South Indian Subtropical Gyre Research Articles

Seasonal and Interannual Variability of the Subtropical South Indian Ocean Sea Surface Salinity Maximum

AbstractThe sea surface salinity (SSS) maximum of the South Indian Ocean (the SISSS‐max) is a high‐salinity feature centered at 30°S, 90°E, near the center of the South Indian subtropical gyre. It is located poleward of a region of strong evaporation and weak precipitation. Using several different satellites and in situ data sets, we track changes in this feature since the early 2000s. The centroid of the SISSS‐max moves seasonally north and south, furthest north in late winter and farthest south in late summer. Interannually, the SISSS‐max has moved on a northeast‐southwest path about 1,500 km in length. The size and maximum SSS of the feature vary in tandem with this motion. It gets larger (smaller) and saltier (fresher) as it moves to the northeast (southwest) closer to (further from) the area of strongest surface freshwater flux. The area of the SISSS‐max almost doubles from its smallest to largest extent. It was maximum in area in 2006, decreased steadily until it reached a minimum in 2013, and then increased again. The seasonal variability of the SISSS‐max is controlled by the changes that occur on its poleward, or southern, side, whereas interannual variability is controlled by changes on its equatorward side. The variations in the SISSS‐max are a complex dance between changes in evaporation, precipitation, wind forcing, gyre‐scale ocean circulation, and downward Ekman pumping. Its motion correlated with SSS changes throughout the South Indian Ocean and may be an indicator of changes in the basin's subtropical circulation.

Effects of iron and light on microbial nitrogen cycles in the primary nitrite maxima of the eastern Indian Ocean

To elucidate the effect of light and iron on the dynamics of nitrogen at the primary nitrite maximum (PNM), observed at the bottom of the euphotic zone of the eastern Indian Ocean, we conducted high-resolution sampling and incubation experiments at three stations. The stations were located at the Bay of Bengal, equatorial region, and South Indian subtropical gyre. The PNM was observed below the subsurface chlorophyll maximum and around the nitracline at all three stations; its magnitude was largest (>1.5 μM) in the Bay of Bengal. The results of the iron enrichment experiments under two different light regimes demonstrated that the growth of phytoplankton communities at the PNM was limited by light availability at all three stations. In the subtropical gyre, co-imitation by iron and light was observed, whereas iron was the secondary limiting factor at the other two stations, suggesting widespread iron limitation on phytoplankton growth in the subsurface waters of the open ocean. However, the changes in the concentrations of ammonium, nitrite, and nitrate suggest that the effect of light and iron deficiency on the release of nitrite by phytoplankton is relatively minor. Instead, active ammonium oxidization by shallow-clade ammonium-oxidizing archaea was observed around the PNM, suggesting that nitrite supply via the tight coupling of ammonium regeneration sustained by high surface productivity and ammonium oxidization maintains the distinct PNM in the subsurface water of the eastern Indian Ocean.

Surface Inorganic Iodine Speciation in the Indian and Southern Oceans From 12°N to 70°S

Marine iodine speciation has emerged as a potential tracer of primary productivity, sedimentary inputs, and ocean oxygenation. The reaction of iodide with ozone at the sea surface has also been identified as the largest deposition sink for tropospheric ozone and the dominant source of iodine to the atmosphere. Accurate incorporation of these processes into atmospheric models requires improved understanding of iodide concentrations at the air-sea interface. Observations of sea surface iodide are relatively sparse and are particularly lacking in the Indian Ocean basin. Here we examine 127 new sea surface (≤10 m depth) iodide and iodate observations made during three cruises in the Indian Ocean and the Indian sector of the Southern Ocean. The observations span latitudes from ∼12°N to ∼70°S, and include three distinct hydrographic regimes: the South Indian subtropical gyre, the Southern Ocean and the northern Indian Ocean including the southern Bay of Bengal. Concentrations and spatial distribution of sea surface iodide follow the same general trends as in other ocean basins, with iodide concentrations tending to decrease with increasing latitude (and decreasing sea surface temperature). However, the gradient of this relationship was steeper in subtropical waters of the Indian Ocean than in the Atlantic or Pacific, suggesting that it might not be accurately represented by widely used parameterizations based on sea surface temperature. This difference in gradients between basins may arise from differences in phytoplankton community composition and/or iodide production rates. Iodide concentrations in the tropical northern Indian Ocean were higher and more variable than elsewhere. Two extremely high iodide concentrations (1241 and 949 nM) were encountered in the Bay of Bengal and are thought to be associated with sedimentary inputs under low oxygen conditions. Excluding these outliers, sea surface iodide concentrations ranged from 20 to 250 nM, with a median of 61 nM. Controls on sea surface iodide concentrations in the Indian Ocean were investigated using a state-of-the-art iodine cycling model. Multiple interacting factors were found to drive the iodide distribution. Dilution via vertical mixing and mixed layer depth shoaling are key controls, and both also modulate the impact of biogeochemical iodide formation and loss processes.

