercurrent Research Articles

Meridional‐Width Variability of Near‐Equatorial Zonal Currents Along 80.5°E on Seasonal to Interannual Timescales in the Indian Ocean

AbstractNear‐equatorial zonal currents along 80.5°E in the upper Indian Ocean are key to tropical inter‐basin hydrographic exchanges. However, their meridional‐width variability and underlying causes have remained elusive. In this study, we use satellite‐derived ocean current data to reexamine the latitudinal structure of Wyrtki jets (WJs), Monsoon Circulation (MC), and South Equatorial Countercurrent (SECC) along 80.5°E on seasonal to interannual timescales. The WJs are characterized by a transient semiannual feature within the latitudinal range of 2.5°S–3°N, whereas the MC and the SECC have a dominant annual period within the latitudinal range of 3°–5.5°N and 2.5°–6°S, respectively. Empirical orthogonal function (EOF) analysis separates the seasonal latitudinal boundaries of the WJs, the MC, and the SECC, and reveals their interannual variability in response to anomalous wind forcing. EOFs1–3 represent the WJ, MC, and SECC modes accounting for 54%, 19%, and 14% of the total variance, respectively. The near‐equatorial zonal currents are asymmetrically responsive to the Indian Ocean Dipole (IOD), with an abnormally strong westward current resulting from the latitudinal superposition of the WJ and SECC modes in the autumn of 1997, 2006, and 2015 during strong positive IOD events. However, the abnormally enhanced eastward WJs occurred only during weak IOD events in the autumn of 1995, 1999, and 2005, accompanied by latitudinal destructive interference of strong westward SECC. Model sensitivity experiments show that the asymmetric response of near‐equatorial zonal currents to climate modes is closely related to the spatial differences in large‐scale wind anomalies associated with wind‐driven and eastern‐boundary‐generated waves.

Two Pathways of Subsurface Spiciness Anomalies in the Subtropical South Pacific

Subduction and migration of density-compensated (warm/salty or cool/fresh) temperature and salinity water-mass anomalies on isopycnals, referred to as spiciness anomalies, are examined in the subtropical gyre of the South Pacific using an observational dataset. The present results demonstrate that the spiciness anomalies are found to follow two pathways from the subtropical region to the tropical area on the 25–25.5σθ isopycnals. The water masses of one pathway subduct south of 20°S and mainly flow westward via the mean geostrophic current to the western boundary region. The water masses of this pathway correspond to the salinity maximum on these isopycnals as well as the bottom of the South Pacific Tropical Water. Positive temperature and salinity trends were prominent along this pathway during the study period. In the other pathway, the water masses subduct north of 20°S and go directly to the tropics through the interior region. Decadal variability of the spiciness anomalies is prominent along this pathway. In both pathways, sea surface salinity variability likely plays an important role in generating the spiciness anomalies on the isopycnals. A passive tracer experiment revealed that the advection by the South Equatorial Countercurrent (SECC) divides these two pathways. Hence, SECC plays a key role in determining whether a spiciness anomaly propagates through the interior region or the western boundary region.

A Shallow Thermocline Bias in the Southern Tropical Pacific in CMIP5/6 Models Linked to Double‐ITCZ Bias

AbstractA basin‐wide strong shallow bias in the thermocline of the southern tropical Pacific is found in an ensemble of models from the coupled model intercomparison project phases 5 and 6 (CMIP5/6). The bias is present in over 90% of models examined and is equally represented in CMIP5 and CMIP6 models. In contrast to observations, where the southern thermocline is far deeper than its northern counterpart, models have a hemispherically symmetric tropical thermocline. The shallow thermocline bias is closely linked to the well known double intertropical convergence zone (ITCZ) bias; more so for the physical thermocline (i.e., depth of maximal vertical thermal gradient) than the commonly used C isotherm thermocline proxy. A shallow thermocline bias is further found to be associated with climatic conditions including wider separation of double ITCZ peaks, a pronounced cold tongue, and a spurious south equatorial counter current.

