Atmospheric sciences dept talk on Saharan drought

FROM STEPHAN HLOHOWSKYJ

Atmospheric sciences dept talk on Saharan drought            

As humans improve their understanding of climate it is becoming more apparent that to fully characterize the Earth we need not only to improve climate models, but also expand and develop observational data. In North American we take for granted the availability of observational networks and records that roughly span the past 100 years (or more in some cases!). Stable governments and economic situations have allowed many different agencies, scientists, and citizens to contribute to climate data. However, in other regions of the world such data do not exist. The difficulty of acquiring climate data can stem from various obstacles such as, dangerous political situations, high cost, war, and famine. This has been the case for many countries, in the Sahara Desert region of Africa. For example the current and ongoing civil war in Mali has prevented western scientists from visiting the region to work with local climatologists. Despite this, there are still many researchers that are making progress to understand, characterize and document climate in Northern Africa.

            Dr. Amato T Evan (Assistant Professor of Climate, Atmospheric Science & Physical Oceanography at Scripps) is part of a research team that has worked on climate in the Sahara Desert. His research has found that the dynamics controlling Sahara Desert temperatures can cause changes in the position of the Sahel (West African) Monsoon, as well as, play a role in Atlantic Ocean surface temperatures. This enormous region (over 9.4 million square kilometers) is still being characterized due to limited observational data sets. A collaborative effort between the Algerian government, French government and Scripps institute have been working to produce a meteorological data beginning in the 1970's in the Western region of the Sahara. Dr. Evan was recently hosted by the department of Atmospheric Sciences at the University of Arizona, to present his findings from analyzing this collaborative data set. At his seminar, Dr. Evan gave new insights into atmospheric processes controlling not only temperatures over the Sahara Desert but also the position of the Sahel Monsoon. His findings suggest that as temperatures increase over the Sahara, moist air is brought in due to convection and the increased water vapor pressure in the atmosphere feeds back on temperature. Much like the urban heat island effect, the increase of water vapor causes temperatures to stay warmer particularly at night. During the day long wave radiation is adsorbed and remitted to the surface of the desert, while short wave radiation is scattered offsetting the heating of the surface. However at night since there is no more shortwave radiation (after sunset) the long wave radiation absorbed by water vapor during the day is remitted. Since the Sahara is already a very dry region, the results suggest that only small changes in water vapor pressure were needed to make a big difference in surface heating. Dr. Evan termed this process as SWAT (Sahara WAter Moisture and Temperature) feedback, since as nighttime temperatures increase, so does daytime convection and therefore more moist air is drawn in from surrounding regions (e.g. Atlantic and Mediterranean). This can effect the position of the Sahel Monsoon, and also likely influence droughts in the region. Since we already know that ocean temperatures are increasing, Dr. Evan predicts that the SWAT feedback coupled with changes in the Atlantic Ocean may cause the monsoon to not travel as far north rather staying around the Gulf of Guinea. This raises the concern that long term “Megadroughts” could become a more common as the SWAT feedback intensifies in concert with anthropogenic climate change over the next 100 years. These findings reiterate the need for increased observational data not only in Northern Africa but all over the world in order to better understand climate change.