**4. Gulf stream variability and frontal filaments**

In a Sea Surface Temperature (SST) NOAA GOES satellite-based study, Pietrafesa [6] reported that the Gulf Stream Current deflects offshore near 31°N, 79°W, meanders laterally thereafter, and its lateral meander variability decreases downstream of this deflection. The seaward deflection of the Gulf Stream Current was determined to be caused by the presence of a topographic irregularity, which became known as the "Charleston Bump," actually Hoyt's Hill in the geological history of the region. The study conjectured that the topographic feature changed the vertical vorticity of the Gulf Stream by shrinking the vertical water column such that the Gulf Stream had to move offshore to deeper water to preserve its angular momentum balance. After the Gulf Stream moves into deeper water, given angular momenta requirements, it reroutes itself toward the coast. The process was speculated to affect the generation of Topographic Rossby Waves (TRWs) in that locale by Rooney et al. [7] and Pietrafesa and Janowitz [3]. These waves were found to propagate to the north along the shelf breaks of SC and NC [3, 8, 9], with periods between 2 and 12 days and propagation speeds of 30–40 km/d. The study of Sun and Pietrafesa [9] also discovered that the Gulf Stream Front has an inherent 8-day baroclinic instability that is a persistent source of downstream propagating waves. John and Schott [10] staged an Eulerian

#### **Figure 4.**

*(a) Left panel depicts the Gulf Stream and its configurations in the North Atlantic Ocean basin; (b) right panel shows a Gulf Stream frontal filament in a NOAA SST mage and its conceptual flow field.*

current meter study on the FL outer shelf and determined that the Gulf Stream meandered laterally, and they concluded that these onshore-offshore motions were northward propagating waves, with dominant wavelengths of 340 km and 170 km, periods of 12 days and 5 days, and propagation speeds of 28 km/d and 36 km/d, respectively. A NOAA GOES SST image is shown in **Figure 4a**, which shows Gulf Stream variability along the Atlantic Seaboard. We note the offshore deflection off Charleston and a variety of frontal features. The horizontal crests of these laterally meandering waves can bring surface layer parcels of water onto the outer continental shelves of the coasts of NC and SC. Moreover, these waves can fold back at their crests.

Pietrafesa and Janowitz [11] and Pietrafesa [12] evaluated Eulerian current meter data off NC and FL, respectively, and provided a detailed current meter–based spatial and temporal map of a TRW meander crest that folded back, which they referred to as a Frontal Filament, wrapped around an offshore cold core eddy (**Figure 4b**). In summary, meanders and the frontal filaments and the eddies they generate serve as the principal form of mesoscale variability along the path of the Gulf Stream Current, on the outer continental shelf within the South Atlantic Bight (between Cape Canaveral, FL, and Cape Hatteras, NC). From this suite of comprehensive studies, the Gulf Stream has been shown to display many degrees of freedom (**Figure 4a**). **Figure 4b** is a beautiful SST representation of a GSF. Warm Gulf Stream water traveling in the crest of a Meander folds back onto the outer shelf and then travels southwestward into and around the filament and then turns toward the northeast and back into the Gulf Stream Front. The Cartoon in **Figure 4b** shows that pictorially.