Microbial functional structure and stable isotopic variation of leptocephali across three current zones in the western South Pacific

The ecology of leptocephali remains poorly known but they appear to feed on marine snow that can vary spatially and temporally according to the food web dynamics. This study provided new information about the position of leptocephali within the functional structure of microbial plankton and other food web components of the western South Pacific (WSP) region at a large geographic scale including the New Caledonia, Fiji, and Samoa islands. The hydrographic structure varied with latitude, and nutrient levels were generally low but somewhat variable. Stable isotopic signatures were examined in relation to the 3 current zones of the eastward flowing South Equatorial Countercurrent (SECC; north), the westward South Equatorial Current (SEC; mid-latitudes), and the eastward South Tropical Countercurrent (STCC; south), and all zones were found to be primarily based on a heterotrophic planktonic functioning that were co-limited by inorganic nitrogen and phosphorus, with biomasses of planktonic groups varying with depth. Isotopic signatures of leptocephali were compared to the signatures of other mesozoplankton, micronekton, and Trichodesmium components of the food web, and in relation to the signatures of particulate organic matter (POM) that varied among the 3 collection depths. The isotopic signatures of six taxa of leptocephali, other taxonomic groups and POM showed interesting variability according to latitude and among some stations. The presence of Trichodesmium at the surface in the STCC zone influenced the isotopic signatures of POM and thus the signatures of leptocephali. The signatures of leptocephali were therefore linked with the overall food web and were consistent with the larvae feeding on marine snow components of POM. The two apparent groups of leptocephali with different isotopic signatures that have also been observed in other oceanic areas may be explained by feeding behavior at different depths or on different types of marine snow.

ORGANIC MATTER AND NUTRIENT PROFILE OF THE TWO-CURRENT-REGULATED-ZONE IN THE SOUTHWESTERN SUMATRAN WATERS (SSW)

The Indian Ocean is influenced by monsoon systems which alter the ocean’s physical and chemical properties. Specifically, the southwestern Sumatran waters in the eastern Indian Ocean are considered a dual current regulated zone i.e. affected by South Equatorial Counter Current (SECC) and South Java Current (SJC). This area is considered as having an important role in the transfer of organic matter or the biological pump. However, the information about this area is minimal, especially in terms of organic matter and nutrient profile. This study will update the recent information about the area, including the profile of particulate organic matter (POM), macro-nutrients, total suspended solids (TSS), macromolecule-degradingbacteria, and soft bottom macrobenthic organisms sampled from 26 stations in both the SECC-regulated zone and the SJC-regulated zone. The physical profile is typical of tropical watersand both zones have a distinct profile of organic matter and nutrients. The particulate organic carbon (POC), particulate organic nitrogen (PON), and TSS of the SECC-regulated zone can be considered higher than those of the SJC-regulated zone. This region is categorized as mesotrophic waters, especially from the surface up to 100 m. The production of nutrients and organic matter in the water column in this area contribute significantly to the abundance of heterotrophic bacteria and benthic organisms.

Intraseasonal Variability of the Surface Zonal Currents in the Western Tropical Pacific Ocean: Characteristics and Mechanisms

