Feature article

Average rainfall rate (mm day-1) during (top) all of 2007 and January 2007 (bottom).

Pacific Intertropical Convergence Zones during 2007

Dr. A. Brett Mullan, National Institute of Water & Atmospheric Research, New Zealand

There are two prominent convergence zones in the Pacific: the ITCZ in the Northern Hemisphere between 5°N–10°N, and the South Pacific Convergence Zone (SPCZ) that extends southeastward from near the Solomon Islands (10°S, 160°E) to near 30°S, 140°W. The Southern Hemisphere counterpart to the ITCZ also occurs near 5°S, but is seen mainly during February through April and east of the Date Line. This feature is suppressed or absent during El Niño years when warmer waters near the Equator disrupt the seasonal convergence patterns. During 2007, the Pacific convergence zones reflected the transition from weak El Niño conditions early in the year to a moderate La Niña by the end of the year.

ITCZ activity during 2007

The ITCZ appears as a fairly broad feature in the annual mean pattern of tropical rainfall due to its normal meridional movement throughout the year. However, in 2007 the ITCZ was closer to the Equator than normal at the beginning of the year in association with El Niño, and further poleward than normal later in the year due to La Niña. A double ITCZ was present from the second half of February through April and, although it appears weak in the annual mean, was nearly as intense as the northern branch during March 2007. West of the Date Line where the ITCZ and SPCZ tend to merge, the annual rainfall totals were comparable to the 1998-2006 Tropical Rainfall Measuring Mission (TRMM) climatology.

SPCZ activity during 2007

The SPCZ was shifted equatorward and eastward of normal in January consistent with El Niño, and produced enhanced convection from north of the Solomon Islands to central French Polynesia. Penrhyn, in the Northern Cook Islands, recorded its second highest January rainfall on record (262% of normal). At the same time, the Caroline Islands of northern Micronesia experienced dry conditions through March 2007. The SPCZ was particularly active in March between 180–150°W, producing record rainfall in Niue, Tonga, and some of the Northern Cook Islands.

Although the NASA ENSO Precipitation Index was persistently negative and consistent with La Niña fromMarch–December 2007, the convergence zones did not stray from their climatological positions until July. The northward displacement of the ITCZ first occurred in the far eastern equatorial Pacific (120–90°W) in July andthen extended to the central Pacific during September.

Information for this article was sourced from “Chapter 4: The Tropics” in “The State of the Climate - 2007”, which is an annual report produced by NOAA/NCDC. The report was published as a supplement to the Bulletin of the American Meteorological Society in July 2008, and was contributed to by over 150 climate scientists from more than 33 institutions worldwide.