Remote Forcing of Tasman Sea Marine Heatwaves

Abstract:

Marine heatwaves (MHWs), which are prolonged extreme oceanic warm-water events, have recently caused substantial impacts to the marine ecosystem, fisheries and aquaculture off Tasmania's east coast. Despite these impacts, our understanding of the drivers (forcing) and potential predictability of MHW events in this region, and in fact most regions of the world, is still in its infancy. in this study, we investigate the role of remote forcing from the interior South Pacific on observed MHW occurrences off southeast Australia from 1994 to 2016 - including the extreme 2015/2016 MHW event which led to oyster and abalone mortality and poor performance in farmed salmon. First, we used an upper-ocean heat budget analysis to show that about 51% of these historical Tasman Sea MHWs were primarily attributed to increased East Australian Current (EAC) Extension poleward transports through the region. Second, we used lagged correlation analysis to empirically connect the EAC Extension intensification to incoming westward-propagating sea surface height (SSH) anomalies from the interior South Pacific. Third, we dynamically analyzed these SSH anomalies using baroclinic and barotropic Rossby wave models forced by wind stress curl changes across the South Pacific. In this study, we also show that associated monthly SSH changes around New Zealand may be a useful index of western Tasman Sea MHW predictability with a lead time of 2-3 years. In conclusion, we find there is potential predictability of western Tasman Sea MHW event likelihoods up to several years in advance if they are caused by ocean advection, due to the deterministic contribution from baroclinic and barotropic Rossby waves in modulating the EAC Extension transports which transport heat into the region.