observing system depiction of circulation in the se us coastal ocean
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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