Micronekton distribution as influenced by mesoscale eddies, Madagascar shelf and shallow seamounts in the south-western Indian Ocean: an acoustic approach

An investigation of the vertical and horizontal distributions of micronekton, as influenced by mesoscale eddies, the Madagascar shelf and shallow seamounts, was undertaken using acoustic data collected during two research cruises at an unnamed pinnacle (summit depth ~240 m) thereafter named “MAD-Ridge”, and at La Pérouse seamount (~60 m) in the south-western Indian Ocean. MAD-Ridge is located to the south of Madagascar, in an “eddy corridor”, known both for its high mesoscale activity and high primary productivity. In contrast, La Pérouse is located on the outskirts of the Indian South Subtropical Gyre (ISSG) province, characterised by low mesoscale activity and low primary productivity. During the MAD-Ridge cruise, a dipole was located in the vicinity of the seamount, with the anticyclone being almost stationary on the pinnacle. Total micronekton acoustic densities were greater at MAD-Ridge than at La Pérouse. Micronekton acoustic densities of the total water column were lower within the anticyclone than within the cyclone during MAD-Ridge. Micronekton followed the usual diel vertical migration (DVM) pattern, except within the cyclone during MAD-Ridge where greater acoustic densities were recorded in the daytime surface layer. The backscatter intensities were stronger at the 38 kHz than at the 70 and 120 kHz frequencies in the daytime surface layer at MAD-Ridge cyclonic stations. These backscatter intensities likely correspond to gas-filled swimbladders of epi- and mesopelagic fish actively swimming and feeding within the cyclone or gelatinous organisms with gas inclusions. Our findings evidenced that the distributions of micronekton and DVM patterns are complex and are influenced significantly by physical processes within mesoscale eddies. The mesoscale eddies’ effects were dominant over any potential seamount effects at the highly dynamic environment prevailing at MAD-Ridge during the cruise. No significant increase in total micronekton acoustic densities was observed over either seamount, but dense aggregations of biological scatterers were observed on their summits during both day and night.

Micronekton distributions and assemblages at two shallow seamounts of the south-western Indian Ocean: Insights from acoustics and mesopelagic trawl data

