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Circulation modelling
Pro~ Oceu.r,¢ ', )I !4, pp 3;2--2"2- I';'~" P n n t ~ J ' n Or~a : B n : a m aAI ,'__'h', r':,cr'.cd
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S U M M E R U P W E L L I N G ON T H E S O U T H E A S T E R N C O N T I N E N T A L S H E L F
OF THE U.S.A. DURING 1981
Circulation Modelling Jo.;,o LORE'4ZZETTi , JoH:', D.W.-~XG
a n d THo'-l-\s N. LEE
".thnt~terto da Ciencta e Tecnologia. lnstimro de Pe~quzsa~ Espactats (I,VPE~ .4 v, dos Astronauta~. 1"758 Catxu Po~tal 515-I220I. SJo Y~s# dos CarnpoY, SP Brazd "'Ro~ensnel School of ),h,rtne and Atmospheric Sc'tence. (_'ntterstO' of 3,1iamt. 46¢)0 Rickenh,acker Causeway. Mramt. FL 3314t), U.S.A.
ABSTRACT Summer upwelling on the continental shelf north of Cape Canaveral, Florida, has been previously observed to r e s u l t from wind forcing. reasonably well and magnitude. forcing
the c h a r a c t e r i s t i c s
of
A two-layer, f i n i t e element model reproduces
the wind-driven upwelling in respect to
location
Model i n v e s t i g a t i o n also shows that upwelling results from offshore current
which is
imposed through an along-shelf sea level slope.
This sea level
slope,
which has been found to be of the order of -10 -7 , represents a mean Gulf Stream e f f e c t . The results suggest that the strongest upwelling events near Cape Canaveral occur when the wind and Gulf Stream forcings act together.
CONTENTS I.
Introduction
2.
Model Description
314
2.1
Lower Layer
316
2.2
Upper Layer
317
3.
4.
314
Model Experiments
320
3.1
Wind driven upwelling
321
3.2
Upwelling from alongshore pressure gradient forcing
324
Conclusions
325
Acknowledgements
326
References
327
313
.~{J.
J. LOP.E'.,ZZET'Ti~.*[~[
I.
INTRODUCTION
Past studies indicate that frequent upwelling c e l l s are present in the continental
with
negative
temperature
shelf region north of Cape Canaveral, Florida, during the
summer season, when the c l i m a t o l o g i c a l wind stress f i e l d tends to be northward and p a r a l l e l to the coastline. (1979)
anomalies oriented
For example, GREEN (1944), TAYLOR and STEWART (1959), and LENING
suggest that northward winds appear to drive a localized upwelling in the v i c i n i t y
of Cape Canaveral during the summer.
BLANTON, ATKINSON, PIETRAFESq and LEE (1981) show
that diverging isobaths north of Cape Canaveral w i l l
cause upwelling during northward flow
episodes and may account f o r the observed l o c a l i z a t i o n of upwelling near the C1pe. and STEWART (1959)
show monthly mean sea surface
TAYLOR
temperature plots obtaine~ from a data
set from 1925 to 1954, in which a d i s t i n c t i v e negative anomaly is observed during the summer months at Daytona Beach and Canova Beach, close to Cape Canaveral. relatively
LEM[NG (1979) documents
large temperature decreases at the 20m isobath north of Cape Canaveral f o r
summer of 1970, and f o r the coastal area around the Cape f or July ]971. clusion was that,
the
His overall con-
although the r e l a t i o n between upwelling favorable winds and temperature
f l u c t u a t i o n s had not been very strong, major temperature depressions occurred during up~elling favorable wind stress,
i n d i c a t i n g that the wind should be an important f a c t o r determining
the upwelling there.
The same author mentions, however,
that there were cases when the
upwelling occurred and yet no upwelling favorable winds were observed.
These cases were
normally associated with onshore displacements of the Flor ida Current over the shelf, suggesting that the summer upwelling might be produced also by intrusions of that current. results
and conclusions are found in SMITH (1981).
Similar
ATKINSON, LEE, BLANTON and PAFFENHOFER
(1988) also show that a l o c a l i z e d upwel]ing occurred in this area when a major cross-shelf intrusion of the Gulf Stream occurred in the lower layer, upwelling favorable winds and the occurrence of a f r o n t a l
due to the combined action of eddy in the o u t e r - s h e l f ,
during
periods when the Gulf Stream was in an onshore pos it io n . Motivated by these studies,
a numerical modelling experiment was implementeJ to quantify
the dynamics governing the upwelling in this region and to investigate a l t e r n a t i v e mechanisms f o r i t s generation. The data set used in this investigation is from the GABEX-II experiment (Georgia Bight Experi ment/Summer 1981), an inter-disciplinary study with the long range objective of investigating the physical, chemical and biological processes in the shelf region between Cape Canaveral and Cape Fear and adjacent Gulf Stream.
The map of the study area with positions where
data were collected is shown in figure I.
2.
MODELDESCRIPTION
The numerical model used in this study derives from a two-layer, f i n i t e element formulation by WANG and CONNOR (1975). data collected
in
It
is w e l l - s u i t e d f o r the study area because the hydrographic
the region during the summer months show that a mid-water thermocline
C~rcuiation modelhn#
5 [5
Ca~ 3~01 f
I
•
BLM moorings
/
Myrtte
\ '\
~ cureent meter mOor,rigs "t-22- Doe "23-25 8LM • NO60 meteorolog~ca~ ouoy • coastal meteOtolocjJcal station coastal sea revel - - - ttClal cmanne~s Cape Rornatn
213°
-i
I /
..-~;
C harlestom,
/ ..... j
25
o~
32°
J
31°
30°
eB
eC
/ /-
2<30 NewSmyrna~l~ 1 L
6each
x'~r:.
Canaveral
J: ..?.
"!'
p
28c
~.
. . . . .
81°
FIG.I.
PO ~ 9 : 3 / 4 - G
I
80 °
79 °
78 °
77 °
Map of the r e g i o n of i n t e r e s t , showing p o s i t i o n s where d a t a were c o l l e c t e d f o r the GABEX-II experiment (from June 1 to October 15, 1981).
76°w
J. LORE>,ZZET'T,I et J~"
3 1#>
(pycnocline) is well developed, and separates *:n_ = water column into two d i s t i n c t layers (ATKINSON, LEE, BLANTON and CHANDLER, 1983; and ATK[NSON, LEE, BLANTON and PAFFENHOFER,
The f l u i d system is assumed to consist of two rotating layers with constant and s l i g h t l y different densities, stably s t r a t i f i e d and separated by a material interface.
Using a right-
handed coordinate system with z-direction upward and with subscript I (2) referring to lower (upper) layer, dependent variables are defined as transports: f.
qix = Z ~i-I
vdz
udz, and qiy -
i:I,2
(I)
~i-I
and layer thicknesses:
Hi = ~l - SO' and where u and v are f l u i d ~l(X,y,t)
: interface
H2 =
velocities
elevation;
~2 - ~I in x and y d i r e c t i o n s ;
and ~ 2 ( x , y , t )
Model equations are derived by v e r t i c a l over each layer to obtain the f o l l o w i n g 2.1
(2) Co(X,y)
= bottom e l e v a t i o n ;
= surface e l e v a t i o n .
integration
of the momentum and c o n t i n u i t y
equations
governing equations:
Lower Layer
continuity
(3)
Hl,t + qlx,x + qly,y = 0
x momentum
qlx,t
- f qly = -(F1p - F l x x ) , x + F1yx,y
(4) + I/~ 1
{~lx -
~Ox +
Pl r-'l,x - PO ~O,x t
y momentum qly, t + f qlx = - ( F l p - F l y y ) , y + F l x y , x
(5) + I/~I
{%ly -
toy +
Pl 51,y - PO 5 O,y
Circulauon rnod¢lhng
2.2
3'"
Zpp~r C~ger
continuity
H2,t + q2x,x + q2y,y x
(6)
0
momentum
q2x,t - fq2y = -(F2p - F2xx),x + F2yx,y
(7) + I/P2
{~2x -
%Ix - Pl ~ l , x
y momentum
q2y,t + fq2x =-(F2p - F2yy),y + F2xy,x
(s) + I/P2 A partial
derivative
{Z2y -
Zly - Pl ¢ l , y }
is indicated by a s u b s c r i p t comma followed by the independent v a r i a b l e .