AbstractThe surface circulation of the tropical Pacific Ocean is characterized by alternating zonal currents, such as the North Equatorial Current (NEC), North Equatorial Countercurrent (NECC), South Equatorial Current (SEC), and South Equatorial Countercurrent (SECC). In situ measurements of subsurface moorings and satellite observations reveal pronounced intraseasonal variability (ISV; 20–90 days) of these zonal currents in the western tropical Pacific Ocean (WTPO). The amplitude of ISV is the largest within the equatorial band exceeding 20 cm s−1 and decreases to ~10 cm s−1 in the NECC band and further to 4–8 cm s−1 in the NEC and SECC. The ISV power generally increases from high frequencies to low frequencies and exhibits a peak at 50–60 days in the NECC, SEC, and SECC. These variations are faithfully reproduced by an ocean general circulation model (OGCM) forced by satellite winds, and parallel model experiments are performed to gain insights into the underlying mechanisms. It is found that large-scale ISV (>500 km) is primarily caused by atmospheric intraseasonal oscillations (ISOs), such as the Madden–Julian oscillation (MJO), through wind stress forcing. These signals are confined within 10°S–8°N, mainly as baroclinic ocean wave responses to ISO winds. For scales shorter than 200 km, ISV is dominated by ocean internal variabilities with mesoscale structures. They arise from the baroclinic and barotropic instabilities associated with the vertical and horizontal shears of the upper-ocean circulation. The ISV exhibits evident seasonal variation, with larger (smaller) amplitude in boreal winter (summer) in the SEC and SECC.

Effects of Southeastern Pacific Sea Surface Temperature on the Double-ITCZ Bias in NCAR CESM1

Abstract The double intertropical convergence zone (ITCZ) is a long-standing bias in the climatology of coupled general circulation models (CGCMs). The warm biases in southeastern Pacific (SEP) sea surface temperature (SST) are also evident in many CGCMs. In this study, the role of SEP SST in the double ITCZ is investigated by prescribing the observed SEP SST in the Community Earth System Model, version 1 (CESM1). Both the double ITCZ and dry equator problems are significantly improved with SEP SST prescribed. Both atmospheric and oceanic processes are involved in the improvements. The colder SST over the SEP decreases the precipitation, which enhances the southeasterly winds outside the prescribed SST region, cooling the ocean via increased evaporation. The enhanced descending motion over the SEP strengthens the Walker circulation. The easterly winds over the equatorial Pacific enhance upwelling and shoal the thermocline over the eastern Pacific. The changes of surface wind and wind curl lead to a weaker South Equatorial Countercurrent and stronger South Equatorial Current, preventing the warm water from expanding eastward, thereby improving both the double ITCZ and dry equator. The enhanced Walker circulation also increases the low-level wind convergence and reduces the wind speed in the tropical western Pacific, leading to warmer SST and stronger convection there. The stronger convection in turn leads to more cloud and reduces the incoming solar radiation, cooling the SST. These competing effects between radiative heat flux and latent heat flux make the atmospheric heat flux secondary to the ocean dynamics in the western Pacific warming.

Seasonal variation of the South Indian tropical gyre

Based on satellite altimeter data and global atlases of temperature, salinity, wind stress and wind-driven circulation we investigate the seasonal variation of the South Indian tropical gyre and its associated open-ocean upwelling system, known as the Seychelles–Chagos Thermocline Ridge (SCTR). Results show a year-round, altimeter-derived cyclonic gyre where the upwelling regime appears closely related to seasonality of the ocean gyre, a relationship that has not been previously explored in this region. An analysis of major forcing mechanisms suggests that the thermocline ridge results from the constructive interaction of basin-scale wind stress curl, local-scale wind stress forcing and remote forcing driven by Rossby waves of different periodicity: semiannual in the west, under the strong influence of monsoonal winds; and, annual in the east, where the southeasterlies prevail. One exception occurs during winter, when the well-known westward intensification of the upwelling core, the Seychelles Dome, is shown to be largely a response of the wind-driven circulation. At basin-scale, the most outstanding feature is the seasonal shrinkage of the ocean gyre and the SCTR. From late autumn to spring, the eastward South Equatorial Countercurrent (SECC) recirculates early in the east on feeding the westward South Equatorial Current, therefore closing the gyre before arrival to Sumatra. We find this recirculation longitude migrates over 20° and collocates with the westward advance of a zonal thermohaline front emerging from the encounter between (upwelled) Indian Equatorial Water and relatively warmer and fresher Indonesian Throughflow Water. We suggest this front, which we call the Indonesian Throughflow Front, plays an important role as remote forcing to the tropical gyre, generating southward geostrophic flows that contribute to the early recirculation of the SECC.

