A Regional Climate Modeling Study Over West Africa During the 2001-2006 Atlantic Hurricane Seasons
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2008-04-02
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Abstract
The 2001-2006 base period is examined using the ICTP REGional Climate Model system version 3 (RegCM3)to investigate African easterly waves (AEWs) during the Atlantic tropical cyclone season (June-November)for the years of 2001-2006. Filtered 700mb meridional wind and relative vorticity are used to investigate AEW activity during the research period. A sea surface temperature (SST) sensitivity study was conducted to determine the impacts and effects of SSTs on rainfall, circulation and AEW strength, frequency and development.
The regional climate model used in this study (RegCM3) does a good job depicting the propagation speed, frequency and development of AEWs over Western Africa and the eastern Atlantic. The AEWs in RegCM3 are stronger than those found in the NNRP1 reanalysis data and the GFS reanalysis data. It is concluded this is the result of two primary factors. First, the higher model resolution in addition to spotty and at times unreliable surface observations would result in reduced precision and accuracy of the reanalysis data in this region, resulting in weaker AEW disturbances. Secondly, the RegCM3 overpredicts precipitation over the research domain and it is concluded that the dynamic processes within the model that result from this are acting to produce stronger AEWs. The exact dynamical mechanism for this is unclear but we suspect it is in response to the baroclinic changes that occur within the area close to the Inter-tropical convergence zone (ITCZ) and the African Easterly Jet (AEJ) which results in stronger AEWs.
A sensitivity study was also conducted to determine the impacts of warm sea surface temperatures (SSTs) in the eastern Atlantic portion of the domain during the historically active 2005 Atlantic hurricane season. To study this, a SST climatology model run was completed using averaged 2001-2006 weekly SSTs. All other variables and parameters remained the same, including using the 2005 NNRP1 reanalysis for boundary forcing. The results from this investigation show the dramatic impact of warm SSTs in the tropical eastern Atlantic on both AEW activity over Western Africa and the eastern tropical Atlantic. We also note a change in the location of the ITCZ through the early part of the study (June through August) in response to a strong SST dipole between the warm (2C+) pool off the Senegal and Mauritania coastlines and the cool tongue extending into the Gulf of Guinea. The location of the ITCZ trended back to normal during the second half of the research period (September through November) as the SST dipole over the eastern Atlantic and Gulf of Guinea weakened due to climatologically average SSTs returning to the Gulf of Guinea. Our results also show a change in the location and strength of the AEJ. When the model was run with observed 2005 SSTs, we note a more northerly and weaker AEJ in comparison to the climatology run. It has been found in previous studies that a more northerly and weaker AEJ corresponds with stronger AEWs and more precipitation over the Sahel region (Grist⁄Nicholson⁄Barcilon). Because of these findings we conclude that the AEJ and ITCZ respond to the strength and location of sea surface temperature anomalies and this manifests itself in determining the strength and frequency of AEWs.
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Keywords
regional climate model, African easterly waves
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
