Recurrent Synoptic Waves Instigated Severe Marine Heatwave in the Southwest Pacific

Abstract Marine heatwaves are devastating to regional ecosystems and often evolve under the influence of persistent atmospheric weather patterns. However, the relative importance of synoptic atmospheric processes in the development of these events remains unclear, as previous studies have predominantly focused on quasi‐stationary systems. Here, we demonstrate that a severe marine heatwave (MHW) over the Southwest Pacific unfolded through two distinct warming stages followed by a cooling phase, all dominated by recurrent synoptic Rossby wave packets propagating through the upper troposphere. During each warming phase, these wave packets organized near‐surface circulation anomalies that inhibited wind‐driven mixing, causing unusually shallow mixed‐layer depths. The resultant shoaling amplified surface warming by intensifying the effects of increased solar radiation, ultimately triggering the extreme MHW. Conversely, during the intermediate cooling phase, the mixed layer returned to its climatological state, allowing the heatwave conditions to dissipate. Strong quasi‐stationary patterns emerged only near the ends of both intensification and decay, downstream of the recurring wave events. These blocking patterns appear to have helped terminate the recurrence by causing potential instability to build up to a point of more widespread release. A diagnostic analysis of the lower tropospheric vorticity budget highlights the central role of dynamical vorticity production tied to the anomalous atmospheric circulation forced by upper‐level waves.