The Response of the Surface Circulation of the Arabian Sea to Monsoonal Forcing

AbstractTwo decades of drifter and satellite data allow the authors to describe the seasonal evolution of the surface circulation of the Arabian Sea, which reverses annually with the Indian monsoon winds. This study finds several features that advance current understanding. Most significantly, northward flow appears along the length of the western boundary, together with a weak anticyclone at 6°N (a precursor to the Great Whirl) as early as March or April, one or two months before the southwest monsoon winds. This circulation is driven by planetary waves, which are initiated by wind curl forcing during the previous southwest monsoon, leading the authors to speculate that there is an oceanic mechanism through which one monsoon may precondition the next. Second, the authors find that the eastward South Equatorial Counter Current (SECC) is present year-round, fed by the northward East African Coastal Current (EACC). During the southwest monsoon the EACC overshoots the equator and splits, feeding both northward into the Somali Current and eastward into the SECC by looping back across the equator. This retroflection of the EACC is what was previously known as the southern gyre. At the surface, this circulation is obscured by strong, locally wind-driven, cross-equatorial transport. The semiannual variability of the SECC is governed by Ekman pumping over the equatorial gyre. Finally, there is broad, strong eastward flow at the mouth of the Gulf of Aden throughout the southwest monsoon, coincident with alongshore winds and a switch in sign of the wind stress curl along the axis of the atmospheric monsoon jet.

Contrasting the Flow Patterns in the Equatorial Pacific Between Two Types of El Niño

Outputs based on the National Centers for Environmental Prediction (NCEP) Global Ocean Data Assimilation System (GODAS) are adopted to contrast the current variations in the equatorial Pacific between two types of El Niño. The model fully resolves the equatorial currents. We found that the central Pacific El Niño (CP-El Niño) corresponds well with previous El Niño studies in that both the eastward Equatorial Undercurrent (EUC) and westward South Equatorial Current (SEC) weaken. On the other hand, the eastern Pacific El Niño (EP-El Niño) displays a distinct circulation pattern. The North Equatorial Countercurrent (NECC) strengthens in the developing phase and persists into the peak of the warm event, whereas the northern branch of the SEC (SECn) also intensifies during the mature phase and lasts for about six months. The South Equatorial Countercurrent (SECC) strengthens during the decaying phase of the EP-El Niño. The shifting of the wind stress curl associated with the thermocline variability is chiefly responsible for the unique current performance of the EP-El Niño. It is worth noting that the air–sea interaction plays an important role in the current variability not only during a CP-El Niño but also during an EP-El Niño. RÉSUMÉ [Traduit par la rédaction] Nous adoptons les sorties basées sur le système GODAS (Global Ocean Data Assimilation System) des NCEP (National Centers for Environmental Prediction) pour mettre en évidence les variations de courant dans le Pacifique équatorial entre les deux types d'El Niño. Le modèle représente complètement les courants équatoriaux. Nous trouvons que l'El Niño du centre du Pacifique (CP-El Niño) correspond bien aux études précédentes sur l'El Niño puisque le sous-courant équatorial (EUC) vers l'est et le courant sud-équatorial (SEC) vers l'ouest faiblissent. D'autre part, l'El Niño de l'est du Pacifique (EP- El Niño) affiche une configuration de circulation distincte. Le contre-courant nord-équatorial (NECC) se renforce dans la phase de développement et persiste jusqu'au maximum du réchauffement, tandis que la branche nord du SEC (SECn) s'intensifie aussi durant la phase de maturité et persiste pendant environ six mois. Le contre-courant sud-équatorial se renforce durant la phase de dissipation de l'EP-El Niño. Le changement du rotationnel de la tension du vent lié à la variabilité thermocline est principalement responsable du comportement particulier du courant de l'EP-El Niño. Il est à remarquer que l'interaction air–mer joue un rôle important dans la variabilité du courant, non seulement durant un CP-El Niño mais aussi durant un EP-El Niño.

