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Investigating the impact of El Niño-Southern Oscillation (ENSO) on subtropical Indian Ocean dipole (SIOD) facilitates understanding their combined effects on African and Australian climate. We found the correlation between ENSO in austral summer and SIOD in following autumn is enhanced after the early 1990s. Before the early 1990s, ENSO only changes the southeastern Indian Ocean (SEIO) temperature by modulating the Walker circulation. In contrast, after the early 1990s, El Niño induces positive sea surface temperature (SST) anomalies in the SEIO and negative SST anomalies in the southwestern Indian Ocean (SWIO), causing the enhanced ENSO-SIOD correlation. This increasing influence of ENSO on the SWIO closely links to the ENSO-related atmospheric teleconnection in the midlatitude Southern Hemisphere (SH). Before the early 1990s, the El Niño-related teleconnection coincides with a low-pressure anomaly to the south of southern Africa. Anomalous westerlies on the northern flank of this low-pressure warm the SWIO SST by increasing the solar radiation and reducing the mixed-layer thickness but cool the SWIO SST by changing the oceanic heat advection. These opposite effects cause a weak ENSO-SWIO connection. After the early 1990s, the low-pressure related to El Niño shifts to the SWIO in January–February–March, causing anomalous local moisture divergence. The increased air–sea humidity difference enhances the latent heat release and then deepens the mixed-layer thickness, cooling the SWIO SST. The interdecadal differences in the ENSO-related SH teleconnection are probably attributed to the westward-shifted ENSO pattern and the reduced response of the Tasman Sea SST to ENSO after the early 1990s. [ABSTRACT FROM AUTHOR]

Aeolian transport of mineral dust is an important carrier of nutrients and trace metals to the surface ocean and, hence, affects the biogeochemical cycles of C and N therein. Studies focusing on the provenances of dust across the latitudinal gradients over the open ocean waters are essential for better constraining the biogeochemical effects of dust deposition. Here, we analyzed the radiogenic isotope composition of Sr (87Sr/86Sr) and Nd (ε Nd (0)) in the silicate fraction of mineral aerosols collected over the pelagic northern and southern Indian ocean waters during the GEOTRACES-India research expeditions (GI-01: March-April 2014 and GI-02: April-May 2014). Both tracers, 87Sr/86Sr and ε Nd (0), exhibited marked spatial variability for dust deposition to different basins, including the Bay of Bengal (BoB), Arabian Sea (AS), equatorial Indian Ocean (EIO), and the southern sector of the Indian Ocean (SIO). The dust deposited in the AS showed a narrow spread in both 87Sr/86Sr (0.71520–0.71642) and ε Nd (0) (−10.9 to −10.1), along with the air mass back trajectories, indicating transport from the Arabian Peninsula. Likewise, the dust collected over the BoB was mainly sourced from the Thar Desert (87Sr/86Sr > 0.72, −16 < ε Nd (0) 〈 −1 3), whereas the EIO received mineral dust from the Thar Desert and sand dunes in northern Australia (87Sr/86Sr: 0.72, ε Nd (0): −11) and Indonesian Archipelago. In contrast, the SIO appeared to receive dust deposition from three different source regions, including dust from Antarctica, western Australia (87Sr/86Sr: 0.712, −20 < ε Nd (0) <−16, and South Africa/South America (87Sr/86Sr: 0.708–0.710; −24.5 < ε Nd (0) >−8.4). These results have important implications for modelling the dust transport to pelagic Indian and southern Indian Ocean waters. [ABSTRACT FROM AUTHOR]

The variability of Indian Ocean shallow meridional overturning circulation (SMOC) is studied using the century long ocean reanalysis simple ocean data assimilation (SODA) data. Though SMOC exhibits stronger southward transport during boreal summer, it displays stronger variability during boreal winter. The spectrum analysis of winter SMOC index reveals presence of highest amplitude between 5 to 7 years at 95% confidence level, suggesting the dominance of intra-decadal SMOC variability. The robustness of intra-decadal SMOC variability is also confirmed in different ocean reanalysis data sets. Composite analysis of filtered upper Ocean Heat Content, sea level, thermocline depth and Sea Surface Temperature anomalies for strong (weak) SMOC years show negative (positive) anomaly over north and East of Madagascar. Correlation analysis, of filtered SMOC index and sea level pressure (zonal winds) over the India Ocean, found significant negative (positive) correlation coefficient north of 40 °S (around 10 °S) and significant positive (negative) correlation coefficient over the 45 °S to 70 °S (20 °S to 50 °S and north of 5 °S). This meridional pattern of correlation coefficient for sea level pressure, manifesting the out of phase relationship between sub-tropics and high latitude mean sea level pressure, resembles with Southern Annular Mode (SAM). We conclude that the intra-decadal variability of mean sea level pressure leads to zonal wind variation around 10 °S modulating SMOC, which in turn affects the upper ocean thermal properties in the east and north of Madagascar. This study for the first time brought out coherent intra-decadal evolution of SAM and SMOC during boreal winter. [ABSTRACT FROM AUTHOR]

