Vertical structures of marine heatwaves and their impacts on primary production in the South China Sea

Abstract

Marine heatwaves (MHWs) are extreme warm‐water events that can strongly perturb marine ecosystems. Although MHWs are known to extend into subsurface and deep waters, their vertical structures and biogeochemical consequences remain poorly quantified. Here, we use a physical‐biogeochemical model to characterize the vertical structures of MHWs in the South China Sea (SCS) and to assess their impacts on primary production (PP). Four distinct types of vertical structure of MHWs (shallow, sub‐reversed, sub‐intensified and deep MHWs) are identified. Deep MHWs are the most common, accounting for nearly 50% of events. Shallow and sub‐reversed MHWs are generally short‐lived and confined to the upper ocean, whereas sub‐intensified and deep MHWs persist longer and penetrate deeper. Heat budget analysis indicates that air–sea heat flux primarily drives the formation of shallow, sub‐reversed, and deep MHWs, while vertical processes dominate the maintenance of sub‐intensified MHWs. Distinct MHW structures modify vertical stratification and nutrient transport, producing spatially and vertically heterogeneous PP responses across the mixed layer and the euphotic zone. Over the past two decades, the occurrence area, duration, and total intensity of all four MHW types increased significantly, and associated PP anomalies exhibited a declining trend. This decline is largely attributable to strengthened mixed‐layer stratification and reduced mixed‐layer PP, with subsurface processes contributing little to the long‐term trend.