Continental shelf surface thermal fronts in winter off the northeast US coast

Transcription

1 Continental Shelf Research 21 (2001) Continental shelf surface thermal fronts in winter off the northeast US coast David S. Ullman*, Peter C. Cornillon Graduate School of Oceanography, University of Rhode Island, Oceanography, Bay Campus, South Ferry Road, Narragansett, RI 02882, USA Received 23 June 2000; accepted 8 August 2000 Abstract Analysis of 12 years ( ) of sea surface temperature (SST) imagery covering the shelf and slope off the northeast US coast has revealed the presence of persistent fronts in winter over the middle shelf. Ongoing work shows that similar fronts occur in other coastal regions, suggesting that these fronts are of more than regional interest. The satellite data from the US east coast make clear that these fronts, which are found with highest frequency in the vicinity of the 50 m isobath, separate cool water inshore from warmer outer shelf water. The temperature step across the fronts, a measure of the frontal strength, is negatively correlated with estimates of heat flux (latent plus sensible) indicating that winter surface cooling plays an important role in their formation. Although, generally, the fronts are oriented parallel to the bottom topography, the region around Nantucket Shoals is a location where fronts oriented in the crossisobath direction occur more often than elsewhere, suggesting that this area is one of enhanced crossisobath flow. The cross-isobath flow manifests itself in the form of cold tongues extending south and west from the shallowest part of the shoals. Historical hydrographic data from the shelf in winter indicate that the fronts typically have a salinity signal, with water on the inshore sides of the fronts having lower salinity and resulting lower density due to the controlling influence of salinity on density. Weak vertical stratification is often present inshore of the fronts suggesting that the fronts may represent the offshore edge of a freshened coastal zone. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thermal fronts; Coastal oceanography; USA; Mid-Atlantic Bight; Gulf of Maine 1. Introduction Recently Ullman and Cornillon (1999) applied an objective edge-detection algorithm (Cayula and Cornillon, 1995) to a 12 year time series of high-resolution (1.2 km) satellite-derived sea surface temperature (SST) imagery and produced a frontal climatology for the shelf and slope off *Corresponding author /01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 1140 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) the northeast US coast (Fig. 1). In addition to the well-known fronts in this region such as the shelfbreak front (Wright, 1976), tidal mixing fronts on Georges Bank and in the Gulf of Maine (Garrett et al., 1978; Yentsch and Garfield, 1981), and fronts defining the edge of the Eastern Maine Coastal Current (Townsend et al., 1987), a previously undescribed class of fronts over the middle shelf was found in this study. These fronts occurring in winter, predominantly in a broad band roughly coincident with the 50 m isobath (Fig. 2), separate cold inner shelf water from warm outer shelf water. Examination of XBT sections across the New York Bight shelf in winter showed that the inner shelf water could be vertically stratified, with cold water overlying warmer water, suggesting the importance of salinity in determining the density field (Ullman et al., 1999). Preliminary results from application of the edge-detection algorithm to 9 km resolution global SST fields indicate that mid-shelf fronts are a common feature in winter over many other mid-latitude shelves (Hickox et al., 2000). In this paper we seek to describe the properties of winter shelf fronts from the perspectives of both satellite SST and historical hydrographic data. The satellite data allow the fronts to be characterized by the cross-frontal temperature step, temperature gradient, length scale, and water depth at the front. The time evolution of these frontal properties over the winter period is presented in Section 2. In this section, the scalar product of the temperature and bathymetry gradient vectors is used as a measure of the orientation of the fronts relative to the bathymetry. Fig. 1. (a) Northeast US shelf region, showing the locations of the National Data Buoy Center buoys (44007; 44008; 44009) and CMAN (alsn6) stations used in this study. The solid lines are the 50 and 200 m isobaths. The vector-averaged wind stress during winter is shown by the arrows at each buoy location. (b) Map delineating the zones referred to in the text: (I) Delaware shelf, (II) New York Bight, (III) Nantucket Shoals, (IV) Gulf of Maine Coast. The thin region delineated within each zone encompasses the vicinity of the 50 m isobath.

