The 2018 Monsoon Onset from Ship-based Measurements across the Air-Sea Interface

In the Bay of Bengal, the warm and dry boreal spring is concluded with the annual onset of the summer monsoon and its accompanying southwesterly winds, periodic heavy rains, and variable air-sea fluxes. Here, we summarize the onset of the 2018 summer monsoon from ship-based, high-resolution measurements that spanned the air-sea boundary in the central Bay of Bengal. The first half of the ~20 day cruise was punctuated by cold outflows, intermittent heavy rainfall, and increasing winds (2-15 m/s), while the second half was marked by relatively dry, clear conditions with persistent 10-12 m/s winds. The strong winds prevented formation of a diurnal warm layer; however, the observed precipitation exhibited a diurnal signal with rain tending to occur during daylight hours. Accumulated rainfall at the ship exceeded 200 mm with 90% of precipitation taking place during the first 8 days of the cruise. One particularly strong system was accompanied by 10 minute rain rates that exceeded 130 mm/hr and an accumulated precipitation of more than 75 mm over 6 hours. During this event, the upper 5 m of the ocean freshened by nearly 0.25 psu. Over the cruise duration, the mixed layer depth increased from 20 m to over 40 m depth with an accompanying decrease (increase) in sea surface temperature (salinity) of more than 1°C (1 psu). The upper ocean heat content relative to the 28°C isotherm reduced by a half to 250 MJ/m³. The coinciding flux of heat was ~200 W/m² out of the upper 40 m, which may be compared to the observed net surface loss of ~100 W/m² averaged over the first half of the cruise. The relatively large fraction of ocean heat loss, when integrated over the large area of the northern Bay, may be an important factor in organization of convection during the monsoon onset.