Big climate cycle means wet decades

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.