A Numerical Study of the Role of Meridional Overturning Circulation in the Gulf Stream Separation
OCCI Project Funded: 2005
Proposed ResearchA western boundary current, the Gulf Stream (GS) flows roughly parallel to the east coast of the United States until it separates from the western boundary region near Cape Hatterasoff the coast of South Carolinaand continues across the Atlantic toward the northeast. It is the upper limb of the density-driven Meridional Overturning Circulation (MOC) in the North Atlantic. The GS separation point has been shown to fluctuate on interannual time scales, and such fluctuations are likely an important component for atmosphere-ocean interactions such as the North Atlantic Oscillation (NAO).
Previous studies have examined aspects of this interaction and found that a greater MOC, in the form of a stronger Deep Western Boundary Current (DWBC), tends to push the Gulf Stream separation point southward. But in those studies, the upper limb of the MOC was held steady, and thus, the effect comes solely from changes in the DWBC. A less-studied aspect of MOC-GS interaction is how the variability of the upper MOC limb affects the GS separation. One study showed that the shift is mostly due to the change of the upper limb of the MOC.
We propose to investigate the interaction between MOC and GS and its impact on GS separation by using an ocean model. The interaction between MOC and GS is complicated, involving multiple processes. In this project, we plan to use idealized numerical models to study how variations from overflow and Labrador Sea convection affect the transport and separation of the GS and how changes in the DWBC affect the GS. We will study how bathymetry and thermocline depth affect this interaction. We hypothesize that the impact from different components of the NADW can be very different. Signals from variability associated with the Nordic Seas overflow would probably propagate mainly along the western boundary, and, thus, likely to have a direct impact of the GS.
Based on indirect observation, it has been reported that the overflow through the Faroe Bank Channel, downstream from the Iceland-Scotland Ridge, has decreased by at least 20 percent in the last five decades. In addition, various studies have shown that the Labrador Sea Water (LSW) formation had changed profoundly in response to NAO forcing. Our idealized modeling study will lay a foundation for constructing a more realistic model to investigate how these observed changes affect the GS dynamics.