Mechanisms of Decadal North Atlantic Climate Variability and Implications for the Recent Cold Anomaly

Marius Årthun,Helene R Langehaug,Robert C J Wills,Léon Chafik,Helen L Johnson

doi:10.1175/jcli-d-20-0464.1

Abstract

AbstractDecadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations, and preindustrial control simulations, we identify a decadal mode of atmosphere–ocean variability in the North Atlantic with a dominant time scale of 13–18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air–sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability.

Highlights

The North Atlantic Ocean displays pronounced decadal variability (Fig. 1; Deser and Blackmon 1993; Czaja and Marshall 2001)
In agreement with previous studies (e.g., Buckley et al 2014), the Gulf Stream extension region is a more heavily damped system where ocean heat transport convergence and surface heat fluxes are strongly anticorrelated (20.75). These results demonstrate that the spatial sea surface temperature (SST) pattern associated with the Atlantic decadal variability (ADV) is not just the time-integrated response to surface heat flux anomalies, and that ocean heat transport plays a key role in decadal SST variability
There has recently been a large focus on identifying the mechanisms responsible for Atlantic multidecadal variability (AMV; e.g., Clement et al 2015; O’Reilly et al 2016; Drews and Greatbatch 2017; Wills et al 2019; Zhang et al 2019)

Summary

Introduction

The North Atlantic Ocean displays pronounced decadal variability (Fig. 1; Deser and Blackmon 1993; Czaja and Marshall 2001). Decadal variations in North Atlantic sea surface temperature (SST) influence climate over adjacent continents and are a major source of skill in climate predictions (Hermanson et al 2014; Msadek et al 2014; Årthun et al 2017; Yeager and Robson 2017). Previous studies have reported a wide range of mechanisms underlying North Atlantic decadal variability (e.g., Grötzner et al 1998; Häkkinen 2000; Eden and Willebrand 2001; Dong and Sutton 2005; Williams et al 2014; Menary et al 2015a; Muir and Fedorov 2017; Nigam et al 2018; Martin et al 2019).

Objectives

Findings

Discussion

Conclusion