Towards a climatology of the SE US coastal ocean
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Transcript of Towards a climatology of the SE US coastal ocean
Towards a climatology of 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, 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
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, 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 with50% or greater coverage are included
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
Mean position of GS 20m
400m
40m
Feb surftemp
Feb bottomtemp
Blantonclimatology
Blantonbottomtempclim.
Feb surfsalinity
Feb botsalinity
Blantonclimatology
Blantonsurfacesalinityclim
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
Blantonbottempclim
July surfsalinity
July botsalinity
BlantonSurfsalinity
Blantonclimatology
Depth-resolved flow- July
20 cm/s
Dep
th (
m)
•Significant vertical shear/veering •Consistent with upwelling•Should promote nutrient delivery from GS
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
Oct surftemp
Oct bottemp
Blantonclimatology
BlantonBotTempclimatology
Oct surfsalinity
Oct botsalinity
BlantonSurfSalinityclim
Blantonclimatology
Depth-resolved flow- October
20 cm/s
Dep
th (
m)
Flow strongest on inner shelfWeak offshore bottom flow
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
Some notion of dynamics:
Wind stress weak – but curl?Alongshore pressure gradient important but possibly non-constantCross-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?
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
Blaha, JGR ’84 found coherent monthly averagedsea level variationsalong shelf (’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