Monitoring the presence of blue whale (Balaenoptera musculus ssp.) stereotyped calls has been a widely used method to assess the different populations' distribution worldwide. All blue whale populations also produce nonstereotyped vocalizations, or D‐calls. Here, we monitored the presence of D‐calls in long‐term records from a large hydrophone array located in the open southern Indian Ocean, using an automated detection method and manual validation of the detections. D‐calls were detected at all sites of the array, which extends from 24°S to 56°S, but the majority of them were detected at the two southernmost sites. We observed a latitudinal shift in their seasonal occurrence, with more D‐calls in the north during austral autumn and winter and more in the south during austral spring. The geographical occurrence of D‐calls compared to that of songs indicates that blue whale acoustic behavior switches from a song‐intensive and sparse‐D‐call emission in the north to song‐moderate and more intensive D‐call emissions in the south. These findings support the hypothesis that both call types are used for different purposes, as D‐calls are mainly detected around foraging grounds and songs in wintering grounds. Monitoring both call types might therefore be a relevant acoustic indicator of blue whale behavior. [ABSTRACT FROM AUTHOR]

Tropical cyclones (TCs), commonly called cyclones in the southern Indian Ocean (SIO), represent one of the most devastating disasters in the oceanfront regions of Africa. The present study explores the long-term tendency of annual mean TC genesis location in the SIO. A notable westward shift is detected in the SIO TC genesis longitude since 1979, which is linked to an increase in the TC genesis frequency in the southwestern SIO and a decrease in the TC genesis frequency in the northeastern SIO. The dipole trend pattern of the TC genesis frequency in the SIO is intimately linked to the weakening of the westerly vertical wind shear over the western SIO and the strengthening of the easterly vertical wind shear over the eastern SIO, resulting from a reduced meridional temperature gradient. The weakened meridional temperature gradient is attributed to the enhanced warming of the subtropical troposphere that is a response of atmospheric temperature to global warming. Our study implies a potential increase in the risks faced by coastal and island countries in eastern Africa.

The Southern Indian Ocean is a major reservoir for rapid carbon exchange with the atmosphere, plays a key role in the world’s carbon cycle. To understand the importance of anthropogenic CO2 uptake in the Southern Indian Ocean, a variety of methods have been used to quantify the magnitude of the CO2 flux between air and sea. The basic approach is based on the bulk formula—the air-sea CO2 flux is commonly calculated by the difference in the CO2 partial pressure between the ocean and the atmosphere, the gas transfer velocity, the surface wind speed, and the CO2 solubility in seawater. However, relying solely on wind speed to measure the gas transfer velocity at the sea surface increases the uncertainty of CO2 flux estimation. Recent studies have shown that the generation and breaking of ocean waves also significantly affect the gas transfer process at the air-sea interface. In this study, we highlight the impact of windseas on the process of air-sea CO2 exchange and address its important role in CO2 uptake in the Southern Indian Ocean. We run the WAVEWATCH III model to simulate surface waves in this region over the period from January 1st 2002 to December 31st 2021. Then, we use the spectral partitioning method to isolate windseas and swells from total wave fields. Finally, we calculate the CO2 flux based on the new semiempirical equation for gas transfer velocity considering only windseas. We found that after considering windseas’ impact, the seasonal mean zonal flux (mmol/m2·d) increased approximately 10%-20% compared with that calculated solely on wind speed in all seasons. Evolution of air-sea net carbon flux (PgC) increased around 5.87%-32.12% in the latest 5 years with the most significant seasonal improvement appeared in summer. Long-term trend analysis also indicated that the CO2 absorption capacity of the whole Southern Indian Ocean gradually increased during the past 20 years. These findings extend the understanding of the roles of the Southern Indian Ocean in the global carbon cycle and are useful for making management policies associated with marine environmental protection and global climatic change mitigation.

Variations in annual accumulated snowfall over the Antarctic ice sheet have a significant and direct impact on mean sea-level change. The interannual variability of the precipitation over coastal Antarctica adjacent to the southern Indian Ocean (SIO) cannot be totally explained by the dominant mode of atmospheric variability in the Southern Hemisphere. This study explores the possible contributions from sea surface temperature (SST) anomalies in SIO on the precipitation over East Antarctica. The results suggest that the winter precipitation in the Lambert Glacier basin (LGB) is closely related to the autumn SST variability in SIO without the influence of El Niño–Southern Oscillation. It is shown that the positive autumn SIO dipole (SIOD) of SST anomalies is usually followed by reduced precipitation in the following winter over the LGB region and vice versa. The positive (negative) autumn SIOD can persist into the winter and excite cyclonic (anticyclonic) circulation and deepen (weaken) SIO low in high latitude, corresponding to an enhanced northward (southward) wind anomaly in LGB and central SIO. This mechanism prevents (promotes) the transportation of warm and moist marine air to the LGB region and hence decreases (increases) the precipitation during the following winter.

The study of ocean bottom pressure (OBP) helps to understand the changes in the sea level budget and ocean deep circulation. In this study, the characteristics and mechanisms of interannual OBP variability in the Southern Indian Ocean are examined using Gravity Recovery and Climate Experiment (GRACE) satellite data from 2003 to 2016. Results show that there are two energetic OBP centers in the Southern Indian Ocean (50°–60°S, 40°–60°E and 45°–60°S, 80°–120°E). The OBP magnitudes at two centers have strong variability on interannual time scales, and their values are larger during austral summer (NDJF) and winter (JJAS). Atmospheric forcing plays an important role in local OBP variability. The high (low) sea level pressure (SLP) over the Southern Indian Ocean benefits positive (negative) OBP anomalies via the convergence (divergence) of Ekman transport driven by local wind. Such SLP anomalies are related to the Southern Annular Mode (SAM), Southern Oscillation (SO) and Indian Ocean dipole (IOD). SAM can influence the OBP changes in both austral summer and winter, while SO and IOD have positive correlations with OBP variability during austral summer and austral winter, respectively. These results are validated by a mass-conservation ocean model, which further confirms the importance of atmospheric forcing on the interannual OBP variations.

