2023 Articles

Role of the Dipole Mode Index in governing the freshwater content within the Bay of Bengal summer Pycnocline.

Bohman, Shannon M.; Gordon, Arnold L.

The Bay of Bengal (BoB) surface layer receives approximately 0.13 million m3/sec (0.13 Sv) of freshwater through a combination of precipitation and river runoff minus evaporation, which then is exported to neighboring seas. Quasi-stationary salinity is established by the import of salty water from the Arabian Sea (AS), primarily within the pycnocline as an estuarine type of circulation. The BoB pycnocline is also affected by low salinity export from the southern Andaman Sea (AndS). We use Argo observations and GODAS reanalysis products to trace the spreading of AS and AndS water within BoB pycnocline during the summer monsoon when the bulk of AS water is imported. We use relative freshwater content (RFWC), which is zero for pure AS water and 1 for pure AndS water. We find significant interannual variability of the RFWC pattern, which relates to the Indian Ocean Dipole, as defined by the Dipole Mode Index (DMI). The position of the Sri Lanka Dome SLD) off the east coast of Sri Lanka varies with DMI: the SLD is farther east during negative DMI, which directs the AS water farther to the east in BoB; whereas during the positive DMI, the AS water is directed to the north.

Highlights:
- Indian Ocean Dipole impacts the freshwater content within Bay of Bengal pycnocline.
- Arabian Sea and Andaman Sea waters regulate the Bay of Bengal pycnocline salinity.
- Salty water spreads eastward (northward) during negative (positive) Dipole Mode.