Variability in reproductive investment of skipjack tuna (Katsuwonus pelamis) in relation to the ocean - climate dynamics in the tropical eastern Indian Ocean

The Indian Ocean is characterised by significant climatic and oceanographic variability such as the seasonal monsoon and the decadal and inter-annual oscillation of the Indian Ocean Dipole (IOD). Understanding the effects of ocean–climate variability on marine top predators is important for both fisheries and ecosystem management. To test the hypothesis that variability in surface and subsurface temperature affects the annual reproductive cycles of skipjack tuna (Katsuwonus pelamis) in the tropical eastern Indian Ocean, we examined the seasonal and inter-annual variations of the gonadosomatic index (GSI) and determined hatch date by using otolith microstructure analysis. The GSI tended to be higher in January–February during the north-eastern monsoon when seawater cooling at the subsurface layer is induced by the seasonal inflow of the South Equatorial Countercurrent (SECC). The mode of the hatch-month distribution was also detected in that season. The generalised linear model (GLM) resulted in a dome-shaped quadratic relationship, with a peak GSI between 24°C and 26°C occurring at a depth of 50 m during austral summer. These findings indicate that seasonal monsoons and associated changes in oceanographic conditions strongly affect the energy-allocation pattern of skipjack tunas.

Environmental effects on forage and longline fishery performance for albacore (Thunnus alalunga) in the American Samoa Exclusive Economic Zone

AbstractThe South Equatorial Counter Current (SECC) strongly influences the American Samoa Exclusive Economic Zone (EEZ) and changes strength on a seasonal and ENSO cycle. A strong SECC is associated with a predominantly anticyclonic eddy field as well as increased micronekton biomass and catch‐per‐unit‐effort (CPUE) for albacore tuna, the economically important target species of the local longline fishery. A strong SECC carries chlorophyll a‐rich waters from upwelling regions at the north coast of New Guinea towards the EEZ, most likely resulting in the observed increase in micronekton biomass, forage for albacore. Relatively stable anticyclonic eddies show a further increase in micronekton biomass, apparently advected from neighboring SECC waters. The presence of forage presumably concentrates albacore, thus resulting in the observed increase in CPUE. High shear regions of neither anticyclonic nor cyclonic eddies correlate with increased micronekton biomass. Areas characterized by South Equatorial Current (SEC) waters correspond to areas with the lowest micronekton biomass and the highest number of aggregative structures, which are most likely small pelagic fish shoals. Micronekton composition in SEC waters differs from that in the SECC. During El Niños, the seasonal signals at the north shore of New Guinea and in the SECC are exceptionally strong and correspond to higher albacore CPUE in the EEZ. My results suggest that the strength of upwelling and the resulting increase in chlorophyll a at New Guinea, as well as the Southern Oscillation Index, could be used to predict the performance of the local longline fishery for albacore tuna in the American Samoa EEZ.

Central South Pacific thermocline water circulation from a high‐resolution ocean model validated against satellite data: Seasonal variability and El Niño 1997–1998 influence

The oceanic circulation in French Polynesia, central South Pacific, is investigated based on a high‐resolution model validated with satellite data. We focus on the upper 500 m, where ocean current variability has been barely documented due to a lack of in situ measurements. Seasonal and El Niño–Southern Oscillation 1997–1999 variability of surface and subsurface currents are characterized. In the north, the South Equatorial Current (SEC) follows a seasonal cycle with winter enhancement corresponding to trade wind strengthening. During summer, the SEC slows and the eastward South Equatorial Countercurrent (SECC) appears across the domain north of an upward Ekman pumping region related to the South Pacific Convergence Zone development in the area. Around the Marquesas Islands, a wind curl dipole is suspected to create westward and eastward (Marquesas Countercurrent, MCC) jets. The eastward Subtropical Countercurrent (STCC) is found south of 20°S. It is disturbed by eddies and broken into westward and eastward branches. During El Niño 1997–1998, due to the reversed winds in the equatorial band, the SEC weakened, while eastward countercurrents appeared. The SECC reinforced and moved northeastward in summer 1998. During La Niña 1998–1999, following the trade wind enhancement, the SEC strengthened. The countercurrents weakened or disappeared and the SECC moved southwestward in summer 1999. During both El Niño and La Niña events, the STCC eddy activity strengthened, while the MCC and associated westward jet were absent. These variations and their dynamical origins are discussed, and a coherent picture of the French Polynesian current system and its variations is proposed.

