Northwestern Arabian Sea Research Articles

Trends in the surface wind-wave heights in the North Indian Ocean based on 50-year NCEP/NCAR reanalysis wind-wave data

Analysis of 50 years of hourly significant wave heights of the Indian Ocean between 30°N and 60°S is carried out and linear trends of mean significant wave heights and maximum significant wave heights are obtained on annual, seasonal and decadal scales. The significant wave heights (SWH) are obtained from a third-generation spectral wind-wave model forced with six-hourly NCEP-NCAR winds over the Indian Ocean. These SWHs are validated with measurements available in the north Indian Ocean and the correlation coefficient is more than 0.87. The climatology obtained from the spectral wind wave model matches well with the climatology reported in the literature using other models and satellite-based datasets. The maximum SWH ranged from 0.25 m to 22.5 m with the max-SWH exceeding 7 m South of 30°S, and greater than 13 m between 45°S-55°S and 50°E-95°E. In the Northern Indian Ocean (NIO), max SWH reached 5.5 m in the central Bay of Bengal and over 8 m in the western Arabian Sea. The annual trend of the mean SWH showed an overall positive trend in the Indian Ocean, except in the west BoB, northwest Arabian Sea, and a part of the central Indian Ocean. The maximum SWH also followed an overall similar positive trend as that of mean SWH albeit with a prominent negative trend in the BoB, southeastern and northwestern Arabian Sea. There was a significant difference in the SWH trend between the seasons both spatially and temporally for all other decades. However, the maximum SWH showed highly varying trends for all the seasons and decades. The variation in the mean SWH decadal trend showed greater variation in the lower latitudes compared to the higher latitudes.

Projected Changes of Wind Energy Input to Surface Waves in the North Indian Ocean Based on CMIP6

This paper explores the effects of climate change on the wind energy input (WEI) to the surface waves (SWs) in the northern Indian Ocean (NIO), a region with great potential for green renewable energy from waves and wind. We used the newly developed Coupled Model Intercomparison Project Phase 6 (CMIP6) model data to predict the spatiotemporal variations of the WEI to the SW. We found that, under the global warming scenario, the WEI to the SWs decreased significantly in most of the NIO, and it will drop by 18% to 27% in the central and southern regions by the end of the 21st century under the SSP5–8.5 scenario. However, the WEI to the SWs increased in the Red Sea, Persian Gulf, northwestern Arabian Sea, and northern Bay of Bengal, with the largest increase in the Persian Gulf region (up to 27%). We also examined the interannual and interdecadal variability characteristics of the WEI to the SW after the accumulation of the whole study region and found that it showed a long-term increasing trend only under the SSP1–2.6 scenario, while it showed a significant decreasing trend under the SSP2–4.5 and SSP5–8.5 scenarios. Furthermore, we show that the WEI to the SWs in the Indian Ocean mainly occurs in summer, followed by winter.

The increased summer monsoon rainfall in Northwest India: Coupling with the Northwestern Arabian Sea warming and modulated by the Silk Road Pattern since 2000

This study investigates changes in rainfall patterns over Northwest India (NWI) and the warming of the Northwestern Arabian Sea (NWAS) since 2000. These changes are attributed to the Silk Road Pattern (SRP) phase change in the late 1990s. The SRP positive phase triggers geostrophic deformation over the Tibetan Plateau (TP) by inducing an anomalous circulation pattern along the westerly jet, at 200 hPa. This deformation along the westerly Jet leads to Jet streaks and thermally indirect ageostrophic circulation over TP. In addition, the intensified heating over the TP led to a westward shift of the South Asian High (SAH). Associated with this, the interaction of positive temperature advection and northward ageostrophic wind (Jet entrance) over south-central Asia generates a southward component of ageostrophic wind (Jet Exit), causing upper-level convergence at southeast TP, excites the sinking cold air thereby strengthen the surface pressure below, over TP. This amplifies the anomalous anticyclone effect over there resulting in strong easterly wind anomalies along the monsoon trough at 850 hPa, disrupting the monsoon circulation. These anomalies, combined with a poleward shifting moisture-laden Low-Level Jet (LLJ), contribute to the warming of the NWAS and increased rainfall over NWI. The NWAS has shown significant summer warming over the past two decades. The poleward shift of the LLJ and SRP-induced circulation changes also impact the ocean dynamics. Given the occurrence of severe floods in this region of the Indian subcontinent, these findings have broader implications for predicting and projecting monsoon variability and extremes.

