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The Variability of the Temperature, Salinity and Density Fields in the Upper Layers of the Baltic Sea
489
THE VARIABILITY OF THE TEMPERATURE, S A L I N I T Y AND DENSITY FIELDS I N THE UPPER LAYERS OF THE BALTIC SEA A. Aitsam,
J. Pavelson
.'
I n s t i t u t e o f Thermophysics and E l e c t r o p h y s i c s , Academy o f Sciences o f t h e Estonian S.S.R. INTRODUCTION I n r e c e n t years, t h e use o f a towed CTD has proved successful i n d e t e r mining t h e s t r u c t u r e o f t h e temperature;' s a l i n i t y and d e n s i t y f i e l d s .
The
s p a t i a l d i s t r i b u t i o n s o f f i e l d s obtained by t h i s method i n a comparatively s h o r t t i m e have an e s s e n t i a l l y h i g h e r r e s o l u t i o n than those obtained from observations a t d i s c r e t e p o i n t s .
Depending on t h e aims o f t h e i n v e s t i g a -
t i o n , t h r e e towing regimes a r e p o s s i b l e : 1. 2.
t h e CTD moves a t a f i x e d depth (Gargett, 1978), t h e CTO performs wavelike motion between two l e v e l s ( A l l e n e t a l . 1980),
3.
t h e CTD moves a l o n g a f i x e d isotherm o r isopycnal (Katz,
1973,
1975). I n most o f t h e s t u d i e s j u s t mentioned,
t h e s p a t i a l s t r u c t u r e o f the
temperature and s a l i n i t y f i e l d s was obtained w i t h o u t s e p a r a t i n g t h e "background" o f i n t e r n a l waves.
However, when s t u d y i n g t h e v a r i a b i l i t y o f these
v a r i a b l e s , t h e r e l a t i v e p a r t due t o i n t e r n a l waves should be determined. This i s i m p o r t a n t f o r a c o r r e c t e v a l u a t i o n o f t h e c h a r a c t e r i s t i c s o f t h e non-wave
perturbations.
As a f i r s t approximation,
an isopycnal a n a l y s i s
m i g h t be used as i n r e c e n t s t u d i e s by Woods and M i n n e t t (1979), and Cairns (1980).
Despite i t s shortcomings,
t h i s method i s t h e b e s t one a t present.
The aim of t h e p r e s e n t study i s t o determine t h e main c h a r a c t e r i s t i c s o f t h e temperature, s a l i n i t y and d e n s i t y f i e l d s on h o r i z o n t a l scales l a r g e r than 1 km i n t h e upper l a y e r s o f t h e open p a r t o f t h e B a l t i c Sea.
F i r s t , we
s h a l l describe t h e experiments and t h e processing o f t h e data.
Then, the
most t y p i c a l r e s u l t s w i l l be presented and discussed.
F i n a l l y , some hypothe-
ses about t h e o r i g i n o f t h e observed s t r u c t u r e w i l l be formulated.
EXPERIMENTS AND DATA PROCESSING
All
the
experimental
data were obtained u s i n g t h e towed measuring
device c o n s t r u c t e d a t t h e I n s t i t u t e o f Thermophysics and E l e c t r o p h y s i c s o f t h e Academy o f Sciences o f t h e Estonian S.S.R. 1981).
(Pavelson and Portsmuth,
The i n s t r u m e n t c o n s i s t s o f an underwater u n i t , t h e "FISH",
onboard system ( F i g u r e 1).
The " F I S H "
and an
i s equipped w i t h a CTD MARK I11
490
2
MINI -
INTERFACE
z
PL 0 T TER .''
. COMPUTER H P 9825A
HP 9862A
i
r - - ---
-- 1
I
I
CABLE
%
CT D
b
DECK UNIT
TAPE RECORDER KENNEDY 9832
1
U N D E R WATER UNIT
'FISH'
F i g u r e 1.
Block diagram o f t h e towed measuring device.
( N B I S ) and guided by means o f small wings a c t i v a t e d by a m i n i a t u r e e l e c t r i c motor.