Estimation of the primary productivity (PP) for the area of the Southern Indian Ocean has been made on the base of assessment of interannual variability in concentration of silicic acid in the euphotic layer. Mean for different sublatitudinal zones (subtropical, subantarctic and antarctic) summary for the vegetative period and daily PP, varied in the range 13.0-29.8 and 0.03-0.23 g C m-2 relatively. Kerguelen Plato was the most productivity area, where summary PP reached 165 g C m-2 and daily - 0.9.

Near the southern tip of the African continent, the Agulhas Current retroflects eastward to feed the Agulhas Return Current, giving rise to the Agulhas Retroflection (AR). The AR is dynamically unstable and radiates eddies and rings that constitute a portion of the Agulhas leakage. In this study, by analyzing 26 years (1993–2018) of updated satellite altimetry data, the spatial and temporal characteristic of the surface AR point are revisited. By adopting a refined identification algorithm of the AR point, we show that the AR longitude (XAR) fluctuates widely between 7° and 27°E on timescales ranging from days to months, without a stable seasonal cycle. The XAR variability also shows evident temporal asymmetry with shorter residence time at western positions. The retreat of the AR from the western to eastern positions tends to be swift, with 46% of the events completed in one day. In comparison, the intrusion from the eastern to western positions is much slower, with 32% of the events taking more than one month. The energy transfer from the mean flow to mesoscale eddies is likely the primary regime underlying the AR variability. Further analysis suggests that the AR variability causes sea surface temperature anomalies of ∼0.5°C through anomalous advection and thereby gives rise to turbulent heat flux anomalies of ∼10 W m−2, exerting potential impacts on the local climate. Plain Language Summary: The Agulhas Retroflection (AR) is a key feature of the Agulhas Current system, and the choke point for the inter‐basin exchange between the Indian and Atlantic Oceans. The AR also serves as the origin of the Agulhas leakage that is constituted by northwestward moving eddies and rings radiated from the AR. Although there has been literature exploring the dynamics of the Agulhas Current and Agulhas leakage, our understanding of the AR variability is still incomplete. Using 26 years of updated satellite observation data of 1993–2018, we show that the mean longitude of the surface AR point is 17.5°E, and its fluctuation can be as wide as 7°−27°E. Owing to its strong variability and irregular behaviors, the AR does not possess a robust seasonal cycle. The AR behaviors are asymmetric: its residence time in the west tends to be much shorter than in the east. The interaction between the mean flow and mesoscale eddies is likely an essential regime controlling the AR variability. We also show that the AR variability can cause strong anomalies in regional sea surface temperature and air‐sea heat fluxes, which has potential influence on the local climate. Key Points: The mean longitude of the AR is 17.5°E with a standard deviation of 2.6°E and shows temporal asymmetry in westward and eastward phasesThe energy transfer from mean flow to eddies is likely the primary mechanism regulating the AR variabilityThe AR variability causes local SST anomalies of ∼0.5°C and surface heat flux anomalies of ∼10 W m−2 [ABSTRACT FROM AUTHOR]

Background and aims - Despite the ongoing taxonomical revision of the entire (sub)-Antarctic diatom flora, our knowledge on the ecology and community associations of moss-inhabiting diatoms is still rather limited. In the present study, our research aim was to survey the diversity together with the environmental factors structuring the epiphytic moss diatom communities on Ile Amsterdam (TAAF), a small volcanic island in the southern Indian Ocean. Material and methods - A morphology-based dataset and (physico)chemical measurements were used for the ecological and biogeographical analysis of moss-inhabiting diatom flora from Ile Amsterdam. In total, 148 moss samples were examined using light microscopy. Key results - The analysis revealed the presence of 125 diatom taxa belonging to 38 genera. The uniqueness of the Ile Amsterdam diatom flora is mainly reflected by the species composition of the dominant genera Pinnularia, Nitzschia, Humidophila, and Luticola, with a large number of unknown and often new species. This highly specific diatom flora, together with differences in the habitats sampled and the isolated position of the island, resulted in very low similarity values between Ile Amsterdam and the other islands of the Southern Ocean. From a biogeographical point of view, 40% of the taxa have a typical cosmopolitan distribution, whereas 22% of all observed species can be considered endemic to Ile Amsterdam, with another 17% species showing a restricted sub-Antarctic distribution. The NMDS analysis, based on a cluster dendrogram, divides the samples into six main groups. For each group, indicator species were determined. Both environmental data and diatom distributions indicate that apart from elevation, specific conductance, pH, and moisture are the major factors determining the structure of moss-inhabiting diatom communities on Ile Amsterdam. Conclusion - The isolated geographic position and unique climatological and geological features of the island shaped the presence of a unique diatom flora, characterised by many endemic species. The results of the study are of prime importance for further (palaeo-)ecological and biogeographical research. [ABSTRACT FROM AUTHOR]

