Twentieth-century warming of the tropical Atlantic Main Development Region: a model-based assessment

The tropical Atlantic Main Development Region (MDR) is an important source region for major Atlantic hurricanes. Over the 20th century, surface temperatures in the MDR have warmed by about 0.6 degrees Celsius. The MDR is characterized by pronounced multi-decadal variability, apparently associated with the Atlantic Multi-decadal Oscillation. However, on the century time scale the MDR warming approximately mirrors that of the global mean temperature, but with additional multi-decadal variability superimposed.

To assess possible causal factors for the long-term warming, temperatures in the MDR are compared with climate model simulations which include various combinations of external forcings. The historical climate simulations are for the period 1861-2000, and use two new GFDL global climate models (CM2.0 and CM2.1). "All-forcing" runs include the effects of changes in well-mixed greenhouse gases, ozone, sulfates, black and organic carbon, volcanic aerosols, solar flux, and land-cover. Indirect effects of tropospheric aerosols on clouds and precipitation processes have not yet been included but could have important impacts on the solution. Ensembles of size 3 (CM2.0) and 5 (CM2.1) with all forcings are analyzed, along with smaller ensembles of natural-only and anthropogenic-only forcing, and multi-century control runs with no external forcing.

The observed surface warming trends in the MDR are simulated fairly realistically in the all-forcing and anthropogenic-only forcing runs. The simulated trends (1901-2000) in those runs are not significantly different from the observed trend over the period. However, the simulated trends in runs without anthropogenic forcing are significantly different from the observations. This provides model-based support for the hypothesis that anthropogenic forcing has been an important causal factor in the 20th century warming of the MDR. This conclusion is dependent on a number of factors, including the model's simulation of internal climate variability in the tropical Atlantic, the specification of climate forcing history for the simulations, the model's response to the forcing, and the reliability of the observed temperatures.

Some comparative analysis of 20th century warming trends in other tropical ocean basins will also be presented, as well as an update on further analyses relevant to the MDR.