Micronekton distributions and assemblages were investigated at two shallow seamounts of the south-western Indian Ocean using a combination of trawl data and a multi-frequency acoustic visualisation technique. La Pérouse seamount (summit depth ~60 m) is located on the outskirts of the oligotrophic Indian South Subtropical Gyre (ISSG) province with weak mesoscale activities and low primary productivity all year round. The “MAD-Ridge” seamount (thus termed in this study; ~240 m) is located in the productive East African Coastal (EAFR) province with high mesoscale activities to the south of Madagascar. Higher micronekton species richness was recorded at MAD-Ridge compared to La Pérouse. Resulting productivity at MAD-Ridge seamount was likely due to the action of mesoscale eddies advecting productivity and larvae from the Madagascar shelf rather than local dynamic processes such as Taylor column formation. Mean micronekton abundance/biomass, as estimated from mesopelagic trawl catches, were lower over the summit compared to the vicinity of the seamounts, due to net selectivity and catchability and depth gradient on micronekton assemblages. Mean acoustic densities in the night shallow scattering layer (SSL: 10–200 m) over the summit were not significantly different compared to the vicinity (within 14 nautical miles) of MAD-Ridge. At La Pérouse and MAD-Ridge, the night and day SSL were dominated by common diel vertically migrant and non-migrant micronekton species respectively. While seamount-associated mesopelagic fishes such as Diaphus suborbitalis (La Pérouse and MAD-Ridge) and Benthosema fibulatum performed diel vertical migrations (DVM) along the seamounts’ flanks, seamount-resident benthopelagic fishes, including Cookeolus japonicus (MAD-Ridge), were aggregated over MAD-Ridge summit. Before sunrise, mid-water migrants initiated their vertical migration from the intermediate to the deep scattering layer (DSL, La Pérouse: 500–650 m; MAD-Ridge: 400–700 m) or deeper. During sunrise, the other taxa contributing to the night SSL exhibited a series of vertical migration events from the surface to the DSL or deeper until all migrants have reached the DSL before daytime. Possible mechanisms leading to the observed patterns in micronekton vertical and horizontal distributions are discussed. This study contributes to a better understanding of how seamounts influence the DVM, horizontal distribution and community composition of micronekton and seamount-associated/resident species at two poorly studied shallow topographic features in the south-western Indian Ocean.

Micronekton diel migration, community composition and trophic position within two biogeochemical provinces of the South West Indian Ocean: Insight from acoustics and stable isotopes

Spatial distribution, community composition and trophic roles of micronekton (crustaceans, fishes and squids) were investigated in the Indian South Subtropical Gyre (ISSG) province and the East African Coastal province (EAFR), by combining acoustic surveys, mid-water trawls and stable isotope analyses from scientific cruises conducted in 2009 and 2010. Mesopelagic micronekton performed diel vertical migrations in both provinces, from deep (400–740 m) to surface (0–200 m) layers at dusk and in the opposite direction at dawn, with some species migrating below 740 m. The EAFR province was more dynamic than the oligotrophic ISSG province, with enhanced eddy activity and enhanced yearly productivity. The active enrichment mechanisms in the EAFR, in terms of available primary production, led to high micronekton acoustic density (as a proxy of micronekton abundance) and large micronekton weight and abundance estimates from trawl data. Particulate organic matter in the EAFR exhibited greater enrichment in 13C and 15N compared to the ISSG and, consequently, tissues of selected micronekton organisms in the EAFR were more enriched in 15N (higher δ15N values). In both provinces, micronekton encompassed a wide range of isotopic niches, with large overlaps between species. Micronekton and swordfish in the EAFR had an overlapping range of δ15N values, contrasting with the ISSG province where swordfish were two trophic levels higher than the sampled micronekton. Our results provide some evidence that the combined action of riverine input and the dynamics of eddies might influence productivity in the EAFR, and hence the abundance of micronekton and the enrichment of tissues in 15N, compared to the oligotrophic ISSG province.

The impact of circulation and dust deposition in controlling the distributions of dissolved Fe and Al in the south Indian subtropical gyre

