Differences in the annual stratification cycle over short spatial scales on southern Georges Bank

Continental Shelf Research, Vol. 12, No. 2/3, pp. 415-435, 1992.

0278-4343/92 $5.00 + 0.00 Pergamon Press plc

Printed in Great Britain.

Differences in the a n n u a l stratification cycle over short spatial scales on southern Georges B a n k JAMES J. BISAGNI*

(Received 9 August 1990; accepted 30 October 1990) Abstract--Analysis of hydrographic data collected during 1977-1978 at six locations on southern Georges Bank revealed differences in the sinusoidally-modeled mean seasonal hydrographic cycles over short cross-bank spatial scales. In depths of less than 60 m, vertical stratification remained weak year-round, with no seasonal cycle. At greater depths, a transition to a stratified water column occurred from late spring to early autumn. However, statistically-significant differences exist between the mean hydrographic cycles derived from data collected in two contiguous seasonally-stratified zones located within the deeper region. In 60--80 m depths: (1) the mean cycle of the maximum density gradient in the seasonal pycnocline was dominated by a one cycle-per-year model; (2) the maximum density gradient varied by almost two orders of magnitude, from a January minimum to a late July maximum; and (3) the mean depth of the maximum density gradient remained near 20 m year-round. In 80-100 m depths: (1) the mean cycles of both the maximum density gradient and pycnocline depth were dominated by a one cycle-per-year model; (2) the maximum density gradient varied by only one order of magnitude; and (3) the mean depth of the maximum density gradient varied from 20 m in early June to 60 m in late December when the water column is well mixed. The cross-bank transition from well-mixed to seasonally-stratified waters agrees with the Simpson-Hunter stratification parameter computed by other workers for the study domain and, thus, to first order is consistent with existing one-dimensional models for tidally-mixed seas. Variations between hydrographic cycles noted within the seasonally-stratified waters may be due to the presence of the "cold-band" and on-bank advection of slope waters, thus modifying the one-dimensional assumptions.

INTRODUCTION

GEORGESBank is a large shoal area located east-southeast of Cape Cod, Massachusetts, in the western North Atlantic. The Bank faces the open North Atlantic ocean to the south, forming a barrier between the Atlantic and the Gulf of Maine. The Bank's northern margin dips steeply into the Gulf of Maine while, depth increases more slowly on the b r o a d e r s o u t h e r n f l a n k ( U c H u P I a n d AUSTIN, 1987). S e a w a r d o f t h e 100-m c o n t o u r o n t h e s o u t h e r n flank, d e p t h i n c r e a s e s r a p i d l y a c r o s s t h e s h e l f b r e a k . D u e to t h e i r l o c a t i o n e a s t o f t h e N o r t h A m e r i c a n c o n t i n e n t a l l a n d m a s s , G e o r g e s B a n k w a t e r s a r e s u b j e c t e d to s t r o n g a i r - s e a i n t e r a c t i o n , r e s u l t i n g in t h e t r a n s f e r o f h e a t a n d momentum between the atmosphere and the ocean by various processes acting over a v a r i e t y o f s p a c e a n d t i m e scales (HOPKINS a n d RAMAN, 1987). F u r t h e r m o r e , t h e s p a t i a l scales o f t h e p r o c e s s e s r e s p o n s i b l e f o r this t r a n s f e r a r e g e n e r a l l y l a r g e w i t h r e s p e c t to t h e

*NOAA/National Marine Fisheries Service, Narragansett, RI 02882, U.S.A. 415

416

J . J . BISAGNI 1987 YEAR DAY 80

120

200

160

I

L

I

I

80 I

40

~

o

,7._

' ~ -40 m -80 I

80 40 "?

-40 ' ~

80

o

40

~4

0

I

i

I

'

I

i

I

'

-80

-40 -80 '

80

120

160

200

1987 YEAR DAY Fig. 1. Computed u (+eastward/-westward) and v ( + n o r t h w a r d / - s o u t h w a r d ) tidal current components and current magnitude for one location (41°29'N, 67°15'W) on central Georges Bank for year-days 91-181 (1 April through 30 June) 1987.

Bank itself and include: (1) the seasonal meteorological cycle; (2) "weather", i.e. the passage of synoptic high and low-pressure systems; and (3) diurnal fluctuations. Of equal importance to the waters of Georges Bank is the intense stirring generated by bottom friction caused by the strong tidal currents associated with the Bank. Since the Gulf of Maine is in near-resonance with the M2 semidiurnal tidal component (GARRETT, 1972, 1974), tidal currents on Georges Bank are dominated by the M2 component. These tidal currents are rotary in nature, with the major axis of the tidal current ellipses generally oriented cross-bank, and increasing as depth decreases onto the shoal. Analysis of the K 1 diurnal tide by BROWN and MOODY (1987) shows that diurnal tidal currents are much reduced with respect to the more energetic semidiurnal tide and are aligned more along the isobaths than across them. Semidiurnal tidal current speeds are greatest on the northeast portion of the Bank, and can approach 100 cm s -1, while speeds of <50 cm s -1 are more characteristic of the southern flank area (BROWN and MOODY, 1987). Thus, considerable spatial variability exists in the tidal current speeds and tidal stirring over spatial scales comparable to the Bank itself. Interference between the M2 and other semidiurnal tidal components on Georges Bank results in significant 14.8 and 27.6 day spring-neap modulations of the semidiurnal tidal current. A 3-month (1 April through 30 June 1987) synthetic tidal record (Fig. 1), computed using the M2, N2 and $2 semidiurnal and the Ot and K1 diurnal tidal constituents for station 9 (41°20'N, 67°15'W) on Georges Bank (see Fig. 4 in BUTMANand BEARDSLEY,

The annual stratificationcycleon southern Georges Bank

417

1987a, for location) shows the tidal current components and magnitude and the influence of the spring-neap modulation. At this location, the computed currents varied almost by a factor of two over the spring-neap cycle. The sum of the physical forcing functions on Georges Bank waters, should be a response which varies over a wide bandwidth of frequency and wavenumber. In an attempt to begin to understand this type of response, simplified one-dimensional models have been applied to shallow seas bordering the eastern and western North Atlantic. SIMPSON and HUNTER (1974) noted that a marked discontinuity, i.e a front, often existed in the western Irish Sea during the summer season. This front separated a stratified water column, containing a warm, highly-stable surface layer resulting from heat input during the warmer seasons, from an isothermal water column caused by intense tidal stirring on the cold side of the front. They noted that continued warming during summer resulted in reduced temperature contrast as water temperatures in the well-mixed regime approached those of surface waters on the stratified side of the front. Neglecting advection and horizontal diffusion, their data suggested to them that the location of the frontal region was controlled simply by the strength of tidal stirring needed to supply sufficient kinetic energy to mix away the buoyancy addition, caused by heat input at the ocean surface. Therefore, given a sinusoidal tidal current, SIMPSONand HUNTER(1974) proposed that the governing equation is based on the balance between the time rate of change of the potential energy lost in the water column as surface water is heated dV _ gaQh

dt

(1)

