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Chapter 11 Tidal Heights and Currents in Hudson Bay and James Bay
205 Chapter 11
TIDAL HEIGHTS AND CURRENTS I N HUDSON BAY AND JAMES BAY S.J.
PRINSENBERG AND N.G.
FREEMAN
INTRODUCTION
Although t h e t i d a l regime along t h e southern coasts o f Hudson Bay and western James Bay coasts was described by Manning i n 1950 and 1951, i t was not u n t i l 1966 t h a t c o t i d a l c h a r t s f o r t h e e n t i r e Hudson Bay system were constructed by Dohler.
Dohler (1967) a l s o summarized t h e e a r l y v i s u a l observaGodin (1972) used a one-dimensional numer-
tions o f currents i n Hudson Bay.
i c a l model of James Bay and observations around t h e Bay t o prepare c o t i d a l charts f o r several t i d a l constituents. He f u r t h e r modified t h e r e s u l t s when more t i d a l height data became a v a i l a b l e (Godin, 1974).
H y d r o e l e c t r i c power
developments i n northern Quebec prompted studies t o be made o f t h e physical oceanography o f Hudson Bay and James Bay.
Numerical t i d a l modelling (Freeman
and Murty, 1976) provided t h e f i r s t c o t i d a l charts o f several c o n s t i t u e n t s f o r the t o t a l area.
Later, o f f s h o r e c u r r e n t meter and t i d a l height data
became a v a i l a b l e f o r comparison w i t h model r e s u l t s . This chapter presents results o f the t i d a l model, comparison t o t h e o f f s h o r e observations, and a discussion o f t h e e f f e c t o f t h e i c e cover on t i d a l heights and currents. Also, an analysis i s presented o f t h e mixing and s t r a t i f i c a t i o n e f f e c t s caused by the i n t e r a c t i o n o f t h e t i d e s and freshwater plumes near t h e mouths o f rivers. TIDAL HEIGHTS The mainly semidiurnal t i d e enters Hudson S t r a i t w i t h an amplitude o f about 3 meters, and reaches t h e entrance t o Hudson Bay and Foxe Basin about 3 t o 4 hours l a t e r .
The M, t i d e i s a K e l v i n wave t h a t propagates a n t i c l o c k w i s e
around Hudson Bay, p a r t o f which enters i n t o James Bay and p a r t proceeds north along t h e east coast.
It i s reduced t o 10% i n amplitude and j o i n s t h e
incoming t i d e some 25 hours l a t e r (Fig. 11.1; Freeman and Murty, 1976).
As
the Kelvin wave propagates around Hudson Bay, i t s o f f s h o r e components i n t e r f e r e and cancel each o t h e r out a t two l o c a t i o n s (amphidromic points); westcentral and east-central Hudson Bay.
I n between these amphidromic p o i n t s ,
the offshore t i d a l components t h a t t r a v e l along t h e northern and southern shores r e i n f o r c e each o t h e r and produce a small t i d e (10-25 cm) i n t h e c e n t r e o f Hudson Bay.
The t i d a l wave propagates south around t h e Belcher Islands
206 where r e f l e c t i o n from t h e shallow bar t o t h e south produces a standing wave pattern. The maximum M, t i d a l amplitude o f 1.25 metres occurs along t h e west coast o f Hudson Bay: i t decreases t o 10 cm near t h e degenerate node along
950
800
75
65O
65
60
60'
55
550
Offshore M2 Tidal D a t a stn.
1
51'
phase
ampl.
deg
cm
196
101
2
35
64
3
337
59
4
310
36
51°
Figure 11.1. M c o - o s c i l l a t i n g t i d a l r e s u l t s from Freeman and Murty (1976) w i t h copiase l i n e s i n GMT +5 (degrees) as s o l i d l i n e s and co-amplitude l i n e s (cm) as dashed l i n e s . Data l i s t e d on f i g u r e i n s e r t i s o f o f f s h o r e t i d e gauge l o c a t i o n s represented by
*.
the east coast o f James Bay and t h e southeast coast o f Hudson Bay. t i d a l charts f o r t h e o t h e r semidiurnal t i d e s (S, and N,)
The co-
do not d i f f e r s i g -
n i f i c a n t l y from t h e M, c o t i d a l chart, although t h e i r amplitudes are o n l y 1/4 t h a t o f t h e M, t i d e .
S e n s i t i v i t y a n a l y s i s o f t h e model t o f r i c t i o n a l v a r i a -
tions indicates t h a t t h e r e i s a n e g l i b l e change i n t h e l o c a t i o n o f t h e cophase l i n e s and t h e amphidromic p o i n t s when t h e small l i n e a r f r i c t i o n c o e f f i c i e n t i s decreased by an order o f magnitude.
However, an order o f magnitude
increase i n t h e f r i c t i o n c o e f f i c i e n t a l t e r s t h e l o c a t i o n o f t h e amphidromic points.
High water a r r i v e s e a r l i e r w h i l e t h e amplitude i s reduced.
Model r e s u l t s compared favourably w i t h shore-based s t a t i o n data from around Hudson Bay.
The M, phase agreed g e n e r a l l y w i t h i n loo and t h e ampli-
tude t o w i t h i n a few percent (Freeman and Murty, 1976).
Exceptions occurred
i n James Bay, where t h e r e was about a 30% reduction i n amplitude due t o t h e lack o f r e s o l u t i o n i n t h e t i d a l model.
The o f f s h o r e t i d a l data, c o l l e c t e d i n
the e a r l y 1980's a l s o agrees w i t h t h e model r e s u l t s except i n two areas: (1) James Bay where t h e M, t i d a l model amplitude i s underestimated by about 40%; and (2) around Coates and Manse1 Islands where t h e model phase lags t h e o f f shore t i d a l data by 15' t o 30" and amplitude i s l a r g e r by 30%. I n t h e f i r s t area the model i s d e f i c i e n t as both t h e shore-based and o f f s h o r e data show consistently higher amplitudes than those produced by t h e model.
A finer
g r i d r e s o l u t i o n i n t h e model f o r James Bay should obviate t h i s problem. I n the second area, t h e problems causing t h e discrepancies i n phase and amplitude between o f f s h o r e and inshore records can o n l y be corrected by a much denser network o f o f f s h o r e t i d e gauges i n t h e entrance t o Hudson Bay so t h a t the proper phase and amplitude f o r c i n g can be a p p l i e d a t t h e open boundary. Figure 11.2 shows t h e cophase and coamplitude l i n e s f o r t h e K, c o n s t i -
A s i n g l e amphidromic p o i n t i s l o c a t e d near t h e center o f Hudson Bay as would be expected, since t h e f r e e g r a v i t y wavelength o f 2700 km f o r t h e K, tuent.
frequency equals t h e circumference o f t h e Bay.
The model p r e d i c t s t h e
degenerate node i n t h e northeastern corner o f James Bay as described e a r l i e r by Godin (1972). Observed o f f s h o r e K, t i d a l c o n s t i t u e n t s a t t h e p o i n t s shown on Figure 11.2 agree g e n e r a l l y w i t h t h e model r e s u l t s but have s i m i l a r s i z e discrepancies as described above f o r t h e M, t i d e except t h a t f o r t h e K , t i d e the observed t i d e i s now l a r g e r and comes i n l a t e r a t a l l o f f s h o r e locations. The t i d a l model o f Freeman and Murty (1976) was a l s o used t o p r e d i c t t h e independent t i d e s o f Hudson Bay t h a t are generated d i r e c t l y by t h e t i d a l forcing functions r a t h e r than by t h e t i d e progressing i n t o t h e Bay from t h e A t l a n t i c Ocean.
The amplitude o f t h e M, independent t i d e i s an order of
magnitude l e s s than t h a t o f t h e c o - o s c i l l a t i n g t i d e w h i l e t h e amplitude o f the K, independent t i d e i s about 30% t h a t o f t h e c o - o s c i l l a t i n g t i d e .
