Interdecadal Variability of El Niño Onset and Impact on Monsoon Systems over Areas Encircling the Pacific Ocean

Based on previous study by Xu and Chan (2001), two types of El Niño distinguished by the onset time, a Spring (SP) type and a Summer (SU) type, have been investigated from 1871 through 2011. As can be classified by the spatial patterns of sea surface temperature anomaly (SSTA) into the Warm Pool (WP) and Cold Tongue (CT) El Niño, the temporal features of the CT are dominated by the SP events whereas the SU events mostly display the spatial pattern of WP or Mixed events. The approximate 140-year data analysis shows that the frequency of SP events tends to increase in the most recent 30 years (1980-2009) while the SU events show very strong activity in the beginning of the 20th century (1900-1929), which are closely associated with the decadal changes in oceanic and atmospheric background conditions. The air-sea processes indicate that the pattern of sea surface temperature (SST) gradient between tropical and extratropical Pacific Ocean on decadal time scales is related to the sea level pressure (SLP) distribution, which tends to produce wind anomalies. The wind anomalies in turn affect the SST anomalies on interannual time scales over the equatorial areas and finally result in the early onset of El Niño in spring time (SP) or late onset of El Nino in summer time (SU). A spring onset El Niño favors a Kelvin wave that propagates across the basin and a summer onset favors a Kelvin wave that does not traverse the basin or the related effects are not strong enough. The early or late onset of El Niño significantly impacts the precipitation distribution correlated with the monsoon systems including the Asian - Australian monsoon (AAM) and North - South American monsoon (NSAM). The El Niño - monsoon relationship is modulated by decadal changes in atmospheric and oceanic background conditions. The precipitation in the monsoonal area circling the Pacific Ocean exhibits characteristic quasi-biennial variations that are closely associated with the onset time of El Niño events, especially with the early onset of El Niño. For the Spring (SP) type, drought is observed over the central China, Australia, southwestern North America and northern South America in boreal summer, but the opposite pattern appears in the subsequent summer of the following year.