Feature article
Correlation between the annual Southern Oscillation Index (SOI) and annual mean SLP, surface temperature and percentage of normal precipitation, for the most recent negative and positive phase of the IPO.
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Part 2 – Interdecadal Pacific Oscillation – Modulating ENSO
A Gosai, B Mullan, J Renwick and J Salinger
The Interdecadal Pacific Oscillation (IPO) modulates teleconnections with ENSO in a complex way, strengthening relationships in some areas and weakening them in others.
Although the IPO and ENSO operate on different time scales, they are similar in their expression in tropical Pacific SSTs, suggesting that the IPO exerts a modulating effect on ENSO (Power et al., 1999; Salinger et al., 2001). Based on the correlations between the IPO and SST, on average the IPO is positively associated with enhanced and more frequent El Niño events.
During the positive phase of the IPO, the intensity and frequency of El Niño increases. The frequency of El Niño months increases from 12% to 27% for all months and the SOI intensifies from –1.25 to –1.82. These results are based on analysis of the last two phases of the IPO (1940 – 1998). A recent change from positive to negative IPO may have occurred in 1998.
The June ICU discussed changes in climatic averages between the two most recent IPO phases. However, recent research has also suggested there are systematic changes in year to year variability associated with the El Niño-Southern Oscillation (ENSO). Thus it appears the ENSO teleconnections vary with the phase of the IPO.
Power et al (1999) found that strong correlations between the SOI and Australian rainfall and temperature occurred only during the negative IPO – the sign of the correlations implying drier and warmer during El Niños. Salinger et al (2001) examined SOI-climate correlations for New Zealand and the South Pacific, and Figure 1 shows key results.
The basic ENSO teleconnection pattern is present in both IPO phases: that is, during El Niños the pressure is higher over Australia, more southwesterly winds occur over New Zealand, the South Pacific Convergence Zone shifts northeastward with cooler and drier conditions south of it and warmer and wetter conditions to the north. However, the intensity of this pattern is quite different in places between the IPO phases.
In positive IPO, the correlations strengthen between SOI and mean Sea Level Pressure (SLP) in the Coral Sea and north Tasman Sea and southeast of New Zealand. During El Niño (La Niña) events, SLP is lower (higher) in the southeast, and higher (lower) in the Coral Sea and Tasman Sea regions during the positive IPO phase.
Thus the El Niño cold southwesterlies across and east of New Zealand are much more pronounced during the positive IPO period. Along the equator, sea surface temperatures are below normal west of the dateline during positive IPO, but not significantly abnormal during negative IPO.
The SOI-rainfall correlations are more complex. Near Fiji and Tonga, the positive correlation (e.g. drier during El Niño) weakens during the positive IPO period. Near Tuamoto Island in French Polynesia, the negative correlation (wetter during El Niño) also weakens in positive IPO.
References
Power S, Casey T, Folland C, Colman A, Mehta V. 1999. Interdecadal modulation of the impact of ENSO on Australia. Climate Dynamics 15: 319-324.
Salinger MJ, Renwick JA, Mullan AB. 2001. Interdecadal Pacific Oscillation and South Pacific Climate. International Journal of Climatology 21:1705-1721.