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The wind driven currents on the Middle Atlantic Bight inner shelf
Continental Shelf Research 19 (1999) 757 — 773
The wind driven currents on the Middle Atlantic Bight inner shelf Kuo-Chuin Wong College of Marine Studies, University of Delaware, Newark, DE 19716, USA Received 10 March 1998; accepted 28 August 1998
Abstract A set of month-long current meter data from two moorings deployed at the 20 and 30 m isobaths to the southeast of Atlantic City, New Jersey are examined to characterize the structure of the wind-induced subtidal currents in the Middle Atlantic Bight under summer time conditions. The wind stress and currents are dominated by variability at the 2—4 d time scales. The majority of the wind stress variance is oriented in the along-shelf direction (35°T), but the subtidal currents at both moorings also show substantial across-shelf variability, with the standard deviation of the across-shelf current component exceeding 50% of that of the along-shelf current component. Furthermore, there is an appreciable reduction in magnitude and a change in orientation of the subtidal current vector with depth. The currents from the mooring located at the 30 m isobath are significantly coherent with the wind stress, with surface current rotating clockwise and bottom current rotating anticlockwise of the wind. With an upwelling favorable wind, there is a significant offshore flow in the upper layer and an onshore flow in the lower layer, consistent with Ekman transport. The situation reverses with a downwelling favorable wind. The depth-averaged current is dominated by variability in the along-shelf direction. Wind stress and along-shelf surface slope are the leading terms in the depth-integrated along-shelf momentum balance, but bottom stress also plays an important role in the balance. The currents at the inner shelf mooring (20 m isobath) are much less coherent with the wind, particularly for the surface current. The reduced linear correlation between the wind and the observed subtidal current there may be caused by the influence of the buoyancy-driven coastal current originating from the Hudson River estuary to the north of the mooring site. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction During the past two decades, a considerable amount of research (e.g. Beardsley et al., 1976; Boicourt and Hacker, 1976; Masse, 1988) has been conducted to examine the characteristics of the subtidal currents in the Middle Atlantic Bight (MAB). These 0278—4343/99/$ — See front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 02 7 8— 4 34 3 ( 98 ) 0 01 0 7— 1
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studies show that the subtidal currents on the inner shelf are strongly influenced by meteorological forcing. Mayer (1982) reported that wind-coherent motions represent about 50% of the low frequency energy in the northern section of the New York Bight (NYB) off Long Island and as much as 70% in the southern section of NYB. In addition to the effect of local wind stress, southward propagating free waves also contribute to the subtidal sea level and current fluctuations in the MAB (Noble et al., 1983). A number of studies have looked into the vertical and/or across-shelf structure of the currents in different parts of the MAB. Bennett and Magnell (1979) have conducted a dynamical analysis of currents near the New Jersey coast using data from two current meters (at 5 and 10 m below the surface) deployed in water depth of about 13 m off Little Egg Inlet, New Jersey. They found that the depth-averaged subtidal current is dominated by variability in the along-shelf direction. They also found significant vertical shear in the along-shelf current. Han and Mayer (1981) have examined current records from a deployment of 17 current meters across a ridge and swale topography 6.5 km off the coast of Long Island, New York. Through empirical orthogonal modes analysis, they found that 79% of the variance is in the first barotropic mode with a strong vertical shear that is probably related to friction. The flow field is nearly parallel, and the orientations of the currents at different depths and locations are within 5° of one another. Mayer et al. (1982) have examined the currents in the Hudson Shelf Valley (HSV) during both the stratified and unstratified seasons. They did not find any significant relationship between the subtidal currents in the upper and lower parts of the water column during the stratified season. However, there is good vertical coherence throughout the low frequency bands during the unstratified winter season. The whole water column is in phase and the currents at different depths are roughly aligned along the same direction. As for the horizontal variability between currents in the inner valley and mid-shelf, they found that the near-bottom currents at the two sites are significantly coherent with each other for both the stratified and unstratified seasons. Above the pycnocline the currents in the inner valley are not coherent with those at mid-shelf in the meteorological band during the stratified season. More recently, Manning et al. (1994) have examined the structure of the near-bottom-currents around the NYB 12-mile dump site based on current meter moorings deployed in water depths from 20 m (near the mouth of New York Harbor) to 53 m (within HSV). They found that wind is most efficient in driving the subtidal currents in the 2—10 d time scales during winter. They also found substantial spatial variability in the bottom currents around the 12-mile dump site. In light of these developments, the present study examines some details in the subtidal currents in the MAB off the New Jersey coast under stratified summer time conditions. The strength and characteristics of the along-shelf and acrossshelf current variabilities are determined. This study examines the importance of the wind-induced variability on the observed currents and the way in which the structure of the wind-driven current changes as a function of depth and acrossshelf position. Furthermore, this study examines the leading factors which control the depth-integrated along-shelf momentum balance in the study area.
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2. Data sources A set of month-long current meter records are available from two moorings deployed by the National Ocean Service (NOS) between mid-June to mid-July 1984 over the inner continental shelf to the southeast of Atlantic City, New Jersey (Fig. 1). Mooring A was deployed at 18.3 m water depth and mooring B was deployed at a water depth of 30.5 m. Each mooring contains three Grundy current meters. The meters are located at 7.3, 10.4, and 14.9 m from the surface at mooting A. The current meters are mounted at 7.6, 15.2 m, and 27.4 m from the surface at mooring B. The placement of the current meters is designed to provide near-surface, mid-depth, and
Fig. 1. Location map of the study area. Current data are available from moorings A and B (marked by solid circles). Sea level data are available from Sandy Hook, Atlantic City, and Cape Henlopen. Wind data is available from environmental buoy 44009 (marked by open triangle). Bathymetry is in m.
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Fig. 2. The near-surface, mid-depth, and near-bottom subtidal current vectors at moorings A and B. Also shown are the subtidal kinematic wind stress vector and sea level at Sandy Hook (solid line), Atlantic City (dotted line), and Cape Henlopen (dash-dotted line).
near-bottom current measurements (Fig. 2). In addition to current speed and direction, the current meters also provide temperature and conductivity measurements which can be used to calculate salinity values (Fig. 3). The mid-depth current meter at mooring A malfunctioned halfway into the survey, resulting in a shortened record there. Since the measurements from this particular current meter are much shorter
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Fig. 3. The near-surface (solid line), mid-depth (dashed line), and near-bottom (dash-dotted line) subtidal temperature and salinity variations at moorings A and B.
than those from the other current meters, the data from mid-depth at mooring A is only used in a very limited fashion in the analysis. Details about the NOS survey can be found in Klavens et al. (1986). In addition to the data derived from the current meters, coastal sea level data at Sandy Hook, New Jersey, Atlantic City, New Jersey, and Cape Henlopen, Delaware are also obtained from the NOS (Fig 2). Furthermore, surface wind data from an environmental buoy (44009) located some 30 km offshore of Delaware Bay are obtained to characterized the wind conditions over the study area. Wind stress is calculated based on the formula of Wu (1980), and kinematic wind stress is then computed as wind stress divided by the water density (Masse, 1988). Since the emphasis here is on the subtidal variability, all the time series data are passed through a Lanczos low-pass filter with a cut-off period of 36 h. Details about the characteristics of the filter can be found in Bloomfield (1976).
