Introduction

The Water Mass Transformation (WMT) framework was initially put forward by Walin (1982) to describe the relationship between surface heat fluxes and interior ocean circulation (building on his earlier paper on salinity transformations in estuaries). A series of studies further refined the framework to include the effect of haline-driven buoyancy forcing (e.g., Tziperman 1986; Speer and Tziperman, 1992) and account for the role of interior mixing (e.g., Nurser et al., 1999; Iudicone et al., 2008). A comprehensive overview of past studies in WMT and details of how WMT is derived from diapycnal processes can be found in Groeskamp et al. 2019.

A detailed account of best practices for computing full water mass budgets in finite-volume ocean models is given by Drake et al. 2025, which was written alongside this package. This package handles the calculation of water mass transformations, generalizing an earlier version of the package that was limited to surface-forced transformations and developed alongside Tesdal et al. (2023). A companion python package, xwmb, handles the other terms in the water mass budget: water mass tendencies, lateral overturning transports, boundary mass fluxes, and spurious numerical mixing.

Package Objectives

The goal of this package is to provide various WMT routines to derive key metrics related to water masses in the ocean and the rates at which they are transformed.

Disclaimer

xwmt does not employ any checks to ensure conservation of heat and salt. It is the user’s responsibility to ensure that budgets for heat and salt are properly closed in the datasets. Improperly conserved fields will yield incorrect results.