2Cp

and the time rate of change of kinetic energy lost by the tide to mixing, part of which increases the potential energy in the water column dE _ p4keU 3

dt

3:r

(2)

where: V = potential energy of the water column; E = available kinetic energy of the tide; a = coefficient of thermal expansion; Q = rate of heat input; g = gravitational acceleration; h = water depth; C o = specific heat; p = water density; k = coefficient of bottom friction; = fraction of kinetic energy available for mixing; U = amplitude of the depth-averaged tidal current. By setting (1) equal to (2) and rearranging terms, SIMPSONand HUNTER (1974) obtained h _ 8Cppke U3 3:rgctQ

(3)

thus showing that the transition from well-mixed to stratified conditions occurs for some critical value of h / U 3 based on the local heat flux into the ocean surface and bottom friction

418

J.J. BISAGNI

values. SIMPSONand HUNTER(1974) determined critical values of 65-100 and 55 m -2 S3 in the southwestern and northern portions of the Irish Sea, respectively. Using both hydrographic data and numerical models, other workers (SIMPSON, 1976; JAMES, 1977; GARREIq"et al., 1978) have determined similar critical values of the order of 100 m -2 s3 for the Celtic Sea and the Gulf of Maine/Bay of Fundy. The original model, together with a surface, wind-induced mixing term, proportional to the cube of the wind speed, was extended by SIMPSONand BOWERS(1981) to show that changes in the location of the front separating the well-mixed and stratified regions might be caused by variability of the stirring rate, resulting from tidal variations induced by the fortnightly spring-neap tidal cycle. Analyses of hydrographic data collected on Georges Bank reveal a physical oceanographic regime similar to the paradigm given above for the Irish and Celtic seas. Specifically, on southern Georges Bank during the late spring, summer and early autumn seasons, there exists a sea surface temperature front which closely parallels the 60-m isobath and separates isothermal waters in depths <60 m from seasonally-stratified waters located over deeper areas (FLAGG,1987). Like the fronts in the Irish and Celtic seas, the Georges Bank front has been shown to result from the balance between buoyancy forcing and tidal stirring (GARRETTet al., 1978), with a secondary consequence being the yearround, isothermal Georges Bank Water observed over shallower portions of the Bank, (HOPKINSand GARFIELD,1981). Using a numerical model and typical heat flux estimates for July and August, GARRETTet al. (1978) computed a critical value of 72.4 m 2 s3 in the vicinity of the Georges Bank front, similar to values found in the Irish and Celtic seas. Departures from the one-dimensional Simpson-Hunter model on southern Georges Bank may be related to advection of waters originating over the continental slope (FLAGG et al., 1982; FLAGG, 1987) and thus, are not related to direct air-sea interaction or tidal stirring. The surface position of the shelf/slope front, i.e. the front separating waters on the southern flank of Georges Bank from slope waters, is generally located near the shelf break, close to the 200-m isobath. However, the near-bottom position of the shelf/slope front is often located some distance shoalward of its surface outcropping, i.e. within a 1520 km wide band centered on the 100-m isobath (WRIGHT, 1976; FLAGGet al., 1982). WRIGHT(1976) also reported that the shelf/slope front for the southern New England area showed a seasonal onshore (northward) movement from winter to autumn. Other higher frequency, wave-like motions of the shelf/slope front have characteristic amplitudes and periods of 20 km and 10 days, respectively, and may be related to local baroclinic instabilities (FLAGGand BEARDSLEY,1978), wind stress (CSANADY,1978) and warm-core Gulf Stream rings (MORGANand BISHOP, 1977; BISAGNI,1983; RAMPet al., 1983; GARFIFED and EVANS,1987). Departures from the Simpson-Hunter model may also be due to a recurring cold hydrographic feature on southern Georges Bank, shown to be located below the seasonal pycnocline within the seasonally-stratified region on the southern flank. The feature is termed the "cold-band" or "cold pool" and is composed of cold shelf waters (FLAGG,1987). During the stratified season this feature has been shown to occur along most of the northeastern continental shelf, from the northeast peak of Georges Bank to Chesapeake Bay during the stratified season (FLAGGet al., 1982; HOUGHTONet al., 1982). On Georges Bank, the feature appears to be centered on the 80-m isobath (FLAGG,1987). Documented southwestward flow within the cold band has shown the feature to be advective in nature, requiring replenishment from the east, possibly by Gulf of Maine Intermediate Water,

The annualstratificationcycleon southernGeorgesBank

419

(BUTMANet al., 1982). Complete vertical mixing of the water column during autumn results in the disappearance of this feature. Although earlier work has described a statistical representation of the mean seasonal cycle of temperature at one location on the southern flank of Georges Bank (BUTMANand BEARDSLEY,1987b), this paper extends that work by computing the mean seasonal density cycle at several locations on the southern flank. Furthermore, this paper investigates both the along-bank and cross-bank differences in these cycles over a portion of the southern flank, including water depths between 40 and 100 m. Similar to the earlier work, the mean seasonal density cycles reported below were determined from statistical modeling of hydrographic data. The data used for this analysis, however, were collected over a longer, 10-year time period, at several fixed locations. METHODS From summer 1977 to autumn 1987, the National Marine Fisheries Service conducted or assisted in 49 nearly-seasonal cruises as part of the field effort in support of the Marine Resources, Monitoring, Assessment and Prediction (MARMAP) program. MARMAP was designed to measure seasonal and interannual variability of the physical and biological environment at 184 locations along the eastern continental shelf of the United States, north of Cape Hatteras, North Carolina (SHERMAN,1980). Although not all 184 locations were sampled on any single cruise, repetitive sampling at fixed locations imposed a constraint which increased the value of the data. Analyses reported below are based upon the assumption that hydrographic data from a given location, although collected on different year-days over the 10-year sampling period, may be used to compute a locationspecific, mean seasonal hydrographic cycle. A limitation of the MARMAP sampling scheme is the low vertical resolution of the hydrographic data due to the use of sample bottles and reversing thermometers for measuring temperature and salinity only at standard depths and near bottom. This discrete method was used throughout nearly the entire MARMAP program, except during 1987 when a continuous-profiling conductivity/temperature/depth (CTD) system largely replaced the earlier protocol. We defined three contiguous southwest-to-northeast trending depth zones of <60, 6080 and 80-100 m, which we designated zones 1, 2 and 3, respectively, to span the region of southern Georges Bank (Fig. 2) where the maximum cross-bank gradient in the SimpsonHunter stratification parameter has been shown to occur (GARRErr et al., 1978). The northern and southern boundaries of these zones were obtained from detailed U.S. Geological Survey bathymetric data (USGS/NOAA, 1990), while their eastern and western boundaries were drawn to isolate the study area from effects related to the Northeast and Great South channels. Of the total 24 MARMAP stations located on Georges Bank, one pair of stations was located within each of the three depth zones (Table 1), for a total of six stations (Fig. 3). Four parameters were extracted from the MARMAP hydrographic data for each of the six selected stations: sea surface temperature (SST) and three others relating to the pycnocline. The three pycnocline parameters are: (1) the absolute value of the maximum vertical density gradient (SP); (2) depth of the maximum vertical density gradient (ZP); and (3) the difference between the near-bottom and surface water density (ST) at each station. Sea water density, used for determing SP and ST was calculated as at from

