An account of the key diatom frustules from the surface sediments of the Central and Eastern Arabian Sea and their biogeochemical significance.

The south Asian monsoon is a complex climatic phenomenon characterized by seasonal shifts in winds, rainfall, and ocean currents. The monsoon is a key driver of regional climate and ecosystems. The reconstruction of the monsoon variations in the past helps in understanding the potential drivers of the monsoon. The eastern Arabian Sea is a dynamic marine region strongly influenced by the advection of nutrient- rich upwelled water from the western Arabian Sea during the summer season, and convective mixing during the winter season. Here, we have used the relative abundance and stable oxygen isotopic ratio of planktic foraminifera in sediments drilled as part of the International Ocean Discovery Program Expedition 355, at Site U1457 in the northeastern Arabian Sea to reconstruct the summer and winter monsoon variability during the last 580 kyr. Globigerinita glutinata was the most abundant species, prior to Marine Isotopic Stage (MIS) 12. Interestingly, its relative abundance as well as the glacial-interglacial difference in abundance consistently decreased post MIS 12, suggesting a consistent weakening of the winter monsoon. At the same time, the relative abundance of Globigerina bulloides was not only higher during the warmer interglacials, but also its peak abundance during the interglacials increased continuously, indicating intensified summer monsoon. From the cumulative relative abundance of summer and winter monsoon assemblages, it is evident that the south Asian summer (winter) monsoon consistently strengthened (weakened) during the last 580 kyr. The several folds higher relative abundance of the winter monsoon assemblage indicates that the northeastern Arabian Sea was dominantly influenced by the winter monsoon throughout the last 580 kyr. The summer monsoon was strongly influenced by eccentricity, whereas both the eccentricity and precession affected the south Asian winter monsoon. The findings will help in improving the long-term monsoon projections, in the wake of global warming.

The monsoon currents in the north Indian Ocean

Role of physical processes in determining the nature of fisheries in the eastern Arabian Sea

Particle fluxes were measured using six time-series sediment traps at three sites in the western (16°20' N; 60°30' E), central (14°31' N; 64°46' E) and eastern (15°31' N; 68°43' E) Arabian Sea. Trap deployment depths were between 900 and 3000 m and collection period was from December 1992 to February 1994. Annual particle fluxes showed an east-west trend with minimum fluxes (22.25 gm-2) in the eastern Arabian Sea and maximum fluxes (69.81 g m-2) in the western Arabian Sea. Carbonates, contributed mainly by foraminifers and coccolithophorids, are the dominant component in all the traps. Opal fluxes were maximum in the western Arabian Sea. At all the locations, lithogenic percentages increased with depth whereas organic carbon percentages decreased. Particle flux patterns show a strong seasonality with peak fluxes during the southwest (SW) monsoon (June to September). Relatively high fluxes were also observed during the northeast (NE) monsoon (December to February). In the western Arabian Sea, particle fluxes are dominated mainly by carbonates during the early SW monsoon but by biogenic silica during the fate SW monsoon. The increase in particle fluxes during the early SW monsoon is related to variations in the mixed layer depth which, in turn, is controlled by the strength of the Findtater Jet and the curl of the wind stress. The increase in biogenic silica fluxes during the late SW monsoon is related to the advection of nutrient-rich water from the Oman and Somali upwelling areas. In the eastern Arabian Sea, particle fluxes are high during the NE monsoon due to the effects of winter cooling.