Dense mineral distribution in the northwestern Arabian Sea: insights into sediment provenance and paleoclimate

Dense mineral distribution in the northwestern Arabian Sea: insights into sediment provenance and paleoclimate

Paleoclimatic Influence on the Evolution of the Late Quaternary Clastic Sediments, Northwestern Arabian Sea

Abstract Sedimentological characteristics of the clastic sediments deposited in the Northwestern Arabian Sea at the ODP core sites 723A and 722B are used to reconstruct the paleoclimatic changes recorded during the last 150 kyrs. ODP site 723A is situated at 808 m water depth on the continental margin of Oman, whereas ODP site 722B is located at 2028 m water depth on the Owen Ridge. Clay minerals such as palygorskite, illite, chlorite, smectite, kaolinite, and non-clay minerals, quartz and feldspar, are present in both cores. However, palygorskite and illite dominated the clay mineral assemblage at core 723A, and illite dominated over palygorskite in the clay mineral assemblage at core 722B. Illites are dominantly well crystalline, with slightly poorer crystallinity occurring during Holocene and inter-glacial stages. Chemically illites are Mg-rich with slightly Al-rich illites associated with aforesaid stages. The higher values of all moisture-clay based humidity tools (ratios of kaolinite/ illite, kaolinite/chlorite, smectite /(illite+chlorite), kaolinite/ palygorskite) are dominantly associated with stronger monsoon based weathering during the interglacial stages, Holocene and interstadial events. The increased total carbonate, elemental concentrations (calcium, barium and phosphorous) and decreased magnetic susceptibility values correlate with the enhanced monsoon intensities during the above events. Glacial stages and millennial-scale drier events (Heinrich, Younger Dryas and stadial events) are marked by the massive influx of wind borne terrigenous material, dominated by clay minerals with higher silicon and aluminium contents; they have increased sediment accumulation rates at both sites. The clay proxies and other sedimentological data demonstrate monsoonal oscillations at 40, 23, 6.3, 4.4 and 3.5kyr cycles in Milankovitch and sub-Milankovitch time scales. The present study suggests that the detritus are mainly eolian and primarily originated from the nearby continental areas of Oman, the Arabian Peninsula, Persian Gulf and Iran-Makran regions. The northwesterly wind played a significant role in sediment transportation into the present sites throughout the studied period. The southwest summer and northeast winter monsoon winds played minor roles in the above activity during the interglacial and glacial stages, respectively. The Indus River discharges (predominantly of glacial origin), although low, have contributed some clay minerals in the deeper water core site.

An annotated checklist of the Amphipoda (Crustacea) of Kuwait, north-western Arabian Gulf

Kuwait’s intertidal and subtidal habitats were surveyed from 2013 to 2016 for amphipods, and earlier records from Kuwait and the rest of the Gulf were compared to provide the first annotated checklist of Amphipoda in Kuwait. A total of 82 sites in mainland and islands were surveyed including intertidal and subtidal zones. The resulting data identified 86 amphipod taxa belonging to 54 genera, 32 families and 3 suborders, including 20 new records to science that are currently being described. A total of 46 amphipod species, 34 genera and 19 families constituted new regional records either for Kuwait or the entire Gulf area. Information on local distribution, occurrence, and habitat are provided for each species. Within the tidal zonation, 50 species inhabited both the intertidal and subtidal areas, 19 were intertidal, 12 subtidal, and 3 species were found in the supratidal zone. Sandy and rocky shores supported the most diverse amphipod assemblages. A gradient of increasing amphipod species richness from north to south along Kuwait’s mainland coastline was revealed. The high level of regional endemism supported notable taxonomic distinction of the amphipod fauna of Kuwait from the other Gulf areas. Compositional dissimilarity among amphipod faunas increased with distance within a wide longitudinal gradient across the Gulf, Sea of Oman, and north-western Arabian Sea.