The South Indian Subtropical Gyre (SISG) is one of the least studied gyre systems of the world ocean with respect to trace elements. Here we report dissolved (<0.45μm) Fe and Al measurements collected during two high-resolution US-CLIVAR CO2 Repeat Hydrography sections, which transected the upper 1000m of the gyre zonally along ~32°S (I05) and meridionally along ~30°E (I06S). Particulate Fe and Al concentrations in waters influenced by the Agulhas Current are also presented. The distributions of dissolved Fe and Al in the gyre are primarily impacted by mineral dust deposition at the surface and the large-scale circulation patterns of the gyre at depth. Using mean mixed layer dissolved Al concentrations, we estimate that the deposition and partial dissolution of mineral dust emanating from South Africa and Australia vary from 60 to 685mg (dust) m−2yr−1 across the 32°S transect. This translates into a dust source of dissolved Fe ranging from 1.7 to 20μmolFem−2y−1. The zonal patterns of aeolian deposition and those of N*, an indirect geochemical tracer for nitrogen fixation, show remarkable similarities along the I05 transect, suggesting that aeolian delivery of Fe may regulate nitrogen fixation rates in the SISG. In the western SISG (west of 60°E), which receives some of the highest aeolian Fe fluxes of the 32°S section, the Fe:AOU ratio in Indian Central Water was elevated relative to that observed in the Indian Central Water occupying the eastern Indian Ocean. These elevated Fe:AOU ratios may reflect the remineralization of Fe-rich organic material from nitrogen fixing organisms at the western end of the basin. Below the mixed layer, the distribution of dissolved Al appears to trace the principal features of the large-scale circulation of the SISG. Elevated subsurface concentrations of dissolved Al (>4nM) in the southwest Indian Ocean west of 45–50°E are most likely sustained by leakage of Al-rich waters from the Agulhas Return Current. Along the southeast African margin, the elevated particulate Fe (up to 230nM) and Al (up to 690nM) concentrations reflect the resuspension and transport of shelf sediments by the highly energetic Agulhas Current. However, while the particulate inputs at the margin are massive and appear to supply modest amounts of dissolved Fe, the distribution of dissolved Al is decoupled from the particulate phase. This observation suggests that the elevated subsurface dissolved Al concentrations observed near the African shelf are not the result of sediment resuspension processes occurring in situ along I05 but are more likely an advected signal originating from the upper reaches of the Agulhas Current.

Dust deposition in the eastern Indian Ocean: The ocean perspective from Antarctica to the Bay of Bengal

AbstractAtmospheric deposition is an important but still poorly constrained source of trace micronutrients to the open ocean because of the dearth of in situ measurements of total deposition (i.e., wet + dry deposition) in remote regions. In this work, we discuss the upper ocean distribution of dissolved Fe and Al in the eastern Indian Ocean along a 95°E meridional transect spanning the Antarctic margin to the Bay of Bengal. We use the mixed layer concentration of dissolved Al in conjunction with empirical data in a simple steady state model to produce 75 estimates of total dust deposition that we compare with historical observations and atmospheric model estimates. Except in the northern Bay of Bengal where the Ganges‐Brahmaputra river plume contributes to the inventory of dissolved Al, the surface distribution of dissolved Al along 95°E is remarkably consistent with the large‐scale gradients in mineral dust deposition and multiple‐source regions impacting the eastern Indian Ocean. The lowest total dust deposition fluxes are calculated for the Southern Ocean (66 ± 60 mg m−2 yr−1) and the highest for the northern end of the south Indian subtropical gyre (up to 940 mg m−2 yr−1 at 18°S) and in the southern Bay of Bengal (2500 ± 570 mg m−2 yr−1). Our total deposition fluxes, which have an uncertainty on the order of a factor of 3.5, are comparable with the composite atmospheric model data of Mahowald et al. (2005), except in the south Indian subtropical gyre where models may underestimate total deposition. Using available measurements of the solubility of Fe in aerosols, we confirm that dust deposition is a minor source of dissolved Fe to the Southern Ocean and show that aeolian deposition of dissolved Fe in the southern Bay of Bengal may be comparable to that observed underneath the Saharan dust plume in the Atlantic Ocean.

Dissolved Fe and Al in the upper 1000 m of the eastern Indian Ocean: A high‐resolution transect along 95°E from the Antarctic margin to the Bay of Bengal

