Mechanism of Interannual to Decadal Variability of the North Atlantic Circulation

Abstract

A model of the Atlantic Ocean was forced with decadal-scale time series of surface fluxes taken from the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis. The bulk of the variability of the oceanic circulation is found to be related to the North Atlantic oscillation (NAO). Both realistic experiments and idealized sensitivity studies with the model show a fast (intraseasonal timescale) barotropic response and a delayed (timescale about 6-8 yr) baroclinic oceanic response to the NAO. The fast response to a high NAO constitutes a barotropic anticyclonic circulation anomaly near the subpolar front with a substantial decrease of the northward heat transport and an increase of northward heat transport in the subtropics due to changes in Ekman transport. The delayed response is an increase in subpolar heat transport due to enhanced meridional overturning and due to a spinup of the subpolar gyre. The corresponding subpolar and subtropical heat content changes could in principle act as an immediate positive feedback and a delayed negative feedback to the NAO.

Intrinsic Variability of the Atlantic Meridional Overturning Circulation at Interannual-to-Multidecadal Time Scales

“…Gulf Stream surface velocity observations (Ezer et al 2013) confirm the actual existence of this latter peak, which might therefore be of intrinsic origin. Correspondence between seasonally and fully forced AMOC spectral peaks is less clear but may also be present in the subpolar gyre, around 8 yr between 458 and 608N; this peak could therefore be of intrinsic origin but also may be excited by the North Atlantic Oscillation (NAO; see Eden and Willebrand 2001). In this region, we may hypothesize that the interannual forcing excites and paces intrinsic variability modes that spontaneously emerge in the S run.…”

The role of Atlantic Ocean-atmosphere coupling in affecting North Atlantic oscillation variability

“…Various coupled GCMs or ocean only integrations have shown substantial variability of the MOC at interdecadal timescales, with associated current, temperature and salinity anomalies extending all the way to the equator [e.g., Delworth et al, 1993;Timmerman et al, 1998;Häkkinen, 1999;Eden and Willebrandt, 2001;Eden and Jung, 2001;Dong and Sutton, 2001]. The dynamics of such basin-scale changes were discussed in detail in Visbeck et al [this volume].…”

“…Various observational studies have shown a fast response of the Gulf Stream/recirculation system to NAO variability (about a 1-year lag), leading to localized SST anomalies north of the Gulf Stream extension [e.g., Taylor and Stephens, 1998;Frankignoul et al, 2001b]. On longer (decadal and interdecadal) timescales, the ocean response involves basin-scale currents and SST anomalies, as hinted at in various numerical studies [e.g., Visbeck et al, 1998;Krahman et al, 2001;Eden and Jung, 2001;Eden and Willebrandt, 2001]. In addition, like the atmosphere, the ocean circulation displays intrinsic variability on a broad range of timescales (e.g., interannual to decadal associated with the wind-driven gyres, interdecadal timescales associated with the meridional overturning circulation (MOC)).…”

Oceanic control of multidecadal variability in an idealized coupled GCM

“…Several hypotheses have been proposed which include either ocean-atmosphere coupled modes (Timmermann et al 1998;Weaver and Valcke 1998), oceanic modes that are excited by atmospheric noise associated with synoptic weather (Griffies and Tziperman 1995;Sévellec et al 2009;), oceanic response to variable atmospheric forcing (Delworth and Greatbatch 2000;Eden and Jung 2001;Eden and Willebrand 2001), or intrinsic oceanic modes where the energy source originates from an internal instability of the large-scale ocean circulation (Colin de Verdière and Huck 1999;Te Raa and Dijkstra 2002).…”

“…The idea that the North Atlantic Oscillation (NAO, Hurrell 1995) forcing is the main driver of Atlantic multidecadal variability was thus explored in a number of studies (e.g. Eden and Jung 2001;Eden and Willebrand 2001;Mecking et al 2014). Although the processes and timescales involved in the oceanic response may depend on both the exact nature of the forcings and the model configuration, the conclusion was reached that the NAO forcing is essential to the oceanic multidecadal variability and to reproduce part of the observed North Atlantic SST signal.…”