Initiation and amplification of the Ningaloo Nino

Marine heat waves along the Western Australian coast are potentially damaging to the marine environment especially coastal fisheries and the Ningaloo Reef. Initiation and amplification mechanisms for marine heat waves (referred to as ‘Ningaloo Niño’ events) are explored using ocean and atmosphere reanalyses for the period 1960–2011. We find that the onset stage from October to November is promoted by wind-evaporation-SST feedback that operates to the northwest of the coast on the north-eastern flank of the Mascarene subtropical high: cyclonic anomalies act to reduce the surface wind speed and warm the ocean surface, thereby driving increased rainfall and stronger cyclonic anomalies. The growth and southward expansion of positive SST anomalies along the Australian west coast is further supplemented by anomalous poleward advection of heat by the Leeuwin Current, which is coupled with the cyclonic anomalies off the coast. The strongest Ningaloo Niño events, such as the record strong 2011 event, occur in conjunction with La Niña conditions in the Pacific, which drives westerly wind anomalies to the northwest of Australia that can promote the WES feedback and accelerate the Leeuwin Current via transmission of thermocline anomalies from the western Pacific onto the west Australian coast. However, many Ningaloo Niño events occur independent of La Niña and some Ningaloo Niño events even occur during certain El Niños. We explain this general independence from ENSO because the triggering of Ningaloo Niño events from the Pacific is most sensitive to antecedent SST anomalies in the far western Pacific, rather than in the central Pacific where ENSO typically has greatest magnitude.