420

J.J. BISAGNI Longitude

71°W ,

690W

,

,

,

67"W

e4

'

/

65"W

44ON--

40*N~1- I . -, Fig. 2.

GSC].L"~~5/' • " "

.

I

Locations of depth zones 1-3 on Georges Bank. Also shown is the 200-m isobath (dotted) and the locations of Great South Channel (GSC) and Northeast Channel (NEC).

temperature and salinity using the formulation given by MILLEROand POISSON(1981). SP was determined using a simple finite-difference technique between consecutive bottle sample depths, while ZP was computed as the mean depth of the two discrete sample depths used to calculate SP. Since the stability of the water column, E, as defined by HESSELBERG(1918) and described by PONDand PICKARD(1983), is to a first approximation given by E -

(4)

1 Oat p Oz

where Table l.

Georges Bank M A R M A P station characteristics

Station number

Depth zone

Depth (m)

Station coordinates (°N Lat., °W Long.)

123 148 149 155 153 178

1 1 2 2 3 3

38 40 67 68 97 89

41.18,68.13 41.26,67.68 40.93,67.68 41.21,66.93 40.76,67.31 41.50,66.33

The annual stratification cycle on southern Georges Bank

421

Longitude 68"W I

I

6 6 TM I

I

I

]

42ON

40ON

Fig. 3. Locations of depth zones 1-3 on Georges Bank together with the locations of six MARMAP stations (circles). Also shown is the 200-m isobath (dotted) and those portions the 60, 80 and 100-misobaths used to define the three depth zones.

ot

=

[v(s,t,0)

-

1.0] × 1000

(5)

and p = in situ water density; z = water depth; p(s,t,O) = water density at zero ambient pressure; then SP, the discrete formulation of 8crt/Oz, the partial derivative contained in (4), is also assumed to be related to the stability value of the water column at the M A R M A P stations. However, because both the SP and ZP parameters are influenced by the chosen bottle sample depths with respect to the true pycnocline, and the distance between bottles, they were greatly affected by uncertainties introduced by sample bottle location. F u r t h e r m o r e , although the ST p a r a m e t e r is m o r e of a "bulk" measure of stability, it is not sensitive to either sample depths or sample spacing, because the samples used to compute ST remain "fixed" near the water column boundaries. Based on these considerations, the total uncertainty due to the sum of instrumental and sampling error associated with each of the four parameters, was estimated (Table 2). The mean seasonal hydrographic cycle was computed for each of the six stations and four parameters, using the 10-year data set and a least squares procedure described by FOFONOFF and BRYDEN (1975). This method attempted to fit up to a third-order annual harmonic to the data of the form

422

J . J . BISAGN1

Table 2. Uncertainties of sea surface temperatures and pycnocline parameters derived from MARMAP hydrographic data

Parameter SST ZP SP ST

y=

Maximum total estimated uncertainty 0. I°C 4.5 m (ZP <35 m) 14.5 m (ZP >50 m) underestimates true SP (ot-m l) by 2-5 times 0.04 (at units)

~ ~j + A j c o s ( 2 : r X l + B / s i n ( 2 : r x / j=t,3 \ TjJ \ T//

(6)

where: y = parameter value; x = year day; T / = period o f j t h harmonic; A/, B / = amplitudes. An initial set of mean seasonal cycles was fit iteratively, first using the longest period harmonic and proceeding to thew next higher order only if the lower order fit was significant at the 95% level. The final fit was completed employing the same iterative technique, but used only data points within +2 standard deviations of the initial fit, thus ensuring that statistical "outliers" were not used. In this way, the final seasonal cycle was defined by up to three sets of values, i.e. a mean and the sine and cosine amplitudes, for each of the three harmonics. RESULTS Table 3a gives the basic statistics of the observations for each of the four hydrographic parameters at each of the six M A R M A P stations. Table 3b gives the coefficients of the first-, second- and third-order harmonic fits used by (6) to model each of the four parameters at each of the six M A R M A P stations. A significant seasonal cycle was not detected when all sinusoidal coefficients for a given parameter, i.e. A/and B/, are all equal to zero. Table 3c gives the standard deviations of the four observed hydrographic parameters about the harmonic fits, using all three sets of coefficients given in Table 3b. Any differences in the parameter means given in Tables 3a and b are due to the elimination of statistical "outliers" from, the fitting procedure, as described above. The results show that significant mean SST seasonal cycles were detected at all six stations in all three depths zones (Fig. 4). All six of the SST cycles appear to be similar, with minima and maxima occurring near year-days 75 (16 March) and 250 (7 September), respectively. Because the SST cycles for Stas 149 and 153 included a semi-annual harmonic, they appear to be slightly different from the other four stations. In addition, SST standard deviations were almost twice as large at stations in zone 3 (Stas 153 and 178) as in zone 1 (Stas 123 and 148). Results for each of the three pycnocline parameters (Fig. 5) varied greatly, showing large differences among the three depths zones. For instance, at Stas 123 and 148 in zone 1,

The annual stratification cycleon southern Georges Bank

423

Table 3a. Statistics of the four observed hydrographic parameters for each of the six MARMA P stations

Station number 123

148

149

155

153

178

SST N Mean standard deviation (°C) 41 10.3 4.1 42 9.9 3.9 39 9.7 3.8 31 9.1 3.4 36 10.6 4.4 32 8.9 3.6

