Tropical Cyclone Activity in the Northwest Pacific in Relation to the El Niño/Southern Oscillation Phenomenon

The interannual variations in tropical cyclone activity in the northwest Pacific (NWPAC) and their relationships with the El Niño/Southern Oscillation (ENSO) phenomenon were studied using the method of spectral analyses. Time series of a Southern Oscillation Index (SOI, 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 SOI and tropical cyclone activity show significant coherence in these two spectral peaks. The dominant peak is at the Southern Oscillation frequency with the SOI 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 SOI 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 SOI. The establishment of an anomalous Walker Circulation shifts areas of enhanced or suppressed convection, leading to the observed variations in cyclone activity.