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HB-1-5

Lateral exchange of heat and freshwater between the western boundaryand the North Atlantic Current

PhD student :Tilia Breckenfelder
Project supervisors :M. Rhein (Germany)
P. Myers (Canada)
Key hypothesis
The lateral flux of freshwater and heat between the Western Boundary Current and the North Atlantic Current needs to be taken into account for the heat and freshwater flow in the subpolar gyre.
 
The subpolar North Atlantic is one of the key climate relevant regions of the ocean. Here, the deep water masses of the cold limb of the Atlantic Meridional Overturning Circulation (AMOC) are formed or significantly modified. An important element of this circulation is the Western Boundary Current (WBC), hugging the Canadian continental slope, and transporting cold and fresh water masses to the south. In the northward flowing upper limb of the AMOC warm and salty water of tropical/subtropical origin is imported into the subpolar gyre in the North Atlantic Current (NAC). There are several NAC pathways in the western part of the gyre, but eventually the NAC crosses the Mid Atlantic Ridge. Some of this water flows into the Nordic Seas, but the major part turns westward, forming the anticlockwise subpolar gyre. Near 47°N, both currents, the WBC and the NAC are swift and narrow and in close proximity due to the relatively steep topography of the continental shelf. This leads to strong lateral exchanges not only in the surface waters, but throughout the water column, caused by meandering of both currents. The meandering of the WBC is expressed in the variance maximum at about 12-15 days in the cross-stream velocity. The circulation in the interior of the basin has up to now been studied with shipboard data. Our LADCP measurements (6 repeats) at 47°N reveal a very strong inflow of the NAC close to the WBC followed by a recirculation pathway further east. After cold and fresh water from the Canadian shelf and newly formed Labrador Sea Water leave the boundary current through lateral exchange with the NAC, they are transferred into the interior of the Newfoundland basin. The water either remains in the subpolar gyre and continues to the northeast, or is exported in the NAC recirculation to the south. How much heat and freshwater is exchanged and how the exchange is related to the strength of the currents and their variability is not known. Under a climate warming scenario, we expect long-term changes in the circulation and a weakening of the currents. Whether this has an influence on the lateral heat and freshwater fluxes from the WBC into the basin’s interior is an open question. The Bremen Oceanography group is familiar with the research area and study deep water formation rates, circulation, transport variability and spreading of anomalies as well as the strength of diapycnal mixing in the interior and the boundary current. The interpretation encompasses own shipboard observations and moorings as well as altimetry and other remote sensing data, data from the Argo program and model results. Paul Myers (CA) work in the area has included study of the historical variability of the hydrography, circulation and transport of the boundary currents, property and water formation changes in the interior as well as issues of modeling and model drift . The modeling has been carried out in a framework based on the NEMO model.
The objectives are to quantify the cross-frontal exchange of volume, heat and freshwater between the Western Boundary Current and the North Atlantic Current and follow the circulation of the anomalies in the subpolar gyre, and study the role of the involved processes (tides, meanders, velocity fluctuations on different time scales). T/S and velocity time series from WBC moorings and time series from moored PIES at 47°N together with shipboard hydrographic observations (including CFC and SF6 distributions) will be combined with Argo data and altimetry to study the cross - frontal exchange of volume, heat and freshwater. Argo data and time series from PIES at the Mid-Atlantic Ridge as well as hydrographic and tracer data will be used to determine the circulation of these anomalies in the subpolar gyre, and especially study the role of the NAC. The results will be compared with the output of a high resolution numerical model. The project is closely linked with projects HB-6 and -7 and CA-6 and -7.