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Several damaging tropical cyclones (TCs) have occurred recently over the South Indian Ocean (SIO) region, causing enormous social and economic losses. Yet, while many studies have examined SIO TC characteristics using observations and reanalysis, only a few have assessed these characteristics specifically for this region in climate models, and fewer have investigated their projections under climate change. Here we do this for a historical (1980–2010) and future (2020–2050) period, using multimodel simulations from the High Resolution Model Intercomparison Project, as well as examine biases in the historical period relative to a reanalysis (ERA5). The models have horizontal resolutions of 25–50 km, which has enabled an improved ability to represent tropical cyclones globally in previous studies. TempestExtremes software is employed to detect tropical storm and cyclone tracks. In cases where TempestExtremes cannot be applied due to a lack of requisite variables in a dataset, we instead examine extreme wind speeds in that dataset. For the historical period, we find considerable variation in model biases compared to ERA5, which itself exhibits realistic spatial patterns of tracks and their monthly distribution. Models do at least agree on positive biases in track frequency east of Madagascar and somewhat in the Mozambique Channel. However, the models and ERA5 only produce Category 3 tropical cyclones at best. Wind speeds for 25 km resolution models have much larger positive biases than for 50 km ones, suggesting the former can simulate even higher‐category tropical cyclones. Considerable intermodel variation is also found in track changes between the future and historical periods. No systematic intercategory pattern of change exists, and low signal‐to‐noise may obscure any such patterns in the limited timespan of available data. Thus, no meaningful conclusions can be drawn regarding changes in track intensity. Nevertheless, track frequency broadly decreases across models for the region, as does accumulated cyclone energy. An east‐to‐west shift in track location from east of Madagascar toward the Mozambique Channel is also implied by track frequency and wind speed changes. Our findings provide information to potentially improve storm resiliency in this vulnerable region. [ABSTRACT FROM AUTHOR]

It has been reported that the subtropical South Indian Ocean (SIO) has been rapidly warming over the past two decades and can therefore be characterized as one of the major heat accumulators among the oceanic basins. However, this strong warming is not uniformly distributed in the vertical direction. In comparison to the decade‐long warming in the upper layer (0–300 m) in 2004–2013, the intermediate layer (300–1,000 m) displays a shorter warming during 2004–2009 and an intense cooling during 2010–2016. By decomposing temperature variations into heaving and spice components, and performing a heat budget analysis, we show that temperature variations in the intermediate layer during these two periods are primarily contributed by isopycnal migrations driven by local wind forcing. Local wind change in the subtropical SIO can be explained by the Indian Ocean Dipole and El Niño–Southern Oscillation during 2004–2016, while Southern Annular Mode (SAM) favors anomalous wind change in mid‐latitudes and the formation of basin‐wide wind change in the SIO. Additionally, wind forcing in the Subantarctic Mode Water (SAMW) formation region, which is closely linked to the SAM, modulates the anomalous spreading of SAMW into the interior of the subtropical SIO. This, therefore, leads to the SAMW intrusion being of secondary importance to the quasi‐decadal temperature variability. Our findings demonstrate the independence of wind‐driven temperature changes on the quasi‐decadal scale in the intermediate layer of the subtropical SIO under the overall warming background of SIO waters. Plain Language Summary: Compared to the decade‐long warming in the upper layer of the South Indian Ocean (SIO), which has been studied extensively, our understanding of temperature change in the intermediate layer is relatively limited. This study reveals a quasi‐decadal temperature cycle in the intermediate layer of the subtropical SIO during the Argo period, which is characterized by a shorter warming period during 2004–2009 and subsequent cooling during 2010–2016. Decomposition of temperature changes suggests that this quasi‐decadal temperature variability is primarily driven by the heaving component, which is tightly associated with local wind variability driven by local and remote forcings, whereas the spice change largely contributed by the SAM‐related water mass transmission from higher latitudes, is of secondary importance. Thus, this study expands our knowledge of temperature variability in the SIO and demonstrates that the quasi‐decadal variability of intermediate layer temperatures in the subtropical SIO serves as a crucial archive for both global and local climate change. Key Points: Quasi‐decadal temperature variations occur in the intermediate layer (300–1,000 m) of subtropical South Indian Ocean (SIO)Local wind‐driven heaving process is the major driver, spice component arising from the Subantarctic Mode Water intrusion is of secondary importanceThe local wind change in the subtropical SIO can be well explained by the combined effects of El Niño–Southern Oscillation, Indian Ocean Dipole and Southern Annular Mode [ABSTRACT FROM AUTHOR]

This study investigates the influence of the Southern Annular Mode (SAM) on chlorophyll- a (Chl- a ) concentrations and the underlying mechanisms governing their associated environmental variations in the mid-latitude (35–50° S) ocean from 1998 to 2021. The intensification of westerly winds during positive SAM phases influences meridional water transport and mixed layer depth (MLD), which are both critical factors that affect surface nutrient availability. A marked contrast in the relationship between the meridional current anomaly and the SAM was observed, with reduced northward transport of nutrient-rich water in regions north of 50° S during positive SAM phases. This reduction could be attributed to the poleward migration of the westerly winds, which impeded the meridional current from reaching the mid-latitudes. The relationship between SAM and MLD south of 50° S was positive whereas that in the mid-latitude eastern (60–110° E) South Indian Ocean and eastern (90–140° W) South Pacific Ocean was negative or weak. The immediate effect of a more positive SAM on Chl- a in the mid-latitude ocean was reduced productivity caused by enhanced nutrient depletion. However, in the mid-latitude eastern South Pacific Ocean, the northward migration of the zonal mean meridional current anomaly closely aligned with the lagged correlation pattern between SAM variability and Chl- a over time, suggesting that the delayed northward transport of nutrient-rich waters may partially counterbalance the immediate effects of the SAM on ocean productivity. This mechanism was not present in the mid-latitude eastern South Indian Ocean, implying that future climate change may variably affect these regions. Our findings emphasize the importance of considering regional differences and temporal lags when evaluating the influence of SAM variability on ocean productivity and nutrient dynamics in the context of climate change.

