Section 1.31.2.1 February
The winter subtropical ridge in the Northern Hemisphere is located at 25-30 °N (Fig. 2.2). Equatorward of the ridge, the northeasterly trade winds dominate the Northern Hemisphere tropics. The general circulation in the eastern and western halves of the Pacific are markedly different.
In the western half, the northeasterly trades cross the equator, become northwesterlies, and converge into the Intertropical Convergence Zone (ITCZ) and monsoon trough near 12°S. This flow is known as the Australian summer monsoon. East of the dateline, however, the cross equatorial flow is from the south and the ITCZ is located near 5°N. This section of the ITCZ is actually only a trade wind confluent zone and very little convective activity takes place during this time of the year. The other feature of note is the Southern Pacific Convergence Zone (SPCZ) which lies near 20°S from 175°E to 160°W. While the SPCZ is often viewed as an extension of the Australian monsoon trough, they are separate features and are not always connected.
1.2.2 May
In May the major synoptic features shift northward from the February positions (Fig. 2.3). The Northern Hemisphere (NH) subtropical ridge has moved north of 3O°N and the ITCZ in the eastern North Pacific is now located near 10°N. The ITCZ has also changed its character. From Central America to the col at 110°W and 10°N, the ITCZ is more active as the northerly and southerly flows begin to oppose each other. It is in this region where most cyclogenesis takes place. West of the col, the ITCZ becomes a confluence zone with decreased convective activity.
In the western Pacific, the Southern Hemisphere ITCZ and near equatorial trough or buffer zone and SPCZ have moved equatorward. The western North Pacific ITCZ and near equatorial trough or buffer zone begins to appear at 4°N near the Caroline Islands (approximately 150°E). Twin cyclones, one in the Northern and one in the Southern Hemisphere, are common at this time of year.
1.2.3 August
By August, the Northern Hemisphere subtropical ridge has migrated to its northernmost latitude near 40°N ( Fig. 2.4). Cross equatorial flow from the Southern Hemisphere extends across the entire Pacific, converging into a nearly continuous ITCZ and monsoon trough in the Northern Hemisphere.
The ITCZ in the eastern North Pacific has now moved north of 10°N. The active portion of the ITCZ now extends west to 125°W. The confluent portion of the ITCZ extends farther west to the point where it is nearly connected with the monsoon trough in the western North Pacific.
The monsoon trough of the western North Pacific is poorly depicted in a long-term mean chart because it is often migratory. In general, it typically displays a southeast-northwest orientation. However, there are instances where the eastern portion of the trough migrates northward, reversing the orientation. Tropical cyclone formation in the western North Pacific is largely confined to the monsoon trough because enchancement of the low level flow on either side of the monsoon trough increases the relative vorticity, divergence and moisture concentration that make conditions more favorable for TC formation (He and Yang, 1981; Love, 1985; Ding, 1994).
In the Southern Hemisphere, southeasterly trades dominate the entire tropical ocean equatorward of the subtropical ridge centered at 25°S. The SPCZ still exists, although it is weak and rarely associated with tropical cyclogenesis.
1.2.4 November
As expected, the transition season exhibits a general southward movement of the major synoptic features (Fig. 2.5). The eastern North Pacific ITCZ is once again located at or south of 10°N and the active portion is confined east of the col near 95°W and 9°N. As in May, the ITCZ and near equatorial trough exist on both sides of the equator in the western Pacific. The SPCZ again becomes more pronounced and linked to the Australian ITCZ.
Chapter 2
