The internal variability modulates marine heatwave duration changes by day-to-day ocean memory in the recent decades

The duration of marine heatwaves (MHWs) determines the cumulative thermal stress imposed on marine organisms, thereby strongly influencing the severity of ecological consequences. Recently, day-to-day ocean memory, quantified by a first-order sea surface temperature (SST) autocorrelation model, has been found as an effective metric to depict the duration of MHWs, due to its consistent climatological distribution and trends with MHW duration in the observations. However, the relative importance of external forcing and internal variability in modulating the day-to-day ocean memory and, more importantly, its relationship with MHW duration, remain unclear. In this study, we employ 100 ensemble members of CESM2 and 32 CMIP6 models to analyze the roles of external forcing and internal variability in the day-to-day ocean memory and MHW duration. We find that internal variability dominates the trends of day-to-day ocean memory in the recent decades and further modulates the trends of MHW duration, particularly in the tropical oceans. The zonal gradient of SST trends over the tropical Pacific emerges as a key feature of the internal variability. By combining a stochastic climate model framework with a mixed-layer heat budget, we further reveal that ocean mixing and dynamics are the primary processes through which internal variability modulates trends in day-to-day ocean memory, whereas SST-surface heat flux feedback provides a secondary contribution and variations in mixed-layer depth play negligible roles. This study demonstrates that internal variability exerts a considerable influence on MHW duration through its modulation on day-to-day ocean memory, providing crucial evidence and a new perspective for understanding and predicting MHW duration in a warming climate.