J. Atmos. Sci., 62, 1157-1174.
Goddard Earth Sciences and Technology Center, University of Maryland Baltimore County, and Laboratory for Atmospheres, NASA/Godddard Space Flight Center, Greenbelt, MD
Robert F. Adler
Laboratory for Atmospheres, NASA/Godddard Space Flight Center, Greenbelt, MD
Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY
The 6-year (1998-2003) rainfall products
from the Tropical Rainfall Measuring Mission (TRMM)
are used to quantify the Intertropical Convergence Zone (ITCZ)
in the eastern Pacific (defined by longitudinal averages over
$90^oW$-$130^oW$) during boreal spring (March-April).
The double ITCZ phenomenon, represented by the occurrence of
two maxima with respect to latitude in monthly mean rainfall,
is observed in most but not all of the years studied.
The relative spatial locations of maxima in sea surface
temperature (SST), rainfall, and surface
pressure are examined. Interannual and weekly variability are
characterized in SST, rainfall, surface convergence,
total column water vapor, and cloud water.
There appears to be a competition for rainfall
between the two hemispheres during this season.
When one of the two rainfall maxima
is particularly strong, the other tends to be weak,
with the total rainfall integrated over the two varying less than does
the difference between the rainfall integrated over each separately.
There is some evidence for a similar competition between the SST maxima
in the two hemispheres, but this is more ambiguous, and
there is evidence that some variations in the relative strengths of the
two rainfall maxima may be independent of SST.
Using a 25-year (1979-2003) monthly rainfall dataset from the Global Precipitation Climatology Project (GPCP), four distinct ITCZ types during March-April are defined, based on the relative strengths of rainfall peaks north and south of, and right over the equator. Composite meridional profiles and spatial distributions of rainfall and SST are documented for each type. Consistent with previous studies, an equatorial cold tongue is essential to the existence of the double ITCZs. However, too strong a cold tongue may dampen either the southern or northern rainfall maximum, depending on the magnitude of SST north of the equator.