AbstractA high‐resolution section of dissolved iron (dFe) and aluminum (dAl) was obtained along ~95°E in the upper 1000 m of the eastern Indian Ocean from the Antarctic margin (66°S) to the Bay of Bengal (18°N) during the U.S. Climate Variability and Predictability (CLIVAR) CO2 Repeat Hydrography I08S and I09N sections (February–April 2007). In the Southern Ocean, low concentrations of dAl (&lt;1 n M) reflect the negligible dust inputs impacting the water masses subducted in the circumpolar domain. The low dAl concentrations characterizing the Southern Ocean terminate near 45°S, probably because of the advection of subtropical water masses that received dust and sedimentary inputs in their formation region. Our subsurface dFe data near the southern Kerguelen Plateau were significantly higher than historical observations in other Indian sectors of the Southern Ocean. We surmise that the offshore advection of dFe‐rich waters along the western flank of the southern Kerguelen plateau and enhanced vertical mixing could contribute to this elevated subsurface dFe inventory. Elevated subsurface particulate and dFe levels downstream of the northern Kerguelen Plateau may reflect long‐range lateral transport from the plateau's sediments and/or remineralization inputs. At the northern edge of the south Indian subtropical gyre, the deposition of Australian dust, possibly combined with the advection of dAl‐enriched waters from the Indonesian Throughflow, creates a region of elevated dAl in the upper 400 m but without a corresponding enrichment in dFe. In the northern Indian Ocean, the South Equatorial Current constitutes a remarkable biogeochemical front separating the oxygen‐rich and dFe‐poor subtropical gyre waters from the dFe‐rich and oxygen‐depleted waters of the northern Indian Ocean. By tracing the accumulation of macronutrients and dFe along the advective pathway of Indian Central Water, we show that the central waters of the northern Indian Ocean receive excess dFe in addition to that produced by remineralization inputs. The resuspension of shelf sediments and release of pore waters probably contribute to the elevated dFe and dAl levels observed below the highly stratified upper layers of the Bay of Bengal.

Pelagic cephalopods in the western Indian Ocean: New information from diets of top predators

Using a combination of diverse large predatory fishes and one seabird, we collected information on the cephalopod fauna of the western Indian Ocean. We analyzed the stomach contents of 35 fishes representing ten families (Xiphiidae, Istiophoridae, Scombridae, Carangidae, Coryphaenidae, Alepisauridae, Dasyatidae, Carcharhinidae, Alopiidae and Sphyrnidae) and of the sooty tern Onychoprion fuscata of the Mozambique Channel from 2000 to 2010. Both fresh and accumulated beaks were used for identifying cephalopod prey. Cephalopods were important prey for twelve predators; swordfish Xiphias gladius had the highest cephalopod proportion; sooty tern (O. fuscata) and bigeye tuna (Thunnus obesus) had high proportions too. We recovered 23 cephalopod families and identified 38 species. Ten species from four Teuthida families (Ommastrephidae, Onychoteuthidae, Histioteuthidae and Ancistrocheiridae) and two Octopoda families (Argonautidae and Bolitaenidae) occurred very frequently in the stomach contents, while Sepiida were rare. Ommastrephidae were the most cephalopod food sources: the purpleback flying squid Sthenoteuthis oualaniensis was the most prevalent prey by far, Ornithoteuthis volatilis was important for eleven predators and few but large specimens of the neon flying squid Ommastrephes bartramii were recovered in the stomachs of swordfish in the Indian South Subtropical Gyre province only. Predators' groups were identified based on cephalopod prey composition, on depth in which they forage, and on prey size. Surface predators' diets were characterized by lower cephalopod diversity but greater average numbers of cephalopod prey, whereas the deep-dwelling predators (swordfish and bigeye tuna) preyed on larger specimens than surface predators (O. fuscata or yellowfin tunas Thunnus albacares). Our findings emphasized the usefulness of a community of marine predators to gain valuable information on the biology and the distribution of the cephalopod forage fauna that are discussed with regard to biogeographic province, marine predator, fishing gear to catch the large pelagic fishes, and size of the beaks recovered in the stomachs.

Regional Feeding Patterns of the Longnose Lancetfish (Alepisaurus ferox Lowe, 1833) of the Western Indian Ocean

The food composition of the longnose lancetfish (Alepisaurus ferox Lowe, 1833) was studied in two areas belonging to different biogeographic provinces of the western Indian Ocean: Indian Monsoon Gyres Province (MONS) and Indian South Subtropical Gyre Province (ISSG). A total of 158 lancetfish were sampled during two research longline cruises in 1986-1987: 50 within the MONS, from the western equatorial area (between the EEZs of Kenya and Seychelles) and 108 within the ISSG, from the northern part of the EEZ of Mauritius (Saya-de-Malha Bank – Agalega Islands area). In the equatorial area, 135 prey items (of 18 families or higher taxa) were found in the lancetfish stomachs. In the waters of Mauritius, 476 prey items of 53 taxa were recorded. Regional variability in the feeding habits reflects province-specific differences in the prey abundance and the structure of pelagic communities of the intermediate trophic levels. Large predatory crustaceans dominate in the waters of MONS, while they are absent in the poor waters of ISSG. Foraging success of the lancetfish on non-evasive prey was 3.2 times higher in the MONS than in the ISSG. In overall, swimming crab, Hyperiidea, conspecifics, barracudina, hatchetfish, hammerjaw, and Polychaeta were the predominant prey of the lancetfish. Great differences in the food composition of ‘small’ (FL < 100 cm) and ‘large’ (FL ≥ 100 cm) lancetfish were recorded. For large lancetfish, the cannibalism rate and the occurrence of large evasive prey and Sargassum algae floating at the sea surface were greater than for small lancetfish.