SP N Mean standard deviation (at m l) 41 0.01 0.05 42 0.01 0.01 40 0.03 0.05 31 0.03 0.04 36 0.04 0.04 33 0.04 0.05

ZP N Mean standard deviation (m) 41 12.1 9.3 42 15.5 9.8 40 19.0 14.1 31 19.7 14.8 36 40.4 27.1 33 29.2 22.3

ST N Mean standard deviation (at) 21 0.06 0.20 24 0.02 0.04 27 0.44 0.50 24 0.25 0.34 31 0.91 0.77 23 0.69 0.57

no mean seasonal cycles were detected for any of the three pycnocline parameters. Further, the data from Stas 123 and 148 are indicative of a water column possessing very low year-round stability. At the greater depths (60-80 m) in zone 2, seasonal cycles were detected in all but one of the parameters (ZP) from Stas 149 and 155. Moreover, although SP varied by nearly two orders of magnitude and ST values as great as 1.0 occurred at both stations in zone 2, ZP remained constant near 20 m at Sta. 149 throughout the year, while ZP oscillated weakly about a mean depth of approximately 20 m at Sta. 155. Zone 3 exhibited seasonal cycles in all three parameters at both Stas 178 and 153. However, the seasonal cycles of SP and ZP appeared to differ from those in nearby zone 2 in that SP varied over only one order of magnitude, ZP varied over a much larger depth range and ST increased to about two (9"t units during the stratified season. Of the four stations located in zones 2 and 3, only Sta 155 displays a significant phase difference between SP and ST maxima. The remaining three stations in these zones show SP and ST maxima which are in phase and occur almost simultaneously, sometime between year-days 200 and 225. The anomalous phase difference of Sta. 155 may be an artifact, caused by a large gap in the data input to the harmonic modeling procedure between year-days 175 and 250. This large data gap does not exist, and data are much more evenly distributed throughout the year for the other five stations, located in all three zones. In order to decide if the observed differences among depth zones for each parameter were significant, a two-dimensional Kolmogorov-Smirnov (KS) statistical test was performed using the extracted hydrographic data values for each of the four parameters. This

424 Table 3b.

J, J. BISAGNI Coefficients of the frst, second and third-order harmonic fits to the four observed hydrographic parameters for each of the six M A R M A P stations

Station number 123

148

149

155

153

SP

ZP

ST

Yt A1 B1

Yl Al Bl Y2 A2 B2

Yl AI BI Y2 A2 B2

~VIAt BI Y2 A2 B2

Y3 A3 B3

Y3 A3 B3

.v3 A3 B3

Y2 A2 B2 Y3 A3 B3 10.1 0.0 0.0 10.0 0.0 0.0 10.2 0~2 0.0 9.7 0.0 0.0 10.4

O.1 178

SST

0.0 9.4 0.0 0.0

-2.3 0.0 0.0 -2.1 0.0 0.0 -2.4 0.4 0.0 -1.9 0.0 0.0 -2.8 0.4 0.0 -2.3 0.0 0.0

-5.7 0.0 0.0 -5.8 0.0 0.0 -5.6 0.6 0.0 -5.2 0.0 0.0 -5.1 0.9 0.0 -4.9 0.0 0.0

-2.4 0.0 0.0 -2.4 0.0 0.0 -1.9 0.0 0.0 -1.8 0.0 0.0 -1.5 0.0 0.0 -1.5 0.0 0.0

0,0 0,0 0,0 0,0 0.0 0.0 -0.7 0.0 0.0 -0.8 0.0 0.0 -0.4 0.0 0.0 -0.5 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 -0,3 0,0 0,0 -0,3 0,0 0.0 -0.2 0.0 0.0 -0.3 0.0 0.0

10.8 0.0 0.0 15.4 0.0 0.0 17.2 0.2 0.0 19.5 0.0 0.0 37.3 0.0 0.0 25.5 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 -1.7 0.0 0.0 23.8 0.0 0.0 8.9 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.6 0.0 -11.8 0.0 0.0 -5.3 0.0 0.0 -14.5 0.0 0.0

0.0 0.0 0.0 0.2 0.0 0.0 0.3 0.0 0.0 0.3 O.1 0.0 0.9 0.1 0.0 0.6 0.0 0.0

11.0 0.0 0.0 0.0 0.0 0.0 0.11 0.0 0.0 0.0 0.0 0.0 -0.3 -0.3 0.1 0.2 0.0 0.0 - 1 1 . 3 -0.3 -11.1 0.2 0.0 0.0 - 1 1 . 7 -0.7

-0. l

I).2

0.0 -0.6 0.0 0.0

0.0 -0.5 0.0 0.0

Table 3c, Standard deviations of the four observed hydrographic parameters about the harmonic fits for each of the six M A R M A P stations

Station number

SST Standard deviation (°C)

SP Standard deviation [lOgl0 (Or m-l)]

ZP Standard deviation (m)

ST Standard deviation (or)

123 148 149 155 153 178

0,5 0.7 0,9 0.7 1.1 0.9

0.46 0,49 0.42 0.34 0.19 0.36

7.4 9.8 11.0 10.8 18.8 16.1

0.01 0.43 0.23 0.12 0.22 0.28

t e s t , d e s c r i b e d b y PRESS a n d TEUKOLSKY ( 1 9 8 8 ) , is a t w o - d i m e n s i o n a l i m p l e m e n t a t i o n o f the standard one-dimensional KS test. As used herein, the test decides whether or not the t w o i n p u t d a t a s e t s t o b e t e s t e d w e r e d r a w n f r o m t h e s a m e p r o b a b i l i t y d i s t r i b u t i o n , e v e n if t h e d i s t r i b u t i o n i t s e l f is u n k n o w n . T h e t e s t y i e l d s t w o v a l u e s : " D " , a m e a s u r e o f t h e