3. Basic statistics of wind and currents The currents at both mooring sites show significant subtial fluctuations with magnitudes up to 20 cm/s, and the kinematic wind stress exhibits subtidal fluctuations with magnitudes up to 2 cm/s (Fig. 2). A right-handed coordinate system is adopted here such that a positive current/wind along the horizontal axis indicates eastward flow and a positive current/wind along the vertical axis indicates northward flow. The along-shelf direction, as defined by the general orientation of the shoreline and
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Table 1 Statistics of the currents and kinematic wind stress (q). Standard deviations (std dev) are computed for the along-shelf and across-shelf components of the currents and wind stress as well as the components along the principal and minor axis directions. Standard deviation for the current is in cm/s while that for the kinematic wind stress is in cm/s. Statistics for the mid-depth current at mooring A are computed using shortened records due to instrument malfunction A(s)
A(m)
A(b)
Alongshelf (std dev) Across-shelf (std dev) Principal axis (std dev) Minor axis (std dev)
7.2 3.3 7.2 3.3
6.5 3.9 6.9 3.2
3.8 3.5 4.9 1.6
Orientation of principal axis (°T)
34.9
18.6
358.1
B(s)
q
B(m)
B(b)
9.3 6.5 10.4 4.7
5.4 2.9 5.4 2.8
4.0 2.2 4.3 1.6
0.6 0.2 0.6 0.2
68.9
46.4
16.8
35.6
isobaths around the current meter moorings, is about 35°T. The principal axis of the wind stress vector, defined as the direction with the maximum wind stress variance, is aligned with the along-shelf direction. The standard deviation of the along-shelf wind stress component is a factor of three higher than that of the across-shelf wind stress component (Table 1). In contrast, the subtidal currents exhibit appreciable variance in the across-shelf direction. While the along-shelf current variability is stronger than the across-shelf current variability, the standard deviation of the across-shelf flow exceeds 50% of that of the along-shelf flow in most cases. Given the strength of the across-shelf current, the principal axis direction usually deviates substantially from the along-shelf direction. Furthermore, there are significant variations in the directions of the principal axes as a function of depth in the water column. At mooring B, for example, the orientation of the principal axis changes more than 50° between the near-surface and near-bottom currents. Generally speaking the standard deviation of the current decreases with depth, consistent with the effect of bottom friction. The currents are not rectilinear, as the standard deviation of the minor-axis current component may be as high as 50% of that along the principal axis. To establish the relationship between currents and wind stress, one can resolve the current and wind stress vectors into two orthogonal components and examine the relationship between individual pairs of wind stress/current components. One might resolve the wind stress/current vectors into their along-shelf and across-shelf components or into components along the principal and minor axes directions. However, given the substantial variations in the principal axes directions and the fact that the currents are not rectilinear, an analysis based on the wind stress/current components will be sensitive to the choice of the components involved. It is more desirable to examine the wind stress and currents in a way such that the results are independent of the rotation of the coordinate system used to resolve the vectors into components. For this reason, rotary spectral analysis for complex-valued vector time series (Gonella, 1972; Mooers, 1973) is performed to examine the overall relationship between wind stress and currents.
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4. The rotary spectrum Based on the analysis of Gonella (1972) and Mooers (1973), The tip of the current/wind vector in each frequency band traces out a hodograph ellipse over time. This elliptical motion can be further decomposed into a superposition of two counterrotating circular motions, one rotating clockwise and the other anticlockwise. The variance associated with the anticlockwise rotating circular motion is represented by the inner-autospectrum of the current vector at positive frequencies and the variance associated with the clockwise rotating motion is represented by the inner-autospectrum at negative frequencies. In the limiting case where the two counterrotating circular motions are equal in strength, the elliptical motion degenerates into rectilinear motion. In the other limiting case where one circular motion overwhelms the other counter-rotating circular motion, the elliptical motion approaches circular motion. The inner coherence squared measures the overall linear relationship between the co-rotating motion of two vectors. The inner phase (f (p)) between vectors 1 and 2 measures the temporal lag between the co-rotating clockwise circlar motions (negative frequency p(0) and the anticlockwise circular motions (positive frequency p'0) of these two vectors. To interpret the physical meaning of this quantity, it is more instructive to examine the difference in ellipse orientation (*a ) and ellipse phase difference (*b ). Since *a and *b are quantities associated with the elliptical motion, not the counter-rotating components of the ellipse, they are only defined for the modulus of the frequency s""p" (Mooers, 1973). They are computed as *a (s)"1/2 [f (s)#f (!s)] and *b (s)"1/2 [f (s)!f (!s)]. *a (s) rep resents the angular difference in the orientations of the hodograph ellipses between vectors 1 and 2, and *b (s) represents the temporal lag between the elliptical motions.
5. The wind-driven currents To examine the relationship between wind stress and currents, the inner coherence squared and inner phase between the wind stress vector and the current vectors at different depths are computed (Fig. 4). The abbreviated record at mid-depth at mooring A is not included in the analysis. The results show that currents at mooring B are significantly coherent with the wind stress, especially at the surface and the bottom. The inner coherence squared exceeds 0.8 in the positive frequency range, indicating that the anticlockwise co-rotating motion of the wind stress and current are highly correlated with each other. The currents at the inner shelf mooring (A) are less coherent with the wind stress. This decrease in coherene is especially evident for the near-surface current at mooring A. Based on the distribution of inner phase with frequency, the difference in ellipse orientation (*a) and ellipse phase difference (*b) between the wind stress vector and current vectors are computed (Fig. 5). At mooring B there is a significant difference in the ellipse orientation (about 30°) between the surface current and wind stress. The positive angular difference in the ellipse orientation indicates that the surface current vector rotates clockwise relative to the wind
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Fig. 4. The inner coherence squared and inner phase between wind stress and the near-surface (solid line), mid-depth (dotted line), and near-bottom (dash-dotted line) currents. The dashed line indicates the 95% significance level.
stress vector. At mid-depth there is no significant difference in the ellipse orientation between the current and wind stress, indicating that at this depth the current vector is roughly aligned with the wind stress vector. At the bottom the difference in the ellipse orientation indicates that the current veers anticlockwise from the wind stress by about 20°. At mooring A the distribution of *a indicates that the wind stress ellipse and the current ellipse from the upper-most instrument are oriented along the same direction over the entire subtidal frequency band. At the bottom the difference in ellipse orientation indicates that the current veers anticlockwise from the wind stress by about 40°. The distribution of *b indicates that the current and wind stress are nearly in-phase at very low frequencies, but there is a tendency for *b to increase with frequency (Fig. 5). The dependence of *b with frequency is approximately linear, indicating a roughly constant time lag between wind stress and the currents over the subtidal time scales. At mooring A the linear distribution of *b with frequency corresponds to a constant time lag of about 12 h. At mooring B the phase lag between the current and
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Fig. 5. The difference in ellipse orientation and ellipse phase difference between wind stress and the near-surface (solid line), mid-depth (dotted line), and near-bottom (dash-dotted line) currents.
wind stress is smaller at the surface than at mid-depth or the bottom, but the average time lag between the current and wind stress is again of the order 12 h. At mooring B the current meters are placed at the scaled depths (z/h) of 0.25, 0.50, and 0.90. The placement of these instruments provides a reasonable representation of the currents near the surface, mid-depth, and bottom. The data indicates that the surface current veers clockwise from the wind stress and the currents at mid-depth and the bottom veer anticlockwise from the surface current at mooring B. The fact that the near-surface current veers to the right of the wind and the currents at depth veer to the left of the surface current suggest the presence of surface and bottom Ekman layers. During the study period the principal axis of the wind stress vector is aligned with the along-shelf direction. Fluctuations in the wind stress along this direction would result in across-shelf Ekman transport in the surface Ekman layer, causing coastal setup/setdown and consequently inducing along-shelf geostrophic currents. Both the Ekman and the geostrophic currents are present in the surface Ekman layer, and the resultant current vector is expected to veer clockwise from the wind stress vector by an angle between 0 and 90°. The along-shelf geostrophic current would persist beneath the surface Ekman layer until it feels the effect of bottom friction. As this current
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Fig. 6. Coherence squared and phase between temperature and salinity at the near surface (solid line), mid-depth (dotted line), and near-bottom (dash-dotted line) current meters. The dashsed line represents the 95% significance level.
