Can ENSO combined with low-frequency SST signals enhance or suppress rainfall in Australian sugar-growing areas?

Abstract

The use of three low-frequency (LF) sea surface temperature (SST) signals were combined with an El Niño/Southern Oscillation (ENSO) signal in this diagnostic study to assess if a suppression or enhancement of rainfall was observed in seven sugarcane growing regions along the east coast of Australia during the harvesting period (Jun-Nov). LF signals utilized included the Interdecadal Pacific Oscillation (IPO), Pacific Decadal Oscillation (PDO) and a decadal-frequency signal (DEC). These were stratified into cool and warm phases based on their sign (+ or –) and were combined with cool (La Niña) and warm (El Niño) ENSO phases. Box plots displaying a 95% confidence interval for the median were developed for early (Jun-Aug) and late (Sep-Nov) harvest season periods. These were used to determine if discriminatory ability among rainfall distributions is improved when combining ENSO and LF indices as apposed to using individual ENSO phases or individual LF phases only. Results suggest the ENSO signal is strong across all regions for Sep-Nov, but has only minor to moderate effects during Jun-Aug. All three LF indices display very little discriminatory ability across both harvesting periods. When ENSO and the LF indices are combined, additional suppression or enhancement effects could not be established. Thus, from results obtained, combining any of the three LF indices with ENSO does not appear to improve upon existent knowledge of rainfall distributions in sugar growing regions along the east Australian coast.