Interdecadal variability of the meridional overturning circulation as an ocean internal mode

Abstract

The meridional overturning circulation (MOC) in the coupled ECHAM5/MPIOM exhibits variability at periods of near 30 years and near 60 years. The 30-year variability, referred to as interdecadal variability (IDV), exist in an ocean model driven by climatological atmospheric forcing, suggesting that it is maintained by ocean dynamics; the 60-year variability, the multidecadal variability (MDV), is only observed in the fully coupled model and therefore is interpreted as an atmosphere-ocean coupled mode. The coexistence of the 30-year IDV and the 60-year MDV provides a possible explanation for the widespread time scales observed in climate variables. Further analyses of the climatologically forced ocean model shows that, the IDV is related to the interplay between the horizontal temperature-dominated density gradients and the ocean circulation: temperature anomalies move along the cyclonic subpolar gyre leading to fluctuations in horizontal density gradients and the subsequent weakening and strengthening of the MOC. This result is consistent with that from less complex models, indicating the robustness of the IDV. We further show that, along the North Atlantic Current path, the sea surface temperature anomalies are determined by the slow LSW advection at the intermediate depth.

Decadal windstorm activity in the North Atlantic-European sector and its relationship to the meridional overturning circulation in an ensemble of simulations with a coupled climate model

“…In particular, all runs exhibit periods with significant variability around 30 years and around 60 years, in line with the results of Zhu and Jungclaus (2008), who also reported MOC variability with periods around 30 and 60 years in the MPIOM model. The simulations analysed here additionally include variability with shorter periods (around 10 and 20 years), which reaches statistically significant levels in 2 out of the 3 simulations.…”

“…Variability in the MOC of the ECHAM5-MPIOM model has been analysed by Zhu and Jungclaus (2008). They identified two distinct periods of approximately 60 years and of approximately 30 years.…”

“…Latif et al (2004) found that multidecadal variations in the MOC lag multidecadal changes in the NAO by about a decade. In the MPIOM ocean model, which is used for this study, internally generated (30-year variability) and coupled ocean-atmosphere variations (60-year variability) in the MOC co-exist (Zhu and Jungclaus 2008).…”

The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation

“…Key to the MOC variability in these coupled models is the horizontal transport of salt into the convective regions and its strong influence on deep convection, a substantial phase lag between gyre and MOC changes, and, when there is a spectral peak, the role of the MOC-induced meridional heat transport to reverse the phase of the oscillations. In a few cases, however, temperature dominates the subpolar density fluctuations, as in the 30-yr variability of a lower resolution version of ECHAM5/MPIOM, which seems to be an ocean internal mode (Zhu and Jungclaus 2008) While the observational record is insufficient to document MOC changes, the role of salinity and the link between MOC and gyre changes can be tested with ocean general circulation model (OGCM) hindcasts. The OGCMs generally are of higher resolution and are forced by more realistic atmospheric fields.…”

Geostrophic dynamics of meridional transport variability in the subpolar North Atlantic

“…Additionally, Spall [2008] concludes that low-frequency MOC variability can be generated by many factors other than buoyancy fluxes, including lateral advection, acting on the baroclinic pressure gradient in the mixed layer. Moreover, it has been suggested that MOC variability can be generated internally, with no external atmospheric forcing, through the interaction of horizontal pressure gradients and the ocean circulation [Zhu and Jungclaus, 2008].…”