3 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) Fig. 2. Front detection probability for winter (January March) averaged over The probability (expressed in percent) is the ratio of the number of times a pixel was a frontal pixel to the number of times the pixel was clear. For clarity, probabilities 53% are not shown. The dashed lines are the 50 and 200 m isobaths. We suggest that regions of cross-isobath transport can be identified as those where the proportion of fronts oriented across isobaths is relatively large. The relationship between the frontal strength and meteorological forcing is explored using correlation analysis in Section 3. Using wind stress and sensible and latent heat fluxes estimated from National Data Buoy Center (NDBC) buoy data, we test the hypothesis that the SST step across mid-shelf fronts is forced by strong surface cooling occurring during the winter. Although the data are consistent with this hypothesis, analysis of historical hydrographic observations (Section 4) shows that the across-frontal density contrast is primarily associated with salinity changes, suggesting that temperature is dynamically unimportant and serves merely as a tracer for salinity changes. This is illustrated with a simple model of surface cooling over a shelf. The model, discussed in Section 5, demonstrates that in response to cooling, an SST front develops at approximately the location of an initial salinity front. 2. Frontal properties 2.1. Seasonal variability For each SST image, the front-detection algorithm of Cayula and Cornillon (1995) provides the coordinates of the pixels delineating each front. At each frontal pixel, we also compute the SST gradient, across-frontal temperature step, water depth, and bathymetry gradient. These quantities are subsequently averaged over all pixels of a given front to provide the mean properties of that front. The properties of wintertime SST fronts will be presented with respect to the four shelf regions shown in Fig. 1: the shelf off the Delaware Bay mouth (DS), the New York Bight shelf (NYB), the vicinity of Nantucket Shoals (NS), and the western Gulf of Maine coast (GM). The offshore boundaries of these regions were chosen to include the vicinity of the 50 m isobath (the locus of the major shelf frontal band in Fig. 2) and, for the three southern regions, to avoid onshore

4 1142 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) excursions of the shelfbreak front. However, as discussed below, we were not entirely successful in meeting the latter criterion in the case of the Delaware shelf because of the proximity of the 50 m isobath to the shelfbreak in that region. The seasonal evolution of frontal properties during the winter (defined as week 49 through week 13) is presented in the form of mean weekly time series. These time series are derived by first averaging the particular frontal property over each week of the entire 12 years ( ). The mean weekly time series is then the average over all years of the individual weekly means. Fig. 3 illustrates the winter time evolution of the mean cross-frontal temperature step, DT, mean gradient, rt, an estimate of the cross-frontal length scale, L derived from the step and gradient (L ¼ DT=rT), and the mean water depth at fronts, H. The mean cross-front temperature step and gradient typically increase to a maximum in January to February with a slow decrease during late winter. Mean temperature steps range from 1 to 28C; with higher values often occurring on the Delaware Shelf. Gradients range from 0.15 to 0:308C=km. The computed cross-frontal length scale exhibits no significant variability over the winter period with length scales of 5 6 km found in the three northernmost regions and values of 6 8 km observed on the Delaware Shelf. The larger cross-frontal temperature step and length scale on the Delaware shelf may result from sporadic onshore excursions of the much stronger shelfbreak front, the mean position of which is very close to the offshore boundary of the Delaware Shelf region. In all regions, the mean water depth at which fronts occur increases as the winter progresses. Mean depths are lowest over the Delaware Shelf (30 40 m) consistent with Fig. 2 which shows a broad band of high probability offshore of the mouth of Delaware Bay well inshore of the 50 m isobath. The mean depth is largest in the Gulf of Maine ( m) where the shelf is steeply sloped. The mean cross-frontal length scales from Fig. 3 can be compared with an estimate of the internal radius of deformation. Hydrographic data from the vicinity of fronts, discussed in Section 4, suggest a radius of deformation of 1 2 km which is quite small due to the weak stratification present in winter. Observed cross-frontal length scales are thus approximately 2 7 internal radii, which are larger than values of 41 radius predicted for the width of the surface expression of the frontal interface by models of geostrophic adjustment (Ou, 1984). Note however, that the resolution of the satellite SST fields of 1.2 km is insufficient to resolve scales of the order of the internal radius in winter. In addition, mixing effects due to winds and tidal currents, which are neglected in the model of Ou (1984), might be expected to increase the frontal width substantially Frontal orientation relative to bathymetry At subinertial time scales, the flow associated with a density front is expected to be approximately in geostrophic balance, with the flow predominantly in the along-front direction. If SST fronts are assumed to have a density signature, then the orientation of the SST front indicates the direction of the along-front subinertial flow. The mean frontal orientation is taken to be normal to the direction of the temperature gradient vector averaged along the length of the front. Ullman and Cornillon (1999) used the relationship of the temperature and bathymetry gradients to classify fronts for descriptive purposes into cold and warm fronts. Here we use the gradient information in a more quantitative manner.

