Observing system depiction of circulation in the SE US coastal

ocean

H. Seim, University of North Carolina at Chapel HillL Leonard, University of North Carolina at Wilmington

M. Fletcher, University of South CarolinaD. Savidge, Skidaway Institute of Oceanography

C. Edwards, University of North Carolina at Chapel Hill

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

Hare et al., 2007

A specific interest – design of Marine Protected Areas – are they connected?

Winter/Spring Summer Fall

Depiction of Seasonal Cycle by Lee, Yoder and Atkinson (1991),Based on big DOE-funded deployments in ‘70s and ‘80s

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

Only variability

Blanton et al. 2004 – digital model climatology, forced by mass field and climatological winds (COADS) – inner shelf regime hard to distinguish, noOnslow Bay

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?

• 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

Coverage over time in the ‘climatology’ for ADCPs– only months with50% or greater coverage are included

15m

50m

•Weak mean flow (5 cm/s or less)inshore of 30 m isobath, divergent

•GS-influenced poleward flow seawardof 40 m isobath

•Near-zero flow S off SC•Topographic steering – flow largely

along isobaths•Mean winds are weak and variable

CapeFear

Depth-averaged mean currentsand average winds

0.005 N/m2

Lentz, JGR, 2008

MAB depth-averaged mean current – equatorward and relatively uniform

Seasonal depiction – consider:

• winds

• Limited temperature/salinity time series

• Depth-averaged currents

• Depth-varying currents

Wintertime

Fairly uniform SE wind stressDominated by cold-air outbreaks

0.03 N/m2

Wintertime

•Similar to mean•Reasonable comparison to model

20 cm/s

Depth-averaged flow

Temp (deg C)

Feb surftemp

Feb bottomtemp

Blantonclimatology

Dep

th (

m)

Depth-resolved flow - February

•Generally little vertical structure•Exception at nearshore stations

Summer

Bermuda-high dominatedNorthward wind stress

Summer

Whole shelf in motion to NEMinimum flow off SC – signature of gyre?Model underestimates inner shelf flow

SC

Depth-averaged flow

Jul surftemp

Jul bottomtemp

Blantonclimatology

Temp (deg C)

Depth-resolved flow- July

20 cm/s

Dep

th (

m)

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

SC

Fall

Strong southward wind stressStrength increases seaward

Fall

Reduced flow at 40 m isobathSouthward flow on middle, inner shelfMinima off SC againSchematic captures flow wellModel misrepresents inner, middle shelf

SC

Depth-averaged flowGA

Depth-resolved flow- December

20 cm/s

Dep

th (

m)

Confused flow, strongly divergentVeering mostly in opposite senseOffshore bottom flow – convection?

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.

Coast

Shelf

Gulf Stream

Average transport

Low transport

Offshore Fixed “Hinge”

Coast

Shelf

Gulf Stream

High transport

Average transport

Offshore Fixed “Hinge”

Low transport,higher CSL

High transport,lower CSL

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

• 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

SSW

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

MONTHLY MEAN ALONG- AND CROSS-SHORE CURRENT

NNE On-shore

Off-shore

ALONG CROSS

Dep

th (

m a

bo

ve b

ott

om

)

Dep

th (

m a

bo

ve b

ott

om

)

At StationOff GA

Baringer/Larsen

40 cm/s

Jan

40 cm/s

Feb

40 cm/s

Mar

40 cm/s

Apr

40 cm/s

May

40 cm/s

Jun

40 cm/s

Jul

40 cm/s

Aug

40 cm/s

Sep

40 cm/s

Oct

40 cm/s

Nov

40 cm/s

Dec