Multidecadal-scale Adjustment of the Ocean Mixed Layer Heat Budget of the Tropical and Subtropical Oceans

Multiple ocean and atmospheric reanalysis products are used to understand how the components of the ocean mixed layer heat budget across the tropics have adjusted to decadal-scale trends in recent decades. Atmospheric reanalyses used are the ERAI, JRA-55, and NCEP2 products, and ocean reanalyses are GODAS, GECCO, ORAS-3 and ORAS-4.

In the tropical Pacific, linear trends of warming in the west and cooling in the east are accompanied by a strengthening of the Pacific Walker circulation. Two components of the ocean mixed layer heat balance provide primary support for the cooling trends in the eastern and central Pacific. One is increased cooling through an enhanced latent heat flux (evaporation) over much of the tropical Pacific. Despite reductions in the latent heat flux associated with the ocean mixed layer cooling, increases in the trade winds drive net increases in the latent heat flux. Cooling along the equator in the central Pacific is due to enhanced equatorial upwelling, also associated with stronger low-level easterly flow. Increases in the equatorial undercurrent also occur, but the associated warm water advection occurs primarily beneath the ocean mixed layer.

Similar trends and adjustments occur in the tropical Atlantic basin, with increases in surface wind speeds playing a crucial role, but only in the Southern Hemisphere. In the tropical North Atlantic, the trade winds are not accelerating. Instead, there is a region of weakening flow off the northeast coast of South America, decreases in the latent heat flux out of the oceans, and mixed layer warming. Primary heating in the equatorial and Northern Hemisphere subtropical Atlantic is from positive trends in the net longwave radiative flux into the ocean. Temperature trends in the ocean mixed layer of the Indian Ocean are positive and more uniform than in the tropical Pacific and Atlantic. This Indian Ocean warming is associated with increases in the net heat flux into the atmosphere as thermally- and, especially, dynamically-driven latent heat fluxes increase over the warmer ocean surface.