Transformation of climatologically anomalous water masses

Ran Liu , Gaël Forget, Zijie Zhao , Henri Drake

Wederiveanovelanomalous water mass budget framework in which water masses are defined by bounds on tracer anomalies rather than absolute tracer values, extending Water Mass Transformation (WMT) theory to ocean variability and extremes. The material derivative of tracer anomalies introduces three additional transformation terms absent from the standard WMT framework: advection of climatological tracer gradients by velocity anomalies, the convergence of climatological-mean transient advective heat fluxes, and contributions from time mean tendency. The framework is first tested using an analytical model, confirming budget closure and providing a clear physical interpretation of each term. Application to the ECCOv4r4 ocean state estimate over 1993–2017 then reveals that the global volume of anomalously warm water (defined by Θ′ ≥3.5◦C) exhibits two prominent peaks coinciding with the 1997/98 and 2015/16 El Ni˜ no events. Budget analysis identifies anomalous advection of the climatological temperature gradient by velocity anomalies as the dominant driver of warm water volume variability, consistent with the Bjerknes feedback mechanism, while surface heat flux anomalies consistently act as a damping term throughout both events, and other terms are relatively small. Negligibly small residuals confirm the numerical consistency of the framework when applied to a dynamically consistent ocean state estimate. These results establish the budget of anomalous water mass as a rigorous theoretical framework that opens new avenues for systematically investigating the dynamical mechanisms governing the generation, evolution, and dissipation of ocean anomalies associated with climate variability and extreme events.