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Upwelling is a physical phenomenon that occurs globally along the eastern boundary of the ocean and supports pelagic fishery which is an important source of protein for the coastal population. Though upwelling and associated small pelagic fishery along the eastern Arabian Sea (EAS) is known to exist at least for the past six decades, our understanding of the factors controlling them are still elusive. Based on observation and data analysis we hypothesize that upwelling in the EAS during 2017 was modulated by freshwater-induced stratification. To validate this hypothesis, we examined 17 years of data from 2001 and show that inter-annual variability of freshwater influx indeed controls the upwelling in the EAS through stratification, a mechanism hitherto unexplored. The upper ocean stratification in turn is regulated by the fresh water influx through a combination of precipitation and river runoff. We further show that the oil sardine which is one of the dominant fish of the small pelagic fishery of the EAS varied inversely with stratification. Our study for the first time underscored the role of freshwater influx in regulating the coastal upwelling and upper ocean stratification controlling the regional pelagic fishery of the EAS. [ABSTRACT FROM AUTHOR]

In the Arabian Sea, two extremely severe cyclonic storms occurred back to back during October-November 2015. Using a suite of ocean and atmospheric data, we examined the upper ocean responses of tropical cyclones Chapala and Megh, the latter originated immediately after the dissipation of the former. Cyclones Chapala and Megh cooled the sea surface by 1.5°C and 1.0°C, respectively, which was also captured by Bio-Argo float in the vicinity of their tracks. The cyclone-induced chlorophyll a enhancement was 6 and 2 times respectively from their precyclone value of 0.36 and 0.30 mg/m3, while the net primary productivity showed an increase of 5.8 and 1.7 times respectively from the precyclone values of 496.26 and 518.63 mg C/m²/day after the passage of Chapala and Megh. The CO2 flux showed a sixfold and twofold increase respectively compared to the precyclone value of 2.69-3.58 and 6.78 mmol/m²/day. We show that the anomalous co-occurrence of the positive phase of Indian Ocean Dipole and the strong El Niño supported large-scale warming in the western Arabian Sea. Subsequent altered Walker circulation played a decisive role in the preconditioning of both atmosphere and ocean, which led to a situation congenial for cyclogenesis. What triggered the formation of the cyclone Chapala was the arrival of eastward-propagating atmospheric convection associated with the Madden-Julian oscillation. The generation of the second cyclone Megh within 8 days appears to be associated with the oceanic Rossby wave that led to an increase in the upper ocean heat content by deepening the 26°C isotherm in the vicinity of the origin of cyclones. [ABSTRACT FROM AUTHOR]

A butterfly type of repeat track cruise was carried out in the South Eastern Arabian Sea (off Minicoy) onboard INS Sagardhwani during July 2016 to Aug 2016. We have also made use of the data from OMNI buoy, AD09, which is about 6 km close to the centre station of butterfly track. Air sea flux, the horizontal current data from AD09 and the time series data collected from the butterfly experiment were analysed to compute the mixed layer heat and salt budget. The short-term thermo-haline variability off Minicoy, relative contribution of heat/salt budget terms in MLD and its effects on acoustic propagation are addressed in this paper. In this study, most dominating term in the mixed layer heat budget estimation is net surface heat flux followed by the advective terms has been found. However the salinity in the mixed layer is dominated by the contribution of buoyancy mixing due to night time evaporative cooling. During the calm, sunny day, the so-called afternoon effect due to the diurnal heating restricts the sonar range. But during the windy day, the wind/wave mixing prevents the warming of the surface layer which in turn enhances the sonar range. Similarly, the night time cooling also enhance the acoustic propagation range. The presence of Arabian Sea High Salinity Watermass in the surface layer also enhances the acoustic propagation. [ABSTRACT FROM AUTHOR]