Impact of tropical Indian Ocean warming on the surface phytoplankton biomass at two significant coastal upwelling zones in the Arabian Sea

The Sea Surface Temperature (SST) of the Arabian Sea undergoes large seasonal variations owing to the monsoonal forcing and upwelling. Warming of the ocean adversely affects its biological productivity. The present study examined the role of the rapid warming of the tropical Indian Ocean and associated changes in physical forcing parameters on phytoplankton biomass in the Arabian Sea. SST during the summer monsoon period (June-September) of 1971–2020 showed an increasing trend of 0.6 °C in the southeastern Arabian Sea (SEAS) and 1.6 °C in the northwestern Arabian Sea. During the recent summer monsoon period of 1998–2020, surface chlorophyll-a (chl-a) concentration showed a decreasing trend of – 0.32 mg m−3 in the SEAS and an increasing trend of 0.56 mg m−3 in the northwestern Arabian Sea. High-resolution satellite and reanalysis data of physical forcing parameters such as surface winds, SST, Sea Level Anomaly (SLA) that influence the surface chl-a concentration at two distinct upwelling locations in the Arabian Sea, were analysed for the recent two decades (1998–2020). Multiple linear regression (MLR) analysis showed that SLA was the most important parameter that determines the surface chl-a variability in the SEAS, whereas alongshore wind stress was dominant in the northwestern Arabian Sea. An epochal analysis showed that in the most recent decade, SLA in the SEAS became less favourable for upwelling, whereas summer monsoon winds became increasingly favourable for upwelling in the northwestern Arabian Sea. These differences corroborate the contrasting trends in surface chl-a in the two locations in the Arabian Sea. The present study has shown that the inconsistent response of surface chl-a at distinct locations within the Arabian Sea depends on the relative strength of the influencing physical forcing mechanisms.

A vignette of mitogenome and evolutionary status of the giant form of Sthenoteuthis oualaniensis (Lesson, 1830) endemic to the northwestern Arabian Sea

A vignette of mitogenome and evolutionary status of the giant form of Sthenoteuthis oualaniensis (Lesson, 1830) endemic to the northwestern Arabian Sea

Climatology, trend of aerosol-cloud parameters and their correlation over the Northern Indian Ocean

Aerosols are one of the important atmospheric constituents and exert indirect impact on climate through the modification of microphysical and radiative properties of clouds that in turn perturb the precipitation pattern. Thus, the long term quantification of changes in aerosol and cloud characteristics and their interactions on both temporal as well as spatial scale will provide a crucial information for the better assessment of future climate change. In present study, 18 years (2003–2020) MODerate Resolution Imaging Spectro-radiometer (MODIS) derived aerosol-cloud dataset over the Northern Indian Ocean (NIO) were analysed to assess climatology and trend of aerosol, cloud characteristics and their correlation. We found a strong heterogeneity in spatio-temporal variation of aerosol and cloud parameters over the NIO that are more prominent for the coastal region. The climatological mean of aerosol loading is found high (AOD ≥ 0.5) over the outflow region along the Indian sub-continent and low (AOD ≤ 0.2) over the northern equatorial open ocean. The climatological mean of cloud properties shows dominance of optically thicker deep convective (CTP < 600 hPa and CTT < 260 K) clouds over the southern Bay of Bengal (BoB) and thinner shallow (CTP > 700 hPa and CTT > 273 K) over the northwestern Arabian Sea (AS). Similarly, bigger effective radii (>17 µm) observed along the equatorial open ocean whereas smaller CER (<17 µm) were found over Indian sub-continental coastline and western AS. Further, trend analysis reveals an increasing pattern in AOD (0.002 yr−1), CER (0.051 µm yr−1), LWP (0.033 gm−2 yr−1) and CF (0.002 yr−1) while COD, CTT and CTP show negative trend in order of −0.005 yr−1, −0.094 K yr−1 and −1.160 hPa yr−1, respectively. We also perform similar analysis for seven sub-region of interest (R1 to R7) across the NIO and results show a decreasing pattern in AOD (−0.001 yr−1) at R4 against maximum mean AOD (0.44 ± 0.03). However, coastal sub-regions R1 and R5 illustrate maximum increase in aerosol loading (>0.003 yr−1) suggesting a significant impact of sub-continental outflow over the regions. The spatial correlation of cloud properties with respect to AOD shows a positive slope for CER (0.14) and CF (0.48) and a negative for COD (−0.19), LWP (−0.18), CTT (−0.37), CTP (−0.41). The present study provides in-depth information about the aerosol-cloud characteristics for a long term scale over NIO and could be useful in regional aerosol-cloud interaction induced climate forcing estimation.

