Extended Abstract (240K)3B.2
Tropical-extratropical interactions causing precipitation in Northwest and West Africa: Part II: Dynamics
Peter Knippertz, University of Wisconsin, Madison, WI; and A. H. Fink and J. E. Martin
The synoptic evolution of a particular type of tropical-extratropical interaction and its relevance for precipitation in subtropical northwest Africa and tropical West Africa have been presented in a companion paper. Here, the large-scale dynamics leading to the formation of deep upper-level troughs to the west of Africa, instrumental in the export of moisture from the Tropics and the generation of precipitation in the two extreme cases described in Part I (January 2002 in Senegal/Mauritania; March/April 2002 in Morocco/Algeria), are investigated using ECMWF analyses and output from numerical simulations made using the UW-NMS model at 75 km horizontal grid spacing.
Both cases are characterized by an upper-level equatorial trough over the East Pacific, a ridge over Central America and another trough over northern South America several days before the actual event. To the east of the ridge, a positive vorticity tendency generated by the irrotational part of the wind field forces strong equatorward flow and a zonal contraction of the downstream trough. The resulting increase in the flow’s Rossby number leads to localized rotational wind imbalance associated with strong deceleration, sharp cyclonic turning, and subsequent strong southwesterly acceleration along the gradient of geopotential, in one case accompanied by the formation of a tropical cloud plume. The entrance region of the developing subtropical jet (STJ) is associated with a pronounced convergence-divergence couplet. The convergence is further enhanced through superposition with the right exit region of the midlatitude jet to the north of the Central American ridge. Positive vorticity tendencies generated by this convergence lead to a Rossby-wave breaking over the Atlantic Ocean associated with the penetration of an extratropical trough into the Tropics. This trough incorporates the strong Atlantic STJ flow and is directly responsible for the generation of the extreme precipitation. In both cases the trough associated with the record precipitation is preceded by a prior upper-level disturbance over West Africa, which is generated by a similar, but less intense succession of events. Although this first trough does not produce rainfall, it initiates a mid-level poleward transport of moisture from the Tropics, thereby modifying the thermodynamic environment upon which the vertical motion field of the next wave can act.
Session 3B, Monsoon I
Monday, 3 May 2004, 1:30 PM-3:30 PM, Napoleon I Room
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