Long term observed changes of the global and Southern Ocean surface layer

The surface layer of the ocean is the gateway for all exchanges between the atmosphere and the deep ocean. Change in the surface oceans have therefore widespread impact by regulating the penetration of climate signal to the deep-seas. The surface layer of the oceans are currently undergoing rapid changes, which increases its stability and impact large-scale circulation patterns, ocean carbon and heat cycles, the rate of ocean oxygenation and acidifications, as well as surface primary productivity. However, despite its glaring importance for climate, current changes in surface layer stability, and associated mixed-layer depth are still poorly documented and understood. In this study, we document and quantify global increase of the surface layer stratification over the past 50 years, as well as, concurrent change of mixed-layer depth. Interestingly the Southern Ocean departs from the global average picture with a counter-intuitive significant increase of the stability of the Southern Ocean surface layer in summer, along with a deepening of the summer mixed-layer. The long-term deepening of the summer mixed-layer depth is primarily explained by increasing momentum flux associated with intensifying westerly winds. In contrast with summer changes, deep mixed-layers associated with large water mass formation in the Southern Ocean Pacific basin appear to have significantly shallowed over the past twenty years. Our results have important consequences for biogeochemistry and its future, as well as for the ventilation of the deep ocean.