The annual stratificationcycleon southern Georges Bank

425

difference between the two data sets; and "P", a measure of the significance of the difference. Larger D values indicate larger differences between the two data sets, while P values of <-0.05 indicate significance at or greater than the 95% confidence level. The two-dimensional KS test was first used to test the SST parameter for differences between each of the station pairs located in the same depth zone, i.e intra-zonal tests. Table 4a gives the results of these intra-zonal tests which showed that the SST data distributions for station pairs contained in the same depth zone did not differ greatly (D = 0.1-0.2) and that the differences were not significant at the 95% level. On the basis of this result, data from within-zone station pairs were combined, and the same KS test was done on the "lumped station pairs" to perform a series of inter-zonal tests. Again, the differences were both small and insignificant (Table 4b). The same procedure was applied to each of the three pycnocline parameters, generating a similar series of intra-zonal and inter-zonal KS tests. The intra-zonal tests (Table 5a) showed that the differences between station pairs located in the same zone were, again, both small and insignificant. However, Table 5b shows that of the nine inter-zonal tests performed, seven exhibited D values >0.3, with P values indicating significance >99% level of confidence, while only one of the tests showed a D value of <(I.25 which was insignificant at the 90% level. The remaining value, resulting from the test for ZP between zones 2 and 3, yielded a D value of 0.25 which was significant at the 94% level. DISCUSSION In general, the cross-bank relationships revealed by analyses of both SST and the seasonal pycnocline are in good agreement with the notions of large-scale physical forcing and with the one-dimensional physical models described above. Specifically, because seasonal meteorological forcing causes seasonal changes in buoyancy addition/extraction due to air-sea interaction over scales which are large with respect to Georges Bank, the first-order spatial uniformity exhibited by the mean SST seasonal cycle is expected. The offshore-directed (from zone 1 to zone 3), increase in the noise level of the SST data, if related to advective processes resulting from increased cross-bank movement of waters on the outer portions of southern Georges Bank, is also expected. Over spatial scales comparable to those of the Bank itself, the significant differences in the annual cycles of the three pycnocline parameters, at least to a first-order approximation, appear to be related to the strong cross-bank gradients in tidal mixing energy and the Simpson-Hunter stratification parameter which exist across the study domain. Consistent with the one-dimensional models which defined the Simpson-Hunter parameter for tidally-mixed regions, results for stations located in zone 1 at depths <60 m on southern Georges Bank showed vertical stratification remaining weak year-round with no significant seasonal cycle. Density differences between the surface and bottom waters were close to zero year-round. Thus, zone 1 is indicative of an energy regime where mixing due to tidal dissipation is complete and can easily overcome any buoyancy addition due to air-sea interaction, as predicted by the Simpson-Hunter stratification parameter. At greater depths, and also consistent with theory, there occurred a transition to a seasonally-stratified regime, which possessed a mean annual hydrographic cycle. However, statistically significant differences were demonstrated between the mean hydrographic cycles derived from data collected in zones 2 and 3, both of which are located within the deeper (>60 m) region on southern Georges Bank. The differences between

426

J.J. BISAGNI

26

I

,

1

,

I

,

I

,

I

,

I

,

I

,

I

,

26

I

,

I

,

I

I

'

50

1O0

I

,

I

,

I

,

'

I

,

I

'

I

24 22

22

20

20

18

18

5" L

?...14

16 14

~2

12

r~ lO

10 8

6 4

2 0 50

,

26

1O0

150

200

250

300

350

'

0

I

I

I

150

200

250

300

YEAR D A Y

YEAR DA Y

STATION 123 (Zone 1)

STATION 148 (Zone 1)

1

,

I

i

I

,

I

J

I

,

I

,

I

26

~

I

,

I

,

I

J

I

~

I

,

I

1

550

,

t

I

24-

22

22-

2,

5"

20-

20

18-

18

16

16-

5" L 14

~..14

12r~ 10

108

8

~

6 4. 2 0

0 50

100

150

200

250

300

350

I

0

'

I

50

'

I

1O0

'

I

150

'

I

'

200

I

250

'

I

300

YEAR D A Y

YEAR D A Y

STATION 149 (Zone 2)

STATION 155 (Zone 2)

'

I

350

F i g . 4. M e a n s e a s o n a l S S T c y c l e s f o r t h e p a i r s o f s t a t i o n s l o c a t e d in e a c h o f d e p t h z o n e s 1 , 2 a n d 3 ( s o l i d l i n e s ) a l o n g w i t h t h e _+ 1 s t a n d a r d d e v i a t i o n l i m i t s ( d a s h e d l i n e s ) o f e a c h m o d e l fit t o t h e i n p u t data (circles).

zones 2 and 3 and possible reasons for the existence of these differences are discussed below. For stations located in zone 2, with bottom depths between 60 and 80 m: (1) the mean cycle of the maximum gradient of the seasonal pycnocline was dominated by a one cycleper-year model; (2) the mean depth of the maximum gradient did not display a distinct seasonal cycle, but remained fixed, or nearly so, near 20 m year-round and; (3) the mean cycle of the maximum gradient varied annually by almost two orders of magnitude, from a minimum in late January to a maximum in late July. All of these observations suggests that zone 2 is a "transition" regime as proposed by FLAGG (1987), showing characteristics of

The annual stratification cycleon southern GeorgesBank , l ~ l ~ l ~ l

26

I , l ~

26

h

I

~

I

~

427

I

I

,

I

L

I

,

24-

24

O

22

22-

20

20-

18

18

O

,2_.14 t~ 10 8

8 6-

6

4~

4 0

20

I

0

2 '

50

I

1O0

'

'

150

I

'

200

l

250

'

I

I

300

350

o

1

50

'

I

1O0

'

I

150

;

I

'

200

1

250

'

I

300

YEAR D A Y

YEAR DA Y

STATION 153 (Zone 3)

STATION 178 (Zone 3)

F i g . 4.

350

Continued)

both the well-mixed central Bank waters and those waters overlying the deeper portions of the Bank. Observation (1) shows that in an average sense, during spring and summer, buoyancy addition due to air-sea interaction dominates tidal stirring, with the result being development of a seasonal pycnocline. Further support for the transitional nature of zone 2 is given by observation (2), which might result from greater average mixing, caused by tidal stirring of sufficient strength to limit the depth of the mean pycnocline, as described by PINGREE et al. (1975) for the English Channel. Observation (3) results from: the development of strong summer stratification, yielding SP values which are similar to those from zone 3 during summer; equally-strong buoyancy extraction and mixing during the late autumn and early winter, with the entire water column becoming as well-mixed as described above for zone 1, with SP values that are indistinguishable from those for zone 1. If zone 2 is transitional as the above observations suggest, it follows that late spring and summer buoyancy addition and tidal forcing there, are on average, more closely balanced. Based on this and speculating further, the pycnocline in this transition regime might then be expected to respond strongly to the 28 day spring-neap tidal cycle as proposed by JAMES (1977) and observed in the shallow seas surrounding the United Kingdom (SIMPSONand BOWERS,1981). This is because the tidal stirring is directly proportional to the cube of the depth-averaged tidal current, U, as shown by equation (2), and also the change in Ucaused by the 28 day spring-neap cycle at a nearby station on Georges Bank is approximately a factor of two (Fig. 1). Based on this, the result would be a factor of eight increase in tidal stirring, occurring every 28 days, during spring tides. If any translation of the tidallyinduced front on southern Georges Bank due to the spring-neap cycle is small ( - 4 km), as noted for fronts around the United Kingdom (SIMPSON and BOWERS,1981), the hydrographic cycles described for zone 2 may imply that a large portion of the increase in tidal stirring during periods of spring tides might cause a decrease in both the magnitude and depth of the pycnocline. Conversely, an increase in the same parameters would occur during neaps. However, given the nearly-seasonal sampling of the M A R M A P hydrographic data, only the mean state of the water column could be resolved; resolution of the

