36 Thermodynamic and Dynamic Mechanisms in the Hydrological Response to Idealized SST Warming in an Aquaplanet Model

Monday, 26 June 2017
Salon A-E (Marriott Portland Downtown Waterfront)
Gang Chen, UCLA, Los Angeles, CA; and J. Norris, J. D. Neelin, and J. Lu

Extreme weather events are responsible for a disproportionally large part of climate-related damage. While extreme weather events are often identified by the probability distribution function (PDF) of meteorological variables (e.g., temperature or moisture) or the pattern of large-scale circulations (e.g., jet stream meandering or blocking), recent works have noted the non-Gaussian tails are common to the PDFs of dynamical or chemical tracers subject to the advection-diffusion processes. Thus, an investigation of weather extremes in tracer coordinates can provide new insights to the processes responsible for these events. In this work, the moisture budget is examined using column integrated water vapor (CWV) as a coordinate. More specifically, the moisture budget is conditioned onto the values of CWV, which allows to separate the effect of moisture on precipitation from that of the lower level convergence. Analysis is performed for the response in precipitation extremes to idealized SST warming in an aquaplanet model with the horizontal resolution increased from 240km to 30km. The change in precipitation is attributed to a change in the mean increase in CWV, a change in the higher moments of CWV, and a change due to atmospheric circulation. The simulations display robust increases in moisture convergence at high CWV and moisture divergence at low CWV that can be associated with increases in CWV in a warming climate. In contrast, there is a robust drying trend in the subtropics that is associated with weakened lower-level convergence, likely due to increased stratification or the poleward shift of atmospheric circulation. Results are qualitatively similar when the moisture budget is conditioned onto the values of precipitation. These demonstrate that the moisture budget in the CWV coordinate provides a useful framework for the moisture and circulation effects on the hydrological cycle in a warming climate, extending the breakdown of thermodynamic versus dynamic mechanisms commonly used for the climatology to the full probability distribution of CWV.
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