The Warmer The Ocean Surface, The Shallower The Mixed Layer. How Much Of This Is True?

Ocean surface warming is commonly associated with a more stratified ocean, less productive and oxygenated. Such assertion is mainly made based on its consistency with increased near-surface stratification and shallower mixed layers under global warming scenarios. However, here, the analysis of long term hydrographic time-series at mid-latitudes does not support such simplified relationships, and the generalized surface warming is found concurrent with deepening mixed layers depths (MLD) at rates from 5.8 to 17.8 m/decade. This result relies on the best fit of density profiles of hydrographic time-series at HOTS (22.8°N, 158°W;North Pacific Subtropical Gyre), BATS (32.2°N, 64.5°W;North Atlantic Subtropical Gyre) and Santander (43.8°N, 3.8°W; eastern North Atlantic) stations sampled monthly since the early 1990s to a predefined ‘‘ideal’’ functional form. Through this approximation, apart from the MLD, we get information on other parameters of hydrographic profiles, such as the stratification (pycnocline gradient) and the distance between the MLD and the main pycnocline. Combining this information with estimated MLDs from buoyancy fluxes, MLD variability reveals itself as a subtle interplay between circulation and air-sea exchanges at mid-latitudes. In the North Atlantic Subtropical Gyre, the MLD deepens but less than expected from changes on densification of the ocean surface during the winter while the distance between the MLD and the main pycnocline increases. It is coherent with slower subduction rates and circulation in the basin, consistent with the sustained decrease of the Atlantic Meridional Overturning Circulation during the last decade. Conversely, in the North Pacific Subtropical Gyre, the deepening of the MLD is higher than expected, and the distance between the MLD and the main pycnocline decreases. An increase of the subduction rates and circulation could explain it. Concerning the effects of deeper MLDs on ocean productivity, just the maintenance of the nutrient supply would lead to an increase of primary production in a warmer upper ocean under the assumption of a bottom-up control and nutrient-limited production.