Science Briefs

Climate Responses to the Ocean Temperature Maximum in the Tropical Eastern Atlantic Ocean

Two plots of ocean surface temperature and summer accumlated rainfall

June-September mean ocean surface temperatures of the tropical eastern Atlantic Ocean, represented by black contours (°K), superimposed on satellite-observed precipitation accumulations (mm), represented by color shading, for the same period. Top: June-September 2009; Bottom: June-September 2010.

The eastern tropical North Atlantic Ocean (adjacent to West Africa) is a breeding ground for tropical storms that sometimes develop into hurricanes, so scientists are interested in how ocean surface temperatures (OST) interact with climate elements. This region features maximum temperatures usually in the range of 28-29°C (82.4-84.2°F) in the area between 5-8° north of the equator during the summer, although peak values vary from year to year.

The black curves in the figures show contours of ocean surface temperatures for June-September during two example years, 2009 and 2010. The ocean surface was on the average about 1°C warmer in 2010 within the belt of maximum temperature. The color shading in the figures represents total June-September precipitation accumulations as observed by the Tropical Rainfall Measuring Mission (TRMM) satellite. These estimates indicate that precipitation accumulations for June-September 2010 over the area of the OST maximum were about 35% higher than over the cooler ocean during June-September 2009.

To study the sensitivity of the summer climate to ocean surface temperature variability in the eastern tropical North Atlantic Ocean, we conducted two idealized climate model experiments. Simulations were made with a regional, limited-area climate model, first using the observed OST for June-September 2006 and second with the OST maximum replaced by values 3°C cooler.

Simulation results imply that the eastern tropical North Atlantic Ocean OST maximum adds about 74% to precipitation accumulations over the ocean area shown in the figure. Some of the precipitation enhancement results from the added stimulus of upward motion over warmer water. In addition, the simulations showed that increases in precipitation also derive from stronger near-surface winds that transport moisture toward the rain belt at higher rates, compared with the hypothetical scenario without the OST maximum. While storm intensities are somewhat enhanced by the OST maximum, model results imply that the timing and trajectories of storm tracks are not sensitive to the OST pattern.

Reference

Druyan, L., and M. Fulakeza, 2011: The sensitivity of African easterly waves to eastern tropical Atlantic sea-surface temperatures. Meteor. Atmos. Phys., 113, 39-53, doi:10.1007/s00703-011-0145-9.

Contact

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