continental shelf researchjaustin/publications/savidge_etal_pt1_2012.pdf(lentz, 2010; castelao et...

Continental Shelf Research ] (]]]]) ]]]–]]]

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Continental Shelf Research

0278-43

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Research papers

Variation in the Hatteras Front density and velocity structure Part 1: Highresolution transects from three seasons in 2004–2005

Dana K. Savidge a,n, Jay A. Austin b, Brian O. Blanton c

a Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, United Statesb Large Lakes Observatory, University of Minnesota, Duluth, United Statesc Renaissance Computing Institute, North Carolina, United States

a r t i c l e i n f o

Article history:

Received 27 January 2012

Received in revised form

5 November 2012

Accepted 7 November 2012

Keywords:

Coastal circulation

Seasonal variability

Cross-shelf transport

Buoyancy effects

Coastal fronts

43/$ - see front matter & 2012 Elsevier Ltd. A

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esponding author. Tel.: þ1 912 598 3340; fax

ail address: [email protected] (D.K. S

e cite this article as: Savidge, D.K., eects from three seasons in 2004–20

a b s t r a c t

On the continental shelf near Cape Hatteras, cool fresh Mid-Atlantic Bight and warm salty South

Atlantic Bight shelf waters converge alongshelf 90% of the time, causing strong alongshelf gradients in

temperature, salinity, and density known as the ‘Hatteras Front’. Mechanisms of shoreward transport in

this region have long been a topic of interest, since many commercially important species spawn on the

outer shelf, but utilize the adjacent Albemarle and Pamlico Sounds for nurseries, requiring some

physical transport mechanism to move the eggs and larvae from the outer shelf to these nursery areas.

One mechanism providing such shoreward transport is strong shoreward velocity along the cross-shelf

oriented ‘nose’ of the Hatteras Front. The Frontal Interactions near Cape Hatteras (FINCH) project used

shipboard ADCP and a towed undulating CTD to examine Hatteras Front property, density and velocity

fields in August 2004, January 2005, and July 2005. Strong property gradients were encountered across

the nose of the Hatteras Front in all cases, but the density gradient evolved in time, and along with it

the dynamic height gradient driving the observed along-front cross-shelf velocities in the nose of the

Front. In August and January FINCH data, MAB shelf waters on the north side of the Hatteras Front are

less dense than SAB shelf waters, driving shoreward velocities along the Hatteras Front. By July, MAB

shelf waters are slightly more dense than SAB shelf waters, with areas of weak seaward and shoreward

velocities within the Hatteras Front. As Part 1 of a pair of contributions, this article focuses on FINCH

data to illustrate the range of density gradients encountered and resulting cross-shelf velocities.

Whether these observations are typical of variability in the Hatteras Front is explored in a second

article, Part 2.

& 2012 Elsevier Ltd. All rights reserved.

1. Background

On the continental shelf and slope near Cape Hatteras, Mid-Atlantic Bight (MAB) and South Atlantic Bight (SAB) shelf watersconverge alongshelf 90% of the time in daily alongshelf transports(Savidge and Bane, 2001). Since shelf waters derived from north andsouth of Cape Hatteras have large differences in temperature (T) andsalinity (S) characteristics, this convergence supports a strongalongshelf gradient in T and S expressed across the cross-shelforiented ‘‘Hatteras Front’’ (Stefansson et al., 1971; Pietrafesa et al.,1994; Berger et al., 1995). Such convergence requires offshelf exportof shelf waters through continuity, examined for the MAB shelf watercomponent in several field studies, including the Shelf EdgeExchange Projects (SEEP and SEEP-II) and the Ocean Margins Project(OMP) (Walsh et al., 1988; Biscaye et al., 1994; Verity et al., 2002).

Mechanisms of shoreward transport in this region have alsobeen a topic of interest (Checkley et al., 1988; Shanks, 1988;

ll rights reserved.

: þ1 912 598 2310.

avidge).

t al., Variation in the Hatte05. Continental Shelf Resea

Stegmann and Yoder, 1996; Quinlan et al., 1999). Many commer-cially important species spawn on the outer shelf, but utilize theadjacent Albemarle and Pamlico Sounds for nurseries, requiringsome physical transport mechanism to move the eggs and larvaefrom the outer shelf to these nursery areas. Persistent interest ingas and oil deposits on the shelf and slope raises the question ofhow any pollution resulting from such activities might reach andaffect the ecologically and economically important sounds andbeaches of North Carolina.

Velocities along the Hatteras Front provide one effectiveshoreward conduit in winter, first demonstrated by Savidge(2002) using mooring records (Fig. 1). In that study, the T and Sgradients across the Hatteras Front did not completely compen-sate, such that cold, relatively fresh MAB shelf water was lessdense than warmer, saltier SAB shelf water. The resulting dynamicheight gradient across the Front was estimated to be of sufficientmagnitude to drive observed shoreward alongfront velocities.

The Frontal Interactions near Cape Hatteras (FINCH) projectwas designed and carried out to investigate this shorewardtransport mechanism. Using shipboard ADCP combined with atowed undulating CTD, the circulation and density fields

ras Front density and velocity structure Part 1: High resolutionrch (2012), http://dx.doi.org/10.1016/j.csr.2012.11.005

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mailto:[email protected]

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Fig. 1. Cape Hatteras region and measurement locations. Panel A: Cape Hatteras (CH) MMS field study site (March, 1992–February, 1994), with mooring locations (black

diamonds) along three cross-shelf lines (lines A–C in panel A) and two additional shelf-edge moorings (between lines A and B). The hydrographic data shown in Figs. 6–8

were collected on cross-shelf transects along the lines defined by the A moorings and the C moorings. A schematic Hatteras Front (HF) shows both the cross-shelf oriented

‘nose’ of the Front and its more along-shelf oriented ‘seaward flank’. The approximate mean position and width of the Gulf Stream is also shown. The lower panels show

the FINCH transects in August 2004, January and July 2005 used within this paper. Red lines show locations of transects in Fig. 3. Together with the black lines, they

indicate the locations of all transects included in the stream coordinate means shown in Fig. 5. (For interpretation of the references to color in this figure caption, the reader

is referred to the web version of this article.)

