THE IMPACT OF SURFACE FORCINGS ON THE ATLANTIC MULTIDECADAL VARIABILITY

Abstract

The North Atlantic sea surface temperature (SST) anomalies associated with the Atlantic multidecadal variability (AMV) have major impacts on the regional and global climate. Therefore, understanding the AMV mechanisms can help us understand and predict the long-term climate variability. In addition to the SST anomalies, AMV is also manifested as persistent upper ocean heat content (HC) anomalies in the North Atlantic and low-frequency fluctuations of the Atlantic meridional overturning circulation (AMOC). It is important to understand what physical processes generate these subsurface oceanic anomalies and how they connect with the deep overturning on multidecadal time scales. Some previous studies show that the surface heat flux (SHF) anomalies play a dominant role in driving the AMOC multidecadal variability while the surface momentum flux (SMF) anomalies are more important in its seasonal and interannual variability. In this study, we first reexamine the above conclusion with a longer forcing in a newer version model. Then we further examine whether the upper ocean HC anomalies are generated differently by the anomalous SHF and SMF forcings and how the potentially different HC anomalies affect the AMOC fluctuations on multidecadal time scales. To answer these questions, a series of 600-year simulations are conducted using an ocean general circulation model (OGCM) forced by prescribed monthly atmospheric state variables from a community earth system model (CESM) pre-industrial run. Using these OGCM runs, we diagnose how the different surface forcings contribute to the coupled model-simulated multidecadal variability. The results of these experiments confirm the results from previous studies that the SHF anomalies are dominant in driving the AMOC multidecadal variability. However, it is demonstrated that SMF anomalies can also generate substantial AMOC multidecadal variability, although its amplitude is weaker than that generated by SHF. Moreover, it is shown that both SHF and SMF can generate basin-wide HC anomalies but with distinctive spatial distributions. The HC variability generated by SHF is border and occupies most of the northern North Atlantic. In the SMF run, intense HC variability is confined to the Gulf Stream extension region and weakened quickly further north. It is further demonstrated that these HC anomalies are then advected to form a characteristic dipole pattern, which modulates the North Atlantic Current and affects the upper branch of the AMOC. On multidecadal time scales, these HC anomalies get strengthened in the North Atlantic through advection while the local SHF anomalies play a damping role to the existing HC dipole pattern. This local damping effect is stronger in the SMF run than the SHF run. A further analysis shows that the time mean and perturbation flows play compensating roles in the heat advection and the geostrophic current is more dominant than the Ekman flow.