Oceanographic investigation of the American Samoa albacore (Thunnus alalunga) habitat and longline fishing grounds

AbstractThe American Samoa fishing ground is a dynamic region with strong mesoscale eddy activity and temporal variability on scales of <1 week. Seasonal and interannual variability in eddy activity, induced by baroclinic instability that is fueled by horizontal shear between the eastward‐flowing South Equatorial Counter Current (SECC) and the westward‐flowing South Equatorial Current (SEC), seems to play an important role in the performance of the longline fishery for albacore. Mesoscale eddy variability in the American Samoa Exclusive Economic Zone (EEZ) peaks from March to April, when the kinetic energy of the SECC is at its strongest. Longline albacore catch tends to be highest at the eddy edges, while albacore catch per effort (CPUE) shows intra‐annual variability with high CPUE that lags the periods of peak eddy activity by about 2 months. When CPUE is highest, the values are distributed toward the northern half of the EEZ, the region affected most by the SECC. Further indication of the possible importance of the SECC for longline performance is the significant drop in eddy variability in 2004 when compared with that observed in 2003 – resulting from a weak SECC – which was accompanied by a substantial drop in albacore CPUE rates and a lack of northward intensification of CPUE. From an ecosystem perspective, evidence to support higher micronekton biomass in the upper 200 m at eddy boundaries is inconclusive. Albacore's vertical distribution seems to be governed by the presence of prey. Albacore spend most of their time between 150 and 250 m, away from the deep daytime and shallow nighttime sonic scattering layers, at depths coinciding with those of small local maxima in micronekton biomass whose backscattering properties are consistent with those of albacore's preferred prey. Settling depths of longline sets during periods of decreased eddy activity correspond to those most occupied by albacore, possibly contributing to the lower CPUE by reducing catchability through rendering bait less attractive to albacore in the presence of prey.

On the circulation of water masses across the Mascarene Plateau in the South Indian Ocean

The South Equatorial Current (SEC) is the major westward current in the South Indian Ocean. It crosses the Mascarene Plateau, an extensive range of banks and islands, near 60°E, but how this occurs has until now been unclear. Here, we present the results of a recent survey during June–July 2002 using a suite of modern instrumentation, and provide a detailed examination of this process, and the water masses involved. Upstream from the Plateau the SEC carries 50–55 Sv (1 Sv=10 6 m 3/s) westwards between 10 and 16°S. As it approaches the Plateau, 25 Sv of this is constricted to pass over a narrow sill (about which we provide new information) between the Saya De Malha and Nazareth Banks at 12–13° S. This then forms a northern core to the SEC between 10 and 14°S downstream from the Plateau (25 Sv). The remainder of the inflow passes either around the northern edge of the Saya De Malha Bank (8–9°S) or between Mauritius and the Cargados Carajos Bank (18–20°S). The former may retroflect to flow eastwards near 8°S, joining the South Equatorial Counter Current (SECC), whereas the latter, strengthened near Mauritius by further flows from the south, forms a southern core to the SEC downstream from the Plateau (20–25 Sv between 17 and 20°S). The overall effect of the Plateau is to split the SEC into two cores. On reaching Madagascar, these cores may then form the Northeast and Southeast Madagascar Currents. The SEC also forms a sharp boundary between upper and intermediate level water masses. Subtropical Surface Water (STSW), Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are present on the southern side of the SEC, whereas Arabian Sea High Salinity Water (ASHSW) and Red Sea Water (RSW) are found on its northern side. As they approach the Plateau, the STSW and SAMW are partially drawn northwards, and Tropical Surface Water (TSW) is drawn southwards, in order to flow across the sill near 12–13°S. At deeper levels, North Indian Deep Water (NIDW) passes southwards below the SEC on the western side of the Plateau, and while there is no indication of North Atlantic Deep Water, Antarctic Bottom Water (AABW) is present west of the Plateau. Finally, there is evidence of significant mixing in the upper and intermediate waters (the TSW, STSW, SAMW and RSW) as they pass across the sill at 12–13°S, and also in the deeper waters on the eastern side of the Plateau above the rough bottom topography of the Central Indian Ridge.

