The temperatures of the global oceans and atmosphere have been increasing, and the increase is particularly pronounced in the past two decades from 1980s through 1990s. One intuitive consequence of the global warming is that the moisture holding capacity of the atmosphere should go up following the Clausius-Clapeyron equation. While warmer air holds more moisture, warmer sea surfaces cause more evaporation. Indeed, a 50-year time series derived from a recent project of the Objectively Analyzed air-sea Fluxes (OAFlux) at the Woods Hole Oceanographic Institution (WHOI) (http://oaflux.whoi.edu) has an upward trend in global oceanic evaporation starting from the late 1970s. The trend has a large-scale, spatially coherent structure, being most significant in the tropical Indo-Pacific warm pool and the mid-latitude boundary current regions.

The ocean is the source of 86% of the global evaporation, and plays a key role in the global hydrological and energy cycles. Changes in oceanic evaporation have important implications for weather and climate changes over the global scales. This study investigates the pattern of global variations in oceanic evaporation and the cause of the variations. The study finds that the increase in oceanic evaporation is in concert with the rise of sea surface temperature (SST), suggesting an atmospheric response to oceanic forcing. However, the exact mechanism that leads to the close relationship between SST and oceanic evaporation is not the air-sea moisture gradient but the wind speed. In situ and satellite observations do show that global surface winds have been strengthening in the recent decades. The enhanced wind speed impacts the oceanic evaporation via two ways. The first way is direct: the greater wind speed induces more evaporation by carrying water vapor away from the evaporating surface to allow the air-sea moisture gradient reestablished at a faster pace. The second way is indirect: the enhanced surface wind strengthens the wind-driven gyre circulation. A greater transport by the western boundary currents is resulted, which in turn warms up the boundary regions and leads to more evaporation. The oceanic and atmospheric observations that support the increase in oceanic evaporation will be discussed. The differences between the evaporation estimates from the OAFlux project and those from the NCEP and ERA40 reanalyses products will also be examined.