This study reveals the detailed mechanism behind the winter-time barrier layer (BL) formation on an interannual timescale in the eastern and western Bay of Bengal (BoB) from an ocean dynamics perspective. The interannual variability (IAV) of winter-time barrier layer thickness (BLT) over the coastal waveguide and eastern BoB is mostly controlled by the isothermal layer depth (ILD) variation induced by the second downwelling Kelvin and associated Rossby wave activities, respectively, which is highly influenced by the El Niño-Southern Oscillation and Indian Ocean Dipole events. The influence of the Kelvin waves on the BLT variation reduces over the western BoB due to high mixed layer depth (MLD) variability linked with freshwater distribution and vertical mixing. The composite analysis of years with extreme BLT (positive BLT years) and those with the least BLT (negative BLT years), identified by an empirical orthogonal function analysis of BLT anomalies over the western BoB, reveals contrasting mixing dynamics at the mixed layer (ML) base. During the positive BLT years, an enhanced (reduced) vertical mixing at the mixed layer base over the north (south) of 15°N is noted, which reduces the freshwater export beyond 15°N, resulting in shallow (deep) MLD at the northern (southern) bay. A deeper ILD and a shallower MLD over the north lead to the formation of a thick BL in these years. During the negative BLT years, reduced vertical mixing and anomalous poleward coastal currents bring anomalously high-saline water into the bay, which increases the MLD and decreases the BLT.

This study investigates the interannual variability (IAV) of the Bay of Bengal (BoB) thermohaline structure in terms of mixed layer depth (MLD), isothermal layer depth (ILD), and barrier layer thickness (BLT) over a 65-year period spanning from 1958 to 2022. This study offers a comprehensive understanding of the mentioned IAV and underlying mechanism using the ORAS5 and ERA5 reanalysis data products. Although previous studies have explored seasonal and year-to-year variability in this region, this study delves into unexplored dynamics like differential spatial response to the plausible drivers of the IAV. An empirical orthogonal function (EOF) analysis conducted on monthly anomalies of MLD, ILD, and BLT reveals that the first EOF accounts for 44.5% of the variance in ILD, 23.7% in MLD, and 22.3% in BLT, and the first principal component (PC) shows a good correlation to Niño3.4 index and dipole mode index (DMI). This analysis further reveals that the influence of El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) is restricted to the southern and eastern boundaries of the bay. The composite analysis shows that the ILD exhibits negative (positive) anomalies in the equatorial and the eastern BoB during El Niño (La Niña) years, whereas the MLD does not show a distinct response to the ENSO events. The negative (positive) ILD anomalies are also prominent in the eastern BoB during positive (negative) IOD events. Unlike the ENSO years, negative (positive) MLD anomalies are visible in the Southern BoB during positive (negative) IOD years. The above anomalous variation in the MLD and ILD results in an anomalous decrease (increase) in BLT in the eastern side during El Niño and positive IOD years (La Niña and Negative IOD years). The response mentioned above in the MLD, ILD, and BLT is linked to the interannual response of the Kelvin waves and associated Rossby wave radiation to the ENSO and IOD forcing. In the northern, central, and western BoB, salinity exerts a strong influence on barrier layer formation, likely driven by the freshwater influx through evaporation, precipitation, and river runoff; however, the exact role of river discharge in modulating the IAV of BLT remains poorly understood. Since southern BoB acts as a persistent heat source, meridional heat transport (MHT) via the eastern boundary plays a critical role in enhancing the deepening of ILD, with heat being advected northward by oceanic currents. The role of the East India Coastal Current (EICC) is also evident, with freshwater advection contributing significantly to MLD variability along the east coast of India. The influence of monsoon current is observed in the second EOF of both MLD and ILD, capturing 9% and 8.5% of the variance respectively. A strong monsoon current brings high-saline, relatively cooler Arabian Sea water into the southwest BoB, resulting in deeper MLD and shallow ILD.

Using a spatiotemporal dataset of dissolved lead (dPb) from the subtropical oceans surrounding South Africa, this study quantifies the exchange of dPb between the Indian and Atlantic Oceans. Despite the absence of a major Pb source within the South Atlantic sector and the complete phase-out of leaded petroleum in Southern Africa, the ecologically important southeast Cape Basin shows an elevated surface dPb concentration (21–30 pmol kg −1 ). We estimated up to 90% of the measured dPb in surface waters of the Cape Basin was delivered from the Indian Ocean via the Agulhas Current (AC). Eddy dynamics and leakage at Agulhas retroflection result in an increased Pb flux from winter to summer, while a long-term (2008–2019) temporal change in dPb in the AC-derived water of Cape Basin was contemporaneous to a change in atmospheric Pb emissions from South Africa. The South African-origin atmospheric Pb, however, contributes first to the Agulhas waters in the West Indian Ocean, which is then transported to the South Atlantic, thereby regulating the dPb inventory of the Cape Basin. This indirect mechanism of Pb transfer emphasizes the importance of regulating Pb emissions from Southern Africa to protect rich fishing grounds associated with the Benguela marine ecosystem.

