Earth Warming Unevenly: Tropical Atlantic Sees Weaker Trade Winds and More Rainfall
Earth's global temperature has been rising gradually over the last decades, but the warming has not been the same everywhere. Scientists are therefore trying to pin down how the warming has affected regional climates because that is what really matters to people, and to adaptation and mitigation strategies. Their efforts, however, had hit a roadblock because the necessary observations of the winds over the oceans were biased.
Developing a new method to remove the bias, Hiroki Tokinaga and Shang-Ping Xie at the International Pacific Research Center, University of Hawaii at Manoa, found that their corrected observations show the trade winds in the tropical Atlantic have weakened and the pattern of ocean surface temperature has changed. As a result, the equatorial Amazon and the Guinea Coast are seeing more rainfall and the Sahel less. The findings are published online in the February 6, 2011, issue of Nature Geoscience.
The raw observations of winds over the ocean suggest that the winds have grown stronger during the last 60 years. The trend is, however, largely due to a change in the placement of the anemometers, the instruments measuring wind speed. Ships are the main source of wind data over the ocean, and ships have increased in height and so has the anemometer placement. Tokinaga and Xie corrected this wind bias using wind-wave heights. The tropical Atlantic has three major ship routes along which ships provide meteorological data.
Applying their new correction technique to observations along these routes from 1950 to 2009 together with other observations, they found the trade winds in the tropical Atlantic had weakened significantly during this period.
Although ocean surface temperature in the tropical Atlantic has risen, the pattern has changed and with it the climate. The cold tongue of water that stretches out from the eastern tropical Atlantic coast has warmed more than the western part of the basin. At the same time, the weakened trade winds have resulted in less upwelling of cold water and nutrients in the eastern tropical Atlantic. These latter changes could impact marine life.
The graphic shows changes over the last 60 years in the August marine-cloudiness and land-rainfall over the tropical Atlantic. Yellow to dark red indicates more cloudiness/rainfall; light blue to gray, less cloudiness/rainfall. Over land, dark red = 40 mm/month more rain, and grey = 40 mm/month less rain. Note the significant increase in rainfall on the Guinea Coast. (Credit: Hiroki Tokinaga, IPRC/SOEST/UHM)
Article continues: http://www.sciencedaily.com/releases/2011/02/110206132902.htm