Intra-annual variability of the Arabian Sea high salinity water mass (ASHSW) in the South Eastern Arabian Sea (SEAS) and Gulf of Mannar (GoM) are addressed in this paper by utilising the monthly missions carried out onboard INS Sagardhwani during 2016-17. Our observations revealed that the ASHSW was evident along the SEAS irrespective of seasons, whereas in the GoM the presence of ASHSW was observed during winter. The processes such as downwelling/up-welling, coastal currents, intrusion of low saline waters, stratification are clearly affect the spreading of the ASHSW. The characteristics such as core salinity value, depth and thickness of ASHSW exhibited remarkable spatio-temporal variability. Lateral mixing with the low saline waters in the region during winter reduces its core salinity. The intrusion of low saline waters was clearly seen upto 15 °N but the intrusion of low saline waters is not flowing through the GoM. The interface between the ASHSW and the prevailing low saline waters showed strong horizontal gradients of salinity. The presence of the ASHSW makes difference in the SLD and the below layer gradient which is sufficient to complicate or influence sound transmission. The spatio temporal variability of the ASHSW and its acoustic relevance are documented in this paper. [ABSTRACT FROM AUTHOR]

ABSTRACT: The Arabian Sea (AS) has undergone a regime shift after 1995, manifesting the disruption of its natural decadal cycle over‐riding a secular rise in its sea‐surface temperature (SST). Using a suite of in situ and remote sensing data, we show that the rate of post‐1995 accelerated warming is spatially different, due to regionally and seasonally differing oceanographic processes. Along parts of the western boundary, the accelerated warming was mediated by a reduction in the post‐1995 linear trend of wind speed during summer monsoon leading to reduced upwelling and also reduced evaporation trend. In the northeast AS, post‐1995 increase in specific humidity during late winter, lead to a decrease in the evaporative cooling and subsequent convective mixing resulting in the increased rate of warming. The continued warming seen in the northeast AS until spring inter‐monsoon was due to the increasing net heat flux trend into the ocean along with reducing trend in evaporation. The accelerated warming in the central AS was linked to the reduced wind speed trend. [ABSTRACT FROM AUTHOR]

The variability of Chlorophyll-a(Chl-a) distribution derived from MODIS (on Aqua and Terra platforms) and MERIS sensors have been compared with SeaWiFS data in the Arabian Sea. MODIS Aqua has overestimated the SeaWiFS Chl-awithin 25–32% in the coastal turbid (eutrophic) waters and underestimated in open ocean waters with error within 20%. However, there is no significant bias (−0.1 on log-scale) observed as the slope is well within 0.97-1.1 (log transformed). MODIS-Terra has underestimated the Chl-aconcentration in open ocean waters by about 29–31%, which is higher than MODIS-Aqua. MODIS-Terra is observed to be more accurate than MODIS-Aqua in the coastal waters. MERIS is overestimating the SeaWiFS Chl-awith log RMS error of ∼0.15 and log bias of ∼0.13–0.2. The differences in the Chl-aestimates between each sensor are possibly due to differences in the sensor design, bio-optical algorithms and also due to the time differences between the satellites over passes. We have examined that the MERIS is performing similar to SeaWiFS and the MODIS-Aqua (Terra) data are reliable in open ocean (coastal) waters. However, Chl-aretrieval algorithms need to be improved especially for coastal turbid waters to continue with SeaWiFS data for long-term studies. [ABSTRACT FROM PUBLISHER]

Abstract: The response to the tropical cyclone Phyan, which developed in the eastern Arabian Sea during 9–11 November 2009, was rapid cooling of sea surface temperature (SST), enhancement of chlorophyll a and two-fold increase in net primary productivity (NPP). Cooling of SST was immediate in response to the strong wind-mixing, and the subsequent upward Ekman pumping sustained the cooling even after the dissipation of Phyan. The biological response mediated by the upward Ekman pumping driven vertical transport of subsurface nutrient showed a time lag of 3–4 days. The CO2 flux to the atmosphere associated with Phyan was 0.123 Tg C, which accounted for ∼85% of the total out-gassing from the eastern Arabian Sea during November. Thus, an increased occurrence of cyclones in a warming environment will lead to an enhanced biomass production and also increase in CO2 out-gassing. [Copyright &y& Elsevier]

