The Role of Mixed Layer Depth in Summer Extensive Marine Heatwaves in the Kuroshio‐Oyashio Extension Region

Abstract The Kuroshio‐Oyashio Extension (KOE) region has experienced increasingly frequent extensive summer marine heatwaves (MHWs), yet the governing physical mechanisms remain unclear, particularly regarding the role of mixed layer depth (MLD). This study investigates seven major MHW events between 2001 and 2024 using a composite heat budget analysis, providing a generalized basin‐scale assessment of dominant drivers beyond single‐event perspectives. Anomalies in the surface heat flux term, calculated as the net surface heat flux divided by the MLD, explain approximately 65% of the warming during the development phase, while the same term (34%) and vertical processes (39%) contributed comparably to cooling during the decay phase. MLD variability modulates upper ocean warming through two distinct processes. Shallow MLD increases the anomalous surface heat flux term, and MLD shoaling induces the detrainment effect. These MLD‐related contributions (47%) are comparable in magnitude to the surface heat flux anomaly effect (36%). In the decay phase, the surface heat flux anomaly effect (40%) and entrainment associated with MLD deepening (36%) both contribute comparably to the total cooling. Reduced low cloud cover and intensified wind speed played key roles in driving these processes during the development and decay phases, respectively. This study highlights not only the multiple atmospheric and oceanic processes that shape the evolution of extensive summer MHWs in the KOE region, but also the distinct and quantitatively assessed role of MLD, which can exert an influence comparable to that of surface heat flux.