A Regime Shift in Atlantic Surface Currents Reveals a Step-like Decline of the Meridional Overturning Circulation

TL;DR

This study uncovers a significant regime shift in Atlantic surface currents in 2009, linked to a nonlinear weakening of the Atlantic Meridional Overturning Circulation, with implications for climate variability and ocean dynamics.

Contribution

It identifies a new basin-scale phase, the Atlantic Convergence Divergence Mode, and links its shift to a step-like decline in AMOC, advancing understanding of oceanic climate responses.

Findings

A regime shift in 2009 marked by weakened vertical water exchange.

The Atlantic Convergence Divergence Mode (ACDM) acts as a proxy for AMOC fluctuations.

The 2009 shift signifies a nonlinear, step-like weakening of AMOC.

Abstract

The Atlantic surface currents associated with the Atlantic Meridional Overturning Circulation (AMOC) play a central role in regulating Earth's climate, yet their large scale dynamical response to climate variability remains poorly understood. Here we identify a previously unrecognized basin scale phase of Atlantic surface circulation, termed the Atlantic Convergence Divergence Mode (ACDM), characterized by a convergence divergence pattern in the North Atlantic and coherent meridional flows in the South Atlantic. We show that the ACDM experienced a pronounced regime shift in 2009, marked by weakened vertical water exchange and reduced meridional transport. This transition closely coincides with direct RAPID MOCHA AMOC observations and is driven by AMOC modulated multicale forcing: a low frequency oceanic thermal reorganization that preconditions the system, and episodic atmospheric shocks that trigger the shift. By identifying the ACDM variability as a sensitive and physically grounded proxy for interannual AMOC fluctuations, we reveal that the observed 2009 shift signifies a nonlinear, step like weakening of AMOC that triggered a fundamental basin scale reorganization of Atlantic surface currents. Our results offer a dynamical explanation for the AMOC's recent decline and demonstrate its inherently nonlinear nature, highlighting the need to account for step like transitions in assessing its stability and future evolution.