enters the bottom Ekman layer, it is expected to decrase in amplitude and veer anticlockwise relative to the along-shelf direction. However, the water depth at mooring B is only 30 m, which is of the same order as the Ekman depth. For unstratified water the surface and bottom Ekman layers would overlap, and the clockwise rotation of the surface Ekman spiral would largely cancel the anticlockwise rotation of the bottom Ekman spiral, resulting in currents which flow predominately in the wind direction (Ekman, 1905). However, the surface and bottom Ekman layers can be decoupled in shallow waters if the water column is sufficiently stratified. The temperature and salinity time series from mooring B (Fig. 3) show a mean surface to bottom temperature difference of 12.2°C and a salinity difference of 2.1 ppt. There are significant salinity and temperature fluctuations at the surface, but these fluctuations are drastically diminished at the bottom. A visual inspection reveals that the temporal variations in temperature are correlated with those of salinity. To further examine this point, the coherence squared and phase between salinity and temperature are computed at all three depths (Fig. 6). The results show that salinity and temperature are significantly coherent but nearly 180° out of phase over the subtidal frequency bands, especially at the surface and mid-depth. During summer the discharge from the estuaries adjacent to the inner shelf carries estuarine water which is fresher and warmer than the shelf water. This estuarine discharge may affect the salinity and temperature distributions in the study area. At the bottom the coherence between salinity and temperature decreases at frequencies lower than 0.2 cycles/d, suggesting that at very low frequencies the temperature at the bottom is not strongly influenced by the estuarine discharge. Given the low temperature involved (7°C), the
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temperature of the bottom water at mooring B is largely influenced by the characteristics of the cold pool on the MAB. Since the surface water is substantially warmer and fresher than the bottom water, both the effects of salinity and temperature give rise to strong stratification. This stratification may permit the decoupling of the surface and bottom Ekman layers and facilitate the observed clockwise rotation between the wind stress and the near-surface current vectors and the anticlockwise rotation between the surface current vector and current vectors at depth. This type of rotation in the current direction has been observed by Blanton and Atkinson (1983) on the inner continental shelf of the southeastern United States where a northward wind stress produces offshore transport near the surface and onshore transport near the bottom. At mooring A there is no siginficant directional variation between the current from the upper meter and wind. The placement of this current meter, at z/h"0.4, may not provide an adequate description of the near-surface current. Similar to the nearbottom current at mooring B, the near-bottom current at mooring A also shows substantial anticlickwise rotation from the wind. The salinity and temperature time series at mooring A may offer a clue to the weakened relationship between currents at this site and wind, especially for the near-surface current. At mooring A the water is less stratified, with a mean surface to bottom salinity difference of 0.8 ppt and temperature difference of 5.1°C. Compared to the situation at mooring B, the surface water at mooring A shows much stronger subtidal variations in salinity (Fig. 3). Furthermore, the surface to bottom salinity difference also shows substantial variation over the study period. These large variations in salinity and degree of stratifiation may indicate the influence of the buoyancy-driven coastal current originating from the Hudson River estuary to the north of the mooring site. The interactions between the buoyancy-driven coastal current and the wind-driven current may therefore reduce the linear correlation between the observed subtidal current and the wind. The reduction in coherence between current and wind is most evident for the upper layer flow at mooring A, as the coastal current is expected to have the strongest influence near the shore and the surface. The difference in the response of the current to wind is expected to produce spatial variability in the subtidal currents. To examine the across-shelf variation in the structure of the currents, the inner coherence squared, inner phase, difference in ellipse orientation and ellipse phase difference are computed between the near-surface currents at moorings A and B as well as those between the near-bottom currents (Fig. 7). It can be seen that the near-bottom current vectors are highly coherent and nearly in phase between the two sites some 30 km apart in the across-shelf direction. There is a 20° difference in the orientation of the current ellipses between the near-bottom currents. This 20° difference may be partly a result of the differences in local topographic steering. The near-surface currents at the two sites are much less coherent than those at the bottom. The currents in the upper layer are nearly in phase with each other, and there is a sgnificant difference (40°) in the orientation of the current ellipses between the two sites. The results thus show that that while the bottom currents exhibit only modest across-shelf variations, there are more signifiant across-self variations in the current vectors in the upper part of the water column. The
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Fig. 7. The inner coherence squared, inner phase, difference in ellipse orientation, and ellipse phase difference between the surface currents at moorings A and B (solid line) and between the bottom currents at moorings A and B (dash-dotted line). The dashed line indicates the 95% significance level.
low coherence between the near-surface currents is a direct result of the weakened linear relationship between wind stress and current at mooring A.
6. The depth-integrated alongshelf momentum balance The availability of uninterrupted data near the surface, mid-depth, and the bottom at mooring B permits the calculation of a depth-averaged current (¼ "u ?TE ?PE #iv ) there for the entire study period. The principal axis of ¼ deviates from the ?TE ?TE principal axis of the local wind stress (the along-shelf direction) by only about 14°. The standard deviation of the along-shelf component of ¼ is 2.5 times that of its ?TE across-shelf component. At mooring B the surface current veers clockwise of the predominately along-shelf wind stress and the bottom current veers anticlockwise of the wind. These counter-rotating motions are largely canceled out in the depthaveraging process. As a result, the along-shelf variability of ¼ is much stronger ?TE than its across-shelf variability. The dominance of along-shelf variability in the
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depth-averaged wind-driven flow off the New Jersey coast has also been demonstrated in an earlier study by Bennett and Magnell (1979). Given the dominance of the along-shelf component of ¼ , it is instructive to ?TE examine the relationship between this current component and the possible forcing mechanisms in more detail. The linearized depth-integrated alongshelf momentum balance can be simplified as h(*u /*t)"!gh(*g/*x)#(q !q ). (1) ?TE UV @V The local acceleration term on the left-hand side of Eq. (1) can be calculated based on the observed current. The first term on the right hand side of Eq. (1) is the depth integrated along-shelf barotropic pressure gradient which can be calculated based on the differences in the coastal sea level fluctuations between Cape Henlopen and Sandy Hook. q is the along-shelf component of the kinematic wind stress. q represents the UV @V kinematic bottom stress, and it can be computed as (r u ) where r is the linearized ?TE resistance coefficient. Following Pettigrew’s (1980) analysis on the appropriate linearized resistance coefficient for shelf waters, r is chosen to be 0.05 cm/s. Both the along-shelf pressure gradient and the kinematic wind stress terms show subtidal fluctuations with similar magnitudes up to about $2 cm/s. It is apparent that the along-shelf pressure gradient is not independent of the along-shelf wind stress. These two quantities are significantly coherent and out of phase with each other, indicating that a positive along-shelf wind stress is associated with a negative alongshelf pressure gradient. This is not surprising, as Lentz (1994) has shown through an examination of the wind-driven circulation in the northern California inner shelf that the along-shelf wind stress and along-shelf pressure gradient tend to be similar in magnitude but opposite in direction. In addition to these two leading terms, the bottom stress term also has a non-negligible contribution to the momentum balance.
Fig. 8. The left panel shows the along-shelf wind stress (solid line), the along-shelf depth-integrated barotropic pressure gradient (dashed line), and the along-shelf bottom stress (dash-dotted line). The right panel shows the sum of the along-shelf wind stress, bottom stress, and pressure gradient (solid line) and the depth-integrated local acceleration (dashed line).
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The right panel of Fig. 8 shows a comparison between the sum of the three terms on the right-hand side of the momentum equation and the local accelerations term. The discrepancy between the two sides of the momentum equation (Fig. 8, right panel) reflects either an inadequate estimation of the terms included in Eq. (1) or the effect of the terms not included there, such as the baroclinic pressure gradient, Coriolis, and nonlinear terms.
