Journal of Climate: Vol. 14, No. 5, pp. 743-751.

Evolutionary Structure of the Eastern Pacific Double ITCZ Based on Satellite Moisture Profile Retrievals

Christopher E. Lietzke

Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

Clara Deser

Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado

Thomas H. Vonder Haar

Department of Atmospheric Science, Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado

(Manuscript received 18 June 1999, in final form 23 November 1999)

Abstract

For about a month near the boreal vernal equinox, the eastern Pacific intertropical convergence zone (ITCZ) is observed to form two troughs quasi-symmetrically situated about the equator near 5°–7° latitude during years when an equatorial sea surface cold tongue is present (e.g., La Niña years). The three-dimensional structure and temporal evolution of the eastern Pacific double ITCZ is documented using weekly cloud liquid and ice water fields and relative humidity profiles retrieved from Special Sensor Microwave/Temperature-2 measurements. The depth of convection in the southern branch of the double ITCZ, as determined by the coincident presence of cloud liquid and ice as well as by upward motion inferred from the relative humidity field, is observed to be sensitive to both the underlying SST and subsidence from the northern branch. The equatorial sea surface cold tongue appears to be the determining factor regulating the formation of a double ITCZ in the eastern Pacific. Areas of deep convection within the double ITCZ are accompanied by surface wind convergence maxima. However, the coincident maxima in deep convection and surface convergence are located several degrees of latitude equatorward of the highest sea surface temperatures.
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Hongjun Zhang: zhangho@ucar.edu