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Shelf and slope circulation inshore of the Charleston Bump Shelf and slope circulation inshore of the Charleston Bump

Shelf and slope circulation inshore of the Charleston Bump - PowerPoint Presentation

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Shelf and slope circulation inshore of the Charleston Bump - PPT Presentation

H Seim W Stark UNC Chapel Hill C Edwards Skidaway Institute Of Oceanography Ryan and Yoder 1996 found maximum wintertime Chl off Long Bay outside frontal eddy decay regions What supports the productivity in this area ID: 559604

shelfbreak shelf upper slope shelf shelfbreak slope upper water filaments flow warm strong offshore response bottom deflection filament onshore

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Slide1

Shelf and slope circulation inshore of the Charleston Bump

H. Seim, W. Stark, UNC Chapel HillC. Edwards, Skidaway Institute Of OceanographySlide2

Ryan and Yoder (1996) found maximum wintertime Chl

offLong Bay, outside frontal eddy decay regions. What supports the productivity in this area?

Ryan and Yoder (1996)

MODIS imagery (from J. Nelson, 2010)Slide3

Nutrient Input

Possible mechanisms:Gyre/Warm Filaments – strong circulation over upper slope - onshore component?Wind-driven exchange – Ekman layer deepening and transport? (but how is this unique to Long Bay)Slumping-driven exchange – dense water formation on the shelf

Internal tide-driven mixing – does absence of Gulf Stream on upper slope enhance this process?Slide4

Strong deflection

Weak deflection

Bane and Dewar, 1988

Gulf Stream deflection – based on position relative to 600

m

isobathSlide5

Warm filaments – onshore component of frontal eddies, may promote cross-shelf exchange in eddy decay regions (not in Long Bay).

Grey lines: 100

m and 600 mDashed black: mean Gulf StreamShoreward front position

(Miller, 1993) Slide6

Filament example

Feb 3

Feb 8

Feb 9

Seen along the

shelfbreak

(75

m

)

Saw rapid SW progression

o

f some detached filaments Slide7

2012 field season

GS deflection

weak

strong

Filament strength

none

Moving SW

stationary

Moving NESlide8

Field program – (January – March 2012)

- mooring, glider and shipboard sampling- will examine mooring observations (barotropictide removed from

currents)- nutrient concentrationsforthcoming but generally

high N=low Temp

Cross-shelf array:

LB1 – 30

m

LB2 – 75

m

LB3 – 175

m

Along

AcrossSlide9

Variance ellipses

– full and detided

- relatively strong cross-shore tidal currents on shelf, weaken offshore- use detided

ellipse to define alongshore orientation

-

shelfbreak

mooring orientation different, still unclear of origin

latitude

longitudeSlide10

Strong filaments – in gray – brings:

- 0.4 m/s equatorward flow over slope and shelfbreak- warm bottom temperatures at these sites

- limited velocity signal on 30m isobath, associated with initial arrival- limited correlation of currents across the shelf

- suggests filaments flood

shelfbreak

with warm (low nutrient?) water

LB1 – 30

m

LB2 – 75

m

LB3 – 175

m

Vel

Along – depth-

avg

Bot. temp.

filamentsSlide11

Wind forcing and cross-shelf flow

– different response across the shelf.- at mid-shelf, due to varying stratification (well-mixed or weakly stratified)- at shelfbreak

and over slope, filament response superimposed-

Height above bottom (

m

)Slide12

Mean velocity profiles

– referenced to sea surface- across-shelf: upper 60 m

- offshore over upper slope, onshore at shelfbreak, filament response; bottom offshore flow at shelfbreak

-

along-shelf

: upper 60

m

– positive vertical shear, though

equatorward

offshore,

poleward

over shelf

shelf

shelfbreak

Upper slope

Onshore

poleward

Offshore

equatorwardSlide13

Glider density section – typical of conditions observed over winter - dense water on inner shelf, decreases moving horizontally offshore

- consistent with mean vertical shear of alongshore current (thermal wind shear of 0.002 1/s)- may explain pronounced offshore bottom flow at shelfbreakSlide14

Summary

No obvious onshore flow of deep upper slope waterFilaments flood upper slope and shelfbreak with warm water, dominant during observation periodWind response complicated by shallowness of water but no clear transport mechanismInner shelf dense water formation and slumping appears to be persistent (even during very anomalous winter) – but not unique to Long Bay

Internal tide/inertial response – see Edwards posterThanks to crew of the R/V Savannah for field effort, Sara Haines, Julie Amft

, Trent Moore,

Chris Calloway

f

or initial processing and analysis and

Dongsik

Chang and

Klimka

Szwaykowska

for help piloting the gliders