Particle flux data obtained by time series sediment traps deployed at water depths of approximately 3000 m in the western, central, and eastern Arabian Sea since 1986 were compared with wind speeds derived from measurements made by microwave radiometer flying on polar orbiting satellites and sea surface temperatures (SSTs) provided by the Physical Oceanography Distributed Active Archive Center at Jet Propulsion Laboratory. This comparison has allowed us to trace the link between the oceanographic and biological processes related to the development of the SW monsoon with the pattern and interannual variability of particle fluxes to the interior of the Arabian Sea. We could recognize the well‐known upwelling systems along the coasts of Somalia and Oman as well as open ocean upwelling at the beginning of the SW monsoon. Both open ocean upwelling and coastal upwelling off Oman cause a cooling of surface waters at our western and central Arabian Sea stations. When SSTs fall below their long‐term average, an increase in fluxes which are dominated by coccolithophorid‐derived carbonates occurs. The timing of this increase is determined by the rate of surface water cooling. Further intensification of upwelling as the SW monsoon progresses causes additional increases in biogenic opal fluxes denoting diatom blooms in the overlying waters. The total fluxes during this period are the highest measured in the open Arabian Sea. At the central Arabian Sea location the fluxes are only randomly affected by these blooms. The particle flux in the eastern Arabian Sea is as high as in the central Arabian Sea but is influenced by a weaker upwelling system along the Indian coast. The observed interannual variability in the pattern of particle fluxes during the SW monsoons is most pronounced in the western Arabian Sea. This is controlled by the intensity of the upwelling systems on the one hand and the transport of cold, nutrient‐poor, south equatorial water into the Oman region on the other. The latter effect, which is strongest during the SW monsoon with highest recorded wind speeds, reduces the influence of upwelling and the related particle fluxes at the western Arabian Sea station, where highest fluxes occur during SW monsoons with moderate wind speeds. Thus coastal and open ocean upwelling are most effective in transferring biogenic matter to the deep sea during the SW monsoons of intermediate strength.

H.C. Starck's tantalum still conflict free

The Indian monsoon is a complex climatic phenomenon characterized by seasonal shifts in winds, rainfall, and ocean currents. The monsoon strongly influence the climate and ecosystems of the region. The reconstruction of the past monsoon variations can help to understand the potential drivers of the monsoon. The eastern Arabian Sea is a dynamic marine region that is influenced by various oceanographic processes, including the advection of upwelled water from the western Arabian Sea during the summer and convective mixing during the winter season. We used the relative abundance of planktic foraminifera from the sediments drilled during the International Ocean Discovery Program Expedition 355 Site U1457 to reconstruct the summer and winter monsoon variability during the last 580 kyr. Globigerinita glutinata was the most abundant species, prior to Marine Isotopic Stage (MIS) 12. Its relative abundance decreased post MIS12, suggesting weakened winter monsoon. We further report that the relative abundance of G. glutinata was high during the glacial cycles as compared to the interglacials, suggesting strengthened winter monsoon induced increased convective mixing in the eastern Arabian Sea. The relative abundance of Globigerina bulloides was higher during interglacials, suggesting intensified summer monsoon. From the cumulative relative abundance of summer and winter monsoon assemblages, it is evident that the winter monsoon consistently weakened during the last 580 kyr.

Two sediment cores from the western (905; 10.46°9′N, 51.56°4′E, water depth 1586 m) and eastern (SK17; 15°15′N, 72°58′E, water depth 840 m) Arabian Sea were used to study past sea surface temperatures (SST) and seawater δ18O (δ18Ow) variations for the past 35 ka. We used coupled Mg/Ca‐δ18O calcite variability in two planktonic foraminiferal species: Globigerinoides ruber, which thrives throughout the year, and Globigerina bulloides, which occurs mainly when surface waters contain high nutrients during upwelling or convective mixing. SSTs in both areas based on Mg/Ca in G. ruber were ∼3 to 4°C lower during the Last Glacial Maximum (LGM; ∼21 ka) than today and the Holocene period. The SST records based on G. bulloides also indicate general cooling, down to 18°C in both areas. SSTs in the western Arabian Sea based on G. bulloides were always lower than those based on G. ruber, indicating the presence of strong seasonal temperature contrast during the Holocene and LGM. We interpret the consistent presence of this seasonal temperature contrast to reflect a combination of seasonal summer upwelling (SW monsoon) and winter convective mixing (NE monsoon) in the western Arabian Sea. In the eastern Arabian Sea, G. bulloides‐based SSTs were slightly lower (about 1°C) than G. ruber‐based SSTs during the Holocene, indicating the almost absence of a seasonal temperature gradient, similar to today. However, a large seasonal temperature contrast occurred during the LGM which favors the assumption that strong NE monsoon winds forced winter upwelling or convective mixing offshore Goa. SST and δ18Ow reconstructions reveal evidence of millennial‐scale cycles, particularly in the eastern Arabian Sea. Here, the stadial periods (Northern Hemisphere cold periods such as Younger Dryas and Heinrich events) are marked by increasing SSTs and salty sea surface conditions relative to those during the interstadial periods. Indeed, the δ18Ow record shows evidence of low‐saline surface waters during interstadial periods, indicating increased freshwater runoff from the Western Ghats as a consequence of enhanced SW monsoon intensity.

