Summertime increases in upper ocean stratification and mixed layer depth

The surface mixed layer of the world ocean regulates global climate by controlling heat and carbon exchanges between the atmosphere and the oceanic interior(1–3). The mixed layer also shapes marine ecosystems by hosting most of the ocean’s primary production(4) and providing the conduit for oxygenation of deep oceanic layers. Despite these important climatic and life-supporting roles, possible changes in the mixed layer during an era of global climate change remain uncertain. Here, we use oceanographic observations to show that from 1970-2018 the density contrast across the mixed-layer base increased and that the mixed layer itself deepened. The summertime density contrast increased by 8.9±2.7% dec(-1) (10(-6)-10(-5) s(-2) dec(-1), depending on region), more than six times greater than previous estimates due to our use of a more physically-based definition of mixed layer stability following the differing dynamical regimes across the global ocean. While prior work has suggested that a thinner mixed layer should accompany a more stratified ocean(5–7), we instead find that the summertime mixed layer deepened by 2.9±0.5% dec(-1) or several meters per decade (typically 5-10m dec(-1), depending on region). A detailed mechanistic interpretation is challenging, but the concurrent stratification and deepening of the mixed layer are related to an increase in stability associated with surface warming and high latitude surface freshening(8,9), accompanied by a wind-driven intensification of upper-ocean turbulence(10,11). Our results are based on a complex dataset with incomplete coverage of a vast area; we found our results to be robust within a wide range of sensitivity analyses, but important uncertainties remain, such as sparse coverage in the early years. Nonetheless, our work calls for reconsideration of the drivers of ongoing shifts in marine primary production, and reveals stark changes in the world’s upper ocean over the past five decades.