208
950
800
65'
60'
55'
stn.
phase
ampl.
deg
cm
51 95'
900
850
800
K, c o - o s c i l l a t i n g t i d a l r e s u l t s from Freeman and Murty Figure 11.2. (1976) presented as M, r e s u l t s were i n Figure 11.1. TIDAL CURRENTS
Observations a t o f f s h o r e l o c a t i o n s i n Hudson Bay i n d i c a t e t h a t t h e major I n t h e chapter
p o r t i o n o f t h e current energy i s associated w i t h t h e t i d e s .
on c i r c u l a t i o n (Chapter 10) a time s e r i e s o f c u r r e n t meter data was shown f o r a l o c a t i o n 150 km northeast o f C h u r c h i l l (Fig. 10.6;
Prinsenberg, 1986a).
209 The major feature o f t h e c u r r e n t i n t h i s area i s t h e semidiurnal t i d a l o s c i l l a t i o n which reaches amplitudes o f 30 cm s-l (Prinsenberg and Weaver, 1983). Larger semidiurnal t i d a l c u r r e n t s are observed a t t h e entrance t o Hudson Bay (90 cm 5 - l ) and a t the entrance t o James Bay (50 cm s - l ) . S i m i l a r currents, on the order of 100 cm s - l , were observed by Dohler (1967) from s h i p ' s d r i f t at the entrance t o Hudson Bay.
Wind generated i n e r t i a l c u r r e n t s (13.8 h period) can reach d a i l y maximum amplitudes o f up t c 30 cm s-l which equal t h e
t i d a l amplitudes observed w i t h i n Hudson Bay i t s e l f (Table 10.3; Prinsenberg, 1986a). However, t i d a l currents recur d a i l y w h i l e i n e r t i a l currents occur only a f t e r storms.
T i d a l c u r r e n t s a l s o decrease slowly w i t h depth whereas i n e r t i a l currents are r e s t r i c t e d t o t h e surface layer. '
Tidal stream analysis has been used t o p a r t i t i o r ! t h e observed c u r r e n t s
i n t o t i d a l and short- and long-period residual comronents.
For Hudson Bay,
the t i d a l components are mainly made up o f t h e semidiurnal t i d a l c o n s t i t u e n t s M,,
S,, and N, analogously w i t h t h e t i d a l heights. The d i u r n a l c o n s t i t u e n t s (K, and 0,) are an order o f magnitude smaller, t y p i c a l l y 5 t o 10% o f t h e
t o t a l predictable t i d a l c u r r e n t (Table 10.1;
Prinsenberg, 1986a).
Like the
t i d a l heights, t i d a l c u r r e n t s are c l a s s i f i e d as semidiurnal (Dohler, 1966). Observed (summer) and model (Freeman and Murty, 1976) v e l o c i t y amp1 i t u d e s along the major and minor axes o f t h e M, t i d a l c u r r e n t e l l i p s e s are shown i n Figure 11.3.
The observed values are a mean o f surface and subsurface (70
-
100 m) current meter values (Prinsenberg and Deys, 1979 and Prinsenberg and
The strongest M, c u r r e n t s occur a t the entrance t o Hudson
Fleming, 1982).
Bay where averaged magnitudes o f up t o 45 cm
5-l
were measured.
They a l i g n
with the channel a x i s owing t o t h e strong i n f l u e n c e o f t h e topography on t h e current d i r e c t i o n .
I n t h e i n t e r i o r o f Hudson Bay, t h e two observed t i d a l
e l l i p s e s show weaker t i d a l currents (20 cm s - l ) s i m i l a r t o model predictions. A t the shallow entrance t o James Bay, t h e observed t i d a l c u r r e n t s increase t o
22 and 29 cm s-l, values not predicted by t h e model because o f t h e lack o f g r i d resolution. SEASONAL VAR IAT1 ONS The annual i c e cover o f Hudson Bay and other l o c a t i o n s i n t h e Canadian Arctic a l t e r s t h e t i d a l signal. Some s t a t i o n s are apparently unaffected while those i n Hudson Bay and t h e Beaufort Sea experience smaller t i d e s during t h e ice-covered season and t h e phase o f t h e t i d e i s a l t e r e d (Godin and Barber, 1980). I n Hudson Bay, t h e phase o f t h e t i d e i s advanced during t h e ice-covered season w h i l e i n t h e Beaufort Sea t h e t i d e i s retarded (Henry and Foreman, 1977).
Further i n v e s t i g a t i o n o f p o s s i b l e m o d i f i c a t i o n o f t h e t i d e
i n Hudson Bay showed t h a t t h e d i s t o r t i o n increased as t h e t i d e propagated around Hudson Bay and i n t o James Bay.
65
60'
....
\
... ....... . . . . . . . . -
.....
... ..
....
55 '
M2 TIDAL
CURRENTS
observed
,+,
0model scale cms-' 0
. 40 .
80
51a
9 5'
Figure 11.3.
900
850
800
Predicted and observed M, t i d a l currents.
To determine t h e annual changes i n magnitude and phase o f t h e height o r
current due t o t h e i c e cover, cross spectral a n a l y s i s was c a r r i e d out w i t h t h e observed data and data from a reference s t a t i o n located i n a year-round i c e - f r e e area.
The parameter used i s t h e amplitude and phase o f t h e admit-
tance which i s t h e r a t i o o f t h e cross spectrum and t h e power spectrum o f t h e reference s t a t i o n f o r a given t i d a l frequency band (Godin and Barber, 1980).
211 If the components o f t h e observed data f l u c t u a t e r e l a t i v e t o those a t t h e reference s t a t i o n , t h e admittance should r e f l e c t these f l u c t u a t i o n s .
For
t i d a l currents and heights, both t h e d i u r n a l and semidiurnal frequency bands were used t o f o l l o w t h e seasonal change i n amplitude and phase f o r monthly record lengths. The reference s t a t i o n data used was t h e predicted t i d e a t Halifax, although o t h e r ice-free l o c a t i o n s could have been used, as l o n g as they had energy spectra s i m i l a r t o those o f t h e observed data.
Tidal height
r e s u l t s show t h a t t h e semidiurnal t i d a l amplitude decreases i n t h e w i n t e r c o n t i n u a l l y anticlockwise around Hudson Bay r e l a t i v e t o t h e summer t i d a l amplitudes (Godin and Barber, 1980).
A t C h u r c h i l l , t h e t i d a l height de-
creased by 7% and advanced by 20 minutes during the w i n t e r compared t o summer data.
S i m i l a r seasonal changes i n phase and amplitude were found i n the o f f -
shore current meter data (Prinsenberg and Weaver, 1983).
The admittance
results o f t h e t h r e e current meter records from t h r e e depths (25, 53, 90 in) were averaged t o o b t a i n a mean change i n amplitude and phase advance.
These
results are presented i n Figure 11.4 and show t h a t t h e amplitude o f t h e semidiurnal t i d a l current decreases i n amplitude by 10% and comes i n 20 minutes e a r l i e r r e l a t i v e t o t h e s u m e r t i d a l currents.
The a n a l y s i s o f t h e d i u r n a l
currents shows t h e same seasonal pattern; amplitudes are reduced by 30% and the phase i s advanced by 30 minutes during t h e winter.
However, t h i s l a t t e r
analysis i s not as accurate since t h e d i u r n a l signal i s weak and a l o c a l l y generated t i d e o f comparable amp1 i t u d e i s present. The a r r i v a l times o f t h e t i d e and t i d a l c u r r e n t s are advanced i n Hudson Bay during the w i n t e r w h i l e i n t h e Beaufort Sea t h e t i d e a r r i v e s l a t e r .
The
difference i n t h e t i d a l response t o t h e i c e cover i s due t o t h e wave form o f the t i d e i n each area.
I n t h e Beaufort Sea, t h e K e l v i n wave propagates more o r less along a s t r a i g h t coast and f o l l o w s t h e general theory o f a damped
Kelvin wave whose amplitude and phase v e l o c i t y decrease when f r i c t i o n i n creases (Prinsenberg, 1986b).