The nonlinear advective terms are neglected because scale analysis indicated they are small in comparison to other that an f - p l a n e
terms.
Horizontal
approximation can be used.
scales of motion are assumed to be small enough The integrated pressure terms can be expressed
as:
_
Fip
1 Pi
f~i ¢i-I
Pidz = I 2
gH~ + I__ Pi piHi
with Pi = layer d e n s i t y ; g = g r a v i t a t i o n a l
i=1,2
acceleration;
(9)
and Pi'
the pressure
at
z
= ~i
given by PO = P2gH2 + PlgHI = pressure at bottom, = pressure at i n t e r f a c e ,
Pl = P2gH2 P2 = O
:
I n t e r n a l stresses due to f l u i d F. = : ~ ixx 5i-I
~
dz, xx
and
pressure at surface, assumed equal to zero. eddy v i s c o s i t y are integrated as: F. : lyy
: i Ci-I
~
dz YY
(~o)
5[, ~
J. L O R E ' , * Z Z E T T I
et" ~l
r7.
F. E F = .r l , Ixy lyx ~i-I
~ixx'
~y,
stress,
~
xy
Tiy x are the t u r b u l e n t
~I is the i n t e r f a c e stress,
dz
(11)
stresses
acting on v e r t i c a l
planes.
t 0 is the
bottom
and #2 is the surface (wind) stress.
Bottom and i n t e r f a c e stresses are parameterized, as
ox
= Pl CB (~2 + F 2 ) I / 2
~Ix: and s i m i l a r l y city, ient.
°I
C1 [ ( ~ I
~2 )2
+
(#I
f o r the y-components.
CB is the bottom f r i c t i o n
-
#2 ) 2 ] I / 2
(u 2
-
u I)
(12)
The overbar v a r i a b l e s represent the average layer v e l o -
coefficient,
and CI is the i n t e r f a c i a l
shear stress c o e f f i c -
The wind stress is parameterized in the usual way as
#2 : P a i r CO where
-
Ul
Pair is the a i r
I~I01~I0 density,
(13) UIO is the wind v e l o c i t y ,
normally measured at a height of
lOm above the mean sea l e v e l , and CD is the wind stress c o e f f i c i e n t , CD
=
(1.1
+
0.0536
IJlo l) x lO -3,
lUlo I
given by
in m s -1
(14)
as proposed by WANG and CONNOR (1975). The i n t e g r a t e d i n t e r n a l stresses Fixx, Fiyy , and Fixy are parameterized as:
• - m qik + -3x Flxkx m = Ekm( -~x k qim )'
i = 1,2
(15)
where x I = x and x 2 = y and qik is the t r a n s p o r t l a y e r i in the d i r e c t i o n k. ontal eddy v i s c o s i t y tensor. The system of equations is solved n u m e r i c a l l y using a f i n i t e t r i a n g u l a r elements f o r
the s p a t i a l
The f i n i t e
element f o r m u l a t i o n with l i n e a r
d e r i v a t i v e s and a " s p l i t - t i m e "
to advance the s o l u t i o n to the next time step. can be found
in WANG and CONNOR (1975)
Ekm is a h o r i z -
finite
d i f f e r e n c e scheme
A d e t a i l e d d e s c r i p t i o n of these proedures
and LORENZZETTI, WANG, LEE and PIETRAFESA (1986).
element g r i d used in the numerical experiments is shown in f i g u r e 2.
from F l o r i d a to
Model parameters used are: layer density
I t extends
North C a r o l i n a and from the coast up to the s h e l f break at the 75m isobath. time step = 300s, C o r i o l i s
parameter f = O.753xlO-4s - I ,
OI = I025.5kg m-3, upper l a y e r d e n s i t y P2 = I023.5kg m-3,
thickness H2 = 17m, bottom f r i c t i o n i e n t CI : 0 . 4 x i 0 -3.
coefficient
CB = 2 . 3 x i 0 -3,
initial
lower
upper l a y e r
and i n t e r f a c e stress c o e f f i c -
Cir~ulanon modelhn~
3 lg
Wilmington • 123/ 110 SPONGE LAYER Chadeston 92.
Savannah
68i
I
Jacksonvdle •
lOOkm
so~'-.p-j{--,Id~ Daytona Beach,33 ~
C. Canaver~3~tSPONGELAYER
FIG.2.
F i n i t e element grid f o r SAB. Number of nodes = 144 and number of elements : 231. Some of the nodes discussed in the t e x t are indicated by t h e i r nodal numbers.
Because the same model has been used to study the summer c i r c u l a t i o n of the e n t i r e shelf region,
LORENZZETT[, WANG, LEE and PIETRAFESA (1986),
the model grid covers a much larger
area than our present region of i n t e r e s t : the neighborhood of Cape Canaveral. The model bottom topography is shown in f i g u r e 3. 17m below the surface,
so that
it
intersects
The observed thermocline depth is about
the bottom about lOkm from the coastline.
In the model there is, therefore, only one layer in the row of elements closest to the coast. The normal flow is specified equal to zero at the coastal boundary and an adiabatic boundary condition
(fixed
allow f o r
some wave energy r a d i a t i o n ,
layer depths)
is used at the along-shelf oceanic boundary. two sponge layers were implemented
In order to
at the north and
south cross shelf boundaries of the domain according to the methodology explained in LORENZETTI and WANG (1986).
-:-~'}
]
LORE'~ZZET-rI rr ~II
C. Fear
Canaveral
FIG. 3.
Bottom topography used in Contours are in meters.
3.
the numerical experiments.
MODELEXPERIMENTS
The described two-layer model and domain d i s c r e t i z a t i o n have been tested successfully against a number of d i f f e r e n t
conditions including tides,
winter wind-driven c i r c u l a t i o n and mean
summer wind forcing (WANG, KOURAFALOUand LEE, 1984; KOURFALOU, WANG and LEE, 1984; LORENZETTI WANG, LEE and PIETRAFESA, 1986).
The primary objective here is to model the upwelling c e l l s
near Cape Canaveral during real
wind events observed at the coastal
stations of Daytona
Beach, Jacksonville, Savannah, Charleston, Wilmington, and at the NOAA meteorological buoys located at the shelf break (Fig. l ) . dary f r i c t i o n
Wind speed measured at the coast was adjusted f o r boun-
effects by m u l t i p l y i n g by a f a c t o r of 2 as suggested by KOURAFALOU, WANG and
LEE (1984) and SCHWING and BLANTON (1984).
The objective analysis scheme described in ALLEN-
DER (1977) was used to i n t e r p o l a t e the wind stress calculated at the f i v e coastal stations and two offshore buoys to each of the model grid points.
Circulation modelhng
321
3.1 The period chosen f o r modelling spans g days from june 23 1981, 0600 hours (EST), to July 2 1981, 0000 hours
(EST).
This i n t e r v a l
was chosen because i t
in the wind stress f i e l d which starts northward, s h i f t s back to north at the end of the experiment.
It
contains a marked change
toward the south, and then returns
also covers part of the period when' the
major subsurface intrusion of the summer was flooding the shelf with cold Gulf Stream water (intrusion
#I, ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988).
been more desirable to
start
In retrospect i t would have
the model at the beginning of the intrusion,
however, when
the model was run, the significance of the i n tr u s ion was not f u l l y comprehended. Preliminary testing
with
v a r i a b l e winds
showed that
boundary conditions caused short period internal intersect
the bottom or
surface,
resulting
i n c o m p a t i b i l i t y between i n i t i a l
waves to form causing the
in the breakdown of the model.
and
interface to This problem
was overcome by using a horizontal eddy v i s c o s i t y (Exx, Eyy), with an i s o t r o p i c c o e f f i c i e n t equal to 5xlO3m2s- I . The interface elevation f i e l d Each p l o t
is shown in f i g u r e 4 f or the period from June 26 to June 29.
is separated by 12 hours.
