Roles of External Forcing and Internal Variability in Global Marine Heatwaves Change During 1982–2021

As discrete prolonged extreme warm water events, marine heatwaves (MHWs) have become more frequent, stronger and longer‐lasting during the past several decades. The relative contributions of external forcing and internal variability to these changes and their underlying drivers remain unclear. Here, analyses of 90 simulations in CESM2 reveal that external forcing dominates the increasing frequency by causing the mean warming of sea surface temperature (SST), accounting for 82% of the observed trends. Both the mean warming and increased variance of SST contribute to the longer‐lasting MHWs during 1982–2021, with external forcing contributing 38% of the increase in the SST variance for global average. Internal variability, especially the Inter‐decadal Pacific Oscillation (IPO), is closely associated with regional MHW changes. The observed negative IPO trend during 1982–2021 is related to increasing, strengthening and longer‐lasting MHW over Kuroshio Extension, but decreasing and shorter‐lasting MHW over the Northeast Pacific Coast. Plain Language Summary Marine heatwaves (MHWs) are discrete prolonged extreme warm water events, with devastating and profound impacts on the health of marine ecosystems and biological systems. During the past several decades, MHWs have become more frequent, stronger, and longer‐lasting. Both external forcing and internal variability have been argued to play important roles in these changes, but their relative contributions are still largely unknown. Here, we use the large ensemble simulations from CESM2 with a total of 90 members to identify the role of external forcing and internal variability in the global MHW changes. It is found that external forcing dominates the increasing MHW frequency, accounting for 82% of the observed global averaged increase during 1982–2021. This increase is mainly caused by the mean SST warming. In addition to the rising mean SST, the increased SST variance also plays a role in the longer‐lasting MHWs. However, the MHW intensity change is dominated by the internal variability via the SST variance change. In particular, the IPO is closely associated with the MHW changes in Kuroshio Extension and Northeast Pacific Coast. These findings underscore the importance of considering both external forcing and internal variability in predicting and projecting the MHW change in the future. Key Points External forcing dominates the increasing frequency of marine heatwave (MHW), accounting for 82% of the observed