Evolution of the Atlantic Multidecadal Variability in a Model with an Improved North Atlantic Current

Abstract

This article investigates the dynamics and temporal evolution of the Atlantic multidecadal variability (AMV) in a coupled climate model. The model contains a correction to the North Atlantic flow field to improve the path of the North Atlantic Current, thereby alleviating the surface cold bias, a common problem with climate models, and offering a unique opportunity to study the AMV in a model. Changes in greenhouse gas forcing or aerosol loading are not considered. A striking feature of the results is the contrast between the western and eastern sides of the subpolar gyre in the model. On the western side, anomalous heat supply by the ocean plays a major role, with most of this heat being given up to the atmosphere in the warm phase, largely symmetrically about the time of the AMV maximum. By contrast, on the eastern side, the ocean anomalously gains heat from the atmosphere, with relatively little role for ocean heat supply in the years before the AMV maximum. Thereafter, the balance changes with heat now being anomalously removed from the eastern side by the ocean, leading to a reduced ocean heat content, behavior associated with the establishment of an intergyre gyre at the time of the AMV maximum. In the warm phase, melting sea ice leads to a freshening of surface waters northeast of Greenland that travel southward into the Irminger and Labrador Seas, shutting down convection and terminating the AMV warm phase.

A Review of the Role of the Atlantic Meridional Overturning Circulation in Atlantic Multidecadal Variability and Associated Climate Impacts

“…For example, the amplitudes of internal variability in subpolar North Atlantic SST and associated winter NAO in a CGCM's historical simulation are much weaker than observed at multidecadal timescales (Figures a and b), which are linked to the underestimated low‐frequency internal AMOC variability (Kim, Yeager, Chang, & Danabasoglu, ). Correcting the mean state North Atlantic SSS and AMOC biases in CGCMs might improve the pattern and amplitude of AMV‐related SST and surface turbulent heat fluxes (Drews & Greatbatch, , ; Park et al, ). The impact of AMV on the anticorrelated multidecadal variability in winter NAO is substantially underestimated in most CGCMs (e.g., Peings et al, ; Ting et al, ), due to the underestimated internally generated AMV signal and associated surface turbulent heat flux anomalies (Peings et al, ).…”

On the persistence and coherence of subpolar sea surface temperature and salinity anomalies associated with the Atlantic multidecadal variability

“…[] shows that the fundamental equation and mechanism governing the AMV in fully coupled models is well distinguished from that in slab ocean models. At low frequency there is a negative correlation/regression between net surface heat flux (or surface turbulent heat flux) and SST anomalies in subpolar NA in both observations and fully coupled models, indicating the important role of ocean dynamics in the AMV [ Gulev et al ., ; O ' Reilly et al ., ; Zhang et al ., ; Drews and Greatbatch , , ]. However, using a red noise model with no oceanic damping, Clement et al .…”

A Multivariate AMV Index and Associated Discrepancies Between Observed and CMIP5 Externally Forced AMV

“…The leading mechanism of the observed Atlantic multidecadal variability (AMV) is often thought to be linked to the low‐frequency variability of the Atlantic Meridional Overturning Circulation (AMOC; e.g., Ba et al, , ; Delworth & Mann, ; Delworth et al, ; Drews & Greatbatch, , ; Kim, Yeager, Chang, et al, ; Kim, Yeager, & Danabasoglu, ; Knight et al, ; Kushnir, ; Latif et al, ; McCarthy et al, ; Park et al, ; Yan et al, , ; Zhang, ; Zhang et al, , ). However, some recent studies suggest that AMV is mainly a direct response of the North Atlantic sea surface temperature (SST) to changes in external radiative forcings (e.g., Bellomo et al, ; Bellucci et al, ; Booth et al, ; Murphy et al, ).…”