Protistan assemblages across the Indian Ocean, with a specific emphasis on the picoeukaryotes

Protists, and among them the picoeukaryotes (cells<3 μm), have been described as significant contributors to both carbon biomass and production in oligotrophic regions of the oceans. However, protist assemblages remain largely undescribed in pelagic ecosystems and in particular in the Indian Ocean. In the present work, we investigated protists along an eastward transect across the sub-tropical gyre of the Indian Ocean (from South Africa to Australia), with a particular focus on picoeukaryotes. We combined inverted and epifluorescence microscopy, flow cytometry, pigment analysis, denaturing gel gradient electrophoresis (DGGE), 18S rDNA clone libraries, and fluorescent in situ hybridization (FISH). Overall the picophytoplankton fraction contributed 88% and 90% of total Chl a at the surface and DCM, respectively, with picoeukaryotes accounting for 38% and 50% of total Chl a at the surface and DCM. Considering only the Indian South Subtropical Gyre (ISSG) province, we observed greater shifts in the picoeukaryotic assemblage throughout the upper 200 m of the water column than along the ca. 10,000 km cruise track. In terms of taxonomic diversity and contribution of each taxon to the picoeukaryotic community, prasinophytes were well represented at more coastal stations with the genus Micromonas reaching densities up to 750 cell mL −1 in coastal waters and less than 100 cell mL −1 at open ocean stations. Haptophytes (56% and 45% of picoeukaryotic pigments at surface and DCM, respectively) and possibly pelagophytes (28% and 40% of picoeukaryotic pigments at surface and DCM, respectively) appeared to be dominant at open ocean stations. Other groups and in particular organisms affiliated to chrysophytes, and to a lesser extent to cryptophytes, appear as clear targets for future qualitative and quantitative studies. Moreover, the occurrence of many sequences related to radiolarians (5% and 27% at surface and DCM, respectively) will require further investigation.

Air–sea interaction mechanisms and low-frequency climate variability in the South Indian Ocean region

Long-term observations indicate that the Indian Ocean displays significant low-frequency variability in mean sea-level pressure, near-surface wind, cloud and sea-surface temperature (SST). A general circulation model is used to study the response of the atmosphere to an idealized SST anomaly pattern (warm in southern mid-latitudes, cool in southern tropics) that captures the essence of observed multidecadal SST variability as well as that associated with ENSO in the South Indian Ocean. The major objectives are to investigate air–sea interaction mechanisms potentially associated with the variability and whether the atmospheric response to the SST is likely to lead to maintenance or damping of the original SST anomaly pattern, and on what time scale. Two types of experiment are performed to tackle these objectives. An ensemble of roughly 1-year-long integrations suggests that the seasonal-scale response of the atmosphere to the imposed SST anomaly includes reduced genesis and density of cyclones in the mid- to higher latitudes, and an indication of a shift in their tracks relative to climatology. It is argued that these changes together with those to the near-surface winds could be expected to lead to variations in surface fluxes that would tend to reinforce the original SST anomaly pattern on seasonal scales. A 21 year integration of the model with the SST anomaly pattern imposed throughout indicates that a low is generated near, and downstream of, the warm mid-latitude anomaly. On decadal/multidecadal scales, the associated changes to the surface winds are argued as being likely to lead to changes in surface fluxes and in the strength of the South Indian subtropical gyre that would oppose the original anomaly. The current and previous model results together with the observations then support the idea that the observed multidecadal variability in atmospheric circulation and SST of the South Indian Ocean during the past century may have arisen through a combination of basin scale atmosphere–ocean interaction and a remotely forced component. © 1998 Royal Meteorological Society