The decrease i n phase v e l o c i t y w i l l cause a
delay i n t h e a r r i v a l t i m e o f t h e t i d e and t i d a l currents. I n Hudson Bay t h e phase v e l o c i t y o f t h e r e f l e c t i n g K e l v i n wave a l s o decreases due t o increased f r i c t i o n , but now t h e sumnation o f an i n c i d e n t and a r e f l e c t i n g K e l v i n wave causes t h e phase and amplitude p a t t e r n s (amphidronic point) t o move southward as f r i c t i o n increases.
One sees p r o p o r t i o n a l l y more
o f the i n c i d e n t wave than t h e r e f l e c t e d wave which causes an advancement o f the a r r i v a l time o f t h e i r sum ( t i d e o r c u r r e n t s ) and o f f s e t s t h e small delay
o f a r r i v a l time o f each c o n t r i b u t o r due t o t h e decrease i n phase v e l o c i t y . Along a s t r a i g h t coast such as i n t h e southern Beaufort Sea t h i s does not occur and only a delay i n a r r i v a l t i m e i s seen i n t h e w i n t e r due t o the decrease i n phase v e l o c i t y .
However i n t h e southeastern corner o f t h e
212
T
AMPLITUDE SEMIDIURNAL
0.6I < SEP
40
SEP
NOV
1
1
NOV
JAN
1
1
JAN
MAR
1
1
MAY
*
MAR
1
1
MAY
JUL
1
1
JUL
1
F i g u r e 11.4. Seasonal v a r i a t i o n i n phase and amplitude o f semidiurnal and d i u r n a l t i d a l c u r r e n t northeast o f C h u r c h i l l .
Beaufort Sea some t i d a l wave r e f l e c t i o n must occur as no change i n a r r i v a l time o f t h e t i d e was n o t i c e d t h e r e by Godin and Barber (1980). TIDAL M I X I N G AT FRONTS AND PLUMES The s t r a t i f i c a t i o n i n Hudson Bay varies from v e r t i c a l l y well-mixed zones such as t i d a l f r o n t s , t o h i g h l y s t r a t i f i e d zones such as freshwater plumes l o c a t e d o f f r i v e r mouths. Using a two-dimensional model o f t h e semidiurnal t i d e G r i f f i t h s e t al. (1981) c a l c u l a t e d t h e r a t i o o f t h e s t a b i l i z i n g e f f e c t o f the surface buoyancy f l u x on t h e production o f t u r b u l e n t k i n e t i c energy t o p r e d i c t where f r o n t a l regions separate zones o f v e r t i c a l t i d a l m i x i n g from zones where summer s t r a t i f i c a t i o n exists.
These f r o n t a l regions are o f
213 p a r t i c u l a r importance as they are thought t o increase t h e b i o l o g i c a l product i v i t y as a r e s u l t o f t h e upward n u t r i e n t f l u x ( G r i f f i t h s e t al.,
1981).
The r e s u l t s show t h a t c e n t r a l Hudson Bay has no t i d a l f r o n t s as i t s t i d a l currents are t o o weak.
The o n l y well-mixed zones r e s u l t i n g from strong t i d a l
currents occur i n t h e shallow regions o f James Bay and along t h e southeast coast o f Hudson Bay.
Small-scale f r o n t s a l s o occur near headlands and i n
channels where l o c a l l y increased t i d a l c u r r e n t s cause v e r t i c a l l y mixed conditions i n areas t h a t would otherwise be w e l l s t r a t i f i e d . o f G r i f f i t h s e t al.
However, t h e model
(1981) was based on North Sea experience, and exclude
wind stress as a source f o r mixing and r u n o f f as a source f o r t h e buoyancy flux; thus it i s o n l y a p p l i c a b l e t o o f f s h o r e regions o f Hudson Bay.
Wind
mixing and r u n o f f are important c o n t r i b u t o r s t o t h e pycnocl i n e development and thus s t r a t i f i c a t i o n o f Hudson and James bays (Prinsenberg, 1983). Wind mixing may e s p e c i a l l y become important i n exposed areas such as t h e Belcher Islands where much b i o l o g i c a l a c t i v i t y occurs. Runoff, on t h e other hand, increases t h e s t a b i l i t y parameter i n protected areas such as i n southern James Bay where s t r a t i f i e d c o n d i t i o n s do occur i n t h e summer. Freshwater plumes are observed o f f r i v e r mouths i n Hudson and James bays, both i n the summer and t h e winter.
The plumes remain coherent up t o 70 km
from t h e i r source and reach widths o f 20 t o 30 km.
The existence o f an i c e
cover i n t h e w i n t e r i n h i b i t s wind-induced mixing and allows t h e plumes t o spread out f u r t h e r than under t h e i c e - f r e e c o n d i t i o n s o f t h e summer.
Even
though the r u n o f f r a t e s are an order o f magnitude l e s s i n w i n t e r than i n t h e suinner, the w i n t e r plumes a t t a i n t w i c e t h e surface area o f t h e summer plumes, p r i n c i p a l l y due t o t h e i n s u l a t i o n from wind-induced turbulence by t h e i c e cover (Freeman, 1982).
During t h e ice-covered period t i d a l d i s s i p a t i o n i s
the main source o f plume mixing. One o f t h e plunes, o f f t h e La Grande River, was e x t e n s i v e l y s t u d i e d as i t s r i v e r discharge r a t e was a1 t e r e d by h y d r o e l e c t r i c development (Freeman e t a l . , 1982 and Messier e t al., 1986). The buoyant La Grande River p l m e spreads out under t h e i c e l a t e r a l l y over about 20 km, and i s p u l l e d northward by the mean c i r c u l a t i o n t o a distance o f 40 t o 50 km. (Fig. 11.5 from Freeman, 1982).
It varies i n thickness from 2 t o 5 m and acquires s a l t as i t
moves away from t h e r i v e r mouth. Maximum t i d a l v e l o c i t i e s w i t h i n t h e plume are less than 10 cm s - l , which t r a n s l a t e s i n t o a h o r i z o n t a l plume motion o f only 1.5 km (Fig. 11.5).
The l a r g e v e l o c i t y a t t h e r i v e r mouth (43 cm 5 - l )
drops t o 12 cm s-l i n t h e f u l l y s t r a t i f i e d region some 3.5 km downstream and i n t h e f r o n t a l region some 40 kin downstream. I n t h e f u l l y s t r a -
t o 5 cm s'l
t i f i e d region t h e r e i s l i t t l e exchange w i t h t h e bottom layer.
The freshwater
spreads out due t o buoyancy and advection over t h e slow moving bottom layer. Farther downstream, i n t h e f r o n t a l region, t h e plume undergoes t h e maximum
214
54:
30
54 10
54
oc
FULLY STRATIFIED REGION
F i g u r e 11.5. Surface s a l i n i t y contours f o r LaGrande R i v e r plume during March 1-9, 1980 (from Freeman, 1982). I n s e r t s show 25-hr averaged p r o f i l e s o f c u r r e n t s , d e n s i t y and temperature s t r u c t u r e f o r t h r e e regions whose l o c a t i o n s along t h e plume's a x i s a r e shown by t h e l a r g e arrows.
215 r a t e o f s a l t exchange.
I n t h e middle region, t h e t i d a l k i n e t i c energy below
the plume i s much l a r g e r than i n t h e plume i t s e l f (Fig. 11.5),
which shows
how the S t r a t i f i c a t i o n i s o l a t e s t h e t i d a l current (and mixing) from t h e plume even though t h e plume moves up and down w i t h t h e t i d e .
Seaward o f t h e
f r o n t a l region, a well-mixed region e x i s t s where t h e t i d a l k i n e t i c energy i n and below t h e plume increases. V e r t i c a l mixing increases, breaking down t h e stratification.
