The upper ocean vertical structure in a changing climate
Ocean surface warming is commonly associated with a more stratified ocean, less productive and oxygenated. Such assertion is commonly made based on its consistency with increased near-surface stratification and shallower mixed layers under global warming scenarios. In contrast to model simulations, concurrent with the observed surface warming at mid-latitudes, ocean observations show that stratification is not unequivocally increasing and MLD shoaling. We find that while SST increases at three study areas at mid-latitudes, stratification both increases and decreases, and MLD deepens with enhanced deepening of winter MLDs. MLD is found deepening even when stratification increases. Why is this so? It is true that increased stratification from warming and freshening hinders the ventilation of the ocean. However, the ventilation of the ocean depends also on the strength of the wind-driven divergence able to compensate the effects of ocean warming and increasing stratification. Apart from the certain warming of the upper ocean over the last 30 years, a poleward migration of the maximum westerlies is taken place on both hemispheres with changes also in its intensity. With that in mind, during the presentation I would like to discuss with you, both observationalist and modellers, how all these changes are affecting the upper ocean vertical structure and determining MLD and stratification changes that may oppose the expected changes due to warming from the overlying atmosphere. On this discussion, I will find particularly interesting if we can address two issues: the ability of ocean models to reproduce the upper ocean vertical structure and of different stratification indexes to represent the tendency/opposition of a water column to be mixed.