5 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) Fig. 3. Mean weekly time series of (top to bottom) cross-frontal temperature step, cross-frontal temperature gradient, cross-frontal length scale, and depth at front for each of the four regions averaged over The error bars denote the 95% confidence intervals for the mean values. The orientation of a front relative to the local bathymetry is measured by the scalar product of the temperature gradient and bathymetry gradient vectors. We define a normalized scalar product: P ¼ rt rh jrtjjrhj ; where T is temperature and H is water depth (H50). The value of P ranges from 1 to1 with the minimum (maximum) value corresponding to the case of cold (warm) water on the shallow side of a front oriented parallel to isobaths. A value of zero indicates a front oriented perpendicular to the local isobaths, and with the above assumptions represents a case of cross-isobath flow. We focus on the vicinity of the 50 m isobath (the areas delineated by the thin lines in Fig. 1 which were drawn to enclose the 50 m isobath probability bands seen in Fig. 2) and the time ð1þ

6 1144 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) Fig. 4. Percent of the total number of fronts with normalized scalar product, P within the specified range for fronts in the vicinity of the 50 m isobath for January February for each of the four regions. The error bars represent the 95% confidence intervals for the computed percentages. Cold (warm) fronts have cold (warm) water on the shallow side of the front. period of January February when the shelf fronts are strongest and most likely to be detected. Fig. 4 presents the percentage of fronts within each region that have normalized scalar products within each of five ranges. In all regions, the majority of fronts satisfy 14P4 0:6 and, using the terminology of Ullman and Cornillon (1999) are defined as cold fronts. This shows that not only the probability bands (Fig. 2) but also individual fronts tend to align with the bathymetry. The suggestion from Fig. 2 is that these fronts represent the outer edge of a coastal transport pathway extending from the Bay of Fundy to Cape Hatteras. The Nantucket Shoals region appears to be unique in that there are significantly more fronts aligned in the cross-isobath direction ( 0:25P50:2) than in any of the other regions. If the alignment of fronts indicates the direction of flow, then Nantucket Shoals is a region of cross-isobath transport and may represent a leak in the hypothesized coastal pathway. Further evidence that cross-isobath flow occurs in the vicinity of Nantucket Shoals can be found by visual examination of individual AVHRR images. Fig. 5 provides a striking example of a tongue of cold water extending from the shallowest part of the shoals across the 50 m isobath and then downshelf at least 100 km. While Fig. 5 is a particularly vivid example of apparent crossisobath flow, many other such events can be seen in the 12 year record, indicating that this is a robust and recurring feature. These events appear to be unique to the Nantucket Shoals region as we have not observed their occurrence elsewhere. In fact, direct evidence of cross-isobath flow in Nantucket Shoals area comes from current measurements from the Nantucket Shoals Flux Experiment (Beardsley et al., 1985), where a 1 2 cm=s offshore component was observed in winter in the vicinity of the 60 m isobath, and from the Coastal Mixing and Optics Experiment (Lentz et al., 1999) where currents averaged from fall through spring exhibited an offshore component of 1 6 cm=s.

