From SYNOP to AMOC: Stirring by deep cyclones and the evolution of Denmark Strait Overflow Water observed at Line W

Shipboard velocity and property data from 18 transects across the North Atlantic Deep Western Boundary Current (DWBC) near 40˚N are analyzed to study the evolution of the Denmark Strait Overflow Water (DSOW) component of the DWBC and its mixing with the interior. The transects were made between 1994 and 2014 and lie along Line W, which reaches from the continental shelf south of New England to Bermuda. Measurements comprise velocity from lowered acoustic Doppler current profilers (LADCPs), CTD profiles, and trace gas chlorofluorocarbon (CFC) concentrations from bottle samples at discrete depths at 26 regular stations or a subset of these stations. In each transect, DSOW exhibits a distinct CFC concentration maximum in the abyssal ocean (> 3000 m depth) along the sloped western boundary. Sea surface height (SSH) maps from satellite altimetry indicate that quasi-stationary meander troughs of the Gulf Stream path in the upper ocean were present at Line W during 5 of the 18 sections. For these 5 sections, the LADCP velocity sections suggest the upper ocean trough is accompanied by a large cyclone in the deep ocean in the DSOW density layer. The occurrence of deep cyclones in conjunction with Gulf Stream troughs as inferred from the LADCP sections along Line W is consistent with previous observations (from 1988 to 1990) in the region from a moored array in the Synoptic Ocean Prediction (SYNOP) experiment. The SYNOP array suggested deep cyclones are present here about 35% of the time. The composite velocity section produced from the 5 Line W transects sampling through a Gulf Stream trough suggests that a typical cyclone reaches swirl speeds of greater than 30 cm/s at 3400 m depth and has a radius (distance between the center and the maximum velocity) of ~75 km. The tracer data suggest that these cyclones affect not only the deep velocity structure along Line W, but also provide a mechanism for water exchange between the DWBC and the interior.