SCIENCE OF SEASONAL CLIMATE PREDICTION

Intra-seasonal and decadal variability: Implications for seasonal prediction
by Harry Hendon, BMRC

Dr Harry Hendon received his PhD in Atmospheric Science from the University of Washington, Seattle USA in 1985. He since has held research positions at CSIRO, University of Colorado, and the Climate Diagnostics Center, USA. He has been Principal Research Scientist at BMRC since 2001. His interests include tropical intra-seasonal variability and its prediction, monsoon dynamics, and tropical oceanatmosphere interaction. He is currently co-chair of the WMO WCRP Asian-Australian Monsoon Panel and member of the USCLIVAR MJO Working Group.

Variability with shorter and longer periods than those normally associated with El Niño- Southern Oscillation (ENSO) influence our ability to predict seasonal climate anomalies. Shorter and longer period variability may also be predictable, thereby contributing to improved seasonal prediction. This review will focus on (i) the dominant mode of tropical intra-seasonal variability, the 40-50 day oscillation or Madden-Julian Oscillation (MJO) and (ii) decadal ENSOlike modes.

The MJO accounts for about one third of the intra-seasonal variation of convection and winds across much of the equatorial Indian and Pacific Ocean. It influences onset and breaks of the Australian summer monsoon, tropical cyclone formation, and rainfall in subtropical Australia and elsewhere. Empirical prediction schemes for the MJO demonstrate useful skill out to about 15 days. Assessment of predictability of the MJO using a ‘perfect model’ suggests potential predictability with lead-time ~ 20-30 days, indicating the future possibility for enhanced prediction at ~1 month lead time. Seasonal variability of MJO activity, which is largely unpredictable, is an important source of noise for the coupled evolution of ENSO but also accounts for as much seasonal variance of rainfall and winds as does El Niño. Improvements of the representation of the physical processes that control the MJO (primarily moist convection and the interaction with the upper ocean) offer the hope of improved short-range dynamical seasonal prediction and will also contribute to more reliable El Niño prediction via more realistic ensemble spread.

ENSO’s impact on Australia during the twentieth century exhibits large changes from decade to decade, and these changes are statistically linked to the Inter-decadal Pacific Oscillation (IPO). The IPO or the closely related Pacific Decadal Oscillation can be explained as the ‘reddened’ ocean response to atmospheric ‘weather’ and ENSO. Even though the IPO/PDO might well have limited predictability beyond interannual time-scales, substantial modulation of the Australian ENSO teleconnection can occur in association with the IPO. This occurs partly because the relationship between ENSO and all-Australia rainfall is non-linear. Physical explanations for this non-linearity, how it influences the character of decadal variability, and its implications for improved ENSO prediction will be reviewed.