The effects of a change in the latitudinal sea surface temperature gradient are investigated in several GCM experiments. Sea surface temperatures are increased/decreased in the tropics and polar regions, with little change in the global average surface air temperature. Then the experiments are repeated with colderlwarmer conditions globally. Expectations generated from these runs are compared with the resulting climate changes in a doubled CO2 experiment and show overall agreement. Results show that the latitudinal temperature gradient governs the Hadley cell intensity, eddy energy properties, and eddy transports other than latent heat. The global mean temperature governs moisture and cloud cover, Hadley cell extent, and total rainfall. The degree of tropical and subtropical moisture changes depend on both the latitudinal gradient and the mean temperature. The Aleutian low is particularly sensitive to gradient changes, while the Icelandic low (and therefore the Greenland ice core region) is not, possibly due to orographically induced constraints in the North Atlantic. The results are then compared with paleoclimate evidence to deduce what has happened to latitudinal gradients and climate in the past. It is estimated that low-latitude temperature gradients similar to today's may have occurred in the Mesozoic and in the Little Ice Age; reduced gradients were more likely in the Pliocene, Eocene, Younger Dryas, and Last Glacial Maximum. At higher latitudes, warm climates likely had reduced temperature gradients, and cold climates increased gradients. Observed equator to pole gradients were increased in the 1980s relative to the 1950s, and the simulated climate changes consistent with observations include warming of Alaska and Asia, drying in the subtropics, and moisture variability in the United States. Assignment of causes to past latitudinal gradient changes is problematic due to uncertainties concerning CO2 and ocean heat transports, but tentative conclusions based on this analysis support the likelihood of a future, higher-C02 climate exhibiting a large low-latitude gradient and ample precipitation at middle latitudes.
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