Oceanic-versus-terrestrial influences on land humidity: simulations and theory

Humidity over land is a key climate variable that is strongly coupled to mean and extreme temperatures, to precipitation and evapotranspiration, and to wildfires. Understanding the processes controlling the climatology of land humidity and its response to a changing climate is a fundamental scientific question with important societal implications. Here we use a global climate model with tagged water tracers to directly diagnose the sources of land specific humidity over a range of climate states. The simulations isolate the contributions to land humidity from water evaporated: (i) from the land surface ("terrestrial source"); and (ii) from the ocean surface ("oceanic source"). The control simulation reveals that land humidity in most regions and for most months of the year is dominated by the oceanic source, i.e. water evaporated from the ocean and advected over land. The terrestrial source is important in some inland regions, for example Eurasia, and during Jun-Jul-Aug, when advection is weaker in the northern hemisphere. Under climate change, the oceanic source dominates changes in land humidity at all latitudes but with a non-negligible contribution from the terrestrial source. The results are interpreted using a conceptual box model which predicts that the terrestrial and oceanic moisture sources scale equally with warming, implying equal fractional changes in land and ocean humidity. Implications of these new results for understanding the large biases in observed versus simulated land humidity trends over the historical period are discussed.