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Analysis of sea surface temperature (SST) data robustly detects dipole‐pattern decadal anomalies in the southern Atlantic and Indian Oceans (SAIOs) that co‐occur like twins, termed the Atlantic‐Indian Twin Dipoles (AITDs). The mechanisms governing the AITDs are investigated based on observational data sets, climate model simulations, and ocean model experiments. We show that the AITDs are coupled with alterations in subtropical highs that involve a variety of regional air‐sea processes. Specifically, while cloud‐controlled radiative heating plays key role in the Atlantic, wind‐driven turbulent heat flux change is essential in the Indian Ocean. Wind‐driven ocean dynamics are also important near the eastern boundaries of the SAIOs. Interdecadal Pacific Oscillation and Southern Annular Mode are conducive to the AITDs by modulating the subtropical highs. Our results highlight the trans‐basin linkage between the SAIOs, providing implications for predicting the Southern Hemisphere climate and regional extremes.

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.

In this study, we discuss the influence of the Southern Indian Ocean Dipole (SIOD) on the location variation of the Intertropical Convergence Zone (ITCZ) over the Indian Ocean and its possible reasons using observations and the Community Earth System Model version 1.0.4 (CESM1.0.4). The results indicate that positive SIOD (PSIOD) and negative SIOD (NSIOD) events in boreal winter have a considerable influence on the ITCZ location over the Indian Ocean in the following year, especially in boreal spring and summer. The observations suggest that the ITCZ shifts northward in the following spring and summer after PSIOD events, while shifts southward after NSIOD events. Numerical simulations with CESM can well reproduce the observed results. We further analyze the correlation between the winter sea surface temperature anomalies over the southern Indian Ocean in SIOD years and the ITCZ location over the Indian Ocean during the following spring and summer, and the results not only confirm that the PSIOD and the NSIOD drive the ITCZ to shift northward and southward, respectively, but also prove that SIOD events show a greater impact on the Indian Ocean ITCZ location in the following spring and summer. In addition, numerical experiments show that the northeast SST of SIOD has a greater impact on ITCZ than the southwest SST. After NSIOD events, the Mascarene high and the Australian high weaken in the following spring and summer, and northwesterly cross-equatorial flow anomalies appear in the equator around 50°E-90°E. Moreover, there is remarkable and anomalous ascending motion over 0°S-20°S, and the ascending branch of the Hadley circulation is southward. Simultaneously, a positive precipitation anomaly appears south of the equatorial Indian Ocean, and the ITCZ over the Indian Ocean moves southward. However, after PSIOD events, the effects on the Indian Ocean ITCZ location in the following spring and summer are roughly opposite to those of NSIOD events.

Southern Indian Ocean dipole (SIOD) is a dipole SST anomaly in the Southern Indian Ocean that plays an important role in tropical climate variability. However, previous studies focused on the interannual and interdecadal scales. In this paper, the characteristics on sub-seasonal scale of SIOD are revealed from December to February (DJF) based on the ERA-Interim reanalysis dataset, and the excitation mechanisms of SIOD are also discussed. The results show that the two dominant modes of SST in the Southern Indian Ocean are the spatial distribution of Southwest-Northeast direction dipole (SIOD) and triple (SIOT) respectively, which has obvious period of quasi 50–60 days. Moreover, the intensity and zonal oscillation of the Mascarene high are conducive to the formation of SIOD (SIOT). The air–sea interaction during the formation process is composed of three stages. In the first stage the atmospheric forces the ocean to result in the Mascarene high westward (eastward) and enhances the abnormal anticyclone in the Southern Indian Ocean. There are abnormal northerly (southerly) flows on the west (east) sides of the abnormal anticyclone respectively, which weakens (enhances) the southeast trade wind in the climatology. The surface latent heat flux release decreases (increases) and SST is warmed (cooled). During the following stage, the ocean feedback the atmosphere. The warm SST continues to increase, resulting in low-level convective enhancement, which weakens the abnormal anticyclone. The third stage is again the atmosphere forcing the ocean. The abnormal anticyclone gradually turns into an abnormal cyclone and the meridional wind direction is reversed. The release of the latent heat flux increases (decreases) significantly which leads to the cooling (warming) of SST on the west (east) sides of an abnormal cyclone. In addition, the formation and extinction of SIOT are easier affected by the southern annular mode (SAM) than SIOD. The abnormal zonal wave train with wavenumber 4 (3) propagates the Southern Indian Ocean by the westerly jet waveguide and results in an SST anomaly of SIOD (SIOT), accompanied by an obvious sub-seasonal meridional variation of the precipitation in Southern Africa.

While the El Niño-Southern Oscillation and Indian Summer Monsoon Rainfall (ISMR) relationship is weak in recent years, a strong correlation between May Southern Annular Mode Index (SAMI) and June–July (JJ) ISMR is a southern hemispheric source of ISMR predictability. Here, using observed and reanalysis data, we find that the SAMI–ISMR relationship is non-stationary with a potential multi-decadal variability. Both during high/low correlation periods (1980–2010)/(1949–1979), a Southern Indian Ocean Dipole (SIOD) pattern of JJ sea surface temperature anomaly is found to reverse sign during strong and weak SAMI years. The changes in the strength and location of the northern pole of SIOD during the two time blocks are consistent with corresponding changes in the cross equatorial flow and monsoon south-westerlies together with change in SAMI–ISMR correlations. Our analysis indicates teleconnection pathways through which the Atlantic Multidecadal Oscillation (AMO) may be responsible for the multi-decadal swings of SAMI–ISMR correlations through modulation of the SIOD.

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.

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.

Stable isotope (δ18O and δD) and salinity measurements were made on the surface waters collected from the Southern Indian Ocean during the austral summer (25 January to 1 April 2006) onboard R/V Akademik Boris Petrov to study the relative dominance of various hydrological processes, viz. evaporation, precipitation, melting and freezing over different latitudes. The region between 41°S and 45°S is a transition zone: the region lying north of 41°S is dominated by evaporation/precipitation process whereas that south of 45°S (up to Antarctica) is dominated by melting/freezing processes. Further, the combined study of stable oxygen and hydrogen isotope (δ18O and δD) confirms that the Southern Indian Ocean evaporates in non-equilibrium conditions.

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.

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.