Multi-year marine cold-spells off the west coast of Australia and effects on fisheries

Abstract

Ocean temperature variability off the west coast of Australia is affected by the El Niño Southern Oscillation from the Pacific, the Indian Ocean Dipole, the Southern Annular Mode, and the regional air-sea coupling mode – the Ningaloo Niño. The modes of climate variability affect the strength of the Leeuwin Current, a warm, poleward-flowing eastern boundary current, and air-sea heat exchanges off the coast, impacting on the upper ocean heat balance and sea surface temperature (SST) variability. After the unprecedented 2010–2011 Ningaloo Niño (extreme marine heatwave) event, the region experienced more marine heatwaves in the following two austral summers, before SST off the coast eventually switched to a multi-year cold phase. This marine cold-spell (MCS) commenced with the 2015–16 El Niño and continued to 2019, when the Niño-4 W region SST was warmed up and Leeuwin Current was weakened. The upper ocean heat content in the southeast Indian Ocean was lowered (thermocline being elevated), due to the reduction of the Indonesian Throughflow transport, and alongshore northward winds strengthened during this period, which allowed thermocline waters to be more efficiently entrained into the surface layer to cool the SST. Thus, several factors might have contributed in the multi-year MCSs. The MCSs was associated with increased primary production in the Leeuwin Current and helped the recovery of benthic species of local fisheries such as Roe's abalone, saucer scallops and Shark Bay's blue swimmer crabs that were significantly affected by the damaging heat stress during the 2011–2013 marine heatwave period. • Marine cold-spells (MCSs) occurred during 2015–2019 off the west coast of Australia • MCSs commenced with 2015–16 El Niño and were tightly related to Niño-4 W SST • MCSs were associated with weaker Leeuwin Current, shallow thermocline depth • MCSs were associated with stronger northward winds and increased primary production • MCCs helped recovery of invertebrate fisheries affected by the 2011 marine heatwave