By affecting precipitation efficiency, that process can potentially alter the water vapor content of the atmosphere. In addition, dust that absorbs solar radiation can affect precipitation through the circulation that arises in response to such heating.

Mechanisms centered in the tropics, particularly the tropical Pacific, have received increasing attention in recent years. The tropics have a compelling advantage over the North Atlantic as a mediator of global climate change. Because of the relatively weak confining influence of the earth’s rotation at low latitudes, the effect of local changes in sea-surface temperature is communicated almost instantly through the atmosphere to the entire tropical band. This confining influence is due to the earth’s rotation, which causes the well-known Coriolis effect in which all atmospheric and oceanic flows turn relative to the surface beneath them (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere) rather than proceeding directly from regions of high to low pressure. The degree of turning is proportional to the sine of the latitude, so turning disappears at the equator. The strong high-latitude tendency for wind and ocean currents to turn slows the transmission of information through the climate system, whereas the weaker turning tendency near the equator allows rapid communication.

Tropical forcings create circulation patterns that have a major remote impact in the middle-latitude and polar regions, again communicated through the atmosphere. The tropical atmosphere-ocean system offers a rich palette of possible amplifiers and switches that could in principle lead to abrupt climate change. The dominant atmospheric circulation in the tropics is an overturning motion known as the Hadley cell, with air rising in warmer regions near the equator, spreading at high altitude, sinking in the subtropics, and returning along the surface. This circulation has a profound effect on tropical water vapor, cloudiness, and convection, with rain forests under the rising limb and deserts under the descending air. The Coriolis turning of the return flow along the surface yields the surface easterlies that drive the ocean. The upper branch of the circulation affects middle-latitudes through its influence on the upper-level subtropical jet.

Shifts in the position of the rising branch (the intertropical convergence zone, or ITCZ) can lead to major changes in the strength of the circulation (Hou and Lindzen, 1992; Lindzen and Hou, 1988), and it has been suggested that ITCZ shifts could amplify abrupt climate change (Clement et al., 2000). Model results suggest that changes in North Atlantic temperature associated with cutoff of the THC cause substantial changes in the Hadley circulation, propagating the influence of THC shutdown into the



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