Equatorial Mid-Tropospheric Easterly Jet over the eastern Pacific: Comparison from the ECMWF and NCEP-NCAR reanalyses

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Abstract

[1] The upper-air data set of the European Centre for Medium-Range Weather Forecasts (ECM) 1979–93 Reanalysis is evaluated in comparison to the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCP) Reanalysis for the same period. Attention is focused on the Equatorial Mid-Tropospheric Easterly Jet (EMTEJ) over the eastern Pacific, first noted from a short series of rawin soundings at Galapagos, and then corroborated in its broad spatial context from the NCP 40-year Reanalysis. The present follow-up study contributes a comparison of evidence from the two sources with different model assimilations. The major characteristics of the EMTEJ are confirmed, although the winds from ECM are overall slower and those from NCP faster than the measurements over Galapagos. In both ECM and NCP the easterly speed maximum along the equator is over the eastern Pacific located around 600–700 mb and further westward at progressively lower elevations. Divergence prevails in the realm of the speed maximum and is primarily fed by subsidence from aloft. The ECM evidence differs from NCP by a more westward location of the easterly speed maximum and overall weaker flow.

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