Andrea Sealy - Land-atmosphere interactions and Sahel precipitation: the impact of dynamic vegetation on dust and Sahel precipitation.
The Sahel geographically extends from the Atlantic Ocean to the African “Horn” and is the transition zone between the arid Sahara Desert to the north and the wetter tropical regions to the south (Figure 1). The Sahel wet season is primarily during the months of June through September during the onset of the West African Monsoon. The efforts to understand rainfall variability and predictability in this region are not only relevant to atmospheric science policy, because the livelihood for many in this region is pastoral, but are also scientifically challenging.
Sea surface temperature (SST) and its impact on seasonal variability and predictability of precipitation has been the focus of climate model studies in recent years. Various regions respond to anomalous SST patterns such as the El Niño/Southern Oscillation phenomenon (ENSO).
It has been suggested that Sahel precipitation is linked to Gulf of Guinea, Tropical Atlantic and Indian Ocean SSTs, however, other factors such as land state variables (soil moisture, vegetation cover, albedo) and aerosols (such as dust) may also contribute to precipitation variability in this region.
I am working with Natalie Mahowald (on leave from NCAR/CGD and currently at Cornell University) to examine the interaction of vegetation dynamics with desert dust and Sahel precipitation using the Community Atmosphere Model (CAM). The configuration of the model simulations that we are analyzing differ as to whether the runs include dust radiative forcing and feedback (both shortwave and longwave), the SST forcing (whether observed sea surface temperature/AMIP or interactive SST from the Slab Ocean Model/SOM) and if coupled with the Dynamic Global Vegetation Model (DGVM). Comparison of CAM/DGVM to uncoupled CAM simulations suggests that there is a response of precipitation and dust to dynamic vegetation. The interaction with and response of dust and precipitation to dynamic vegetation also appears to vary with observed SST forcing versus interactive SST.
Preliminary results indicate that for runs forced with SOM SST and coupled with DGVM (both with and without dust radiative forcing and feedback), the June-July-August-September (JJAS) precipitation over the Sahel is on average higher than those runs using the default vegetation (Figure 2, left).
However for the cases forced with observed SST the coupling with DGVM produces more complex results. The JJAS precipitation is lower for the DGVM cases over sub-Saharan Africa, but in the coastal regions closer to the Gulf of Guinea, the JJAS precipitation is higher (Figure 2, right).
Cases |
Mean (mm/day) |
SOMnd |
4.465595 |
SOMndDV |
5.564331 |
SOMwfb |
4.31044 |
SOMwfbDV |
5.600183 |
AMIPnd |
4.491285 |
AMIPndDV |
3.846207 |
AMIPwfb |
4.475676 |
AMIPwfbDV |
4.032315 |
OBS |
3.095098 |
| Table 1: Area averages (12°N-18°N, 18°W-20°E) of mean JJAS precipitation for all cases. | |
In examining the area average over 12°N-18°N, 18°W-20°E the SOM cases with DGVM have higher mean JJAS precipitation than those cases without DGVM, for the AMIP cases the result is the opposite (Table 1). For both AMIP and SOM cases, the amount of dust decreases when CAM is coupled with DGVM (not shown).
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ASP Spotlight September 2007
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