The data are t r a n s m i t t e d t o t h e onboard t e r m i n a l and s t o r e d on a
tape r e c o r d e r KENNEDY-9832 w i t h t h e h e l p o f s p e c i a l i n t e r f a c e s . o f t h e "FISH"
The c o n t r o l
motion and t h e p r e l i m i n a r y p l o t t i n g o f temperature s e c t i o n s
a r e performed by an HP-9825-A computer. For t h e experiments r e p o r t e d i n t h i s paper, t h e computer was programmed t o give the "FISH"
a wavelike motion.
The t o w i n g speed was between 5 and 7
knots, t h e CTD was lowered t o a depth o f 40 m, and t h e l e n g t h o f t h e h o r i z o n t a l c y c l e v a r i e d from 370 t o 500 m. hertz,
the v e r t i c a l
Using a measuring frequency o f 30
r e s o l u t i o n was b e t t e r than 5 cm,
since the v e r t i c a l
v e l o c i t y d i d n o t exceed 1.5 m s-I. Experiments u s i n g t h e towed CTD were performed d u r i n g t h e 9 t h and 1 5 t h c r u i s e s o f t h e R/V "Ayu-Dag" Sea.
i n t h e c e n t r a l and southern p a r t s o f t h e B a l t i c
The l o c a t i o n s o f t h e CTD s e c t i o n s a r e shown i n F i g u r e 2.
9 t h c r u i s e (1978),
t i o n s 1, 2 and 3; t h e l e n g t h s o f these s e c t i o n s were 90, respectively.
During t h e
we worked i n t h e extended BOSEX area and o b t a i n e d sec-
D u r i n g t h e 1 5 t h c r u i s e (1979),
85 and 120 km
measurements were made along
s e c t i o n 4 (200 km), extending from t h e BOSEX area t o t h e I s l a n d o f Bornholm. Hydrometeorological c o n d i t i o n s d u r i n g these two c r u i s e s were various.
The
measurements o f t h e 9 t h c r u i s e t o o k p l a c e a t t h e beginning o f August, i . e . ,
491 a t a t i m e o f weak winds and s t r o n g thermal s t r a t i f i c a t i o n .
During t h e 1 5 t h
c r u i s e ( a t t h e end o f September), s t r o n g v a r i a b l e winds and a negative heat f l u x p r e v a i l e d and combined t o d e s t r o y t h e s t r a t i f i c a t i o n o f t h e upper l a y ers.
F i g u r e 2.
Locations o f t h e "FISH" tow sections.
A c h a r a c t e r i s t i c f e a t u r e o f measurements made w i t h a towed h i g h f r e quency device, when s t u d y i n g s y n o p t i c scale phenomena, i s t h e accumulation For example, some o f o u r s e r i e s have up
o f a g r e a t amount o f i n f o r m a t i o n .
6 t o 16 x 10 data p o i n t s . i n essential
losses
It i s e v i d e n t t h a t subsampling t h e s e r i e s r e s u l t s
i n the determination o f isolines.
method i s n o t used here.
this
The d a t a processing c y c l e includes a p r e l i m i n a r y
and t h e d e t e r m i n a t i o n o f c h a r a c t e r i s t i c p e r t u r b a t i o n s o f t h e
processing
temperature and s a l i n i t y f i e l d s .
First,
syncroerrors a r e e l i m i n a t e d from
t h e CTD data; t h e r e a r e about 300 syncroerrors p e r hour. i n c o r r e c t values o f pressure P,
A t t h e same stage,
temperature T and c o n d u c t i v i t y C a r e r e Those Pi,
moved, u s i n g t h e f o l l o w i n g c r i t e r i a :
Ti and Ci which do n o t meet
the conditions
-
Pi-l < 1 dbar, a r e considered i n c o r r e c t . Pi
Therefore,
Ti
-
Ti-l
< l0C,
Ci
-
Ci-l
-1 < 1 mmho cm
I n t h e second step, a l l t h e s e r i e s a r e smoothed, t o lessen t h e i n f l u ence o f t h e v e r t i c a l m i c r o s t r u c t u r e and t h e noise l e v e l .