This study provides a quantitative assessment of steric changes associated with sea level rise in the upper (0–500 m) South Indian Ocean (SIO) during 1993–2017, using the latest ocean state estimate of Estimating the Circulation and Climate of the Ocean (ECCO) combined with in‐situ observations from Argo. Both the observations and ECCO estimate show a sea level rise in the low‐latitude (0°–30°S) SIO that is faster than its South Pacific and South Atlantic counterparts by a factor at least two. Much of this fast sea level rise is due to warming and freshening of the upper ocean, with no significant contribution from the deeper layers (>500 m). We emphasize the importance of halosteric effect, whose contribution to sea level rise is comparable with that of thermosteric effect in the eastern basin. On interannual time scale, up to 90% of the region's sea level variability can be explained by steric changes that in turn are dominated by upper ocean convergence. Plain Language Summary: Sea level has been rising steadily in the low‐latitude South Indian Ocean (SIO), roughly, between 0° and 30°S, at a rate about two time faster than its South Pacific and South Atlantic counterparts during 1993–2017. This fast sea level rise is largely due to warming and freshening of the upper ocean (0–500 m) that is observed only in the low‐latitude SIO. As is the case over much of the global ocean, thermal expansion has the dominant contribution to sea level rise, except in the eastern basin, where contribution from salinity changes is of comparable strength. The upper SIO has also experienced a large interannual variability in heat and freshwater content that directly contribute to the region's sea level variability. Much of this variability is due to water convergence of the upper ocean, while contribution from surface heat and freshwater fluxes and small‐scale processes is relatively small. Key Points: Sea level in the low‐latitude South Indian Ocean has been rising twice as fast as its South Pacific and South Atlantic counterpartsMuch of this fast sea level rise is due to warming and freshening of the upper ocean, with little contribution from the deeper layersThe region's interannual sea level variability is dominated by steric changes associated with water convergence of the upper ocean [ABSTRACT FROM AUTHOR]

The 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. Plain Language Summary: The ocean surface is saltiest in the mid‐latitude subtropics where evaporation is strong and precipitation is weak. Each ocean basin has such a region, centered 20°–30° from the equator. These regions are sensitive indicators of a balance between the effects of the wind, evaporation, precipitation, and ocean currents. In this study, we focus on this salty region of the South Indian Ocean. It is centered around 30°S, 90°E, west of the coast of Australia. We have found variations in the size, saltiness, and position of the feature. It shifts over a northeast‐southwest path on a year‐to‐year basis and gets saltier and fresher at the same time. Our conclusion is that this is largely a result of changes in the wind field and the shape and strength of the underlying ocean subtropical gyre. The motion we see is synchronized with similar motion of the analogs feature in the South Pacific, bringing up the likelihood of more global processes controlling both features. Key Points: We have studied the seasonal and interannual variability in the size and position of the South Indian Ocean sea surface salinity maximumThe feature moves on a northeast‐southwest track, with correlated changes in sizeVariability seems to be associated with changes in gyre‐scale flows and wind forcing, not surface freshwater input/output [ABSTRACT FROM AUTHOR]

The positive phase of the subtropical Indian Ocean dipole (SIOD) is one of the climatic modes in the subtropical southern Indian Ocean that influences the austral summer inter-annual rainfall variability in parts of southern Africa. This paper examines austral summer rain-bearing circulation types (CTs) in Africa south of the equator that are related to the positive SIOD and the dynamics through which specific rainfall regions in southern Africa can be influenced by this relationship. Four austral summer rain-bearing CTs were obtained. Among the four CTs, the CT that featured (i) enhanced cyclonic activity in the southwest Indian Ocean; (ii) positive widespread rainfall anomaly in the southwest Indian Ocean; and (iii) low-level convergence of moisture fluxes from the tropical South Atlantic Ocean, tropical Indian Ocean, and the southwest Indian Ocean, over the south-central landmass of Africa, was found to be related to the positive SIOD climatic mode. The relationship also implies that positive SIOD can be expected to increase the amplitude and frequency of occurrence of the aforementioned CT. The linkage between the CT related to the positive SIOD and austral summer homogeneous regions of rainfall anomalies in Africa south of the equator showed that it is the principal CT that is related to the inter-annual rainfall variability of the south-central regions of Africa, where the SIOD is already known to significantly influence its rainfall variability. Hence, through the large-scale patterns of atmospheric circulation associated with the CT, the SIOD can influence the spatial distribution and intensity of rainfall over the preferred landmass through enhanced moisture convergence. [ABSTRACT FROM AUTHOR]

Abstract This study provides a quantitative assessment of steric changes associated with sea level rise in the upper (0–500 m) South Indian Ocean (SIO) during 1993–2017, using the latest ocean state estimate of Estimating the Circulation and Climate of the Ocean (ECCO) combined with in‐situ observations from Argo. Both the observations and ECCO estimate show a sea level rise in the low‐latitude (0°–30°S) SIO that is faster than its South Pacific and South Atlantic counterparts by a factor at least two. Much of this fast sea level rise is due to warming and freshening of the upper ocean, with no significant contribution from the deeper layers (>500 m). We emphasize the importance of halosteric effect, whose contribution to sea level rise is comparable with that of thermosteric effect in the eastern basin. On interannual time scale, up to 90% of the region's sea level variability can be explained by steric changes that in turn are dominated by upper ocean convergence.

Despite the rapid warming in the south Indian Ocean (SIO) during the recent two decades, the upper 300 m of the tropical southwest Indian Ocean (SWIO) experienced significant cooling during 2005–2011. The decreasing rate of the observed temperature reached −0.50 ± 0.1°C/decade. Heat budget analysis reveals that the cooling trend over SWIO results from the competing effect between local air‐sea heat flux and horizontal heat advection. Moreover, all the three observational datasets indicate that in the subtropical SIO, a prolonged cooling event occurred beneath the thermocline during 2010–2016. Although there exist some discrepancies between the observations and estimating the circulation and climate of the ocean (ECCO) reanalysis before 2010, the observations and ECCO reanalysis agree in terms of the post–2010 temperature variability in the intermediate layer of the subtropical region, probably because of the better constraint of the ECCO assimilation product by the increased sampling of Argo floats in the deeper ocean. The heat budget analysis indicates that during the period 2010–2016, the decadal cooling in the 400–1,000 m layer is primarily modulated by vertical ocean advection, and the horizontal ocean advection makes a secondary contribution. This study expands our knowledge of the decadal temperature variability and heat balance in the SIO during the Argo era. Plain Language Summary: The south Indian Ocean (SIO) has been suggested to be one of the major heat accumulators owing to its strong warming during the past two decades. Nevertheless, the temperature increase is not evenly distributed but occurred along with two significant decadal cooling events in the SIO. First, we found that compared with the remarkable warming in the upper layer of the southeast Indian Ocean during the recent global surface warming slowdown, the upper layer of the tropical southwest Indian Ocean undergoes a cooling trend during 2005–2011. This cooling trend is jointly forced by latent heat flux and horizontal heat advection associated with the changes in the surface atmosphere circulation. Second, the observational datasets suggest that the intermediate layer of the subtropical SIO displays significant cooling during the period 2006–2016. This cooling trend was stronger in the second half stage (2010–2016) mainly because of the upward motion of relatively colder water from the deeper ocean. The present study focuses on the cooling events in the SIO during the Argo era, the complexity of mechanisms that drive the temperature changes in the SIO, and the importance of more reliable observations in the intermediate and deeper ocean. Key Points: Two decadal cooling events occurred in the south Indian Ocean during the Argo eraAir‐sea heat flux and ocean heat advection jointly induce the upper‐ocean cooling in the tropical southwest Indian Ocean during 2005–2011Upwelling of colder water from the deeper ocean dominates the cooling in the intermediate layer of subtropical region during 2010–2016 [ABSTRACT FROM AUTHOR]

