Feature article
Figure 1. Phases of the Interdecadal Pacific Oscillation from 1950 to 2000.
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Figure 2. Trends in annual mean temperature (0.01°C) between the most recent positive and negative phase of the IPO.
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Figure 3. Trends in mean annual precipitation (%) between the most recent positive and negative phase of the IPO.
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The Interdecal Pacific Oscillation – Pacific Climate Shifts
By Ashmita Gosai, Dr Jim Salinger and Dr Brett Mullan, NIWA
Climate variability observed in the Pacific is dominated by changes in the tropics on timescales ranging from interannual to interdecadal.
The El Niño-Southern Oscillation (ENSO) dominates interannual climate variability not only over the Pacific but globally. Research shows that the Interdecadal Pacific Oscillation (IPO) modulates South Pacific climate on the decadal time scale.
The IPO is an ‘ENSO-like’ feature of the climate system that operates on time scales of several decades. There is a tight coupling between the ocean and atmosphere. The main centre of action in SST departures is in the north Pacific centred near the dateline at 40 oN, with an opposing weaker centre just south of the equator in the eastern Pacific north of the Easter Island at 10 oS. There is also another weaker centre of action, in the south west Pacific centred near the Cook Islands at 20 oS, which is in the same phase as the north Pacific centre. The matching atmospheric sea level pressure pattern (SLP) is one of an east/west seesaw at all latitudes, but again centred over the north Pacific, with the centre of action over the Aleutian Islands.
In the positive phase of the IPO, over the south west Pacific region southerly winds are more prevalent, with south westerlies more frequent in the New Zealand area. The negative phase of the IPO brings a tendency for a reversal of the SST and SLP anomalies for the positive phase, and more frequent northerly winds for the south west Pacific, and north easterly winds in the New Zealand area.
Three phases of the IPO have been identified during the 20th century, being: positive (1922-1944), negative (1946-1977) and another positive phase (1978-1998). Recent findings show that there has been another shift towards a negative IPO since 1999 (Figure 1). These changes cause climate shifts in the South Pacific.
Changes in mean annual surface air temperatures between the most recent negative (1947-77) and most recent positive (1977-98) IPO period are shown in Figure 2.
There is warming over much of the region, which is to be expected with the global warming trend of 0.2°C/decade that has occurred since 1976 compared with no warming during the period 1946-1975.
The warming in the region south of 20°S is similar to the average global mean surface air temperature trends. However, in the north east of the region increases in mean temperatures exceed this amount, and in the north west there is less, or no warming. This is consistent with higher pressures and more anomalous southerly flow in the northwest, and lower pressures and more anomalous northerly flow in the north east.
The changes in annual rainfall from the negative to positive phase of the IPO show the north east of the region much wetter during the positive phase (Figure 3). Rainfall in the extreme north east increases by more than 50 percent. The line of zero change runs from Funafuti, north of Apia and then south eastwards to Rapa. Annual rainfall shows decreases south west of the zero line. The annual rainfall changes are consistent with increases in atmospheric pressure in the west and south of the region and decreases in the north east of the region with a change to the positive IPO phase.