Wintertime convection and ventilation of the upper pycnocline in the northernmost Arabian Sea

Abstract

Mined regional observations from the northernmost Arabian Sea show winter convection probably annually forming a well-ventilated salinity maximum with σ t of ∼25 kg m ―3 . This Arabian Sea High Salinity (NASHSW) replaces the Arabian Sea High Salinity Water (ASHSW) of the central and eastern Arabian Sea (σ t ∼24 kg m ―3 ). It is subducted similarly to high-salinity waters at the subtropical convergences elsewhere and interleaves below the ASHSW. Poleward of 21°―22°N, the core is usually centered between 22° and 23°C. In data from March to April 1990, the medians of depth, temperature, density, and O 2 of the NASHSW and ASHSW differ at p < 0.01. North of 21°―23°N during at least the first trimester, the NASHSW ventilates the pycnocline down to the 25.5 kg m ―3 isopycnal (∼200 m) or somewhat deeper. Between 21° and 22°C and 24°―25°C, NASHSW's O 2 content tends to be 1―2 mL L ―1 higher than in the ASHSW adjoining to the south. It persistently excludes denitrification. Its O 2 may degrade settling organic matter at shallow depths. In the pycnocline top above the NASHSW, a thin (decameters) salinity minimum is frequent, which in one third of the data is less aerated than the first depth of the NASHSW. This layer may be deep, high-nutrient water isolated by interleaving NASHSW. It is impacted first by wintertime convection. In 30 years with data between 1960 and 1999, NASHSW occurred almost annually at and poleward of 21°―22°N. In more than one half of the cases, it extended to 20°N and in one third, to 18°N.