Eastern Equatorial Indian Ocean Research Articles

Seasonal and interannual variabilities of chlorophyll across the eastern equatorial Indian Ocean and Bay of Bengal

Seasonal and interannual variabilities of surface and subsurface chlorophyll (Chl) in the eastern equatorial Indian Ocean (IO) and Bay of Bengal were examined based on vertical Chl inferred from satellite data. Four biotic zones were identified by Empirical orthogonal function analysis, which represented different types of mixed layer processes. The areas south of Sri Lanka indicated the most significant Chl increase in the surface and subsurface during summer due to coastal upwelling. Surface Chl highs in the Sri Lanka Dome occurred every summer during the time series. Chl increases were also recorded in the southwestern Bay of Bengal during fall–winter in response to wind-induced divergence and convective mixing. The other two zones in south IO basin between 5 and 15°S exhibited seasonal surface Chl highs in summer and subsurface Chl maximum during fall–winter. Wind-induced divergence plays a major role in the maintenance of this upwelling zone year-round; however, stratification inhibited the surface Chl increase in winter and results in enhanced Chl in the subsurface. The Chl interannual variability was related with Indian Ocean dipole (IOD) associated physical field oscillations. In positive IOD, wind divergence increased significantly along the eastern equatorial IO coast that contributed to Chl enhancement there, it decreased in Bay of Bengal and South IO and acted to reduce the Chl in these regions. In negative IOD, reversals in the forcing anomalies drove increasing Chl in Bay of Bengal and South IO and decreasing of that in the eastern equator. This resulted in significant correlations between IOD index and Chl confined in these regions. There are pronounced variance of phytoplankton carbon biomass in the Bay of Bengal in IOD events, however, less changes of that were seen over the South IO during IOD. Our findings imply that physiological changes in cellular pigmentation are the dominant cause of satellite-observed interannual variations in Chl in the South IO. Our results highlight the role of physiological processes in regulating Chl distributions, and support the use of Chl-based indices as indicators of climatic feedbacks.

The Mixed Layer Salinity Budget in the Central Equatorial Indian Ocean

AbstractThe oceanic surface mixed layer salinity (MLS) budget of the central and eastern equatorial Indian Ocean during boreal fall and winter is studied using in situ and remote sensing measurements. Budgets on roughly 100 km scale were constructed using data from two DYNAmics of the Madden–Julian Oscillation and two Research Moored Array for African‐Asian‐Australian Monsoon Analysis and Prediction moorings near 79°E during September 2011 to January 2012. The horizontal advective salinity flux plays a significant role in the seasonal variation of equatorial MLS. In boreal fall, the equatorial and 1.5°S MLS increases due to horizontal advection and turbulent mixing, despite the freshening surface flux associated with MJOs. In boreal winter, with larger sub‐monthly variation and uncertainties, the decreasing of equatorial MLS is accounted by freshening zonal advection and surface flux, abated by salty meridional advection; the 1.5°S MLS is explained by the combination of freshening meridional advection and surface flux, and salty zonal advection. Budgets between 2011 and 2015 are investigated using data products from Tropical Rainfall Measuring Mission, Aquarius, Ocean Surface Current Analyses Real‐time, Objectively Analyzed air‐sea Fluxes, and Argo mixed layers over a wider region. The eastward development of the equatorial salinity tongue in the central to eastern Indian Ocean in boreal fall and the westward retreat in boreal winter are largely determined by the equatorial zonal current. The meridional migration of ITCZ rainfall plays a secondary role. In order to improve model prediction skills of MLS changes in the equatorial Indian Ocean, both zonal and meridional salinity advective fluxes, at a spatial scale of 1° longitude and latitude and a timescale less than days, need to be properly simulated.