7. Discussion The present study shows that the subtidal current fluctuations on the Middle Atlantic Bight inner shelf off New Jersey are coherent throughout the water column, but there is a significant anticlockwise rotation of the current direction with depth. When compared to the local wind, the surface current vector veers significantly clockwise of the wind stress vector and the bottom current veers significantly anticlockwise of the wind stress. With the wind stress aligning roughly in the along-shelf direction, the clockwise and anticlockwise rotations of the current vectors result in very significant across-shelf flow at the surface and the bottom. The measurements indicate the presence of significant Ekman veering in the wind-driven subtidal circulation on the inner shelf off the New Jersey coast during the stratified season. In contrast, an earlier study by Mayer et al. (1982) found no clear relationship between the near surface current and wind stress in the Hudson Shelf Valley HSV) during the stratified season. They also did not observe any relationship between the subtidal currents in the upper and lower pats of the water column under stratified conditions. Mayer et al. (1982) did find wind-coherent low frequency motion throughout the water column during the unstratified season, but the currents at different depths are roughly aligned along the same direction in all the energetic frequency bands. The presence of wind-induced Ekman veering has been previously observed in coastal regions other than the New Jersey inner shelf. Smith (1977) has examined the near-bottom across-shelf currents in 13.5 m of water some 4.7 km off the central Texas Gulf coast. He found that the across-shelf current at 0.9 m above the bottom is coherent with the along-shelf component of the wind stress, suggesting a response to wind-induced Ekman transport. In a separate study off the central Texas Gulf coast, Smith (1978) deployed two current meters at 2 and 10 m above the bottom at the 17 m isobath 21.5 km off the coast of Port O’Connor, Texas. He found that the strong along-shelf current, of the order 70—80 cm/s, is produced by a combination of wind stress and along-shelf presure gradient. He also found evidence of Ekman veering in that onshore flow is observed at the upper meter and offshore flow is observed at the lower meter during a 10-day period with downwelling favorable wind conditions. While reporting the presence of Ekman veering, Smith (1978) also noted that the current at either depth is still overwhelmingly dominated by the along-shelf flow. A progressive vector calculation over the 10-day period indicates that the across-shelf flow is much weaker (by more than a factor of 4) than the along-shelf flow. Smith (1982) has also examined the current data from two current meters (9 m below the surface and 2 m above the bottom) at the 26 m isobath on the Florida inner shelf some
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23 km offshore of the Fort Pierce Inlet. He found similar patterns in the subtidal current fluctuations at the upper and lower layers. The vertical shears in the currents are due primarily to differences in speed rather than direction. He noted that to the extent that there is a turning of the current with increasing depth, it appears to be a slight turning to the right. Schwing et al. (1983) have examined the flow on the South Carolina continental shef by installing a current meter 10 km offshore at mid-depth in 10 m of water. They found that most of the subtidal current variability is concentrated within a frequency band on the order of 2—12 d, similar to the frequency band for wind events in the South Atlantic Bight (Kjerfve et al., 1978). Both the along-shelf and across-shelf currents are found to be significantly coherent with the along-shelf wind, suggesting the effect of wind-induced Ekman transport. Similar to the findings of Smith (1978), Schwing et al. (1983) also found that the variance of the along-shelf current to be much stronger (by a factor of 3) than the across-shelf current. Blanton (1981) has examined the current profiles at two closely spaced sites between the 12 and 14 m isobaths off Ossabaw Sound, Georgia. His findings show that the structure of the wind-induced current can be quite complicated. The average surface current is across-shelf at one site and almost along-shelf at the other site. Currents at both sites decreae and veer anticlockwise with depth, but the degree of turning is drastically different between the two sites. At one site the surface and bottom currents are only off by about 5°, but the orientations of the surface and bottom currents differ by more than 90° at the other site. Lentz (1994) has examined the vertical structure of the subtidal currents at the 30 m isobath over the northern California inner shelf under relatively unstratified conditions. He found that the along-shelf current components are 2.5—6.5 times stronger than the across-shelf components. Through EOF analysis, he found that the largest mode, which is oriented roughly along-shelf, can explain 89.2% of the total current variance. For this mode the current at any given depth only deviates from the along-shelf direction by less than 5°. The second EOF mode accounts for 7.7% of the total variance and is oriented roughly 30° clockwise relative to the local isobaths, with surface and bottom currents flowing in opposite directions. For the second EOF mode, there are very substantial variations in the orientation of the current with depth, with current rotating clockwise from surface to bottom. The findings of the present study are consistent with those from Bennett and Magnell (1979) and Lentz (1994) in the sense that the depth-averaged current is dominated by variability in the along-shelf direction. Furthermore, the subtidal flow is driven by both the along-shelf wind stress and the along-shelf pressure gradient which tend to be similar in magnitude but opposite in sign. This first-order balance is qualitatively consistent with the findings of Smith (1978) and Lentz (1994). However, the results from the present study also differ from some of the above mentioned studies in two areas. First of all, the present study shows that the across-shelf currents at the surface and the bottom are quite significant relatie to the along-shelf currents. At these depths the standard deviation of the across-shelf flow may exceed 60% of that of the along-shelf flow. Secondly, both the surface and bottom currents veer significantly relative to the wind, with the surface current rotating clockwise of the wind and
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bottom current rotating anticlockwise of the wind. Both of these features may be attributed to the presence of surface and bottom Ekman layers under stratified conditions. The separation of the two Ekman layers permits substantial turning of the currents with depth in each layer, thus resulting in significant across-shelf flow in the upper and lower parts of the water column. A comparison of the structure of the near-bottom subtidal currents reveals very little across-shelf variation between the two mooring sites. The near-surface currents are, however, only marginally coherent between the two sites. These features, including the high across-shelf coherence below the pycnocline and low coherence above the pycnocline, are qualitatively consistent with those found between the inner Hudson Shelf Valley and the mid-shelf in New York Bight (Mayer et al., 1982). Of all the currents measured, the one in the upper layer at the inshore site (mooring A) is the least coherent with wind stress. This low coherence suggests that the subtidal current there may be significantly influenced by the buoyancy-driven coastal current originating from the Hudson River estuary. The enhanced influence of the coastal current in the near shore region may play a significant role in the large across-shelf variation in the subtidal current structure above the pycnocline.
Acknowledgements This study was funded by the National Science Foundation under grant OCE9000158 and NOAA-Sea Grant under grant NA56RG0147 (project R/ME-16). Dr. Joy Moses-Hall performed the preliminary data processing. Jacqueline Bijansky assisted in typing the manuscript. The comments of two anonymous reviewers are valuable in improving the manuscript.
References Beardsley, R.C., Boicourt, W.C., Hansen, D.V., 1976. Physical oceanography of the Middle Atlantic Bight. American Society of Limnology and Oceanography Special Symposium 2, 20—34. Bendat, J.S., Piersol, A.G., 1971. Random Data: Analysis and Measurement Procedure. Wiley-Interscience, New York, 407 pp. Bennett, J.R., Magnell, B.A., 1979. A dynamical analysis of currents near the New Jersey Coast. Journal of Geophycal Research 84, 1165—1175. Blanton, J.O., 1981. Ocean currents along a nearshore frontal zone on the continental shelf of the southeastern United States. Journal of Physical Oceanography 11, 1627—1637. Blanton, J.O., Atkinson, L., 1983. Transport and fate of river discharge on the continental shelf of the southeastern United States. Journal of Geophysical Research 88, 4730—4738. Bloomfield, P., 1976. Fourier Analysis of Time Series: An Introduction. Wiley, New York, 258 pp. Boicourt, W.C., Hacker, P.W., 1976. Circulation on the Atlantic Continental Shelf of the United States, Cape May to Cape Hatteras. Memoire dela Societe Royale Science Liege 6 (10), 187—200. Ekman, V.W., 1905. On the influence of the earth’s rotation on ocean currents. Arkiv Matematik. Astronomi och Fysik, 2 (11), 52 pp. Gonella, J., 1972. A rotary-component method for analyzing meteorological and oceanographic vector time sseries. Deep Sea Research 19 (12), 833—846.
