Recent changes in the upper oceanic water masses over the Indian Ocean using Argo data

Abstract

Utilizing Argo data from 2003 to 2019, we examine thermohaline changes in the Indian Ocean within the upper 700 m. Widespread warming is observed except in the Southern Indian Ocean. Increasing salinity is obtained over all regions except the Bay of Bengal and Southern Indian Ocean. Thermohaline trends in regional water masses at various depths are first decomposed into spice and heave components, and then linked to processes like pure heave, pure freshening and pure warming. Three consistent patterns across all seven regions are: (1) Below 300 m spice dominates heave; (2) The freshening process within the spice component is the primary driver below 300 m; (3) Spice primarily influences salinity changes along isobars. The warming of Arabian Sea’s Subsurface Minima and the Indian Equatorial Water are primarily dictated by spice and heave, respectively. Freshening of the Bay of Bengal Water is linked to heave changes under pure freshening and pure heave processes. In the upper 250 m of the western equatorial, southern Indian Ocean, and Seychelles–Chagos Thermocline Ridge, salinity rises due to spice under pure freshening. The southern Indian Ocean’s advected mode water shows freshening and cooling trends due to pure freshening.