• Indonesian Throughflow waters condition the extent of the surface plankton bloom of the Seychelles Chagos Thermocline Ridge. • Strong inputs of Indonesian Throughflow waters suppress surface phytoplankton, reducing the surface chl-a concentrations. • The westward propagation of Indonesian Throughflow waters are closely associated with downwelling Rossby waves. The Seychelles-Chagos Thermocline Ridge (SCTR) is a biologically important region of open ocean upwelling within the south west Indian Ocean (5–10°S and 45–90°E), driven by the tropical gyre. The SCTR refers to an elongated feature that joins two local minima in thermocline depth; the Seychelles Dome (SD) and Chagos Dome (CD). Entering the ocean basin from the east, the Indonesian Throughflow (ITF) has been shown to interact with the upwelling region, although the relationship between the phytoplankton bloom associated with the SCTR and the ITF are so far unexplored. Using in situ observations and remotely sensed data, the buoyancy fluxes from the ITF are shown to strongly condition surface chlorophyll-a (chl-a) concentrations over the Chagos Dome, the eastern extreme of the SCTR, at seasonal and interannual scales. Accordingly, we find a significant inverse correlation (r = −0.43) between the altimeter-derived volume transport of the ITF and the surface chl-a concentrations. This inverse correlation increases (r = −0.61) when only the 10th and 90th percentile of the ITF volume transport anomalies are considered, indicating the influence of the ITF may be overcoming other physical drivers, especially under extreme ITF events. We hypothesise that the buoyancy flux of a strong ITF input 'caps' the Chagos Dome with warm, less saline waters, suppressing surface phytoplankton and reducing the surface chl-a concentrations. This hypothesis is supported by a strong, significant correlation (r = 0.66) between remotely sensed surface salinity and surface chl-a over the region. This relationship is not found over the Seychelles Dome, where the ITF has a weaker direct impact over the bloom. These results suggest that the westward travel of ITF waters may condition the eastward expansion of the SCTR and, therefore, the zonal extent of the associated chl-a bloom. This happens at seasonal and interannual time-scales concomitantly with the propagation of downwelling Rossby waves, deepening the thermocline and facilitating the westward advance of ITF waters. This is visible through a combination of remotely sensed and in situ observations at depth from the RAMA mooring array at the eastern domain of the SCTR, where intrusions of warm, less saline waters, typical of ITF waters, coincide with downwelling Rossby waves deepening the thermocline. Thus, both the westward travel of ITF waters and the propagation of downwelling Rossby waves shape the eastward expansion of the SCTR and, therefore, the zonal extent of the associated surface chl-a bloom on a year-to-year basis. [ABSTRACT FROM AUTHOR]

Spiciness anomalies generated in the salinity maxima region are important for several atmospheric and oceanic factors as they move along the geostrophic pathways towards the equator and resurface. Subduction and injection mechanisms are responsible for the spiciness generation in the South Indian Ocean (SIO) salinity maxima region. Using ECCO data from 1992 to 2017, here we study monthly variations of spiciness associated with both of these mechanisms in the SIO salinity maxima region. Using a Lagrangian approach, we estimated the monthly evolution of the subduction rate. A maximum subduction rate of 35–38 m/mon occurs during September and consequently decreases towards the end of the year. The effective subduction rate in the salinity maxima region shows the dominance of temporal induction (mixed layer tendency) term, with a sharp gradient in total subduction rate along the 30∘S associated with large mixed layer depth variation. Further, a high Turner angle (> 66∘) to the south of 30∘S confirms the generation of spiciness by injection mechanism. We found that the decrease in mixed layer salt (MLS) coincides with the increase in salinity below the mixed layer. To explore the significance of MLS changes in spiciness generation, we further addressed the monthly evolution of spiciness through MLS budget. Our results suggest that the entrainment and meridional advection terms are key to monthly variations in MLS changes and thus the spiciness. [ABSTRACT FROM AUTHOR]

In the South Indian Ocean (SIO), high surface chlorophyll concentrations are often observed in mesoscale anticyclonic eddy cores. Yet the role of submesoscale dynamics in modulating phytoplankton distributions remains largely unknown in the region. By using a biogeochemical‐Argo float and a high‐resolution model, we show that elevated chlorophyll concentrations were evident in a frontal region, and the depth‐integrated chlorophyll in this frontal region can be higher than that in the anticyclonic eddy core. Submesoscale frontogenesis was shown to inject nutrients vertically and enhance phytoplankton growth in the frontal region. Frontal dynamics tend to facilitate large phytoplankton such as diatom growth, which consequently increases diatom fraction. Over the entire central SIO, we found a significant correlation between the phytoplankton composition index and frontogenesis. However, the correlation between total phytoplankton biomass and frontogenesis was not significant. These results demonstrate the complex role of frontal dynamics in structuring phytoplankton distributions in the central SIO. Plain Language Summary: Mesoscale eddies and submesoscale processes are widely distributed in global oceans and are often dynamically connected. In the central South Indian Ocean (SIO), mesoscale anticyclonic eddies have been suggested to contribute greatly to the observed high concentration of surface phytoplankton chlorophyll. Yet the influence of submesoscale processes on phytoplankton distribution has not been fully studied. We used a biogeochemical‐Argo profiling float and a high‐resolution ocean model to evaluate this influence. We show that the chlorophyll in the submesoscale frontal region can be higher than that in the anticyclonic eddy core. A frontal region is associated with vertical nutrient supply, which favors phytoplankton growth, especially for large phytoplankton. Thus, for the entire central SIO, submesoscale processes play an important role in structuring phytoplankton distributions Key Points: Localized chlorophyll enhancement and submesoscale structures in a frontal region were captured by a biogeochemical‐Argo float in the central SIOHigh‐resolution simulations revealed that frontogenesis leads to nutrient injections and induces high chlorophyll in the frontal regionAt the basin scale, a significant correlation between frontogenesis and the phytoplankton composition index was found in the central SIO [ABSTRACT FROM AUTHOR]

