Easterly Wave Variability, Sahel Drought and Atlantic Hurricanes

Principal Investigator: Dr. Christopher W. Landsea
Collaborating scientist(s):
Stan Goldenberg
Lixion Avila (NOAA/NHC)
Objective: To quantify and uncover the mechanisms for intra and inter-annual variability of African easterly waves.
Rationale: Why would easterly wave variations be important? Strong statistical links have been demonstrated for interannual associations between seasonal Atlantic hurricane activity and seasonal rainfall over North Africa's Sahelian region. One hypothesis that would physically link the two phenomena is that weaker than normal easterly waves would move over the Atlantic during Sahelian drought years, making it less likely that tropical cyclones could be spun up. Conversely, in wetter than normal years in the Sahel, easterly waves may have larger cyclonic vorticity and more persistent, deep convection and thus more of a chance to undergo tropical cyclogenesis. Also there is the likely possibility that easterly wave variability may be linked to Atlantic hurricanes independent of the connection to the Sahelian drought. A direct link of easterly waves to hurricane activity would not be unexpected because of the very large contribution of easterly waves to the formation of tropical cyclones (e.g. over 80% of intense hurricanes in the Atlantic basin originate from African easterly waves).
Method: The research plan is to investigate the year-to-year variability of the African easterly waves utilizing surface and upper air data that have been archived and analyzed during the last several years at the NOAA/AOML Hurricane Research Division. Characteristics to be investigated include easterly wave vorticity and divergence fields, sea level pressure minima, wave frequency, wavelength, and location. Wind data may be supplemented by computing high density satellite-derived cloud drift winds over the eastern Atlantic Ocean and North Africa. METEOSAT IR imagery will also be utilized to quantitatively measure amounts of deep convection associated with individual easterly waves. The years of focus include 1988 to 1995, as these provide quite a range of both hurricane activity and Sahelian rainfall.

As an addition to the observational portion of this research project, numerical modeling sensitivity studies will be conducted to determine various environmental effects on easterly wave structure. The linear primitive equation model of Stevens and Ciesielski will be utilized for such experiments. This model has already proven to be useful in providing realistic easterly wave structure.

Accomplishment:
Key references:
Landsea, C. W., and W. M. Gray, 1992: The strong association between western Sahelian monsoon rainfall and intense Atlantic hurricanes. J. Climate, 121, 1703-1713.

Landsea, C.W., W. M. Gray, P. W. Mielke, Jr., and K. J. Berry, 1992: Long-term variations of Western Sahelian monsoon rainfall and intense U.S. landfalling hurricanes. J. Climate, 5, 1528-1534.

Last modified: 10/9/96