Three statistical prediction models for the summer extreme precipitation frequency (EPF) in the middle and lower reaches of the Yangtze River valley (MLYRV) based on the winter sea surface temperature in the southern Indian Ocean (SIO‐SST; Scheme‐SST), the spring sea‐ice concentration in the Beaufort Sea (Scheme‐SIC), and both predictors (Scheme‐SS), are established by using the year‐to‐year increment method. The winter SIO‐SST may affect the SST anomaly in the east of Australia via a teleconnection pattern. The east of Australia SST signal in winter continues until the following summer and then affects the summer EPF in the MLYRV via modulation of the meridional Hadley circulation in the western North Pacific. The positive Beaufort SIC anomaly may result in a negative summer SST anomaly by the increased surface albedo. In response, the atmospheric circulation presents a dipole anomaly distribution in the Beaufort Sea and the Barents Sea. The Arctic dipole anomaly may bring about frequent extreme precipitation in the MLYRV by adjusting the position of the East Asian westerly jet and Eurasian teleconnection pattern. The prediction skill of the summer EPF for Scheme‐SS is higher than that of Scheme‐SST and Scheme‐SIC in the cross‐validation test during 1962–2017 and the independent hindcast during 1992–2017. Scheme‐SS shows a higher prediction skill of the summer EPF than that of Scheme‐SST and Scheme‐SIC, with a time correlation coefficient of 0.62, accounting for 39% of the total variance, of which 35% is the winter SIO‐SST and 4% the spring Beaufort SIC. Scheme‐SS not only shows a rather high predictive ability for the anomalous summer EPF but can also reproduce the increasing trend of extremely heavy precipitation. [ABSTRACT FROM AUTHOR]

Biophysical interactions are influential in determining the scale of key ecological processes within marine ecosystems. For oceanic predators, this means foraging behaviour is influenced by processes shaping the distribution of prey. However, oceanic prey is difficult to observe and its abundance and distribution is regionally generalised. We use a spatiotemporally resolved simulation model to describe mid‐trophic prey distribution within the Southern Ocean and demonstrate insights that this modelled prey field provides into the foraging behaviour of a widely distributed marine predator, the southern elephant seal. From a five‐year simulation of prey biomass, we computed climatologies of mean prey biomass (average prey conditions) and prey biomass variability (meso‐scale variability). We also compiled spatially gridded metrics of seal density and diving behaviour from 13 yr of tracking data. We statistically modelled these metrics as non‐linear functions of prey biomass (both mean and variability) and used these to predict seal distribution and behaviour. Our predictions were consistent with observations (R2adj = 0.23), indicating that seals aggregate in regions of high mesoscale activity where eddies concentrate prey. Here, seals dived deeper (R2marg = 0.12, R2cond = 0.51) and spent less time hunting (R2marg = 0.05, R2cond = 0.56), likely targeting deep but profitable prey patches. Seals generally avoided areas of low eddy activity where prey was likely dispersed. Most seals foraged south of the Subantarctic Front, despite north of the front exhibiting consistently high simulated prey biomasses. This likely reflects seal prey or habitat preferences, but also emphasises the importance of mesoscale prey biomass variability relative to regionally high mean biomass. This work demonstrates the value of coupling mechanistic representations of prey biomass with predator observations to provide insight into how biophysical processes combine to shape species distributions. This will be increasingly important for the robust prediction of species' responses to rapid system change. [ABSTRACT FROM AUTHOR]

In the Southern Ocean, the term "winter waters" (WWs) refers to a water mass characterized by a subsurface layer of minimum temperature that plays an important ecological role for marine ecosystems, and in particular for top predators. Given that the Southern Ocean is experiencing warming and intense marine heatwaves (MHWs), particularly at subantarctic latitudes, we investigate here how different levels of warming might impact the presence, depth and minimum temperature of WWs in the Indian sector of the Southern Ocean. In particular, we assess how WWs are impacted by surface MHWs using in situ Argo hydrographic observations and biologging data. The results indicate that WWs are substantially reduced, deeper and warmer during the presence of MHWs. Using the most recent climate projections, we find a significant, but scenario-dependent, southward shift of WWs under global warming. Potential impacts of such WW shifts on pelagic ecosystems, at different timescales (from daily to decadal), are discussed. • Marine heatwaves are associated with a decreased probability of presence of winter waters. • Winter water distribution shifts southward linearly with global warming west of Kerguelen Plateau. • Winter water distribution southward shift is weak east of Kerguelen up to 2.6 °C of global warming. [ABSTRACT FROM AUTHOR]