7 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) Fig. 5. AVHRR SST image from February 5, 1993 showing a plume of cold water emanating from Nantucket Shoals. Clouds are white and the black lines denote the 50 and 100 m isobaths. 3. Meteorological forcing Time series of wind, air temperature, and water temperature from 1985 to 1996 at the buoys and CMAN station marked in Fig. 1, obtained from the National Data Buoy Center, were used to estimate wind stress and surface heat flux (sensible and latent). Bulk formulae were used in the computation of wind stress and sensible heat flux (Gill, 1982). The neutral stability drag coefficient formula of Large and Pond (1981) was used, with observed winds adjusted to 10 m height using the iterative method outlined by Blanton et al. (1989). The heat transfer coefficient was taken to be equal to the estimated drag coefficient. Latent heat flux was derived from the computed sensible heat flux and an estimate of the Bowen ratio (Q Sens =Q Lat ) using the functional form given by Blanton et al. (1989). Correlation coefficients between weekly averaged frontal temperature step and wind stress and heat flux time series were computed for the winter period (weeks 49 13) for each of the four

8 1146 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) regions. Correlation coefficients were tested for significance using estimates of the effective number of degrees of freedom computed from the autocorrelation functions of the two input time series (Emery and Thomson, 1997). The ratio of the number of data points to the effective number of degrees of freedom was typically about 3, indicating that approximately every third weekly value was independent. The correlations were computed as a function of lag in weeks (frontal time series lag) and wind direction. For all regions, significant negative correlation is found between heat flux and temperature step for lags ranging from 0 to 5 weeks (Fig. 6). Maximum correlation magnitudes in the range occur at 1 3 weeks lag in all regions. The negative correlation indicates that surface cooling (negative heat flux) is associated with increased across-front temperature step, suggesting that the fronts are, at least partly, a response to strong winter cooling. Low but significant correlations are observed between wind stress and temperature step (Fig. 7) for wind directions of T. Lags at which maximum correlation magnitude occur are in the range of 1 4 weeks, roughly equivalent to the lag of temperature step with respect to heat flux. The correlation with wind is such that offshore (onshore) winds are associated with stronger (weaker) fronts. Offshore winds in winter are associated with outbreaks of cold continental air into the coastal zone. Because of the strong surface cooling associated with these outbreaks, the wind stress and heat flux time series are highly correlated (r ¼ 0:520:8) at zero lag making it difficult to separate the effects of wind and heat flux. However, as will be discussed below, we argue that the observed phasing between wind stress and frontal strength is inconsistent with existing knowledge of the response of fronts to wind (Csanady, 1978), suggesting that heat flux is more important than winds in increasing the cross-frontal temperature step. Fig. 6. Correlation between weekly averaged heat flux (sensible plus latent) and weekly averaged frontal temperature step for each of the four regions during winter (weeks 49 13). Correlations are shown as a function of lag in weeks, with the heat flux time series leading. Asterisks denote correlation coefficients that are significantly different from zero at the 95% level.

9 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) Fig. 7. Correlation between weekly averaged wind stress and weekly averaged frontal temperature step for each of the four regions during winter (weeks 49 13). Correlations are shown as a function of lag in weeks, with the wind stress time series leading, and wind direction, with wind direction denoting the direction toward which the wind blows. The contour interval is 0.1 and negative contours are dashed. Asterisks denote correlation coefficients that are significantly different from zero at the 95% level. Csanady (1978) found that longshore winds opposing the surface geostrophic flow associated with a coastal front tended to weaken the front and vice versa. The basic idea is that if the pressure field associated with the wind-driven setup opposes (augments) the preexisting pressure field due to the front, the front weakens (strengthens). For the case of cross-shore winds (perpendicular to the fronts), offshore (onshore) wind tends to lower (raise) sea level at the coast (Csanady, 1982) and creates an onshore (offshore)-directed surface pressure gradient. If the winter fronts we observe were density fronts resulting only from temperature variations, then the presence of cold water inshore would imply a surface pressure gradient directed onshore which would be augmented by the pressure field produced by an offshore-directed wind. Using the Csanady (1978) results as a guide, this would suggest frontal strengthening. However, as will be shown in Section 4, the observed mid-shelf fronts are dominated by salinity, with typically an increase in density in the offshore direction. For these fronts, the surface pressure gradient is opposite to that produced by an offshore wind, thus weakening of the front is expected under offshore winds. It is thus difficult to explain the observed correlation between wind and frontal temperature step on the basis of response to wind alone. 4. Hydrographic properties of fronts The cross-frontal temperature salinity characteristics of the winter shelf SST fronts were examined using hydrographic observations archived by the National Oceanographic Data Center