D.K. Savidge et al. / Continental Shelf Research ] (]]]]) ]]]–]]]2

associated with the cross-shelf nose of the Hatteras Front wereintensively sampled in three field seasons: August 8–11, 2004,three 2–3 day forays in late January 2005, and July 19–21, 2005.The August data are described in detail in Savidge and Austin(2007), verifying strong property (T and S) and density gradientsacross the Front, and strong geostrophic shoreward velocitiesalong its nose.

MAB shelf waters are known to undergo strong seasonal Tvariability, in both T ranges and strength of vertical stratification(Lentz, 2010; Castelao et al., 2008). SAB shelf water T also varies

Please cite this article as: Savidge, D.K., et al., Variation in the Hattetransects from three seasons in 2004–2005. Continental Shelf Resea

with season, with more modest stratification changes, asdescribed in Blanton et al. (2003). Salinity evolution is much lessseasonal than T in either SAB or MAB shelf waters, but both MABand SAB T and S are subject to interannual variability (Blantonet al., 2003; Mountain, 2003; Castelao et al., 2008). Such seasonalor interannual variability may effect the density gradient acrossthe Hatteras Front, and therefore the geostrophically drivenshoreward velocities along its nose.

In this article, data collected in the January and July FINCHfield seasons are compared to the August data to examine how


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density and velocity fields in the Hatteras Front changed with thechanging property fields in the converging MAB and SAB shelfwaters. Several archived shipboard transects taken near CapeHatteras are also examined as examples of property and densityevolution on both sides of the Hatteras Front through spring,summer and fall. These snapshots imply both a potentiallyrepeatable seasonal evolution of density and velocity within theHatteras Front and the possibility of interannual variability indensity contrast across the Front. For that reason, this articleconstitutes ‘Part 1’ of a pair of contributions. In the companionarticle, Savidge et al. (this issue), hereinafter ‘Part 2’, several long-term archived data sets are examined to determine whether thedensity evolution described here in Part 1 is characteristic oftypical seasonal variability within the Hatteras Front. Seasonal orinterannual variability could have important ramifications forspecies that are dependent upon Hatteras Front associated circu-lation for cross-shelf transport.

2. Data and methods

The August 2004 and July 2005 FINCH data from the nose ofHatteras Front were collected from the R/V Fay Slover, a 55 ftdayboat owned and operated by Old Dominion University, whilethe January 2005 FINCH data were collected from the R/V

Savannah, a 92 ft UNOLS vessel owned and operated by Skidaway

Fig. 2. Wind records from CMAN station CLKN7 for FINCH field seasons. Northward w

which the wind was blowing, in the oceanographic conventional sense.


Institute of Oceanography (Fig. 1 shows transect locations dis-cussed herein). On all cruises, CTD data were collected with aSeaBird SBE-19plus at high vertical and horizontal resolutionusing an undulating towed vehicle, the SeaSciences Acrobat. TheSBE-19p samples at 4 Hz, and was equipped with a Wetlabs C-Star transmissometer and Wet-Star fluorometer. The Acrobatprofiled within 3 m of the bottom and surface between the15 and 58 m isobaths, with 0.25 km between up and down ‘casts’along the zigzag flight path. The R/V Slover is also equipped with ahull-mounted 600 kHz RDI ADCP, while the R/V Savannah isequipped with a hull-mounted 300 kHz RDI ADCP. The datashown here are in 1 m vertical bins, and have been averaged over20 s, for an alongtrack resolution of � 60 m. The shipboard ADCPdata were detided using alongtrack tidal predictions from theADCIRC model (Luettich and Westerink, 1992; Westerink et al.,2008). Tides and inertial motions are relatively small near CapeHatteras, with M2 magnitudes typically less than 10 cm/s(Pietrafesa et al., 1985; Lentz et al., 2001; Berger et al., 1995;Savidge et al., 2007). Preliminary plots with undetided datashowed similar features to those examined below in the detideddata—that is, the tidal variability is sufficiently small that it doesnot mask the Hatteras Front associated circulation. The CTD datawere linearly interpolated onto a uniform grid coincident with theADCP data grid for plotting purposes. Wind information duringFINCH was collected from the NDBC Cape Lookout CMAN station(CLKN7, Fig. 2). More relevant CMAN data from Diamond Shoals

inds are aligned with the positive y-axis. Sticks represent the direction towards


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off Cape Hatteras (previously tower station DSLN7, replaced bybuoy 41025 in 2003) was repeatedly disrupted by instrument andbuoy failure during FINCH.

An additional valuable dataset near Cape Hatteras comes froma two-year-long mooring, drifter, and hydrographic study fundedby the Minerals Management Service (MMS) (Berger et al., 1995).Moorings were maintained from March, 1992 to February, 1994,with seasonal hydrographic surveys at three to four monthintervals during the experiment. Shipboard hydrography fromtransects along the A and C moorings lines (Fig. 1A) are discussedherein.