Sea Surface Temperature Variability over the Tropical Indian Ocean during the ENSO and IOD Events in 2016 and 2017

2016 and 2017 were marked by strong El Niño and weak La Niña events, respectively, in the tropical East Pacific Ocean. The strong El Niño and weak La Niña events in the Pacific significantly impacted the sea surface temperature (SST) in the tropical Indian Ocean (TIO) and were followed by extreme negative and weak positive Indian Ocean Dipole (IOD) phases in 2016 and 2017, which triggered floods in the Indian subcontinent and drought conditions in East Africa. The IOD is an irregular and periodic oscillation in the Indian Ocean, which has attracted much attention in the last two decades due to its impact on the climate in surrounding landmasses. Much work has been done in the past to investigate global climate change and its impact on the evolution of IOD. The dynamic behind it, however, is still not well understood. The present study, using various satellite datasets, examined and analyzed the dynamics behind these events and their impacts on SST variability in the TIO. For this study, the monthly mean SST data was provided by NOAA Optimum Interpolation Sea Surface Temperature (OISST). SST anomalies were measured on the basis of 30-year mean daily climatology (1981–2010). It was determined that the eastern and western poles of the TIO play quite different roles during the sequence of negative and positive IOD phases. The analysis of air-sea interactions and the relationship between wind and SST suggested that SST is primarily controlled by wind force in the West pole. On the other hand, the high SST that occurred during the negative IOD phase induced local convection and westerly wind anomalies via the Bjerknes feedback mechanism. The strong convection, which was confined to the (warm) eastern equatorial Indian Ocean was accompanied by east–west SST anomalies that drove a series of downwelling Kelvin waves that deepened the thermocline in the east. Another notable feature of this study was its observation of weak upwelling along the Omani–Arabian coast, which warmed the SST by 1 °C in the summer of 2017 (as compared to 2016). This warming led to increased precipitation in the Bay of Bengal (BoB) region during the summer of 2017. The results of the present work will be important for the study of monsoons and may be useful in predicting both droughts and floods in landmasses in the vicinity of the Indian Ocean, especially in the Indian subcontinent and East African regions.

Diversity of picoeukaryotes in the eastern equatorial Indian Ocean revealed by metabarcoding

We used 18S rRNA gene metabarcoding to investigate picoeukaryotic diversity and distribution at the surface and deep chlorophyll maximum (DCM) of 4 stations in the eastern equatorial Indian Ocean (EEIO). The results showed that picoeukaryotic communities were dominated by 5 phyla: Dinoflagellata, Radiolaria, Chlorophyta, Ochrophyta and Ciliophora. The picoeukaryotic communities were classified into 3 groups matching their water mass origins and depth: (1) Group I was in the surface waters of the Bay of Bengal, which had low salinity, and was dominated by Radiolaria Group A, Spirotrichea and marine stramenopiles; (2) Group II was in the DCM within the intrusion of Arabian Sea high salinity water, in which Chloropicophyceae and Pelagophyceae were more abundant; and (3) Group III was located in the 0°-5°S surface water, which was enriched by Dinophyceae. In addition, Caecitellaceae paraparvulus was abundant at 4°S, where weak vertical mixing occurred. This study provides the first baseline of picoeukaryotic diversity in the EEIO.

Generating a High-Resolution Time-Series Ocean Surface Net Radiation Product by Downscaling J-OFURO3

The ocean surface net radiation ( $R_{n}$ ) characterizing ocean surface radiation budget is a key variable in ocean climate modeling and analysis. In this study, a downscaling scheme was developed to generate a high-resolution (0.05°) time-series (2002–2013) daily ocean surface $R_{n}$ from the third-generation Japanese Ocean Flux Data Sets with Use of Remote-Sensing Observations (J-OFURO3) at 0.25° based on the Advanced Very-High-Resolution Radiometer (AVHRR) top-of-atmosphere (TOA) observations (AVH021C) and other ancillary information (Clearness Index and cloud mask). This downscaling scheme includes the statistical downscaling models and the residual correction post-processing. A series of angle-dependent downscaling statistical models were established between the daily ocean surface $R_{n}$ in J-OFURO3 and the AVHRR TOA data, and then, the residual correction was conducted to the model estimates $R_{n}$ _AVHRR_est to obtain the final downscaled data set $R_{n}$ _AVHRR. Validation against the measurements from 57 moored buoy sites in six ocean observing networks shows the high accuracy of the downscaled estimates $R_{n}$ _AVHRR_est with a $R^{2}$ of 0.88, RMSE of 23.44 $\text{W}\cdot \text{m}^{-2}$ , and bias of $-0.14\,\,\text{W}\cdot \text{m}^{-2}$ under all-sky condition. The results of the spatio-temporal analysis in $R_{n}$ _AVHRR and intercomparison with Cloud and the Earth’s Radiant Energy System (CERES) and the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) products also indicated that the superior of the $R_{n}$ _AVHRR with more detailed information especially in the hot spot regions, such as central tropical Pacific (warming pool), Atlantic and Equatorial Eastern Indian Ocean (EIO).