10 1148 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) (NODC). The coordinates of all CTD and bottle casts from 1985 to 1996 within each of the four shelf regions were extracted from the 1998 World Ocean Database (Conkright et al., 1998). For each winter day of the 12 year record daily composite frontal maps were constructed, upon which the positions of all stations occupied that day were plotted. This allowed the identification of pairs of hydrographic stations located on either side of an SST front. This was a somewhat subjective procedure in that, often a choice of stations was available. In selecting representative stations on either side of a front, we tried to minimize the separation distance while ensuring that each station was clearly away from the front. Therefore, in the analysis that follows, note that the separation distance between the two stations was variable. In several instances, more than one pair of stations was identified for a particular front. In this case, the inshore and offshore hydrographic properties for that front were taken as the average of those from the different station pairs. The surface across-front hydrographic properties are summarized in Fig. 8. The surface temperature and salinity on either side of a front were used to compute the across-front change in density due to temperature and due to salinity. In Fig. 8, we plot the density change (offshore inshore) due to temperature versus that due to salinity. Essentially all fronts fall within the fourth quadrant of these plots, with a negative density change due to temperature and a positive change due to salinity. This indicates that the fronts separate cold fresher inshore water from warm saltier offshore water with temperature and salinity changes having opposing effects on density. The Fig. 8. Surface density change across fronts from selected NODC hydrographic profiles. Plotted is the density change due to temperature versus that due to salinity. The dashed lines represent the case in which temperature and salinity changes exactly compensate, resulting in zero across-front density step.

11 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) dashed lines in Fig. 8 separate the regime in which temperature dominates and the inshore water is denser from that in which salinity is dominant and the offshore water is denser. Fronts whose properties lie along the line exhibit no cross-frontal density step. In all regions except Nantucket Shoals, the points tend to lie in the salinity dominated regime with denser offshore water. In the Nantucket Shoals region, the effect of temperature is more important in a relative sense, often completely counteracting the effect of salinity. The fronts in this region tend to have a very small density step, with a number of fronts characterized by cold dense inshore water. Vertical stratification in the hydrographic profiles was examined by estimating the mean vertical property gradient using the shallowest and deepest depths of the profile. Although the continental shelf in winter is generally considered to be vertically well mixed, this is not always the case. This is illustrated by Fig. 9, showing vertical temperature gradients versus salinity gradients for inshore and offshore stations, and Fig. 10, where the vertical density gradient offshore is plotted against the inshore gradient. While there are certainly profiles for which the vertical gradients are essentially zero, in many cases there is weak stratification, typically with cold fresh water overlying warm salty water (Fig. 9). For all regions except Nantucket Shoals, it is the inshore stations that are typically more strongly stratified, suggesting the effects of coastal freshwater outflows on the water column structure. Fig. 10 shows that fronts in these regions often separate well-mixed water offshore from stably stratified water inshore. At other times, however, there is essentially no vertical stratification on either side of the front, suggesting the importance of episodic mixing, Fig. 9. Vertical temperature gradient versus vertical salinity gradient for stations inshore (crosses) and offshore (circles) of fronts from selected NODC hydrographic profiles. The vertical coordinate z is positive upward.