To emphasize the density space contrasts illustrated by theFINCH undulating CTD data, T-S diagrams are presented asfrequency plots. Frequencies were calculated by sorting the CTDdata into regularly spaced bins (S from 28 to 38 psu by 0.1 psu, Tfrom 7 to 29 1C by 0.1 1C) and then counting the total number ofvalues within each bin. Frequency distributions have beensmoothed over S and then T bins with a 3 point Hanning filter,and actual numbers of data points in each smoothed bin werethen normalized by the overall maximum. The natural log (zerosexcluded) of the results were then contoured. Normalized datafrequency o5% is not contoured for August and January, ando2% is not contoured for July. The presentation of the Augustdata differs slightly between Savidge and Austin (2007) and thispaper, since the values in that article were not smoothed ornormalized and small quantity bins were not eliminated from thecontouring. The contours of the highest frequency samples clearlymatch between versions (their Fig. 6 and Fig. 4 herein). Thedarkest bands indicate the T–S space that was sampled mostfrequently. No colorbars are provided, as the actual numbers ofsamples, which depend on the sampling rate and time spent inand out of the water masses of interest, is not relevant to thedefinition of the T–S ranges and density characteristics of thetarget water masses.

To further illustrate the features discussed below in individualsections, stream coordinate mean density and velocity fields werecalculated from FINCH transects for August, January and July.Methods are described in Savidge and Austin (2007). Briefly, toestimate a stream coordinate mean from a sequence of obliquetransects across a front in motion, a local coordinate system foreach transect needs to be defined that is perpendicular to theorientation of the front, and whose origin is consistently locatedrelative to some common feature within the transects. Here theorientation of the Front relative to each transect is taken as thedirection of the (spatial) mean ADCP velocity in each transectwithin a density range that approximately maximizes the magni-tude of that velocity. ADCP velocity sections are then rotated intoalong-front and across-front components. The origin is defined asthe location of a specific density contour at a specific depth alongthe transect (Table 1). The realized transect density and ADCPvelocity data are then projected onto a perpendicular to thefront’s orientation, aligned across the Front to the selected origin,smoothed slightly with an optimal interpolation, and averaged.The density ranges used to define the orientation of the front andthe origin definition changed between August and January, andbetween the three January forays (Table 1). With only weak

Table 1Values used to define cross-Front direction and origin for stream coordinate

means.

FINCH foray # sections sT range Contour ðsT Þ Depth (m)

August 2004 10 22.1–23.1 23.1 7.5

January 2005 5 25.6–26.4 25.8 15

July 2005 4 NA 23.8 15


velocities measured in July, a default frontal orientation wasassumed to be east to west, and transect ADCP velocities anddensities were projected onto south to north oriented cross-front lines.

The first objective in each FINCH excursion was to determinethe alongshelf position of the Hatteras Front for sampling. Thoughconvergent in the mean, SAB and MAB shelf water alongshelftransport variability is correlated with alongshelf wind, so thatthe location of the Hatteras Front shifts alongshelf with alongshelfwind forcing (Savidge and Bane, 2001; Savidge, 2002). Whiletemperature contrast between the SAB and MAB shelf waters insatellite SST imagery can be useful for locating the Front, cloudcover at Cape Hatteras often obscures the coastal ocean there. Oncloud-free days, the Front can also be obscured by the warmsurface layer in the MAB that develops in summer, or by thepresence of a fresh cap of Chesapeake Plume waters, which carriesa T signature of its own. In early August 2004, the Hatteras Frontpropagated rapidly south past Cape Hatteras under the influenceof Hurricane Alex, which skirted the barrier islands there onAugust 3. The strong winds (Fig. 2) mixed the warm surface layerMAB shelf water down into the cold bottom layer, making the Tcontrast between MAB and SAB shelf waters immediately appar-ent in satellite SST imagery. In January, cloud cover obscured theCape Hatteras shelf in satellite imagery almost continuously. Asequence of strong wind events affected the Front’s location,making shipboard CTD sections the most reliable method oflocating the Front. Cloud cover also obscured satellite detectedSST during July sampling, so shipboard CTD sections were againuseful for locating the Front. The primary goal of the July fieldwork was to recover several moorings, so cross-Front samplingwith undulating CTD and shipboard ADCP was accomplished only1–2 times on each of three consecutive days (July 19–21).

3. Results

Undulating CTD sections across the Hatteras Front in August2004, and January and July 2005 show large S and T gradientsacross the Front in all sampled seasons. From August throughJanuary, and again from January through July, large temperaturechanges in the MAB and SAB shelf waters shifted the densityspace in which both shelf waters resided. These shifts led to anunanticipated evolution in the density contrast between the MABand SAB shelf waters. Along with the density gradient changethrough time, the resulting dynamic height gradient and along-front velocity in the nose of the Front changed between the threefield seasons. In both August and January, strong shorewardvelocities were encountered across the nose of the Hatteras Front,while in July, much weaker shoreward and seaward velocitieswere measured. In the following, the August 2004 results ofSavidge and Austin (2007) are reviewed, then January and Julymeasurements are compared to the August results. To sample arepresentative range of densities resulting from the seasonal Tevolution in MAB and SAB shelf waters, several MMS-studyshipboard transects from late spring, summer and fall are alsopresented.

