Partitioning Ocean Warming at the Mixed Layer Base

Atmospheric warming enters the ocean via surface buoyancy fluxes. Our study analyzes temperature anomalies at the depth where these anomalies penetrate the ocean interior, by focusing at the winter mixed layer (WML) base. A this interface, we separate temperature anomalies that are attributed to wind-driven gyre dynamics from density-compensated anomalies that penetrate the ocean along isopycnals. Multidecadal temperature trends over the past four decades from an objectively analyzed hydrographic dataset EN4 are partitioned between a component related to isopycnal movement (heave) and a component along isopycnals (spice). Our analysis reveals that the largest warming occurs in subtropical gyres and western boundary currents; this warming projects onto the heave component as expected from the previously observed strengthening of gyre circulation. However, in subtropical regions of surface salinity maxima, warm anomalies are observed along isopycnals.

A Lagrangian 1D model of a water column allows us to study in detail the processes behind the projection of anomalous surface heat and freshwater fluxes onto the spice and heave components of the permanent pycnocline. Using the 1D model, the mechanisms driving the redistribution of anomalous surface fluxes are related to the advective and diffusive components of the subduction. In particular, the subduction of temperature anomalies under a warming climate is studied in the subtropical gyres where there is a density-compensated layer.