An analysis of the wind f i e l d ,
i l l u s t r a t e d in f i g u r e
5, shows that on June 25 the wind was toward the north in a p o s i t i v e alongshore d i r e c t i o n everywhere in the domain.
A strong pulse of p o s i t i v e alongshore wind stress occurred in
the northern part of the domain inducing a set down of sea level at the coast of 15cm which -I then propagated southward at a speed of about 500cm s In f i g u r e 4 i t is seen that, associated with
the northward wind event, some closed patches of p o s i t i v e in t e r f a c e anomalies
(upwellings) developed in the southern region from Cape Canaveral to about Daytona Beach on dune 27.
The strongest upwelling c e l l
occurred just
north of Cape Canaveral and had
upward v e r t i c a l v e l o c i t i e s in the i n t e r fa c e of about +7xlO-3cm s- I .
At 1800 hours on June 26,
strong southward
wind developed in the northern region and propagated southward again at
about 500cm s - l ,
accompanied by a coastal
set up of approximately 15cm.
The set up was
followed by the development of a long patch of negative int er f ac e displacement (downwell{ng) p a r a l l e l to the coastline and extending from inner to mid-shelf.
The upwelling c e l l s that
previously developed north of Cape Canaveral decayed, and in t h e i r
place the development
of a very strong patch of downwelling is observed with a magnitude of several meters. These numerical results indicate that in response to the upwelling favorable winds a moderate cooling should occur in the l o c a l i z e d area north of Cape Canaveral from June 26-28, followed by a stronger warming a f t e r June 28 as downwelling winds occur. series collected at moorings 2, 3 and 4 (see F i g . l ) pattern as described above.
Bottom temperature time
and shown in f i g u r e 6 indicate the same
Bottom temperature maps from the moored array and hydrographic
data (see Figs. 6 and 7 of ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988) indicate that the modelled upwelling was part of
a major subsurface int r us io n of deeper, cold Gulf Stream
water that began on June I0 with upwelling favorable winds and continued u n t i l June 28 when the winds reversed, causing a current reversal that advected the in t r u s ion southward and o f f the shelf by July 7.
3]]
J LOR~',,ZZETFI V~' ~./,
6/28
FIG.4.
06:00
6/28
18:00
6/29
Interface e l e v a ti o n plots f o r the wind event experiment. Solid lines represent p o s i t i v e and dashed lines negative values. Contours are in l-meter i n t e r v a l s . The numbers below each p l o t represent the date and local time.
06:00
CLrculatlon mod~lhn~
6/25
323
6J26
18:00
18:00
Jf
6/27 FIG. 5.
ff
18:00
6/28
18:00
Time series plots of the interpolated wind stress f i e l d used in the wind event experiment.
20
I
i
i
I
i
l
i
I
2 --
19
~
a 18
~0
16
13
4
24
25
26
27
JUNE
28
29 30 119811
1
2
JULY
FIG.6. Time series of bottom temperatures at moorings 2, 3 and 4 during the upwelling/downwelling event.
";~ '
J, LORE%ZZETTI e'r ~/
i : has been argued (LEMING, 1979; SMITH, 1981; ATKINSON, LE~, BLANTON and PIETRAFESA, 1988; LE~ and PlETRAFESA, 1988) that fo r observed
the summer season,
:~e most intense upwelling events
in the inner and mid-shelf region north of Ca~e Canaveral are associated with a
combined resoonse to both upwelling favorable winds and co!~ intrusions from the Gulf Stream. The model could not forecast the formation of Gulf Stream intrusions. indicate that
the strength
However, model results
and location of upwelling/do~nwelling events can at
p a r t l y determined by the Ekman response to the local wind f i e l d
least be
in ~ r e a l i s t i c topographic
se~ting including Cape Canaveral.
3.2
It
:'~uel~ing ~ro~ alongshore pressure grad¢en~ ~orc~<2 has been suggested
through numerical modelling, data analysis,
and a n a l y t i c a l studies
that effects of the deep ocean flow (Gulf Stream) on the shelf c i r c u l a t i o n can be parameterized through the imposition of an along-shelf pressure gradient at the shelf break (SEMTNER and MINTZ, 1977; BEARDSLEY and WINANT, 1979; CSANADY, 1978). 1974) and momentum balance analysis made f o r
Gulf Stream l e v e l l i n g (STURGES,
our region of
i n t e r e s t using current meter
and hydrographic data indicates that sea level slopes toward the north at the shelf break with a gradient on the order of -10 -7 during a l l KOURAFALOU and WANG, 1984).
Using t h i s
seasons (LEE and PIETRAFESA, 1988; LEE,
information,
an along-shelf sea level slope
of
- l . 6 x l O -7 was applied at the alongshore open boundary of the model domain to study the shelf response to this forcing.
The wind f i e l d
of alongshore slope forcing only.
was set to zero so as to not confuse the results
The model was integrated f o r three days, at which time
a steady state solution was reached. Figure 7 shows the free surface and i n t e r f a c e elevation contours and upper and lower layer v e l o c i t y f i e l d s at the end of the experiment.
Sea level dropped about 6cm from shelf break
to coast at the widest part of the domain o f f Savannah (120km wide), generating a crossshore sea level gradient of 5xlO -7, shore current of 7cm s- I . to increasing water depths.
which is in geostrophic balance with an average along-
Observe that the currents increase towards the shelf break
due
The free surface elev a t io n p l o t shows that the negative along-
shelf pressure gradient is confined over the outer shelf region where the bottom slope steepens (see Fig.3).
A small, p o s i t i v e ,
along-shelf sea level slope is observed at the coast
in contrast to the conclusion of CSANADY (1978),
who found that for
the case of a shelf
with a constant bottom slope the along-shelf pressure gradient imposed at the shelf break would extend over the e n t i r e coastal region without change.
The same type of topographic
trapping of the along-shelf pressure gradient over the slope was also obtained a n a l y t i c a l l y by WANG (1982). An i n t e r e s t i n g feature of the model solution is observed in the int er f ac e elevation p l o t . Similar to the northward impulsive wind case, Canaveral in this
figure.
An average v e r t i c a l
an in t e r f a c e rise of 5m in three days.
an upwelling c e l l
is evident north of Cape
v e l o c i t y of +2x10-3cm s- I
is indicated by
The generation of upwelling north of Cape Canaveral
by forcing the model with only a negative along-shelf sea level slope imposed at the shelf break confirms f i e l d
observations that northward winds are not the only mechanism capable
Circulation rnodelhn~
325
n Contour
Int
=
1 0 cm
C o n t o u r int.
•]ST
~mls
FIG.7.
I 0 m
~_] ~ cm/s
Free surface and interface elevation contours (upper panels) and upper and lower layer velocity plots (lower panels) generated by an along-shelf pressure gradient of -l.6xlO " / , imposed at the shelf break.
of inducing the Cape Canaveral summer upwelling. break, or,
:
A negative sea level slope at the shelf
equivalently, an along-shelf northward flowing current,
increasing toward the
shelf break is capable of producing a similar, localized, upwelling response, with vertical velocities of the same order of magnitude as for wind driven upwelling.
4.
CONCLUSIONS
The results of the numerical modelling experiments permit us to make some conclusions regarding the summer upwelling in the Cape Canaveral region. barotropic and f i r s t
A two-layer model, with only the
baroclinic vertical modes, contains the essential dynamics for two
localized upwelling mechanisms. The wind forced experiments showed that the model predicted the upwelling f i e l d in reasonably good agreement with what has been observed. The model was capable of reproducing a transient
3In
J. LO~E~ZZETFIer ~.'
real upwelling/downwelling event localized in tne region just north of Cape Canaveral with upwelling/down~elling velocites ranging from about 10-2 to 10-3cm s-I. The modelled current and sea level response to transient local wind forcing appears to occur as an arrested topographic wave (CSANADY, 1978) with southward phase speeds of about 500cms-I matching that of the wind forcing.
