The Atlantic interhemispheric thermal gradient in the 20th century—an asymmetric warming and its possible forcings

The Tropical Atlantic interhemispheric gradient in sea surface temperature (SST) is thought to be important in determining the climate of the tropical Atlantic sector, including Northeast Brazil and West Africa. Decadally-averaged indices of this gradient (tropical south SST minus tropical north SST) derived from observational SST datasets are found to have an upward trend before the 1980s, with stronger warming in the South relative to the North. A similar long-term trend is detected in the CMIP3 models' SSTs from forced 20th century simulations. Comparison to long-term preindustrial simulations show that the 20th century trends are found to be unlikely due to model internal variations, indicating that at least a portion of the upward trend is forced. Examination of single forcing experiments from two of the CMIP3 models available to us suggests the sulfate aerosols to be the most likely source. This suggestion is consistent with the all-forcing 20th century simulations that tend to show a larger upward trend in models that have the indirect aerosol effect. CMIP3 models simulated sea level pressure (SLP) interhemispheric indices from 20th century climate experiments have a downward trend, in correspondence to the simulated SST indices' upward trend. Simulated oceanic northward heat transport is shown to be increasing through out most part of the 20th century, suggesting that part of this trend may be due to forced changes in the ocean circulation.