The Agulhas retroflection (AR) region possesses the highest eddy kinetic energy (EKE) level in the Indian Ocean. However, mechanisms regulating EKE of the AR remain uncertain. Here, by analyzing an eddy-resolving coupled model simulation with improved EKE representation, we show that the upper-ocean EKE of the AR is mainly generated through barotropic instability in its upstream and leakage zones and is by nonlocal transport in its downstream zone. The interaction between mesoscale eddies and local winds plays a key role in EKE dissipation. The lack of eddy-wind interaction results in flawed EKE budget in the leakage zone in ocean-alone models, leading to severe biases in EKE distribution with overestimation and over-strong penetration into the South Atlantic. Our results highlight the essence of mesoscale air-sea interaction in the dynamics of the AR, with implications for understanding the inter-basin transport of the Agulhas leakage.

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This study assesses the capability of 12 models in the Phase 6 of the Coupled Models Intercomparison Project (CMIP6) in simulating the Southeast Indian Subantarctic mode water (SEISAMW) by comparing to Argo observations. The results show that all of the analyzed CMIP6 coupled models can reproduce the SEISAMW and its seasonal cycle, albeit with discrepancies of formation region and properties among the models. The SEISAMW subduction rate shows significant interannual variability, which is primarily induced by lateral induction in these models, in agreement with observations. Furthermore, the longterm trends of the SEISAMW show volume loss in accordance with the descending trend of the subduction rate, which are co‐occurred with the ascending trend of the Southern Annular Mode (SAM) indices in most of the analyzed CMIP6 models, similar to those in the CMIP3 and CMIP5 coupled models. Meanwhile, the potential density of the SEISAMWs show decreasing trends in these volume loss models, which could be explained by the warming (SAM0‐UNICON, CESM2‐WACCM, CAS‐ESM2‐0 and CIESM), the freshening (IPSL‐CM6A‐LR, CAMS‐CSM1‐0, MRI‐ESM2‐0, and FGOALS‐f3‐L) or the both (FIO‐ESM‐2‐0, E3SM‐1‐0, and CanESM5) trends of the SEISAMWs in different CMIP6 coupled models. Decreases in the projected subduction rate and volume of SEISAMW imply a slowdown of Southern Indian Ocean circulation in the future, reducing the heat and carbon transport from atmosphere to ocean interior contributed by SEISAMW. Plain Language Summary: Subantarctic mode water (SAMW) is formed between the subantarctic front and the subtropical front. The SAMW that formed in the South Indian Ocean is called Southeast Indian Subantarctic Mode Water (SEISAMW). When it forms, seawater at the base of mixed layer subducted into the ocean interior in late winter, providing subsurface oceanic reservoirs of heat, carbon, and other properties that can significantly affect the global climate. Using data from the historical simulation in 12 CMIP6 coupled models and Argo; we assess the capability of these models in simulating SEISAMW and reveal the interannual and longterm trend in SEISAMW properties (e.g., subduction rate, volume, temperature, salinity, and density). The results indicate that all of the analyzed CMIP6 models are able to reproduce the SEISAMW and its seasonal cycle. The interannual variability of the SEISAMW is attributed to lateral induction in all of the analyzed CMIP6 models, in agreement with observations. Both the subduction rate and volume of the SEISAMW show decreasing trends, which are co‐occurred with enhanced Southern Annular Mode (SAM), in most of the CMIP6 historical simulations. The decrease of subducted SEISAMW implies a slowdown of the Southern Ocean circulation under the anthropogenic forcing, due to surface warming and freshening. Key Points: The analyzed Coupled Models Intercomparison Project Phase 6 (CMIP6) models reproduce Southeast Indian subantarctic mode water (SEISAMW) and its seasonal cycle, albeit with inter‐model differencesThe simulated SEISAMWs show significant interannual variability dominated by lateral induction, consistent with observationsThe simulated SEISAMWs show volume loss during the period of the historical simulation in most of the analyzed CMIP6 models [ABSTRACT FROM AUTHOR]

In the past three decades, altimeter-based remote sensing has been a widely used system to estimate ocean surface currents. However, it remains a great challenge to effectively resolve scales below ∼100 km at high latitudes and ∼ 300 km at mid-latitudes. In this study, we propose a scheme that utilizes geostrophic equilibrium and surface quasigeostrophy theory (SQG) to improve surface current resolution by incorporating remote sensing sea surface temperature (SST), sea surface height (SSH), and sea surface salinity (SSS) observations. The scheme separately characterizes the larger-scale flows and smaller-scale motions of surface currents. A case study encompassing the Agulhas surface current demonstrates that the smaller-scale motions associated with temperature fronts are well captured by introducing high spatial-temporal resolution SST data. Furthermore, the reconstructed surface current is systemically evaluated by using surface drogued drifters and a Lagrangian synthetic particle tracking tool throughout the South Indian Ocean (SIO) for 2011–2015. Notably, the reconstructed zonal velocity component is closer to the drifter observations than the meridional counterpart and corresponding velocity phase. Regionally, the Antarctic Circumpolar Current (ACC) showcases superior reconstruction performance, with higher skill scores and lower Lagrangian separation distances. However, a relatively large uncertainty is observed around the Agulhas Retroflection (AR) and Greater Agulhas System (GAS), which are linked to complicated regional dynamic regimes. We finally conduct four simulation experiments to explore the effect of different SST products on surface current reconstruction within the subdomain AR. The results indicate the varying potentials of the four evaluated SST products for informing surface current applications. Specifically, the MWIRSST enhances the likelihood of particles reaching the target field, while DMI OI shortens the average deviation distance of the arrived particles. • A new scheme estimating the surface current is proposed based on the SQG theory. • Small-scale motions are resolved by introducing high-resolution SST. • Uncertainty of the estimated currents partially depends on different SST products. [ABSTRACT FROM AUTHOR]

