The South Pacific Convergence Zone

The second main regional climatic influence is the South Pacific Convergence Zone (SPCZ). This is a wide band of low pressure with an approximate northwest to southeast orientation over the southwest Pacific, extending diagonally from near Solomon Islands, across to Samoa, the Cook Islands and beyond (Salinger et al. 1995). The SPCZ marks the boundary between the Southeast Trade Winds and the Divergent Easterly Winds farther to the northeast, produced by a high pressure system that sits over the eastern part of the southwest Pacific on a semi-permanent basis. Since the SPCZ is a low-pressure trough, it is associated with cloud and rain.

An important feature of the South Pacific Convergence Zone is its seasonal migration (Hay et al. 1993, Vincent 1994). It generally lies equator-wards, i.e. to the north of its average position, in mid-winter (July), and moves to occupy a more southerly position by mid-summer (January) (Fig. 1.4). During the summer, the SPCZ tends to be better defined and have

July

1 Kiribati

Tuvalu

Fiji

-Tokelau

Divergent Easterlies in,.

Trade Winds

Islands Ofy

Polynesia

January

Kiribati

. Samoa

Cook Islands

Divergent Easterlies

Southeast Trade Winds

Southeast Trade Winds

2000

Fig. 1.4. Seasonal migration of the South Pacific Convergence Zone (SPCZ) from mid-summer (January) to mid-winter (July). Adapted from Nunn (1994).

South Pacific Convergence Zone

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec

Fig. 1.5. Long-term (1905-1999) monthly rainfall for Alofi, the capital of Niue. Data Source: Niue Meteorological Office.

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec

Fig. 1.5. Long-term (1905-1999) monthly rainfall for Alofi, the capital of Niue. Data Source: Niue Meteorological Office.

more active convergence, often producing thick stratiform and cumulus clouds, and associated showery weather. Sometimes very large cumulonimbus towers of cloud may form, bringing thunderstorms and intense rain.

The seasonal north-to-south shifting and alternate weak and strong activity of the SPCZ are reflected in the distinctly seasonal pattern of the annual rainfall across the islands of the South Pacific. A wet-dry seasonality is experienced by all tropical islands in the southwest Pacific. For example, on Niue island, approximately 67% of the total 1,992 mm of rainfall in a year arrives in the summer wet season when the strong SPCZ lies nearby, and the remaining 33% arrives during the winter dry season when the SPCZ weakens and moves away (Terry 2004a). Monthly rainfall for Alofi, the capital of Niue, is shown in Fig. 1.5.

Continue reading here: The El Nino Southern Oscillation

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