Changes in mid-latitude orographic precipitation due to global warming

Climate models suggest that the globally averaged relative humidity will remain approximately constant as the temperature rises, implying through the Clausius-Clapeyron (CC) equation a specific humidity increase of approximately 7% per kelvin of surface warming. Yet due to global energy balance constraints, climate models also suggest that the globally averaged precipitation will only increase by about 2%/K. Local changes in precipitation can, of course, deviate substantially from the global mean. Here we investigate the dynamics responsible for changes in precipitation over mesoscale regions on the windward slopes of mid-latitude mountain ranges.

We simulated the influence of four 2-km high north-south mountain ridges at mid-latitudes on islands in an aqua-planet. While global mean precipitation in simulations increases by 1.6%/K when the CO2 concentration is doubled, the mean precipitation on the windward sides of the mountains increases by 5%/K if mountains are put between 40-60 N, but it becomes the same as global mean response, 1.6%/K, if mountains sit between 35--55 N. In both the 35-55N and 40-60N cases, local precipitation increases larger than 7%/K are reached along the northern parts of the windward sides of mountains. The maximum local increase can be as large as 12%/K. In addition, the frequency of extreme orographic precipitation events at higher mid-latitudes is doubled as temperature increases.

Although orographic precipitation increases faster than CC scaling at northern mid-latitudes, it decreases faster than zonal mean precipitation at southern mid-latitudes. The north-south asymmetry of these responses is related to the poleward shift of storm tracks. To understand changes in orographic precipitation we used a simple diagnostic model, which showed that the changes were mainly determined by three factors: the moist adiabatic lapse rate of saturation specific humidity, the cross-mountain wind component, and the saturated vertical displacement. The first factor is temperature dependent, and it contributed about 4%/K of total changes in orographic precipitation. The other two factors are both affected by storm track shift and thermodynamic changes, so their effects show strong dependence on latitude.