Temperature Change in Mode Water Source Regions

Abstract

Mode waters, characterized by vertically uniform temperature and salinity beneath the mixed layer, play a crucial role in ventilating anthropogenic heat and carbon into the ocean interior. However, the global comparison of their temperature evolution and governing processes under a global closed-budget framework remains to become more systematically undertaken, as required to make mechanistic progress. Using two global ocean datasets OCCA2 and IAPv4, we examine temperature changes in 11 mode waters from 1980 to 2022. Most exhibit statistically significant warming, strongest in North Pacific Central Mode Water (NP-CMW) and North Atlantic Subtropical Mode Water (NA-STMW), where the datasets agree closely. A hemispheric contrast emerges: decadal (interannual) variability dominates in the Northern (Southern) Hemisphere,. Mode water temperature is regulated by both advective ocean heat transport convergence (adiabatic) and water-mass transformation (diabatic) associated with air–sea heat fluxes and convective mixing. The closed heat budget in OCCA2 reveals that the diabatic term controls variability in NA-STMW, North Pacific Eastern Subtropical Mode Water (NP-ESTMW), and South Atlantic Subtropical Mode Water (SA-STMW), with advection compensating. Conversely, advection dominates in North Pacific Subtropical Mode Water (NP-STMW), South Pacific Eastern Subtropical Mode Water (SP-ESTMW), and Subantarctic Mode Water (SAMW). In North Atlantic Madeira Mode Water (NA-MMW), North Atlantic Subpolar Mode Water (NA-SPMW), NP-CMW, and Southern Pacific Western Subtropical Mode Water (SP-WSTMW), both processes contribute comparably. Across all eleven mode waters, we find that the degree of compensation between adiabatic and diabatic processes and which process dominates temperature variability are timescale-dependent, highlighting the interplay between mixed-layer transformation and interior ocean heat transport on different time scales.