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

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 58-78 latitude during years when an equatorial sea surface cold tongue is present (e.g., La Nina 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.