I n t h i s area t h e r e i s evidence o f a bottom r e t u r n f l o w as
found i n estuarine c i r c u l a t i o n (Fig. 11.5).
I n t h e well-mixed region a l a r g e increase i n dissolved n u t r i e n t s near t h e i c e occurs and an i c e b i o t a comnuni t y i s observed which i s not observed inshore.
Since t h e areal extent o f
surface plumes seems t o be s t r o n g l y dependent on t h e discharge r a t e s as found by numerical plume model 1i n g (Freeman, 1982), t h e increased LaGrande River discharge w i l l l i m i t n u t r i e n t supply t o t h e surface l a y e r over a l a r g e r area and s i m i l a r e f f e c t s should be expected i n plumes whose r u n o f f r a t e s are modif i e d by h y d r o e l e c t r i c developments. CONCLUSION Tides i n Hudson Bay are c l a s s i f i e d as semidiurnal and o n l y a small r e g i o n along the eastern shore i s c l a s s i f i e d as mixed but s t i l l mainly semidiurnal. The semidiurnal t i d a l amplitude ranges from 1.25 m along t h e western shore t o 0.10 m along t h e eastern shore. values o f 8 cm.
The d i u r n a l t i d a l amplitude only reaches
The maximum t i d a l currents are observed a t t h e entrance t o
Hudson Bay where they reach v e l o c i t i e s o f 90 cm s-l; smaller maximum values
of 30 cm 5-l occur w i t h i n Hudson Bay.
During t h e winter, t h e i c e Cover
dampens the t i d a l c u r r e n t s and heights and advances t h e i r a r r i v a l times. Tidal and wind-generated c u r r e n t s determine t h e l o c a t i o n where strong mixing occurs and where t h e strong s t a b i l i t y o f freshwater plumes breaks down. Further oceanographic research should c o l l e c t o f f s h o r e t i d a l height and current meter data a t t h e entrances t o Hudson Bay and James Bay i n order t o resolve phase questions r a i s e d by t h e present data and provide b e t t e r boundary conditions f o r f u t u r e t i d a l modelling.
The modelling research should
address not only t h e e f f e c t t h e i c e cover has on t h e t i d e but a l s o t h e d i s crepancy now seen i n t h e o f f s h o r e data r e l a t i v e t o model r e s u l t s . Year-round offshore data w i t h i n Hudson and James Bays are required i n order t o o b t a i n r e l i a b l e amplitudes and phases o f o f f s h o r e t i d a l heights and currents. ACKNOWLEDGEMENTS The authors thank Drs. G. Ingram, D. Wright and D. Greenberg f o r t h e i r helpful comments on t h e manuscript, and t h e s t a f f o f the B a y f i e l d Laboratory, D.F.O.,
i n Burlington, Ontario f o r t h e i r e f f o r t s i n c o l l e c t i n g t h e data dur-
ing the f i e l d programs and processing t h a t are used i n t h i s manuscript.
216 Mr. R.E.
Walker and Mr. A.D.
graphy, i n Dartmouth, N.S.,
Cosgrove o f t h e Bedford I n s t i t u t e o f Oceanoa s s i s t e d w i t h t h e c a r t o g r a p h i c work and Dr. P.
M a r t i n i w i t h t h e e d i t o r a l work. REFERENCES Dohler, G.C., 1966. Tides i n Canadian Waters. Dept. Mines Tech. Surv., Can. Hydrogr. Serv., Mar. Sci. Branch, Rep. M54-966: 17 pp. Dohler, G.C., 1967. Tides and t i d a l c u r r e n t s i n Hudson Bay. I n : C.S. Beals ( E d i t o r ) , Science and H i s t o r y o f Hudson Bay, Dept. Energy, Mines and Resources, Ottawa, pp. 824-837. Freeman, N.G., 1982. Measurements and m o d e l l i n g o f freshwater plumes under an i c e cover. Ocean Science and Surveys, Dept. o f F i s h e r i e s and Oceans, B u r l i n g t o n , Manus. Rept. Ser. No. 14, 155 pp. Freeman, N.G. and Murty, T.S., 1976. Numerical m o d e l l i n g o f t i d e s i n Hudson Bay. J. Fish. Res. Bd. Canada, 33: 2345-2361. Roff, J.C. and P e t t , R.J., 1982. Physical, chemical, and Freeman, N.G., b i o l o g i c a l f e a t u r e s o f r i v e r plumes under an i c e cover i n James and Hudson Bays. Le N a t u r a l i s t e canadien, 109(4): 745-764. G r i f f i t h s , D.K., Pingree, R.D. and S i n c l a i r , M., 1981. Summer t i d a l f r o n t s i n t h e n e a r - a r c t i c regions o f Foxe Basin and Hudson Bay. Deep-sea Res. 28A: 865-876. Godin, G., 1972. The t i d e s o f James Bay. Dept. Environ., Mar. Sci. Branch, Tech. Rep. 24: 97-142. Godin, G., 1974. The t i d e i n e a s t e r n and western James Bay, A r c t i c 27: 104-110. Godin, G. and Barber, F.G., 1980. V a r i a b i l i t y o f t h e t i d e a t some s i t e s i n t h e Canadian A r c t i c . A r c t i c 33: 30-37. Henry, R.F. and Foreman, M.G.G., 1977. Numerical model s t u d i e s o f semid i u r n a l t i d e s i n t h e southern Beaufort Sea. P a c i f i c Marine Sc. Rept. 77-11. I n s t . o f Ocean Sciences, V i c t o r i a , B.C. 71 pp. Manning, T.H., 1950. Notes on t h e t i d e s along t h e south Hudson Bay and west James Bay coast. A r c t i c 3: 95-100. Manning, T.H., 1951. Remarks on t h e t i d e s and d r i f t w o o d s t r a n d l i n e s along t h e east coast o f James Bay. A r c t i c 4: 122-130. Messier, D., Ingram, R.G., D. and Savard, J.P., 1986. Physical and b i o l o g i c a l oceanographic m o d i f i c a t i o n s i n response t o La Grande H y d r o e l e c t r i c Complex. I n : I.P. M a r i t i n i ( E d i t o r ) , Canadian I n l a n d Seas, E l s e v i e r , Amsterdam. Prinsenberg, S.J., 1983. E f f e c t s o f t h e h y d r o e l e c t r i c developments on t h e oceanographic s u r f a c e waters o f Hudson Bay. Atmosphere-Ocean, 21: 418430. Pr nsenberg, S.J., 1986a. C i r c u l a t i o n p a t t e r n and c u r r e n t s t r u c t u r e o f Hudson Bay. In: I.P. M a r t i n i ( E d i t o r ) , Canadian I n l a n d Seas. E l s e v i e r , Amsterdam. P r nsenberg, S.J., 1986b. E f f e c t s o f t h e annual i c e cover on t h e t i d a l c u r r e n t s and freshwater content o f Canadian i n l a n d seas. I n : Canadian East Coast Workshop on Sea Ice, January 7-9, 1986. Can. Tech. Rept. o f Fish. and Aquat. Sci., Dept. o f F i s h e r i e s and Oceans, Dartmouth, N.S. Pr nsenberg, S.J. and Deys, F.W., 1979. Hudson Bay oceanographic data r e p o r t 1975, volume 2. Ocean and Aquatic Sc., Dept. o f Fish. and Oceans, B u r l i n g t o n , Data Rep. Ser. No. 82-1, 255 pp. P r nsenberg, S.J. and Fleming, B.M., 1982. Hudson Bay/James Bay oceanographic survey 1976 Data Report, Volume 1. Ocean Sci. and Surv., Dept. o f Fish. and Oceans, B u r l i n g t o n , Data Rep. Ser. No=-1, 255 p. P r nsenberg, S.J. and Weaver, R.K., 1983. Hudson Bay oceanographic data r e p o r t 1982. Ocean Sci. and Surv., Dept. o f Fish. and Oceans, B u r l i n g t o n , Data Rep. Ser. No. 83-5, 221 pp.