Biogeochemical cycling of dissolved manganese in the Arabian Sea

The study of marine biogeochemical cycling of Mn, a bio-essential trace metal, is crucial to decipher its relative role in controlling the production and diversity of marine phytoplankton. Furthermore, Mn oxides and oxyhydroxides act as sinks for other trace elements in the water column, and Mn redox cycling may exert a major control on the vertical distribution of these elements. Dissolved Mn (dMn) concentrations are measured along two GEOTRACES-India (GI) transects, GI05 and GI10, for a basin-wide investigation of biogeochemical controls on the dMn distribution in the Arabian Sea (AS). Surface dMn distribution shows a strong spatial gradient, with relatively dMn-rich levels in the eastern AS (3.4–5.7 nmol kg−1) compared to the central and western AS (1.2–2.3 nmol kg−1). This correlates with the higher model estimates of soluble Mn input from the atmospheric dust deposition in the eastern AS (∼1.5–2.0 times of input in the western and central AS), and probable additional Mn input from riverine discharge and shallow shelf region of the eastern AS. In the south-eastern AS, a correlated increase in surface dMn with a decrease in salinity indicates the Mn input by the advection of dMn-rich, low-salinity Bay of Bengal (BoB) surface waters to the AS under the influence of the Northeast Monsoon Current. Our mass balance estimates for dMn in the euphotic waters (upper 100 m) suggest important contributions from atmospheric Mn deposition (14–99% of the total estimated input flux), and dMn input/dilution (13–74% of the total estimated input or removal flux) due to lateral/vertical advective-eddy diffusive mixing. Mn-oxidation and passive scavenging are determined to be the dominant processes for the dMn removal (61–99%) from the euphotic zone (upper 100 m), while export via Mn bio-assimilation is comparatively negligible.In the AS denitrification zone, the observed dMn maxima in the upper thermocline depths (∼200–400 m) result from the in situ reductive dissolution of Mn oxides and oxyhydroxides, and lateral advective-eddy diffusive Mn flux from the reducing sediments of the western Indian margin. Outside the denitrification zone, in the northern and western AS, a shift of dMn maxima to the lower thermocline waters (∼500 m) indicates the change in the source region and dMn input from the reducing margins of the north (Pakistan Margin) and north-western AS (Oman Margin), intersecting the core of the oxygen minima at the depth range of 400–600 m.Deep water (>1500 m) dMn concentrations are low (<0.3 nmol kg−1) in most of the AS regions. Anomalous dMn enrichment in the deep waters is observed in the southern and western AS, presumably indicating the influence of hydrothermal plumes emanating from the active hydrothermal vents over the Carlsberg Ridge and spreading ridges in the Gulf of Aden. A correlated and localized increase in dMn and dAl levels close to the Murray Ridge hints at dMn input from the sediment resuspension over the ridge and subsequent Mn dissolution in low-oxygen waters (DO < 20 µM) and/or diffusive flux from the Mn-rich pore waters.