428

J.J. B1SAGNI

o

o

~

~

I I I I

I

o

I ~3

--

(~)

0

-

.e...t c.I

}

P4

19

)

0 S

c~

{)

}

,1""

c~ )

i

"tzi'~

-~.- ~ _

z

0

0

--


,

C

(,o I

I

(~)

I

I

.,{

.=.

,S

~'C D

--

-

-

I

I

I

I

o

~E

'az

e',

° I

I

x

I

~' ?

I

ii 0

T

T

t

"

I

[

l

N

Z {2

~{, -T-

I

0

e.~ a} ~} ..~

e,,l o e.

D

z

e~

0

r.

) O-~

[.., u']

eD

r~

0 I

I

I

(~,) 'az

]

I

'

1

I

I

~

"2 o b.l

)C @ C () {

{9

o

"N

0~) 'xs {D

"~ e-

The annualstratificationcycleon southernGeorgesBank

No

o° -

.< F-

( ~ ) '~s

(uO '~z

(j_~, ~o)OJ6oz '
I l l l l

II

&

I

I

I ~o

g

g w)

g

g~ i '0 o

V-

I l l

( ~ ) ',~z

I

I

I

I

o

429

430

J.J.

B1SAGNI

(l_w ~)OtSoi 'dS I

° I 1 /

I

7 ? ? T IA I I I

I /~ho

°1~0~~ t o

i

8

°i

0

0

0 8 o o o

8

("0 'zs

-

(u,)

'az

,,4

(t_~ ~)OJ6o~ ',IS

r~.

,!o

o

~N

~ Lt~

o

o o o o o

(~) 'az

8

~

I

I

~

~

I

-

o

o

The annual stratification cycle on southern Georges Bank

431

Table 4a. Georges Bank intra-zonal sea surface temperature Kolmogorov-Smirnov test results for station pairs Depth zone (depth range) 1 (<60 m) 2 (60-80 m) 3 (80-100 m)

D SST

P SST

0.12 11.24 0.23

0.95 0.32 0.38

Table 4b. Georges Bank inter-zonal sea surface temperature Kolmogorov-Smirnov test results for lumped station pairs Depth zones tested

D SST

P SST

1,2 2,3 1,3

0.13 0.11 0.17

0.64 0.86 0.37

fortnightly spring-neap cycle from these data is not possible. Lastly, combining the effects of air-sea interaction and the spring-neap tidal cycle may yield another important process, i.e. perhaps during spring tides, the hypothesized decreased depths and magnitude of the pycnocline in zone 2 may mean that the pycnocline may be more susceptible to disruption, caused by strong mixing resulting from air-sea interaction. An alternative explanation, consistent with observation (2) which is unrelated to the one-dimensional energy balance described by SISIPSONand HUNTER(1974) is the constant Table 5a. Georges Bank intra-zonal pycnocline parameters Kolmogorov-Smirnov test results for station pairs

Depth zone (depth range) 1(<60m) 2(60-80m) 3 (80-100 m)

D

P

SP, ZP, ST

SP, ZP, ST

0.19 0.12 0.20

0.19 0.18 0.20

0.19 0.20 0.19

0.52 0.98 0.61

0.50 0.82 0 . 7 1 11.73 /).58 /I.79

Georges Bank inter-zonal pycnocline parameters Kolmogorov-Smirnov test results for lumped station pairs

Table 5b.

Depth zones tested 1,2 2,3 1,3

D

P

SP, ZP, ST

SP, ZP, ST

0.33 0.40 0.66

0.22 0.25 0.39

0.62 0.44 0.83

1/.00 0.12 0.00 0.06 0.00 0.00

0.00 0.00 0.00

432

J . J . BISAGNI

addition of negative potential energy present in the "cold-band" as it flows onto southern Georges Bank, centered near the 80-m isobath as described by FLACC(1987). Although the cold-band initially develops as a parcel of "remnant" winter shelf water, which becomes abruptly isolated from further air-sea interaction through development of the seasonal pycnocline during spring, measurements have shown a mean 10 cm s-~ alongbank flow to the southwest within the "cold-band" (BUTMANet al., 1982), requiring replenishment from the east. This advective input of negative potential energy might also result in, or help maintain, the constant pycnocline depth observed in zone 2. Stations located in zone 3, in depths of between 80 and 100 m, display characteristics which indicate reduced mixing due to tides, an increase in the relative importance of buoyancy addition through air-sea interaction processes, and a lack of advective inputs of negative potential energy related to the "cold-band", although other inputs of negative potential energy, through cross-bank advection of denser slope water may be indicated. These characteristics include: (1) mean cycles of both the maximum gradient and depth of the pycnocline which were dominated by a one cycle-per-year model; (2) the mean depth of the maximum gradient, which increased steadily from 20 m in early June to 60 m in late December when the upper water column is well-mixed; (3) the mean cycle of the maximum gradient, which varied annually by only one order of magnitude. Inputs of negative potential energy, caused by cross-bank advection of denser slope water, may in part, be the cause of minimum pycnocline magnitude values which are almost a factor of 10 higher than those in zone 2, thus indicating that the water column is never completely vertically mixed to bottom. Reduced vertical mixing and the large excursion of the mean annual cycle in the depth of the maximum vertical gradient (ZP), may exclude development of the "cold-band" in this region. Based on this work, the sum of the physical forcing on southern Georges Bank as revealed by cross-bank differences in the mean annual stratification cycles, shows that Georges Bank is not a simple, shallow, tidally-stirred sea, i.e. responding only to the energy balance between buoyancy input and tidal stirring. Rather, the data suggests that the mean density stratification on southern Georges Bank may be strongly modified by the presence and spatial distributions of different water masses, which have been shown to exist on this portion of the Bank. The interaction of these different water masses with the physical forcings described above probably have a profound effect on the chemical and biological oceanography in this region of the Bank as well. Both the "cold-band" and incursions of slope water represent sources of nutrient-rich water which have been shown to be in close proximity to both the seasonal pycnocline and the well-mixed waters on the central portion of the Bank. This relationship is clearly shown from summertime hydrographic measurements which display strong nitrate gradients corresponding closely to the position of the tidally-mixed front and the pycnocline during the stratified season on southern Georges Bank (PAsTUSZAKet al., 1982; WALSHet al., 1987; HORNEet al., 1989). One result of this hydrographic structure might be the flux of nutrients into both the wellmixed region on central Georges Bank and the seasonal pycnocline on the Bank's southern flank and thus be a controlling factor with respect to primary production in these regions as suggested by LODERand PEAT (1985). In support of this hypothesis, both of these regions have been shown to be areas of sustained high primary production between May and September (O'REILLYet al., 1987). Mechanisms responsible for the flux might include the residual cross-bank circulation caused by tidal rectification (LODER and WmCHT, 1985; TEE, 1985), frictionally-induced, ageostrophic force balances (GARRETTand LODER, 1981),