TIDAL HEIGHTS AND CURRENTS I N HUDSON BAY AND JAMES BAY S.J.
PRINSENBERG AND N.G.
FREEMAN
INTRODUCTION
Although t h e t i d a l regime along t h e southern coasts o f Hudson Bay and western James Bay coasts was described by Manning i n 1950 and 1951, i t was not u n t i l 1966 t h a t c o t i d a l c h a r t s f o r t h e e n t i r e Hudson Bay system were constructed by Dohler.
Dohler (1967) a l s o summarized t h e e a r l y v i s u a l observaGodin (1972) used a one-dimensional numer-
tions o f currents i n Hudson Bay.
i c a l model of James Bay and observations around t h e Bay t o prepare c o t i d a l charts f o r several t i d a l constituents. He f u r t h e r modified t h e r e s u l t s when more t i d a l height data became a v a i l a b l e (Godin, 1974).
H y d r o e l e c t r i c power
developments i n northern Quebec prompted studies t o be made o f t h e physical oceanography o f Hudson Bay and James Bay.
Numerical t i d a l modelling (Freeman
and Murty, 1976) provided t h e f i r s t c o t i d a l charts o f several c o n s t i t u e n t s f o r the t o t a l area.
Later, o f f s h o r e c u r r e n t meter and t i d a l height data
became a v a i l a b l e f o r comparison w i t h model r e s u l t s . This chapter presents results o f the t i d a l model, comparison t o t h e o f f s h o r e observations, and a discussion o f t h e e f f e c t o f t h e i c e cover on t i d a l heights and currents. Also, an analysis i s presented o f t h e mixing and s t r a t i f i c a t i o n e f f e c t s caused by the i n t e r a c t i o n o f t h e t i d e s and freshwater plumes near t h e mouths o f rivers. TIDAL HEIGHTS The mainly semidiurnal t i d e enters Hudson S t r a i t w i t h an amplitude o f about 3 meters, and reaches t h e entrance t o Hudson Bay and Foxe Basin about 3 t o 4 hours l a t e r .
The M, t i d e i s a K e l v i n wave t h a t propagates a n t i c l o c k w i s e
around Hudson Bay, p a r t o f which enters i n t o James Bay and p a r t proceeds north along t h e east coast.
It i s reduced t o 10% i n amplitude and j o i n s t h e
incoming t i d e some 25 hours l a t e r (Fig. 11.1; Freeman and Murty, 1976).
As
the Kelvin wave propagates around Hudson Bay, i t s o f f s h o r e components i n t e r f e r e and cancel each o t h e r out a t two l o c a t i o n s (amphidromic points); westcentral and east-central Hudson Bay.
I n between these amphidromic p o i n t s ,
the offshore t i d a l components t h a t t r a v e l along t h e northern and southern shores r e i n f o r c e each o t h e r and produce a small t i d e (10-25 cm) i n t h e c e n t r e o f Hudson Bay.
The t i d a l wave propagates south around t h e Belcher Islands
206 where r e f l e c t i o n from t h e shallow bar t o t h e south produces a standing wave pattern. The maximum M, t i d a l amplitude o f 1.25 metres occurs along t h e west coast o f Hudson Bay: i t decreases t o 10 cm near t h e degenerate node along
950
800
75
65O
65
60
60'
55
550
Offshore M2 Tidal D a t a stn.
1
51'
phase
ampl.
deg
cm
196
101
2
35
64
3
337
59
4
310
36
51°
Figure 11.1. M c o - o s c i l l a t i n g t i d a l r e s u l t s from Freeman and Murty (1976) w i t h copiase l i n e s i n GMT +5 (degrees) as s o l i d l i n e s and co-amplitude l i n e s (cm) as dashed l i n e s . Data l i s t e d on f i g u r e i n s e r t i s o f o f f s h o r e t i d e gauge l o c a t i o n s represented by
*.
the east coast o f James Bay and t h e southeast coast o f Hudson Bay. t i d a l charts f o r t h e o t h e r semidiurnal t i d e s (S, and N,)
The co-
do not d i f f e r s i g -
n i f i c a n t l y from t h e M, c o t i d a l chart, although t h e i r amplitudes are o n l y 1/4 t h a t o f t h e M, t i d e .
S e n s i t i v i t y a n a l y s i s o f t h e model t o f r i c t i o n a l v a r i a -
tions indicates t h a t t h e r e i s a n e g l i b l e change i n t h e l o c a t i o n o f t h e cophase l i n e s and t h e amphidromic p o i n t s when t h e small l i n e a r f r i c t i o n c o e f f i c i e n t i s decreased by an order o f magnitude.
However, an order o f magnitude
increase i n t h e f r i c t i o n c o e f f i c i e n t a l t e r s t h e l o c a t i o n o f t h e amphidromic points.
High water a r r i v e s e a r l i e r w h i l e t h e amplitude i s reduced.
Model r e s u l t s compared favourably w i t h shore-based s t a t i o n data from around Hudson Bay.
The M, phase agreed g e n e r a l l y w i t h i n loo and t h e ampli-
tude t o w i t h i n a few percent (Freeman and Murty, 1976).
Exceptions occurred
i n James Bay, where t h e r e was about a 30% reduction i n amplitude due t o t h e lack o f r e s o l u t i o n i n t h e t i d a l model.
The o f f s h o r e t i d a l data, c o l l e c t e d i n
the e a r l y 1980's a l s o agrees w i t h t h e model r e s u l t s except i n two areas: (1) James Bay where t h e M, t i d a l model amplitude i s underestimated by about 40%; and (2) around Coates and Manse1 Islands where t h e model phase lags t h e o f f shore t i d a l data by 15' t o 30" and amplitude i s l a r g e r by 30%. I n t h e f i r s t area the model i s d e f i c i e n t as both t h e shore-based and o f f s h o r e data show consistently higher amplitudes than those produced by t h e model.
A finer
g r i d r e s o l u t i o n i n t h e model f o r James Bay should obviate t h i s problem. I n the second area, t h e problems causing t h e discrepancies i n phase and amplitude between o f f s h o r e and inshore records can o n l y be corrected by a much denser network o f o f f s h o r e t i d e gauges i n t h e entrance t o Hudson Bay so t h a t the proper phase and amplitude f o r c i n g can be a p p l i e d a t t h e open boundary. Figure 11.2 shows t h e cophase and coamplitude l i n e s f o r t h e K, c o n s t i -
A s i n g l e amphidromic p o i n t i s l o c a t e d near t h e center o f Hudson Bay as would be expected, since t h e f r e e g r a v i t y wavelength o f 2700 km f o r t h e K, tuent.
frequency equals t h e circumference o f t h e Bay.
The model p r e d i c t s t h e
degenerate node i n t h e northeastern corner o f James Bay as described e a r l i e r by Godin (1972). Observed o f f s h o r e K, t i d a l c o n s t i t u e n t s a t t h e p o i n t s shown on Figure 11.2 agree g e n e r a l l y w i t h t h e model r e s u l t s but have s i m i l a r s i z e discrepancies as described above f o r t h e M, t i d e except t h a t f o r t h e K , t i d e the observed t i d e i s now l a r g e r and comes i n l a t e r a t a l l o f f s h o r e locations. The t i d a l model o f Freeman and Murty (1976) was a l s o used t o p r e d i c t t h e independent t i d e s o f Hudson Bay t h a t are generated d i r e c t l y by t h e t i d a l forcing functions r a t h e r than by t h e t i d e progressing i n t o t h e Bay from t h e A t l a n t i c Ocean.
The amplitude o f t h e M, independent t i d e i s an order of
magnitude l e s s than t h a t o f t h e c o - o s c i l l a t i n g t i d e w h i l e t h e amplitude o f the K, independent t i d e i s about 30% t h a t o f t h e c o - o s c i l l a t i n g t i d e .
