UC Irvine

The intertropical convergence zone (ITCZ) in the east Pacific is located north of the equator during most of the year. In daily data it is most variable in March-April when it may be located north of the equator (nITCZ), on both sides of the equator (dITCZ), or south of the equator (sITCZ), or it may be absent (when convection does not take on a zonally elongated form). Additionally, in strong El Niño years it is located on the equator during the boreal winter half-year. Here the focus is on conditions when the ITCZ has a presence south of the equator (dITCZ, sITCZ) and composites of various fields are compared to "normal conditions" [i.e., when the ITCZ is north of the equator (nITCZ)]. Composites of sea surface temperature (SST), precipitation, outgoing longwave radiation, and the upper-level circulation show very similar patterns for dITCZ and sITCZ days, where the latter cases have almost double the amplitude of the former. The sITCZ state is viewed as an extreme case of the dITCZ state. Both are found to be related to the central Pacific (CP) La Niña with anomalous positive SST and atmospheric heating over the western tropical Pacific and anomalous negative SST and cooling over the central tropical Pacific. Ocean-atmosphere interaction plays an important role in developing the dITCZ and sITCZ anomalies. These daily composite patterns can be reproduced by the regression of monthly fields on the cold CP El Niño-Southern Oscillation mode, suggesting that the interannual rather than day-to-day variability dominates in contributing to the patterns of the composites.