Is Preconditioning Effect On Strong Positive Indian Ocean Dipole by a Preceding Central Pacific El Niño Deterministic?

AbstractThe 2018 Central Pacific (CP) El Niño preceded the 2019 strong positive Indian Ocean Dipole (pIOD). The robustness of a CP El Niño preconditioning a strong pIOD has not been investigated. Here, we show that March‐April‐May (MAM) easterly anomalies over the equatorial eastern Indian Ocean (EIO) induced by a preceding CP El Niño drive a thermocline shallowing conducive to an early equatorial EIO cooling. However, the same winds also generate off‐equatorial downwelling Rossby waves in the southern EIO, which reflect as downwelling Kelvin waves, able to weaken the initial anomalies. Furthermore, zonal winds in June‐July‐August (JJA) can either be beneficial or unfavorable for the developing pIOD. Only when the equatorial easterlies, EIO cooling, and thermocline shallowing are sufficiently amplified by JJA does a strong pIOD occur. Despite this, a multi‐century model simulation suggests that development of majority of strong pIOD events is facilitated by a preceding CP El Niño.

The Composition and Primary Metabolic Potential of Microbial Communities Inhabiting the Surface Water in the Equatorial Eastern Indian Ocean.

Simple SummaryMarine microbes are regarded as the most diverse organisms in the biosphere and drive biogeochemical cycles through their metabolism. It is essential to understand the structure and metabolic function of microbial communities. The Indian Ocean is the third largest ocean in the world, and it possesses unique hydrographical properties. So far, assessments of microbial diversity and metabolism need to be improved in the Indian Ocean. Therefore, we carried out a series of investigations in the equatorial eastern Indian Ocean in order to clarify the local microbial communities and detect the genetic potential for microbial functions. The obtained results suggested Cyanobacteria was the dominant microbial group, and predicted the Calvin cycle and the assimilatory nitrate and nitrite reduction played important role in the pathway of carbon fixation and nitrogen metabolism respectively. This study provides insights into microbial community structures as well as the metabolic potential that may be active in the local environment, and lays the groundwork for understanding the roles of microbes in energy and resource cycling in this habitat.Currently, there is scant information about the biodiversity and functional diversity of microbes in the eastern Indian Ocean (EIO). Here, we used a combination of high-throughput sequencing of 16S rRNA genes and a metagenomic approach to investigate the microbial population structure and its metabolic function in the equatorial EIO. Our results show that Cyanobacterial Prochlorococcus made up the majority of the population. Interestingly, there were fewer contributions from clades SAR11 (Alphaproteobacteria) and SAR86 (Gammaproteobacteria) to microbial communities than contributions from Prochlorococcus. Based on functional gene analysis, functional genes rbcL, narB, and nasA were relatively abundant among the relevant genes. The abundance of Prochlorococcus implies its typically ecological adaptation in the local ecosystem. The microbial metabolic potential shows that in addition to the main carbon fixation pathway Calvin cycle, the rTCA cycle and the 3-HP/4-HB cycle have potential alternative carbon fixation contributions to local ecosystems. For the nitrogen cycle, the assimilatory nitrate and nitrite reduction pathway is potentially the crucial form of nitrogen utilization; unexpectedly, nitrogen fixation activity was relatively weak. This study extends our knowledge of the roles of microbes in energy and resource cycling in the EIO and provides a foundation for revealing profound biogeochemical processes driven by the microbial community in the ocean.