The annual stratification cycle on southern Georges Bank

433

density-driven circulation (KULLENBERG, 1983) and externally-imposed, along-bank pressure gradients as summarized by BUTMANet al. (1987). Recent evidence indicates that middepth (33 m) on-bank eddy fluxes, related to the semidiurnal M2 tidal period were responsible for much of the on-bank nitrate transfer at a station on the northern flank of Georges Bank (HORNE et al., 1989). An important result of invoking any of these proposed mechanisms may be that variations in the nutrient fluxes probably occur over a variety of time scales. This could result in periods of enhanced and reduced nutrient fluxes from the deeper, nutrient-rich waters into both the well-mixed waters of the central Bank and the stable region within the pycnocline on the southern flank. Three examples of such time scales could be the 2- to 5day band and the 14- and 28-day bands resulting from episodic wind mixing and periodic increased tidal mixing caused by the 14- and 28-day spring-neap tidal cycles, respectively. In any case, such episodic or periodic enhanced fluxes might be optimal for production of a series of phytoplankton blooms as reported by PINGREE et al. (1975) for the English Channel. Further field sampling to collect time series of both nutrient and chlorophyll concentrations should be designed to test this hypothesis in this region of Georges Bank. Clearly, the role of water column stability, as affected by air-sea buoyancy fluxes and tidal mixing is probably a large one with respect to controlling the overall productivity on Georges Bank. However, its role, and indeed water column stability itself, are likely strongly coupled to the presence of the various, juxtaposed, water masses possessing different physical and chemical properties on southern Georges Bank.

CONCLUSIONS

(1) Harmonic analyses of hydrographic data revealed that the scales of the cross-bank spatial variability for both SST and stratification are generally in good agreement with both physical forcing and theory. (2) To a first approximation the cross-bank and along-bank differences in the mean seasonal SST cycle were both small and insignificant, consistent with larger scale meteorological forcing extant. (3) Cross-bank differences in the mean seasonal stratification cycle were large and significant and appeared related to both the pronounced cross-bank gradient in the Simpson-Hunter stratification parameter and cross-bank differences in sub-surface water masses which are known to occur on southern Georges Bank. Along-bank differences were small and insignificant. (4) In depths <60 m the mean seasonal stratification cycles indicate that the water column does remain unstratified year-round. (5) At depths of 60-80 m, although stratification is indeed established, results indicate that the depth of the pycnocline is strongly constrained to about 20 m year-round and the magnitude of variation of the maximum vertical density gradient is large, varying annually by at least two orders of magnitude. (6) At depths of 80-100 m, the mean seasonal stratification cycle showed large (40 m) fluctuations in the depth of the pycnocline, while the magnitude of the maximum vertical density gradient varied annually by only one order of magnitude. Acknowledgements--The author would like to thank Dr D. Mountain and J. Manning, NOAA/National Marine Fisheries Service, Woods Hole, Massachusetts, for providing the MARMAP hydrographic data and Dr D.

434

J.J. B1SAGNI

Twichell, U.S. Geological Survey, Woods Hole, Massachusetts, for providing the detailed bathymetry. Special thanks are due to Dr M. C. Ingham and R. S. Armstrong, NOAA/National Marine Fisheries Service, Narragansett, Rhode Island, for guidance and useful discussion throughout the data analysis and production of the manuscript. The author would also like to thank two anonymous reviewers for their critical reading of the manuscript.

