Diagnosing Spice and Heave Contributions to Temperature Variability and Subsurface Marine Heatwaves

Abstract Marine heatwaves (MHWs) occur not only at the ocean surface but also in the subsurface, where they are less directly influenced by air–sea heat fluxes. While the drivers of surface MHWs are well studied, the mechanisms causing subsurface events remain poorly understood. Here, we separated subsurface MHW temperature and salinity changes into components driven by density-surface movements (“heave”) and along-isopycnal water mass changes (“spice”) to better identify their causes. Using this spice–heave framework, we explored the dominant mechanisms causing temperature and salinity variabilities across monthly, seasonal, and interannual time scales, based on gridded Argo observations in the upper 2000 m of the open ocean. Seasonal and interannual analyses provide context for interpreting spice and heave and their underlying processes. We focused on four documented MHW regions: the northeast Pacific, eastern equatorial Pacific, western Tasman Sea, and Southern Ocean south of Australia. In the northeast Pacific region, MHWs below the mixed layer were primarily spice driven, likely linked to subduction following surface buoyancy fluxes, while MHWs near the thermocline were mainly heave driven, tied to thermocline depth variations. In the Tasman Sea and equatorial regions, MHWs were dominated by isopycnal heave associated with eddies from the East Australian Current extension and planetary waves, respectively. Vertically coherent MHWs below the thermocline were dominated by heave across great depths, linked to meridional frontal movements and long-lived deep eddies, as seen in the northeast Pacific and the Southern Ocean. The spice–heave framework offers a useful approach for understanding subsurface MHWs globally, with implications for assessing their ecosystem impacts. Significance Statement Marine heatwaves (MHWs) can occur far below the ocean surface, yet their driving mechanisms remain poorly understood. This study applies a spice–heave decomposition framework to gridded Argo observations to distinguish between subsurface MHWs driven by water mass changes (“spice”) and those caused by isopycnal movements (“heave”). Across four key ocean regions, we found that spice-driven MHWs are likely linked to surface-driven subduction, while heave-driven MHWs are tied to thermocline variability, eddy activity, planetary waves, and frontal shifts. These results reveal region- and depth-dependent drivers of subsurface MHWs and highlight the importance of large-scale ocean dynamics. Our approach provides a diagnostic tool for identifying MHW mechanisms with implications for climate variability and marine ecosystem impacts.