Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly

Abstract

<p>There has recently been a large focus on identifying the mechanisms responsible for Atlantic multidecadal variability (AMV). However, decadal-scale variability embedded within the AMV has received less attention, despite being a prominent feature of observed North Atlantic sea surface temperature (SST) and important for the climate of adjacent continents. These decadal fluctuations in the North Atlantic Ocean are also a key source of skill in decadal 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 pre-industrial 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 strongly suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability, and that we might expect it to become less pronounced over the next few years.</p>

The Potential Mechanisms of AMOC Multidecadal Periods in CMIP6/CMIP5 Preindustrial Simulations

“…3. The usage of band-pass filters helps isolate the timescale of interest, which has been applied in previous studies (e.g., Muir and Fedorov, 2017;Gastineau et al, 2018;Årthun et al, 2021). In most models, there are clear westward temperature propagations with alternative positive and negative signs across the entire subpolar North Atlantic basin, consistent with the results in other CMIP5 models (Muir and Fedorov, 2017).…”

Multistability in a Coupled Ocean-Atmosphere Reduced Order Model: Non-linear Temperature Equations

“…Potential mechanisms for LFV in the midlatitudes include coupling of the ocean with the atmosphere where temperature, wind stresses, and moisture are transferred from one to another (Holton, 2004). This coupling has been observed in data (Årthun et al, 2021;Wu and Liu, 2005;Czaja and Frankignoul, 2002;Czaja and Marshall, 2001).…”

Observed mechanisms activating the recent subpolar North Atlantic Warming since 2016

“…Since around 2016, and following the cold SPNA period that lasted for nearly a decade [18][19][20][21][22][23][24], including intense heat loss events between 2013 and 2015 that induced the North Atlantic cold blob [25], a shift in ocean circulation is reported to have initiated a new SPNA warming phase [26]. The findings from the latter study indicate that this warming is caused by enhanced advection of subtropical waters to the eastern SPNA (region east of the Reykjanes Ridge, figure 1b), and to some extent by a reduction in surface net heat loss.…”