3.1. August 2004

August FINCH measurements occurred just after HurricaneAlex had travelled through the study site, vertically mixing bothSAB and MAB shelf water on the mid-shelf. The relatively uniformsurface to bottom mixed MAB shelf water was cooler, fresher, andless dense than the warmer saltier SAB shelf water, so that adensity gradient existed across the Front. Several example cross-front transects were discussed in Savidge and Austin (2007). One


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Fig. 3. Property, density and velocity (positive shoreward) transects across the Hatteras Front in August 2004, January 2005 and July 2005. Transect locations are shown in

red on the maps in Fig. 1. Left panels: August 9, 2004, middle panels: January 20 2005, and right panels: July 19 2005. From top to bottom, the panels are: salinity (contour

interval 0.5 psu); temperature (contour interval 0.5 1C); density (contour interval 0.25 sy units); cross-shelf velocities (positive shoreward in orange, contour interval

0.05 m/s, bold line is 0.0 contour). The bold gray dashed lines are schematic boundaries of the ‘jet’ within the Front, traced roughly along the 15.0 cm/s contour in the

bottom panels for August and January, and repeated on the upper three panels. No such jet exists for July.


example is repeated in the left panels of Fig. 3 (transect location isshown as the red line in Fig. 1B), which shows strong S, T anddensity fronts. While the strong narrow T gradient located about6 km along the section is co-located with a relatively stronglocally narrow S gradient, in fact the strongest S and densitygradients are co-located somewhat farther along near the middleof the transect, with fresher MAB water situated on the lighterside of the density front. The T gradient is also strong near themiddle of the transect, but in both subregions is compensated forby the S gradient. At the left, the T and S almost completelycompensate, and the density gradient is small. But in the middleof the transect, the S gradient controls the sign of the densitygradient, despite the strong opposing T gradient there.

Strong shoreward alongfront velocities were recorded in theHatteras Front with the shipboard ADCP in August, reachingmagnitudes of over 30 cm/s. Estimated dynamic height relativeto 20 m depth increased by several centimeters crossing theHatteras Front from the SAB waters into the MAB waters overapproximately 10 km. This was of sufficient magnitude to accountfor the observed alongfront velocities. This centrally locateddensity gradient is collocated with strong surface intensifiedshoreward velocities. A schematic boundary of this ‘jet’ within


the Front has been sketched along the position of the 15.0 cm/scontour in the bottom panel, which is repeated on the upper threepanels as well. In August 2004, the fastest velocities existedwithin the fresh and cool MAB shelf water side of the densityand property fronts, and thus preferentially carried MAB shelfwater shoreward.

The density contrast across the Hatteras Front in the August2004 FINCH data is illustrated in a composite T–S plot (Fig. 4A),showing the strong MAB-SAB T and S contrasts, with densityranging from 22 to 23.5 sy units across the Front. The slightlycooler, much fresher MAB shelf water is significantly lighter thanthe SAB shelf water because of the S contrast, and in spite of thedifference in T.

3.2. January 2005

In late January 2005, three separate forays of two to three dayseach were completed between several strong wind forcing events(Fig. 2 shows vector winds). One transect collected during the firstforay on January 19–21 (Fig. 3, transect location is shown as thered line in Fig. 1C) illustrates the strong T and S contrasts acrossthe Hatteras Front in January. SAB and MAB shelf waters were


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Fig. 4. Upper panels: T–S frequency plots from the 2004–2005 FINCH project, taken across the nose of the Hatteras Front with the undulating CTD. The darkest grey bands

encompass the properties of the MAB shelf water at the cold fresh end, and the SAB shelf water at the salty warm end. Lower panels: T–S diagrams from the MMS study

hydrographic sections in 1992 and 1993, taken from the 5th CTD stations seaward along the A line and C line density sections shown in Figs. 6–8, located near moorings A2

and C2 in Fig. 1. August, November and May data are represented by circles, squares, and triangles, respectively. SAB samples are shown as dark gray filled symbols, while

upper and lower MAB water column samples are shown as open and filled light gray symbols, respectively. May 1992 MAB water column was well mixed at this location

so open and closed triangles overlay one another. Boundaries of different water masses defined for the Hatteras region by Flagg et al. (2002) (their Fig. 3) are plotted as gray

boxes in the upper right panel. Delineated regions are labeled Virginia Coastal Water (VCW, summer and winter), Mid Atlantic Bight Water (MABW, summer and winter),

South Atlantic Bight Water (SABW, summer and winter), and Gulf Stream Water (GSW, summer and winter). Their upper and deep slope water categories have not been

included in this figure.


� 10 1C cooler than in August, consistent with the magnitude ofseasonal cooling documented in Blanton et al. (2003) for SAB shelfwaters and in Lentz (2010) and Castelao et al. (2008) for MABshelf waters. No large change from August to January in salinityrange covered by either MAB or SAB shelf water componentappears (T–S diagram in Fig. 4A). This is consistent with relativelysmall seasonal salinity signals documented by Blanton et al.(2003) for SAB shelf waters and by Mountain (2003) andCastelao et al. (2008) for MAB shelf waters. Density across theHatteras Front in January, as in August, was not entirely compen-sated, with density ranging from 24.5 to 26.5 sy units across theFront. As in August, MAB shelf water was lighter than SAB shelfwater, due to the fresher MAB S, despite cooler MAB T.

Strong alongfront velocities were also measured in January,directed shoreward along the portion of the Hatteras Front orientedcross-shelf (Fig. 3). As with the August data, estimated dynamicheight relative to 20 m depth increased by several centimeterscrossing the Hatteras Front from the SAB waters into the MABwaters over approximately 10 km. This was of sufficient magnitudeto account for the observed alongfront velocities. The schematicboundary within the Front sketched along the position of the15.0 cm/s contour shows that in the January case, SAB shelf watersreside in the portion of the Front where the highest shorewardvelocities occur. This is in contrast to the August 2004 case, whenthe strongest velocities in the Front transported MAB watersshoreward. Whether this represents a seasonal progression or


storm response is unknown from the limited data. However, itdoes demonstrate that there are occasions when either shelf watercomponent may be transported shoreward within the Front.