A hydrographic section across the subtropical South Indian Ocean

Features of the water-property and circulation fields at the southern limit of the continentally bounded Indian Ocean are described on the basis of a transoceanic hydrographic section occupied along roughly Lat. 32°S by the R.R.S. Charles Darwin in November-December 1987. Primary observations consisted of 106 full-depth CTD/O 2 stations with discrete measurements of the concentrations of dissolved silica, phosphate and nitrate. The section lies in the southern part of the South Indian subtropical gyre; water-property features in the upper kilometer indicate that the northward interior flow is predominantly in the eastern half of the ocean there, consistent with the forcing pattern of wind-stress curl. The southward return flow is the Agulhas Current, whose transport at Lats 31–32°S is estimated as 85 × 10 6 m 3 s −1. Circumpolar Deep Water flows northward to fill the greater deep Indian Ocean by means of western-boundary currents in the Crozet Basin, Central Indian Basin and Perth Basin. North Atlantic Deep Water entering directly from the mid-latitude South Atlantic is almost entirely confined to the south-western Indian Ocean (Mozambique Basin, Natal Valley) by the topography of the Madagascar Ridge and Mozambique Channel. Geostrophic transport figures are presented based on a zero-velocity surface constructed along the section from the tracer-property evidence of where deep water was moving northward and where southward. Ekman transport, deduced from shipboard acoustic-Doppler profiler measurements, as well as synoptic and historical wind stress data, is found to be small (about 1 × 10 6 m 3 s −1 northward). Net transport (geostrophic and Ekman) across the section is estimated to be 7 × 10 6 m 3 s −1 southward, which implies a similarly sized Indonesia throughflow. Ambiquity in the geostrophic referencing scheme, and the magnitude of baroclinic eddy noise on the section, suggest this figure in uncertain by at least ±10 × 10 6 mm 3 s −1. The calculations obtain a figure for net transport of water below 2000 dbars of 27 × 10 6 m 3 s −1 northward, which specifies an average upwelling speed at the 2-km level north of 30°S of 6.9 × 10 −5 cm s −1. This estimate, perhaps uncertain by 20–30%, nonetheless contributes to growing evidence for an anomalously vigorous meridional circulation in the Indian Ocean. The associated calculations of heat and fresh water flux divergences demonstrate that the Indian Ocean thermohaline circulation essentially expresses a conversion of bottom and deep water to mid-depth thermocline, and near-surface water.