Convection over the eastern equatorial Indian Ocean and Iranian branch of Asian summer monsoon anticyclone modulates the wet and dry phases of summer monsoon rainfall over Kerala

AbstractThis study examined factors affecting wet and dry phases of monsoon rainfall over Kerala, using modern‐era retrospective analysis for research and applications (MERRA 2) data set. Based on circulation pattern, our study shows that the wet and dry phases of monsoon over Kerala are related to modulation of convective activity over the eastern equatorial Indian Ocean (EEIO) and the Iranian branch (Western flank) of Asian Summer monsoon anticyclone (ASMA). Outgoing longwave radiation (OLR) and cloud top pressure (CTP) show an anomalous dipole (quadrupole) pattern over the Equatorial Indian Ocean (EIO) during the wet (dry) phase of Kerala summer monsoon rainfall (KSMR). We found that KSMR is enhanced during intensification of the Iranian branch of ASMA and suppressed by anomalous deep convection over the EEIO. Further, anomalous lower level cyclonic circulation over the Western North Pacific (WNP) is pulling low‐level jet (LLJ) through Southern Peninsular India, hence playing a vital role in establishing the KSMR wet phase. Contrarily, the dry phase of KSMR is associated with weakening of the Iranian branch of ASMA in the upper level.

Correlation between subsurface salinity anomalies in the Bay of Bengal and the Indian Ocean Dipole and governing mechanisms

Abstract. Lead–lag correlations between the subsurface temperature and salinity anomalies in the Bay of Bengal (BoB) and the Indian Ocean Dipole (IOD) are revealed in model results, ocean synthesis, and observations. Mechanisms for such correlations are further investigated using the Hamburg Shelf Ocean Model (HAMSOM), mainly relating to the salinity variability. It is found that the subsurface salinity anomaly of the BoB positively correlates to the IOD, with a lag of 3 months on average, while the subsurface temperature anomaly correlates negatively. The model results suggest the remote forcing from the equatorial Indian Ocean dominates the interannual subsurface salinity variability in the BoB. The coastal Kelvin waves carry signals of positive (negative) salinity anomalies from the eastern equatorial Indian Ocean and propagate counterclockwise along the coasts of the BoB during positive (negative) IOD events. Subsequently, westward Rossby waves propagate these signals to the basin at a relatively slow speed, which causes a considerable delay of the subsurface salinity anomalies in the correlation. By analyzing the salinity budget of the BoB, it is found that diffusion dominates the salinity changes near the surface, while advection dominates the subsurface; the vertical advection of salinity contributes positively to this correlation, while the horizontal advection contributes negatively. These results suggest that the IOD plays a crucial role in the interannual subsurface salinity variability in the BoB.

Long-term variability of Sea Surface Temperature in the Tropical Indian Ocean in relation to climate change and variability

Abstract The long-term change in the sea surface temperature (SST) of the Tropical Indian Ocean (TIO) and the underlying mechanisms are examined in detail by analysing multiple reanalysis datasets and historical simulations of a global climate model. A robust basin-wide warming signal was found in SST, with the highest long-term warming trend (~0.09 ± 0.015 °C decade−1) seen over the north-western and equatorial Indian Ocean regions. The robust warming trend of TIO SST is a dominant warming signal of the global tropical oceans and is largely in phase with similar warming in western Pacific and Atlantic Oceans. The central equatorial Indian Ocean and south TIO regions went through significant warming during recent decades. The weakened cross-equatorial flow and the associated cyclonic winds over the Arabian Sea region have resulted in equatorial westerlies with an enhanced loss of latent heat over the central and eastern equatorial Indian Ocean. The anomalous cyclonic wind stress curl, in turn, caused mixed layer stratifications in the western Indian Ocean. The advective heat transfer from the south-eastern Indian Ocean to the western Indian Ocean and reduced oceanic cooling by vertical processes, overcome the cooling by the net loss of surface heat fluxes, and favor surface warming the TIO.

The Extreme Positive Indian Ocean Dipole of 2019 and Associated Indian Summer Monsoon Rainfall Response

AbstractThe positive Indian Ocean Dipole (IOD) event in 2019 was among the strongest on record, while the Indian Summer monsoon (ISM) was anomalously dry in June then very wet by September. We investigated the relationships between the IOD, Pacific sea surface temperature (SST), and ISM rainfall during 2019 with an atmospheric general circulation model forced by observed SST anomalies. The results show that the extremely positive IOD was conducive to a wetter‐than‐normal ISM, especially late in the season when the IOD strengthened and was associated with anomalous low‐level divergence over the eastern equatorial Indian Ocean and convergence over India. However, a warm SST anomaly in the central equatorial Pacific contributed to low‐level divergence and decreased rainfall over India in June. These results help to better understand the influence of the tropical SST anomalies on the seasonal evolution of ISM rainfall during extreme IOD events.