The pelagic zone is home to a large diversity of organisms such as macrozooplankton and micronekton (MM), connecting the surface productive waters to the mesopelagic layers (200-1000 m) through diel vertical migrations (DVM). Active acoustics complement net sampling observations by detecting sound-scattering layers (SL) of organisms, allowing to monitor the MM dynamics with a high spatio-temporal resolution. Multi-frequency analyses are a pertinent approach to better integrate the rich diversity of organisms composing SLs and their respective dynamics. However, analysing simultaneously emitted acoustic signals with distinct depth ranges and separating spatial from temporal variability is challenging and needs adapted tools to be fully exploited. This study examines the pelagic realm in a transition zone between the Southern Ocean and the subtropical Indian Ocean, crossing the Saint-Paul and Amsterdam islands' natural reserve. We extended a Multivariate Functional Principal Component Analysis (mfPCA) to analyse the joint vertical variation of five frequencies from two oceanographic cruises (2016 and 2022), allowing the decomposition of the acoustic dataset into orthogonal vertical modes (VM) of variability. We found the first VM to be linked to the temporal variability due to DVM, while the following majorly depict patterns in spatial distribution. Overall, from the subantarctic to the subtropical zones, we observed (i) enrichment of densities in the surface layer (0–100 m), (ii) a decrease in densities in the intermediate layer during the daytime (100–300 m) and (iii) the apparition of an intensive deep scattering layer on the 38 kHz. We explored VMs' connection with in-situ environmental conditions by clustering our observations into three distinct environmental-acoustic regions. These regions were compared with vertically integrated nautical area scattering coefficient distribution, a proxy for marine organisms' biomass. Additionally, we analysed species assemblage changes from complementary cruises to further elucidate the observed acoustic distribution. We show that the mfPCA method is promising to better integrate the pelagic horizontal, vertical and temporal dimensions which is a step towards further investigating the control of the environment on the distribution and structuring of pelagic communities. • Functional data analysis identifies variance in SL distribution via vertical modes. • First mode is tied to temporal dynamics, next modes display spatial patterns. • 3 regions link environmental surface data and SL from South Ocean to Southern Indian Ocean. • 18 and 38 kHz differ in integrated acoustic density spatial patterns. • Species assemblage help explain acoustic patterns in transition zones. [ABSTRACT FROM AUTHOR]

As a new type of pollutant, microplastics widely exist in the marine environment and have attracted a lot of attention from the international community. In order to study the distribution of microplastics and the influence of ocean current, microplastic samples in seawater of the southern Indian Ocean were collected using a peristaltic pump equipped on-board and concentrated on site. Qualitative and quantitative analyses of microplastics were performed using a stereo-microscope and a micro-Fourier transform infrared spectroscope attenuated total reflection. The results showed that the average abundance of microplastics in seawater of the southern Indian Ocean was 2.3 ± 2.1 items/m3, which was consistent with that in other oceans. Polyethylene terephthalate (PET), polyethylene (PE), Rayon, polyamide (PA), and polyvinylidene chloride (PVDC) were the main polymers of microplastics in the southern Indian Ocean. The size range of all detected microplastics was 108.2–4703.0 µm. All microplastics had different colors, such as black, red, yellow, gray, blue, green, purple, and transparent. Fiber was the dominant shape of microplastics. The abundance distribution of microplastics fluctuated in the latitudinal direction. The abundance of microplastics from the present study area was higher in the coastal region of the South Africa continent and the Indian Ocean garbage patch, with an average abundance of 4.0 items/m3. The average abundance of microplastics was relatively high in the convergence area of the circulation, which revealed that the ocean current facilitated the agglomeration and transportation of microplastics.

Antarctic sea ice has experienced an increasing trend in recent decades, especially in the Ross Sea and Indian Ocean sectors. Sea ice variability affects greatly the maritime airmass transport from high latitude to Antarctic continent. Here we present a new ice core record of sea salt sodium (ssNa+) concentration at annual-resolution in the Princess Elizabeth Land spanning from 1990 to 2016, showing that this marker could be used as a potential proxy for reconstructing the sea ice extent (SIE) in the Southern Indian Ocean (SIO) given their significant correlation (R = −0.6, P <0.01) over the past 27 years. The correlation and composite analyses results show that the ssNa+ at the 202 km inland from Zhongshan Station and the SIE changes in SIO are closely related to the Indian Ocean Dipole (IOD) and Southern Annular Mode (SAM). The northward wind in central SIO occurs during positive IOD and the strengthened westerlies occurs during positive SAM, both of which favor increased sea ice in SIO and lead to the decreased ssNa+ concentration at the coastal site. [ABSTRACT FROM AUTHOR]

Satellite altimetry data and high‐resolution model output are used to investigate the mechanism of interannual variations of the solitary meander in the Agulhas Current, called the Natal Pulse. It is shown that most of the Natal Pulses are triggered by anticyclonic eddies originating from the Southeast Madagascar Current (SEMC) and anticyclonic eddies of the Mozambique Channel are the secondary source. In addition to the barotropic conversion suggested by previous studies, the advection of eddy kinetic energy (EKE) plays a statistically significant role in the growth of the Natal Pulse. The number of anticyclonic eddies arriving at the Natal Bight largely explains the interannual variations in the Natal Pulse. The barotropic conversion from the SEMC, whose interannual variations are likely related to the remote wind forcing, is the main EKE source and corresponds well with the interannual variations of anticyclonic eddies generated off the southern coast of Madagascar. It is also shown that the interannual variations of the Natal Pulse are linked to large‐scale wind variability in the southern Indian Ocean. Plain Language Summary: Natal Pulse is the large meander of the Agulhas Current originating in the Natal Bight and it affects both the local marine environment and global ocean circulation. Previous observational studies suggested that the occurrence of the Natal Pulse varies from year‐to‐year. Since oceanic eddies tend to trigger the Natal Pulse, we investigate the relationship between the Natal Pulse and incoming eddies in this study. We show that variations of the Natal Pulse are mainly explained by variations of eddies propagating from the south of Madagascar and the Mozambique Channel. We also show that variations in the Natal Pulse are linked to the large‐scale wind variability over the southern Indian Ocean. This study is the first to show the mechanism of the year‐to‐year variations in the Natal Pulse. Key Points: Interannual variations of the Natal Pulse are explained by the number of anticyclonic eddies arriving at the Natal BightSoutheast Madagascar Current is the main source of eddies which propagate to the Natal Bight, and thus the main source of the Natal PulseSignificant relationship between interannual variations of the Natal Pulse and wind stress curl field in the southern Indian Ocean is found [ABSTRACT FROM AUTHOR]