12 1150 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) Fig. 10. Vertical density gradient offshore of front versus vertical density gradient inshore of front from selected NODC hydrographic profiles. The dashed line in each plot represents the case of equal vertical gradient on both sides of a front. presumably due to wind and=or surface cooling. Brown and Beardsley (1978) present a hydrographic section across an example of the former type of front along the Maine coast in December Onshore of the front, the water column was stratified with cooler, fresher water near the surface, while the water offshore of the front was well mixed down to a depth of about 100 m, which they attributed partly to strong surface cooling. One month later, the water column was essentially well mixed everywhere, with only a weak horizontal density gradient present. In the Nantucket Shoals area, stations inshore of fronts tend to be well mixed (Figs. 9 and 10), consistent with the fact that the area shallower than approximately 50 m over the shoals is known to be tidally well mixed throughout the year (Limeburner and Beardsley, 1982). In a number of cases, stations offshore of fronts in this region display vertical stratification, possibly due the intrusion of the shelfbreak front into the mid-shelf area as has recently been described by Lentz et al. (1998). 5. Shelf cooling model Although the winter fronts are detected in SST, the analysis of hydrographic data shows that the cross-frontal density difference is controlled mainly by salinity changes. Temperature appears

13 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) to act mainly as a tracer for the presence of low salinity water over the shelf. To illustrate how a temperature front can develop in response to surface cooling in the presence of a salinity front, an extremely simplified model will now be introduced. The model is one-dimensional (vertical) and is forced with a specified surface heat flux (cooling). Initial temperature and salinity profiles are prescribed. The only mechanism for vertical mixing is the production of static instabilities which are removed by mixing vertically to a depth just sufficient to eliminate the instabilities. A two-dimensional shelf cross-section is treated as a series of one-dimensional models. The initial configuration is as shown in Fig. 11 (top panels) with a linearly sloping shelf and a lens of low-salinity water near the coast with top to bottom salinity difference of 0.4 psu. The temperature is initially isothermal across the shelf, and at t ¼ 0a spatially uniform cooling flux of 300 W=m 2 is applied and held constant for the 8 day duration of the calculation. During the initial stages of cooling, the low-salinity water near shore cools faster than the wellmixed water offshore because the stable salinity stratification there confines the heat loss to the thin lens. The result is a temperature jump of 1228C at the offshore side of the lens. In this example, after 8 days, sufficient cooling has occurred to convectively overturn the initially stably stratified lens. This destroys the temperature and salinity fronts by mixing warm salty water from below the pycnocline with the cooler fresher lens water. The horizontal temperature gradient at this time increases monotonically towards the coast. The vertically uniform density field, however, exhibits a maximum just offshore of the location of the initial salinity front. Although the present model cannot address this question, the final density structure would have a tendency to adjust under gravity, with the nonlinearity of the density field contributing to frontogenesis (Simpson and Linden, 1989). 6. Discussion Isotopic analysis indicates that shelf water in the Gulf of Maine and Mid-Atlantic Bight (MAB) is a mixture of freshwater and slope water, with the majority of the former derived from arctic sources (Chapman and Beardsley, 1989). These authors presented evidence that the observed mean southwestward flow in the MAB is a continuation of a large-scale coastal buoyancy-forced current originating off the southern Greenland coast. The finding, in the present study, of a persistent mid-shelf frontal zone suggests the existence of an inner shelf zone where local buoyancy sources are of greater importance. We have not, however, detected significant lagged correlation between frontal strength and river discharge records. Among the many reasons for this are the fact that our frontal strength is a measure of the cross-frontal temperature step, which according to our hypothesis may be enhanced by a low-salinity plume, but is ultimately produced by surface cooling. The temperature signature of the mid-shelf fronts appears to be a result of winter cooling of a nearshore trapped band of low-salinity (low-density) water. As the simplified model demonstrates, the effect of this cooling is to reduce the already low-density contrast across the front. Eventually, the water column inshore becomes well mixed, and depending upon the initial state an SST front may still be present. In fact, many SST fronts are detected when the water column is well mixed on both sides of the front (Fig. 10) suggesting that this is often the case. However, a counter-example

14 1152 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) Fig. 11. Model simulation of salinity (left panels), temperature (center panels), and sigma-t (right panels) for a cooling flux of 300 W=m 2 over a sloping shelf with initial salinity stratification. is clearly shown by the observations of Brown and Beardsley (1978) where a month of surface cooling is found to effectively destroy an initial coastal front resulting in a well-mixed water column inshore as well as offshore. Although the simplified model can explain the formation of a temperature front, the front is short-lived, disappearing in about a week with the initial conditions and forcing used here. This

