The Gulf Stream near 73°W: views from observations and a primitive-equation model

A model/data comparison was made between the data from two Gulf Stream programs, the UNC/URI Gulf Stream Dynamics Experiment (April 1983–May 1985) and the URI Pegasus Program (September 1980–May 1983), and one year of numerical output from the NRL Limited-Area, North Atlantic Model. The purpose of this study was to examine the realism of the modeled Gulf Stream, as well as to study the Gulf Stream near 73°W. The data included one year of moored current meter measurements from 20 instruments, two years of IES (inverted echo sounder) measurements at 20 sites, and 17 repeated cross-stream profiles of velocity and temperature. Spectral properties, coherence spectra between the Gulf Stream lateral displacement and deep flow, and the Gulf Streamʹs transport, width and maximum speed have been compared between the model and observations. It was found that the Gulf Stream variability near 73°W is much weaker at timescales of about 10–30 days in the model than in the observations. Both the observations and the model showed that deep flow near 73°W is dominated by topographic Rossby waves with scales ranging from 10 to a few tens of days. Weekly-scale meanders were observed in both the model and the data, and the model shows that these meanders are developed along the inshore (cyclonic) flank of the Gulf Stream. A strong coupling is present between the Gulf Stream meandering and underlying deep flow in the model, but not in the observations, suggesting an over-estimated coupling between the Gulf Stream and deep flow in the model. It was also found that the model Gulf Stream is stronger, wider and faster than that observed. Model deficiencies such as the constant boundary forcing, the two-layer vertical structure and the limited model domain are suggested to have contributed to the differences found between the model and the data.