Source: Cai, W. and van Rensch (2012) GRL, Vol 39, L08702

By Kelly

Further to Claire‘s announcement (on behalf of the Australian Government) that we are ‘officially’ out of drought, CSIRO scientists are further unravelling the complexities of the climate system. In a recent paper, the authors discuss the southeast Queensland (SEQ) extreme rainfall and flooding that accompanied the extraordinarily strong La Nina event in January 2011. They provide three lines of evidence that this La Nina event  indicates a transition of the Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscillation (IPO) into its negative phase. They argue that it is the coincidence of both ENSO and PDO/IPO phases that brought about the extreme flooding events in 1974, and again last year in 2011. So what is this PDO/IPO?

The ENSO cycle is a well-known climate phenomena that crossed from scientific jargon into the vernacular as Australians took great interest in….and then denied climate change (she shakes her fist at the sky/general public). The Pacific Decadal Oscillation describes anomalies in sea-surface temperature (SST) over the Pacific Ocean (north of 20N) that vary over 20-30 year time scales. The negative phase that the authors discuss is detected by warmer SST in the western Pacific and cooler temperatures in the east. The Interdecadal Pacific Oscillation is detected in both the north and south Pacific gyres, with the largest anomalies occurring away from the equator. Both the ENSO and PDO manifest in variable sea level pressure, surface winds and SST. However it is important to note that they operate on very different timescales, with ENSO events persisting for as little as one year, where the PDO lasts up to 30 years. The effect that climate change is having on this large-scale climate feature is an area of active, although inconclusive, research.

The authors state that while the SEQ weather events are affected by the ENSO cycle, they are further modulated by the state of the PDO/IPO. Forthcoming data will confirm this transition, however the large rainfall and value of the Southern Oscillation Index (ie strength of the La Nina), the re-establishment of a correlation between SEQ rainfall and ENSO, aswell as the decadal circulation state (in particular tropical convection), are all reminiscent of previous negative PDO/IPO states. If this is the case then La Nina events in the coming decade may be accompanied by similar high summer rainfall events for the Sunshine State.

To see the abstract/download the article, click on the link here.