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
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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