| Summary: | Australia experienced record high rainfall in austral spring 2010, which has previously been attributed to the concurrence of a strong La Nia event and a strong positive excursion of the Southern Annular Mode (SAM). In this study, we examine the role of the sea surface temperature (SST) trend over the recent 50 years, which has large warming over the tropical Indian, western Pacific and North Atlantic Oceans, in driving the extraordinary climate conditions of spring 2010, using the Australian Bureau of Meteorology coupled model seasonal forecast system. Four forecast sensitivity experiments were designed by using randomly chosen atmospheric initial conditions but with: (1) observed ocean initial conditions for 1 September 2010; (2) the same ocean initial conditions except the linear temperature trend over the period 1960-2010 was removed; (3) ocean initial conditions in which the trend was added to the climatological ocean state for 1 September; and (4) climatological ocean conditions only. A synergistic response to the La Nia SST anomalies and the SST trend was detected: the tropical rainfall anomalies were amplified over the western side of the Indo-Pacific warm-pool, which led to a significant increase of tropical upper tropospheric warming and a resultant increase of meridional temperature gradient in the Southern Hemisphere (SH) extratropics. Consequently, the SH eddy-driven jet was shifted poleward (i.e. positive phase of the SAM), which induced rainfall over subtropical Australia. Our findings highlight that the interaction of interannual anomalies and the trend may play an important role in the amplification of extreme events.
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