Equatorial Mid-Tropospheric Easterly Jet over the Eastern Pacific

Abstract

The NCEP-NCAR 40-year upper-air dataset has beer used to reassess the circulation over the equatorial Pacific. The discovery from this data source of an Equatorial Mid-Tropospheric Easterly Jet (EMTEJ) is supported by radiosoundings at Galapagos. The EMTEJ is consistent with a mid-tropospheric trough along the Equator resulting from the lower-tropospheric thickness pattern controlled by the near-equatorial tongue of cold surface waters. The EMTEJ is part of a zonal circulation cell along the Equator, consisting of ascending motion and upper-tropospheric eastward outflow over the western to central Pacific, convergent inflow from the West into the upper troposphere and mid-tropospheric subsidence over the eastern Pacific, and the EMTEJ in large part fueling the upward motion to the West. The lower troposphere does not participate in this zonal circulation. During the low/warm as compared to the high/cold phase of the Southern Oscillation (SO), the EMTEJ is weaker, commensurate with reduced upper-tropospheric convergence and mid-tropospheric subsidence. The EMTEJ thus contrasts with the classical model of a Pacific Walker Cell.