Big climate cycle means wet decades

The big floods in Queensland and Victoria have ebbed away, for now, but there may be more floods over the next 20 to 30 years or so thanks to a gigantic climate cycle in the Pacfic Ocean.

For a number of researchers point to the Pacific Decadal Oscillation (PDO) or Interdecadal Pacific Oscillation (IPO), as an over arching climate cycle which governs the number and duration of the shorter, better known La Niña and El Niño climate cycles that directly affect rainfall in eastern Australia.

After careful analysis of the historical weather records, as well as the likes of tree rings and ancient coral reefs they say that they have established a link back through thousands of years worth of rainfall changes.

The PDO had “flipped” into its cool mode, and researchers believe that means more and stronger la Ninas in coming decades.

The issue is still a matter of some scientific dispute, with the Australian Bureau of Meterology doubting the link and, above all, saying that there is not yet enough evidence to forecast changes in the sequence of el Ninos and la Ninas.

However, investigations into the link between the PDO and rainfall are, in turn, part of efforts to uncover patterns in the great oceanic cycles which seem to govern climate over decades.

As is well known, the El Niño and La Niña climate patterns are characterised by changes in sea surface temperatures that drive shifts in atmospheric circulation and cloud patterns. When a La Niña effect rules, as it does now, the sea surface in the central and western Pacific is generally cooler, and that shift in sea surface temperatures sweeps rain bearing clouds towards Australia.

In an El Niño the opposite occurs, resulting in drier weather that may turn into a drought.

An El Niño or La Niña cycle can be anywhere between six to 18 months long, and occur once every three to seven years, but are not predictable.

To date the best that scientists can do is give a few months warning of an onset of a new cycle by watching for telltale signs of changes in sea surface temperatures.

Together known as the El Niño-Southern Oscillation (ENSO), the cycles are tracked by the Southern Oscillation Index (SOI), which is the difference between air pressure readings in Tahiti and Darwin. When it is high a la Nina rules, when it is low an el Nino  brings drier weather.

In contrast to the comparatively short- lived ENSO phases, the PDO, first noticed as a pattern by salmon fishery researchers in the 1990s, "flips" between cool and warm modes every 20 to 30 years. In April 2008, after examining patterns of sea surface temperatures identified by satellite, researchers at NASA's Jet Propulsion Laboratories announced the PDO had "flipped" into its cool mode.