Meeting Documents

Water Mass Contributions and Eddy Effects on AMOC Variability

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental component of the global climate system, redistributing heat and regulating climate. Understanding its variability is therefore critical. While a weakening of the AMOC has been suggested by proxy data since the last century, and is predicted to continue into the future by most climate models, key knowledge gaps remain regarding the specific drivers of this decline. Here, we focus on (1) disentangling the contributions of individual water mass components and (2) quantifying the role of mesoscale eddy transports in AMOC transport variability. We use the ECCOv4-r4 dataset (1997–2018) to analyse temperature, salinity, and volume transport across three key water masses: Antarctic Intermediate Water (AAIW), North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW).

Over the last two decades, AAIW and NADW generally exhibit warming and salinification, though NADW salinity decreases between 20°S and 30°S. AABW shows predominantly cooling and freshening. A central finding is a significant weakening of NADW transport between 10°N and 30°N (~0.1 Sv/year), likely driven by internal variability. AAIW transport also weakens in the Southern Hemisphere, while AABW transport responses are mixed. Further analysis reveals these trends to be driven by the large-scale velocity field, with parametrised eddy-induced transport partially compensating them.

Overall, the weakening of northward AAIW and southward NADW transports suggests a negative feedback loop operating on multi-decadal timescales: reduced AAIW transport lowers freshwater input to the North Atlantic, while reduced NADW transport lowers salt export out of the North Atlantic. Both effects are theoretically expected to increase North Atlantic salinity and, consequently, possibly the strength of its overturning circulation.