What Controls Near-Surface Relative Humidity Over the Ocean?

Abstract

Observations and models show that near-surface relative humidity is nearly constant at ∼80% over the ocean in the current climate, and almost invariant in the global mean in projected future climates. Here, this behavior is investigated through the development of a simple theoretical model for near-surface relative humidity by considering the moisture balance above a uniform ocean surface. The relative humidity is predicted to depend on only the near-surface wind speed, air-surface temperature difference, surface wetness and large-scale moisture convergence. Although developed in the context of moist over-ocean convection, the theory is able to determine the relative humidity in a suite of idealized simulations over both wet and dry surfaces with a root-mean-square error of less than 3%. The theory also predicts the climatology of relative humidity over the ocean with a root-mean-square error of less than 3%. The theory thus provides a theoretical basis for investigating changes in relative humidity over the ocean, water vapor feedbacks and the water cycle in current and future climates.