Variations of Primary Productivity in the Northwestern Arabian Sea During the Last 23,000 Years and Their Paleoclimatological Implications

AbstractThe Arabian Sea (AS) is one of the most productive oceanic regions in the world due to several monsoon‐related processes that can increase nutrients contents in the euphotic zone. Previous studies of the imprint of oceanic primary productivity (PP) in AS sediments yielded diverse results depending on the studied area and the chosen proxies, with unprecise paleoclimatic interpretations. Here, we provide multi‐decennial PP and coastal upwelling dynamic records off northern Oman over the last 23 Kyr, based on the analysis of coccoliths from sediment core MD00‐2354. Our results have been compared with previous paleoenvironmental records as well as new modeling data to get precise paleoclimatic interpretations. We document higher PP and weaker coastal upwelling during the Last Glacial Maximum relative to the Holocene, and significant millennial‐scale variations over the last deglaciation corresponding to the fluctuations of the Atlantic Meridional Overturning Circulation strength. Higher PP and weaker upwelling are found during the cold stadials, while lower PP and stronger upwelling during the warm interstadial. We propose that the increases of PP were driven by increased bioavailable nutrient content in surface waters under both stronger winter monsoon conditions that strengthened the convective mixing, and higher aeolian inputs. Over the Holocene, stronger upwelling and slightly lower PP are found during the Early‐Mid Holocene, when higher summer insolation triggered stronger summer monsoon. At that time, the lower PP was probably the result of restricted advection of eutrophic summer upwelling seawater under negative wind stress curls and less aeolian inputs.

Distribution and cycling of dissolved aluminium in the Arabian Sea and the Western Equatorial Indian Ocean

Dissolved aluminium (dAl) distribution has been studied over the full vertical water column profiles along the GEOTRACES-India (GI) transect, GI-05, in the Arabian Sea (AS) and the Western Equatorial Indian Ocean (W-Eq.IO) during the fall inter-monsoon period. Surface dAl distribution in the AS demonstrates an east-west gradient, i.e., elevated dAl (6.5–21.6 nM) close to the Indian coastal region and low dAl (1.5–3.6 nM) along the western boundary of the AS. Rapid surface dAl removal due to relatively high biological productivity and a decrease in atmospheric dust deposition during the fall inter-monsoon result in low surface dAl levels in the western AS. Mass balance for surface dAl variation in the AS reveals very short scavenging removal timescales (0.01–0.47 yr) between the mid-summer and fall inter-monsoon period. Further, dAl input/dilution due to surface water advection is found to play an important role in controlling the surface dAl variation, varying between 190 and 300% of the dust-supported dAl input in different regions of the AS. These results have important implications for the use of surface dAl as a proxy of dust deposition in the AS. In the W-Eq.IO, a relative increase observed in the surface dAl concentrations, compared to the western AS and the central equatorial region, suggests a local dAl input, presumably, due to the dust influx from the Somali coast. Probable mechanism for this could be dust input from Somalia to the coastal western equatorial region and subsequent advection of the dAl enriched coastal waters to the offshore sampling sites, facilitated by mesoscale eddies.The intrusions of the high salinity water masses (the Arabian Sea High Salinity Water and the Persian Gulf Water) in the thermocline depths (~75–300 m) are observed to carry the dAl-rich signal of their formation regions to the open AS. This dAl enrichment in thermocline waters is, however, mostly restricted to the northern and north-western AS during the study period. Correlated dissolved Al and Fe maxima observed in the deep water over the Murray Ridge in the northern AS suggest Al and Fe release from reactive clay minerals (e.g., illite), found abundantly in the sediments deposited over the ridge. Further, elevated dAl levels were also seen near the Laxmi-Panikkar-Palitana ridge system (~10.0 nM) and the Carlsberg Ridge (~4.5 nM) in the AS and the W-Eq.IO, respectively.

