The South Atlantic Bight

Cape Hatteras

Cape Canaveral

Inner Shelf (0-20m)

Middle Shelf (20-40m)

Outer Shelf (40-70m)

Lee et al. (1991)

(Lee et al., 1991)

Upwelling on the SAB

Upwelling on the SAB is primarily in response to eddies and meanders in the Gulf Stream Front.

– The distance waters penetrate across the shelf depends on wind velocity, local topography and the density of resident shelf waters (Pietrafesa et al., 1985; Lee et al., 1985; Atkinson et al., 1985).

– There are two types of upwelling on the SAB

• Type 1: During all seasons eddies and meanders associated with the Gulf Stream front cause upwelling at the shelf break with periods of 2-14 days.

• Type 2: When the shelf is thermally stratified in May-October, upwelled waters penetrate across the shelf as subsurface intrusions of cold, nutrient-rich water.

(Yoder, 1985)

(Signorini & McClain, 2006)

Seasonal surface currents from mean sum geostrophic components and Ekman.

Red vectors indicate the general direction of mean winds for each season.

Black vectors indicate mean surface current speed and direction (5-10 cm/s)

Winter

FallSummer

Spring

fT y

x

fT x

y

2

2H

kT y

y

2

2H

kT x

x

Ekman equations: Deep water

Ekman equations: Shallow water

yyT

y

xT

)( Hzk

v y

)( Hzk

u x

)4

sin()exp( azazVu o

)4

cos()exp( azazVv o

V

V

02

2

ifwz

wk

SAB summary

• 3 distinct bathymetrically separated regions– Inner, middle and outer shelves.

• Gulf stream is the primary method of onshore nutrient transport/upwelling.

• Coastal currents are in the direction of mean winds, which all vary seasonally.

Questions?

• References:

– Janowitz, G.S., and L.J. Pietrafesa (1982), The effects of alongshore variation in bottom topography on a boundary current—Topographically induced upwelling, Cont. Shelf Res., 1, 123-141.

– Lee, T. N., J. A. Yoder, and L. P. Atkinson (1991), Gulf Stream Frontal Eddy Influence on Productivity of the Southeast U.S. Continental Shelf, J. Geophys. Res., 96(C12), 22,191–22,205.

– Signorinni, S. R., and C. McClain (2006), Remote versus local forcing on chlorophyll variability in the South Atlantic Bight, NASA Tech. Mem., 2006-214145.

– Yoder, J.A. (1985), Environmental Control of Phytoplankton Production on the Southeastern U.S. Continental Shelf, in Oceanography of the Southeastern U.S. Continental Shelf, Coastal and Estuarine Sciences 2, edited by L.P. Atkinson et al., pp.93-103, AGU, Washington, D.C.

Seasonal climatology maps of AVHRR SST.

The inner shelf has the coldest surface temperatures in winter, which are around 10 oC.

Rapid winter cooling is due to convective overturn and tidal and wind mixing.

Warmest surface temperatures in summer reach 28-30oC on the shelf.

(Signorini & McClain, 2006)

Winter

FallSummer

Spring

Seasonal maps of SeaWiFS surface Chl-a.

Chlorophyll is generally higher on the inner shelf due to available terrestrial nutrients.

Maximum concentrations occur in late winter early spring due to nutrient availability from increased river discharge.

Minimum concentrations are found in summer due to nutrient depletion and increased stratification.

(Signorini & McClain, 2006)

Winter

FallSummer

Spring