Over the past 60 years the easterly trade winds over the tropical Atlantic Ocean have decreased in strength, say Hiroki Tokinaga and Shang-Ping Xie of the University of Hawai. The pattern suggests a permanent ‘Atlantic El Niño’, changed precipitation over the Sahel region of Africa and the Amazon basin and may even influence US hurricanes and European weather patterns.
The findings were published in this week’s edition of Nature Geoscience.
Strange thing about the news is we previously thought the Atlantic trade winds were actually picking up. This was down to erroneous measurements, the researchers say. Many meteorological measurements are carried out on commercial ships, which have grown considerably in size. Ocean wind speeds would now be measured at higher altitudes, leading to higher recorded wind speeds. Once this error was corrected, the local wind trend proved to be declining.
Global trends, local shifts
The authors suggest though that the effect could be a temporal shift in a generally warming global climate. Aerosol emissions at higher latitude mask some of the warming there – whereas over the tropical oceans the true force of the greenhouse gas radiative forcing is being felt. With possibly decreasing aerosol emissions these regional differences could dissipate.
Current La Niña
The current La Niña episode in the Pacific Ocean also tends to weaken the Atlantic trade winds, as they are being blocked by high pressure systems off the Latin American west coasts. Over a period of months this leads to a smaller build-up of warm waters in the Caribbean that now sees negative sea surface temperature anomalies.
This connection between East Pacific and Caribbean water temperatures is described as the Western Hemisphere Warm Pool (WHWP), although during the La Niña phase ‘Cool Pool’ would be more appropriate.
The warm pool trade winds connection also has ramifications for the Atlantic hurricane season. Warmer waters off the coast of Africa stimulate the formation of tropical storms. And without the wind sheer of overly strong trade winds, these have ample time for vertical development into hurricanes.
It is the reason La Niña years are usually accompanied by relatively strong Atlantic hurricane seasons – and, if the trend of weakening trade winds were to continue, hurricane frequency in the Caribbean and along the US coasts could increase.
Atlantic Gulf Stream
With cooler Caribbean waters the Atlantic Gulf Stream cools down too, propagating fair weather over Western Europe, although positive phases in the North Atlantic Oscillation (NOA) would prevail, as negative temperature anomalies reinforce the Azores high pressure system. As the build-up of Caribbean waters is smaller in volume too, this may even have an effect on Gulf Stream speeds [these are however mostly driven by Arctic deep water formation].
© Rolf Schuttenhelm | www.bitsofscience.org