Examining the global record of interannual variability in stratification and marine productivity in the low-latitude and mid-latitude ocean

Strengthened stratification of the upper ocean, associated with anthropogenic or climate-driven warming, is generally expected to inhibit marine primary productivity in light-replete, nutrient-limited environments, essentially, in the low and middle latitude ocean, based on the supposition that increased water column stability will inhibit vertical mixing and consequently the upward entrainment of deep nutrients into the euphotic zone. Herein, we examine the local stratification control of productivity on interannual timescales over the global subtropical and tropical ocean by directly comparing in situ measures of stratification (from hydrographic profile data) with contemporaneous values of ocean chlorophyll (from satellite data). In the subtropical ocean, we find no evidence of a strong local correlative relationship between these properties over the observational record, a result that challenges the widely held view that stratification variability is a primary driver of interannual variability in nutrient supply and productivity in these waters. A strong negative relationship is observed, however, in the tropical Pacific, suggesting that previously reported correlations between globally averaged stratification and productivity variability are driven by strong associations in this region. An examination of the long-term changes in our profile data also reveals trends of decreasing stratification scattered across the low-latitude and mid-latitude ocean, driven by faster rates of warming in the subsurface relative to the surface. This observation seemingly undercuts a fundamental assumption of the paradigm of local stratification control, namely that increases in upper ocean heat content necessarily produce strengthened stratification.