Gulf Stream (GS) meander structure and propagation offshore of Cape Hatteras are investigated by integrating current measurements from a bottom-moored Acoustic Doppler Current Profiler (ADCP) with high-frequency radar (HFR) surface current measurements and satellite Sea Surface Temperature (SST) images during November 2014. The ADCP measurements provide well-resolved current observations throughout most of the water column, while hourly surface current measurements from HF radars and available satellite SST images provide spatial context to the GS orientation, meander propagation, circulation, and shear structure in the region of the ADCP mooring. The observations provide new insights about meander propagation and evolution in this important transition region. ADCP measurements observed that the increase and deepening intervals of the downstream current with approaching meander crests were typically longer than those for the decrease and shoaling of downstream current, consistent with prominent skewed crests near the surface. The transition time from trough to crest is much greater than that from crest to trough, reflecting the asymmetry in the downstream velocity structure. Vertical shears in the downstream and cross-stream velocity components are indicative of a cold dome centered downslope and offshore of the ADCP. Local maxima in downstream current and bottom temperature at the ADCP occur simultaneously, are accompanied by large vertical velocities, and are led by offshore currents in the upper water column. The mean meander phase speed estimated with HFRs is 48 km/day. Meander periods during the month are about 5-6 days. Where the maxima are seen in the water column, downstream currents reach 2.5 m/s, with current reversals sometimes in excess of 0.5 m/s. Downstream currents occupy an increasing portion of the water column as a crest approaches, and a decreasing fraction as a trough approaches. The deepening increase in downstream velocities with approaching crests is often accompanied by an increase in upstream velocities near the bottom.
A method to extract characteristics of the Gulf Stream (GS) surface flow field using High-frequency radar (HFR) derived currents is described. Radial velocity measurements, from radar installations near Cape Hatteras, NC serve as input, chosen because of the greater spatial and temporal coverage provided compared to total velocity fields. The landward GS edge, jet axis, orientation, and Cyclonic Shear Zone (CSZ) width are identified along bearings within the radar footprint. The method is applied to observations from two radar installations from November 2014 and provides GS estimates with daily temporal resolution. Results along eight bearings provide a consistent representation of GS variability dominated by the passage of meanders. Average distance to the GS edge along bearings varies from 50-100 km; distance estimate quality degrades with range from the radars. Monthly mean GS jet axis locations from satellite Sea Surface Height (SSH) and the algorithm are consistent. Cross-correlations between estimates of GS characteristics in the same region vary from 0.37-0.73 for the GS edge. Estimates of radar distance to the GS edge are negatively correlated with current velocity measurements nearest the surface from a moored 150 kHz Acoustic Doppler Current Profiler and vary between −0.58 and − 0.71. GS CSZ width metrics range from mean values of 29 to 31 km. Daily GS orientation estimates are affected by the crossing angle of the radial bearing relative to the GS. Lags from the cross-correlations of monthly mean properties suggest meander propagation speed estimates increase from 43.2 km day−1 south of the cape, to 136.8 km day−1 just east of it.
Multi-year measurements of current velocity, salinity, and temperature from fixed and vessel-mounted sensors quantify Gulf Stream (GS) marine hydrokinetic energy (MHK) resource variability and inform development off Cape Hatteras, NC. Vessel transects across the GS demonstrate a jet-like velocity structure with speeds exceeding 2.5 m/s at the surface, persistent horizontal shear throughout the jet, and strongest vertical shears within the cyclonic shear zone. Persistent equatorward flow at the base of the GS associated with the Deep Western Boundary Current (DWBC) produces a local maximum in vertical shear where stratification is weak and is postulated to be a site of strong turbulent mixing. Repeated transects at the same location demonstrate that the velocity structure depends upon whether the GS abuts the shelf slope or is offshore.Currents from a fixed acoustic Doppler current profiler (ADCP) deployed on the shoreward side of the GS exceed 1 m/s 64% of the time 40 m below the surface. The 3.75-year time series of currents from the ADCP mooring document large, roughly weekly variations in downstream and cross-stream speed (−0.5 to 2.5 m/s) and shear (± 0.05 s−1) over the entire water column due to passage of GS meanders and frontal eddies. Current reversals from the mean GS direction occur several times a month, and longer period variations in GS offshore position can result in reduced currents for weeks at a time. Unresolved small-scale shear is postulated to contribute significantly to turbulent mixing.
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