EASTERLY WAVE VARIABILITY, SAHEL DROUGHT AND ATLANTIC HURRICANES
Principal Investigator:
Christopher W. Landsea
Collaborating scientist(s):
Lloyd
Shapiro, CSU
Stan Goldenberg, NOAA AOML/HRD
William Gray, CSU
Objective:
To quantify the intra- and interannual variability of African easterly waves in relationship to the Sahel drought and Atlantic hurricanes. Mechanisms for the African easterly wave variability will also be indentified.
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 (see Figure 1).
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upper air data that have been archived and analyzed during the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:
This project is just being initiated as of September 1995. No results are available yet.
Key reference:
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.
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