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

Hare et al., 2007 A specific interest – design of Marine Protected Areas – are they connected?

Winter/SpringSummer 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 ( 40m), and the Gulf Stream. Cartoon depicts Gulf Stream, outer and mid shelf. No mean flow presentation Only variability

Blanton et al – 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 , 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

Wintertime Fairly uniform SE wind stress Dominated by cold-air outbreaks 0.03 N/m 2

Wintertime Similar to mean Reasonable comparison to model 20 cm/s Depth-averaged flow Mean position of GS 20m 400m 40m

Feb surf temp Feb bottom temp Blanton climatology Blanton bottom temp clim.

Feb surf salinity Feb bot salinity Blanton climatology Blanton surface salinity clim

Depth (m) Depth-resolved flow - February Generally little vertical structure Exception at nearshore stations

Summer Bermuda-high dominated Northward wind stress

Summer Whole shelf in motion to NE Minimum flow off SC – signature of gyre? Model underestimates inner shelf flow SC Depth-averaged flow

Jul surf temp Jul bottom temp Blanton climatology Blanton bot temp clim

July surf salinity July bot salinity Blanton Surf salinity Blanton climatology

Depth-resolved flow - July 20 cm/s Depth (m) Significant vertical shear/veering Consistent with upwelling Should promote nutrient delivery from GS SC

Fall Strong southward wind stress Strength increases seaward

Fall 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 SC Depth-averaged flow GA

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 20 cm/s Depth (m) Flow strongest on inner shelf Weak offshore bottom flow

15m 50m 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 Cape Fear Depth-averaged mean currents and average winds N/m 2

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-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?

SSW Climatological along-shore monthly mean wind (scaled 1cm/s:1m/s) MONTHLY MEAN ALONG- AND CROSS-SHORE CURRENT NNEOn- shore Off- shore ALONG CROSS Depth (m above bottom ) At Station Off GA

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. 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