208
950
800
65'
60'
55'
stn.
phase
ampl.
deg
cm
51 95'
900
850
800
K, c o - o s c i l l a t i n g t i d a l r e s u l t s from Freeman and Murty Figure 11.2. (1976) presented as M, r e s u l t s were i n Figure 11.1. TIDAL CURRENTS
Observations a t o f f s h o r e l o c a t i o n s i n Hudson Bay i n d i c a t e t h a t t h e major I n t h e chapter
p o r t i o n o f t h e current energy i s associated w i t h t h e t i d e s .
on c i r c u l a t i o n (Chapter 10) a time s e r i e s o f c u r r e n t meter data was shown f o r a l o c a t i o n 150 km northeast o f C h u r c h i l l (Fig. 10.6;
Prinsenberg, 1986a).
209 The major feature o f t h e c u r r e n t i n t h i s area i s t h e semidiurnal t i d a l o s c i l l a t i o n which reaches amplitudes o f 30 cm s-l (Prinsenberg and Weaver, 1983). Larger semidiurnal t i d a l c u r r e n t s are observed a t t h e entrance t o Hudson Bay (90 cm 5 - l ) and a t the entrance t o James Bay (50 cm s - l ) . S i m i l a r currents, on the order of 100 cm s - l , were observed by Dohler (1967) from s h i p ' s d r i f t at the entrance t o Hudson Bay.
Wind generated i n e r t i a l c u r r e n t s (13.8 h period) can reach d a i l y maximum amplitudes o f up t c 30 cm s-l which equal t h e
t i d a l amplitudes observed w i t h i n Hudson Bay i t s e l f (Table 10.3; Prinsenberg, 1986a). However, t i d a l currents recur d a i l y w h i l e i n e r t i a l currents occur only a f t e r storms.
T i d a l c u r r e n t s a l s o decrease slowly w i t h depth whereas i n e r t i a l currents are r e s t r i c t e d t o t h e surface layer. '
Tidal stream analysis has been used t o p a r t i t i o r ! t h e observed c u r r e n t s
i n t o t i d a l and short- and long-period residual comronents.
For Hudson Bay,
the t i d a l components are mainly made up o f t h e semidiurnal t i d a l c o n s t i t u e n t s M,,
S,, and N, analogously w i t h t h e t i d a l heights. The d i u r n a l c o n s t i t u e n t s (K, and 0,) are an order o f magnitude smaller, t y p i c a l l y 5 t o 10% o f t h e
t o t a l predictable t i d a l c u r r e n t (Table 10.1;
Prinsenberg, 1986a).
Like the
t i d a l heights, t i d a l c u r r e n t s are c l a s s i f i e d as semidiurnal (Dohler, 1966). Observed (summer) and model (Freeman and Murty, 1976) v e l o c i t y amp1 i t u d e s along the major and minor axes o f t h e M, t i d a l c u r r e n t e l l i p s e s are shown i n Figure 11.3.
The observed values are a mean o f surface and subsurface (70
-
100 m) current meter values (Prinsenberg and Deys, 1979 and Prinsenberg and
The strongest M, c u r r e n t s occur a t the entrance t o Hudson
Fleming, 1982).
Bay where averaged magnitudes o f up t o 45 cm
5-l
were measured.
They a l i g n
with the channel a x i s owing t o t h e strong i n f l u e n c e o f t h e topography on t h e current d i r e c t i o n .
I n t h e i n t e r i o r o f Hudson Bay, t h e two observed t i d a l
e l l i p s e s show weaker t i d a l currents (20 cm s - l ) s i m i l a r t o model predictions. A t the shallow entrance t o James Bay, t h e observed t i d a l c u r r e n t s increase t o
22 and 29 cm s-l, values not predicted by t h e model because o f t h e lack o f g r i d resolution. SEASONAL VAR IAT1 ONS The annual i c e cover o f Hudson Bay and other l o c a t i o n s i n t h e Canadian Arctic a l t e r s t h e t i d a l signal. Some s t a t i o n s are apparently unaffected while those i n Hudson Bay and t h e Beaufort Sea experience smaller t i d e s during t h e ice-covered season and t h e phase o f t h e t i d e i s a l t e r e d (Godin and Barber, 1980). I n Hudson Bay, t h e phase o f t h e t i d e i s advanced during t h e ice-covered season w h i l e i n t h e Beaufort Sea t h e t i d e i s retarded (Henry and Foreman, 1977).
Further i n v e s t i g a t i o n o f p o s s i b l e m o d i f i c a t i o n o f t h e t i d e
i n Hudson Bay showed t h a t t h e d i s t o r t i o n increased as t h e t i d e propagated around Hudson Bay and i n t o James Bay.
65
60'
....
\
... ....... . . . . . . . . -
.....
... ..
....
55 '
M2 TIDAL
CURRENTS
observed
,+,
0model scale cms-' 0
. 40 .
80
51a
9 5'
Figure 11.3.
900
850
800
Predicted and observed M, t i d a l currents.
To determine t h e annual changes i n magnitude and phase o f t h e height o r
current due t o t h e i c e cover, cross spectral a n a l y s i s was c a r r i e d out w i t h t h e observed data and data from a reference s t a t i o n located i n a year-round i c e - f r e e area.
The parameter used i s t h e amplitude and phase o f t h e admit-
tance which i s t h e r a t i o o f t h e cross spectrum and t h e power spectrum o f t h e reference s t a t i o n f o r a given t i d a l frequency band (Godin and Barber, 1980).
211 If the components o f t h e observed data f l u c t u a t e r e l a t i v e t o those a t t h e reference s t a t i o n , t h e admittance should r e f l e c t these f l u c t u a t i o n s .
For
t i d a l currents and heights, both t h e d i u r n a l and semidiurnal frequency bands were used t o f o l l o w t h e seasonal change i n amplitude and phase f o r monthly record lengths. The reference s t a t i o n data used was t h e predicted t i d e a t Halifax, although o t h e r ice-free l o c a t i o n s could have been used, as l o n g as they had energy spectra s i m i l a r t o those o f t h e observed data.
Tidal height
r e s u l t s show t h a t t h e semidiurnal t i d a l amplitude decreases i n t h e w i n t e r c o n t i n u a l l y anticlockwise around Hudson Bay r e l a t i v e t o t h e summer t i d a l amplitudes (Godin and Barber, 1980).
A t C h u r c h i l l , t h e t i d a l height de-
creased by 7% and advanced by 20 minutes during the w i n t e r compared t o summer data.
S i m i l a r seasonal changes i n phase and amplitude were found i n the o f f -
shore current meter data (Prinsenberg and Weaver, 1983).
The admittance
results o f t h e t h r e e current meter records from t h r e e depths (25, 53, 90 in) were averaged t o o b t a i n a mean change i n amplitude and phase advance.
These
results are presented i n Figure 11.4 and show t h a t t h e amplitude o f t h e semidiurnal t i d a l current decreases i n amplitude by 10% and comes i n 20 minutes e a r l i e r r e l a t i v e t o t h e s u m e r t i d a l currents.
The a n a l y s i s o f t h e d i u r n a l
currents shows t h e same seasonal pattern; amplitudes are reduced by 30% and the phase i s advanced by 30 minutes during t h e winter.
However, t h i s l a t t e r
analysis i s not as accurate since t h e d i u r n a l signal i s weak and a l o c a l l y generated t i d e o f comparable amp1 i t u d e i s present. The a r r i v a l times o f t h e t i d e and t i d a l c u r r e n t s are advanced i n Hudson Bay during the w i n t e r w h i l e i n t h e Beaufort Sea t h e t i d e a r r i v e s l a t e r .
The
difference i n t h e t i d a l response t o t h e i c e cover i s due t o t h e wave form o f the t i d e i n each area.
I n t h e Beaufort Sea, t h e K e l v i n wave propagates more o r less along a s t r a i g h t coast and f o l l o w s t h e general theory o f a damped
Kelvin wave whose amplitude and phase v e l o c i t y decrease when f r i c t i o n i n creases (Prinsenberg, 1986b).
