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Tracking the South Pacific Convergence Zone: the SPCZ Position Index (SPI)

Dr J Salinger, NIWA

Figure 1. SPCZ Position Index (SPI; normalised Suva minus Apia pressure difference), November - April. Positive values indicate displacement toward Apia and negative values toward Suva.

Figure 2. Phases of the Interdecadal Pacific Oscillation 1920-2005 based on the Hadley Centre IPO index.

The South Pacific Convergence Zone (SPCZ), one of the most extensive features of the atmospheric circulation in the South Pacific, is known to vary its location according to the phase of the El Niño/Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation.

The SPCZ is characterised by a semi-permanent band of enhanced cloudiness and precipitation. Its position can be seen in remote sensing imagery via lower than normal outgoing long-wave radiation (OLR) anomalies, or higher rainfall. It originates from the west Pacific warm pool in the Indonesian region and trends southeast towards French Polynesia. West of the Date Line it is linked to the ITCZ over the western Pacific warm pool. To the east, it is maintained by the interaction of easterly trade winds and the disturbances in the mid-latitude westerlies emanating from the Australasian region. It is most active during the November-April season (wet season) in the Southwest Pacific.

Where it is located is of great importance as those areas directly underneath often receive much above average precipitation, whilst locations well away can receive much lower than normal precipitation. The location of the SPCZ between the Date Line and the longitude of French Polynesia is known to vary systematically with ENSO-related expansions and contractions of the west Pacific warm pool, and moves northeast during El Niño events and south-west during La Nina events. It also varies with the phase of the Interdecadal Pacific Oscillation (IPO). Variations in the mean location of the SPCZ are important for South Pacific climate, as precipitation can vary strongly with the movement. Since 1958 there are plenty of observations and data to enable the seasonal positioning of the mean November to April location from NCEP re-analysis data.

Low-level convergence has been used to define the mean SPCZ location at each longitude over the period 1958-1998. To obtain a comparable century-long time series for the SPCZ, high quality station mean sea-level pressure (MSLP) data from South Pacific island sites were used as a proxy index of the SPCZ position. Two stations were used: Suva in Fiji and Apia in Samoa, as these lie on opposite sides of the mean SPCZ location. The SPCZ position index (SPI) was calculated as the normalised November-April difference in MSLP between Suva and Apia (Figure 1) based on the period 1961-90. It defines the latitude of the SPCZ between longitudes 180 and 170°W.

Relations between the SPCZ, ENSO, and the IPO were analysed using the SPI, the Southern Oscillation Index (SOI), and an IPO, index. When the IPO is in its negative phase, and in a La Niña, the SPI is negative the mean position of the SPCZ is displaced to the southwest towards Suva. During the positive IPO, and in an El Niño, the SPI is positive and the SPCZ is displaced northeast.

Comparisons between Figure 1 and Figure 2 show clearly that when the IPO is in its negative phase, the SPI is negative indicating that the mean position of the SPCZ is displaced to the southwest. During the positive IPO, the SPI is positive indicating that the SPCZ is displaced northeast. Similarly in a La Niña, the mean position of the SPCZ is displaced to the southwest and in an El Niño, the SPCZ is displaced northeast. The SPI is a good index of the SPCZ location.

Trends show that the largest displacement in the SPCZ towards Suva occurred in the mid 1910s, with a run of years showing the largest displacements toward Apia in the early 1940s. Generally the November-April SPI shows the last two IPO phases clearly with displacement toward Suva 1946-1976, and movement toward Apia 1977-1998. Movement of this particularly sensitive section of the SPCZ have occurred in phase of those of the IPO over the 20th century.