A warming climate is predicted to increase the upper ocean stratification in tropical and subtropical ocean which in turn is expected to result in decreases in the primary productivity for these oligotrophic regions. To assess if there is trended change in primary productivity, we use 17-years of satellite chlorophyll (Chl) data for the southern Indian Ocean (IO) with known striking temperature increase. Observations from this oligotrophic region exhibited and surprisingly increasing Chl concentrations. To study potential mechanisms underlying the increased Chl, we used temperature/salinity observations to re-evaluate stratification in the southern IO.  The southern IO experienced basin-wide surface warming over the time series however there was a region of subsurface cooling at 50-100m around 10°S.  In the subtropical IO gyre faster rates of subsurface warming relative to the surface exit. These inhomogeneous trends in the vertical thermohaline structure induced subsurface instabilities that weakened surface stratification. This was particularly true over the southern IO gyre, which experienced sustained increase of surface mixing disturbances over the last decade—resulting in a more likely vertical transport of nutrients into the euphotic zone. A mixed layer nutrient budget analysis suggested that vertical mixing and entrainment due to mixed layer deepening were crucial in delivering nutrients into the gyre’s upper mixed layer, which fueled phytoplankton activity. This emphasizes the importance subsurface instabilities on the surface Chl. This study highlights the importance of a three-dimensional framework for examining stratification to assess future marine ecosystem responses to a changing climate.
 

chlorophyll, subsurface instabilities, stratiffication