Evaporation Change and Global Warming: The Role of Net Radiation and Relative Humidity

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Abstract
The change in evaporation over the oceans in climate models is analyzed from the perspective of air‐sea turbulent fluxes of water and energy. The results challenge the view that the change in evaporation is predominantly constrained by the change in the net radiation at the surface. For fixed net radiation change, it is found that (1) robust increases in near‐surface relative humidity and (2) robust decreases in turbulent exchange coefficient lead to a substantial reduction in evaporation below the rate of increase implied by the net radiation alone. This reduction of evaporation is associated with corresponding changes in the sensible heat flux. In addition, a net imbalance in the surface energy budget under transient greenhouse gas forcing provides a further reduction in the evaporation change in climate models. Further results also suggest that it might be more physical to view the evaporation change as a function of relative humidity change rather than net radiation. In this view, the relative humidity controls the net surface shortwave radiation through changes in low‐level cloudiness and the temperature controls the net surface radiation through the changes in longwave radiation. In addition, the results demonstrate the dominant role of both the air‐sea temperature difference and relative humidity over, for example, wind speed in reducing the evaporation change in climate models below the Clausius‐Clapeyron rate.

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