REFERENCES BISAGNI J. J. (1983) Lagrangian current measurements within the eastern margin of a warm-core Gulf Stream ring. Journal of Physical Oceanography, 13,709-715. BROWN W. S. and J. A. MOODY0987) Tides. In: Georges Bank, R. H. BACKUS,editor, MIT Press, Cambridge, MA, pp. 100-107. BtJTMANB. and R. C. BEARDSLEY(1987a) Physical oceanography. In: Georges Bank, R. H. BACKUS,editor, MIT Press, Cambridge, MA, pp. 88-98. BUTMANB. and R. C. BEARDSLEY(1987b) Long-term observations on the southern flank of Georges Bank. Part I: A description of the seasonal cycle of currents, temperatures, stratification and wind stress. Journal of Physical Oceanography, 17, 367-384. BUTMAN B., R. C. BEARDSLEY,B. MAGNELL,D. FRYE, J. A. VERMERSCH,R. SCHLITZ, R. LIMEBURNER,W. R., WRI6HT and M. NOBLE(1982) Recent observations on the mean circulation on Georges Bank. Journal of" Physical Oceanography, 12, 569-591. BtJTMAN B., 3. W. LODERand R. C. BEARDSLEY(1987) The seasonal mean circulation: Observation and theory. In: Georges Bank, R. H. BACKUS,editor, MIT Press, Cambridge, MA, pp. 125-139. CSANADYG. T. (1978) Wind effects on surface to bottom fronts. Journal of Geophysical Research, 83, 4633--4640. FLA~ C. N. and R. C. BEARDSLEY(1978) On the stability of the shelf water/slope water front south of New England. Journal of Geophysical Research, 83, 4623-4631. FLAGGC. N., B. A. MAGNELL,D. FRYE,J. J. CURA,S. E. McDOWELLand R. I. SCARLET(1982) Interpretation of the physical oceanography of Georges Bank. Final report, Vol. I, prepared for the New York OCS Office, Bureau of Land Management, by EG&G Environmental Consultants, Waltham, MA. FEAGGC. N. (1987) Hydrographic structure and variability. In: Georges Bank, R. H. BACKUS,editor, MIT Press, Cambridge, MA, pp. 108-124. FOFONOFFN. P. and H. BRYDEN(1975) Specific gravity and density of seawater at atmospheric pressure. Journal ol"Marine Research, 33, 69-82. GARFIELDN., III and D. L. EVANS(1987) Shelf water entrainment by Gulf Stream warm-core rings. Journal of Geophysical Research, 92, 13003-13012. GARRETTC. J. R. (1972) Tidal resonance in the Bay of Fundy and Gulf of Maine. Nature, 238,441-443. GARRETTC. J. R. (1974) Normal modes of the Bay of Fundy and the Gulf of Maine. Canadian Journal of Earth Sciences, II, 549-556. GARRETFC. J. R. and J. W. LODER(1981) Dynamical aspects of shallow sea fronts. Philosophical Transactions of the Royal Society of London, Set. A, 302, 563-581. 8 GARRETTC. J. R., J. R. KELLEYand D. A. GREENBERG(1978) Tidal mixing versus thermal stratification in the Bay of Fundy and the Gulf of Maine. Atmosphere-Ocean, 16,403-423. HESSELBERGTH. (1918) Uber die Stabilitatsverhaltnisse bei Vertikalen Verscheibungen in der Atmosphare und im Meer. Annalen der Hydrographie u. Maritme Meteorologie, 118-129. HOPKINST. S. and N. GARFIELD,III (1981) Physical origins of Georges Bank water. Journal of Marine Research, 39, 465-500. HOPKINS T. S. and S. RAMAN(1987) Atmospheric variables and patterns. In: Georges Bank, R. H. BACKUS, editor, MIT Press, Cambridge, MA, pp. 66-73. HORNE E. P. W., J. W. LODER,W. G. HARRISON,R. MOHN, M. R. LEWIS, B. IRWINand T. PLATT(1989) Nitrate supply and demand at the Georges Bank tidal front. In: Topics in marine biology, J. D. Ros, editor, Scientia Marina, Vol. 53, pp. 145-158. HOU6rtTON R. W., R. SCHLITZ,R. C. BEARDSLEY,B. BUTMANand J. L. CrtAMBERLIN(1982) The Middle Atlantic Bight cold pool: evolution of the temperature structure during summer 1979. Journal of Physical Oceanography, 12, 1019-1029. JAMESI. D. (1977) A model of the annual cycle of temperature in a frontal region of the Celtic Sea. Estuarine and Coastal Marine Science, 5,339-353.

The annual stratification cycle on southern Georges Bank

435

KULLENBERG G. (1983) Chlorophyll fluorescence distribution in the Georges Bank area: effects of mixing and topography. International Council for Exploration of the Sea, Biological Oceanography Committee Document C.M 1983/L23:1-5. LODERJ. W. and T. PLATr (1985) Physical controls on phytoplankton production at tidal fronts. In: Proceedings of the 19th European marine biology symposium, P. E. GIBBS, editor, Cambridge University Press, pp. 321, LODER J. W. and D. G. WRIGHT(1985) Tidal rectification and frontal circulation on the sides of Georges Bank. Journal of Marine Research, 43,581-604. MILLERO F. J. and A. POISSON (1981) International one-atmosphere equation of state of seawater. Deep-Sea Research, 28A, 625-629. MORGANC. W. and J. M. BISHOP(1977) An example of Gulf Stream eddy-induced water exchange in the midAtlantic Bight. Journal of Physical Oceanography, 7,472-479. O'REILEY J. E., C. EVANS-ZETLINand D. A. BUSCH (1987) Primary production. In: Georges Bank, R. H. BACKUS.editor, MIT Press, Cambridge, MA, pp. 220-233. PASTUSZAKM., W. R. WRIGHTand D. PATANJO(1982) One year of nutrient distribution in the Georges Bank region in relation to hydrography. Journal of Marine Research, 14, 525-542. PINGREE R. D., P. R. PUGH, P. M. HOLLIGANand G. R. FORSTER(1975) Summer phytoplankton blooms and red tides along tidal fronts in the approaches to the English Channel. Nature, 258,672-677. POND S. and G. L. P~CKARD(1983) Introductory dynamical oceanography, 2nd Edn, Pergamon Press, New York, 329 pp. PRESS W. H. and S. A. TEUKOLS~V(1988) Kolmogorov-Smirnov test for two-dimensional data. Computers In Physics, 1, 74-77. RAMP S. R., R. C. BEARDSLEYand R. LEGECKIS(1983) An observation of frontal wave development on a shelfslope/warm core ring front near the shelf break south of New England. Journal of Physical Oceanography, 13,907-912. SHERMAN K. (1980) MARMAP, a fisheries ecosystem study in the Northwest Atlantic: fluctuations in ichthyoplankton-zooplankton components and their potential for impact on the system. In: Advanced concepts on ocean measurementsfor marine biology, F. P. DIERMER,F. J. VERNBERGand D. Z. MIRKES, cditors, University of South Carolina Press, Columbia, SC, pp. 9-37. SIMPSONJ. H. (1975) A boundary front in the summer regime of the Celtic Sea. Estuarine and Coastal Marine Science, 4, 71-81. SIMPSONJ. H. and R. J. HUNTER(1974) Fronts in the Irish Sea. Nature, 250, 404-406. SIMPSON J. H. and D. BOWERS (1981) Models of stratification and frontal movement in shelf seas. Deep-Sea Research, 28A, 727-738. TEE K.-T. (1985) Depth-dependent studies of tidally induced residual currents on the sides of Georges Bank. Journal of Physical Oceanography, 15, 1818-1846. UCHUPI E. and J. A. AUSTIN Jr (1987) Morphology. In: Georges Bank, J. H. BACKUS, editor, MIT Press, Cambridge, MA, pp. 25-30, USGS/NOAA (1990) Digital bathymetric library. U.S. Geological Survey/National Ocean and Atmospheric Administration Joint Office of Mapping and Research, Reston, VA 22092, U.S.A. WALSH J. J., Z. E. WHITLEDGE,J. E. O'REIEEY, W. C. PHOELand A. F. DRAXLER(1987) Nitrogen cycling on Georges Bank and the New York shelf: a comparison between well-mixed and seasonally stratified waters. In: Georges Bank, R. H. BACKUS,editor, MIT Press, Cambridge, MA, pp. 234-246. WRIGHTW. R. (1976) The limits of shelf water south of Cape Cod, 1941 to 1972. Journal of Marine Research, 34, 1-14.