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Han, G.C., Mayer, D.A., 1981. Current structure on the Long Island inner shelf. Journal of Geophysical Research 86 (C5), 4205—4214. Kjerfve, B., Greer, J.E., Crout, R.L., 1978. Low-frequency response of estuarine sea level to non-local forcing. In: Wiley M. L. (Ed.), Estuarine Interactions. Academic Press, New York, pp. 497—513. Klavens, A., Stone, P., Stoney, G., 1986. Delaware River and Bay circulation survey: 1984—1985. NOS Oceanography Circulation Survey Report, 9. U. S. Department of Commerce, NOAA, 79 pp. Lentz, S.J., 1994. Current dynamics over the northern California inner shelf. Journal of Physical Oceanography 24, 2461—2478. Manning, J.P., Oey, L.Y., Packer, D., Vitaliano, J., Finneran, T.W., You, K.W., Fromm, S., 1994. Observations of bottom currents and estimates of resuspended sediment transport at the New York Bighat 12-mile dumpsite. Journal of Geophysical Research 99 (C5), 10 221—10 239. Masse, A. K., 1988. Estuary-shelf interaction: Delaware Bay and the adjacent inner shelf. Ph.D. dissertation, University of Delaware, 216 pp. Mayer, D.A., 1982. The structure of circulation: MESA physical oceanographic studies in New York Bight, 2. Journal of Geophysical Research 87 (C12), 9579—9588. Mayer, D.A., Han G.C., Hansen, D.V., 1982. Circulation in the Hudson Shelf Valley: MESA physical oceanographic studies in New York Bight, 1. Journal Geophysical Research 87 (C12), 9563—9578. Mooers, C.N.K., 1973. A technique for the cross-spectrum analysis of pairs of complex-valued time series, with emphasis on properties of polarized components and rotational invariants. Deep Sea Research 20, 1129—1141. Noble, M., Butman, B., Williams, E., 1983. On the longshelf structure and dynamics of subtidal currents on the eastern United States continental shelf. Journal of Physical Oceanography 13, 2125—2147. Pattigrew, N.R., 1980. The dynamics and kinematics of the coastal boundary layer off Long Island, Ph. D. thesis, MIT/WHOI Joint Program in Oceanography, Woods Hole, MA, 262 pp. Schwing, F.B., Kjerfve, B., Sneed, J.E., 1983. Nearshore coastal currents on the South Carolina continental shelf. Journal of Geophysiche Research 88, 4719—4729. Smith, N.P., 1977. Near-bottom cross-shelf currents in the northwestern Gulf of Mexico: a response to wind forcing. Journal of Physical Oceanography, 7, 615—620. Smith, N.P., 1978. Longshore currents on the fringe of Hurricane Anita. Journal of Geophysical Research 83, 6047—6051. Smith, N.P., 1982. Response of Florida Atlantic shelf waters to Hurricane David. Journal of Geophysical Research 87, 2007—2016. Wu, J., 1980. Wind-stress coefficients over sea surface near neutral condition — a revisit. Journal of Physical Oceanography 10, 727—740.
The wind driven currents on the Middle Atlantic Bight inner shelf Kuo-Chuin Wong College of Marine Studies, University of Delaware, Newark, DE 19716, USA Received 10 March 1998; accepted 28 August 1998
Abstract A set of month-long current meter data from two moorings deployed at the 20 and 30 m isobaths to the southeast of Atlantic City, New Jersey are examined to characterize the structure of the wind-induced subtidal currents in the Middle Atlantic Bight under summer time conditions. The wind stress and currents are dominated by variability at the 2—4 d time scales. The majority of the wind stress variance is oriented in the along-shelf direction (35°T), but the subtidal currents at both moorings also show substantial across-shelf variability, with the standard deviation of the across-shelf current component exceeding 50% of that of the along-shelf current component. Furthermore, there is an appreciable reduction in magnitude and a change in orientation of the subtidal current vector with depth. The currents from the mooring located at the 30 m isobath are significantly coherent with the wind stress, with surface current rotating clockwise and bottom current rotating anticlockwise of the wind. With an upwelling favorable wind, there is a significant offshore flow in the upper layer and an onshore flow in the lower layer, consistent with Ekman transport. The situation reverses with a downwelling favorable wind. The depth-averaged current is dominated by variability in the along-shelf direction. Wind stress and along-shelf surface slope are the leading terms in the depth-integrated along-shelf momentum balance, but bottom stress also plays an important role in the balance. The currents at the inner shelf mooring (20 m isobath) are much less coherent with the wind, particularly for the surface current. The reduced linear correlation between the wind and the observed subtidal current there may be caused by the influence of the buoyancy-driven coastal current originating from the Hudson River estuary to the north of the mooring site. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction During the past two decades, a considerable amount of research (e.g. Beardsley et al., 1976; Boicourt and Hacker, 1976; Masse, 1988) has been conducted to examine the characteristics of the subtidal currents in the Middle Atlantic Bight (MAB). These 0278—4343/99/$ — See front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 02 7 8— 4 34 3 ( 98 ) 0 01 0 7— 1
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studies show that the subtidal currents on the inner shelf are strongly influenced by meteorological forcing. Mayer (1982) reported that wind-coherent motions represent about 50% of the low frequency energy in the northern section of the New York Bight (NYB) off Long Island and as much as 70% in the southern section of NYB. In addition to the effect of local wind stress, southward propagating free waves also contribute to the subtidal sea level and current fluctuations in the MAB (Noble et al., 1983). A number of studies have looked into the vertical and/or across-shelf structure of the currents in different parts of the MAB. Bennett and Magnell (1979) have conducted a dynamical analysis of currents near the New Jersey coast using data from two current meters (at 5 and 10 m below the surface) deployed in water depth of about 13 m off Little Egg Inlet, New Jersey. They found that the depth-averaged subtidal current is dominated by variability in the along-shelf direction. They also found significant vertical shear in the along-shelf current. Han and Mayer (1981) have examined current records from a deployment of 17 current meters across a ridge and swale topography 6.5 km off the coast of Long Island, New York. Through empirical orthogonal modes analysis, they found that 79% of the variance is in the first barotropic mode with a strong vertical shear that is probably related to friction. The flow field is nearly parallel, and the orientations of the currents at different depths and locations are within 5° of one another. Mayer et al. (1982) have examined the currents in the Hudson Shelf Valley (HSV) during both the stratified and unstratified seasons. They did not find any significant relationship between the subtidal currents in the upper and lower parts of the water column during the stratified season. However, there is good vertical coherence throughout the low frequency bands during the unstratified winter season. The whole water column is in phase and the currents at different depths are roughly aligned along the same direction. As for the horizontal variability between currents in the inner valley and mid-shelf, they found that the near-bottom currents at the two sites are significantly coherent with each other for both the stratified and unstratified seasons. Above the pycnocline the currents in the inner valley are not coherent with those at mid-shelf in the meteorological band during the stratified season. More recently, Manning et al. (1994) have examined the structure of the near-bottom-currents around the NYB 12-mile dump site based on current meter moorings deployed in water depths from 20 m (near the mouth of New York Harbor) to 53 m (within HSV). They found that wind is most efficient in driving the subtidal currents in the 2—10 d time scales during winter. They also found substantial spatial variability in the bottom currents around the 12-mile dump site. In light of these developments, the present study examines some details in the subtidal currents in the MAB off the New Jersey coast under stratified summer time conditions. The strength and characteristics of the along-shelf and acrossshelf current variabilities are determined. This study examines the importance of the wind-induced variability on the observed currents and the way in which the structure of the wind-driven current changes as a function of depth and acrossshelf position. Furthermore, this study examines the leading factors which control the depth-integrated along-shelf momentum balance in the study area.
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2. Data sources A set of month-long current meter records are available from two moorings deployed by the National Ocean Service (NOS) between mid-June to mid-July 1984 over the inner continental shelf to the southeast of Atlantic City, New Jersey (Fig. 1). Mooring A was deployed at 18.3 m water depth and mooring B was deployed at a water depth of 30.5 m. Each mooring contains three Grundy current meters. The meters are located at 7.3, 10.4, and 14.9 m from the surface at mooting A. The current meters are mounted at 7.6, 15.2 m, and 27.4 m from the surface at mooring B. The placement of the current meters is designed to provide near-surface, mid-depth, and
Fig. 1. Location map of the study area. Current data are available from moorings A and B (marked by solid circles). Sea level data are available from Sandy Hook, Atlantic City, and Cape Henlopen. Wind data is available from environmental buoy 44009 (marked by open triangle). Bathymetry is in m.