Using 28-year satellite-borne Special Sensor Microwave Imager observations, features of high-wind frequency (HWF) over the southern Indian Ocean are investigated. Climatology maps show that high winds occur frequently during austral winter, located in the open ocean south of Polar Front in subpolar region, warm flank of the Subantarctic Front between 55°E–78°E, and south of Cape Agulhas, where westerly wind prevails. The strong instability of marine atmospheric boundary layer accompanied by increased sensible and latent heat fluxes on the warmer flank acts to enhance the vertical momentum mixing, thus accelerate the surface winds. Effects of sea surface temperature (SST) front can even reach the entire troposphere by deep convection. HWF also shows distinct interannual variability, which is associated with the Southern Annual Mode (SAM). During positive phase of the SAM, HWF has positive anomalies over the open ocean south of Polar Front, while has negative anomalies north of the SST front. A phase shift of HWF happened around 2001, which is likely related to the reduction of storm tracks and poleward shift of westerly winds in the Southern Hemisphere. [ABSTRACT FROM AUTHOR]

The location of the Antarctic Polar Front (PF) is mapped in the Southern Indian Ocean by decomposing the shape of temperature and salinity profiles into vertical modes using a functional Principal Component Analysis. We define the PF as the northernmost minimum of temperature at the subsurface and represent it as a linear combination of the first three modes. This method is applied on an ocean reanalysis data set and on in situ observations, revealing a seasonal variability of the PF latitudinal position that is most pronounced between the Conrad Rise and the Kerguelen Plateau. This shift coincides with variations in the transport across the Northern Kerguelen Plateau. We suggest that seasonal changes of the upper stratification may drive the observed variability of the PF, with potentially large implications for the pathways and residence time of water masses over the plateau and the phytoplankton bloom extending southeast of the Kerguelen Islands. Plain Language Summary: The Antarctic Polar Front (PF) is a water mass boundary that flows around Antarctica between approximately 48°S and 56°S in the Southern Indian Ocean. The position of the PF in space and time is important to understand the oceanic circulation, the heat and salt exchanges, and also marine ecosystems. In the Indian sector the PF has to cross the Kerguelen Plateau, a major bottom topography feature. The present study develops and then applies a novel method for mapping the PF taking into account the whole hydrographic structure in the upper 300 m of the ocean. We are able to map the PF position and find that it presents large seasonal variations that are more intense just west of the Kerguelen Plateau. Between the Conrad Rise and the Kerguelen Plateau, the PF is essentially zonally orientated in September and found farther south by up to 4° latitude in March. Shifts in the PF position are shown to correlate with a seasonal variation in volume transport between Kerguelen and Heard Islands. We discuss how these seasonal variations in circulation pathways could have an impact on the local marine ecosystems. Key Points: A novel method classifying TS profiles reveals the spatiotemporal variability of the Polar Front (PF) in the Southern Indian OceanWest of the Kerguelen Plateau, the Polar Front follows the 49 degrees south latitude in the Austral spring, while in the autumn it is located at 53 degrees southThis seasonal meandering covaries with the transport over the Plateau, suggesting important variations in circulation pathways [ABSTRACT FROM AUTHOR]

This paper presents an 8-year (2005–2012 inclusive) study of the marine aerosol distribution and variability over the Southern Indian Ocean, precisely in the area 10 ° S − 40 ° S ; 50 ° E − 110 ° E which has been identified as one of the most pristine regions of the globe. A large dataset consisting of satellite data (POLDER, CALIOP), AERONET measurements at Saint-Denis (French Réunion Island) and model reanalysis (MACC), has been used. In spite of a positive bias of about 0.05 between the A O D (aerosol optical depth) given by POLDER and MACC on one hand and the A O D measured by AERONET on the other, consistent results for aerosol distribution and variability over the area considered have been obtained. First, aerosols are mainly confined below 2 km asl (above sea level) and are dominated by sea salt, especially in the center of the area of interest, with A O D ≤ 0 . 1 . This zone is the most pristine and is associated with the position of the Mascarene anticyclone. There, the direct radiative effect is assessed around - 9 W m - 2 at the top of the atmosphere and probability density functions of the A O D s are leptokurtic lognormal functions without any significant seasonal variation. It is also suggested that the Madden-Jullian oscillation impacts sea salt emissions in the northern part of the area considered by modifying the state of the ocean surface. Finally, this area is surrounded in the northeast and the southwest by seasonal Australian and South African intrusions ( A O D > 0.1 ) ; throughout the year, the ITCZ seems to limit continental contaminations from Asia. Due to the long period of time considered (almost a decade), this paper completes and strengthens results of studies based on observations performed during previous specific field campaigns. [ABSTRACT FROM AUTHOR]

