Shelf and slope circulation inshore of the Charleston Bump H. Seim, W. Stark, UNC Chapel Hill C. Edwards, Skidaway Institute Of Oceanography

Ryan and Yoder (1996) found maximum wintertime Chl off Long Bay, outside frontal eddy decay regions. What supports the productivity in this area? Ryan and Yoder (1996) MODIS imagery (from J. Nelson, 2010)

Nutrient Input Possible mechanisms: – Gyre/Warm Filaments – strong circulation over upper slope - onshore component? – Wind-driven exchange – Ekman layer deepening and transport? (but how is this unique to Long Bay) – Slumping-driven exchange – dense water formation on the shelf – Internal tide-driven mixing – does absence of Gulf Stream on upper slope enhance this process?

Strong deflection Weak deflection Bane and Dewar, 1988 Gulf Stream deflection – based on position relative to 600 m isobath

Warm filaments – onshore component of frontal eddies, may promote cross-shelf exchange in eddy decay regions (not in Long Bay). Grey lines: 100 m and 600 m Dashed black: mean Gulf Stream Shoreward front position (Miller, 1993)

Filament example Feb 3 Feb 8 Feb 9 Seen along the shelfbreak (75 m) Saw rapid SW progression of some detached filaments

2012 field season GS deflection weak strong Filament strength none Moving SW stationary Moving NE

Field program – (January – March 2012) - mooring, glider and shipboard sampling - will examine mooring observations (barotropic tide removed from currents) - nutrient concentrations forthcoming but generally high N=low Temp Cross-shelf array: LB1 – 30 m LB2 – 75 m LB3 – 175 m Along Across

Variance ellipses – full and detided - relatively strong cross-shore tidal currents on shelf, weaken offshore - use detided ellipse to define alongshore orientation - shelfbreak mooring orientation different, still unclear of origin latitude longitude

Strong filaments – in gray – brings: m/s equatorward flow over slope and shelfbreak - warm bottom temperatures at these sites - limited velocity signal on 30m isobath, associated with initial arrival - limited correlation of currents across the shelf - suggests filaments flood shelfbreak with warm (low nutrient?) water LB1 – 30 m LB2 – 75 m LB3 – 175 m Vel Along – depth-avg Bot. temp. filaments

Wind forcing and cross-shelf flow – different response across the shelf. - at mid-shelf, due to varying stratification (well-mixed or weakly stratified) - at shelfbreak and over slope, filament response superimposed - Height above bottom (m)

Mean velocity profiles – referenced to sea surface - across-shelf: upper 60 m - offshore over upper slope, onshore at shelfbreak, filament response; bottom offshore flow at shelfbreak - along-shelf: upper 60 m – positive vertical shear, though equatorward offshore, poleward over shelf shelf shelfbreak Upper slope Onshore poleward Offshore equatorward

Glider density section – typical of conditions observed over winter - dense water on inner shelf, decreases moving horizontally offshore - consistent with mean vertical shear of alongshore current (thermal wind shear of /s) - may explain pronounced offshore bottom flow at shelfbreak

Summary No obvious onshore flow of deep upper slope water Filaments flood upper slope and shelfbreak with warm water, dominant during observation period Wind response complicated by shallowness of water but no clear transport mechanism Inner shelf dense water formation and slumping appears to be persistent (even during very anomalous winter) – but not unique to Long Bay Internal tide/inertial response – see Edwards poster Thanks to crew of the R/V Savannah for field effort, Sara Haines, Julie Amft, Trent Moore, Chris Calloway for initial processing and analysis and Dongsik Chang and Klimka Szwaykowska for help piloting the gliders