The destructive nature of tropical cyclones poses a risk to the regions where they occur during and after landfall. Southern hemisphere countries are usually more vulnerable to these risks and impacts due to their lower adaptive capacity. Research for the northern hemisphere has comprehensively explored the nature of tropical cyclones. This information has since been applied to long‐term adaptation plans and short‐term evacuation plans. This study interrogates the characteristics and nature of Southern Hemisphere tropical cyclones and critically explores their differences to Northern Hemisphere storm characteristics. The differences found between the hemispheric storm activities include seasonality, number of storms and the latitudinal position at which these storms undergo genesis and intensification. Similarities were found in storm intensity, which was observed to increase with the time taken to reach maximum lifetime intensity. Higher sea surface temperatures were similarly found to be a major factor associated with increased intensity and lifetime of Southern Hemisphere tropical cyclones. Tropical cyclone seasonality is also centred around the warmer months within each hemisphere. Compared to the Northern Hemisphere the tropical cyclone seasonal length indicates no increase. The magnitude of sea surface temperature increase is also lower in the Northern Hemisphere. These fundamental links observed with temperature in both hemispheres implicates climate warming and its impact on tropical cyclones in both hemispheres, regardless of their seasonality and frequency. [ABSTRACT FROM AUTHOR]

Single-stranded (ss) DNA viruses are ubiquitous and constitute some of the most diverse entities on Earth. Most studies have focused on ssDNA viruses from terrestrial environments resulting in a significant deficit in benthic ecosystems including aphotic zones of the South Indian Ocean (SIO). Here, we assess the diversity and phylogeny of ssDNA in deep waters of the SIO using a combination of established viral taxonomy tools and a Hidden Markov Model based approach. Replication initiator protein-associated (Rep) phylogenetic reconstruction and sequence similarity networks were used to show that the SIO hosts divergent and as yet unknown circular Rep-encoding ssDNA viruses. Several sequences appear to represent entirely novel families, expanding the repertoire of known ssDNA viruses. Results suggest that a small proportion of these viruses may be circular genetic elements, which may strongly influence the diversity of both eukaryotes and prokaryotes in the SIO. Taken together, our data show that the SIO harbours a diverse assortment of previously unknown ssDNA viruses. Due to their potential to infect a variety of hosts, these viruses may be crucial for marine nutrient recycling through their influence of the biological carbon pump., Competing Interests: All authors declare that they have no competing interests., (© The Author(s) 2024. Published by Oxford University Press on behalf of the International Society for Microbial Ecology.)

Salinity of the Indian Ocean Subtropical Underwater (IOSTUW) showed robust decadal variability during the Argo period, with a freshening trend between 2005 and 2013 and a salinification trend between 2014 and 2019. The salinity variability of the IOSTUW originated from both the sea surface salinity maximum region in the subtropical South Indian Ocean and the southeastern tropical Indian Ocean. The mixed layer salinity variabilities in these two regions are dominated by horizontal advection, which is mainly influenced by anomalous local surface currents and salinity anomalies from the western tropical Pacific and Maritime Continent. Both of these two factors have a close relationship with the El Niño‐Southern Oscillation (ENSO) signal. As a result, the mixed layer salinity in those two regions changed dramatically during the strong and long‐lasting ENSO events and thus led to the decadal variability in salinity of the IOSTUW. Plain Language Summary: The Indian Ocean Subtropical Underwater (IOSTUW) is identified as a shallow subsurface salinity maximum within the upper 400 m in the subtropical South Indian Ocean. It is an essential component of freshwater exchanges between the western Pacific, the Indian Ocean, and the South Atlantic. The variability in salinity of the IOSTUW has a close relationship with the global hydrological cycle and is of crucial significance to regional sea‐level change. Here we provide the first observation‐based estimate of the decadal variability in salinity of the IOSTUW and the relevant influencing factors. Our results suggest that the IOSTUW shows robust decadal variability with a freshening trend during 2005–2013 and a salinification trend during 2014–2019. This signal originates from the sea surface salinity maximum region in the subtropical south Indian Ocean and the southeast tropical Indian Ocean. It is mainly modulated by horizontal advection, associated with the advection of salinity anomalies from the western tropical Pacific and Maritime Continent and anomalous local surface currents, both having a close relationship with the El Niño‐Southern Oscillation (ENSO) signal. Key Points: Salinity of the Indian Ocean Subtropical Underwater showed robust decadal variability during 2005–2019This signal resulted from dramatic changes of mixed layer salinity in southeast Indian Ocean during strong and long‐lasting ENSO eventsContributing factors include anomalous local surface currents and anomalies from the western tropical Pacific and Maritime Continent [ABSTRACT FROM AUTHOR]

An analysis of Argo data reveals that the subantarctic mode water (SAMW) in the South Indian Ocean (SIO), characterized by a vertical potential vorticity (PV) minimum, decreases by 10% in volume from 2004 to 2018. Most of this decrease occurs at the 26.8–26.9 kg m−3 density range which forms southwest of Australia, while a slight volume increase occurs at 26.6–26.8 kg m−3. Further analysis indicates that the weakening of the Mascarene High and westerly winds in the SIO reduces the evaporation‐precipitation, surface heat flux, and Ekman pumping and shoals the mixed layer southwest of Australia, which leads to a volume decrease at 26.8–26.9 kg m−3 in approximately 3 years. West of 90°E, the parameters exhibit the opposite change, leading to a volume increase at 26.6–26.8 kg m−3. This result suggests that surface winds play an important role in the variability of the SIO SAMW volume during the Argo period. Plain Language Summary: During austral winter in the subantarctic zone, a water mass called the subantarctic mode water with homogeneous properties (temperature, salinity) forms, sinks, and moves to the interior of the ocean. The formation and transport of the mode water are crucial for the climate, as the water takes up heat, freshwater, carbon, oxygen, and nutrients into the ocean interior. We find a distinct volume loss of the mode water in the South Indian Ocean from 2004 to 2018. Although this mode water forms in broad density ranges in different regions, the volume loss only occurs in the dense mode water south of Australia. Moreover, the volume of the light mode water, which forms in the center of the South Indian Ocean, slightly increases. We find that surface winds control the surface heat flux and water convergence, which determine the volume change here. These winds tend to vary oppositely in the two different mode water formation regions and make the volume vary in opposition phases. This study reveals the layer dependence of the mode water volume variation and reinforces the importance of atmospheric forcing, which would possibly help interpret the heat uptake in the Southern Ocean. Key Points: Argo data reveal a subantarctic mode water volume loss in the South Indian Ocean over the period 2004–2018Most of the volume loss occurs in the density range of 26.8–26.9 kg m−3, while a volume increase occurs at 26.6–26.8 kg m−3Changes in the SAMW volume are controlled by surface forcing, which is closely related to the Mascarene High variation [ABSTRACT FROM AUTHOR]

