Feature article

Reviving of rainfall stations in Vanuatu

Figure 1: Average rainfall rate for March and December

Figure 2: Annual average rainfall rate from TRMM 0.25 degree analysis for 2006

Dr Brett Mullan, NIWA

Three convergence zones can be identified in the Pacific: the Intertropical Convergence Zone (ITCZ) in the Northern Hemisphere at around 5–10°N; its Southern Hemisphere counterpart, denoted separately here as SITCZ; and the South Pacific Convergence Zone (SPCZ) that extends diagonally from around Solomon Islands (10°S, 160°E) to near 30°S, 140°W. The southern ITCZ (SITCZ) only occasionally extends eastwards of about 160°W, and west of this point often merges with the SPCZ, making it diffi cult to separate the two features in the southwestern Pacific.

The year 2006 saw a weak La Niña event end in the first few months, followed by the development of a moderate El Niño that peaked at the end of the year. La Niña conditions in March–April are known to promote the appearance of a ‘double ITCZ’ (Lietzke et al., 2001;Zhang, 2001), and Figure 1 shows a striking example of this. The SITCZ convection for March 2006 is continuous from 85°W to west of the Date Line, is stronger than a rather broken northern ITCZ, and shows a clear separation from a weak SPCZ further south.

A more typical two convergence zone pattern is seen in the rainfall rate for December 2006 (Figure 1, lower panel). A prominent ITCZ is particularly active east of the Date Line, and the SPCZ is bowed northwards near 150°W, both features characteristic of El Niño conditions. For the year as a whole, the TRMM rainfall data suggests that rainfall was close to the 1999–2005 average over much of the tropical Pacific, with the exception of enhanced convection in the SITCZ region (Figure 2).

Rainfall in the ITCZ is higher in the second six months of the year, and this climatological pattern was accentuated during 2006. Rainfall was lower than normal in February (particularly), and higher than normal in October through December 2006. This transition was also apparent in island rainfall records for the year. South of the Equator, rainfall in specific island groups was affected by the ENSO modulation of the South Pacific Convergence Zone. The SPCZ was further south than usual from January through May, as expected with La Niña, and further north than usual in at least some months at the end of 2006 (e.g., December). As noted above, precipitation in the SITCZ region, which occurs mainly February through April, was enhanced during 2006.

Data sets made use of in preparation of this section were the 0.25 degree resolution rainfall data from TRMM (3B–43 product), and NCEP-NCAR reanalysis data. The section also drew on the University of Hawaii PEAC Pacific ENSO Update (http://www.soest.hawaii.edu/MET/Enso/peu/update.html), NCEP climate summary information (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/bulletin/), and the NIWA Island Climate Update (http://www.niwascience.co.nz/ncc/icu).

References

Lietzke, C., E., Deser, C., and Vonder Harr, T.H., 2001. Evolutionary structure of the eastern Pacific double ITCZ based on satellite moisture profi le retrievals. J. Climate, 14: 743–751. Zhang, C. (2001), Double ITCZs. J. Geophys. Res., 106(D11): 11,785–11,792.