The decrease i n phase v e l o c i t y w i l l cause a
delay i n t h e a r r i v a l t i m e o f t h e t i d e and t i d a l currents. I n Hudson Bay t h e phase v e l o c i t y o f t h e r e f l e c t i n g K e l v i n wave a l s o decreases due t o increased f r i c t i o n , but now t h e sumnation o f an i n c i d e n t and a r e f l e c t i n g K e l v i n wave causes t h e phase and amplitude p a t t e r n s (amphidronic point) t o move southward as f r i c t i o n increases.
One sees p r o p o r t i o n a l l y more
o f the i n c i d e n t wave than t h e r e f l e c t e d wave which causes an advancement o f the a r r i v a l time o f t h e i r sum ( t i d e o r c u r r e n t s ) and o f f s e t s t h e small delay
o f a r r i v a l time o f each c o n t r i b u t o r due t o t h e decrease i n phase v e l o c i t y . Along a s t r a i g h t coast such as i n t h e southern Beaufort Sea t h i s does not occur and only a delay i n a r r i v a l t i m e i s seen i n t h e w i n t e r due t o the decrease i n phase v e l o c i t y .
However i n t h e southeastern corner o f t h e
212
T
AMPLITUDE SEMIDIURNAL
0.6I < SEP
40
SEP
NOV
1
1
NOV
JAN
1
1
JAN
MAR
1
1
MAY
*
MAR
1
1
MAY
JUL
1
1
JUL
1
F i g u r e 11.4. Seasonal v a r i a t i o n i n phase and amplitude o f semidiurnal and d i u r n a l t i d a l c u r r e n t northeast o f C h u r c h i l l .
Beaufort Sea some t i d a l wave r e f l e c t i o n must occur as no change i n a r r i v a l time o f t h e t i d e was n o t i c e d t h e r e by Godin and Barber (1980). TIDAL M I X I N G AT FRONTS AND PLUMES The s t r a t i f i c a t i o n i n Hudson Bay varies from v e r t i c a l l y well-mixed zones such as t i d a l f r o n t s , t o h i g h l y s t r a t i f i e d zones such as freshwater plumes l o c a t e d o f f r i v e r mouths. Using a two-dimensional model o f t h e semidiurnal t i d e G r i f f i t h s e t al. (1981) c a l c u l a t e d t h e r a t i o o f t h e s t a b i l i z i n g e f f e c t o f the surface buoyancy f l u x on t h e production o f t u r b u l e n t k i n e t i c energy t o p r e d i c t where f r o n t a l regions separate zones o f v e r t i c a l t i d a l m i x i n g from zones where summer s t r a t i f i c a t i o n exists.
These f r o n t a l regions are o f
213 p a r t i c u l a r importance as they are thought t o increase t h e b i o l o g i c a l product i v i t y as a r e s u l t o f t h e upward n u t r i e n t f l u x ( G r i f f i t h s e t al.,
1981).
The r e s u l t s show t h a t c e n t r a l Hudson Bay has no t i d a l f r o n t s as i t s t i d a l currents are t o o weak.
The o n l y well-mixed zones r e s u l t i n g from strong t i d a l
currents occur i n t h e shallow regions o f James Bay and along t h e southeast coast o f Hudson Bay.
Small-scale f r o n t s a l s o occur near headlands and i n
channels where l o c a l l y increased t i d a l c u r r e n t s cause v e r t i c a l l y mixed conditions i n areas t h a t would otherwise be w e l l s t r a t i f i e d . o f G r i f f i t h s e t al.
However, t h e model
(1981) was based on North Sea experience, and exclude
wind stress as a source f o r mixing and r u n o f f as a source f o r t h e buoyancy flux; thus it i s o n l y a p p l i c a b l e t o o f f s h o r e regions o f Hudson Bay.
Wind
mixing and r u n o f f are important c o n t r i b u t o r s t o t h e pycnocl i n e development and thus s t r a t i f i c a t i o n o f Hudson and James bays (Prinsenberg, 1983). Wind mixing may e s p e c i a l l y become important i n exposed areas such as t h e Belcher Islands where much b i o l o g i c a l a c t i v i t y occurs. Runoff, on t h e other hand, increases t h e s t a b i l i t y parameter i n protected areas such as i n southern James Bay where s t r a t i f i e d c o n d i t i o n s do occur i n t h e summer. Freshwater plumes are observed o f f r i v e r mouths i n Hudson and James bays, both i n the summer and t h e winter.
The plumes remain coherent up t o 70 km
from t h e i r source and reach widths o f 20 t o 30 km.
The existence o f an i c e
cover i n t h e w i n t e r i n h i b i t s wind-induced mixing and allows t h e plumes t o spread out f u r t h e r than under t h e i c e - f r e e c o n d i t i o n s o f t h e summer.
Even
though the r u n o f f r a t e s are an order o f magnitude l e s s i n w i n t e r than i n t h e suinner, the w i n t e r plumes a t t a i n t w i c e t h e surface area o f t h e summer plumes, p r i n c i p a l l y due t o t h e i n s u l a t i o n from wind-induced turbulence by t h e i c e cover (Freeman, 1982).
During t h e ice-covered period t i d a l d i s s i p a t i o n i s
the main source o f plume mixing. One o f t h e plunes, o f f t h e La Grande River, was e x t e n s i v e l y s t u d i e d as i t s r i v e r discharge r a t e was a1 t e r e d by h y d r o e l e c t r i c development (Freeman e t a l . , 1982 and Messier e t al., 1986). The buoyant La Grande River p l m e spreads out under t h e i c e l a t e r a l l y over about 20 km, and i s p u l l e d northward by the mean c i r c u l a t i o n t o a distance o f 40 t o 50 km. (Fig. 11.5 from Freeman, 1982).
It varies i n thickness from 2 t o 5 m and acquires s a l t as i t
moves away from t h e r i v e r mouth. Maximum t i d a l v e l o c i t i e s w i t h i n t h e plume are less than 10 cm s - l , which t r a n s l a t e s i n t o a h o r i z o n t a l plume motion o f only 1.5 km (Fig. 11.5).
The l a r g e v e l o c i t y a t t h e r i v e r mouth (43 cm 5 - l )
drops t o 12 cm s-l i n t h e f u l l y s t r a t i f i e d region some 3.5 km downstream and i n t h e f r o n t a l region some 40 kin downstream. I n t h e f u l l y s t r a -
t o 5 cm s'l
t i f i e d region t h e r e i s l i t t l e exchange w i t h t h e bottom layer.
The freshwater
spreads out due t o buoyancy and advection over t h e slow moving bottom layer. Farther downstream, i n t h e f r o n t a l region, t h e plume undergoes t h e maximum
214
54:
30
54 10
54
oc
FULLY STRATIFIED REGION
F i g u r e 11.5. Surface s a l i n i t y contours f o r LaGrande R i v e r plume during March 1-9, 1980 (from Freeman, 1982). I n s e r t s show 25-hr averaged p r o f i l e s o f c u r r e n t s , d e n s i t y and temperature s t r u c t u r e f o r t h r e e regions whose l o c a t i o n s along t h e plume's a x i s a r e shown by t h e l a r g e arrows.
215 r a t e o f s a l t exchange.
I n t h e middle region, t h e t i d a l k i n e t i c energy below
the plume i s much l a r g e r than i n t h e plume i t s e l f (Fig. 11.5),
which shows
how the S t r a t i f i c a t i o n i s o l a t e s t h e t i d a l current (and mixing) from t h e plume even though t h e plume moves up and down w i t h t h e t i d e .
Seaward o f t h e
f r o n t a l region, a well-mixed region e x i s t s where t h e t i d a l k i n e t i c energy i n and below t h e plume increases. V e r t i c a l mixing increases, breaking down t h e stratification.