The model response is in good agreement with the observ-
ations (LEE and PIETRAFESA, 1988; ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988) and is consistent with the 3rd mode EOF described by HAMILTON (1988) as due to wind forcing. A second experiment, using only alongshore pressure gradient forcing
at
the shelf
edge,
which is analogous to a northward current imposed at the shelf break, also generated upwelling in the v i c i n i t y of Cape Canaveral. by HSUEH and O'BRIEN (]971).
The dynamical explanation f o r this case was expressed
Northward flowing current on the shelf is capable of generating
upwelling due to a loss of geostrophic balance in the bottom boundary layer. layer the alongshore current C o r i o l i s force and produces
is
slowed by bottom f r i c t i o n ,
an onshore acceleration of the f l u i d
to the cross-shore pressure gradient.
Within this
which reduces the cross-shore in the bottom layer due
Continuity then requires an offshore transport
in
the upper layer, r e s u l t i n g in a c i r c u l a t i o n pattern very s i m i l a r to that found in a northward wind upwelling case. The numerical results,
therefore,
indicate that the Cape Canaveral summer upwelling events
can r e s u l t from the separate or j o i n t effects of northward wind forcing and boundary forcing imposed by the alongshore sea level slope of the Gulf Stream.
An onshore position of the
Gulf Stream should r e s u l t in an increased forcing at the outer boundary in the form of increased northward flow and steeper (more negative) alongshore surface slopes, which should produce stronger upwelling at Cape Canaveral. intrusions
with greatest cross-shelf
This may explain why the strongest subsurface
penetrations occurred at times when the Gulf Stream
was in an onshore position (ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988; LEE and PIETRAFESA, 1988).
In a d i t i o n , ATKINSONet a~. (]988) show that major upwelling events, that have t h e i r
roots in the Gulf Stream and extend into the inner shelf as a subsurface intrusion, occur when the above upwelling mechanisms are f u r t h e r
enhanced by the simultaneous occurrence
of upwel]ing in the outer shelf from Gulf Stream f r o n t a l eddies. The explanation f or just
the observed and modelled l o c a l i z a t i o n of the upwelling in the region
north of Cape Canaveral appears to be due to the topography associated with the Cape
r e s u l t i n g in diverging isobaths and upwelling as predicted by BLANTON, ATKINSON, PIETRAFESA and LEE (1981). ACKNOWLEDGEMENTS The authors are grateful for the technical assistance provided by Elizabeth Williams who processed some of the current meter data and Lila Ptak, Gay Ingram and Jere Green who helped in typing the several drafts. data from mooring 2.
We thank L. Pietrafesa for use of the bottom temperature
Support for this study was provided by the Department of Energy
~-
Circulation modelling
(Co~zract OE-ASOB-76EV05163) and Minerals Management Service (Prime Contract AAT851-CT1-25 to Science Applications I n t e r n a t i o n a l Corporation of Raleigh, N.C., Subcontract 11-82029-II). The h. Pietrafesa data were obtained with support from the Department of Energy (Contract DOE-AS09-74-O0902).
REFERENCES ALLENOER, H.A. (1977)
Comparison of model and observed currents in Lake Michigan. J~-~a; o f
Physica~ Oceanography, 7. 711-718. ATKINSON, L.P., T.N. LEE, J.O. BLANTON and W.S. CHANDLER (1983) Climatology of the Southeastern United States Continental shelf waters. Journal of Geophysical Research, 88(C8), 4705-4718. ATKINSON, L.P., T.N. LEE, J.O. BLANTON, G.A. PAFFENHOFER (1988) Summer Upwelling on the Southeaster Continental Shelf of the USA during 1981: Hydrographic Observations. Progress t~ Oceanography, 19, 231-266. BEARDSLEY, R.C. and C.D. Winant (1979) On the mean c i r c u l a t i o n in the Mid-Atlantic Bight Journa~ of Physical Oceanography, 9, 612-619. BLANTON, J.O., L.P. ATKINSON, L.J. PIETRAFESA and T.N. LEE (1981) The intrusion of Gulf Stream water across the Continental Shelf due to topographically-induced upwelling Deep-Sea Research, 28A, 393-405. CSANADY, G.T. (1978) The Arrested Topographic Wave. Journal of Physical Oceanography, 8, 47-62. GREEN, C.K. (1944) Summer Upwelling, Northeast Coast of Florida. Science, 1 0 0 , 516-547. HAMILTON, P. (1988) SummerUpwelling on the Southeastern Continental Shelf of the USA during 1981: The structure of shelf and Gulf Stream motions in the Georgia Bight. Progress in Oceanography, 19, 239-351. HSUEH, Y. and J.J. O'BRIEN (1971) Steady Coastal Upwelling Induced by an Along-shore Current Journal of Physical Oceanography, I, 180-186. KOURAFALOU, V., J.D. WANG and T.N. LEE (1984) C i r c u l a t i o n on the Continental Shelf of the Southeastern United States. Part 111: Modelling the Winter Wind-Driven Flow. Journa~ of Physical Oceanography, 14, 1022-1031. LEE, T.N., W.J. HO, V. KOURAFALOU and J.D. WANG (1984) C i r c u l a t i o n on the Continental Shelf Part I: subtidal response to wind and Gulf Stream forcing. Journal of Physical Oceanography, 14, 1001-1012. LEE, T.N. and L.J. PIETRAFESA (1988) Summer Upwelling on the Southeastern Continental Shelf of the USA during 1981: C i r c u l a t i o n Progress in Oceanography, 19, 267-312. LEMING, T.O. (1979) Observations of Temperature, Current, and Wind Variations of the Central Eastern Coast of F l o r i d a during 1980 and 1971. NOAA Technical Memora~u~.. ?;aFS-S~'C-6 172pp. LORENZZETTI, J.A. and J.D. WANG (1986) On the use of Wave Absorbing Layers in the Treatment of open boundaries in Numerical Coastal C ir c u la t io n Models. Applied Ma:hematical Wodelling, 10, 339-345. LORENZZETTI, J.A., J.D. WANG, T.N. LEE and L. PIETRAFESA (1986) On the Modelling of the Summer C i r c u l a t i o n in the South A t l a n t i c Bight by a Two-Layer F i n i t e Element
Model. Uniuersity of M~ami, Technical Report, U>! RSMAS No.
85002, 200pp.
SCHWING, F.B. and J.O. BLANTON (1984) The use of land and sea based wind data in a simple c i r c u l a t i o n model. Journal of Physical Oeeanographw, 14, 193-197. SEMTNER, A.J. and Y. MINTZ (1977) Numerical Simulation of the Gulf Stream and Mid-ocean Eddies. Journal of Physical Oceanography, 7, 20B-230. SMITH, N.P. (1981) An investigation of Seasonal Upwelling along the Atlantic Coast of Florida In: Eoohydrodgn~ics, J.C.J. NIHOUL, editor, 79-98. STURGES, W. (1974) Sea level slopes along continental boundaries. Journal of Geophysica~
Research,
79, 825-830.
TAYLOR, C.B. and H.B. STEWART, Jr.
(1959) Summer Upwelling along the East Coast of Florida. 64, 33-40. WANG, D-P. (1982) Effects of Continental Slope on the Mean Shelf Circulation. Journal of Physical Oceanography, 12, 1524-1526. WANG, J.D. and J.J. CONNOR (1975) Mathematical Modelling of Near Coastal C i r c u l a t i o n . RM Parsons Laboratory Technical Report. 200, MIT WANG, J.D., V. KOURAFALOU and T.N. LEE (1984) C i r c u l a t i o n on the southeast US Continental Shelf, Part 2 - Model development and application to tidal flow. Journalof Physical Oceanography, 14, 1013-1021.
