The Intertropical Convergence Zone of the eastern Pacific revisited

The Intertropical Convergence Zone (ITCZ) complex over the eastern Pacific is re-examined from the NCEP-NCAR 40 year reanalysis and other data. Consistent with earlier work, the new analysis yields for July-August: a surface position of the near-equatorial wind confluence at 12degreesN; the cross-equatoiial How changing from divergent to convergent at the latitude of recurvature from southeasterly to southwesterly; strongest convergence in the surface layer to the south of the wind confluence, with this feeding vigorous ascending motion and compensating divergence in the upper troposphere; and the interface between cross-equatorial flow and northeast trades sloping southward at 1 : 1500. An easterly speed maximum in the mid troposphere over the equator is consistent with the 4 year rawin record at Galapagos. The new coarse-resolution dataset fails to capture a well-developed Intertropical Divergence Zone. The annual cycle features for the wind confluence and the ITCZ a position closest to the equator in February, a northward migration to June, southward shift in July and August, and a northernmost location in September. This behaviour is paralleled by maxima in the frequency of the Temporal weather systems and rainfall on the Pacific side of Central America in June and September. Regarding the climatic variability, with an anomalously far southerly position of the wind confluence, Central American rainfall tends to be deficient, and such a combination of departures is common in the warm phase of the southern oscillation. The long-term evolution is characterized by a warming trend of equatorial Pacific waters and weakening of cross-equatorial wind, while trends in the latitude position of the wind confluence and in Central American rainfall are not significant. Copyright (C) 2002 Royal Meteorological Society.