Contribution to the circulation climatology of the eastern equatorial Pacific: Lower-atmospheric jets

Abstract

Novel data sets have been evaluated to reappraise the annual cycle of atmospheric circulation over the Pacific. In the eastern Pacific, cross-equatorial surface winds from the southern hemisphere generate an oceanic cold tongue immediately to the south of the equator. The associated lower-tropospheric thickness pattern entails a minimum of absolute topography in the midtroposphere near the equator, and together with the small Coriolis parameter this causes a jet stream of up to 10 ms-1 at 700-600 mbar centered at 0°-2° N. The jet is most intense in late boreal winter because then the near-equatorial surface low-pressure trough is flat. The jet is fed by lower-tropospheric convergence over equatorial South America and by upper tropospheric convergence and subsidence over a narrow equatorial zone of the eastern Pacific. Over the central Pacific a jet of up to 10 ms-1 is found at 10°-12° N and 925-850 mbar, while friction brakes the flow in the layer of steepest meridional pressure gradient near the surface. This lower-tropospheric jet, as the surface pressure gradient, is also strongest in late boreal winter. Overall, the NCEP analyses resolve the near-surface circulation less well than the long-term ship observations. However, the midtropospheric equatorial jet over the eastern Pacific is also borne out by the 1967-1971 radiosoundings over Galapagos, and the lower-tropospheric trade wind jet over the central Pacific was also identified from the intense radiosonde program of the March-April 1967 Line Islands Experiment. Copyright 1998 by the American Geophysical Union.