Section 5.2It has been observed that large scale global variations in atmospheric phenomena are related to changes in the frequency of TC formation. The following section discusses these relationships.
The El Nino and Southern Oscillation (ENSO) phenomenon is covered more thoroughly in Section 2. This section deals with the effects of ENSO on TC formation in various ocean basins. The El Nino event is characterized by warmer SST's in the eastern South Pacific and anomalous winds over much of the equatorial Pacific. It influences TC formation in the western North Pacific (Chan, 1985), South Pacific (Revell and Goulter, 1986), and even the North Atlantic (Gray, 1984a,b).
During the peak phase of the El Nino (usually occurs during the months of July - October), anomalous westerly winds near the equator extend to the dateline in the western North Pacific. This acts to enhance the ITCZ in this area, making it more favorable for formation of TCs. During El Nino years, the equatorial region of 130° W - 180° longitude (i.e. the international date line) should be monitored closely for increased TC activity.
Another effect of the El Nino circulation is an equatorward displacement of the SPCZ (South Pacific Convergence Zone). In addition, the waters of the eastern South Pacific are warmer. Reveil and Goulter (1986) noted that during such years, TCs form closer to the equator and farther east. Regions such as French Polynesia, which are typically unfavorable for TCs due to a strong upper-level trough, experienced six TCs between February-June 1983.
The eastern North Pacific is also affected by the El Nino phenomenon. The ITCZ in this region is displaced south of its normal latitude to approximately 5° N. Additionally, the warm ocean anomaly of El Nino extends to near 20° N, which enhances the possibility of TC formation. The result is an average increase of two tropical cyclones during El Nino years. Cyclones also develop closer to the equator and farther west than during a normal year (Chan, 1985).
Chapter 3