Seasonal variability of the South Equatorial Countercurrent

Using the 1.5‐layer long Rossby wave model forced by the seasonal European Remote Sensing (ERS) satellite scatterometer wind, we demonstrate that the seasonal variability of the South Equatorial Countercurrent (SECC) is due to the interplay of two types of forced Rossby waves: the resonantly forced Rossby waves north of 10°S and the locally forced Rossby waves south of 10°S. The resonantly forced Rossby waves north of 10°S are due to the annually varying tropical Pacific wind, and the locally forced Rossby waves south of 10°S in the western basin are associated with the Western Pacific Monsoon. Both types of waves are intensified westward in the SECC region but have different phases across the SECC; this latitudinal phase jump results in the seasonal variability of the SECC, which reaches its maximum in March and its minimum in August. Favorable agreement between the seasonal SECC signals from the model and from the satellite sea surface height measurements confirms this new insight about the seasonal modulation of the SECC.

Seasonal Modulations in the Eddy Field of the South Pacific Ocean

Abstract Decade-long satellite altimeter data from the Ocean Topography Experiment (TOPEX)/Poseidon and European Remote Sensing Satellite-1 and-2 (ERS-1/2) missions are analyzed to investigate the eddy signals in the South Pacific Ocean. High–eddy kinetic energy (EKE) bands with well-defined annual cycles are detected along the eastward-flowing surface currents of the South Tropical Countercurrent (STCC) between 21°–29°S and the South Equatorial Countercurrent (SECC) centered near 9°S. Overriding the westward-flowing South Equatorial Current (SEC), the STCC layer has the sign of its mean potential vorticity gradient opposite to that in the SEC layer, subjecting the vertically sheared STCC–SEC system to baroclinic instability. In austral winter, the STCC–SEC system is baroclinically more unstable than in other seasons because of the large vertical shear and weak stratification. This seasonal variation in the intensity of baroclinic instability is responsible for the seasonal modulation of the STCC's EKE fi...

Upper-layer circulation in the eastern Equatorial and South Atlantic Ocean in January–March 1995

The upper-layer circulation in the eastern basin of the South Atlantic was studied from hydrographic and direct velocity measurements along WOCE lines A11, A13 and A14. A13 and A14 provide quasi-meridional samplings of the equatorial, subequatorial and subtropical circulation regimes. A13 was carried out along the African coast at about 600 km offshore from it, and A14 along the nominal longitude 9°W. A11 intersects the Cape Basin between 46°S in the west and 30°S in the east. Transport estimates were derived from direct velocity measurements and a box inverse model. In the equatorial eastern Atlantic, the Equatorial Undercurrent (EUC) transport decreases from 25×10 6 m 3 s −1 at 9°W to 13×10 6 m 3 s −1 at 5°E. Re-circulations of the EUC into the northern and equatorial branches of the South Equatorial Current (SEC) are evidenced at 5°E and quantified. In the tropical Atlantic, we estimate 7.5×10 6 and 4.2×10 6 m 3 s −1 for the transports at 9°W of the South Equatorial Undercurrent (SEUC) and South Equatorial Countercurrent (SECC), respectively. Both the SEUC and SECC extend vertically down to intermediate depths and contribute to the northern limb of the Angola Gyre. The Angola Current transport is estimated to be 16±5×10 6 m 3 s −1 for σ 1<32.1 at 13°S. South of the Angola Gyre, the transports show an apparent cyclonic circulation, developed mostly at the intermediate level. The water mass properties suggest that it is, at least partially, a re-circulation of the Benguela Current. Further south and for the subtropical gyre, we estimate 10±5×10 6 m 3 s −1 for the transport of the South Atlantic Current across 9°W and 28±4×10 6 m 3 s −1 for the transport of the Benguela Current at 10°E for σ 1<32.1.