Frontal structure, water masses, and circulation in the Crozet Basin

Due to topographic steering by the Crozet and Kerguelen Plateaus, the Antarctic Circumpolar Current (ACC) core in the Crozet Basin area is shifted to its northernmost position in the Southern Ocean, along the southern limb of the South Indian subtropical gyre. Here the jet‐like current is embedded within a narrow band (&lt; 200 km) of the frontal zone which marks a sharp transition between subtropical and subantarctic waters, hugging the northern flanks of the Crozet and Kerguelen Plateaus. Adding to this topographic control, the confluence of the ACC and the Agulhas Return Current north of Crozet further intensifies downstream baroclinic shear strengths. Eastward transport associated with the Agulhas Return Current at the entrance of the Crozet Basin at 53°E is estimated as 35 Sv (1 Sv = 106 m3/s), most of which recirculating northward into the northern part of the basin before reaching Kerguelen‐Amsterdam passage. About 75% of the ACC transport through Drake Passage pass through Kerguelen‐Amsterdam passage, the quasi‐totality of which being concentrated in the frontal zone composed of two merged fronts, the Subtropical and Subantarctic Fronts; the rest of the current passes south of Kerguelen. The Polar Front in this Crozet Basin area is not associated with any noticeable baroclinic shear and represents only a minor portion (5–7 Sv) of the ACC, in great contrast to that observed in the other sectors of the Southern Ocean. The Crozet Basin provides an important source for the formation and modification of water masses. Crozet Basin Mode Water formed locally just north of the ACC is of subtropical origin but shows significant zonal anomalies in its characteristics, with lighter mode (γθ &gt; 26.5 kg/m3) in the western half of the basin and heavier mode (γθ &lt; 26.7 kg/m3) in its eastern half. These different varieties of Crozet Basin Mode Water are closely correlated with the degree of exchange and mixing between subtropical and subantarctic waters along the dynamically active frontal zone. The area north of the Kerguelen Plateau appears to be an important source of water mass modification. Antarctic Intermediate Water advected from the west also undergoes there significant modification by intrusions across the frontal zone of fresher, oxygen‐richer, colder subantarctic water. Energetic eddy activities observed along the frontal zone likely enhance such cross‐frontal exchange of water masses. Equatorward spreading of the modified thermocline water masses (Mode Water, Antarctic Intermediate Water) occurs as part of the anticyclonic circulation in the South Indian subtropical gyre which is centered within the basin. Strong injection of North Indian Deep Water into the ACC south of Amsterdam Island is clearly evidenced for the first time. This deep oxygen minimum water centered at about 3000 m can be traced to the east of Madagascar, from where it spreads southward through the eastern part of the Crozet Basin. Due to the influence of this water mass, North Atlantic Deep Water characteristics of the Circumpolar Deep Water are significantly altered in Kerguelen‐Amsterdam passage. Our results indicate a cyclonic deep‐level circulation pattern in the Crozet Basin, with a northward flow (9 Sv) of Circumpolar Deep Water / Antarctic Bottom Water as a deep western boundary current and a partially compensating southward flow (3 Sv) of North Indian Deep Water through the eastern part of the basin. It is concluded that the Crozet Basin constitutes an important passage for the exchange of water masses between the Southern Ocean and the Indian Ocean.

Stratification and circulation at the Agulhas Retroflection

The Agulhas Retroflection, as observed in late 1983, consists of the main Retroflection at 21°E with two large warm-core Agulhas rings to its west. The Retroflection position is situated at the eastern-most limit of its range. The ring immediately to the west (Retroflection Eddy) is ellipitical in shape and carries 40 × 10 6 m 3 s −1 relative to the 1500 decibar (db) level; it's core properties are identical to the water enclosed by the Retroflection. These characteristics suggest that the Retroflection Eddy was formed just prior to the field work; thus the west edge of the Retroflection Eddy near 15°E marks the prior position of the main Retroflection, close to the western limit of its range. The second eddy (Cape Town Eddy), centered about 250 km southwest of Cape Town, is circular in form and carries 35 × 10 6 m 3 s 1 relative to 1500 db. It encircles a core of Agulhas water which was highly altered, by the action of the winter atmosphere, from the source Agulhas water. The southwest transport into the main Retroflection through the Agulhas Passage, is 70 × 10 6 m 3s −1 relative to 1500 db (95 × 10 6 m 3 s −1 relative to the sea floor). Most enters the Retroflection, but 10 × 10 6 m 3 s −1 (relative to 1500 db) continues into the South Atlantic within the region between the two eddies and the African mainland. The magnitude of this flow into the South Atlantic is increased somewhat by the inclusion of South Atlantic Water. The Agulhas Return Current, which receives most of the Agulhas Current transport, flows eastward near 40°S and then executes a “S”-shaped meander over the west half of the Agulhas Plateau to pass eastward within the southern side of the Agulhas Passage. The Agulhas Current introduces Indian Ocean water masses into the Retroflection region, though this is limited to water less dense than sigma-φ of 27.5, shallower than 1500 or 2000 m. At greater depth, the water flowing within the Retroflection circulation pattern consists primarily of Atlantic and Circumpolar Deep Water. The Indian Ocean Water entering the Retroflection is drawn from diverse sources: the thermocline of the South Indian subtropical gyre, the thermocline water of the western tropical Indian Ocean and within the lower thermocline, remanants of the high salinity-low oxygen water from the Red Sea. Once in the Retroflection region the Indian Ocean Water is altered by the local atmosphere (forming remnant winter mixed layers, marked by relatively high salinity and oxygen) and by low salinity intrusions of South Atlantic Water.