Whales have been titled climate savers in the media with their recovery welcomed as a potential carbon solution. However, only a few studies were performed to date providing data or model outputs to support the hypothesis. Following an outline of the primary mechanisms by which baleen whales remove carbon from the atmosphere for eventual sequestration at regional and global scales, we conclude that the amount of carbon whales are potentially sequestering might be too little to meaningfully alter the course of climate change. This is in contrast to media perpetuating whales as climate engineers. Creating false hope in the ability of charismatic species to be climate engineers may act to further delay the urgent behavioral change needed to avert catastrophic climate change impacts, which can in turn have indirect consequences for the recovery of whale populations. Nevertheless, whales are important components of marine ecosystems, and any further investigation on existing gaps in their ecology will contribute to clarifying their contribution to the ocean carbon cycle, a major driver of the world’s climate. While whales are vital to the healthy functioning of marine ecosystems, overstating their ability to prevent or counterbalance anthropogenically induced changes in global carbon budget may unintentionally redirect attention from known, well-established methods of reducing greenhouse gases. Large scale protection of marine environments including the habitats of whales will build resilience and assist with natural carbon capture.

Seasonal feeding behaviour of humpback whales ( Megaptera novaeangliae ) has been observed in the coastal waters of the Southern Benguela where the species has been observed forming super-groups during the austral spring in recent years since 2011. Super-groups are unprecedented densely-packed aggregations of between 20 and 200 individuals in low-latitude waters and their occurrences indicate possible changes in feeding behaviour of the species. We accessed published data on super-groups occurrence in the study area in 2011, 2014 and 2015, and investigated oceanographic drivers that support prey availability in this region. We found that enhanced primary production is a necessary but not sufficient condition for super-groups to occur. Positive chlorophyll anomalies occurring one month prior to the super-group occurrences were identified, but only a concurrent significantly reduced water volume export from the region throughout October were conducive to the aggregations in the specific years. Hydrodynamic model results attributed the anomalous decreased volume export to the strength and orientation of the Goodhope Jet and associated eddy activity. The combination of random enhanced primary production typical of the region and emerging anomalous conditions of reduced water export in October since 2011 resulted in favourable food availability leading to the unique humpback whale aggregations. The novelty of this grouping behaviour is indicative of the lack of such oceanographic conditions in the past. Given the recency of the events, it is difficult to attribute this reduction in ocean transport to climatic regime shifts, and the origin should be likely investigated in the distant water mass interaction with the greater Agulhas system rather than in local intensifications of the upwelling conditions. A positive trend in the humpback whale population abundance points to the need to monitor the exposure of the species to the changing climate conditions.

Humpback whales, Megaptera novaeangliae , are a highly migratory species exposed to a wide range of environmental factors during their lifetime. The spatial and temporal characteristics of such factors play a significant role in determining suitable habitats for breeding, feeding and resting. The existing studies of the relationship between oceanic conditions and humpback whale ecology provide the basis for understanding impacts on this species. Here we have determined the most relevant environmental drivers identified in peer-reviewed literature published over the last four decades, and assessed the methods used to identify relationships. A total of 148 studies were extracted through an online literature search. These studies used a combined estimated 105,000 humpback whale observations over 1,216 accumulated study years investigating the relationship between humpback whales and environmental drivers in both Northern and Southern Hemispheres. Studies focusing on humpback whales in feeding areas found preferences for areas of upwelling, high chlorophyll-a concentration and frontal areas with changes in temperature, depth and currents, where prey can be found in high concentration. Preferred calving grounds were identified as shallow, warm and with slow water movement to aid the survival of calves. The few studies of migration routes have found preferences for shallow waters close to shorelines with moderate temperature and chlorophyll-a concentration. Extracting information and understanding the influence of key drivers of humpback whale behavioral modes are important for conservation, particularly in regard to expected changes of environmental conditions under climate change.

High riverine freshwater discharge makes the salinity distribution highly heterogeneous in the Bay of Bengal (BoB). This paper investigates the impact of freshwater discharge from ten major rivers on the seasonal sea surface variability and the coastal Kelvin wave characteristics in the BoB using the Regional Ocean Modeling System (ROMS). Two numerical experiments were conducted (i) with- and (ii) without-incorporating monthly climatology of river runoff into the model under the same forcing conditions. The model was forced by Comprehensive Ocean-Atmosphere DataSet (COADS) climatology for both the experiments. This study demonstrates that incorporation of river discharge into the model simulates a realistic picture of sea surface salinity, seasonal sea level variations, Kelvin wave activities, and equatorward flowing East India Coastal Current (EICC). The amplitude of the second downwelling Kelvin wave is increased most (greater than 4 cm at the Odisha coast and approximately 2 cm near the Chennai coast) due to river discharge. Apart from that, we also found that the equatorward EICC is strengthened and more organized when river runoff is considered. The volume transport by equatorward EICC becomes almost double when river runoff is included in the model in most of the region.

