Changes in Tropical Extreme Precipitation and its Decomposition into Dynamic and Thermodynamic components

Abstract:

Many studies have shown that tropical precipitation is projected to increase in a warming world. Predicting the response of tropical extreme precipitation to global warming, however, remains a challenge. The response of tropical extreme precipitation involves a complex interplay of changes in tropical circulations (dynamic) and changes in atmospheric moisture content due to changes in sea surface temperature (thermodynamic). While a process-based understanding requires high resolution cloud-resolving simulations, GCMs, reanalyses and observations may be used to estimate the contributions of the dynamic and thermodynamic components to extreme precipitation changes.

In this study we derive metrics for extreme precipitation from daily rainfall data from GCMs, reanalyses and observations using indices defined by ETCCDI. To understand how extreme precipitation in the tropics depends on temperature and dynamical regimes, we construct PDFs of these precipitation indices conditioned on SST and vertical velocity (omega at 500 hPa) for present day and climate change scenarios. We find that, besides an overall increase in extreme precipitation in the tropics, the contrast in extreme precipitation between high and low SST regions is amplified in a warmed world.

Further, we use an approach introduced by Bony et al. (Clim. Dyn., 2004) and modified by Emori and Brown (GRL, 2005) to decompose the changes in these precipitation indices due to SST changes into thermodynamic, dynamic and covariation terms. We find that extreme precipitation changes due to SST changes arise due to a combination of thermodynamic and dynamical factors, but their relative contributions depend strongly on SST. Work is ongoing to understand the significance of the SSTs at which such changes occur.