National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2005

Large-scale characteristics of the atmospheric boundary layer in the eastern Pacific cold tongue—ITCZ region

McGauley, M., C. Zhang, and N.A. Bond

J. Climate, 17(20), 3907–3920, doi: 10.1175/1520-0442(2004)017<3907:LCOTAB>2 (2004)

Observations from the Eastern Pacific Investigation of Climate 2001 (EPIC2001) field campaign and a simple mixed-layer model are used to study the large-scale structure and dynamics of the atmospheric boundary layer (ABL) in the eastern Pacific. Vertical and latitudinal distributions of the meridional pressure gradient, winds, and other variables are examined and the momentum balance of the mixed layer is explored for a latitudinal range from 0° to 10°N along 95°W including the equatorial cold tongue, sea surface temperature (SST) front, and convective region at the latitude of the intertropical convergence zone (ITCZ). The surface meridional pressure gradient is partitioned into contributions from the ABL, largely controlled by the gradient in SST, and from the troposphere above the boundary layer, mainly controlled by elevated heating and atmospheric wave activity. The mean meridional gradient in surface pressure is dominated (90%) by the ABL contribution; however, its temporal variability primarily comes from the free troposphere. Outside the convective region, thermal variables (e.g., pressure, temperature, and humidity) are well mixed from the surface to 800–1000 m. Winds are well mixed from the surface to approximately 500 m, except over the cold tongue where strong vertical shear exists near the surface. Considerable wind shear is also found in the upper part of the ABL, owing to the rapid decrease with height in the meridional pressure gradient and its reversal in sign above the ABL. The pressure gradient reversal results in a northerly flow atop the ABL. Relatively large entrainment velocities (~1–2 cm s–1) are estimated, partially because of this strong wind shear. Both observations and simulations from the mixed-layer model demonstrate the indispensable roles of entrainment and vertical mixing in general in the momentum balance of the mixed layer in the region.

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