Upper layer characteristics of the South Eastern Arabian Sea associated with an unusual low saline pool during fag end of southwest monsoon

Observational evidence on an anomalous low saline pool due to excess rainfall and runoff during September 2020 in the South Eastern Arabian Sea (SEAS) is explained based on in-situ and satellite/reanalysis datasets. The study shows the presence of a low saline pool in the coast to offshore section between 10 and 14°N, which is associated with weak winds (3–5 m/s) and surface currents (0.1–0.2 m/s), compared to the long-term average. The pool is observed with strong vertical temperature gradient of 4°C and salinity gradient of 3.5 psu in the upper 20 m column. Cyclonic eddy in the central SEAS influences the distribution of low saline pool which suppress the coastal upwelling in the region. However, active upwelling is observed (surface temperature <26°C) in the southern transects (8–9°N) associated with strong wind (>6 m/s) and surface current (>0.2 m/s) which is also supported with weak rainfall and runoff. The high phytoplankton biomass in the nutrient-rich active upwelling region supports high zooplankton biomass. Contrastingly, the low saline pool region experienced low nitrate and phosphate concentration resulting into less chlorophyll concentration and that might be a plausible reason for the low zooplankton biomass. Research highlights An unusual low saline pool was observed in the mixed layer of SEAS during fag end of SW monsoon 2020. Anomalous rainfall and runoff associated with weak wind, currents lead to the formation of the low saline pool. Cyclonic eddy observed in the region defined the flow within and the overall pattern of the pool region. The low saline pool inhibits coastal upwelling to the subsurface in the central SEAS. The low saline pool resulted in a stratified water column in the upper layer leading to reduced phytoplankton concentration and zooplankton biomass.