Taking a running
average o f t h e d a t a w i t h a 1 m f i l t e r appears t o be t h e b e s t way t o e l i m i nate t h e m i c r o s t r u c t u r e , a l t h o u g h i t r e s u l t s i n a c e r t a i n deformztion o f t h e s y n o p t i c s c a l e thermohaline s t r u c t u r e .
From t h e smoothed s e r i e s , s a l i n i t y
492
and s p e c i f i c d e n s i t y ,
at,
are c a l c u l a t e d based on known formulae f o r t h e
B a l t i c Sea ( P e r k i n and Walker, 1971; M i l l e r o and Kremling, 1976). I n t h e f i n a l step, a l l p o s s i b l e s p a t i a l i s o l i n e s a r e determined.
Time
i s o l i n e s are transformed i n t o s p a t i a l i s o l i n e s w i t h t h e h e l p o f t h e c o r r e s ponding n a v i g a t i o n data.
This i s f o l l o w e d by t h e p l o t t i n g o f s e c t i o n s o f
t h e T, S , and ut f i e l d s i n space P ; f i n a l l y , s e c t i o n s o f t h e T and S f i e l d s a r e p l o t t e d i n space at, i n t e r n a l waves.
i n o r d e r t o e l i m i n a t e t h e kinematic e f f e c t o f
The comparison o f the s e c t i o n s o f b o t h types a l l o w s us t o
d i v i d e t h e f i e l d s i n t o components, t o separate t h e v a r i o u s p e r t u r b a t i o n s and t o evaluate t h e i r c h a r a c t e r i s t i c s .
ERRORS The e s t i m a t i o n o f e r r o r s interpretation. 1.
i s a l s o i m p o r t a n t i n d a t a processing and
The main sources o f e r r o r s are:
t h e thermal i n e r t i a o f t h e pressure sensor, p a r t i c u l a r l y i n case
o f h i g h temperature g r a d i e n t s ;
2. lated;
3.
t h e accuracy w i t h which P,T,C
a r e measured, and S and ut calcu-
t h e use o f T and ut i n s t e a d o f t h e p o t e n t i a l temperature, 8 , and
t h e p o t e n t i a l d e n s i t y ue. The a n a l y s i s o f our c a l c u l a t i o n s leads t o t h e f o l l o w i n g conclusion. When t h e s t r a t i f i c a t i o n i s s t r o n g (e.g.
summer o f 1978), t h e r e c o r d i n g o f a
change i n t h e v e r t i c a l d i r e c t i o n o f t h e CTD motion i s s i g n i f i c a n t l y delayed due t o t h e thermal i n e r t i a o f t h e pressure sensor.
T h i s causes apparent
s h i f t s i n t h e depths o f t h e v a r i o u s i s o l i n e s w i t h i n t h e f o l l o w i n g ranges: up t o 1, 5 and 2 m f o r isotherms, i s o h a l i n e s and isopycnals, r e s p e c t i v e l y . Therefore, we chose t o use o n l y CTD d a t a c o l l e c t e d d u r i n g t h e upward motion o f t h e FISH.
I n doing so,
t h e h o r i z o n t a l r e s o l u t i o n i s decreased by a
f a c t o r two b u t t h i s i s n o t c r i t i c a l i n view o f t h e scales o f t h e phenomena under study. The second source o f e r r o r s i s t h e absolute accuracy o f t h e i n s t r u ments.
I n view o f t h e l o n g term s t a b i l i t y o f t h e p r o b e ' s o p e r a t i o n and
p a r t i a l c a l i b r a t i o n , AT = k 0.005°C,
AC =
*
0.005 mmho cm-'
and AP = k 3 m
may be taken as e r r o r bounds.
The l a t t e r r e f l e c t s mainly t h e thermal i n e r -
t i a o f t h e pressure sensor.
Since we a r e mainly i n t e r e s t e d i n r e l a t i v e
i s o l i n e changes and d a t a a r e c o l l e c t e d . i n one v e r t i c a l d i r e c t i o n o n l y , t h e pressure e r r o r may be considered systematic.
