Why is the Bay of Bengal less productive during summer monsoon compared to the Arabian Sea?

TLDR

The Bay of Bengal is traditionally considered less productive than the Arabian Sea, and nutrient‑rich water advection into the euphotic zone is unlikely there. We explore the reasons for this in the central Bay during summer. Copious summer rainfall and river discharge freshen the Bay’s surface by 3–7 psu and raise SST by 1.5–2 °C, creating a strongly stratified surface layer that weak winds cannot erode, limiting wind‑driven mixing to <20 m and preventing nutrient entrainment from below.

Abstract

The Bay of Bengal is traditionally considered to be a less productive basin compared to the Arabian Sea. We explore the reasons for this in the central Bay during summer. Copious rainfall and river water freshen the upper layers of the Bay by 3–7 psu during summer, and SST was warmer by 1.5–2°C than in the central Arabian Sea. This leads to a strongly stratified surface layer. The weaker winds over the Bay are unable to erode the strongly stratified surface layer, thereby restricting the turbulent wind‐driven vertical mixing to a shallow depth of &lt;20 m. This inhibits introduction of nutrients from below, situated close to the mixed layer bottom, into the upper layers. While advection of nutrients rich water into the euphotic zone makes the Arabian Sea highly productive, this process is unlikely in the Bay of Bengal.