Internal tides Seim, Edwards, Shay, Werner Some background SAB – generation site? Shelf...

Internal tidesSeim, Edwards, Shay, Werner

• Some background

• SAB – generation site?

• Shelf observations

• Towards a climatology?

Munk&Wunsch,DSR, 1998

Estimate of kinetic energy dissipated from M2 tide

Egbert&Ray, GRL, 2003

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

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

Tidal beam on the CA slope andassociated mixing

Waves can be greatly modified after generation via nonlinearities, transform intoWave packets (aka solitons or solibores)

SC

GA

NC

FL

Wilmington

Jackson-ville

Savannah

Charleston

Our study area – SAB

Big complicatio

n – Gulf Stream

Changes in stratification overslope can focus or defocus waveenergy

Other fun piece – Brian Blanton’s work – annual variation in tidal amplitude – why?

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?

SC

GA

NC

FL

Onshore side – have several years of ADCP(current meter) data – any sign on internal tide?

Challenge – tides dominatevelocity variance and shelfso shallow that boundarylayer influences structurethroughout – how to isolateinternal waves?

Describing internal structure in shallow water

Goal: To remove the barotropic tide, including its associated vertical structure due to bottom friction

Method: EOF analysis

Isolates internal structure within energetic barotropic dynamics

Done by: Catherine Edwards

Non-mode0 bandpassed data – lots of info…

Apr-Jul05Jul-Nov05Dec-Feb06Feb-Jun06

Internal current variability – dominated by diurnal (near inertial) but has different structure than semi-diurnal currents (bottom-trapped?)

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)

Towards a summary – time and cross –shore – mode 1

And mode2

In conclusion…

Internal tides – exist in the SAB but not as highly NL form?

Generation – likely on slope but quite variable

Shelf obs – can isolate internal structure, beginning to look at variations x-shelf and with time.