Towards a climatology of 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, Florida State University
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
Depiction of Seasonal Cycle by Lee, Yoder and Atkinson (1991),
Based on big DOE-funded deployments in ‘70s and ‘80s
Winter/Spring
Summer
Only variability
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
Blanton et al. 2004 – digital model climatology, forced by mass field and
climatological winds (COADS) – inner shelf regime hard to distinguish,
limited northern extent
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
- Disparate moorings and data management
systems
Coverage over time in the ‘climatology’ for ADCPs– only months with
50% or greater coverage are included
Seasonal depiction – consider:
•
•
•
•
Winds
Limited temperature/salinity time series
Depth-averaged currents
Depth-varying currents
0.03 N/m2
Wintertime
Fairly uniform SE wind stress
Dominated by cold-air outbreaks
Wintertime
20 cm/s
40m
400m
Depth-averaged flow
•Similar to mean
•Reasonable comparison to model
20m
Mean position of GS
Feb bottom
temp
Feb surf
temp
Blanton
climatology
Blanton
bottom
temp
clim.
Feb surf
salinity
Feb bot
salinity
Blanton
surface
salinity
clim
Blanton
climatology
•Generally little vertical structure
•Exception at nearshore stations
Depth (m)
Depth-resolved flow
- February
Summer
Bermuda-high dominated
Northward wind stress
Summer
SC
Depth-averaged flow
Whole shelf in motion to NE
Minimum flow off SC – signature of
gyre?
Model underestimates inner shelf flow
Jul bottom
temp
Jul surf
temp
Blanton
climatology
Blanton
bot
temp
clim
July surf
salinity
July bot
salinity
Blanton
Surf
salinity
Blanton
climatology
20 cm/s
Depth-resolved flow
- July
•Significant vertical shear/veering
•Consistent with upwelling
•Should promote nutrient delivery from GS
Depth (m)
SC
Fall
Strong southward wind stress
Strength increases seaward
Fall
SC
GA
Depth-averaged flow
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
Oct surf
temp
Oct bot
temp
Blanton
climatology
Blanton
Bot
Temp
climatology
Oct surf
salinity
Oct bot
salinity
Blanton
Surf
Salinity
clim
Blanton
climatology
Depth-resolved flow
- October
Flow strongest on inner shelf
Weak offshore bottom flow
Depth (m)
20 cm/s
Cape
Fear
0.005 N/m2
Depth-averaged mean currents
and average winds
50m
15m
•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
MAB depth-averaged mean current – equatorward and relatively uniform
Lentz, JGR, 2008
Some notion of dynamics:
Wind stress weak – but curl?
Alongshore pressure gradient important but possibly non-constant
Cross-shelf baroclinic gradient - working on it.
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)
• 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
• Reduced mean flow off SC consistent with gyre
influence but gyre not represented in
observations. Other form of GS influence?
MONTHLY MEAN ALONG- AND CROSS-SHORE CURRENT
Depth (m above bottom)
Depth (m above bottom)
Climatological along-shore monthly
mean wind (scaled 1cm/s:1m/s)
At Station
Off GA
SSW
ALONG
NNE
Onshore
CROSS
Offshore
Baringer/Larsen
Blaha, JGR ’84
found coherent
monthly averaged
sea level variations
along shelf (’55-’75
period, heating
and atmos. press
effects removed).
Can be more than 20
cm variation annually.
Postulated due to
Gulf Stream transport
variations.
Noble/Gelfenbaum – modeled coastal SL
impact of GS transport variations.
Low transport
Average transport
Gulf Stream
Coast
Low transport,
higher CSL
Offshore
Fixed “Hinge”
Shelf
Gulf Stream
Average transport
Coast
Offshore
Fixed “Hinge”
High transport
High transport,
lower CSL
Shelf