Big climate cycle means wet decades

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.