15 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) leads to the question of how such a model can explain the observed persistence in the mid-shelf fronts. We believe that the observed SST fronts are probably transient but recurring, thus making them appear persistent in long-term records. Due to the variability in viewing conditions associated with clouds, it is difficult to unambiguously estimate from the satellite data the time scale over which the fronts form and decay. However, the integral time scale derived from the autocorrelation function of the weekly cross-frontal temperature step time series is O(3 5 weeks), which is consistent with the time evolution observed by Brown and Beardsley (1978). The general orientation of the fronts along isobaths suggests that, at least on time scales of a month or so, they represent a barrier to cross-shelf exchange of nearshore freshened water. The exception in the region studied here is in the vicinity of Nantucket Shoals where apparently offshore flowing cold plumes are observed. An estimate of the offshore transport associated with a plume such as the example shown in Fig. 5 can be made using estimates of currents and of the dimensions of the plume. Limited hydrographic observations (not shown) suggest that the plumes are vertically well mixed, therefore, we take the flow depth as 50 m. Plume widths are typically around 20 km. For the velocity scale, we use the observed cross-isobath currents of Lentz et al. (1999) and Beardsley et al. (1985). Based on these sources, we choose a value of 2 cm=s asa representative depth-averaged plume cross-isobath current. The transport computed using these values is thus m 3 =s, and can be placed into perspective by comparison with an estimate of the transport inshore of the front off the Maine coast in the vicinity of buoy (Fig. 1). Winter current observations (November 1974 January 1975) at the 100 m isobath there indicate alongisobath depth mean speeds of Oð5 cm=sþ to the southwest (Vermersch et al., 1979). Hydrographic measurements during the same period (Brown and Beardsley, 1978) indicate that the surface expression of the front was located at about the 100 m isobath ( 20 km offshore). Assuming a current speed of 5 cm=s and a linearly sloping bottom, we estimate the along-shelf transport inshore of the front to be m 3 =s. Thus the estimated offshore transport at Nantucket Shoals, although episodic, is a significant fraction of the nearshore transport at an upshelf location along the Maine coast. Although the mechanism producing localized offshore flow at Nantucket Shoals is unclear at present, nonetheless we can consider some possibilities. The cross-frontal hydrography (Section 4) indicates that Nantucket Shoals is unique in that cooling can be sufficient to produce dense inshore water. This appears to result from the fact that there are no significant river discharges along this stretch of coastline. As shown in Fig. 12, which displays surface salinity at NODC stations inshore of the 50 m isobath versus along-shore distance, low salinities (530 psu) are observed in the vicinity of river mouths but not over Nantucket Shoals. Modeling studies (e.g. Kikuchi et al., 1999) suggest a tendency for dense coastal water produced by surface cooling to move across-shelf. However, while the results of Kikuchi et al. (1999) show that the offshore flow over a steep continental slope is plume-like (as observed in the Nantucket Shoals region), it appears to be more eddy-like over the shelf. Wind stress may also be a factor contributing to the localized nature of the offshore plume flow. The mean winter wind stress at all NDBC stations (Fig. 1) is directed roughly towards the southeast. The orientation of Nantucket Shoals is such that only there is the associated Ekman transport directed offshore (on the southwest side of the Shoals). This is exactly the location in which the cold offshore-directed plumes are observed.

16 1154 D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) Fig. 12. Surface (0 5 m) salinity at NODC stations inshore of the 50 m isobath versus along-shore distance relative to the northernmost extent of the Gulf of Maine zone. 7. Conclusions The properties of a class of SST fronts over the middle shelf in winter have been described. These fronts, separating colder and fresher nearshore water from warmer and saltier offshore water, appear to delineate a boundary between a freshened nearshore zone and the outer shelf. The SST signature of the fronts results from strong surface cooling in winter, and appears to be enhanced by the presence of salinity stratification in the nearshore zone. The cross-frontal density field is typically characterized by less dense water nearshore, consistent with a downshelf (south and west) geostrophic flow associated with the fronts. We have hypothesized the presence in winter of a nearshore coastal pathway for downshelf transport of water freshened by coastal discharges. The SST signature of the fronts bounding this pathway disappears in early spring, so at present we are unable to infer its existence during seasons other than winter. An apparent leak in the pathway occurs at Nantucket Shoals, where plumes of cold water are observed to cross isobaths with resulting transport of cold water from the shoals to the outer shelf.