Nutriclines and nutrient stoichiometry in the eastern Arabian Sea: Intra-annual variations and controlling mechanisms

Seasonal (winter monsoon, intermonsoon and southwest monsoon) and interannual (between southwest monsoon seasons of 1995 and 1996) variations in total carbon dioxide (TCO2) and partial pressure of CO2 (pCO2) were studied in the central and eastern Arabian Sea as a part of the JGOFS (India) Programme. The pCO2 values were computed from the results of coulometric TCO2 and spectrophotometric pH measurements. Seasonal variability in TCO2 is evident with the changing circulation and biological production. In all seasons, the pCO2 is higher in surface waters of the Arabian Sea, except along the Indian coast in the southwest monsoon, than that in atmosphere, and thus this region appears to be a perennial source of atmospheric CO2. Significantly, an average of ~45 Tg y-1 could be ejected to atmosphere from the study region, that seems to far exceed the earlier estimations. The estimated fluxes, however, are in agreement with those from the eastern equatorial Pacific Ocean.

Particulate fluxes of aluminium, iron, magnesium and titanium were measured using six time-series sediment traps deployed in the eastern, central and western Arabian Sea. Annual Al fluxes at shallow and deep trap depths were 0.47 and 0.46 g m-2 in the western Arabian Sea, and 0.33 and 0.47 g m-2 in the eastern Arabian Sea. There is a difference of about 0.9–1.8 g m-2y-1 in the lithogenic fluxes determined analytically (residue remaining after leaching out all biogenic particles) and estimated from the Al fluxes in the western Arabian Sea. This arises due to higher fluxes of Mg (as dolomite) in the western Arabian Sea (6–11 times higher than the eastern Arabian Sea). The estimated dolomite fluxes at the western Arabian Sea site range from 0.9 to 1.35gm-2y-1. Fe fluxes in the Arabian Sea were less than that of the reported atmospheric fluxes without any evidence for the presence of labile fraction/excess of Fe in the settling particles. More than 75% of Al, Fe, Ti and Mg fluxes occurred during the southwest (SW) monsoon in the western Arabian Sea. In the eastern Arabian Sea, peak Al, Fe, Mg and Ti fluxes were recorded during both the northeast (NE) and SW monsoons. During the SW monsoon, there exists a time lag of around one month between the increases in lithogenic and dolomite fluxes. Total lithogenic fluxes increase when the southern branch of dust bearing northwesterlies is dragged by the SW monsoon winds to the trap locations. However, the dolomite fluxes increase only when the northern branch of the northwesterlies (which carries a huge amount of dolomite accounting 60% of the total dust load) is dragged, from further north, by SW monsoon winds. The potential for the use of Mg/Fe ratio as a paleo-monsoonal proxy is examined.

This study resolves the spatial and seasonal variations in prokaryotic abundance (PA) and biomass concerning physicochemical parameters during Spring Inter-Monsoon (April-May), Summer Monsoon (June-September), and Winter Monsoon (November-February) in the Eastern Arabian Sea. PA and biomass distribution estimated using microscopic techniques revealed their peak abundance during Spring Inter-Monsoon, ranging from 2.29-4.41 × 106 Cells mL-1 to 8.39-21.82 μgL-1, respectively. Similarly, high PA and biomass were observed in late Summer Monsoon (September), ranging from 2.01-3.96 × 106 Cells mL-1 to 8.74-16.70 μgL-1, respectively, which was preceded by a higher phytoplankton abundance (chlorophyll a- 14.57 mgm-3) during the peak Summer Monsoon (August). The Winter Monsoon, started with a low PA and phytoplankton abundance. As Winter Monsoon progressed, convective mixing promoted phytoplankton growth in the latter half until March. The decay released dissolved organic carbon (DOC), leading to a rise in PA from January to February, peaking during Spring Inter-Monsoon (first peak). With the advent of Summer Monsoon, upwelling enriched surface layers with nutrients to promote phytoplankton growth in August. The subsequent decaying phase generated higher DOC which enhanced PA by the end of Summer Monsoon (second peak). However, PA declined to its lowest levels by November. Distance-based linear model analysis indicated that temperature and chlorophyll a were the most influential factors affecting PA in the upper photic-zone, while ammonia, dissolved oxygen, and DOC were associated factors. In contrast, nutrients were the major determining factors in disphotic waters (200-2000m). This study highlights the intricate interplay between physicochemical and biological variables in shaping prokaryotic populations during various physical forcings through intense sampling efforts in the Arabian Sea.