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Fig. 2. The near-surface, mid-depth, and near-bottom subtidal current vectors at moorings A and B. Also shown are the subtidal kinematic wind stress vector and sea level at Sandy Hook (solid line), Atlantic City (dotted line), and Cape Henlopen (dash-dotted line).
near-bottom current measurements (Fig. 2). In addition to current speed and direction, the current meters also provide temperature and conductivity measurements which can be used to calculate salinity values (Fig. 3). The mid-depth current meter at mooring A malfunctioned halfway into the survey, resulting in a shortened record there. Since the measurements from this particular current meter are much shorter
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Fig. 3. The near-surface (solid line), mid-depth (dashed line), and near-bottom (dash-dotted line) subtidal temperature and salinity variations at moorings A and B.
than those from the other current meters, the data from mid-depth at mooring A is only used in a very limited fashion in the analysis. Details about the NOS survey can be found in Klavens et al. (1986). In addition to the data derived from the current meters, coastal sea level data at Sandy Hook, New Jersey, Atlantic City, New Jersey, and Cape Henlopen, Delaware are also obtained from the NOS (Fig 2). Furthermore, surface wind data from an environmental buoy (44009) located some 30 km offshore of Delaware Bay are obtained to characterized the wind conditions over the study area. Wind stress is calculated based on the formula of Wu (1980), and kinematic wind stress is then computed as wind stress divided by the water density (Masse, 1988). Since the emphasis here is on the subtidal variability, all the time series data are passed through a Lanczos low-pass filter with a cut-off period of 36 h. Details about the characteristics of the filter can be found in Bloomfield (1976).
3. Basic statistics of wind and currents The currents at both mooring sites show significant subtial fluctuations with magnitudes up to 20 cm/s, and the kinematic wind stress exhibits subtidal fluctuations with magnitudes up to 2 cm/s (Fig. 2). A right-handed coordinate system is adopted here such that a positive current/wind along the horizontal axis indicates eastward flow and a positive current/wind along the vertical axis indicates northward flow. The along-shelf direction, as defined by the general orientation of the shoreline and
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Table 1 Statistics of the currents and kinematic wind stress (q). Standard deviations (std dev) are computed for the along-shelf and across-shelf components of the currents and wind stress as well as the components along the principal and minor axis directions. Standard deviation for the current is in cm/s while that for the kinematic wind stress is in cm/s. Statistics for the mid-depth current at mooring A are computed using shortened records due to instrument malfunction A(s)
A(m)
A(b)
Alongshelf (std dev) Across-shelf (std dev) Principal axis (std dev) Minor axis (std dev)
7.2 3.3 7.2 3.3
6.5 3.9 6.9 3.2
3.8 3.5 4.9 1.6
Orientation of principal axis (°T)
34.9
18.6
358.1
B(s)
q
B(m)
B(b)
9.3 6.5 10.4 4.7
5.4 2.9 5.4 2.8
4.0 2.2 4.3 1.6
0.6 0.2 0.6 0.2
68.9
46.4
16.8
35.6
isobaths around the current meter moorings, is about 35°T. The principal axis of the wind stress vector, defined as the direction with the maximum wind stress variance, is aligned with the along-shelf direction. The standard deviation of the along-shelf wind stress component is a factor of three higher than that of the across-shelf wind stress component (Table 1). In contrast, the subtidal currents exhibit appreciable variance in the across-shelf direction. While the along-shelf current variability is stronger than the across-shelf current variability, the standard deviation of the across-shelf flow exceeds 50% of that of the along-shelf flow in most cases. Given the strength of the across-shelf current, the principal axis direction usually deviates substantially from the along-shelf direction. Furthermore, there are significant variations in the directions of the principal axes as a function of depth in the water column. At mooring B, for example, the orientation of the principal axis changes more than 50° between the near-surface and near-bottom currents. Generally speaking the standard deviation of the current decreases with depth, consistent with the effect of bottom friction. The currents are not rectilinear, as the standard deviation of the minor-axis current component may be as high as 50% of that along the principal axis. To establish the relationship between currents and wind stress, one can resolve the current and wind stress vectors into two orthogonal components and examine the relationship between individual pairs of wind stress/current components. One might resolve the wind stress/current vectors into their along-shelf and across-shelf components or into components along the principal and minor axes directions. However, given the substantial variations in the principal axes directions and the fact that the currents are not rectilinear, an analysis based on the wind stress/current components will be sensitive to the choice of the components involved. It is more desirable to examine the wind stress and currents in a way such that the results are independent of the rotation of the coordinate system used to resolve the vectors into components. For this reason, rotary spectral analysis for complex-valued vector time series (Gonella, 1972; Mooers, 1973) is performed to examine the overall relationship between wind stress and currents.
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4. The rotary spectrum Based on the analysis of Gonella (1972) and Mooers (1973), The tip of the current/wind vector in each frequency band traces out a hodograph ellipse over time. This elliptical motion can be further decomposed into a superposition of two counterrotating circular motions, one rotating clockwise and the other anticlockwise. The variance associated with the anticlockwise rotating circular motion is represented by the inner-autospectrum of the current vector at positive frequencies and the variance associated with the clockwise rotating motion is represented by the inner-autospectrum at negative frequencies. In the limiting case where the two counterrotating circular motions are equal in strength, the elliptical motion degenerates into rectilinear motion. In the other limiting case where one circular motion overwhelms the other counter-rotating circular motion, the elliptical motion approaches circular motion. The inner coherence squared measures the overall linear relationship between the co-rotating motion of two vectors. The inner phase (f (p)) between vectors 1 and 2 measures the temporal lag between the co-rotating clockwise circlar motions (negative frequency p(0) and the anticlockwise circular motions (positive frequency p'0) of these two vectors. To interpret the physical meaning of this quantity, it is more instructive to examine the difference in ellipse orientation (*a ) and ellipse phase difference (*b ). Since *a and *b are quantities associated with the elliptical motion, not the counter-rotating components of the ellipse, they are only defined for the modulus of the frequency s""p" (Mooers, 1973). They are computed as *a (s)"1/2 [f (s)#f (!s)] and *b (s)"1/2 [f (s)!f (!s)]. *a (s) rep resents the angular difference in the orientations of the hodograph ellipses between vectors 1 and 2, and *b (s) represents the temporal lag between the elliptical motions.
5. The wind-driven currents To examine the relationship between wind stress and currents, the inner coherence squared and inner phase between the wind stress vector and the current vectors at different depths are computed (Fig. 4). The abbreviated record at mid-depth at mooring A is not included in the analysis. The results show that currents at mooring B are significantly coherent with the wind stress, especially at the surface and the bottom. The inner coherence squared exceeds 0.8 in the positive frequency range, indicating that the anticlockwise co-rotating motion of the wind stress and current are highly correlated with each other. The currents at the inner shelf mooring (A) are less coherent with the wind stress. This decrease in coherene is especially evident for the near-surface current at mooring A. Based on the distribution of inner phase with frequency, the difference in ellipse orientation (*a) and ellipse phase difference (*b) between the wind stress vector and current vectors are computed (Fig. 5). At mooring B there is a significant difference in the ellipse orientation (about 30°) between the surface current and wind stress. The positive angular difference in the ellipse orientation indicates that the surface current vector rotates clockwise relative to the wind
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Fig. 4. The inner coherence squared and inner phase between wind stress and the near-surface (solid line), mid-depth (dotted line), and near-bottom (dash-dotted line) currents. The dashed line indicates the 95% significance level.
stress vector. At mid-depth there is no significant difference in the ellipse orientation between the current and wind stress, indicating that at this depth the current vector is roughly aligned with the wind stress vector. At the bottom the difference in the ellipse orientation indicates that the current veers anticlockwise from the wind stress by about 20°. At mooring A the distribution of *a indicates that the wind stress ellipse and the current ellipse from the upper-most instrument are oriented along the same direction over the entire subtidal frequency band. At the bottom the difference in ellipse orientation indicates that the current veers anticlockwise from the wind stress by about 40°. The distribution of *b indicates that the current and wind stress are nearly in-phase at very low frequencies, but there is a tendency for *b to increase with frequency (Fig. 5). The dependence of *b with frequency is approximately linear, indicating a roughly constant time lag between wind stress and the currents over the subtidal time scales. At mooring A the linear distribution of *b with frequency corresponds to a constant time lag of about 12 h. At mooring B the phase lag between the current and
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Fig. 5. The difference in ellipse orientation and ellipse phase difference between wind stress and the near-surface (solid line), mid-depth (dotted line), and near-bottom (dash-dotted line) currents.
