Estimating the Response of Mid-latitude Orographic Precipitation to Global Warming

Orographic precipitation is a key source of water for many of the world's civilizations, yet it is far from clear how this water resource might change in a warmer climate. Climate models suggest the relative humidity will remain roughly constant in warmer world, implying through the Clausius-Clapeyron (CC) equation that total water vapor in the atmosphere will increase by about 7 %/K of surface warming. Due to energy-budget constraints, the rate of increase in the global-mean precipitation predicted by climate models is only about 2 to 3 %/K. According to simple upslope models, changes in orographic precipitation might be expected to scale with changes in the impinging water vapor flux, which in the absence of changes in the large-scale flow, would follow the 7 %/K CC scaling. But global climate models do predict changes in the large-scale circulation. The mid-latitude storm tracks, for example, are predicted to intensify and shift poleward.

We analyze high-resolution GCM simulations illustrating how the precipitation on idealized topography changes in response to a doubling of the C02 concentration. There are large regions where the orographic precipitation increases faster than the CC scaling, and even larger regions where the rate of increase is slower than the CC scaling. Our analysis will focus both on the direct effect of the changes in the large-scale circulation on orographic precipitation and on the way the distribution of our idealized mountains modulates the response of the global circulation to increases in CO2.