How Do Ocean Temperatures Affect Developing Tropical Storms?
In recent experiments with a regional climate simulation model, we examined the effect of changes in eastern Atlantic Ocean temperatures on rainfall. In September, sea-surface temperatures (SST) reach maximums of about 28.5°C (83.3°F) along a band centered at about 7°N. A model simulation of daily weather during September 2006 near the coast of West Africa, incorporating realistic SST, generated realistic rainfall patterns.
The simulation was repeated with ocean temperatures replaced by values that were 3°C colder than the actual within the area between the Equator and 15°N. As expected, rainfall rates over the normally warm ocean between 5-8°N were decreased by 5-15 mm per day (see Figure 2). This is because rainfall rates depend somewhat on the strength of upward movement of air, called convection, and cool water is not as favorable for strong convection as warm water.
However, the simulation experiment with the cooler SST also showed a surprising enhancement of rainfall over an adjacent oceanic region, between 9-12°N. Figure 2 shows differences in rainfall rates between the experiment with the colder SST minus the control experiment with realistic SST, averaged between 30°W-17°W. The red area in the figure occurs where lower rainfall was caused by the colder SST and the blue and purple areas indicate the region of rainfall enhancement.
How is the enhancement explained? Storms entering the eastern Atlantic from West Africa move westward within the 9-12°N latitude band. Air spiraling inward toward the storm's low pressure center directly over the ocean surface is forced upward, triggering storm-related precipitation. Since the convection of a storm is forced by convergence of the circulation, the upward momentum gained by heating from the ocean surface is secondary.
Analysis of the climate model simulations reveals that the reduced rainfall over the colder sea surface between 5-8°N allows more moisture to remain in the air and to be transported northward into the storm-related precipitation shield. In effect, the enhanced precipitation over 9-12°N in the experimental simulation is a compensation for the reduced precipitation to the south.
The conclusion is that near-equatorial maximums of SST not only organize convective showers because of the heating from the ocean surface, they also deprive storms of moisture further north. This reduces storm related precipitation rates. Since these tropical storms are energized by the release of latent heat of condensation, lower precipitation rates may mean less energy for storm development. Additional research is needed to explore this theory, but the SST maximums may indirectly inhibit the development of some tropical storms.
Druyan, L.M., M. Fulakeza, P. Lonergan, and E. Noble, 2009: Regional climate model simulation of the AMMA Special Observing Period #3 and the pre-Helene easterly wave. Meteorol. Atmos. Phys., 105, 191-210, doi:10.1007/s00703-009-0044-5.
Please address all inquiries about this research to Dr. Leonard Druyan.