The seasonal variability of phytoplankton biomass in the Arabian Sea, though a well researched topic, its inter-annual variability is less explored and understood. Analysis of the satellite-derived chlorophyll pigment concentration in the Arabian Sea during 1997-2007 showed a weak increasing trend. Contrary to the earlier hypothesis, our analysis showed that this increased phytoplankton biomass was not driven by the strengthening winds during summer monsoon. In fact, the basin-averaged chlorophyll concentrations during summer monsoon tend to decline, whereas those in September-October and during the winter monsoon showed an increasing trend. Based on the analysis of wind and aerosol optical thickness data, we attribute the increased phytoplankton biomass during September-October to dust-induced iron fertilization when there is sufficient buildup of nitrate in the upper ocean. During winter, the enhanced evaporative cooling under the strengthening winds led to the increased convective mixing. Subsequent supply of subsurface nutrients to the euphotic zone coupled with the increased dust delivery support the observed increase in phytoplankton biomass during winter. [ABSTRACT FROM AUTHOR]

The uptake of dissolved inorganic nitrogen by phytoplankton is an important aspect of the nitrogen cycle of oceans. Here, we present nitrate (NO3-) and ammonium (NH4 +) uptake rates in the northeastern Arabian Sea using 15N tracer technique. In this relatively underexplored region, productivity is high during winter due to supply of nutrients by convective mixing caused by the cooling of the surface by the northeast monsoon winds. Studies done during different months (January and late February-early March) of the northeast monsoon 2003 revealed a fivefold increase in the average euphotic zone integrated NO3- uptake from January (2.3 mmol N m-2d-1) to late February-early March (12.7 mmolN m-2d-1). The f -ratio during January appeared to be affected by the winter cooling effect and increased by more than 50% from the southernmost station to the northern open ocean stations, indicating hydrographic and meteorological control. Estimates of NO3- residence time suggested that NO3- entrained in the water column during January contributed to the development of blooms during late February-early March. [ABSTRACT FROM AUTHOR]

Abstract: The response of the Arabian Sea to global warming is the disruption in the natural decadal cycle in the sea surface temperature (SST) after 1995, followed by a secular warming. The Arabian Sea is experiencing a regional climate-shift after 1995, which is accompanied by a five fold increase in the occurrence of “most intense cyclones”. Signatures of this climate-shift are also perceptible over the adjacent landmass of India as: (1) progressively warmer winters, and (2) decreased decadal monsoon rainfall. The warmer winters are associated with a 16-fold decrease in the decadal wheat production after 1995, while the decreased decadal rainfall was accompanied by a decline of vegetation cover and increased occurrence of heat spells. We propose that in addition to the oceanic thermal inertia, the upwelling-driven cooling provided a mechanism that offset the CO2-driven SST increase in the Arabian Sea until 1995. [Copyright &y& Elsevier]

A temporal increase of ∼5‰ in the average nitrogen isotopic composition (δ15NPON) of surface particulate organic nitrogen was observed in the open north-eastern Arabian Sea during January to late February-early March 2003, despite the presence of T. erithraeum (up to ∼11%), a diazotroph that fixes atmospheric N2, in the latter period. Hydrographic conditions and residence time of nitrate in the water column suggested that this increase could be a combined effect of denitrification in the subsurface layer and inefficient utilization of nitrate entrained in the water column during January. [ABSTRACT FROM AUTHOR]

Abstract: The upper layers of the ocean play an important role in the regulation of the ocean–atmosphere as a coupled system. The Arabian Sea, situated in the western part of the northern Indian Ocean, participates in the intense air–sea exchange processes associated with the semi-annually reversing monsoons. During winter monsoon the weak northeast trade wind brings cool, dry continental air, while during the summer monsoon the strong southwest wind brings humid maritime air into the Arabian Sea. It is natural to expect that these semi-annual atmospheric forcings would modulate the thickness of the upper ocean. The seasonal variability of the mixed layer in the central Arabian Sea (equator to 25°N and 62–66°E) was studied using hydrographic data from NODC and INODC, while the atmospheric forcing responsible for the observed changes were explored using meteorological data from COADS and FSU winds. The mixed layer depth (MLD) and sea-surface temperature (SST) in the central Arabian Sea, to a large extent, were regulated by the incoming solar radiation and wind forcing. The Ekman dynamics associated with the Findlater Jet controlled the mixed layer depth during summer monsoon, while in winter the cooling and convective mixing regulated the MLD. We speculate that the deep mixed layer during February–March in the southern region is associated with that of the Rossby wave propagation. During spring and fall intermonsoons, the weak winds along with strong incoming solar radiation lead to the formation of a thin, warm and stratified mixed layer. We show that biological productivity of the Arabian Sea is rigidly coupled to the seasonal cycle of MLD, which regulates the nutrient levels in the euphotic zone. [Copyright &y& Elsevier]

