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Baroclinic Tides off the SE US Coast H. Seim, C. Edwards, T. Shay – UNC Chapel Hill C. Werner – now at Rutgers University Some background Shelf observations.

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Presentation on theme: "Baroclinic Tides off the SE US Coast H. Seim, C. Edwards, T. Shay – UNC Chapel Hill C. Werner – now at Rutgers University Some background Shelf observations."— Presentation transcript:

1 Baroclinic Tides off the SE US Coast H. Seim, C. Edwards, T. Shay – UNC Chapel Hill C. Werner – now at Rutgers University Some background Shelf observations SAB – generation site considerations

2 Munk&Wunsch,DSR, 1998

3 Estimate of kinetic energy dissipated from M 2 tide Egbert&Ray, GRL, 2003

4 Barotropic tidal analysis reveals seasonal cycle to tide – why? Blanton et al, 2004 M 2 tide at Ft. Pulaski, GA

5 SC GA NC FL Study area: South Atlantic Bight – shallow (<50m), up to 100 km wide, bounded by Gulf Stream R4

6 M2 Major axis M2 Minor axis Repeatable seasonal variations apparent in tidal current analysis m/s 40

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8 R2R6 Stations further from shelfbreak show less variability in vertical structure – but analysis captures only phase-locked component of the tide

9 To examine baroclinic tide in shallow water, need to remove the barotropic tide, including its associated vertical structure due to bottom friction. Use EOF analysis to define vertical modes. Associate first mode with barotropic dynamics, sum of higher modes with baroclinic (Edwards and Seim, 2008). 1 st mode=barotropic dynamics

10 Apr-Jul05 Jul-Nov05 Dec-Feb06 Feb-Jun06 Internal current variability –different vertical structure at diurnal and semi-diurnal frequencies Diurnal variance Semi-diurnal variance

11 Seasonal SD baroclinic variance (m 2 /s 2 ) 40 m isobath 32m isobath 25 m Isobath (why larger?)

12 R8 R4 Near shelfbreak magnitudes vary on weekly time scale m/s

13 More obvious seasonal variability at mid-shelf

14 Glider-obs at R4 - SD pycnocline motion m amplitude. More nonlinear at max tide range. N 2 profile suggests 40 cm/s speed phase speed. m/s Depth-averaged current Temp (C)

15 Observations summary Semi-diurnal tidal current varies seasonally Baroclinic tide (defined via EOF) of 5-10 cm/s apparent on mid-outer shelf Vertical structure dominated by mode 1 Magnitude varies on weekly time scales on outer shelf, on seasonal time scales at mid-shelf Consistent with offshore generation and onshore propagation of an internal tide

16 Extract energy from barotropic tide where flow pushes stratified fluid over topography Vlasenko et al.,2005 Amplitude of response depends on: Height and width of obstacle Stratification strength and depth Frequency of forcing

17 Tidal ellipses for the SAB – good potential for generation at shelfbreak Blanton et al., 2004

18 SC GA NC FL Presence of the Gulf Stream: Impacts density structure over shelfbreak Modifies the effective Coriolis parameter Together with varying topography, may lead to large changes in generated internal tide

19 Wintertime chlorophyll image Nelson, SkIO

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21 Changes in stratification over slope can focus or defocus wave energy

22 Critical angle - when the angle of wave propagation matches the bottom slope can get enhanced response and wave beam type structure.

23 Quoddy model runs, nested in basin scale model (Alfredo/Tom/Cisco)

24 Mean temperature structure – shoals over the slope

25 Critical angle – how best assessed? Looks like a match in places Slope angle Wave angle

26 Cross-shore velocity variance – see some maxima near areas of critical slope – generation sites?

27 Measuring the mass field Pycnocline position varies at the semi-diurnal; current structure dominated by the diurnal Glider-measured density near R4

28 Can begin to look at cross-shore variability…from inshore (GR)

29 Across to shelfbreak (R4)

30 Bandpassed sum of higher modes - u v w


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