The impact of Indian Ocean dipole on tropical Indian Ocean surface wave heights in ERA5 and CMIP5 models

AbstractThe present study examines the relationship between the tropical Indian Ocean (TIO) significant wave height (SWH) and Indian Ocean dipole (IOD) during boreal summer season (June through August) in the latest version of European Centre for Medium‐Range Weather Forecasts reanalysis (ERA5) and wave simulations forced with surface winds and sea‐ice fields from the Coupled Model Intercomparison Project version‐5 (CMIP5) models. The interannual variability of SWH shows a significant negative correlation with the IOD over TIO. SWH anomalies display meridional tripole pattern with significant negative (positive) anomalies over eastern equatorial Indian Ocean caused by anomalous easterlies (westerlies), and positive (negative) anomalies over southeastern TIO and the north Bay of Bengal during positive (negative) phase of IOD. The strong wave heights along the east coast of India during positive IOD and the south and southwest coast of India during negative IOD are noticed. CMIP5 models GFDL‐CM3, MRI‐CGCM3 and the multi‐model mean display considerable skill in capturing these teleconnections with substantial magnitude differences. A thorough understanding of the teleconnections between IOD and TIO wave heights is a significant prerequisite for the accurate forecast of surface waves in the Indian Ocean. Hence, this study advocates the importance evaluating the ability of models in representing the SWH and IOD interactions and its implications on Indian coastal regions in the form of inundation, coastal flooding and other vulnerabilities in a changing climate scenario.

Opposite response of strong and moderate positive Indian Ocean Dipole to global warming

A strong positive Indian Ocean Dipole (pIOD) induces weather extremes such as the 2019 Australian bushfires and African floods. The impact is influenced by sea surface temperature (SST), yet models disagree on how pIOD SST may respond to greenhouse warming. Here we find increased SST variability of strong pIOD events, with strong equatorial eastern Indian Ocean cool anomalies, but decreased variability of moderate pIOD events, dominated by western warm anomalies. This opposite response is detected in the Coupled Model Inter-comparison Project (CMIP5 and CMIP6) climate models that simulate the two pIOD regimes. Under greenhouse warming, the lower troposphere warms faster than the surface, limiting Ekman pumping that drives the moderate pIOD warm anomalies; however, faster surface warming in the equatorial western region favours atmospheric convection in the west, strengthening equatorial nonlinear advection that forces the strong pIOD cool anomalies. Climate extremes seen in 2019 are therefore likely to occur more frequently under greenhouse warming. The strength of a positive Indian Ocean Dipole (pIOD) is set by sea surface temperature gradient across the equatorial Indian Ocean. Modelling shows warming will increase strong pIODs but decrease moderate pIODs, as faster surface warming in the west sets up conducive conditions for the strong events.

Latitudinal distribution of the picoplankton community in the eastern equatorial Indian Ocean during the boreal fall intermonsoon period

A flow cytometric analysis of picoplankton ( Prochlorococcus (Pro), Synechococcus (Syn), picoeukaryotes (Peuk) and heterotrophic bacteria (Hbac) was performed along a meridional transect (88°E, 10°S – 4°N) in the eastern equatorial Indian Ocean (EEIO) during the boreal fall intermonsoon period. The transect spanned contrasting physical and chemical conditions, allowing investigation of the effects of environmental gradients on picoplankton distributions. Hbac accounted for more than 80% of the total picoplankton abundance. Among picophytoplankton, Pro was numerically dominant, with abundances at least one order of magnitude greater than those of Syn and Peuk. Vertically, Pro was mainly distributed in the upper mixed layer while the subsurface Peuk abundance maxima usually coincided with the subsurface chlorophyll a maxima. Physical processes controlled the picoplankton distribution patterns. The maximum prokaryotic picoplankton (Pro, Syn and Hbac) abundances occurred at the center of a cyclonic eddy at 2°N. The depth-integrated picoplankton abundances increased northwards along the transect and were inversely related to the stratification strength. This gradient probably resulted from a combination of physical processes, such as the intrusions of the Bay of Bengal runoff and Indonesia throughflow, advection by the Wyrtki jets and strong stratification south of the equator. This study highlighted the dynamic response of picoplankton to complex physico-chemical conditions in the EEIO. • Flow cytometric analysis of picoplankton was performed along a meridional transect (88°E, 10°S – 4°N) in the eastern equatorial Indian Ocean (EEIO) during the boreal fall intermonsoon period. • Hbac accounted for more than 80% of the total picoplankton abundance. Among picophytoplankton, Pro was numerically dominant, with abundances at least one order of magnitude greater than those of Syn and Peuk. • Physical processes controlled the picoplankton distribution patterns. The depth-integrated picoplankton abundances increased northwards along the transect and were inversely related to the stratification strength.

