Baroclinic Tides off the SE US Coast

H. Seim, C. Edwards, T. Shay – UNC Chapel HillC. Werner – now at Rutgers University

• Some background

• Shelf observations

• SAB – generation site considerations

Munk&Wunsch,DSR, 1998

Estimate of kinetic energy dissipated from M2 tide

Egbert&Ray, GRL, 2003

Barotropic tidal analysis reveals seasonal cycle to tide – why?

Blanton et al, 2004

M2 tide at Ft. Pulaski, GA

SC

GA

NC

FL

Study area: South Atlantic Bight – shallow (<50m), up to 100 km wide, bounded by Gulf Stream

R4

R4M2 Majoraxis

M2 Minoraxis

Repeatable seasonal variations apparent in tidal current analysis

m/s m/s

40

R2 R6

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

0.4 0.36

25 30

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).

1st mode=barotropicdynamics

Apr-Jul05Jul-Nov05Dec-Feb06Feb-Jun06

Internal current variability –different vertical structure at diurnal and semi-diurnal frequencies

Diurnalvariance

Semi-diurnalvariance

Seasonal SD baroclinic variance (m2/s2)

40 m isobath

32misobath

25 mIsobath(why larger?)

R8

R4

Near shelfbreak magnitudes vary on weekly time scale

m/s

More obvious seasonal variability at mid-shelf

Glider-obs at R4 - SD pycnocline motion - 2-5 m amplitude. More nonlinear at max tide range. N2 profile suggests 40 cm/s speedphase speed.

m/s

Depth-averaged current

Temp (C)

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

Extract energy from barotropictide where flow pushes stratifiedfluid over topography

Vlasenko et al.,2005

Amplitude of response dependson:Height and width of obstacleStratification strength and depthFrequency of forcing

Tidal ellipses for the SAB – good potential for generation at shelfbreak

Blanton et al., 2004

SC

GA

NC

FL

Presence of the Gulf Stream:

Impacts density structureover shelfbreak

Modifies the effectiveCoriolis parameter

Together with varyingtopography, may leadto large changes in generated internal tide

Wintertime chlorophyll image

Nelson, SkIO

Changes in stratification overslope can focus or defocus waveenergy

Critical angle - when the angle ofwave propagation matchesthe bottom slope can getenhanced responseand wave beam typestructure.

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

Mean temperature structure – shoals over the slope

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

Slope angle

Waveangle

Cross-shore velocity variance – see some maxima near areas of criticalslope – generation sites?

Measuring the mass field

Pycnocline position varies at the semi-diurnal; current structure dominated by the diurnal

Glider-measured density near R4

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

Across to shelfbreak (R4)

Bandpassed sum of higher modes -

u

v

w