Spatial Variability of Atmospheric Boundary Layer Structure over the Eastern Equatorial Pacific

Abstract

Variations in the atmospheric boundary layer structure over the eastern equatorial Pacific are analyzed using 916 soundings collected during the First Global Atmospheric Research Program Global Experiment. Unstable boundary layer structures are observed much more frequently in soundings north of the ocean front located near 2.5°N in the eastern equatorial Pacific than in soundings south of the front. An objective criterion is applied to identify the presence of the transition layer, a weak stable layer near cloud base, in the soundings. The transition is observed in about 45% of the soundings in both the unstable and the inversion categories. A comparison of soundings over the cold tongue with those over the ITCZ indicates that differences in static stability between these regions are limited to the layer from the surface to about 850 mb, which is the mean height of the inversions capping the cloud layer over the cold tongue. The cold tongue soundings on average are found to be drier from the surface to 300 mb than the ITCZ soundings with the largest average difference (?5 g kg?1) between these two groups of soundings observed just above the inversion layer. Compensating subsidence from the ITCZ may account for some of the drying observed just above the cold tongue inversions, although horizontal advection may also be a factor. North?south cross sections (10°S?15°N) of potential temperature, mixing ratio, equivalent potential temperature, and meridional wind across the cold tongue?ITCZ complex (CTIC) were constructed for two longitudinal bands: 95°?105°W and 105°?115°W. There is little latitudinal variation of the average height of the trade inversion and the height of the transition layer across the CTIC. Although the average lifting condensation level (LCL) at 980 mb is located near the average top of the transition layers observed over the cold tongue, the average 980-mb LCL is close to the average height of the base of the transition layers observed over the ITCZ. These differences, while subtle, may have a substantial impact on the coupling between the subcloud and the cloud layer in these two regions. Strong boundary layer meridional winds are observed near the surface at about 7.5°N over the higher SSTs north of the cold tongue. The average meridional winds over the cold tongue show little vertical shear over the lowest 100 mb of the boundary layer.