Abstract
The climate of the eastern Pacific exhibits a pronounced equatorial asymmetry. Boundary layer air originating in the Southern Hemisphere trades crosses the equator and flows into the intertropical convergence zone (ITCZ), whose southern limit is nearly always located at least 4° to the north of the equator. The sea-surface temperature (SST) distribution is characterized by a prominent “cold tongue” centered ∼ 1°S, a strong frontal zone centered ∼ 2°N, and a warm eastward current centered near 5°N. The surface wind field exhibits a pronounced horizontal divergence as the air flows northward across the oceanic frontal zone.
These features vary in strength in response to the annual cycle and the El Niño/Southern Oscillation phenomenon. The northward cross-equatorial surface winds, the cold tongue and the frontal zone all tend to be strongest during the cold season (July through November). During the cold season of the coldest years, when the cold tongue is most prominent, the cross-equatorial flow tends to be weaker than normal and the northward flow across 5°N stronger than normal.
It is shown that within a few degrees of the equator the meridional equation of motion for the surface winds reduces to a balance between the pressure gradient force and the frictional term that involves the vertical derivative of the vertical flux of momentum by subgrid scale processes. Some of the seasonal and interannual variability of the surface winds appears to be a response to the hydrostatic sea-level pressure changes induced by variations in the strength of the cold tongue. However, that the maximum divergence of the surface winds is observed directly above the oceanic frontal zone rather than over the cold tongue appears to be due to the reduction in vertical wind shear within the lowest 100 m that occurs as air parcels pass northward from the cold tongue to the much warmer waters or the North Equatorial Countercurrent. As evidence of the existence of strong vertical wind shear in the stable boundary layer regime over the cold tongue, we note that northward velocities just 100 m above sea level at the Galapagos Islands have been reported to be on the order of 15 m s−1; more than twice as strong as the surface winds.
