Ambiguous Variations in Tropical Latent Heat Flux since the Years around 1998

Abstract

Abstract The tropical latent heat flux (LHF) has experienced a significant increase under the background of global warming in the past four decades. However, since the years around 1998, the long-term LHF variations in the tropics have been found to be quite different in various flux products. Three different trends in the LHF, climbing, near zero, and declining, are suggested by five widely used flux products, which hinders our knowledge of the actual LHF variations. Although there are buoy observations in the tropics, these observations are hard to use to evaluate flux products as they have been assimilated and/or used as benchmarks in the flux data production. This study aims to identify credible long-term LHF variations since 1998. A linear model decomposing the LHF variations into contributions from sea surface wind U and air–sea humidity differences is first applied. The linear model results show that the LHF variations have been more positively connected to U variations since 1998. Evidence from in situ and remote sensing observations is subsequently employed to identify how U has varied recently. Both Global Tropical Moored Buoy Array (GTMBA) buoy observations (from 82 buoys) and a multisensor merged satellite product support a slightly downward trend in U in the last two decades. Such a weakening of U is not conducive to oceanic evaporation and leads to a reduced LHF. Consequently, a declining LHF under a weakening U since the emergence of the global warming “hiatus” in approximately 1998 might be more convincing in the sense of data accuracy and physical consistency. Significance Statement The latent heat flux acts as the language of air–sea interactions. This study aims to examine how the tropical latent heat flux has changed since the emergence of the global warming slowdown in approximately 1998. The most striking finding is that the long-term variations in the tropical latent heat flux are fairly inconsistent in several widely used flux products in the last two decades. The sea surface wind variation is found to be the primary contributor to the latent heat flux variation after 1998. Observational evidence from buoy and remote sensing data is hence employed to clarify the actual sea surface wind and the latent heat flux variations.