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.

The last time it flipped - from cool to warm - was in 1977, or three years after the last serious floods in Brisbane.

From examining historical weather records, scientists believe the PDO previously flipped (from warm to cool) in the mid-1940s, and around 1920 (from cool to warm).

The release from NASA stated the cool phase of the PDO was characterised by cooler water off the US west coast, stretching from Alaska to the equator, forming a horse shoe around a body of warmer water. In the PDO's warm phase, this pattern is reversed.

This has complex effects but the overall result, as noted in the NASA material, is that the cool phase intensifies La Niña or diminishes El Niño effects around the Pacific basin.

To put that in Australian terms, scientists point out when the PDO was in its warm phase between 1977 and 2008, a succession of severe El Niño cycles brought hot, dry weather to eastern Australia. Droughts in the Murray-Darling basin never seemed to end.

When the PDO was in its cool mode between 1945 and 1977, the rain-bearing La Niñas were strong and frequent.

The PDO is already of some notoriety in the sceptics versus warmists climate debate as sceptics occasionally point out that the cycle’s recently ended warm phase about coincides with a notable increase in temperatures, while its previous cool phase (from the 40s to the 70s) also coincided with a known dip in global temperatures.

The warmists have responded in detail to the sceptics, but this article will concentrate on the quite seperate and much less controversial link to on rainfall - which may or may not also be affected by human-induced global warming, if it exists.

Stewart Franks, an hydrologist and associate professor at the University of Newcastle, says now the PDO has flipped, Australia's climate on the eastern seaboard can be generally expected to return to the conditions that existed between the 1940s and mid-1970s.

Apart from the Brisbane floods in the 1970s, the era was characterised by severe floods around Maitland in NSW in the 1950s.

The correlation between the PDO and Australia's sequence of floods and droughts has been traced through centuries and is extremely strong, he says.

Another researcher who agrees that the correlation is strong is Hamish McGowan, at the climate research group at the University of Queensland.

Along with colleagues at the university and researchers at the Snowy Mountains scheme and Laurentian University in Canada, he recently published a paper closely linking the PDO with water flow through the Murray River. (Reconstruction annual inflows to the headwater catchments of the Murray River, Australia, using the Pacific Decadal Oscillation, Geophysical Research Letters, 25 March 2009).

Another paper by almost the same group and published in the same journal in March of last year, established a connection over 6,500 years,

One conclusion McGowan notes in the first paper is that towards the end of a PDO warm cycle, "inflows to river systems such as the Murray River may reach historical lows". By analysing the PDO and correlating it with the known river flows, the researchers suggest inflows into the Murray River before the change in the cycle in 2008 would have been at their lowest for more than five centuries.

However, McGowan also cautions that the interaction between the PDO and the ENSO climate cycles is by no means the full story of Australian climate. There are a number of other cycles, including the Indian Ocean Dipole that affects Western Australia, as well as the Southern Annular Mode (the SAM). These cycles are now also being investigated, he says.

As one example of of how the SAM can affect climate, Milton S. Speer, a visiting research fellow at the University of NSW’s Climate Change Research Centre, says that a complete return to the wet, "cool" period from late 1940s to mid-1970s would require the SAM to also return to its strongly negative phase. (When the SAM is negative, the winds and cold fronts around the antarctic are more likely to move up to Victoria and New Zealand. The SAM has proved difficult to forecast.)

Speer also notes that the PDO was already weak before it turned strongly negative in 2008, but the counting should start from when it became fully negative.

The PDO, SAM and ENSO cycles are, in turn, only a few of a veritable slew of cycles being investigated around the world, including the Atlantic Multidecadal Oscillation and the well-known North Atlantic Oscillation.

 Ian Simmonds, a professor and researcher in climate dynamics at the University of Melbourne's school of earth sciences, says the SOI index is the highest it has been since the 1970s. This, combined with evaporation from record high sea temperatures around Australia, has resulted in the recent extreme events.

The undoubted recent change in the PDO may create changes in the ENSO patterns, but the pattern is also marked by considerable variability, making forecasting difficult, he says.

"Scientists are still coming to grips with the PDO. It involves deep-water circulation and there's a lot they don't know about the workings of the deep ocean," he says.

Despite the interest in the PDO, the Bureau of Meteorology remains unconvinced.

Dr Scott Power, a senior principal research scientist at the bureau's Centre for Australian Weather and Climate Research, says scientists have been looking at the cycle for 12 years now. The bureau has also published material pointing to a link between the PDO and the Southern Oscillation, but further research makes them think the connection is the other way around.

Perhaps the PDO flips from warm to cool because of a sequence of La Niñas or El Niños, and not the PDO affecting the sequence?

Repeated El Niños, for example, would be expected to increase sea-surface temperatures. Whatever the connection, the bureau is not satisfied that the pattern is established or can be used to forecast.

Power says the pattern of El Niños and La Niñas could be distinctly different over different decades or even generations with more of one type of pattern than another.

But then repeated coin tosses would also show similar patterns with more heads than tails for a time before the pattern reverses. Overall, there had been about the same number of El Niños and La Niñas.

Another complication was the present high global temperatures and record high sea temperatures around Australia, Power says.

Whatever may come of all this scientific enquiry, as readers can see, the higher temperatures forecast by the IPCC, if and when they ever occur, may not result in lower rainfall.

In fact, as scientists really don’t know how these cycles work, forecasting just how rainfall may change due to those supposed higher temperatures is almost certainly a waste of time.