The present study examines the role of the Southern Indian Ocean (SIO) warming on the cyclone destruction potential or Power Dissipation Index (PDI) during two contrasting periods of 1980–1998 and 1999–2016. The PDI in the SIO during 1999–2016 is found to have doubled compared to the same during 1980–1998. PDI was computed using the tropical cyclone track data in the SIO region for cyclone category three and above. The increasing trend in PDI during the latter period is primarily due to an increase in the intensity of cyclones and their duration. The increasing PDI is associated with a sea surface temperature warming and an upper ocean heat content increase as well as a significant slowdown in translation speeds. The increase in upper ocean heat content during the recent decades enhances the intensification of cyclones and their duration, which is consistent with the slowdown of cyclones. Analysis of the relevant atmospheric parameters indicates that processes in the atmosphere did not play a major role in the recent decades in increasing cyclone intensity. We show that in the SIO, ocean processes play a major role in the PDI rise during the recent period. Any continued increase in PDI will cause more loss of life and socioeconomic damage to the island countries such as Mozambique, Mauritius, Mascarene Islands and Madagascar, as well as the coastal inhabitants along East Africa.

Abstracts Water samples from nine vertical profiles down to 110 m water depth and 19 samples from the sea-surface were studied for coccolithophore abundance and distribution across oceanic frontal regions of the Indian sector of the Southern Ocean. Sampling was performed along a north-south transect (between 39°S and 65.49°S, ~ 57.3°E) during the 4th Indian Southern Ocean Expedition (between 31st January and 18th February 2010). Coccospheres and coccoliths were counted separately using a Scanning Electron Microscope (SEM). A total of 39 taxa (including morphotypes, types and subspecies) were recorded as intact coccospheres with abundances reaching up to 750 × 10 3 coccospheres/l. In addition, 85 taxa (including varieties, morphotypes) were counted as coccoliths reaching up to 900 × 10 5 coccoliths/l. Emiliania huxleyi was recognized as the most abundant species, accounting for > 86% of the total coccolithophore assemblage at each station. Elevated coccolithophore diversity was observed in the subtropical zone whereas high coccolithophore abundance was observed in the Subantarctic zone. A monospecific Emiliania huxleyi assemblage was recorded within and south of the Polar frontal zone. Three assemblages were recognized based on coccolithophore abundance and diversity. The assemblage of the Agulhas Retroflection frontal zone and Subtropical zone is highly diverse (39 taxa) and can be linked to relatively warm, high saline and oligotrophic waters. The Subantarctic zone assemblage is characterized by a reduced number (thirteen) of coccolithophore taxa, whereas the Polar Frontal zone comprises a monospecific E. huxleyi assemblage (preferentially morphotypes C and B/C). Multivariate statistics indicated that regions with elevated temperature and low nutrient concentration show high coccolithophore diversity whereas regions with high nutrient concentrations and low temperature show a strongly reduced coccolithophore diversity with abundant monospecific E. huxleyi (morphotypes B/C and C) assemblages. [ABSTRACT FROM AUTHOR]

Over the past 50 years, 210 Pb, 137 Cs and 241 Am have been abundantly used in reconstructing recent sediment and peat chronologies. The study of global aerosol-climate interaction is also partially depending on our understanding of 222 Rn- 210 Pb cycling, as radionuclides are useful aerosol tracers. However, in comparison with the Northern Hemisphere, few data are available for these radionuclides in the Southern Hemisphere, especially in the South Indian Ocean. A peat core was collected in an ombrotrophic peatland from the remote Amsterdam Island (AMS) and was analyzed for 210 Pb, 137 Cs and 241 Am radionuclides using an underground ultra-low background gamma spectrometer. The 210 Pb Constant Rate of Supply (CRS) model of peat accumulations is validated by peaks of artificial radionuclides ( 137 Cs and 241 Am) that are related to nuclear weapon tests. We compared the AMS 210 Pb data with an updated 210 Pb deposition database. The 210 Pb flux of 98 ± 6 Bq·m −2 ·y −1 derived from the AMS core agrees with data from Madagascar and South Africa. The elevated flux observed at such a remote location may result from the enhanced 222 Rn activity and frequent rainfall in AMS. This enhanced 222 Rn activity itself may be explained by continental air masses passing over southern Africa and/or Madagascar. The 210 Pb flux at AMS is higher than those derived from cores collected in coastal areas in Argentina and Chile, which are areas dominated by marine westerly winds with low 222 Rn activities. We report a 137 Cs inventory at AMS of 144 ± 13 Bq·m −2 (corrected to 1969). Our data thus contribute to the under-represented data coverage in the mid-latitudes of the Southern Hemisphere. [ABSTRACT FROM AUTHOR]

