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The Pacific Convergence Zones: 2005 and early 2006

A B Mullan, NIWA

Figure 1: Average rainfall rate (mm/day) from TRMM 0.25 degree analysis for months of February (upper) and September (lower) 2005. Contours at uneven of 0.5, 1, 2, 4, 6, 8, 10, 15, and 20 mm/day.

Figure 2: Average rainfall rate (mm/day) for April 2006. The convection pattern clearly shows a double ITCZ straddling the equator, along with the South Pacific Convergence Zone.

Prominent maxima and minima in rainfall, cloudiness, and outgoing long-wave radiation are associated with the Pacific Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). The Pacific ITCZ has its main branch in the northern hemisphere at around 5-10°N, where it is strongest in June-August. There is a branch of the ITCZ south of the equator, near 5°S, which is prominent in December-February near Australia, sometimes referred to as the 'monsoonal trough'. Convection frequently extends northwards across the equator in Indonesian region. The southern ITCZ rarely extends east of about 160°W, except in March and April when it can reach 90°W.

Near the Date Line, the southern ITCZ merges with the SPCZ which trends southeastward across the South Pacific to subtropical latitudes. The SPCZ is most active in the austral summer (December-February) and is located in a region of strong sea surface temperature (SST) gradient south of the maximum SSTs. On the other hand, the ITCZ is located near the axis of maximum SSTs.

The tropical convergence zones are now monitored routinely by satellite. A particularly useful data set, that is providing climatologists with new insights into tropical convection, is the Tropical Rainfall Measuring Mission (TRMM) rainfall data, available from January 1998 onwards on a 0.25° latitude-longitude grid (disc.sci.gsfc.nasa.gov/data/datapool/TRMM/01_Data_Products/02_Gridded/index.html)

In 2005, the year began with the tropical Pacific in a weak El Niño state and ended in a weak La Niña state. February 2005 was particularly unusual in terms of the convergence zones. The northern ITCZ was very weak west of the Date Line, with the islands of Micronesia experiencing very low rainfall. At the same time, the SPCZ was extremely active from about 160°E to 160°W, in association with more extensive equatorial westerlies and several tropical cyclones (Figure 1). There was concern at the time that this rainfall anomaly pattern might persist with the development of an El Niño event, but this did not eventuate. TRMM rainfall data suggest the northern ITCZ was continuous (in the monthly mean) right across the Pacific from 140°E to 90°W in almost all months of the year apart from February. It lay slightly equatorward of its normal location from May to August. In June and August-September, the northern ITCZ was particularly active in the far eastern tropical Pacific between 120°W and 90°W (Figure 1).

For the year as a whole, the TRMM rainfall data suggest that rainfall was slightly above the 1998-2004 average in the northern ITCZ, and also in the southern ITCZ and SPCZ to about 160°W.

In 2006, March and April are particularly interesting because a marked double ITCZ was present (Figure 2). The southern branch of the ITCZ, east of about 160°W, is a feature of the austral autumn, but is really only prominent in La Niña years, such as 1999-2001. The conditions necessary for the formation of a double ITCZ appear to be (a) a general region of low-level wind convergence with a warm sea surface in the deep tropics which occurs every year during March and April, and (b) a narrow sea surface equatorial cold tongue, which is strongest during La Niña years. Colder conditions along the equator at about 140-100°W enhance the SST meridional gradient associated with the southern off-equatorial SST maximum. See www.gsfc.nasa.gov/topstory/20020711itczworks.html, for further reading on the double ITCZ.