The Front was repeatedly sampled on the second and thirdJanuary forays also (January 25–26 and January 31–February 2),showing strong property and density gradients across it duringthose periods, as during the first January 19–20th foray. Proper-ties and densities on both sides of the Front evolved from eachJanuary foray to the next, under the strong storm forcing betweenforays. The three bands of decreasing temperature distributions inFig. 4A are from the three separate January forays, with decreas-ing temperatures with each foray. Along-Front velocities wereapparent in the first and second forays, but were not demon-strated convincingly from the relatively few transects accom-plished during the third foray.

3.3. July 2005

In July 2005, the Hatteras Front was again located and sampledover a three day period. At this time, the Front was positionedover the shelf north of Cape Hatteras, consistent with theapparent more northward position of the Front in summersurmised by Savidge (2002).

One example July transect shows quite cold MAB shelf wateron the north (righthand) side of the transect contrasting withmuch warmer SAB shelf water on the left (Fig. 3, right column,


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upper two panels, location shown as the red line in Fig. 1D). TheT–S diagram for July indicates that middle and lower watercolumn MAB shelf water had warmed only slightly over valuesfound in January, while the SAB water had warmed to valuesmore typical of summertime (Fig. 4B). Near surface watersampled in these transects was of fairly uniform T overlying boththe warm and cold subsurface shelf waters. However, the nearsurface water was significantly fresher than either warm or coldunderlying MAB or SAB shelf water, suggesting a significantcontribution from Chesapeake Bay water.

The strong property gradients in July were again not entirelycompensated in density. However, in contrast to the August 2004and January 2005 measurements, in the July data the middle andlower water column density gradient was reversed, with coldMAB middle and lower layer shelf water now denser than SABshelf water by about one sy unit across the Front (Fig. 3), despitelower MAB salinity. In the near surface water, T and S gradientswere weak, the density gradient is also small, and in the oppositedirection to that of the deeper water column.

There is some question whether the warm subsurface waterencountered on the shelf in July was actually SAB shelf water, orsimply Gulf Stream water stranded on the shelf, due to the lownumber of transects (4) and patchy, cloud-contaminated satelliteimagery. In the transect shown, the warm salty bolus in the centerof the transect is especially suspicious. However, nowhere withinit does S exceed 35.85 psu, and nearby 20 m isobath mooringssuggest the warmer saltier water measured along the Julytransects was predominantly SAB shelf water. Part 2 lends furtherevidence that the July FINCH transects sampled SAB shelf water,showing that FINCH data are consistent with SAB and MAB shelfwater densities and contrasts from the NODC long term shipboardhydrographic database.

Dynamic height estimates from the lower layer densities (at10 m depth relative to 20 m) indicates only slightly lowerdynamic height over MAB water than over SAB water, amountingto a few mm along the 23 km long transect. This implies at mostextremely weak seaward alongfront velocity, while the slope ofthe isopycnals would imply shoreward thermal shear (decreasingseaward flow with increasing depth). Dynamic height at thesurface relative to 20 m reference is dominated by the warmsalty bolus in the center of the transect and the warm fresh layernear the surface. The horizontal gradient is small relative to thevalues seen in August or January, reaching only 0.5–1 cm over the10 km region surrounding the warm bolus, and otherwiseremaining lower than a fraction of a cm. Integration throughthe warm fresh surface layer reverses the slight dynamic heightgradient of the lower layer, so that slightly higher total watercolumn dynamic heights occur over MAB shelf water.

Diagnosing geostrophic alongfront velocities from small den-sity gradients that reverse with depth is obviously subject tomore error than estimates from large gradients with similar signthroughout the water column, as found in August and January.Cross-shelf velocities measured by ADCP along the July transectsare slight, and modestly shoreward in the lower water column,from 0 to 10 cm/s (Fig. 3, bottom right panel), not seaward asimplied by the slight density gradient in the lower layer. Seawardflow is evident in the upper water column and in the warm saltycentral bolus in the transect shown (discussed below). Instead ofresembling the velocity field implied by the density field, this isactually more consistent with a two layer Ekman response to thenortheastward winds measured for this timeframe (Fig. 2). On theother hand, alongshelf velocities are equatorward (not shown).With only four transects and an inability to define the orientationof the Hatteras Front in July, a clear diagnosis of these velocitieshas not been established. At the least, these measurementsdemonstrate a change in the density gradients observed from


that in August and January, and the absence of strong shorewardvelocities within the Front. Both FINCH and the NODC archive (inPart 2) illustrate the possibility that the density contrast acrossthe Front may diminish or reverse in spring of any given year.

3.4. Stream coordinate mean structure

The stream coordinate averages constructed from the snap-shots show strong shoreward velocities associated with thestrong density gradients across the Hatteras Front in August2004 and January 2005 (Fig. 5). January horizontal density,dynamic height and sea surface gradients were of the same orderas those measured in August 2004. The January shorewardvelocities were not surface intensified, as they were in August,but are situated in the middle of the water column, within thedenser part of the horizontal density gradient.

The July stream coordinate density average over four transectsillustrates the reversed density gradient, especially over the deepwater column (Fig. 5, right panels). Note the left edge has veryhigh standard deviation in density, relative to the average valuesat the left edge. The stream coordinate velocity section from ADCPdata illustrates the absence in July of the strong shorewardvelocities in the Front that were evident in August and January.The measured velocities are not seaward, as expected from adynamic height estimate for the lower water column. An inter-esting feature is the wedge of seaward velocity in the middle andupper water slightly north of the center of the transect. This isconsistent with diverging isopleths in the density stream coordi-nate field, and is located approximately where the largest sali-nities and temperatures appeared in the sample July transectshown (Fig. 3). As discussed in Section 3.3, S within that patch iso35:85 psu, below typical Gulf Stream S from Flagg et al. (2002)or Pietrafesa et al. (1994), and T is closer to that measured at themoorings in SAB water than to at least surface Gulf Streamtemperature. Such a bolus could result from recirculation behindthe Hatteras Front, similar to that discussed in Savidge and Austin(2007) (their Figs. 11–13).

