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Ningaloo Nino/Nina is the dominant climate mode in the southeastern Indian Ocean with its center of positive/negative sea surface temperature anomalies attached to Australia. Ningaloo Nino is the major cause of marine heatwaves in the region. Although oceanic variability in this region has long been considered mainly as a response to the El Nino-Southern Oscillation (ENSO), some recent studies have suggested the possible existence of local air-sea feedback processes. Using a state-of-the-art ocean-atmosphere coupled model that realistically simulates Ningaloo Nino/Nina, whether Ningaloo Nino/Nina can occur independently of ENSO is examined. Even in an experiment in which ENSO is suppressed by strongly nudging tropical Pacific sea surface temperatures toward the model climatology, Ningaloo Nino/Nina with a similar magnitude and seasonality still develops, likely through an air-sea interaction off Western Australia amplifying atmospheric stochastic forcing. This study is the first to show that Ningaloo Nino/Nina can develop even without ENSO.

Plain Language Summary Ningaloo Nino/Nina is the major cause of extreme warming/cooling in the southeastern Indian Ocean in austral summer and affects the marine environment and precipitation over the Western Australia. Although it has long been considered that El Nino and La Nina in the Pacific drive ocean extreme events in this region, some recent studies have suggested a possibility that warming/cooling off Western Australia would add to itself by changing the atmospheric condition and creating further heating/cooling (i.e., positive air-sea feedback). Using climate model simulations, we show that, even without El Nino and La Nina, Ningaloo Nino/Nina with a similar magnitude occurs in summer, likely through the positive air-sea feedback off Western Australia amplifying atmospheric random forcing. This study is the first to show that Ningaloo Nino/Nina can develop even without El Nino and La Nina.