Observed Seasonal and Interannual Controls on Coastal Oxygen and Dead Zones in the Indian Ocean

A major concern is that global de-oxygenation will expand Oxygen minimum zones (OMZs) and favor coastal dead zones (DZs) where already low oxygen levels threaten ecosystems and adjacent coastal economies. The northern Indian ocean is home to both intense OMZs and DZs, and is surrounded by many kilometers of biodiverse and commercially valuable coastline. Exchanges between OMZs and shelf waters that contribute to coastal DZs are subject to the strong monsoonal seasonal cycle and the interannual variability of the Indian Ocean Dipole (IOD).  There is, however, no observational constraints on how these exchanges influence coastal DZs at the scale of the entire northern Indian Ocean.

In this work, we examine the timing and processes that favor low-oxygen concentrations along the coasts of the Bay of Bengal (BoB) and Arabian Sea (AS) using multi-decadal time series of oxygen profiles (Bio-Argo, World Ocean Database and repeat hydrography) combined with a suite of satellite data. Seasonally, we show that coastal oxygen is lowest during winter/spring in the BoB and summer/fall in the AS, closely following the seasonal propagation of coastal waves and wind-driven upwelling. Interannually, observations indicate that positive IODs increase coastal O2 in summer/fall in the AS, partly offsetting the seasonal signal; a result in agreement with prior modeling work (Vallivattathillam et al 2017). Observations reveal, however, that positive IODs favor low coastal O2 conditions and increase the risk of coastal DZs year-round in the BoB and in winter/spring in the AS, whereas negative IODs favor low O2 in summer/fall in the AS.