Extratropical Influences on Tropical Paleoclimates

A J Broccoli (NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ 08542; ph. 609-452-6671; fax 609-987-5063; Internet: ajb@gfdl.gov

Changes in tropical climate, including a shift of the intertropical convergence zone and nonuniform changes in tropical temperature, occur in an atmosphere-mixed layer ocean (A-MLO) model simulation of the climate of the last glacial maximum. Qualitatively similar changes, but of the opposite sense, occur in a simulation forced by a doubling of atmospheric carbon dioxide concentration. The role of extratropical forcing in producing these responses is explored through the use of a hybrid A-MLO model in which sea surface temperatures in middle and high latitudes are prescribed. Changes in tropical climate similar to those found in the doubled carbon dioxide simulation are produced in this hybrid model by imposing a warming of the Northern Hemisphere extratropics and a cooling of the Southern Hemisphere extratropics. The stronger pole-to-equator temperature gradient in the Southern Hemisphere causes a strengthening of the Southern Hadley circulation; conversely, the weakening of the Northern Hemisphere temperature gradient results in a weakening of the Northern Hadley circulation. Other changes in climate are dynamically linked to this change in the Hadley circulation. These results suggest the existence of a mechanism through which a hemispherically asymmetric climate signal originating in the extratropics could be transmitted to the tropics.