Journal of Geophysical Research,
,I,".L,~' ~.~ ¢~0 'NI -- 51 L,,p~rLzht ~ [,~xX ? : - z a m , , n P r¢ . . pie
S U M M E R U P W E L L I N G ON T H E S O U T H E A S T E R N C O N T I N E N T A L S H E L F
OF THE U.S.A. DURING 1981
Circulation Modelling Jo.;,o LORE'4ZZETTi , JoH:', D.W.-~XG
a n d THo'-l-\s N. LEE
".thnt~terto da Ciencta e Tecnologia. lnstimro de Pe~quzsa~ Espactats (I,VPE~ .4 v, dos Astronauta~. 1"758 Catxu Po~tal 515-I220I. SJo Y~s# dos CarnpoY, SP Brazd "'Ro~ensnel School of ),h,rtne and Atmospheric Sc'tence. (_'ntterstO' of 3,1iamt. 46¢)0 Rickenh,acker Causeway. Mramt. FL 3314t), U.S.A.
ABSTRACT Summer upwelling on the continental shelf north of Cape Canaveral, Florida, has been previously observed to r e s u l t from wind forcing. reasonably well and magnitude. forcing
the c h a r a c t e r i s t i c s
of
A two-layer, f i n i t e element model reproduces
the wind-driven upwelling in respect to
location
Model i n v e s t i g a t i o n also shows that upwelling results from offshore current
which is
imposed through an along-shelf sea level slope.
This sea level
slope,
which has been found to be of the order of -10 -7 , represents a mean Gulf Stream e f f e c t . The results suggest that the strongest upwelling events near Cape Canaveral occur when the wind and Gulf Stream forcings act together.
CONTENTS I.
Introduction
2.
Model Description
314
2.1
Lower Layer
316
2.2
Upper Layer
317
3.
4.
314
Model Experiments
320
3.1
Wind driven upwelling
321
3.2
Upwelling from alongshore pressure gradient forcing
324
Conclusions
325
Acknowledgements
326
References
327
313
.~{J.
J. LOP.E'.,ZZET'Ti~.*[~[
I.
INTRODUCTION
Past studies indicate that frequent upwelling c e l l s are present in the continental
with
negative
temperature
shelf region north of Cape Canaveral, Florida, during the
summer season, when the c l i m a t o l o g i c a l wind stress f i e l d tends to be northward and p a r a l l e l to the coastline. (1979)
anomalies oriented
For example, GREEN (1944), TAYLOR and STEWART (1959), and LENING
suggest that northward winds appear to drive a localized upwelling in the v i c i n i t y
of Cape Canaveral during the summer.
BLANTON, ATKINSON, PIETRAFESq and LEE (1981) show
that diverging isobaths north of Cape Canaveral w i l l
cause upwelling during northward flow
episodes and may account f o r the observed l o c a l i z a t i o n of upwelling near the C1pe. and STEWART (1959)
show monthly mean sea surface
TAYLOR
temperature plots obtaine~ from a data
set from 1925 to 1954, in which a d i s t i n c t i v e negative anomaly is observed during the summer months at Daytona Beach and Canova Beach, close to Cape Canaveral. relatively
LEM[NG (1979) documents
large temperature decreases at the 20m isobath north of Cape Canaveral f o r
summer of 1970, and f o r the coastal area around the Cape f or July ]971. clusion was that,
the
His overall con-
although the r e l a t i o n between upwelling favorable winds and temperature
f l u c t u a t i o n s had not been very strong, major temperature depressions occurred during up~elling favorable wind stress,
i n d i c a t i n g that the wind should be an important f a c t o r determining
the upwelling there.
The same author mentions, however,
that there were cases when the
upwelling occurred and yet no upwelling favorable winds were observed.
These cases were
normally associated with onshore displacements of the Flor ida Current over the shelf, suggesting that the summer upwelling might be produced also by intrusions of that current. results
and conclusions are found in SMITH (1981).
Similar
ATKINSON, LEE, BLANTON and PAFFENHOFER
(1988) also show that a l o c a l i z e d upwel]ing occurred in this area when a major cross-shelf intrusion of the Gulf Stream occurred in the lower layer, upwelling favorable winds and the occurrence of a f r o n t a l
due to the combined action of eddy in the o u t e r - s h e l f ,
during
periods when the Gulf Stream was in an onshore pos it io n . Motivated by these studies,
a numerical modelling experiment was implementeJ to quantify
the dynamics governing the upwelling in this region and to investigate a l t e r n a t i v e mechanisms f o r i t s generation. The data set used in this investigation is from the GABEX-II experiment (Georgia Bight Experi ment/Summer 1981), an inter-disciplinary study with the long range objective of investigating the physical, chemical and biological processes in the shelf region between Cape Canaveral and Cape Fear and adjacent Gulf Stream.
The map of the study area with positions where
data were collected is shown in figure I.
2.
MODELDESCRIPTION
The numerical model used in this study derives from a two-layer, f i n i t e element formulation by WANG and CONNOR (1975). data collected
in
It
is w e l l - s u i t e d f o r the study area because the hydrographic
the region during the summer months show that a mid-water thermocline
C~rcuiation modelhn#
5 [5
Ca~ 3~01 f
I
•
BLM moorings
/
Myrtte
\ '\
~ cureent meter mOor,rigs "t-22- Doe "23-25 8LM • NO60 meteorolog~ca~ ouoy • coastal meteOtolocjJcal station coastal sea revel - - - ttClal cmanne~s Cape Rornatn
213°
-i
I /
..-~;
C harlestom,
/ ..... j
25
o~
32°
J
31°
30°
eB
eC
/ /-
2<30 NewSmyrna~l~ 1 L
6each
x'~r:.
Canaveral
J: ..?.
"!'
p
28c
~.
. . . . .
81°
FIG.I.
PO ~ 9 : 3 / 4 - G
I
80 °
79 °
78 °
77 °
Map of the r e g i o n of i n t e r e s t , showing p o s i t i o n s where d a t a were c o l l e c t e d f o r the GABEX-II experiment (from June 1 to October 15, 1981).
76°w
J. LORE>,ZZET'T,I et J~"
3 1#>
(pycnocline) is well developed, and separates *:n_ = water column into two d i s t i n c t layers (ATKINSON, LEE, BLANTON and CHANDLER, 1983; and ATK[NSON, LEE, BLANTON and PAFFENHOFER,
The f l u i d system is assumed to consist of two rotating layers with constant and s l i g h t l y different densities, stably s t r a t i f i e d and separated by a material interface.
Using a right-
handed coordinate system with z-direction upward and with subscript I (2) referring to lower (upper) layer, dependent variables are defined as transports: f.
qix = Z ~i-I
vdz
udz, and qiy -
i:I,2
(I)
~i-I
and layer thicknesses:
Hi = ~l - SO' and where u and v are f l u i d ~l(X,y,t)
: interface
H2 =
velocities
elevation;
~2 - ~I in x and y d i r e c t i o n s ;
and ~ 2 ( x , y , t )
Model equations are derived by v e r t i c a l over each layer to obtain the f o l l o w i n g 2.1
(2) Co(X,y)
= bottom e l e v a t i o n ;
= surface e l e v a t i o n .
integration
of the momentum and c o n t i n u i t y
equations
governing equations:
Lower Layer
continuity
(3)
Hl,t + qlx,x + qly,y = 0
x momentum
qlx,t
- f qly = -(F1p - F l x x ) , x + F1yx,y
(4) + I/~ 1
{~lx -
~Ox +
Pl r-'l,x - PO ~O,x t
y momentum qly, t + f qlx = - ( F l p - F l y y ) , y + F l x y , x
(5) + I/~I
{%ly -
toy +
Pl 51,y - PO 5 O,y
Circulauon rnod¢lhng
2.2
3'"
Zpp~r C~ger
continuity
H2,t + q2x,x + q2y,y x
(6)
0
momentum
q2x,t - fq2y = -(F2p - F2xx),x + F2yx,y
(7) + I/P2
{~2x -
%Ix - Pl ~ l , x
y momentum
q2y,t + fq2x =-(F2p - F2yy),y + F2xy,x
(s) + I/P2 A partial
derivative
{Z2y -
Zly - Pl ¢ l , y }
is indicated by a s u b s c r i p t comma followed by the independent v a r i a b l e .
