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January 23, 2009 | While there has been much attention focused on the question of whether climate change influences hurricanes, scientists are also interested in whether the reverse holds true: do hurricanes significantly impact global climate?
NCAR scientists Aixue Hu and Jerry Meehl are analyzing whether changes in hurricane seasons over time can alter the meridional overturning circulation (MOC) in the Atlantic Ocean. This circulation, driven by variations in salt and temperatures in ocean waters, transports heat north from the tropics and moderates the climate of northern Europe. Paleoclimate studies have indicated that past shutdowns of the MOC were associated with abrupt climate change.
Aixue and Jerry used the Community Climate System Model to simulate ocean conditions in three scenarios: no hurricanes, hurricane winds only, and hurricane winds with torrential rains. They found that the winds tend to strengthen the MOC, increasing heat transport to upper latitudes, and warming the subpolar North Atlantic by several tenths of a degree Celsius. The reason has to do with hurricane winds evaporating water from the surface, thereby increasing the salinity of the remaining water while also warming subsurface temperatures—all factors that would increase the northward movement of warmer water.
Hurricane rains, however, have a different impact. The rains reduce salinity, thereby weakening the MOC. This cools sea-surface temperatures in the subpolar North Atlantic by up to 0.5 degrees Celsius.
A key direction for future research is to determine which mechanism is more powerful: the hurricane winds that strengthen the MOC or the associated rains that weaken it. Aixue and Jerry note that the answer may be complex, depending on the dynamics of individual hurricanes. Their analysis indicates that, in cases where water vapor is drawn into a hurricane from a large region relative to the size of the storm, there may be comparatively more weakening of the MOC.
The research will be published in Geophysical Research Letters.