Wednesday, 12 January 2005: 8:45 AM
Tropical regional rainfall impacts under anthropogenic climate change
Climate model projections of anthropogenic climate change exhibit strong sensitivity of regional tropical rainfall, including regions of drought. These potential changes have not been highlighted in part because of poor agreement among models on the magnitude and spatial distribution, and in part because the mechanisms were poorly understood. Recent hierarchical climate modeling has identified a handful of hypothesized mechanisms for the regional precipitation impacts. In the "upped-ante mechanism", a warm troposphere increases the value of surface low-level moisture required for convection to occur. In regions of plentiful moisture supply, moisture increases to meet this new "ante" and maintain precipitation, but in doing so creates a larger moisture gradient relative to neighboring subsidence regions. Rainfall reductions result at the margins of convection zones where low-level inflow from subsidence regions inhibits moisture rising to meet the increased "ante" for convection. The "M-prime mechanism" favors increased precipitation in regions that already have strong convergence, via reductions in the gross moist stability M. Surface flux mechanisms can contribute in regions where the ocean heat transport divergence changes, but sea surface temperature increases at the tropical-average scale are a by-product rather than a forcing, from the point of view of understanding precipitation impacts. Aerosol impacts include long-range effects mediated by the above mechanisms as well as local effects. Implications for assessment of current climate model global warming scenario simulations are outlined.