Daily States of the March–April East Pacific ITCZ in Three Decades of High-Resolution Satellite Data

Abstract

Abstract Zonally elongated areas of cloudiness that make up the east Pacific intertropical convergence zone (ITCZ) can take on several configurations in instantaneous observations. A novel statistical model is used to automatically assess the daily state of the east Pacific ITCZ using infrared satellite images from 1980 to 2012. Four ITCZ states are defined based on ITCZ location relative to the equator: north (nITCZ) and south (sITCZ) of the equator, simultaneously north and south of the equator (dITCZ, for double ITCZ), and over the equator (eITCZ). A fifth ITCZ state is used to classify days when no zonally elongated area of cloudiness is present (aITCZ, for absent ITCZ). The ITCZ states can occur throughout the year (except for the eITCZ, which is not present during June–October), with the nITCZ state dominating in terms of frequency of occurrence. Interannual variability of the state distribution is large. The most striking variability in ITCZ states is observed in spring. During March–April, the dITCZ state occurs on average 34% of the time, second only to the nITCZ state (39%). Composites of observed infrared temperature and precipitation by ITCZ state reveal distinct spatial configurations of cloudiness and rainfall. Strong sea surface temperature anomalies are associated only with eITCZ and sITCZ and they correspond to El Niño and La Niña, respectively. However, all five ITCZ states are associated with distinct atmospheric circulation patterns. A connection is found between the ITCZ and the South Pacific convergence zone (SPCZ), such that activity in the SPCZ is enhanced when the ITCZ is absent in the east Pacific.