wind stress is smaller at the surface than at mid-depth or the bottom, but the average time lag between the current and wind stress is again of the order 12 h. At mooring B the current meters are placed at the scaled depths (z/h) of 0.25, 0.50, and 0.90. The placement of these instruments provides a reasonable representation of the currents near the surface, mid-depth, and bottom. The data indicates that the surface current veers clockwise from the wind stress and the currents at mid-depth and the bottom veer anticlockwise from the surface current at mooring B. The fact that the near-surface current veers to the right of the wind and the currents at depth veer to the left of the surface current suggest the presence of surface and bottom Ekman layers. During the study period the principal axis of the wind stress vector is aligned with the along-shelf direction. Fluctuations in the wind stress along this direction would result in across-shelf Ekman transport in the surface Ekman layer, causing coastal setup/setdown and consequently inducing along-shelf geostrophic currents. Both the Ekman and the geostrophic currents are present in the surface Ekman layer, and the resultant current vector is expected to veer clockwise from the wind stress vector by an angle between 0 and 90°. The along-shelf geostrophic current would persist beneath the surface Ekman layer until it feels the effect of bottom friction. As this current
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Fig. 6. Coherence squared and phase between temperature and salinity at the near surface (solid line), mid-depth (dotted line), and near-bottom (dash-dotted line) current meters. The dashsed line represents the 95% significance level.
enters the bottom Ekman layer, it is expected to decrase in amplitude and veer anticlockwise relative to the along-shelf direction. However, the water depth at mooring B is only 30 m, which is of the same order as the Ekman depth. For unstratified water the surface and bottom Ekman layers would overlap, and the clockwise rotation of the surface Ekman spiral would largely cancel the anticlockwise rotation of the bottom Ekman spiral, resulting in currents which flow predominately in the wind direction (Ekman, 1905). However, the surface and bottom Ekman layers can be decoupled in shallow waters if the water column is sufficiently stratified. The temperature and salinity time series from mooring B (Fig. 3) show a mean surface to bottom temperature difference of 12.2°C and a salinity difference of 2.1 ppt. There are significant salinity and temperature fluctuations at the surface, but these fluctuations are drastically diminished at the bottom. A visual inspection reveals that the temporal variations in temperature are correlated with those of salinity. To further examine this point, the coherence squared and phase between salinity and temperature are computed at all three depths (Fig. 6). The results show that salinity and temperature are significantly coherent but nearly 180° out of phase over the subtidal frequency bands, especially at the surface and mid-depth. During summer the discharge from the estuaries adjacent to the inner shelf carries estuarine water which is fresher and warmer than the shelf water. This estuarine discharge may affect the salinity and temperature distributions in the study area. At the bottom the coherence between salinity and temperature decreases at frequencies lower than 0.2 cycles/d, suggesting that at very low frequencies the temperature at the bottom is not strongly influenced by the estuarine discharge. Given the low temperature involved (7°C), the
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temperature of the bottom water at mooring B is largely influenced by the characteristics of the cold pool on the MAB. Since the surface water is substantially warmer and fresher than the bottom water, both the effects of salinity and temperature give rise to strong stratification. This stratification may permit the decoupling of the surface and bottom Ekman layers and facilitate the observed clockwise rotation between the wind stress and the near-surface current vectors and the anticlockwise rotation between the surface current vector and current vectors at depth. This type of rotation in the current direction has been observed by Blanton and Atkinson (1983) on the inner continental shelf of the southeastern United States where a northward wind stress produces offshore transport near the surface and onshore transport near the bottom. At mooring A there is no siginficant directional variation between the current from the upper meter and wind. The placement of this current meter, at z/h"0.4, may not provide an adequate description of the near-surface current. Similar to the nearbottom current at mooring B, the near-bottom current at mooring A also shows substantial anticlickwise rotation from the wind. The salinity and temperature time series at mooring A may offer a clue to the weakened relationship between currents at this site and wind, especially for the near-surface current. At mooring A the water is less stratified, with a mean surface to bottom salinity difference of 0.8 ppt and temperature difference of 5.1°C. Compared to the situation at mooring B, the surface water at mooring A shows much stronger subtidal variations in salinity (Fig. 3). Furthermore, the surface to bottom salinity difference also shows substantial variation over the study period. These large variations in salinity and degree of stratifiation may indicate the influence of the buoyancy-driven coastal current originating from the Hudson River estuary to the north of the mooring site. The interactions between the buoyancy-driven coastal current and the wind-driven current may therefore reduce the linear correlation between the observed subtidal current and the wind. The reduction in coherence between current and wind is most evident for the upper layer flow at mooring A, as the coastal current is expected to have the strongest influence near the shore and the surface. The difference in the response of the current to wind is expected to produce spatial variability in the subtidal currents. To examine the across-shelf variation in the structure of the currents, the inner coherence squared, inner phase, difference in ellipse orientation and ellipse phase difference are computed between the near-surface currents at moorings A and B as well as those between the near-bottom currents (Fig. 7). It can be seen that the near-bottom current vectors are highly coherent and nearly in phase between the two sites some 30 km apart in the across-shelf direction. There is a 20° difference in the orientation of the current ellipses between the near-bottom currents. This 20° difference may be partly a result of the differences in local topographic steering. The near-surface currents at the two sites are much less coherent than those at the bottom. The currents in the upper layer are nearly in phase with each other, and there is a sgnificant difference (40°) in the orientation of the current ellipses between the two sites. The results thus show that that while the bottom currents exhibit only modest across-shelf variations, there are more signifiant across-self variations in the current vectors in the upper part of the water column. The
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Fig. 7. The inner coherence squared, inner phase, difference in ellipse orientation, and ellipse phase difference between the surface currents at moorings A and B (solid line) and between the bottom currents at moorings A and B (dash-dotted line). The dashed line indicates the 95% significance level.
low coherence between the near-surface currents is a direct result of the weakened linear relationship between wind stress and current at mooring A.
6. The depth-integrated alongshelf momentum balance The availability of uninterrupted data near the surface, mid-depth, and the bottom at mooring B permits the calculation of a depth-averaged current (¼ "u ?TE ?PE #iv ) there for the entire study period. The principal axis of ¼ deviates from the ?TE ?TE principal axis of the local wind stress (the along-shelf direction) by only about 14°. The standard deviation of the along-shelf component of ¼ is 2.5 times that of its ?TE across-shelf component. At mooring B the surface current veers clockwise of the predominately along-shelf wind stress and the bottom current veers anticlockwise of the wind. These counter-rotating motions are largely canceled out in the depthaveraging process. As a result, the along-shelf variability of ¼ is much stronger ?TE than its across-shelf variability. The dominance of along-shelf variability in the
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depth-averaged wind-driven flow off the New Jersey coast has also been demonstrated in an earlier study by Bennett and Magnell (1979). Given the dominance of the along-shelf component of ¼ , it is instructive to ?TE examine the relationship between this current component and the possible forcing mechanisms in more detail. The linearized depth-integrated alongshelf momentum balance can be simplified as h(*u /*t)"!gh(*g/*x)#(q !q ). (1) ?TE UV @V The local acceleration term on the left-hand side of Eq. (1) can be calculated based on the observed current. The first term on the right hand side of Eq. (1) is the depth integrated along-shelf barotropic pressure gradient which can be calculated based on the differences in the coastal sea level fluctuations between Cape Henlopen and Sandy Hook. q is the along-shelf component of the kinematic wind stress. q represents the UV @V kinematic bottom stress, and it can be computed as (r u ) where r is the linearized ?TE resistance coefficient. Following Pettigrew’s (1980) analysis on the appropriate linearized resistance coefficient for shelf waters, r is chosen to be 0.05 cm/s. Both the along-shelf pressure gradient and the kinematic wind stress terms show subtidal fluctuations with similar magnitudes up to about $2 cm/s. It is apparent that the along-shelf pressure gradient is not independent of the along-shelf wind stress. These two quantities are significantly coherent and out of phase with each other, indicating that a positive along-shelf wind stress is associated with a negative alongshelf pressure gradient. This is not surprising, as Lentz (1994) has shown through an examination of the wind-driven circulation in the northern California inner shelf that the along-shelf wind stress and along-shelf pressure gradient tend to be similar in magnitude but opposite in direction. In addition to these two leading terms, the bottom stress term also has a non-negligible contribution to the momentum balance.