Using sea surface temperature (SST) and wind speed retrieved by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), for the period of 1998–2003, we have studied the annual cycle of SST and confirmed the bimodal distribution of SST over the north Indian Ocean. Detailed analysis of SST revealed that the summer monsoon cooling (winter cooling) over the eastern Arabian Sea (Bay of Bengal) is more prominent than winter cooling (summer monsoon cooling). A sudden drop in surface short wave radiation by 57 W m−2 (74 W m−2) and rise in kinetic energy per unit mass by 24 J kg−1 (26 J kg−1) over the eastern Arabian Sea (Bay of Bengal) is observed in summer monsoon cooling period. The subsurface profiles of temperature and density for the spring warming and summer monsoon cooling phases are studied using the Arabian Sea Monsoon Experiment (ARMEX) data. These data indicate a shallow mixed layer during the spring warming and a deeper mixed layer during the summer monsoon cooling. Deepening of the mixed layer by 30 to 40 m with corresponding cooling of 2°C is found from warming to summer monsoon cooling in the eastern Arabian Sea. The depth of the 28°C isotherm in the eastern Arabian Sea during the spring warming is 80 m and during summer monsoon cooling it is about 60 m, while over the Bay of Bengal the 28°C isotherm is very shallow (35 m), even during the summer monsoon cooling. The time series of the isothermal layer depth and mixed layer depth during the warming phase revealed that the formation of the barrier layer in the spring warming phase and the absence of such layers during the summer cooling over the Arabian Sea. However, the barrier layer does exist over the Bay of Bengal with significant magnitude (20–25 m). The drop in the heat content with in first 50 m of the ocean from warming to the cooling phase is about 2.15 × 108 J m−2 over the Arabian Sea.

Abstract. Mode water acts as a barrier layer controlling surface-to-interior fluxes of key climatic properties. In the Arabian Sea, mode water stores heat and provides an oxygen-rich layer for rapid remineralization, and its subduction is a direct pathway for oxygen into the upper oxygen minimum zone. We use float observations to characterize the properties of the Arabian Sea mode water layer (MWL). The MWL forms when springtime warming stratifies the surface layer and caps the deep surface mixed layer formed during the winter monsoon. During the summer monsoon, a second MWL is formed south of 20° N following the cessation of wind-driven mixing. We use 1D and 3D models to disentangle the contributions of atmospheric and oceanic forcing to this water mass. The 1D model accurately represents the mode water's formation and erosion, showing that atmospheric forcing is the first-order driver, in agreement with observations. However, there are regions where advective processes, eddy mixing, or biological heating are essential for the formation and/or erosion of the MWL. For instance, in the eastern Arabian Sea, freshwater-driven stratification advected via the West Indian Coastal Current reduces the potential for deep mixed layers via convective mixing, resulting in a thinner MWL. The 3D model shows that the MW contributes 5 ± 1 % to the oxygen content of the upper ocean, with its maximum during spring in the northern Arabian Sea (40 ± 17 %), thus highlighting the key role of the water mass in storing and transporting heat and oxygen to the interior.

The seasonal and interannual variability of physicochemical (temperature, salinity, dissolved oxygen, and nutrients) and productivity characteristics of the Arabian Sea (AS) are studied using 30 years (1993–2022) of datasets. Chlorophyll-a (Chl-a) variance at specific depths identified four core variability regions: Western AS (WAS), Eastern AS (EAS), Northern AS (NAS), and Central AS (CAS). The highest Chl-a content was observed in the WAS region during the summer monsoon, followed by the NAS region, where the winter monsoon dominated productivity. Nutrient entrainment from the WAS to the open ocean enhances productivity in the CAS, which lags by over a month compared to other regions. It appears that nitrate and phosphate contribute to productivity in all regions. However, silicate has no contribution in the EAS region, but iron does. Parameters like Chl-a, net primary production, nitrate, and phosphate in all regions have decreased, and on the contrary, iron has increased, but increase of silicate in the EAS region. This study also unveils the El-Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) effects on biogeochemistry parameters across AS regions. During ENSO/IOD years, Chl-a anomaly reveals a strong correlation between IOD and EAS suggesting more substantial influence on the productivity of this region.