Arabian sea Oxygen Minimum Zone projections under climate change: sensitivity to forcing and model resolution

The Arabian sea (AS) hosts one of the most intense oxygen minimum zones (OMZ) in the open ocean. This OMZ is formed and maintained by the peculiar geography and the associated monsoonal productivity in the AS and  is highly sensitive to the strength of monsoonal circulation and surface heating. Model projection from the fifth phase of Coupled Model Intercomparison project (CMIP5) indicate significant changes in both the Indian monsoonal circulation, atmospheric heat fluxes and primary productivity under climate change, but the response of the AS OMZ to these changes remain largely ill-understood. The poor representation of the AS OMZ and lack of oxygen diagnostics in the CMIP5 simulations pose major limitations in exploring the response of AS OMZ to future climate change. In this study, we use a set of regional downscaled experiments with a high-resolution configuration of the Regional Ocean Modeling System (ROMS) model coupled to a nitrogen-based NPZD ecosystem model to examine the sensitivity of the AS OMZ response to a range of CMIP5 forcing anomalies and to model resolution. Our downscaled set of experiments are based on a climatological control simulation forced with observed climatological atmospheric and lateral boundary conditions, to which climate change anomalies derived from CMIP5 simulations are added to construct climate change forcing fields. The control simulation has been extensively validated against observations. We explore the sensitivity of the downscaled oxygen distribution and OMZ to the regional model setup by varying the model horizontal resolution from 1/3 - 1/12 degree. In agreement with the set of available coarse resolution CMIP5 projections, our downscaled experiments show a future increase in the oxygen levels within the core of AS OMZ. The downscaled experiments improve the realistic representation of different classes of water (Oxic - O2 > 60mmol/l; Hypoxic - 60mmol/l >= O2 > 4mmol/l; and the Suboxic - 4 mmol/l > O2 > 0 mmol/l) within the 0-1500m depth range. We find that the projected oxygen changes in the AS OMZ are largely driven by the Apparent Oxygen Utilisation (AOU), which vary with forcing and model resolution, leading to a wide spread in the AS OMZ response to climate change.