Science Briefs

Will Climate Change Cause More Atlantic Hurricanes?

North Atlantic hurricanes are major storms which occasionally bring raging winds and torrential downpours to US and Mexican coastal regions and/or Caribbean Islands, usually between July and October.

The expected buildup of atmospheric "greenhouse" gases, such as carbon dioxide and methane, is likely to increase surface air and ocean temperatures and change wind patterns over many regions. Scientists are uncertain about how these climate changes will affect the frequency of occurrence of North Atlantic hurricanes. Nor can they predict whether the storms will become stronger in future decades.

The NASA/Goddard Institute for Space Studies global climate model is a useful tool that can help evaluate changes in relevant environmental conditions as the warming from greenhouse gas buildup is realized. How can such projections be used to anticipate the future behavior of North Atlantic hurricanes?

Many hurricanes evolve from relatively mild weather disturbances, called African waves, which form over West Africa (including the countries of Senegal, Mauritania, Mali, Niger and Burkina Faso) and move westward out over the North Atlantic Ocean. Landsea and Gray (1992) found that significantly fewer North Atlantic hurricanes developed during decades with frequent droughts in West Africa than during rainier periods. This contrast in hurricane frequency is probably related to the year-to-year differences in the characteristics of the precursor African waves, which produce much of the rainfall over West Africa.

As described in Druyan et al. (1997), we detected an intensification of African waves in climate simulations where present concentrations of atmospheric carbon dioxide were doubled. In fact, the counterclockwise (cyclonic) spin of winds speeded up for most disturbances over the coastal waters near West Africa (where hurricanes form) in the climate simulations with doubled CO2 levels than in those with present CO2 levels. Nevertheless, the waves in the doubled CO2 simulation did not occur more frequently. The stronger spin, however, implies that a greater percentage of African waves could eventually strengthen into hurricanes in the future, but additional research will be needed to determine the role of other climate variables in order to formulate more definitive conclusions.

Fig 1. Caption follows below
Fig. 1: The winds at 2.0 km (6500 ft.) above sea level. Alternating rises and dips above and below the centerline indicate the passage of alternating counterclockwise and clockwise circulations. African waves, in this case at 14°N 30°W, are marked by peaks in counterclockwise spin which, according to the diagram, usually occur at intervals of 4-6 days. An important conclusion evident in these results is that most of the simulated waves in the 2×CO2 climate have stronger spin (measured by vertical distance from the centerline) than in the simulation representing the current climate.
Fig 2. Caption follows below
Fig. 2: Differences in cyclonic spin of African waves between 2×CO2 and current climate simulations. Note the axis of positive values along 15°N 17-40°W near Africa, predicting stronger disturbances for the 2×CO2 climate over the region where so-called Cape Verde hurricanes are spawned.

Reference

Druyan, L., P. Lonergan, and T. Eichler. 1997. Simulations of global warming impacts on tropical storm genesis. Planetary and Global Change, submitted.

Landsea, C., and Gray. 1992. The strong association between western Sahel monsoon rainfall and intense Atlantic hurricanes. J. Climate 5, 435-453.

Contact

Please address all inquiries about this research to Dr. Leonard Druyan.