Tropical Cyclone Activity in the Northwest Pacific in Relation to the El Niño/Southern Oscillation Phenomenon
Abstract
The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their
relationships with the EI Nino/Southern Oscillation (ENSO) phenomenon were studied using the method of
spectral analyses. Time series of a Southern Oscillation Index (SOl, defined as the sea-level pressure difference
between Easter Island and Darwin) and tropical cyclone activity in the entire (NWPAC) ocean basin as well
as in different regions of the NWPAC were analyzed. Two spectral peaks are apparent in all these time series.
One corresponds to the generally accepted Southern Oscillation with a period of - 3 to 3.5 years and another
at the quasi-biennial oscillation (QBO) frequency. Cross-spectral analyses between the SOl and tropical
cyclone activity show significant coherence in these two spectral peaks. The dominant peak is at the Southern
Oscillation frequency with the SOl leading typhoon activity by almost a year. At the QBO frequency, the
two series are almost in phase. Cyclone activity in the eastern part of NWPAC, however, is -180' out of
phase with the SOl series at the Southern Oscillation frequency. It appears that fluctuations of cyclone activity at the dominant Southern Oscillation frequency may be
explained in terms of the change in the horizontal and vertical circulations in the atmosphere during periods
of low SOl. The establishment of an anomalous Walker Circulation shifts areas of enhanced or suppressed
convection, leading to the observed variations in cyclone activity.
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