Nonidentical mechanisms behind the North Pacific summer Blob events in the Satellite Era

The unprecedented North Pacific marine heatwave (known as the Blob) in 2013/14 winter has attracted many research interests, considering its dramatic impacts on the ecosystem and regional climate. Meanwhile, the summertime Blob events have also emerged in recent decades. In this study, we identified four Blob events in the summer of 2014, 2015, 2019, and 2020 during the Satellite Era. Here we show diversity existed in the formation of summer Blob events. The atmospheric forcing responsible for the Blob is the weakened North Pacific subtropical high (NPSH), which contributes significantly to the Blob in 2019 via anomalous shortwave radiation. However, the atmospheric forcing alone cannot fully explain all the Blob events, especially the one in 2020 when the weakened NPSH is absent. Other than the weakened NPSH, the preceding spring sea surface temperature (SST) warm anomalies in the northeast Pacific can persist into the following summer due to anomalous mixed layer heat content and significantly contribute to the Blob. A binary linear regression model considering both the SST persistence and the NPSH is constructed, successfully reproducing the observed Blob variation for the period 1982–2020 (r = 0.76, p < 0.001). The model improves the ability to capture the warm peaks in all four summer Blob events, which either single factor cannot fully depict.