Examining multidecadal trends in the surface heat balance over the tropical and subtropical oceans in atmospheric reanalyses

Abstract

AbstractTrends in the components of the annual‐mean surface heat balance for 1979–2018 over the tropical and subtropical oceans are examined in multiple atmospheric reanalyses to understand how they are changing with current sea surface temperature (SST) trends. Confidence in the reanalysis values is evaluated through statistical significance, agreement among datasets, and physical analysis. While the climatology of the net surface heat flux is similar in the three reanalyses examined (ERAI, JRA‐55, and NCEP2), net heat flux trends agree only in the two second‐generation reanalyses. Trends in the 10‐m winds, which are assimilated in JRA‐55 and ERAI but not in NCEP2, are largely responsible for the disagreement. To first order, trends in the latent heat flux explain trends in the net surface heat flux over the tropical oceans. Trends in sensible heat are smaller, and trends in the net radiative components are relevant only regionally. The latent heat flux is decomposed into thermally, dynamically, and hydrologically driven components. Trends in the thermal component of the latent heat flux simply dampen SST trends through the Clausius–Clapeyron relationship. Dynamically driven trends are associated with an intensification of the tropical easterly trade winds, primarily of the equator and with greater enhancement in the Southern Hemisphere. They are generally supported by hydrologically driven trends, which are similar in magnitude to the wind‐driven trends.