Winds Easterlies

equator. This explains why prevailing winds in the mid-latitude northern hemisphere come from the west.

oceanic circulation

An understanding of wind generation helps to explain the operation of ocean currents. There are two general types of ocean circulation and both result from solar heating: (1) WDC, and (2) thermohaline circulation (THC). Differential solar heating generates wind. Wind blowing across the surface of the ocean causes the water to move. The direction of the ocean current is determined in part by the direction of the prevailing winds at a given latitude. WDC is responsible for currents near the ocean's surface.

THC drives deep-water ocean currents and vertical movement of water. THCs occur as water density increases, either through cooling as water moves towards the poles, or because of increased salinity. In both cases, the THC is responsible for carrying warmer surface water to the ocean depths.

There is an interplay, then, between the WDCs and THCs that is exemplified in the Gulf Stream that moves warm water from the Caribbean in a northeasterly direction, along the eastern seaboard of North America, towards northern Europe. As WDCs carry water away from the tropics, the water cools and becomes more saline. It consequently becomes more dense, which causes it to sink along a THC. This mechanism whereby warm surface water is transported towards the poles, descends as it becomes cooler, to be re-circulated southward as cool deepwater currents is sometimes referred to as the North Atlantic conveyor.

global warming and the interplay of wdcs and thcs

The Gulf Stream starts as a WDC because it gets its initial energy from the trade winds. Global warming could affect the Gulf Stream in two ways. The first, and more widely reported mechanism occurs because global warming could melt arctic ice, decreasing sea salinity. This will in turn hinder the ability of the water to descend as part of the THC. The end result is the cooling of northern latitudes.

Global warming could also affect the initial forcing of the Gulf Stream. Some researchers suggest that global warming will disrupt and weaken the trade winds. Diminished trade winds means less energy driving the Gulf Stream northward, also contributing to a cooler climate at high latitudes. This could cause dramatic cooling in northern Europe which needs the North Atlantic Conveyor to moderate its climate. The paradox is that global warming could ultimately cause regional cooling in some parts of the world by affecting both the THCs and WDCs.

sEE ALso: Coriolis Force; El Niño and La Niña; Jet Streams; Trade Winds; Winds, Easterlies; Winds, Westerlies.

BIBLIogRAPHY. Oleg Saenko, et al., "On the Response of the Oceanic Wind-Driven Circulation to Atmospheric CO2 Increase," Climate Dynamics (v.25, 2005.); J.R. Toggweiler, and Joellen Russell, "Ocean Circulation in a Warming Climate," Nature (v.451, 2008).

Lyn Michaud Independent Scholar

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