Control of the Air-Sea Oxygen to Heat Flux Ratio During Deep Convection Events

Earth System Models project a decline of dissolved oxygen in the oceans due to climate warming. Observational studies suggest that the ratio of O₂ inventory to ocean heat content is several fold larger than what can be explained by solubility alone, but the ratio remains poorly understood. In this work, models of different complexity are used to understand the factors controlling the air-sea O₂ flux to heat flux ratio (O₂/heat flux ratio) during deep convection. Our theoretical analysis based on a one-dimensional convective adjustment model indicates that the vertical stratification and distribution of oxygen before the convective mixing determines the upper bound for the O₂/heat flux ratio. Two competing rates, the mean entrainment rate of deeper waters into the mixed layer and the rate of air-sea gas exchange, determine how much the actual ratio departs from the upper bound. The theoretical predictions are tested against the outputs of a regional ocean model. The model sensitivity experiments broadly agree with the theoretical predictions. Our results suggest that the relative vertical gradients of temperature and oxygen at sites of deep water formation are an important local metric to quantify the marginal changes between years with high and lower heat loss.