Globally, baleen whales were severely depleted by historic whaling. Recovering populations have been observed to alter their behaviour. These changes have been attributed to climate change in some cases and raise concerns over the successful recovery of baleen whale populations. Current data‐driven statistical habitat and behavioural models have proven useful for addressing questions of whale distribution changes within their limitations. Given observed changes in oceanic conditions, a new approach to managing baleen whale population recovery is necessary. Model predictions of future whale movements and distributions under climate change scenarios are vital to enable adequate conservation management. This paper presents a new perspective on understanding the impacts of climate change on humpback whales, arguing the need for a system‐based multidisciplinary research approach. Our approach includes coupled, mechanistic models based upon robust ecological principles, and integrates key physical, biogeochemical, biological and ecological modules to address long‐term changes associated with climate change. To illustrate the need for this system‐based multidisciplinary approach, we focus on Southern Hemisphere humpback whales, the recovery of which may be impacted by rapid changes in habitat conditions brought about by anthropogenic climate change.

The presence of large-amplitude Internal Solitary Waves (ISWs or solitons) is quite common in the Andaman Sea, located in the north-eastern Indian Ocean basin. ISWs are known to induce strong vertical velocities which can play an essential role in the mixing transport of nutrients and are proven hazardous to offshore oil platforms. The surface signatures of ISWs can be detected using remote sensing instruments like Synthetic Aperture Radar (SAR) and sunglint true-colour images. The present study makes an effort to delineate as well as detect the possible potential generation locations of mode-1 long living ISWs in the Andaman Sea using remote sensing observations. To accomplish this, the Moderate Resolution Imaging Spectroradiometer (MODIS) true-colour images of Terra/Aqua satellites for the months of March and April during 2014–2016 are used to map the distribution and propagation characteristics of ISWs. These maps along with SAR imgaes from ENVISAT and TerraSAR-X are used to detect the possible generation locations of ISWs. The study considers the possible generation location of ISW as the circumcentre of each wave packet as they radially propagate along a two-dimensional frame. The analysis reveals five potential ISW generation hotspots that are distributed along the Northern Andaman Sea, as well as locations in the discontinuities off the Nicobar Islands and the great passage. The ISWs that form over these regions are hitting the continental shelf within the Andaman Sea. Interestingly, the waves from two potential generation sites between the Nicobar Islands appear to radiate waves in two opposite directions, towards the Andaman Sea and the southern Bay of Bengal.

An investigation of the eddy and coastal Kelvin wave activities in the Bay of Bengal (BoB) is carried out during premonsoon season in two years of Indian summer monsoon deficit in June (2009 and 2012), occurred in the recent warming hiatus period. Using altimeter observations, our study reveals that over the northern BoB cyclonic eddy kinetic energy is reduced by 35% and 50% from the climatology during premonsoon seasons in 2009 and 2012, respectively, while the cyclonic eddy area is reduced by 18% and 24%, respectively. A concurrent reduction is observed in the first upwelling Kelvin wave (uKW) activities in the eastern equatorial Indian Ocean as well as in the coastal BoB for these years. The reduction in the generation of the first uKW in the eastern equatorial Indian Ocean is attributed to the westerly wind anomalies in January-March of these years. Additionally, meridional wind stress anomalies during March-April in these years are found to be southerly, causing anomalous coastal downwelling in the eastern rim of BoB. This coastal downwelling blocks the propagation of the first uKW. The decrease in the first uKW activities in the coastal waveguide of the BoB reduces the radiation of upwelling Rossby waves, thereby decreasing the cyclonic eddy activities in the northern BoB. The results from this letter could be helpful for further understanding of upper ocean mixing processes in the BoB during monsoon deficit years.

ABSTRACTThis study investigates the impact of wind‐induced sea ice drift on sea ice cover over the Indian Ocean sector of the Southern Ocean (IOS) in the contrasting Southern Annular Mode (SAM) years during a summer (February) and a winter (July) month. Analysis of reanalysis wind shows that during positive SAM events, westerlies show stronger and more zonal flow over the west IOS (west of 45°E), while a stronger northerly component is seen over the east IOS, to the south of 55°S during both February and July. This is attributed to the zonally asymmetric feature of sea level pressure over the IOS. A coupled ocean–sea ice model was forced with dynamical wind forcing for positive and negative SAM events during above months. The zonal contrast as seen in wind and surface current is transferred to the sea ice drift. A stronger zonal eastward sea ice drift is apparent over the west IOS, suggesting increased transport of sea ice from the Weddell Sea region in July. The eastward advection of sea ice results in piling of sea ice over west IOS and causes an increase in sea ice concentration and thickness. Over east IOS, the sea ice drift shows a strong southeastward anomaly from the sea ice edge towards the coast. This results in a piling of sea ice near the coast and a divergence of sea ice near the edge. This results in a negative anomaly in sea ice concentration and sea ice thickness over east IOS. Thus, the dynamical SAM forcing leads to a non‐annular response in sea ice cover over the IOS.