Observed strong subsurface marine heatwaves in the tropical western Pacific Ocean

Marine heatwaves (MHWs), which are discrete extreme oceanic warming events, have important impacts on the marine ecosystem, fishery resources, and social economy. Previous studies based on sea surface temperature suggest that MHWs in the tropical western Pacific Ocean are very weak. However, here we show that the MHWs observed by the Tropical Atmosphere Ocean/Triangle Trans-Ocean Buoy Network buoys in the tropical western Pacific Ocean are unexpectedly strong in the subsurface layer (50–300 m depth). The ensemble mean intensity of subsurface MHWs shows a peak of about 5.2 °C at 150 m, and the maximal mean intensity reaches 8.9 °C at 5° N, 137° E. Subsurface MHWs occur almost every year with an ensemble mean duration ranging from 13 to 22 days, and show no statistically significant correlation with the El Niño-Southern Oscillation index although the subsurface MHWs during La Niña events are slightly stronger and more frequent than during El Niño events. It seems that the subsurface MHWs are strong and frequent in April–June but relatively weaker and less frequent in September and October than in other months. Anomalous sea surface convergence and Ekman down-welling play an important role in the development of subsurface MHWs. Strong subsurface MHWs are likely to affect the fishery production of tropical western Pacific.