Energy budget change in the tropics according to the SRES A1B scenario in the IPCC AR4 models

Abstract

The IPCC AR4 model datasets were investigated in order to address how the energy balance will change in the tropics (30°S-30°N) under the SRES A1B scenario. A climate change signal with a well-defined linear trend was extracted using CSEOF analysis; this signal depicts near-linear warming in the tropics. This warming signal, together with the seasonal cycle, explains most of the variance (91.32-94.86%) in the AR4 model datasets. In addition to the warming of the atmospheric column and the surface, the cloud fraction decreased over most of the tropics, except over the equatorial Pacific. Specific humidity generally increased over the entire troposphere. The decreased cloud fraction and the increased specific humidity resulted in a net increase in shortwave radiation (by ~3.42 (±1.89) W m–2) in the atmospheric column. Simultaneously, the increased atmospheric and surface temperatures (resulting from positive water vapor feedback) caused enhanced longwave radiation exchange between the surface and the atmospheric column; net downward longwave radiation increased by ~19.22 (±3.85) W m–2, and net upward longwave radiation increased by ~14.50 (±3.04) W m–2 over 100 years. The second largest change was found in the heat flux leaving the surface, which amounted to ~4.55 (±1.72) W m–2. As a result of the radiation budget change associated with warming and meridional heat transport by the atmosphere and the ocean, net energy gains were found for both the tropical atmosphere (~1.24 (±1.21) W m–2) and the tropical surface (~0.57 (±0.33) W m–2).