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A Numerical Study of the Role of Meridional Overturning Circulation in the Gulf Stream Separation

OCCI Project Funded: 2005

Proposed Research

A 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 Hatteras—off the coast of South Carolina—and 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.

December 2005 Update

In the last 12 months we have investigated the role of the Deep Western Boundary Current (DWBC) in the Gulf Stream (GS) separation by using numerical models with 2 and 3 layers. The model is fully nonlinear and uses an idealized bathymetry which has a sloping bottom along the western boundary but is flat elsewhere. The surface layer, when forced by a prescribed and steady wind stress, produces a double gyre (subtropical and subpolar) structure with a robust GS which separates from the western boundary at the latitude of zero wind-stress curl. We then introduce a simple meridional overturning circulation (MOC). In the lower layers, two types of deep water enter the model, one through the northern boundary (representing the Nordic Seas overflow) and the other from the top layer to the second layer through a specified diapyncal mass flux (representing the open convection in Labrador Sea). A water-mass source is specified in the upper layer along the southern boundary to conserve the mass budget. An outflow condition is also specified in the upper layer in the northwestern corner to represent the flow into the Nordic Seas. The western boundary current (WBC) of the subtropical gyre strengthens and extends further northward (~200-400 km) when the MOC is introduced. In the next 6 months, we plan to examine the role of each DWBC on the GS separation, and the role of topographic in the GS/DWBC interaction. We also plan to examine the GS responses to variability of DWBC. We plan to write a manuscript at the end of this project.

Hannah Longworth, a graduate student from Southampton Oceanography Centre visiting since early October, has been looking at the adjustment of the meridional overturning circulation to anomalous buoyancy forcing in the subtropical and subpolar gyres in a planetary geostrophic general circulation model. The focus has been on the mechanisms of adjustment and the meridional propagation of anomalies resulting from excessive cooling. Two distinct processes have been identified, a frictional adjustment confined within the latitude bands of the buoyancy anomaly, and an advective adjustment that propagates cyclonically around the basin perimeter. We are now looking at the interaction of the advective adjustment with the Gulf Stream separation.

Originally published: January 1, 2005