Based on t h e formulae used i n
t h e c a l c u l a t i o n s , i t can be determined t h a t AS = t 0.010 O/oo and Ao,
= t
0.0085 ut u n i t s . Finally,
l e t us consider t h e problem o f t h e deformation o f t h e i s o -
l i n e s , which i s r e l a t e d t o t h e nonconservative nature o f t h e temperature and
493
density.
under t h e i n f l u e n c e o f
Indeed,
p a r c e l moves up and down. same water
parcel
Cairns (1980),
i n t e r n a l waves,
a given water
A t c e r t a i n depths, because o f compression, t h e
has d i f f e r e n t temperature and d e n s i t y .
According t o
t h e use o f d e n s i t y i n s t e a d o f p o t e n t i a l d e n s i t y leads t o the
f o l l o w i n g e r r o r s i n temperature and s a l i n i t y : AT = AS =
where
ds(K/g)t dz
= c o e f f i c i e n t o f a d i a b a t i c compredsion,
K
5=
displacement o f water p a r c e l due t o i n t e r n a l waves.
I t can be seen t h a t t h e s m a l l e r t h e v e r t i c a l d e n s i t y g r a d i e n t and t h e l a r g e r
t h e amplitude o f t h e i n t e r n a l waves, the g r e a t e r are t h e e r r o r s i n t h e i s o pycnal a n a l y s i s .
Taking values t y p i c a l o f t h e upper l a y e r o f t h e B a l t i c Sea
= 10-1 O C m-', dS/dz = 5 x O/oo m-', and u n i t s m - l ) and f o r t h e amplitude o f t h e i n t e r n a l waves,
f o r t h e g r a d i e n t s (dT/dz dut/dz
u 2 x lo-'
= 2 x lo-'
we g e t AT
-1
and AS -1
OC
O/oo.
We s h a l l add t h e e r r o r s made i n
t h e d e t e r m i n a t i o n o f isotherms and isopycnals i n space P w i t h o u t c o n s i d e r i n g c o m p r e s s i b i l i t y AT
=
2 x
OC
and Aut E 5 x
ut u n i t s .
a r e s m a l l e r than t h e corresponding absolute accuracies.
The l a t t e r
Consequently, i t i s
necessary t o use t h e p o t e n t i a l temperature and d e n s i t y i n isopycnal a n a l y s i s o n l y i n t h e presence o f extremely small d e n s i t y gradients. RESULTS I n t h i s s e c t i o n , we p r e s e n t and discuss t h e r e s u l t s o b t a i n e d along t h e s e c t i o n s described e a r l i e r . experiments i s s t r i k i n g .
The d i f f e r e n c e between t h e 1978 and t h e 1979
During t h e summer experiment (1978), t h e s t r u c t u r e
o f t h e i s o l i n e s o f t h e temperature and s a l i n i t y f i e l d s i s r a t h e r smooth.
In
t h e thermocline r e g i o n , l o n g w a v e l i k e p e r t u r b a t i o n s w i t h l e n g t h s o f 30-40 km and amplitudes o f up t o 3 m can be d i s t i n g u i s h e d .
The r e s u l t s o f t h e sec-
t i o n made along t h e a x i s o f t h e B a l t i c Sea i n autumn (1979) are q u i t e d i f f e r e n t i n character.
We f i n d l a r g e f l u c t u a t i o n s (up t o 10 m) o f t h e i s o -
l i n e s a t a l l depths where measurements were made. p e r t u r b a t i o n s i s 20-25 km, i . e .
The l e n g t h scale o f these
s m a l l e r than i n summer.
Wavelike perturba-
t i o n s o f small scales were n o t studied, s i n c e they are d i s t o r t e d because o f t h e Doppler e f f e c t d u r i n g f i e l d measurements.
T y p i c a l examples o f s e c t i o n s
o b t a i n e d d u r i n g these experiments a r e presented i n Figures 3 and 4.
494
Distance ( km 1
F i g u r e 3.
