Water characteristics, mixing and circulation in the Bay of Bengal during southwest monsoon

Abstract

Influence of the freshwater influx, the wind forcing and the Indian Ocean monsoon drift current on the property distributions and the circulation in the Bay of Bengal during southwest monsoon has been quantified. At the head of the Bay, waters of low salinity, affected by the freshwater influx, occupy the upper 90 m water column. The isohaline 34.0 x 10-3 separating these waters from those of underlying saline waters shoals southward gradually and outcrops around 14N, ION and 6N in the western, central and southeastern regions of the Bay respectively. The wind-stress-cur\-induced upwelling effect is confined to depth limits of 50-100 m as is supported by a band of cold (24°-19°C) water in the central Bay. In the southern and central regions of the Bay, the monsoon drift current feeds the large scale cyclonic gyre apart from maintaining the northward flowing boundary current in the eastern Bay. A warm (27°-23°C), saline (35.0-35.2 x 10-3 ) watermass is advected northeastward along with the monsoon drift current into the Bay up to 14N at the depth limits of 50-100 m. Below this depth, in the western Bay a well-defined southward flow in the form of a boundary current is documented. Intense vertical mixing is inferred at the zones of salinity fronts in the depth limits of 40-100 m and also at deeper depths (> 2200 m) and elsewhere lateral mixing is predominant.

Modeling Salinity Exchanges Between the Equatorial Indian Ocean and the Bay of Bengal

“…If the salinity of the outflow is 33 psu, 34 psu, or 34.5 psu, the averaged required outgoing volume transport is 1.65 Sv, 3.5 Sv, or 6.9 Sv, respectively. Observations show that high-salinity water from the Arabian Sea enters the BoB between 80°E and 90°E during the southwest monsoon season (June to September; e.g., Murty et al, 1992;Vinayachandran et al, 2013;Figure 7a,b in Gordon et al, 2016, in this issue), and this has been replicated in several models (e.g., Vinayachandran et al, 1999;Han and McCreary, 2001;Jensen, 2001). More recently, observations and…”

The JASMINE Pilot Study

“…The Bay of Bengal sector of the Indian Ocean has been poorly sampled until recently. Murty et al (1992) provide unique results based on a coarse array of temperature and salinity observations within the Bay of Bengal during the southwest monsoon. Bhat et al (2001) report on the recent BOBMEX observations during July and August 1999.…”

Seasonal Effects of Indian Ocean Freshwater Forcing in a Regional Coupled Model*

“…This results in a very large lateral haline gradient. From a time series of hydrographic observations conducted in the northern BoB during the summer of 1984 on board ORV Sagar Kanya, Murty et al (1992) reported the large SSS gradient from the open bay (35 psu) toward the northern BoB (16 psu). The WOA05 well captures this spatial distribution with, for example, low salinity from the river runoff (e.g., 20.5 psu in December at 218N, 888E) that increases sharply toward the center of the bay (32.8 psu at 158N, 888E in December).…”