Two oppositely rotating mesoscale eddies, originating from the Leeuwin Current, were captured during a research cruise in the interior southeast Indian Ocean in August 2012, a few hundred kilometers off the west coast of Australia. The two eddies had different vertical structures: the anticyclonic eddy had stronger vertical shear above 200 m and a less stratified thermocline below the mixed layer, while the cyclonic eddy had a more barotropic structure in the upper 500 m. Compared with the same types of eddies sampled in 2003 close to the Australian coast, the 2012 anticyclonic eddy had a larger radius, deeper isothermal depths, and higher peak velocity; whereas the 2012 cyclonic eddy had a similar structure with the 2003 survey. We use an Argo float and three surface drifters deployed and trapped in the anticyclonic eddy to track its evolution over a 4-month period. The eddy was observed to propagate 1000 km further to the west, indicating that the Leeuwin Current properties of the eddy may be preserved for over 2000 km off the west coast of Australia. The eddy appeared to have experienced radial excursions relative to the eddy center, with periodicity of 4–6 days and 15 days, during their anticlockwise motions. The radial movements of the drifters appeared to be constrained by the radial momentum balance within the eddy. The importance of eddy distortions to ocean primary production in anticyclonic eddies in the southeast Indian Ocean warrants further studies. [ABSTRACT FROM AUTHOR]

South Indian Ocean Rossby waves (SIO-RW) are identified in the Global Ocean Data Assimilation System (GODAS) 1.5-7 yr filtered sea surface height (SSH) time series. There is a persistent three-year oscillation in the 5°-15°S latitude band from 55° to 85°E. Field correlations show little coupling at 90°E, but as the SIO-RW undulates westward at approximately 0.19 m s−1 across the mid-basin, a northwest-southeast axis of warm sea surface temperatures (SSTs) and deep convection forms. Many teleconnections in earlier work are confirmed: interannual pulses of zonal wind in the eastern basin trigger the SIO-RW via anticyclonic wind stress curl. New insights derive from an understanding of links with the upper troposphere. As the SIO-RWs move westward with the onset of an El Niño in the Pacific, increased convection over the north Indian Ocean corresponds to reduced evaporation and SST warming. Mid-tropospheric heating T′ > 2°C over the northwest Indian Ocean accelerates the southern sub-tropical jet to greater than 10 m s−1 over the southeast Indian Ocean, reinforcing the anticyclonic vorticity. The downstream acceleration of the jet generates upper-level divergence and moist convection over the western basin, anchoring an atmospheric Rossby wave in a northwest-southeast alignment underpinned by differential propagation of the SIO-RW. As the ocean Rossby wave reaches Africa, the coupling fades and transitions. What distinguishes Indian Ocean from Pacific Ocean Rossby waves are their southern latitude and higher frequency. The tropical mid-tropospheric heating that accelerates the southern sub-tropical jet shifts westward in tandem with the SIO-RW. [ABSTRACT FROM AUTHOR]

Globally, subtropical circulation in the lower troposphere is characterized by anticyclones over the oceans. Subtropical anticyclones locate over subtropical belts, modulating weather and climate patterns in those regions. The Mascarene High is an anticyclone located over the Southern Indian Ocean and has a vital role in weather and climate variability over Southern Africa. The warm Western Indian Ocean is a major source of moisture for the subcontinent also permitting tropical cyclone genesis. In this study, we review the dynamics of the Mascarene High, its interactions with the ocean, and its impact on weather and climate over Southern Africa. We also review studies on the evolution of subtropical anticyclones in a future warmer climate. The links between SST modes over the Indian Ocean and the strengthening and weakening of the Mascarene High have been demonstrated. One important aspect is atmospheric blocking due to the Mascarene High, which leads to anomalous rainfall and temperature events over the subcontinent. Blocking leads to landfall of tropical cyclones and slow propagation of cut-off lows resulting in severe weather and flooding over the subcontinent. Understanding how expansion of the Mascarene High due to warming will alter trade winds and storm tracks and change the mean climate of Southern Africa is crucial.

Abstract: South Indian Ocean eddies (SIDDIES), originating from a high evaporation region in the eastern Indian Ocean, are investigated by tracking individual eddies from satellite data and co‐located Argo floats. A subsurface‐eddy identification method, based on its steric dynamic height anomaly, is devised to assign Argo profiles to surface eddies (surfSIDDIES) or subsurface eddies (subSIDDIES). These westward‐propagating, long‐lived features (>3 months) prevail over a preferential latitudinal band, forming a permanent structure linking the eastern to the western Indian Ocean, that we call the 'SIDDIES Corridor’. Key features have been revealed in the mean thermohaline vertical structure of these eddies. Anticyclonic SIDDIES are characterized by positive subsurface salinity anomalies, with subSIDDIES not exhibiting negative surface anomalies, as opposed to surfSIDDIES. Cyclonic subSIDDIES also occur, but their related salinity anomalies are weaker. SubSIDDIES exhibit two cores of different temperature polarities in their surface and subsurface levels. Cyclonic subSIDDIES have their cores at around 150‐200 m depth along the 25.4‐25.8 kg m−3 potential density layer with anticyclonic subSIDDIES having their cores at 250‐300 m along the 26‐26.4 kg m−3 density layer. The SIDDIES corridor acts as a zonal pathway for both eddy‐types to advect water masses and biogeochemical properties across the basin. This study provides a new insight on heat/salt fluxes, showing that 58% (32%) of the total heat eddy‐flux is ascribed to cyclonic (anticyclonic) subSIDDIES, respectively, in the eastern South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole high‐saline water eddy‐conveyor toward the western Indian Ocean. [ABSTRACT FROM AUTHOR]

Over recent decades, a strong subsurface cooling trend in the South Indian Ocean (SIO) occurred, despite a continuous sea surface warming. Previous studies suggest this long-term (around 1960–2000) cooling trend is mainly driven by remote Pacific atmospheric forcing or local Indian Ocean (IO) forcing. This study reveals that the dominant driver of the SIO subsurface cooling trend in different periods is closely related to the phase of Pacific Decadal Oscillation (PDO). Our results suggest that the local IO wind forcing is responsible for the majority of the subsurface cooling trend and overwhelms a weak warming trend induced by the remote tropical Pacific wind forcing during the negative-phase period of PDO during 1960–76. However, this situation reverses during the PDO positive-phase period during 1977–98. Our analysis suggests that the PDO strengthens/weakens the tropical Pacific trade winds during negative/positive phase periods. Furthermore, the multi-decadal variations in the western Pacific induced by PDO impact the SIO subsurface temperature via baroclinic Rossby waves.