I n t h i s area t h e r e i s evidence o f a bottom r e t u r n f l o w as
found i n estuarine c i r c u l a t i o n (Fig. 11.5).
I n t h e well-mixed region a l a r g e increase i n dissolved n u t r i e n t s near t h e i c e occurs and an i c e b i o t a comnuni t y i s observed which i s not observed inshore.
Since t h e areal extent o f
surface plumes seems t o be s t r o n g l y dependent on t h e discharge r a t e s as found by numerical plume model 1i n g (Freeman, 1982), t h e increased LaGrande River discharge w i l l l i m i t n u t r i e n t supply t o t h e surface l a y e r over a l a r g e r area and s i m i l a r e f f e c t s should be expected i n plumes whose r u n o f f r a t e s are modif i e d by h y d r o e l e c t r i c developments. CONCLUSION Tides i n Hudson Bay are c l a s s i f i e d as semidiurnal and o n l y a small r e g i o n along the eastern shore i s c l a s s i f i e d as mixed but s t i l l mainly semidiurnal. The semidiurnal t i d a l amplitude ranges from 1.25 m along t h e western shore t o 0.10 m along t h e eastern shore. values o f 8 cm.
The d i u r n a l t i d a l amplitude only reaches
The maximum t i d a l currents are observed a t t h e entrance t o
Hudson Bay where they reach v e l o c i t i e s o f 90 cm s-l; smaller maximum values
of 30 cm 5-l occur w i t h i n Hudson Bay.
During t h e winter, t h e i c e Cover
dampens the t i d a l c u r r e n t s and heights and advances t h e i r a r r i v a l times. Tidal and wind-generated c u r r e n t s determine t h e l o c a t i o n where strong mixing occurs and where t h e strong s t a b i l i t y o f freshwater plumes breaks down. Further oceanographic research should c o l l e c t o f f s h o r e t i d a l height and current meter data a t t h e entrances t o Hudson Bay and James Bay i n order t o resolve phase questions r a i s e d by t h e present data and provide b e t t e r boundary conditions f o r f u t u r e t i d a l modelling.
The modelling research should
address not only t h e e f f e c t t h e i c e cover has on t h e t i d e but a l s o t h e d i s crepancy now seen i n t h e o f f s h o r e data r e l a t i v e t o model r e s u l t s . Year-round offshore data w i t h i n Hudson and James Bays are required i n order t o o b t a i n r e l i a b l e amplitudes and phases o f o f f s h o r e t i d a l heights and currents. ACKNOWLEDGEMENTS The authors thank Drs. G. Ingram, D. Wright and D. Greenberg f o r t h e i r helpful comments on t h e manuscript, and t h e s t a f f o f the B a y f i e l d Laboratory, D.F.O.,
i n Burlington, Ontario f o r t h e i r e f f o r t s i n c o l l e c t i n g t h e data dur-
ing the f i e l d programs and processing t h a t are used i n t h i s manuscript.
216 Mr. R.E.
Walker and Mr. A.D.
graphy, i n Dartmouth, N.S.,
Cosgrove o f t h e Bedford I n s t i t u t e o f Oceanoa s s i s t e d w i t h t h e c a r t o g r a p h i c work and Dr. P.
M a r t i n i w i t h t h e e d i t o r a l work. REFERENCES Dohler, G.C., 1966. Tides i n Canadian Waters. Dept. Mines Tech. Surv., Can. Hydrogr. Serv., Mar. Sci. Branch, Rep. M54-966: 17 pp. Dohler, G.C., 1967. Tides and t i d a l c u r r e n t s i n Hudson Bay. I n : C.S. Beals ( E d i t o r ) , Science and H i s t o r y o f Hudson Bay, Dept. Energy, Mines and Resources, Ottawa, pp. 824-837. Freeman, N.G., 1982. Measurements and m o d e l l i n g o f freshwater plumes under an i c e cover. Ocean Science and Surveys, Dept. o f F i s h e r i e s and Oceans, B u r l i n g t o n , Manus. Rept. Ser. No. 14, 155 pp. Freeman, N.G. and Murty, T.S., 1976. Numerical m o d e l l i n g o f t i d e s i n Hudson Bay. J. Fish. Res. Bd. Canada, 33: 2345-2361. Roff, J.C. and P e t t , R.J., 1982. Physical, chemical, and Freeman, N.G., b i o l o g i c a l f e a t u r e s o f r i v e r plumes under an i c e cover i n James and Hudson Bays. Le N a t u r a l i s t e canadien, 109(4): 745-764. G r i f f i t h s , D.K., Pingree, R.D. and S i n c l a i r , M., 1981. Summer t i d a l f r o n t s i n t h e n e a r - a r c t i c regions o f Foxe Basin and Hudson Bay. Deep-sea Res. 28A: 865-876. Godin, G., 1972. The t i d e s o f James Bay. Dept. Environ., Mar. Sci. Branch, Tech. Rep. 24: 97-142. Godin, G., 1974. The t i d e i n e a s t e r n and western James Bay, A r c t i c 27: 104-110. Godin, G. and Barber, F.G., 1980. V a r i a b i l i t y o f t h e t i d e a t some s i t e s i n t h e Canadian A r c t i c . A r c t i c 33: 30-37. Henry, R.F. and Foreman, M.G.G., 1977. Numerical model s t u d i e s o f semid i u r n a l t i d e s i n t h e southern Beaufort Sea. P a c i f i c Marine Sc. Rept. 77-11. I n s t . o f Ocean Sciences, V i c t o r i a , B.C. 71 pp. Manning, T.H., 1950. Notes on t h e t i d e s along t h e south Hudson Bay and west James Bay coast. A r c t i c 3: 95-100. Manning, T.H., 1951. Remarks on t h e t i d e s and d r i f t w o o d s t r a n d l i n e s along t h e east coast o f James Bay. A r c t i c 4: 122-130. Messier, D., Ingram, R.G., D. and Savard, J.P., 1986. Physical and b i o l o g i c a l oceanographic m o d i f i c a t i o n s i n response t o La Grande H y d r o e l e c t r i c Complex. I n : I.P. M a r i t i n i ( E d i t o r ) , Canadian I n l a n d Seas, E l s e v i e r , Amsterdam. Prinsenberg, S.J., 1983. E f f e c t s o f t h e h y d r o e l e c t r i c developments on t h e oceanographic s u r f a c e waters o f Hudson Bay. Atmosphere-Ocean, 21: 418430. Pr nsenberg, S.J., 1986a. C i r c u l a t i o n p a t t e r n and c u r r e n t s t r u c t u r e o f Hudson Bay. In: I.P. M a r t i n i ( E d i t o r ) , Canadian I n l a n d Seas. E l s e v i e r , Amsterdam. P r nsenberg, S.J., 1986b. E f f e c t s o f t h e annual i c e cover on t h e t i d a l c u r r e n t s and freshwater content o f Canadian i n l a n d seas. I n : Canadian East Coast Workshop on Sea Ice, January 7-9, 1986. Can. Tech. Rept. o f Fish. and Aquat. Sci., Dept. o f F i s h e r i e s and Oceans, Dartmouth, N.S. Pr nsenberg, S.J. and Deys, F.W., 1979. Hudson Bay oceanographic data r e p o r t 1975, volume 2. Ocean and Aquatic Sc., Dept. o f Fish. and Oceans, B u r l i n g t o n , Data Rep. Ser. No. 82-1, 255 pp. P r nsenberg, S.J. and Fleming, B.M., 1982. Hudson Bay/James Bay oceanographic survey 1976 Data Report, Volume 1. Ocean Sci. and Surv., Dept. o f Fish. and Oceans, B u r l i n g t o n , Data Rep. Ser. No=-1, 255 p. P r nsenberg, S.J. and Weaver, R.K., 1983. Hudson Bay oceanographic data r e p o r t 1982. Ocean Sci. and Surv., Dept. o f Fish. and Oceans, B u r l i n g t o n , Data Rep. Ser. No. 83-5, 221 pp.