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Observing system depiction of circulation in the SE US coastal ocean. H. Seim, University of North Carolina at Chapel Hill L Leonard, University of North Carolina at Wilmington M. Fletcher, University of South Carolina D. Savidge, Skidaway Institute of Oceanography

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observing system depiction of circulation in the se us coastal ocean

Observing system depiction of circulation in the SE US coastal ocean

H. Seim, University of North Carolina at Chapel Hill

L Leonard, University of North Carolina at Wilmington

M. Fletcher, University of South Carolina

D. Savidge, Skidaway Institute of Oceanography

C. Edwards, University of North Carolina at Chapel Hill

why a circulation climatology
Why a circulation climatology?

In general:

  • Simple characterization of existing data
  • Important source of validation for models
  • Motivate archival scheme

For the SE United States coastline:

  • Confirm existing depictions and develop digital form
  • Examine adequacy of observing system design
  • Study the dynamics of the flow field
slide4

Depiction of Seasonal Cycle by Lee, Yoder and Atkinson (1991),

Based on big DOE-funded deployments in ‘70s and ‘80s

Only variability

Winter/Spring

Summer

Fall

Distinguishes 3 shelf regimes, inner (<20 m), middle (20-40 m) and outer (>40m),

And the Gulf Stream. Cartoon depicts Gulf Stream, outer and mid shelf.

No mean flow presentation

slide5

Blanton et al. 2004 – digital model climatology, forced by mass field and

climatological winds (COADS) – inner shelf regime hard to distinguish, no

Onslow Bay

slide6

Observing System measurement locations (for SABSOON, Caro-COOPs,

CORMP, NCCOOS and NDBC)

19 stations occupied between 2000-2007, inner and mid-shelf

Area under study

In this talk

what s new
What’s new?
  • Bight-wide coverage over 5+ years
  • Better vertical resolution of currents
  • Inclusion of nearshore (10m or less)
  • Not so good:
    • No observations seaward of 40m isobath
    • Widely disparate moorings and data management systems
slide8

Coverage over time in the ‘climatology’ for ADCPs– only months with

50% or greater coverage are included

slide9

Cape

Fear

0.005 N/m2

Depth-averaged mean currents

and average winds

  • Weak mean flow (5 cm/s or less)
    • inshore of 30 m isobath, divergent
  • GS-influenced poleward flow seaward
    • of 40 m isobath
  • Near-zero flow S off SC
  • Topographic steering – flow largely
    • along isobaths
  • Mean winds are weak and variable

50m

15m

slide10

MAB depth-averaged mean current – equatorward and relatively uniform

Lentz, JGR, 2008

seasonal depiction consider
Seasonal depiction – consider:
  • winds
  • Limited temperature/salinity time series
  • Depth-averaged currents
  • Depth-varying currents
slide12

0.03 N/m2

Wintertime

Fairly uniform SE wind stress

Dominated by cold-air outbreaks

slide13

Wintertime

20 cm/s

Depth-averaged flow

  • Similar to mean
  • Reasonable comparison to model
slide14

Feb bottom

temp

Feb surf

temp

Blanton

climatology

Temp (deg C)

slide15

Depth (m)

Depth-resolved flow

- February

  • Generally little vertical structure
  • Exception at nearshore stations
slide16

Summer

Bermuda-high dominated

Northward wind stress

slide17

Summer

SC

Depth-averaged flow

Whole shelf in motion to NE

Minimum flow off SC – signature of gyre?

Model underestimates inner shelf flow

slide18

Jul bottom

temp

Jul surf

temp

Blanton

climatology

Temp (deg C)

slide19

20 cm/s

SC

Depth-resolved flow

- July

Depth (m)

  • Significant veering
  • Consistent with upwelling
  • Should promote nutrient delivery from GS
  • Exception at shallow stations off SC
slide20

Fall

Strong southward wind stress

Strength increases seaward

slide21

Fall

SC

Depth-averaged flow

GA

Reduced flow at 40 m isobath

Southward flow on middle, inner shelf

Minima off SC again

Schematic captures flow well

Model misrepresents inner, middle shelf

slide23

20 cm/s

Depth-resolved flow

- December

Depth (m)

Confused flow, strongly divergent

Veering mostly in opposite sense

Offshore bottom flow – convection?

slide24

Blaha, JGR ’84 found coherent monthly averagedsea level variationsover SAB (’55-’75 period, heatingand atmos. presseffects removed).Can be more than 20 cm variation annually. Postulated due toGulf Stream transportvariations.

noble gelfenbaum modeled coastal sl impact of gs transport variations
Noble/Gelfenbaum – modeled coastal SL impact of GS transport variations.

Low transport

Gulf Stream

Average transport

Coast

Offshore

Fixed “Hinge”

Low transport,

higher CSL

Shelf

Gulf Stream

Average transport

Coast

High transport

Offshore

Fixed “Hinge”

High transport,

lower CSL

Shelf

role of charleston bump
Role of Charleston Bump?
  • Does turn of GS at the Bump change the surface elevation on the shelf?
  • Could explain the slowdown/reversal in alongshelf flow off SC
summary
Summary
  • Assembled ADCP observations largely confirm qualitative depiction of Lee et al (1991) – reduced flow off SC consistent with gyre influence but gyre not represented in observations.
  • Digital climatology of Blanton et al (2004) fails to represent inner shelf and equatorward mid-shelf flows
  • Strong upwelling circulation in summer is evident
  • Downwelling circulation present in fall/winter/spring but not shelf-wide
  • Plan to continue assembly of currents and winds, temperature and salinity measurements
slide28

MONTHLY MEAN ALONG- AND CROSS-SHORE CURRENT

Climatological along-shore monthly mean wind (scaled 1cm/s:1m/s)

At Station

Off GA

Depth (m above bottom)

Depth (m above bottom)

SSW

NNE

On-shore

CROSS

Off-shore

ALONG