4.7 Eastern and Central North Pacific

Of all TC basins in the world, the eastern (east of 14OW) and central (140W - 180 degree longitude) North Pacific was the least understood until the advent of satellite imagery in the 1960's. Since then, it has been discovered that there are nearly twice as many TCs in this basin as previously thought. This makes it the second most active basin in terms of TC development. It also is the most densely concentrated area in terms of TC occurrence. An average of 6 cyclones pass near 15N and 11OW each year.

Nearly all formation in this area takes place in the ITCZ. On mean surface wind charts, there exists a col in the ITCZ. West of the col, the ITCZ is merely a confluent zone, also referred to as a tradewind trough. To the east, the cross equatorial flow turns westerly and directly opposes the flow from the north. Within this region is the main area of formation. The location of this col moves during the season, expanding the formation area to 13OW during the summer and moving to 90W during winter.

As in the western North Pacific, there is some disagreement as to the actual cause of TC formation. Some argue that easterly waves from the Caribbean Sea traverse the isthmus of Central America and generate TCs in the eastern North Pacific. (Avila 1990). Others discount the easterly wave theory and hypothesize that the disturbances generate within the ITCZ itself. Thus, there is a need for detailed observations and case studies to resolve the questions concerning TC formation in this region.

Although the ITCZ in this region remains north of the equator during the entire year ( Fig. 3.11 and 3.12), TC formation is restricted to the months of May - November. Only one cyclone has formed in this region outside this time frame since 1949. During the winter, the ITCZ shifts south, away from the warmest ocean waters, and becomes much weaker. Additionally, vertical wind shear in the area increases during the winter months, making TC formation unlikely.

The trade wind trough of the central North Pacific can initiate TC formation. Development in this area averages 3 cyclones per year. Activity is restricted to the months from July - November.

Section 4.6 Section 4.8

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