The origin and structure of eastward propagating decadal temperature variability in the South Atlantic and Indian Oceans are investigated by using long-term output of coupled general circulation model. Composite analysis during the warm Southwest Indian Ocean (SWIO) years shows that decadal temperature anomalies in the SWIO region originate in the South Atlantic and are associated with density anomalies. Since the density anomalies propagate at the speed of a few cm s−1, slower than the speed of the background eastward Antarctic Circumpolar Current, and exhibit a surface-intensified equivalent barotropic structure, the eastward propagation of the density anomalies may be attributed to quasi-stationary oceanic Rossby waves. The density anomalies are also accompanied with anomalous Ekman pumping, indicating an important contribution from the overlying atmospheric variability. The role of atmospheric variability is further examined by evaluating the mixed-layer heat balance. It is found that the warm temperature anomalies in the South Atlantic are due to anomalous entrainment and meridional advection. These results suggest that the atmospheric variability plays an important role through the ocean mixed layer in generating the eastward propagation of decadal temperature variability from the South Atlantic besides the internal ocean variability as suggested in previous studies. [ABSTRACT FROM AUTHOR]

In this paper, the NCEP-NCAR daily reanalysis data are used to investigate the characteristics of the atmospheric heat source/sink (AHSS) over South Asia (SA) and southern Indian Ocean (SIO). The thermal differences between these two regions and their influence on the outbreak of the Indian summer monsoon (ISM) are explored. Composite analysis and correlation analysis are applied. The results indicate that the intraseasonal variability of AHSS is significant in SA but insignificant in the SIO. Large inland areas in the Northern Hemisphere still behave as a heat sink in March, similar to the situation in winter. Significant differences are found in the distribution of AHSS between the ocean and land, with distinct land-ocean thermal contrast in April, and the pattern presents in the transitional period right before the ISM onset. In May, strong heat centers appear over the areas from the Indochina Peninsula to the Bay of Bengal and south of the Tibetan Plateau (TP), which is a typical pattern of AHSS distribution during the monsoon season. The timing of SA-SIO thermal difference turning positive is about 15 pentads in advance of the onset of the ISM. Then, after the thermal differences have turned positive, a pre-monsoon meridional circulation cell develops due to the near-surface heat center and the negative thermal contrast center, after which the meridional circulation of the ISM gradually establishes. In years of early (late) conversion of the SA-SIO thermal difference turning from negative to positive, the AHSS at all levels over the TP and SIO converts later (earlier) than normal and the establishment of the ascending and descending branches of the ISM's meridional circulation is later (earlier) too. Meanwhile, the establishment of the South Asian high over the TP is later (earlier) than normal and the conversion of the Mascarene high from winter to summer mode occurs anomalously late (early). As a result, the onset of the ISM is later (earlier) than normal. However, the difference in vorticity between early and late conversion only shows in the changes of strong vorticity centers' location in the upper and lower troposphere. [ABSTRACT FROM AUTHOR]

• Under all scenarios, an intensification of warming and marine heatwaves is projected between 40°S and 55°S. • Projected climate velocities are faster in the mesopelagic than at the surface. • SSP1-2.6 and SSP2-4.5 warming and MHW patterns mostly differ in the long term. The Southern Indian Ocean (20–120°E, 70–30°S) hosts an exceptional biodiversity that contributed to the inscription of the French and Australian natural reserves on the UNESCO World Heritage List. This region is a "hot spot" for ocean heat uptake and already experiences intense marine heat waves (MHW), as evidenced in 2011/2012 over the Kerguelen Plateau. In the coming decades, this region is also expected to face supplemental anthropogenic warming, depending on future greenhouse gas emissions, with unknown consequences for its marine ecosystems. Here, we present a regional analysis of ocean warming and MHW based on the analyses of historical observations and Coupled Model Intercomparison Project Phase 6 (CMIP6) climate projections. Consistent with observations over the last decades, we find an intensification through the 21st century of surface warming and MHW over a band located between 40°S and 55°S within the Antarctic Circumpolar Current region. CMIP6 models also project much faster climate velocities (i.e. the speed and direction at which isotherms drift in the wake of climate change) in the mesopelagic (200–1000 m) than at the surface (0–200 m). Lastly, a comparison between the two Shared Socioeconomic Pathways (SSP1-2.6 and SSP2-4.5) analysed in this study shows much larger changes in the second half of the 21st century for the higher emission scenario. These results suggest that the subantarctic islands will probably be mostly affected by warming and MHW under both scenarios, although committing to SSP1-2.6 could substantially alleviate the pressure on ecosystems in the long term. This study also highlights the need to consider a tri-dimensional environment that may evolve at different paces when designing efficient conservation measures. [ABSTRACT FROM AUTHOR]

The change of sea surface temperature (SST) in the southern Indian Ocean (SIO) during the recent six decades has been analyzed based on oceanic reanalysis and model, as well as atmospheric data. The results show that a thermal regime shift in SIO during the 1960s, which is not caught enough attentions, has been of equal magnitude to the linear warming since 1970. Empirical Orthogonal Function (EOF) analyses reveal that a thermal shift is combined with atmospheric changes such as the weakening of westerly during the period of 1960-1967. Inner dynamic connections can be defined that when the westerly winds turn weak, the anticyclonic wind circulation between westerly winds and the trade winds decreases, which further reduces the SST to a negative peak in this period. It is noted that the shifts in the 1960s are also evident for Southern Hemisphere. For example, subtropical high and the entire westerly winds belt at high latitudes both change dramatically in the 1960s. This large-scaled process maybe link to the change of southern annular mode (SAM). [ABSTRACT FROM AUTHOR]