Role of PBL-Top Zonal Winds in East Pacific ITCZ Dynamics
David J. Raymond, NM Tech,
Graciela Raga, UNAM,
Christopher S. Bretherton and Simon de Szoeke, U. Washington,
John Molinari, SUNYA
Previous results from EPIC2001 showed that the surface moist entropy
(or total heat) flux is instrumental in forcing convection in the
east Pacific ITCZ. The major controls on this flux are the surface
wind speed and the sea surface temperature (SST). Of these, only the
wind speed varies significantly on time scales of days to a few weeks.
A common way in which surface wind speed can be enhanced in the ITCZ
region is if the north-south pressure gradient driving the climatological
southwesterlies in the region is enhanced. This pressure gradient
has two components, a relatively steady part due to the north-south
SST gradient, which creates a corresponding gradient in the planetary
boundary layer (PBL), and a component associated with the pressure
gradient just above the PBL. The latter component is highly variable,
but is geostrophically related to the zonal wind at this level. The
origin of zonal wind variations at the PBL top is still a matter of
some discussion, but recent published results suggest that these variations
are at least partly related to the passage of the active phase of
the Madden-Julian oscillation.
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On 20 Aug 2003, 17:01.