The nonlinear advective terms are neglected because scale analysis indicated they are small in comparison to other that an f - p l a n e
terms.
Horizontal
approximation can be used.
scales of motion are assumed to be small enough The integrated pressure terms can be expressed
as:
_
Fip
1 Pi
f~i ¢i-I
Pidz = I 2
gH~ + I__ Pi piHi
with Pi = layer d e n s i t y ; g = g r a v i t a t i o n a l
i=1,2
acceleration;
(9)
and Pi'
the pressure
at
z
= ~i
given by PO = P2gH2 + PlgHI = pressure at bottom, = pressure at i n t e r f a c e ,
Pl = P2gH2 P2 = O
:
I n t e r n a l stresses due to f l u i d F. = : ~ ixx 5i-I
~
dz, xx
and
pressure at surface, assumed equal to zero. eddy v i s c o s i t y are integrated as: F. : lyy
: i Ci-I
~
dz YY
(~o)
5[, ~
J. L O R E ' , * Z Z E T T I
et" ~l
r7.
F. E F = .r l , Ixy lyx ~i-I
~ixx'
~y,
stress,
~
xy
Tiy x are the t u r b u l e n t
~I is the i n t e r f a c e stress,
dz
(11)
stresses
acting on v e r t i c a l
planes.
t 0 is the
bottom
and #2 is the surface (wind) stress.
Bottom and i n t e r f a c e stresses are parameterized, as
ox
= Pl CB (~2 + F 2 ) I / 2
~Ix: and s i m i l a r l y city, ient.
°I
C1 [ ( ~ I
~2 )2
+
(#I
f o r the y-components.
CB is the bottom f r i c t i o n
-
#2 ) 2 ] I / 2
(u 2
-
u I)
(12)
The overbar v a r i a b l e s represent the average layer v e l o -
coefficient,
and CI is the i n t e r f a c i a l
shear stress c o e f f i c -
The wind stress is parameterized in the usual way as
#2 : P a i r CO where
-
Ul
Pair is the a i r
I~I01~I0 density,
(13) UIO is the wind v e l o c i t y ,
normally measured at a height of
lOm above the mean sea l e v e l , and CD is the wind stress c o e f f i c i e n t , CD
=
(1.1
+
0.0536
IJlo l) x lO -3,
lUlo I
given by
in m s -1
(14)
as proposed by WANG and CONNOR (1975). The i n t e g r a t e d i n t e r n a l stresses Fixx, Fiyy , and Fixy are parameterized as:
• - m qik + -3x Flxkx m = Ekm( -~x k qim )'
i = 1,2
(15)
where x I = x and x 2 = y and qik is the t r a n s p o r t l a y e r i in the d i r e c t i o n k. ontal eddy v i s c o s i t y tensor. The system of equations is solved n u m e r i c a l l y using a f i n i t e t r i a n g u l a r elements f o r
the s p a t i a l
The f i n i t e
element f o r m u l a t i o n with l i n e a r
d e r i v a t i v e s and a " s p l i t - t i m e "
to advance the s o l u t i o n to the next time step. can be found
in WANG and CONNOR (1975)
Ekm is a h o r i z -
finite
d i f f e r e n c e scheme
A d e t a i l e d d e s c r i p t i o n of these proedures
and LORENZZETTI, WANG, LEE and PIETRAFESA (1986).
element g r i d used in the numerical experiments is shown in f i g u r e 2.
from F l o r i d a to
Model parameters used are: layer density
I t extends
North C a r o l i n a and from the coast up to the s h e l f break at the 75m isobath. time step = 300s, C o r i o l i s
parameter f = O.753xlO-4s - I ,
OI = I025.5kg m-3, upper l a y e r d e n s i t y P2 = I023.5kg m-3,
thickness H2 = 17m, bottom f r i c t i o n i e n t CI : 0 . 4 x i 0 -3.
coefficient
CB = 2 . 3 x i 0 -3,
initial
lower
upper l a y e r
and i n t e r f a c e stress c o e f f i c -
Cir~ulanon modelhn~
3 lg
Wilmington • 123/ 110 SPONGE LAYER Chadeston 92.
Savannah
68i
I
Jacksonvdle •
lOOkm
so~'-.p-j{--,Id~ Daytona Beach,33 ~
C. Canaver~3~tSPONGELAYER
FIG.2.
F i n i t e element grid f o r SAB. Number of nodes = 144 and number of elements : 231. Some of the nodes discussed in the t e x t are indicated by t h e i r nodal numbers.
Because the same model has been used to study the summer c i r c u l a t i o n of the e n t i r e shelf region,
LORENZZETT[, WANG, LEE and PIETRAFESA (1986),
the model grid covers a much larger
area than our present region of i n t e r e s t : the neighborhood of Cape Canaveral. The model bottom topography is shown in f i g u r e 3. 17m below the surface,
so that
it
intersects
The observed thermocline depth is about
the bottom about lOkm from the coastline.
In the model there is, therefore, only one layer in the row of elements closest to the coast. The normal flow is specified equal to zero at the coastal boundary and an adiabatic boundary condition
(fixed
allow f o r
some wave energy r a d i a t i o n ,
layer depths)
is used at the along-shelf oceanic boundary. two sponge layers were implemented
In order to
at the north and
south cross shelf boundaries of the domain according to the methodology explained in LORENZETTI and WANG (1986).
-:-~'}
]
LORE'~ZZET-rI rr ~II
C. Fear
Canaveral
FIG. 3.
Bottom topography used in Contours are in meters.
3.
the numerical experiments.
MODELEXPERIMENTS
The described two-layer model and domain d i s c r e t i z a t i o n have been tested successfully against a number of d i f f e r e n t
conditions including tides,
winter wind-driven c i r c u l a t i o n and mean
summer wind forcing (WANG, KOURAFALOUand LEE, 1984; KOURFALOU, WANG and LEE, 1984; LORENZETTI WANG, LEE and PIETRAFESA, 1986).
The primary objective here is to model the upwelling c e l l s
near Cape Canaveral during real
wind events observed at the coastal
stations of Daytona
Beach, Jacksonville, Savannah, Charleston, Wilmington, and at the NOAA meteorological buoys located at the shelf break (Fig. l ) . dary f r i c t i o n
Wind speed measured at the coast was adjusted f o r boun-
effects by m u l t i p l y i n g by a f a c t o r of 2 as suggested by KOURAFALOU, WANG and
LEE (1984) and SCHWING and BLANTON (1984).
The objective analysis scheme described in ALLEN-
DER (1977) was used to i n t e r p o l a t e the wind stress calculated at the f i v e coastal stations and two offshore buoys to each of the model grid points.
Circulation modelhng
321
3.1 The period chosen f o r modelling spans g days from june 23 1981, 0600 hours (EST), to July 2 1981, 0000 hours
(EST).
This i n t e r v a l
was chosen because i t
in the wind stress f i e l d which starts northward, s h i f t s back to north at the end of the experiment.
It
contains a marked change
toward the south, and then returns
also covers part of the period when' the
major subsurface intrusion of the summer was flooding the shelf with cold Gulf Stream water (intrusion
#I, ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988).
been more desirable to
start
In retrospect i t would have
the model at the beginning of the intrusion,
however, when
the model was run, the significance of the i n tr u s ion was not f u l l y comprehended. Preliminary testing
with
v a r i a b l e winds
showed that
boundary conditions caused short period internal intersect
the bottom or
surface,
resulting
i n c o m p a t i b i l i t y between i n i t i a l
waves to form causing the
in the breakdown of the model.
and
interface to This problem
was overcome by using a horizontal eddy v i s c o s i t y (Exx, Eyy), with an i s o t r o p i c c o e f f i c i e n t equal to 5xlO3m2s- I . The interface elevation f i e l d Each p l o t
is shown in f i g u r e 4 f or the period from June 26 to June 29.
is separated by 12 hours.