3.5. Summertime stratification in MAB

The range of density contrasts across the Hatteras Frontobserved during FINCH does not span the entire range of possi-bilities, as the strong summertime stratification known to occurin MAB shelf water appears only weakly in the July FINCHmeasurements. An August 1993 transect along the northern Aline from the MMS shipboard hydrographic data illustrates thatthe well-known strong seasonal stratification in MAB shelf waterextends to the southern end of the MAB (Fig. 6, panels A–C).Vertical stratification along the C line August 1993 (Fig. 6D), istypical of the much weaker (than in the MAB) summertimestratification observed for SAB shelf water (Blanton et al., 2003).By the time the October 1993 ship section was taken, the strongT-based seasonal MAB stratification seen along the A-line in Fig. 6had been eroded, replaced by a weaker stratification more typicalof winter (Fig. 7, panels A–C). Stratification in late October wasalso weak in SAB shelf water along the C line farther southward(Fig. 7D).

The density contrast between the MAB and SAB shelf watersbefore and after the fall transition in these MMS sections isillustrated in composite T–S diagrams (Fig. 4C) of data from the5th hydrographic station seaward along both the A and C lines inAugust and October 1993. In August 1993, the strong contrasts inT, S and density between upper MAB, SAB, and lower MAB shelfwaters are evident, with density ranging from 20.5 to 24.4 syfrom upper to lower MAB water, with SAB waters of intermediatedensity of 23–24 sy. The coolest, freshest component is upper


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Fig. 5. Stream-coordinate means and standard deviations of Hatteras Front along-front velocity (positive shoreward) and density fields from transects taken during the

three FINCH field seasons. Transects are oriented such that SAB shelf water appears at the left of the panels, transitioning into MAB shelf water at the right side of the

panels. Left column of panels: August 9, 2004, middle panels: January 19–20 2005, and right panels: July 19–21 2005. From top to bottom, the panels in each column are:

mean ADCP along-Front velocities; mean density; standard deviations of the ADCP along-Front velocities; standard deviations of the densities. Contour interval for velocity

means and standard deviations is 0.05 m/s, bold line is 0.0 contour, with positive shoreward in orange. Contour interval for the density means and standard deviations is

0.2 sy units). Maps of transect locations included in the stream coordinate means are shown in Fig. 1, panels B–D. Notice that the horizontal scale is shorter for August than

for January and July, and that the density ranges change for each month in the second row of plots.


MAB shelf water, the warmest and saltiest component is SAB shelfwater, and lower MAB shelf water is cooler and fresher, and ofsimilar magnitude density to the SAB shelf water. By October, thelight upper and dense lower MAB shelf waters have mixed so thatthe MAB shelf water became lighter than the SAB shelf water(Fig. 4C). Density of the SAB shelf water did not change muchbetween August and October, indicating that little seasonal cool-ing had occurred (Fig. 4C). Note the strong resemblance betweenthe October 1993 T–S diagram and the August 2004 FINCH T–Sdiagram (Fig. 4A), taken immediately after Hurricane Alex passedclose by.

In May 1992, a hydrographic section from an MMS shipboardtransect along the A line shows conditions after seasonal warmingnear Cape Hatteras had commenced, but before strong verticalstratification had developed in the MAB shelf water. The SAB shelfwaters along line C had warmed somewhat at all depths, but the


MAB upper, middle and lower layer waters remained below 10 1C(Fig. 8). In these sections the cool fresh MAB shelf waters alongline A are very nearly the same density as the warm salty SABshelf waters along line C, illustrated in TS plots from the 5thstation seaward along both lines (Fig. 4D). Note the strongsimilarity of this May 1992 T–S diagram to that from the July2005 FINCH transects (Fig. 4B).

4. Synthesis

The snapshots of density contrast and associated velocityfields in the nose of the Hatteras Front suggest an evolution thatmay be seasonal, and therefore annually repeatable. In thefollowing, the suggested seasonality is outlined, relative to theMMS and FINCH data analyses to date. Seasonal aspects of density


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Fig. 6. Summer (August 2–11, 1993) cross-shelf sections from the MMS project, collected during Seaward Explorer cruise SE9309. Top three panels show data collected along

the A mooring line: panel A: temperature, panel B: salinity and panel C: density. Panel D shows density collected along the C mooring line. Sigma-T values of 23 and lower are

plotted in black, of 23.25 and higher are plotted in gray. Temperature contour interval is 1 1C, salinity contour interval is 0.25, density contour interval is 0.25 kg/m3.


evolution include (1) the fall transition from strong summerstratification to lower vertical stratification in winter; (2) coolingthrough winter, (3) spring warming and (4) summer restratifica-tion. Whether these results are consistent with long-term seaso-nal evolution is then examined in detail in Part 2 using archiveddatasets.

