Arabian Sea Mode Water: A Key Player in Surface-to-Interior Exchange

Mode water acts as a barrier layer controlling surface-to-interior fluxes of key climatic properties. In the Arabian Sea, mode water provides an oxygen-rich layer for rapid remineralization, and its subduction is a direct pathway for oxygen into the upper oxygen minimum zone. Using observations from underwater gliders and argo floats, alongside numerical models (GOTM CVmix and MOM4p1-TOPAZ), we characterize the Arabian Sea mode water across temporal and spatial scales, ranging from submesoscale variability to seasonal climatologies.

Mode water forms when surface buoyancy gain and weak winds cap dense, deep mixed layers beneath a stratified surface layer. This process occurs annually in the northern Arabian Sea during winter and biannually south of 20°N following the monsoons. Atmospheric forcing primarily drives mode water formation, except in regions influenced by advective processes (e.g., freshwater influx from the Bay of Bengal via the WICC), or biological modulation of heat uptake at seasonal timescales. Our findings show that mode water contributes up to 30% of the upper ocean (0-250 m) oxygen content in the Arabian Sea, emphasizing its critical role in regulating oxygen storage. On timescales of days to weeks, we demonstrate the significance of mesoscale eddies in eroding the mode water layer leveraging high-resolution glider observations. These results underline the multifaceted drivers shaping mode water dynamics and their pivotal role in regional climate and biogeochemical processes at different temporal and spatial scales.