Jonathon Wright
Applied Mathematics, APAM Department
"Diagnosis of Relative Humidity Changes in a Warmer Climate Using Tracers of Last Saturation"
Climate models indicate that the global mean radiative water vapor feedback is close to that expected from a constant relative humidity. This has sometimes led to the mistaken conclusion that the distribution of relative humidity remains the same as climate warms. In fact, the simulated relative humidity response to warming is not uniform, with important implications for cloud feedbacks and regional climate changes. Here, the relative roles of shifts in atmospheric circulation as compared to inhomogeneous changes in temperature in driving this response are investigated using a nested pair of global models. Two slab ocean climate model simulations are performed, one representative of the late twentieth century and one with doubled carbon dioxide. The output of these simulations is used to drive a three-dimensional tracer transport model that contains both an independent hydrologic cycle and a zonally symmetric last saturation tracer scheme. The tracers are capable of reproducing both the modeled relative humidity field and the relative humidity response to warming. The response of relative humidity near the extratropical tropopause is shown to be controlled primarily by temperature changes; in contrast, the signal in the tropics and subtropics is driven primarily by circulation shifts. The mechanisms by which these circulation shifts influence the relative humidity distribution is illustrated using the tracers of last saturation.
Host: Lorenzo Polvani
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