Impact of anomalous surface forcing on the sub-polar North Atlantic: Water mass formation and circulation.

摘要

A regional eddy-permitting ocean model of the sub-polar North Atlantic under flux forcing, with a weak restoring term on salinity (to parameterize non-represented sea ice processes), has been developed based on a previous version using restoring boundary conditions. The flux forcing is based upon two different climatologies, from the Southampton Oceanography Center and the NCEP-NCAR reanalysis. With addition of a crude parameterization of the effect of high-frequency variability resulting from the passages of synoptic scale events on the heat flux over the convective region of the Labrador Sea during winter months, the model remains stable and all major features of the sub-polar gyre are represented, although temperature and salinity are still higher than observations, as seen in most high-resolution models of the sub-polar gyre, which appears to be dominated by internal advection processes rather than surface forcing.; The model is then utilized to examine the response of the sub-polar North Atlantic to distinct surface forcings corresponding to extreme North Atlantic Oscillation (NAO) phases and the Last Glacial Maximum (LGM). A potentially multi-decadal variability in Labrador Sea Water (LSW) formation and heat transport is observed in a high NAO experiment in contrast to a low NAO experiment. The results therefore suggest that LSW formation and heat transport processes are less stable under the high NAO phase and this feature appears to be brought about by internal oceanic processes, primarily attributable to changes in the strength and pathway of the North Atlantic Current (NAC). During the LGM, the inclusion of sea ice effects in the atmospheric forcing shuts down Labrador Sea deep convection, as well as steers the NAC from its present northeastward path to a zonal path with an apparent southward shift. The meridional transport also reduces significantly. The results therefore emphasize the importance of the insulating effects of sea ice, in contrast to previous work. With such discrepant responses, this study stresses the importance of understanding how ocean circulation will evolve in some extreme circumstances, especially with the possibility for enhanced variability of the climate system under the high NAO phase which, as suggested by some model simulations, is more likely to occur in a warmer world.