Influences of sea level changes and the South Asian Monsoon on southern Red Sea oceanography over the last 30 ka

AbstractThe southern Red Sea is a key region for investigating the effect of climate forces on a semi-closed basin. Detailed micropaleontological (planktic foraminifera) and isotopic (δ18Ο, δ13C) analyses along with reconstructions of sea surface temperature and salinity on a sediment core from the Farasan banks revealed the imprints of sea level changes and the South Asian Monsoon on the area. Comparison with surrounding records provided insights on the Red Sea-Gulf of Aden (GoA)-Northwestern Arabian Sea (NWArS) water exchange pattern over the last 30 ka. During glacial sea-level lowstand, flow of water from the GoA prevented hypersalinity in the southern Red Sea. Deglacial sea level rise improved water mass exchange between the NWArS, GoA and the entire Red Sea, resulting in relatively similar surface water conditions during the early Holocene when sea-level rise slowed. Thus, sea level change is the major driver of Red Sea δ18O variability. Southwest Monsoon (SWM), which was dominant during the late glacial and Early–Middle Holocene, enhanced surface productivity in the southern Red Sea. Northeast Monsoon (NEM) dominated around Heinrich stadial 1, as indicated by a nearly aplanktonic zone that was probably caused by restricted GoA inflow and low productivity.

Response of sea surface temperature, chlorophyll and particulate organic carbon to a tropical cyclonic storm over the Arabian Sea, Southwest India

The response of sea surface temperature (SST), chlorophyll (Chl) and particulate organic carbon (POC) to the tropical cyclonic storm Ockhi between 29th November and 6th December 2017 over the Arabian Sea, Southwest coast of India was studied. The unique observation is that the cyclone overturned northeastwards (clockwise), the wind speed in north-northwestwards (8–12 m s−1) which caused cold patches (< 28.5 °C) in the northwestern Arabian Sea compared to the southwest coast of India. Sea surface cooling (3–4 °C) was observed in concurrence with significant Chl response (0.5–7.5 mg m−3) along the Ockhi cyclone path. The post cyclonic low SST and high Chl from cyclone centre to farther areas suggested the extent of upwelled nutrient rich cold waters for plankton proliferation. POC levels showed a 3-fold increase, affected by significant biomass production due to subsurface phytoplankton and nutrient sources evidenced by deepening of mixed layer depth. The cyclone induced contrasting surface Chl (< 1 mg m−3) and POC (80–160 mg m−3) concentration substantiated the dominance of less labile refractory organic particulates in the offshore waters. The sub surface cooling and Chl bloom was noticed up to 50 m indicating the strong relation of vertical bio-physical structure during tropical cyclones. This study revealed that the tropical cyclones can act in such a way that it improves the positive magnitude of Chl and POC whereas reverses the SST behavior, on either side of the cyclone path within a short period in the oligotrophic waters of Arabian Sea.

Calcareous peloids in the north-western Arabian Sea: implications of late Quaternary marine sedimentation and paleoclimate

Calcareous peloids in the north-western Arabian Sea: implications of late Quaternary marine sedimentation and paleoclimate

Indian monsoon wind variability since ~11 kyr in the northwestern and northeastern Arabian Sea

• We studied varying monsoon wind stress in the Arabian Sea during the Holocene . • Surface productivity was high during the early Holocene . • Summer monsoon was weak and winter monsoon was strong in the middle Holocene. • Monsoon winds show high spatial variability in strength in the Arabian Sea. In this paper, we use planktic foraminifera and mixed-layer species from cores SK-240/327, SK-243/I- and ABP-25/03, northeastern Arabian Sea to investigate monsoon wind variability. Planktic foraminifera include Globigerina bulloides, a monsoon proxy and mixed-layer species Globigerinoides ruber, Gs. sacculifer, Gs. obliquus and Gs. extremus from cores SK-240/327, SK-243/I-1 and ABP-25/03 encompassing last ~11 calibrated kilo years before the Present (~11 cal kyr BP). The population abundances of Globigerina bulloides and mixed-layer species were compared with published Globigerina bulloides percentages from the western Arabian Sea Ocean Drilling Program Holes 723A, 724B and 727B, and eastern Arabian Sea core ABP-25, 02 to understand variations in Indian monsoon winds during the Holocene. In the eastern Arabian Sea, upwelling induced productivity was higher during 10–6.5 cal kyr BP which decreased during 6.5–3 cal kyr BP. In the northwestern Arabian Sea, we observe a similar scenario but not every site captured this change owing to spatial diverseness in monsoon wind stress. The productivity increased since 3 cal kyr BP due to intense summer monsoon winds in the northeastern Arabian Sea, whereas productivity began to increase at ~1.5 cal kyr BP in the northwestern Arabian Sea. The upwelling intensity shows large spatial and temporal variations in the eastern and western Arabian Sea, indicating varying monsoon wind strength across the Arabian Sea during the Holocene. The spectral analysis performed on Globigerina bulloides shows cyclicity which is associated with solar variability during the past 11 kyr record in the northeastern Arabian Sea.