New observations of the intermediate depth circulation in the tropical Atlantic

The intermediate depth (around 1000 m) circulation in the interior tropical Atlantic has been described as several narrow flow bands. Due to a lack of data, these currents have previously been poorly resolved in space and time. Recent observations, obtained during the mid-1997 Seward Johnson cruise and from PALACE floats which cover the period Summer 1997 to Spring 2000, allow a more detailed description of the intermediate depth circulation in the tropical Atlantic. The PALACE trajectories display several well defined currents between the equator and 4N at 800 to 1100 m. Two regimes separated by the eastern edge of the Mid-Atlantic Ridge seem to exist at these latitudes. Velocities in the eastern regime are lower than in the western regime and at some latitudes, the zonal flow in the two regimes is going in opposite directions. Farther south, between 4S and 2S, westward velocities of the central South Equatorial Current dominate the circulation. The flow north of 4N and south of 4S is governed by up to several month-long periods of eastward or westward flow, with only weak preferences for either direction. The southern region is characterized by the (meandering) transition between the central South Equatorial Current and the South Equatorial Countercurrent. It has been proposed earlier that these two currents do not extend eastward beyond about 10W, and that the intermediate water follows a cyclonic path east of 10W between about 5S and 25S. This could be interpreted as an intermediate expression of the Angola Gyre. Such a circulation is not found in the present data set. It is also noted that no significant cross-equatorial flow is found in the PALACE data.

Seasonal variability of upper-layer geostrophic transport in the tropical Indian Ocean during 1992–1996 along TOGA-I XBT tracklines

Upper layer (0–400 m) geostrophic volume transport associated with the major current systems of the tropical Indian Ocean along the near-meridional TOGA-India XBT tracklines from Mumbai to Mauritius during 1992–1996 is discussed. The transport of the eastward flowing Indian Monsoon Current (IMC) varies between 9 and 14 Sv (1 Sv=10 6 m 3 s −1) during southwest monsoon. The westward flowing North Equatorial Current (NEC) transports about 8 Sv during boreal winter. The westward flowing south equatorial current (SEC) has its lowest transport (10–12 Sv) during austral fall and spring and highest transport (18–21 Sv) during austral winter and summer. The northern boundary of the SEC extends equatorward as far north as for 4–5°S during southwest monsoon from its usual location of 8–10°S. The eastward flowing South Equatorial Counter Current (SECC) exhibits highest transport (37 Sv) during peak austral summer (January) and between 5 and 20 Sv during the rest of the year. In March and May, eastward undercurrents are noticed near the equator. In June and July, the undercurrents are present south of Sri Lanka away from the equator. The equatorial flow in the zonal belts 1–2°N and 1–2°S exhibits westward transport of 6–20 Sv during January–February. This broad westward flow and the SECC constitute an anticlockwise equatorial gyre during boreal winter. In addition to the above current systems, signatures of a warm-core anticyclonic eddy centered at 8°N, 72.5°E and a cold-core cyclonic eddy centered at 10.5°N, 72.5°E are also noticed in February and October, respectively, off the southwest coast of India. The volume transport associated with the warm-core eddy is about 10 Sv directed equatorward and that with the cold-core eddy is 2.5 Sv directed poleward.