Based on observational and reanalysis data for 1979-2015, the possible impacts of spring sea surface temperature anomalies (SSTA) over the South Indian Ocean on the inter-annual variations of summer rainfall in Guangdong and Guangxi Provinces (i.e., the Guangdong-Guangxi area, GG) were analysed in this study. The physical mechanism behind these impacts was explored. Two geographic regions over [65°E-95°E, 35°S-25°S] and [90°E-110°E, 20°S-5°S] were defined as the western pole region and the eastern pole region, respectively, for the GG summer precipitation (P)-related South Indian Ocean dipole SSTA pattern (R-SIODP). The difference between springtime SST anomalies averaged over the western pole region and that averaged over the eastern pole region was defined as the R-SIODP index. The correlation between the spring R-SIODP index and GG summer precipitation can reach up to 0.52. In the spring of positive R-SIODP anomaly, southerly winds over the western pole of the R-SIODP weaken, whereas the southeast trade winds over the eastern pole strengthen. By means of the wind-evaporation-SST feedback mechanism, the enhanced southeast trade winds can weaken the evaporation over the western pole of the R-SIODP and enhance the evaporation over the eastern pole. This results in a sustained positive SSTA in the western pole of the R-SIODP and a sustained negative SSTA in the eastern pole, whereby the distribution of the SSTAs maintains until summer. The SST dipole abnormally enhances the cross-equatorial airflow near 105°E, which intensifies the anomalous anti-cyclonic circulation over South China Sea at 850 hPa and simultaneously results in abnormal enhancement of water vapour transport to GG. Additionally, the SST dipole promotes abnormal divergence in the lower troposphere and abnormal convergence in the upper troposphere over the maritime continent (MC) region. Moreover, the low-level convergence in GG is enhanced, which results in abnormal enhancement of ascending motion in the GG that is conducive to positive summer rainfall anomaly in this region. In this study, the spring R-SIODP index, the SST to the east of Australia and to the east of southern Africa, and the North Atlantic oscillation (NAO) index were used to construct a statistical prediction model for the inter-annual variability of the GG summer rainfall anomaly. This model can well predict the accuracy of the inter-annual variation of GG summer rainfall. [ABSTRACT FROM AUTHOR]

Based on the merged satellite altimeter data and in- situ observations, as well as a diagnosis of linear baroclinic Rossby wave solutions, this study analyzed the rapidly rise of sea level/sea surface height (SSH) in the tropical Pacific and Indian Oceans during recent two decades. Results show that the sea level rise signals in the tropical west Pacific and the southeast Indian Ocean are closely linked to each other through the pathways of oceanic waveguide within the Indonesian Seas in the form of thermocline adjustment. The sea level changes in the southeast Indian Ocean are strongly influenced by the low-frequency westward-propagating waves originated in the tropical Pacific, whereas those in the southwest Indian Ocean respond mainly to the local wind forcing. Analyses of the lead-lag correlation further reveal the different origins of interannual and interdecadal variabilities in the tropical Pacific. The interannual wave signals are dominated by the wind variability along the equatorial Pacific, which is associated with the El Niño-Southern Oscillation; whereas the interdecadal signals are driven mainly by the wind curl off the equatorial Pacific, which is closely related to the Pacific Decadal Oscillation. [ABSTRACT FROM AUTHOR]

Since its first description by Gilbert and Cramer (1897) (originally as Melanostoma argyreum), Synagrops argyreus was considered to be endemic to Hawaii. New data from the southern Indian Ocean in the Madagascar Channel and the Coral and Tasman Seas reveal a considerable wider geographical distribution of the species, adding to the growing number of pseudoceanic species with a wide distribution pattern in the Indo- Pacific. [ABSTRACT FROM AUTHOR]

The interannual variability of East African 'short rains' (EASR) and its link with the Mascarene High (MH) variation are explored, using observations and reanalysis data. Correlation and composite analyses for flood and drought events reveal that the EASR variability is strongly linked to the MH zonal displacement, in particular, the zonal movement of the MH eastern ridge. When the MH eastern ridge is anomalously displaced to the west (east) of its normal position, the south east (SE) trade winds over the South Indian Ocean (SIO) anomalously strengthen (weaken). This enhances (reduces) the relatively cool and dry SE trade winds and induces cold (warm) sea surface temperature anomaly in the SIO. As a result, convection over the western equatorial SIO is suppressed (enhanced) and leads to rainfall deficits (excess) over East Africa. Droughts in East Africa are associated with a westward migration of the MH eastern ridge, while the relationship is less clear for flood events and their link to an eastward migration of the MH. Therefore, the zonal migration of the MH eastern ridge provides a novel indicator for the EASR extremes especially droughts. This revelation has immense social application for rainfall forecast over East Africa where rainfall deficits have become more prevalent against the background of deteriorating conventional forecasts for EASR droughts. [ABSTRACT FROM AUTHOR]

The mean velocity field of south Indian Ocean has been derived by combining high resolution maps of sea level anomalies and the surface drifter data from the Global Drifter program from 1993 to 2012 with a resolution of 1/3 x 1/3 degrees in latitude and longitude. The estimated mean velocity field exhibits strong western boundary currents, zonal currents and eastern boundary currents. The Agulhas Current shows a velocity of above 1.5 m s-1 at around 35°S. The distribution of energy associated with the fluctuating motion and the mean flow illustrates that mesoscale variability are particularly relevant in the Mozambique Channel, south of Java and around 40°S. Advection of mesoscale features along the western boundary is evident in the distribution of eddy fluxes. The long-term average monthly surface velocity field exhibits large variations in surface currents. The most change is observed in the South Equatorial Current which shows spatial and temporal variations. [ABSTRACT FROM AUTHOR]