An analysis of the wind f i e l d ,
i l l u s t r a t e d in f i g u r e
5, shows that on June 25 the wind was toward the north in a p o s i t i v e alongshore d i r e c t i o n everywhere in the domain.
A strong pulse of p o s i t i v e alongshore wind stress occurred in
the northern part of the domain inducing a set down of sea level at the coast of 15cm which -I then propagated southward at a speed of about 500cm s In f i g u r e 4 i t is seen that, associated with
the northward wind event, some closed patches of p o s i t i v e in t e r f a c e anomalies
(upwellings) developed in the southern region from Cape Canaveral to about Daytona Beach on dune 27.
The strongest upwelling c e l l
occurred just
north of Cape Canaveral and had
upward v e r t i c a l v e l o c i t i e s in the i n t e r fa c e of about +7xlO-3cm s- I .
At 1800 hours on June 26,
strong southward
wind developed in the northern region and propagated southward again at
about 500cm s - l ,
accompanied by a coastal
set up of approximately 15cm.
The set up was
followed by the development of a long patch of negative int er f ac e displacement (downwell{ng) p a r a l l e l to the coastline and extending from inner to mid-shelf.
The upwelling c e l l s that
previously developed north of Cape Canaveral decayed, and in t h e i r
place the development
of a very strong patch of downwelling is observed with a magnitude of several meters. These numerical results indicate that in response to the upwelling favorable winds a moderate cooling should occur in the l o c a l i z e d area north of Cape Canaveral from June 26-28, followed by a stronger warming a f t e r June 28 as downwelling winds occur. series collected at moorings 2, 3 and 4 (see F i g . l ) pattern as described above.
Bottom temperature time
and shown in f i g u r e 6 indicate the same
Bottom temperature maps from the moored array and hydrographic
data (see Figs. 6 and 7 of ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988) indicate that the modelled upwelling was part of
a major subsurface int r us io n of deeper, cold Gulf Stream
water that began on June I0 with upwelling favorable winds and continued u n t i l June 28 when the winds reversed, causing a current reversal that advected the in t r u s ion southward and o f f the shelf by July 7.
3]]
J LOR~',,ZZETFI V~' ~./,
6/28
FIG.4.
06:00
6/28
18:00
6/29
Interface e l e v a ti o n plots f o r the wind event experiment. Solid lines represent p o s i t i v e and dashed lines negative values. Contours are in l-meter i n t e r v a l s . The numbers below each p l o t represent the date and local time.
06:00
CLrculatlon mod~lhn~
6/25
323
6J26
18:00
18:00
Jf
6/27 FIG. 5.
ff
18:00
6/28
18:00
Time series plots of the interpolated wind stress f i e l d used in the wind event experiment.
20
I
i
i
I
i
l
i
I
2 --
19
~
a 18
~0
16
13
4
24
25
26
27
JUNE
28
29 30 119811
1
2
JULY
FIG.6. Time series of bottom temperatures at moorings 2, 3 and 4 during the upwelling/downwelling event.
";~ '
J, LORE%ZZETTI e'r ~/
i : has been argued (LEMING, 1979; SMITH, 1981; ATKINSON, LE~, BLANTON and PIETRAFESA, 1988; LE~ and PlETRAFESA, 1988) that fo r observed
the summer season,
:~e most intense upwelling events
in the inner and mid-shelf region north of Ca~e Canaveral are associated with a
combined resoonse to both upwelling favorable winds and co!~ intrusions from the Gulf Stream. The model could not forecast the formation of Gulf Stream intrusions. indicate that
the strength
However, model results
and location of upwelling/do~nwelling events can at
p a r t l y determined by the Ekman response to the local wind f i e l d
least be
in ~ r e a l i s t i c topographic
se~ting including Cape Canaveral.
3.2
It
:'~uel~ing ~ro~ alongshore pressure grad¢en~ ~orc~<2 has been suggested
through numerical modelling, data analysis,
and a n a l y t i c a l studies
that effects of the deep ocean flow (Gulf Stream) on the shelf c i r c u l a t i o n can be parameterized through the imposition of an along-shelf pressure gradient at the shelf break (SEMTNER and MINTZ, 1977; BEARDSLEY and WINANT, 1979; CSANADY, 1978). 1974) and momentum balance analysis made f o r
Gulf Stream l e v e l l i n g (STURGES,
our region of
i n t e r e s t using current meter
and hydrographic data indicates that sea level slopes toward the north at the shelf break with a gradient on the order of -10 -7 during a l l KOURAFALOU and WANG, 1984).
Using t h i s
seasons (LEE and PIETRAFESA, 1988; LEE,
information,
an along-shelf sea level slope
of
- l . 6 x l O -7 was applied at the alongshore open boundary of the model domain to study the shelf response to this forcing.
The wind f i e l d
of alongshore slope forcing only.
was set to zero so as to not confuse the results
The model was integrated f o r three days, at which time
a steady state solution was reached. Figure 7 shows the free surface and i n t e r f a c e elevation contours and upper and lower layer v e l o c i t y f i e l d s at the end of the experiment.
Sea level dropped about 6cm from shelf break
to coast at the widest part of the domain o f f Savannah (120km wide), generating a crossshore sea level gradient of 5xlO -7, shore current of 7cm s- I . to increasing water depths.
which is in geostrophic balance with an average along-
Observe that the currents increase towards the shelf break
due
The free surface elev a t io n p l o t shows that the negative along-
shelf pressure gradient is confined over the outer shelf region where the bottom slope steepens (see Fig.3).
A small, p o s i t i v e ,
along-shelf sea level slope is observed at the coast
in contrast to the conclusion of CSANADY (1978),
who found that for
the case of a shelf
with a constant bottom slope the along-shelf pressure gradient imposed at the shelf break would extend over the e n t i r e coastal region without change.
The same type of topographic
trapping of the along-shelf pressure gradient over the slope was also obtained a n a l y t i c a l l y by WANG (1982). An i n t e r e s t i n g feature of the model solution is observed in the int er f ac e elevation p l o t . Similar to the northward impulsive wind case, Canaveral in this
figure.
An average v e r t i c a l
an in t e r f a c e rise of 5m in three days.
an upwelling c e l l
is evident north of Cape
v e l o c i t y of +2x10-3cm s- I
is indicated by
The generation of upwelling north of Cape Canaveral
by forcing the model with only a negative along-shelf sea level slope imposed at the shelf break confirms f i e l d
observations that northward winds are not the only mechanism capable
Circulation rnodelhn~
325
n Contour
Int
=
1 0 cm
C o n t o u r int.
•]ST
~mls
FIG.7.
I 0 m
~_] ~ cm/s
Free surface and interface elevation contours (upper panels) and upper and lower layer velocity plots (lower panels) generated by an along-shelf pressure gradient of -l.6xlO " / , imposed at the shelf break.
of inducing the Cape Canaveral summer upwelling. break, or,
:
A negative sea level slope at the shelf
equivalently, an along-shelf northward flowing current,
increasing toward the
shelf break is capable of producing a similar, localized, upwelling response, with vertical velocities of the same order of magnitude as for wind driven upwelling.
4.
CONCLUSIONS
The results of the numerical modelling experiments permit us to make some conclusions regarding the summer upwelling in the Cape Canaveral region. barotropic and f i r s t
A two-layer model, with only the
baroclinic vertical modes, contains the essential dynamics for two
localized upwelling mechanisms. The wind forced experiments showed that the model predicted the upwelling f i e l d in reasonably good agreement with what has been observed. The model was capable of reproducing a transient
3In
J. LO~E~ZZETFIer ~.'
real upwelling/downwelling event localized in tne region just north of Cape Canaveral with upwelling/down~elling velocites ranging from about 10-2 to 10-3cm s-I. The modelled current and sea level response to transient local wind forcing appears to occur as an arrested topographic wave (CSANADY, 1978) with southward phase speeds of about 500cms-I matching that of the wind forcing.