4.1. Fall transition

The destruction of summer stratification with increasingwinds and decreasing surface heat fluxes into the coastal oceanin fall is well known, and has been examined in both MAB andSAB settings. Figs. 6 and 7 show the strong contrast in stratifica-tion in particularly MAB water, but also in SAB shelf water beforeand after the fall transition in 1993. The contribution of thismixing of especially the MAB shelf water is illustrated quite


clearly in panel C of Fig. 4. Before the transition in the August1993 MMS data, upper layer MAB shelf water is much lighterthan either upper or lower layer SAB shelf water. Lower layerMAB shelf water does not occupy significantly different densityspace than SAB shelf water, and is roughly equivalent toupper MAB water in S, but not in T or density. After mixing, asseen in the November 1993 MMS data in Fig. 4C, MAB upper andlower layer shelf waters are less dense than SAB shelf water, dueto the mixing of the light upper MAB water into the denser lowerMAB water. That is, by mixing away the significant densitydifference between upper and lower MAB shelf water that isdue to T, the strong S contrast between mixed MAB and mixedSAB shelf water results in a strong density contrast betweenthem.

The August 2004 FINCH data of either Fig. 3 or Fig. 4A showswell mixed conditions on both the MAB and SAB shelf water sides


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Fig. 7. Fall (October 28–November 9, 1993) cross-shelf sections from the MMS project, collected during Seaward Explorer cruise SE9316. Top three panels show data

collected along the A mooring line: panel A: temperature, panel B: salinity and panel C: density. Panel D shows density collected along the C mooring line. Sigma-T values

of 23 and lower are plotted in black, of 23.25 and higher are plotted in gray. Temperature contour interval is 1 1C, salinity contour interval is 0.25, density contour interval

is 0.25 kg/m3.


of the Hatteras Front. The density contrast between these shelfwaters is large and is due to the S difference, as the lighter MABcomponent is slightly cooler than the SAB component. Themeasured shoreward along front velocities of August FINCH areconsequently not representative of summer conditions, but ratherof conditions prevailing after the fall transition. The August 2004mixing of the MAB waters by Hurricane Alex during FINCH mayhave occurred earlier and affected a more limited geographicalarea than mixing from a series of region-wide fall wind eventswould have. It is unknown whether MAB shelf water restratifiedafter mixing by Alex. However, MAB mixing does occur every fall,and will alter the density space occupied by the MAB shelf waters,and therefore its contrast to the density range occupied by SABshelf water in fall.


4.2. Winter cooling

Equally as well known as the fall transition is the seasonalcooling of shelf water in the SAB and MAB through fall and winter.This is shown in T time series in Savidge et al. (2007) (their Fig. 3)from the two year MMS shelf moorings north and south of CapeHatteras, and is quite evident in the T contrast between Augustand January FINCH values shown in Fig. 4A. The density contrastbetween MAB and SAB shelf water across the Hatteras Frontpersisted through fall cooling and the sequence of storms thatpunctuated the January FINCH sampling.

It is conceivable that the integrated winter-long effect ofcooling in the more northern MAB could exceed that in the SABsufficiently to erode the S-based density discrepancy between


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Fig. 8. Spring (April 29–May 6, 1992) cross-shelf sections from the MMS project, collected during Cape Henlopen cruise CH9222. Top three panels show data collected

along the A mooring line: panel A: temperature, panel B: salinity and panel C: density. Panel D shows density collected along the C mooring line. Sigma-T values of 23 and

lower are plotted in black, of 23.25 and higher are plotted in gray. Temperature contour interval is 1 1C, salinity contour interval is 0.25, density contour interval is

0.25 kg/m3.


MAB and SAB shelf water. Indeed the August 2004–January 2005cooling appears larger in MAB water than in SAB water (Fig. 4A).However, for MAB salinities near 32, density contours becomeincreasingly vertical with cooler temperatures in T–S diagrams. Inthat case, large changes in T are necessary to result in smallchanges in density, so that differential cooling between SAB andMAB shelf waters would have to be large to overcompensate forthe large S difference contribution to density. Episodes of strongshoreward velocities attributed to Hatteras Front density con-trasts persist through April and arguably into May in the 1992and 1993 mooring data examined by Savidge (2002), their Fig. 5.In those winters at least, sufficiently large cooling in MAB watercompared to SAB water apparently did not occur.


4.3. Spring warming

Warming of coastal waters in spring and summer is also a wellknown aspect of the seasonal cycle. If SAB warming precedes orexceeds MAB warming, which is conceivable due to its moresouthern exposure, there is opportunity to reduce the S-baseddensity discrepancy between SAB and MAB shelf waters in spring.For SAB salinities near 35–36, density contours become lessvertical with warmer temperatures in T–S diagrams, so thatrelatively smaller changes in T result in larger changes in densitythan for fresher MAB water. In Fig. 4B, it appears that July 2005upper and lower SAB waters have warmed to T characteristic ofsummer, and that lower layer MAB shelf water remains cold.


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T time series from the MMS shelf moorings shown in Savidgeet al. (2007) (their Fig. 3) suggests this is typical, since springtimewarming commenced in April in the SAB, and cold lower layerMAB T persisted through summer. In 2005 sampling, this resultedin slightly denser MAB water in the lower layer. In other years,whether the wintertime density gradient in the lower layer iseliminated, not quite eliminated, or reversed would be subject tothe magnitude and timing of both heat and freshwater fluxes inboth shelf waters in spring. This suggests the potential forinterannual variability in the density gradient and resultingvelocities associated with the springtime Hatteras Front.

4.4. Summer restratification

Upper layer MAB shelf water does not stay cool throughout thesummer, of course, another seasonal aspect of the temperatureevolution that is well described. Upper layer MAB water warmingin the southern MAB commences by late April in 1993, butapparently not until June in 1992, in the two year MMS T mooringdata shown by Savidge et al. (2007). Upper layer MAB shelf waterwas not frequently sampled in July FINCH, as indicated by theabsence of an upper layer MAB component in the frequency plot ofFig. 4B. However, a warm fresh light layer does appear in theindividual transects for July 2005 (see the example shown inFig. 3), which includes a strong thermocline above the lower layercold MAB water. Strongly stratified MAB conditions were in placeby May in 1996, as described from OMP data by Flagg et al. (2002).Their diagnosis indicated that the strong summertime T stratifica-tion was triggered by initial S stratification, which then facilitatedrapid strong warming of the upper layer MAB shelf water.

