Desertification
The deserts of the world shift and change size over time, in response to both
natural and anthropogenic fluctuations. Possible feedbacks between desert
margin changes and the regional climate have been hypothesized (Charney et al., 1977) and are an active area of research. Our previous work on the problem
of the prolonged Sahel drought has shown that, while there may be a link
between the global SST and decadal precipitation regions over Africa, there
is also strong evidence that the expansion of the Sahara Desert is a
self-perpetuating climate change. We are continuing to refine this study
as well as looking at prolonged drought in other regions and the possible
climatic consequences of worldwide desert expansion.
Sahel drought
We have begun using observational data to calibrate the COLA AGCM with SSiB. The
observational data are from the Sahelian Energy Balance Experiment (SEBEX) and the
Hydrological and Atmospheric Pilot Experiment (HAPEX-SAHEL). We invited Dr. S. Allen
(Institute of Hydrology, U.K.), who was involved in the Sahel field campaign, to visit COLA.
Dr. Y. Xue also visited the Institute of Hydrology, U.K. to discuss the use and application of
these observational data. Meanwhile, we have begun to use the HAPEX-SAHEL data through
Dr. S. Prince at the University of Maryland. SSiB has been validated using these data sets. This
is the first time that a biosphere model like SSiB has been validated using the data from a semi-
arid area. The data quality control issue is of primary concern. In a preliminary sensitivity
study, we found that the model underestimated the evaporation rate, and overestimated the
sensible heat flux. This bias was also confirmed in a COLA AGCM simulation. Using the
observational data, the sensitivity of the SSiB model to the parameter changes has been tested.
These results were presented in the European GEWEX meeting in London and will be presented
at the 1995 AMS Conference on Hydrology.
Long term (16 month) integrations at R40L18 resolution have been completed. The results are
consistent with those from our seasonal integrations and show that the land surface changes
significantly impact the annual hydrologic cycle. We also used the R15L18 model to test the
effects of SST on the Sahel drought. The low resolution model showed very poor performance
in the region, and was unable to simulate the observed seasonal variations in the Sahel.
Inner Mongolian grassland
Motivated by the Inner Mongolia Grassland-Atmosphere Surface Study (IMGRASS), an
observational campaign in China, we conducted a number of numerical experiments to test the
impact of desertification in Inner Mongolia on the East Asian summer monsoon. This study
shows that after the land surface conditions were changed in the Inner Mongolian grassland, the
rainfall in northern China was significantly reduced. To the south of the dry area, there was an
area with a positive rainfall anomaly. A second dry area was located further south. This pattern
is consistent with the rainfall difference between the 1980s and the 1950s in the east Asian
region. This work was presented at the 1993 Fall AGU meeting, the 1994 European GEWEX
conference and will be updated at the 1995 AMS Conference on Global Change.
Doubled desert
The COLA GCM with SSiB is used to investigate the climatic effect of doubling the extent of
the earth's deserts (Dirmeyer and Shukla, 1994b). Control and anomaly integrations are
performed for 10 years. In the anomaly case, deserts are expanded over North Africa, South
Africa, Australia, south-central Asia, southwestern North America, and parts of South America.
In the anomaly case, the troposphere is cooler across most of the tropics and subtropics,
including all areas where desertification occurs. Annual mean precipitation is reduced over
North and South Africa, south-central Asia, and Australia (Figure 1). Precipitation over the
eastern Sahel is reduced by over 60%, and droughts persist in all seasons over northern and
southern Africa. Asia and Australia exhibit weaker monsoons, but there is little effect during
winter. Precipitation increases over central Africa between the two desertified regions, over the
ocean equatorward of Asia and Australia during those regions' monsoons, and over the
desertified areas of western North America. Regions of increased rainfall occur over the ocean to
the west of the desert regions of North and South America, Australia, and South Africa. Remote
effects in the winter circulation include a pronounced trough over northern Europe, and increased
geopotential heights over the southern oceans.
Personnel: Dirmeyer, Shukla, Xue (Allen)
Summary prepared by: Dirmeyer, Xue
Figure Caption
1. Mean precipitation anomalies (double desert minus control) for last eight years of
integrations as a percentage of control rainfall, contour interval is 10%. Zero contours are
omitted, Shading indicates significance at 95%.
last update: 5 May 1995