The objective of this study is to identify and understand the long-term change of the tropical hydrological cycle in twentieth-century climate simulations provided by 16 Coupled General Circulation Models (CGCMs) participating in the Intergovernmental Panel on Climate Change (IPCC), Fourth Assessment Report (AR4). This work is focused on trends of precipitation and clouds over the tropical ocean and land.
We find that the CGCMs are reasonably consistent in depicting aspects of long-term changes in twentieth century climate and the tropical hydrological cycle, including (1) a relatively good simulation of trends in surface warming over both the tropical ocean and land, in agreement with observations; (2) increasing precipitation over the tropical ocean and decreasing precipitation over tropical land; (3) an increasing trend in heavy and light rain, but a decreasing trend in moderate rain, over the tropical ocean; and (4) a reduction in total cloud cover in the tropics. These successful simulations are encouraging and enhance our confidence in the future climate projections provided by these models. On the other hand, the current generation of CGCMs still has a number of deficiencies, particularly in modeling clouds and their interactions with radiation. Although most CGCMs are able to simulate the correct sign of trends in precipitation and clouds, they significantly underestimate the magnitudes of the observations. These underestimations likely cause gross errors in model simulations of the global water and energy balance. A better understanding of physical processes and an improvement in the CGCM representations of physical processes associated with clouds and their interaction with radiation are needed. Copyright © 2005 Royal Meteorological Society.