North Atlantic Interdecadal Variability: Oceanic Response to the North Atlantic Oscillation (1865–1997)

Abstract

In contrast to the atmosphere, knowledge about interdecadal variability of the North Atlantic circulation is relatively restricted. It is the objective of this study to contribute to understanding how the North Atlantic circulation responds to a forcing by the North Atlantic oscillation (NAO) on interdecadal timescales. For this purpose, the authors analyze observed atmospheric and sea surface temperature (SST) data along with the response of an ocean general circulation model to a realistic monthly surface flux forcing that is solely associated with the NAO for the period 1865-1997. In agreement with previous studies, it is shown that the relationship between the local forcing by the NAO and observed SST anomalies on interdecadal timescales points toward the importance of oceanic dynamics in generating SST anomalies. A comparison between observed and modeled SST anomalies reveals that the model results can be used to assess interdecadal variability of the North Atlantic circulation. The observed/modeled developments of interdecadal SST anomalies during the periods 1915-39 and 1960-84 against the local damping influence from the NAO can be traced back to the lagged response (10-20 yr) of the North Atlantic thermohaline circulation and the subpolar gyre strength to interdecadal variability of the NAO. Additional sensitivity experiments suggest that primarily interdecadal variability in the surface net heat flux forcing associated with the NAO governs interdecadal changes of the North Atlantic circulation.

A Damped Decadal Oscillation in the North Atlantic Climate System

“…Readers are referred to appendix A for a more detailed description of the model and the coupling procedure. The OGCM is part of the Family of Linked Atlantic Model Experiments (FLAME) hierarchy of models, which have been used with good success (e.g., Eden and Jung 2001;Eden and Willebrand 2001) to simulate interannual to interdecadal variability in the North Atlantic. The OGCM is coupled to a simple stochastic atmosphere described by Bretherton and Battisti (2000, hereafter BB model).…”

“…13). Associating T a with the NAOI, we then force the OGCM with NAO-related surface heat flux and wind stress anomalies, as in Eden and Jung (2001) and Eden and Willebrand (2001). The ocean is able to feedback on T a , the NAOI, by the term bT o in the tendency equation for T a in both the original BB model and our modified setup.…”

“…In particular, changes in the ocean circulation in response to the forcing variability changes the oceanic heat transport in a way that affects the SST. In the North Atlantic, changes in heat transport are associated with changes in the thermohaline driven (overturning) circulation (Böning et al 1995;Eden and Jung 2001;Eden and Willebrand 2001). It appears therefore natural to relate the variability in the coupled model to a certain overturning (THC) index.…”

“…The ocean model used by Eden et al (2002) is essentially the same as in Eden and Jung (2001) and Eden and Willebrand (2001). Those papers describe the per-VOLUME 16 J O U R N A L O F C L I M A T E formance of the model in comparison with observations.…”

“…In addition, Dewar (2001) has suggested the importance of nonlinear processes associated with mesoscale eddies in the ocean for driving variability in the SST and the overlying atmosphere. Eden and Willebrand (2001) and Eden and Jung (2001) found that variability in the thermohaline circulation (THC) plays an important role in the dynamical response to variability in the atmospheric forcing related to the NAO. Eden and Willebrand (2001) speculated that (viscous) boundary waves play a role in variability of the THC.…”