The model response is in good agreement with the observ-
ations (LEE and PIETRAFESA, 1988; ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988) and is consistent with the 3rd mode EOF described by HAMILTON (1988) as due to wind forcing. A second experiment, using only alongshore pressure gradient forcing
at
the shelf
edge,
which is analogous to a northward current imposed at the shelf break, also generated upwelling in the v i c i n i t y of Cape Canaveral. by HSUEH and O'BRIEN (]971).
The dynamical explanation f o r this case was expressed
Northward flowing current on the shelf is capable of generating
upwelling due to a loss of geostrophic balance in the bottom boundary layer. layer the alongshore current C o r i o l i s force and produces
is
slowed by bottom f r i c t i o n ,
an onshore acceleration of the f l u i d
to the cross-shore pressure gradient.
Within this
which reduces the cross-shore in the bottom layer due
Continuity then requires an offshore transport
in
the upper layer, r e s u l t i n g in a c i r c u l a t i o n pattern very s i m i l a r to that found in a northward wind upwelling case. The numerical results,
therefore,
indicate that the Cape Canaveral summer upwelling events
can r e s u l t from the separate or j o i n t effects of northward wind forcing and boundary forcing imposed by the alongshore sea level slope of the Gulf Stream.
An onshore position of the
Gulf Stream should r e s u l t in an increased forcing at the outer boundary in the form of increased northward flow and steeper (more negative) alongshore surface slopes, which should produce stronger upwelling at Cape Canaveral. intrusions
with greatest cross-shelf
This may explain why the strongest subsurface
penetrations occurred at times when the Gulf Stream
was in an onshore position (ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988; LEE and PIETRAFESA, 1988).
In a d i t i o n , ATKINSONet a~. (]988) show that major upwelling events, that have t h e i r
roots in the Gulf Stream and extend into the inner shelf as a subsurface intrusion, occur when the above upwelling mechanisms are f u r t h e r
enhanced by the simultaneous occurrence
of upwel]ing in the outer shelf from Gulf Stream f r o n t a l eddies. The explanation f or just
the observed and modelled l o c a l i z a t i o n of the upwelling in the region
north of Cape Canaveral appears to be due to the topography associated with the Cape
r e s u l t i n g in diverging isobaths and upwelling as predicted by BLANTON, ATKINSON, PIETRAFESA and LEE (1981). ACKNOWLEDGEMENTS The authors are grateful for the technical assistance provided by Elizabeth Williams who processed some of the current meter data and Lila Ptak, Gay Ingram and Jere Green who helped in typing the several drafts. data from mooring 2.
We thank L. Pietrafesa for use of the bottom temperature
Support for this study was provided by the Department of Energy
~-
Circulation modelling
(Co~zract OE-ASOB-76EV05163) and Minerals Management Service (Prime Contract AAT851-CT1-25 to Science Applications I n t e r n a t i o n a l Corporation of Raleigh, N.C., Subcontract 11-82029-II). The h. Pietrafesa data were obtained with support from the Department of Energy (Contract DOE-AS09-74-O0902).
REFERENCES ALLENOER, H.A. (1977)
Comparison of model and observed currents in Lake Michigan. J~-~a; o f
Physica~ Oceanography, 7. 711-718. ATKINSON, L.P., T.N. LEE, J.O. BLANTON and W.S. CHANDLER (1983) Climatology of the Southeastern United States Continental shelf waters. Journal of Geophysical Research, 88(C8), 4705-4718. ATKINSON, L.P., T.N. LEE, J.O. BLANTON, G.A. PAFFENHOFER (1988) Summer Upwelling on the Southeaster Continental Shelf of the USA during 1981: Hydrographic Observations. Progress t~ Oceanography, 19, 231-266. BEARDSLEY, R.C. and C.D. Winant (1979) On the mean c i r c u l a t i o n in the Mid-Atlantic Bight Journa~ of Physical Oceanography, 9, 612-619. BLANTON, J.O., L.P. ATKINSON, L.J. PIETRAFESA and T.N. LEE (1981) The intrusion of Gulf Stream water across the Continental Shelf due to topographically-induced upwelling Deep-Sea Research, 28A, 393-405. CSANADY, G.T. (1978) The Arrested Topographic Wave. Journal of Physical Oceanography, 8, 47-62. GREEN, C.K. (1944) Summer Upwelling, Northeast Coast of Florida. Science, 1 0 0 , 516-547. HAMILTON, P. (1988) SummerUpwelling on the Southeastern Continental Shelf of the USA during 1981: The structure of shelf and Gulf Stream motions in the Georgia Bight. Progress in Oceanography, 19, 239-351. HSUEH, Y. and J.J. O'BRIEN (1971) Steady Coastal Upwelling Induced by an Along-shore Current Journal of Physical Oceanography, I, 180-186. KOURAFALOU, V., J.D. WANG and T.N. LEE (1984) C i r c u l a t i o n on the Continental Shelf of the Southeastern United States. Part 111: Modelling the Winter Wind-Driven Flow. Journa~ of Physical Oceanography, 14, 1022-1031. LEE, T.N., W.J. HO, V. KOURAFALOU and J.D. WANG (1984) C i r c u l a t i o n on the Continental Shelf Part I: subtidal response to wind and Gulf Stream forcing. Journal of Physical Oceanography, 14, 1001-1012. LEE, T.N. and L.J. PIETRAFESA (1988) Summer Upwelling on the Southeastern Continental Shelf of the USA during 1981: C i r c u l a t i o n Progress in Oceanography, 19, 267-312. LEMING, T.O. (1979) Observations of Temperature, Current, and Wind Variations of the Central Eastern Coast of F l o r i d a during 1980 and 1971. NOAA Technical Memora~u~.. ?;aFS-S~'C-6 172pp. LORENZZETTI, J.A. and J.D. WANG (1986) On the use of Wave Absorbing Layers in the Treatment of open boundaries in Numerical Coastal C ir c u la t io n Models. Applied Ma:hematical Wodelling, 10, 339-345. LORENZZETTI, J.A., J.D. WANG, T.N. LEE and L. PIETRAFESA (1986) On the Modelling of the Summer C i r c u l a t i o n in the South A t l a n t i c Bight by a Two-Layer F i n i t e Element
Model. Uniuersity of M~ami, Technical Report, U>! RSMAS No.
85002, 200pp.
SCHWING, F.B. and J.O. BLANTON (1984) The use of land and sea based wind data in a simple c i r c u l a t i o n model. Journal of Physical Oeeanographw, 14, 193-197. SEMTNER, A.J. and Y. MINTZ (1977) Numerical Simulation of the Gulf Stream and Mid-ocean Eddies. Journal of Physical Oceanography, 7, 20B-230. SMITH, N.P. (1981) An investigation of Seasonal Upwelling along the Atlantic Coast of Florida In: Eoohydrodgn~ics, J.C.J. NIHOUL, editor, 79-98. STURGES, W. (1974) Sea level slopes along continental boundaries. Journal of Geophysica~
Research,
79, 825-830.
TAYLOR, C.B. and H.B. STEWART, Jr.
(1959) Summer Upwelling along the East Coast of Florida. 64, 33-40. WANG, D-P. (1982) Effects of Continental Slope on the Mean Shelf Circulation. Journal of Physical Oceanography, 12, 1524-1526. WANG, J.D. and J.J. CONNOR (1975) Mathematical Modelling of Near Coastal C i r c u l a t i o n . RM Parsons Laboratory Technical Report. 200, MIT WANG, J.D., V. KOURAFALOU and T.N. LEE (1984) C i r c u l a t i o n on the southeast US Continental Shelf, Part 2 - Model development and application to tidal flow. Journalof Physical Oceanography, 14, 1013-1021.
Journal of Geophysical Research,