Focuses on a study which discussed the physical forcing of biological productivity in the Northern Arabian Sea during the northeast monsoon. Factors which are critical in the dynamics of the upper-ocean and influence biogeochemistry of the region; Methods; Results of the study.

Presents physical, chemical and biological data that indicates that sea surface cooling drives convection processes. The injection of nutrients into the surface waters of the northeastern Arabian Sea during winter; The mechanism of nutrient supply being a dominant control on winter productivity; Observed seasonal changes in bacterial and microzooplankton populations; A possible explanation for the Arabian Sea paradox.

Abstract: New productivity measurements using the 15N tracer technique were conducted in the north-eastern (NE) Arabian Sea during six expeditions from 2003 to 2007, mostly in winter. Our results indicate that the NE Arabian Sea has a potential for higher new productivity during blooms. Nitrate uptake by plankton is the highest during late winter. New productivity and f-ratios in the NE Arabian Sea are mainly controlled by hydrodynamic and meteorological parameters such as wind strength, sea surface temperature (SST), mixed layer depth (MLD) and mixed layer nitrate. Deepening of the mixed layer supplies nitrate from below, which supports the observed nitrogen uptake. Higher f-ratios during blooms indicate the strong coupling between surface layers and sub-surface layers. Deepening of mixed layer below 100m (from its inter-monsoon value between 30 and 40m) transferred often more than 100mmol N–NO3 m−2 into the surface layers from below. The observed winter blooms in the region are supported by such input and are sustained for more than a month. Higher new productivity has been found in late winter, whereas transport of nitrate is maximum in early winter. In general, new production varies progressively during winter. Diurnal cycling of the mixed layer could be the reason for the under utilization of entrained nitrate during early winter. New productivity values and wind strength show significant differences during Feb–Mar 03 and Feb–Mar 04. These differences indicate that the winter cooling and parameters related the biological productivity also vary inter-annually. However, the difference between the new productivity values between Feb–Mar 03 and Feb–Mar 04 is much lower than the difference between Jan 03 and Feb–Mar 03. The results suggest that amplitude of seasonal variation is higher than the inter-annual variation in the region. During spring, Fickian diffusive fluxes of nitrate into the surface layer range from 0.51 to 1.38mmol N–NO3 m−2 day−1, and can account for 67% and 78% of the observed nitrogen uptake in the coastal and open ocean regions, respectively. We document the intra-seasonal and inter-annual variations in new productivity during winter and identify sources of nitrate which support the observed productivity during spring. [Copyright &y& Elsevier]

Abstract: Nutrient characteristics of four water masses in the light of their thermohaline properties are examined in the eastern Equatorial Indian Ocean during winter, spring and summer monsoon. The presence of low salinity water mass with “Surface enrichments” of inorganic nutrients was observed relative to 20m in the mixed layer. Lowest oxygen levels of 19μM at 3°N in the euphotic zone indicate mixing of low oxygen high salinity Arabian Sea waters with the equatorial Indian Ocean. The seasonal variability of nutrients was regulated by seasonally varying physical processes like thermocline elevation, meridional and zonal transport, the equatorial undercurrent and biological processes of uptake and remineralisation. Circulation of Arabian Sea high salinity waters with nitrate deficit could also be seen from low N/P ratio with a minimum of 8.9 in spring and a maximum of 13.6 in winter. This large deviation from Redfield N/P ratio indicates the presence of denitrified high salinity waters with a seasonal nitrate deficit ranging from −4.85 to 1.52 in the Eastern Equatorial Indian Ocean. [ABSTRACT FROM AUTHOR]