Abstract: The shapes of 268 tropical cyclone tracks in the South West Indian Ocean (SWIO) basin are investigated over the period 1977 to 2011 using an established metric for track sinuosity. Original cyclone position data from the RSMC La Réunion area of responsibility were accessed from the IBTrACS archive maintained by NOAA. Cyclone track sinuosity was measured within a GIS environment and the filtered results were normalised with a cube-root transformation function to reduce positive skew in the output sinuosity distribution. Several key findings from spatial and temporal analysis of sinuosity results may be highlighted. To assess geographical patterns, mapping tracks within designated quartile categories (straight, quasi-straight, curving and sinuous tracks) indicates which land areas in the SWIO are more likely to be affected by cyclones following sinuous tracks that are more difficult to forecast, in particular Madagascar and the islands of Réunion and Mauritius. Over the past three decades, the temporal sinuosity pattern shows a high degree of variability that will probably continue into the future. Yet crucially, a number of conspicuous episodes of relatively magnified or subdued sinuosity are recognised compared to long-term averages. This may present opportunities for identifying major climatic controls on regional anomalous cyclone migration behaviour. Within the average yearly cyclone season, the early months (September–December) have a tendency to produce more predictable straighter-moving cyclones, whereas January stands out as the singular month marking an abrupt swing towards a greater proportion (64%) of curving and sinuously-moving storms. This finding is of importance for vulnerability assessment, because a strong positive correlation is also identified between track sinuosity and cyclone longevity, such that storms steering less predictable sinuous courses are also those that tend to survive for longer durations. [Copyright &y& Elsevier]

The Southwest Indian Ridge (SWIR) is poorly understood in terms of local variability in hydrographical characteristics because of a relative lack of data. Furthermore, previous hydrographic studies in SWIR were restricted to the west of 60°E region. We present a new set of hydrographic data which were collected during a survey for the search of hydrothermal plume/activity in between Mt. Jourdanne and Rodrigues Triple Junction (RTJ) over SWIR. The primary goal of our study is to understand the segment-scale hydrography in and above the SWIR rift valley between 63°E to 69°E, with emphasis on water mass propagation and variability of their physical properties. CTD/Oxygen profiles were examined along a ~650 km distance of SWIR axial valley segments. The properties of Antarctic Bottom Water (AABW) were identified in the rift valley section which passes through deep sills from the Crozet basin. Along the ridge, the physical parameters especially at deep and bottom layer depths have observed gradients formed by the obstruction of flow at sills in the valley, along with a unidirectional density driven flow. • Topography blockage of bottom water movement. • Observed density driven unidirectional flow along the SWIR segments. • Presence of modified AABW in the study region. • Southward propagating NIDW observed west of RTJ. [ABSTRACT FROM AUTHOR]

Electronic tagging provides unprecedented information on the habitat use and behaviour of highly migratory marine predators, but few analyses have developed quantitative links between animal behaviour and their oceanographic context. In this paper we use archival tag data from juvenile southern bluefin tuna ( Thunnus maccoyii, SBT) to (i) develop a novel approach characterising the oceanographic habitats used throughout an annual migration cycle on the basis of water column structure (i.e., temperature-at-depth data from tags), and (ii) model how the vertical behaviour of SBT altered in relation to habitat type and other factors. Using this approach, we identified eight habitat types occupied by juvenile SBT between the southern margin of the subtropical gyre and the northern edge of the Subantarctic Front in the south Indian Ocean. Although a high degree of variability was evident both within and between fish, mixed-effect models identified consistent behavioural responses to habitat, lunar phase, migration status and diel period. Our results indicate SBT do not act to maintain preferred depth or temperature ranges, but rather show highly plastic behaviours in response to changes in their environment. This plasticity is discussed in terms of the potential proximate causes (physiological, ecological) and with reference to the challenges posed for habitat-based standardisation of fishery data used in stock assessments. [ABSTRACT FROM AUTHOR]

The article presents a study which determines the factor that drives rainfall anomalies during Benguela Niño events. It states that the mechanisms behind rainfall anomalies during the events are not well understood as to whether the rainfall anomalies are driven by sea surface temperature (SST) forcing in the South Indian Ocean or the tropical southeast Atlantic SST anomalies. Accordingly, an atmospheric general circulation model (AGCM) is used to investigate the atmospheric circulation and rainfall over southern Africa to different regional SST patterns during Benguela Niño events. Moreover, the study suggests that southwest Indian Ocean SST forcing may increase or decrease the Southern rainfall anomalies during Benguela events depending on its sign, magnitude and location.

Anthropogenic 90Sr, 239,240Pu and 241Am were used as tracers of water mass circulation in the Crozet Basin of the South Indian Ocean, represented by three main water fronts—Agulhas (AF), Subtropical (STF) and Subantarctic (SAF). Higher 90Sr concentrations observed north of 43°S were due to the influence of AF and STF, which are associated with the south branch of the Subtropical gyre, which acts as a reservoir of radionuclides transported from the North to the South Indian Ocean. On the other hand, the region south of 43°S has been influenced by SAF, bringing to the Crozet Basin Antarctic waters with lower radionuclide concentrations. The 238Pu/239,240Pu activity ratios observed in water and zooplankton samples indicated that, even 35 years after the injection of 238Pu to the Indian Ocean from the burn-up of the SNAP-9A satellite, the increased levels of 238Pu in surface water and zooplankton are still well visible. The radionuclide concentrations in seawater and their availability to zooplankton are responsible for the observed 210Po, 239,240Pu and 241Am levels in zooplankton. [ABSTRACT FROM AUTHOR]

Abstract: This article compares the seasonal variability patterns of the South Indian Ocean circulation derived from a global, eddy-permitting, numerical model and altimeter observations. The seasonal variability of the Indian Ocean circulation is driven by the inflow from the Indonesian Passages and by the local wind forcing. Our analysis indicates that the influence of the Indonesian throughflow is confined to the easternmost portion of the basin, while the influence of the wind stress forcing is important everywhere. Model and observations indicate that, between ~105°E and 75°E, the seasonal variability is characterized by the southwestward propagation of an annual wave over a period of ~4 months. Preliminary calculations using Pathfinder data also indicate that, in the western region, there are seasonal perturbations that originate in the tropics and propagate poleward through the Mozambique Channel. Our calculations, however, did not find the connections between the tropical and the Agulhas Current variability suggested by earlier modeling studies. [Copyright &y& Elsevier]