17 Acknowledgements D.S. Ullman, P.C. Cornillon / Continental Shelf Research 21 (2001) This research was supported by the National Oceanographic and Atmospheric Administration s Sea Grant Program (NA66RG0303), the National Aeronautics and Space Administration (NAG53736), and the National Oceanographic Partnership Program (N ). Salary support for P. Cornillon was provided by the State of Rhode Island and Providence Plantations. The software used for image processing was developed by R. Evans, O. Brown, J. Brown, and A. Li of the University of Miami. Their continued support is greatly appreciated. We would like to acknowledge Richard Garvine and an anonymous reviewer whose comments have helped to improve the presentation of this paper. References Beardsley, R.C., Chapman, D.C., Brink, K.H., Ramp, S.R., Schlitz, R., The Nantucket Shoals Flux Experiment (NSFE79). Part I: a basic description of the current and temperature variability. Journal of Physical Oceanography 15, Blanton, J.O., Amft, J.A., Lee, D.K., Wind stress and heat fluxes observed during winter and spring, Journal of Geophysical Research 94, 10,686 10,698. Brown, W.S., Beardsley, R.C., Winter circulation in the western Gulf of Maine: Part 1. Cooling and water mass formation. Journal of Physical Oceanography 8, Cayula, J.-F., Cornillon, P., Multi-image edge detection for SST images. Journal of Atmospheric and Oceanic Technology 12, Chapman, D.C., Beardsley, R.C., On the origin of shelf water in the Middle Atlantic Bight. Journal of Physical Oceanography 19, Conkright, M.E., Levitus, S., O Brien, T., Boyer, T.P., Stephens, C., Johnson, D., Stathoplos, L., Baranova, O., Antonov, J., Gelfeld, R., Burney, J., Rochester, J., Forgy, C., World Ocean Database 1998: Documentation and Quality Control, Version 1.2. Internal Report 14, National Oceanographic Data Center. Csanady, G.T., Wind effects on surface to bottom fronts. Journal of Geophysical Research 83, Csanady, G.T., Circulation in the Coastal Ocean. D. Reidel, Dordrecht. Emery, W.J., Thomson, R.E., Data Analysis Methods in Physical Oceanography. Elsevier, New York. Garrett, C.J.R., Keeley, J.R., Greenberg, D.A., Tidal mixing versus thermal stratification in the Bay of Fundy and Gulf of Maine. Atmosphere and Ocean 16, Gill, A.E., Atmosphere Ocean Dynamics. Academic Press, San Diego. Hickox, R., Belkin, I.M., Cornillon, P., Shan, Z., Climatology and seasonal variability of ocean fronts in the East China, Yellow and Bohai Seas from satellite SST data. Geophysical Research Letters 27, Kikuchi, T., Wakatsuchi, M., Ikeda, M., A numerical investigation of the transport process of dense shelf water from a continental shelf to a slope. Journal of Geophysical Research 104, Large, W.G., Pond, S., Open ocean momentum flux measurements in moderate to strong winds. Journal of Physical Oceanography 11, Lentz, S.J., Anderson, S.P., Plueddemann, A., Edson, J., Evolution of the thermal stratification in the Mid-Atlantic Bight during the Coastal Mixing and Optics Program, August 1996 June EOS, Transactions of the American Geophysical Union 79 (Suppl.), 100. Lentz, S.J., Plueddemann, A., Anderson, S., Edson, J., Current variability on the New England shelf during the Coastal Mixing and Optics program, August 1996 June EOS, Transactions of the American Geophysical Union 80 (Suppl.), 24. Limeburner, R., Beardsley, R.C., The seasonal hydrography and circulation over Nantucket Shoals. Journal of Marine Research (Suppl.) 40,

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