The T and depth of the surface warm layer presumably bothincrease over the summer, though details of that evolution inthese data sets is limited by the sparse vertical coverage typical ofmost mooring or shipboard data. The July 2005 warm layer is only5–10 m thick, whereas the late summer example from MMSAugust 1993 shows an upper layer that fills half the water columnacross the middle MAB shelf. With the limited summertimeFINCH sampling accomplished, it is unknown whether the warmlayer development is typical for mid-July, or if summer stratifica-tion became established somewhat later in 2005 than observed inother field programs.

In any case, the measurements shown suggest that interannualvariability in the timing of stratification onset, its eventualestablishment and ultimate strength may depend on interannualwind, S and T variability in both MAB and SAB settings. Thefurther issue is that the July FINCH data and the August MMS dataillustrate a density gradient across the Hatteras Front that is notof consistent sign in the upper and lower water columns. Asdescribed above, more complicated pressure gradient fieldsresult, with correspondingly more difficult to diagnose geos-trophic velocities along the Front in spring and through summer.The July 2005 ADCP data suggest that complexity, but otherwiseFINCH, MMS and the OMP data are insufficient to detail thesummer evolution of alongfront velocities.

5. Conclusions

The August and January FINCH shipboard undulating CTD andADCP sections across the nose of the Hatteras Front documentstrong property and density gradients across a relatively narrowfront, and strong shoreward velocities along the Front. Densestwaters reside on the SAB shelf water side, where the warm andsalty waters are several sy units denser than the MAB shelf water.Dynamic height gradients across the Front estimated from theAugust and January density fields are sufficient to support


geostrophic velocities of the order measured in the Front, andare taken to be the cause.

During the August FINCH observations, the shelf water deliv-ered shoreward by the alongfront surface intensified jet wasprimarily MAB shelf water, while in the January FINCH sections,the strong shoreward directed flow along the Front carriedprimarily SAB shelf water (Fig. 3). Since larval assemblages havebeen shown to be quite disparate across the Hatteras Front(Grothues and Cowen, 1999), this implies the Front may prefer-entially deliver MAB or SAB shelfwater larval assemblages shore-ward at different times, depending on the evolving density fieldor wind forcing, in ways which are presently undetermined.

The results from August and January FINCH are of interestregardless of whether the process is primarily a seasonal progres-sion or storm response. In either case, Savidge (2002) hasdemonstrated that strong alongfront shore directed velocitiesare a frequent occurrence in winter on the Hatteras shelf.Seasonal variability in T and its effects on the evolving densitysuggests that the density evolution may be repeated on a yearlybasis. This hypothesis is examined in Part 2.

The July 2005 FINCH sections indicate that for this particularyear, MAB shelf water density exceeded that of SAB shelf water.The resulting density gradient across the Front was of very lowmagnitude, and reversed in direction from the August and Januarycases. Velocities measured along the Front were weakly shore-ward in the lower layer, opposite that predicted from dynamicheight estimates from the density gradient. However, the dis-crepancy is small, given the low magnitudes of both predictedand measured velocities. The near equivalence of densities in theJuly FINCH and the MMS May 1992 data demonstrates thepossibility of reduced or reversed cross-shelf transport in thenose of the Front in spring and summer. Interannual variability inheat and freshwater fluxes within the MAB or SAB may determinethe sign and magnitude of the density gradient in spring andsummer.

The strong property gradients measured across the HatterasFront in all FINCH excursions are consistent with the mooringdata from the earlier MMS field program, and are to be expected,based on the significantly different life histories of the shelfwaters impinging on Cape Hatteras from the north and south.What had not been anticipated by prior work on the HatterasFront was that the density contrast across the Front where theseshelf waters meet might change significantly, as it did betweenthe three FINCH excursions.

In Part 2, several archived data sets are examined. That studyillustrates that the density evolution in MAB and SAB shelf watersseen in these FINCH and MMS hydrographic sections is character-istic of the climatological evolution of density with seasonal Tvariability in MAB and SAB shelf waters. The large densitycontrast in fall and winter defined here applies in the climatologythroughout the winter, from the fall mixing of MAB shelf watersuntil springtime warming begins to occur. Spring and summerdensity contrast between cold lower layer MAB shelf water andSAB upper and lower shelf waters in the climatology is relativelyweaker. This is consistent with the possibility of interannualvariability in the strength and direction of the density gradientbetween MAB and SAB shelf waters in spring, due to interannualvariability in S or T over either the MAB or SAB shelf.

Cross-shelf transport in the nose of the Hatteras Front impliedby the density fields of Part 2 is consistent with transportsnapshots shown here, with strong shoreward along Front velo-cities in winter, diminished and perhaps reversed in spring.Summertime velocities in the Front are still undetermined in signor vertical shear, and will vary with the differing contrastsbetween lower MAB and SAB shelf water and upper MAB andSAB shelf water. Both the implied robust shoreward transport in


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fall and winter and the weak or interannually variable cross-shelftransport in spring or summer have important implications forcross-shelf transport of biological or chemical constituents withinthe shelf waters near Cape Hatteras.

Acknowledgements

FINCH data collection was funded by NSF grant OCE-0406543.Anna Boyette’s work on graphics is greatly appreciated. Com-ments of two anonymous reviewers significantly improved thisarticle.

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