A mid-shelf, mean wave direction climatology for southeastern Australia, and its relationship to the El Niño - Southern Oscillation since 1878 A.D

Coastal systems behave on timescales from days to centuries. Shelf and coastal wave climatological data from the Tasman Sea are only available for the past few decades. Hence, the records are too short to investigate inter- and multidecadal variability and their impact on coastal systems. A method is presented to hindcast monthly mid-shelf mean wave direction (MWD) for southeastern Australia, based on the monthly, trans-Tasman mean sea-level pressure (MSLP) difference between northern NSW (Yamba) and the north island of New Zealand (Auckland). The MSLP index is calibrated to instrumental (Waverider buoy) MWD data for the Sydney shelf and coast. Positive/negative trans-Tasman MSLP difference is significantly correlated to southerly/easterly Sydney MWD, and to long/short mean wave periods. The 124-year Sydney annual (MWD) time series displays multidecadal variability, and identifies a significant period of more southerly annual MWD during 1884 to 1914 than in the period since 1915. The Sydney MWD is significantly correlated to the Southern Oscillation Index (SOI). The correlation with the SOI is enhanced during periods when the Interdecadal Pacific Oscillation (IPO) is in its negative state and warm SST anomalies occur in the southwest Pacific region. The Sydney MWD was found to be associated with Pacific basin-wide climate fluctuations associated with the El Niño-Southern Oscillation (ENSO). Southerly/easterly Sydney MWD is correlated with low/high MSLP anomalies over New Zealand and the central Pacific Ocean. Southerly/easterly Sydney MWD is also correlated with cool/warm SST anomalies in the southwest Pacific, particularly in the eastern Coral Sea and Tasman Sea.