A Unique Feature of the 2019 Extreme Positive Indian Ocean Dipole Event

AbstractDuring austral spring of 2019, an extreme positive Indian Ocean Dipole (pIOD) event occurred, with cold sea surface temperature (SST) anomalies over the eastern equatorial Indian Ocean (EEIO) and warming in the west. Although the growth of the EEIO cold anomalies involves forcing by equatorial nonlinear advection, unique to the 2019 pIOD is an air‐sea heat flux that was a forcing to the EEIO cold anomalies, rather than a damping as in previous extreme events. This unique thermodynamic forcing is due to a large latent cooling, which is supported by an unusually strong wind speed contributed by a large southerly anomaly as part of a long‐term trend. The wind trend is underpinned by a mean state SST change featuring slower warming off Sumatra‐Java. Given that a similar SST trend pattern is projected under greenhouse warming, the likelihood of such thermodynamical forcing operating more frequently in the future needs to be considered.

Oceanic Processes in Ocean Temperature Products Key to a Realistic Presentation of Positive Indian Ocean Dipole Nonlinearity

AbstractA positive Indian Ocean Dipole (pIOD) refers to a sea surface temperature anomaly pattern with cold anomalies in the equatorial eastern Indian Ocean and warm anomalies in the west, leading to floods in the eastern African countries and droughts and bushfires in Indonesia and Australia. The pIOD displays strong inter‐event differences, ranging from an extreme event dominated by westward‐extended strong cold anomalies along the equator, to a moderate event with weakened cooling confined to region off Sumatra‐Java. Representation of the extreme pIOD varies vastly across ocean temperature products. Here we show that products generated in a system explicitly involving subsurface oceanic processes capture the nonlinear dynamics of the extreme pIOD, i.e., the equatorial nonlinear zonal and vertical advection, and systematically produce a more realistic extreme pIOD. Thus, our study identifies ocean temperature products that are more suitable for studying extreme pIOD and its climatic impacts.

A sea-level monopole in the equatorial Indian Ocean

In this study, we show the relationship between sea-level anomalies (SLA) and upper-ocean parameters in the Equatorial Indian Ocean (EIO). This work also focuses on the variability of SLA obtained from satellite altimeter data in different spatial and temporal scales and its relationship with computed ocean heat content (OHC), dynamic height (DH), and thermocline depth (20 °C isotherm: D20) during 1993–2015. SLA showed low Pearson’s correlation coefficient (CC) with upper-ocean parameters over central EIO resembling a “Monopole” pattern. The Array for Real-time Geostrophic Oceanography (ARGO) in situ profile data in the central EIO also confirmed this. SLA over this monopole showed low correlations with all parameters as compared with eastern and western EIO. These findings show a clear signature of a persisting sea-level monopole in the central EIO. Oscillating SLA over western and eastern EIO during summer and winter monsoon months is found to be responsible for locking this monopole in the central EIO. Both SLA and OHC increased in EIO during 2006–2015 compared with 1993–2005. The month of January showed different east–west trends at different times. This trend during 1993–2015 is neutral, but it shifted from negative during 1993–2005 to positive during 2006–2015.