Fig. 8. The left panel shows the along-shelf wind stress (solid line), the along-shelf depth-integrated barotropic pressure gradient (dashed line), and the along-shelf bottom stress (dash-dotted line). The right panel shows the sum of the along-shelf wind stress, bottom stress, and pressure gradient (solid line) and the depth-integrated local acceleration (dashed line).
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The right panel of Fig. 8 shows a comparison between the sum of the three terms on the right-hand side of the momentum equation and the local accelerations term. The discrepancy between the two sides of the momentum equation (Fig. 8, right panel) reflects either an inadequate estimation of the terms included in Eq. (1) or the effect of the terms not included there, such as the baroclinic pressure gradient, Coriolis, and nonlinear terms.
7. Discussion The present study shows that the subtidal current fluctuations on the Middle Atlantic Bight inner shelf off New Jersey are coherent throughout the water column, but there is a significant anticlockwise rotation of the current direction with depth. When compared to the local wind, the surface current vector veers significantly clockwise of the wind stress vector and the bottom current veers significantly anticlockwise of the wind stress. With the wind stress aligning roughly in the along-shelf direction, the clockwise and anticlockwise rotations of the current vectors result in very significant across-shelf flow at the surface and the bottom. The measurements indicate the presence of significant Ekman veering in the wind-driven subtidal circulation on the inner shelf off the New Jersey coast during the stratified season. In contrast, an earlier study by Mayer et al. (1982) found no clear relationship between the near surface current and wind stress in the Hudson Shelf Valley HSV) during the stratified season. They also did not observe any relationship between the subtidal currents in the upper and lower pats of the water column under stratified conditions. Mayer et al. (1982) did find wind-coherent low frequency motion throughout the water column during the unstratified season, but the currents at different depths are roughly aligned along the same direction in all the energetic frequency bands. The presence of wind-induced Ekman veering has been previously observed in coastal regions other than the New Jersey inner shelf. Smith (1977) has examined the near-bottom across-shelf currents in 13.5 m of water some 4.7 km off the central Texas Gulf coast. He found that the across-shelf current at 0.9 m above the bottom is coherent with the along-shelf component of the wind stress, suggesting a response to wind-induced Ekman transport. In a separate study off the central Texas Gulf coast, Smith (1978) deployed two current meters at 2 and 10 m above the bottom at the 17 m isobath 21.5 km off the coast of Port O’Connor, Texas. He found that the strong along-shelf current, of the order 70—80 cm/s, is produced by a combination of wind stress and along-shelf presure gradient. He also found evidence of Ekman veering in that onshore flow is observed at the upper meter and offshore flow is observed at the lower meter during a 10-day period with downwelling favorable wind conditions. While reporting the presence of Ekman veering, Smith (1978) also noted that the current at either depth is still overwhelmingly dominated by the along-shelf flow. A progressive vector calculation over the 10-day period indicates that the across-shelf flow is much weaker (by more than a factor of 4) than the along-shelf flow. Smith (1982) has also examined the current data from two current meters (9 m below the surface and 2 m above the bottom) at the 26 m isobath on the Florida inner shelf some
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23 km offshore of the Fort Pierce Inlet. He found similar patterns in the subtidal current fluctuations at the upper and lower layers. The vertical shears in the currents are due primarily to differences in speed rather than direction. He noted that to the extent that there is a turning of the current with increasing depth, it appears to be a slight turning to the right. Schwing et al. (1983) have examined the flow on the South Carolina continental shef by installing a current meter 10 km offshore at mid-depth in 10 m of water. They found that most of the subtidal current variability is concentrated within a frequency band on the order of 2—12 d, similar to the frequency band for wind events in the South Atlantic Bight (Kjerfve et al., 1978). Both the along-shelf and across-shelf currents are found to be significantly coherent with the along-shelf wind, suggesting the effect of wind-induced Ekman transport. Similar to the findings of Smith (1978), Schwing et al. (1983) also found that the variance of the along-shelf current to be much stronger (by a factor of 3) than the across-shelf current. Blanton (1981) has examined the current profiles at two closely spaced sites between the 12 and 14 m isobaths off Ossabaw Sound, Georgia. His findings show that the structure of the wind-induced current can be quite complicated. The average surface current is across-shelf at one site and almost along-shelf at the other site. Currents at both sites decreae and veer anticlockwise with depth, but the degree of turning is drastically different between the two sites. At one site the surface and bottom currents are only off by about 5°, but the orientations of the surface and bottom currents differ by more than 90° at the other site. Lentz (1994) has examined the vertical structure of the subtidal currents at the 30 m isobath over the northern California inner shelf under relatively unstratified conditions. He found that the along-shelf current components are 2.5—6.5 times stronger than the across-shelf components. Through EOF analysis, he found that the largest mode, which is oriented roughly along-shelf, can explain 89.2% of the total current variance. For this mode the current at any given depth only deviates from the along-shelf direction by less than 5°. The second EOF mode accounts for 7.7% of the total variance and is oriented roughly 30° clockwise relative to the local isobaths, with surface and bottom currents flowing in opposite directions. For the second EOF mode, there are very substantial variations in the orientation of the current with depth, with current rotating clockwise from surface to bottom. The findings of the present study are consistent with those from Bennett and Magnell (1979) and Lentz (1994) in the sense that the depth-averaged current is dominated by variability in the along-shelf direction. Furthermore, the subtidal flow is driven by both the along-shelf wind stress and the along-shelf pressure gradient which tend to be similar in magnitude but opposite in sign. This first-order balance is qualitatively consistent with the findings of Smith (1978) and Lentz (1994). However, the results from the present study also differ from some of the above mentioned studies in two areas. First of all, the present study shows that the across-shelf currents at the surface and the bottom are quite significant relatie to the along-shelf currents. At these depths the standard deviation of the across-shelf flow may exceed 60% of that of the along-shelf flow. Secondly, both the surface and bottom currents veer significantly relative to the wind, with the surface current rotating clockwise of the wind and
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bottom current rotating anticlockwise of the wind. Both of these features may be attributed to the presence of surface and bottom Ekman layers under stratified conditions. The separation of the two Ekman layers permits substantial turning of the currents with depth in each layer, thus resulting in significant across-shelf flow in the upper and lower parts of the water column. A comparison of the structure of the near-bottom subtidal currents reveals very little across-shelf variation between the two mooring sites. The near-surface currents are, however, only marginally coherent between the two sites. These features, including the high across-shelf coherence below the pycnocline and low coherence above the pycnocline, are qualitatively consistent with those found between the inner Hudson Shelf Valley and the mid-shelf in New York Bight (Mayer et al., 1982). Of all the currents measured, the one in the upper layer at the inshore site (mooring A) is the least coherent with wind stress. This low coherence suggests that the subtidal current there may be significantly influenced by the buoyancy-driven coastal current originating from the Hudson River estuary. The enhanced influence of the coastal current in the near shore region may play a significant role in the large across-shelf variation in the subtidal current structure above the pycnocline.
Acknowledgements This study was funded by the National Science Foundation under grant OCE9000158 and NOAA-Sea Grant under grant NA56RG0147 (project R/ME-16). Dr. Joy Moses-Hall performed the preliminary data processing. Jacqueline Bijansky assisted in typing the manuscript. The comments of two anonymous reviewers are valuable in improving the manuscript.
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