Benthic foraminiferal response to changes in the northwestern Arabian Sea oxygen minimum zone (OMZ) during past $$\sim$$145 kyr

Benthic foraminiferal response to changes in the northwestern Arabian Sea oxygen minimum zone (OMZ) during past $$\sim$$145 kyr

Late Quaternary Monsoon and Productivity Variability in the Northwestern Arabian Sea

Late Quaternary Monsoon and Productivity Variability in the Northwestern Arabian Sea

Surface chlorophyll anomalies associated with Indian Ocean Dipole and El Niño Southern Oscillation in North Indian Ocean: a case study of 2006-2007 event.

North Indian Ocean witnesses varied dynamical response due to independent climate modes such as Indian Ocean Dipole (IOD)/El Niño Southern Oscillations (ENSO) and their co-occurrences. These modes have a significant impact on ocean productivity, which in turn shows feedback for the strengthening of these patterns. Keeping this in view, the present work attempts to analyze the biological activity during the combined influence of positive IOD with El Niño during 2006-2007 event. To divulge the biological variability along with the dynamical response, the study includes intra-annual variability surface chlorophyll anomaly with D20 anomaly using satellite observations. Here, the individual role of IOD and ENSO on both surface chlorophyll and D20 is segregated through partial regression analysis for a period of 25years (1993-2017). By the regression method, it can be seen varied chlorophyll response for the 2006-2007 event with the IOD forcing leads to the major spatial and temporal variability with positive anomalies in Eastern Equatorial Indian Ocean (EEIO) (generally oligotrophic), Northwestern Bay of Bengal (NWBoB), and Northwestern Arabian Sea (NAS2) where production begins in fall intermonsoon and peaks up during November. On the other hand, negative anomalies are observed around the southern tip of India (SBoB) and the Northern Arabian Sea (NAS1). While ENSO depicts the high surface chlorophyll variability in the Western Indian Ocean (WIO1, WIO2) with negative anomalies of surface chlorophyll. This study observed an asymmetric response of chlorophyll variability over the North Indian Ocean during the 1997-1998 and 2006-2007 events with a major influence of IOD mode compared with the El Niño. Therefore, understanding the chlorophyll anomalies during different climate modes will help us to better understand the interannual variability and improve the predictability of chlorophyll productivity regions.

Eddy‐Induced Temperature and Salinity Variability in the Arabian Sea

AbstractThe eddy field in the Arabian Sea experiences seasonal wind‐driven intensification during the summer monsoon season from June through September. These strong eddies strengthen local currents like the Somali Current and strongly impact regional upper‐level vertical and lateral advection. To investigate the multivariate response to eddying, we apply a closed‐contour eddy‐tracking algorithm to sea level anomaly maps and then examine sea surface temperature and salinity of the identified eddies to infer whether they are surface or subsurface intensified in the Arabian Sea during the summer monsoon. A complete understanding of the temperature and salinity signatures reveals how Arabian Sea eddies alter upper‐ocean stratification. Though both intensification types are identified, we find a dominance of likely surface‐intensified eddies characterized by relatively warm and fresh cores for anticyclonic eddies and relatively cool and saline cores for cyclonic eddies, particularly in the northwestern Arabian Sea and Somali Current region.