Interannual variability in the barrier layer and forcing mechanism in the eastern equatorial Indian Ocean and Bay of Bengal

Interannual variability (IAV) in the barrier layer thickness (BLT) and forcing mechanisms in the eastern equatorial Indian Ocean (EEIO) and Bay of Bengal (BoB) are examined using monthly Argo data sets during 2002–2017. The BLT during November–January (NDJ) in the EEIO shows strong IAV, which is associated with the Indian Ocean dipole mode (IOD), with the IOD leading the BLT by two months. During the negative IOD phase, the westerly wind anomalies driving the downwelling Kelvin waves increase the isothermal layer depth (ILD). Moreover, the variability in the mixed layer depth (MLD) is complex. Affected by the Wyrtki jet, the MLD presents negative anomalies west of 85°E and strong positive anomalies between 85°E and 93°E. Therefore, the BLT shows positive anomalies except between 86°E and 92°E in the EEIO. Additionally, the IAV in the BLT during December–February (DJF) in the BoB is also investigated. In the eastern and northeastern BoB, the IAV in the BLT is remotely forced by equatorial zonal wind stress anomalies associated with the El Nino-Southern Oscillation (ENSO). In the western BoB, the regional surface wind forcing-related ENSO modulates the BLT variations.

Intraseasonal modulation of Wyrtki jet in the eastern Indian Ocean by equatorial waves during spring 2013

A strong spring Wyrtki jet (WJ) presents in May 2013 in the eastern equatorial Indian Ocean. The entire buildup and retreat processes of the spring WJ were well captured by two adjacent Acoustic Doppler Current Profilers mounted on the mooring systems. The observed zonal jet behaved as one intraseasonal event with the significant features of abrupt emergence as well as slow disappearance. Further research illustrate that the pronounced surface westerly wind burst during late-April to mid-May, associated with the active phase of a robust eastward-propagating Madden-Julian oscillation in the tropical Indian Ocean, was the dominant reason for the rapid acceleration of surface WJ. In contrasting, the governing mechanism for the jet termination was equatorial wave dynamics rather than wind forcing. The decomposition analysis of equatorial waves and the corresponding changes in the ocean thermocline demonstrated that strong WJ was produced rapidly by the wind-generated oceanic downwelling equatorial Kelvin wave and was terminated subsequently by the westward-propagating equatorial Rossby wave reflecting from eastern boundaries of the Indian Ocean.

Warming Trends in the Central Equatorial Indian Ocean and the Associated Coupled Feedback Processes

Mohapatra, S. and Gnanaseelan, C., 2020. Warming trends in the central equatorial Indian Ocean and the associated coupled feedback processes. In: Sheela Nair, L.; Prakash, T.N.; Padmalal, D., and Kumar Seelam, J. (eds.), Oceanic and Coastal Processes of the Indian Seas. Journal of Coastal Research, Special Issue No. 89, pp. 39-45. Coconut Creek (Florida), ISSN 0749-0208.The recent increasing trends in the tropical Indian Ocean (TIO) temperature have strong impact on the regional as well as global climate variability. In this work we studied the changes in the boreal summer monsoon winds and the associated monsoon currents on modulating the surface and subsurface temperature of the equatorial Indian Ocean. The reduction in the meridional sea surface temperature (SST) gradient and strengthening of zonal SST gradient over the Indian Ocean reduced the northward propagation of south-westerly wind, thereby enhancing the surface westerlies over the equator. The strengthened surface westerlies increased the equatorial downwelling Kelvin waves and deepened the thermocline. The present study unravels the causes for the observed confinement of strong SST trend over the central equatorial Indian Ocean (CEIO) even though surface westerlies and the associated downwelling extend upto the eastern equatorial Indian Ocean (EEIO). In contrast to the CEIO surface warming, EEIO experienced strong subsurface warming at thermocline. Our analysis suggests that the upper ocean stratification plays an important role in the observed warming trend of both CEIO surface and EEIO subsurface. The eastward equatorial currents bring warm salty water from Arabian Sea and WEIO towards east and warms CEIO, however the high saline surface water is found to sink in the EEIO, thereby warming the subsurface instead of surface. The composite of temperature anomaly for the period 1978-2002, the period of warm Pacific Decadal Oscillation, shows patterns similar to temperature trends, which clearly suggests the dominance of decadal variability on the TIO temperature trends.