Detectability of an AMOC Decline in Current and Projected Climate Changes

D Lobelle,F Sévellec,V Livina,E Frajka‐Williams,C Beaulieu

doi:10.1029/2020gl089974

Abstract

AbstractDetermining whether the Atlantic Meridional Overturning Circulation (AMOC)'s transport is in decline is challenging due to the short duration of continuous observations. To estimate how many years are needed to detect a decline, we conduct a simulation study using synthetic data that mimics an AMOC time series. The time series' characteristics are reproduced using the trend, variance, and autocorrelation coefficient of the AMOC strength at 26.5°N from 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) models under the RCP8.5 future scenario, and from RAPID observations (2004–2018). Our results suggest that the 14‐year RAPID length has just entered the lower limits of the trend's “detection window” based on synthetic data generated using CMIP5 trends and variability (14–42 years; median 24 years), but twice the length is required for detectability based on RAPID variability (29–67 years; median 43 years). The annual RAPID trend is currently not statistically significant (−0.11 Sv yr−1, p > 0.05).

Highlights

Coupled ocean-atmosphere numerical models generally predict a decline of the Atlantic Meridional Overturning Circulation (AMOC) under the influence of anthropogenic warming in the 21st century (IPCC, 2019)
Our results suggest that the 14-year RAPID length has just entered the lower limits of the trend's “detection window” based on synthetic data generated using Coupled Model Intercomparison Project Phase 5 (CMIP5) trends and variability (14–42 years; median = 24 years), but twice the length is required for detectability based on RAPID variability (29–67 years; median = 43 years)
The 20 CMIP5 parameter values range from −0.02 to −0.12 Sv yr−1 for ω, 0.32 to 1.29 Sv2 for σε2, and −0.17 to 0.48 for φ (see Table S1 for each model's values and results from the Durbin and Watson (1950) and Engle (1982) residual analyses; Texts S1 and S2)

Summary

Introduction

Coupled ocean-atmosphere numerical models generally predict a decline of the Atlantic Meridional Overturning Circulation (AMOC) under the influence of anthropogenic warming in the 21st century (IPCC, 2019). Heat is absorbed in surface waters of the tropical Atlantic and carried northward to be released to the atmosphere over the northeast Atlantic. At 26.5◦N, the RAPID-MOCHA (Rapid Climate Change/Meridional Overturning Circulation and Heatflux Array) program (hereafter, RAPID) has continuously monitored the AMOC since 2004 (Rayner & Kanzow, 2011). These observations suggested that a decline in AMOC transport could be occurring (Robson et al, 2014; Smeed et al, 2014), and later a sustained reduced mean transport post-2008 was detected (Smeed et al, 2018). Coupled Model Intercomparison Project Phase 5 (CMIP5) data and other studies using proxy indicators suggest that a long multidecadal declining trend could occur (Caesar et al, 2018; Cheng et al, 2013; IPCC, 2019; Rahmstorf et al, 2015; Robson et al, 2014; Sévellec et al, 2017; Stocker et al, 2013)

Objectives

Results

Discussion

Conclusion