Temperature,
(summer 1978). density:
from tow s e c t i o n 1
temperature: l0C; s a l i n i t y : 0.05 O/oo;
0.2 at u n i t s .
F i g u r e 4 ( f a c i n g page). s e c t i o n 4 (autumn 1979).
0.05 O/oo;
s a l i n i t y and d e n s i t y f i e l d s
Contour i n t e r v a l s :
density:
Temperature, s a l i n i t y and d e n s i t y f i e l d s f r o m tow Contour i n t e r v a l s :
0 . 1 at u n i t s .
temperature:
l0C; s a l i n i t y :
495
I
10
I
Distance I k m ) 20 30 40 50 Isotherms
I
I
I
1
Isdhalines
I
I s 0pycnals
496
On t h e b a s i s o f temperature s e c t i o n s , i t i s p r a c t i c a l l y impossible t o i d e n t i f y p e r t u r b a t i o n s o f non-wave salinity field, only.
Therefore,
method,
origin.
However,
on s e c t i o n s o f t h e
such p e r t u r b a t i o n s can be d i s t i n g u i s h e d , b u t q u a l i t a t i v e l y
however,
we
shall
use t h e isopycnal
has a grave shortcoming.
analysis
hereafter.
This
I f non-wave p e r t u r b a t i o n s o f
temperature and s a l i n i t y do n o t f u l l y compensate each o t h e r , d e n s i t y sect i o n s w i l l n o t r e f l e c t a " p u r e l y " wavelike p i c t u r e .
When t h i s i s t h e case,
t h e study o f t h e temperature and s a l i n i t y f i e l d s i n at-space
may n o t y i e l d
r e l i a b l e estimates o f t h e dimensions and amplitudes o f t h e p e r t u r b a t i o n s . As a f i r s t approximation,
l e t us assume t h a t t h e observed p e r t u r b a t i o n s
o f temperature and s a l i n i t y a r e d e n s i t y compensated.
Then, we may i d e n t i f y
p e r t u r b a t i o n s w i t h t h e f o l l o w i n g s t a t i s t i c s f o r t h e 1978 summer experiment (Table 1). Table 1 C h a r a c t e r i s t i c s o f t h e temperature f i e l d p e r t u r b a t i o n s , summer 1978
Maximum length (h)
Mean distance (km)
8
10
19
28
0.6
13
4
8
21
0.8
No.
Upper l a y e r (16°C) Intermediate l a y e r (4°C)
Mean temperature change ("C)
Mean length (h)
The f o l l o w i n g conclusions can be drawn from these r e s u l t s . and l o n g p e r t u r b a t i o n s dominate i n t h e upper l a y e r .
Relatively rare
I n the intermediate
l a y e r , below t h e thermocline, t h e r e are more p e r t u r b a t i o n s , b u t t h e i r dimensions a r e c o n s i d e r a b l y smaller.
Note t h a t t h e mean temperature change i s
l a r g e r i n t h e i n t e r m e d i a t e l a y e r t h a n i n t h e upper l a y e r . We were unable t o c a l c u l a t e analogous s t a t i s t i c s f o r t h e 1979 autumn experiment because t h e isotherms v a r i e d g r e a t l y ( t h e thermocline i s a t t h e bottom o f t h e l a y e r under study over p a r t o f t h e s e c t i o n ) .
I n spite o f
t h a t , temperature p e r t u r b a t i o n s w i t h l e n g t h scales o f 4 t o 12 km and average amplitudes o f
l0C
may be i d e n t i f i e d a g a i n s t a background o f s m a l l - s c a l e
"noise" ( p e r t u r b a t i o n s s h o r t e r t h a n 4 km w i t h amplitudes o f 0.3"C).
A considerable non-wave v a r i a b i l i t y o f s a l i n i t y i s observed m a i n l y i n t h e upper l a y e r . temperature.
I n most cases, t h e p e r t u r b a t i o n s a r e s i m i l a r t o those o f
For example i n F i g u r e 3, we see a s a l i n i t y p e r t u r b a t i o n which
i s about 18 km long.
There i s a sharp s a l i n i t y g r a d i e n t on b o t h s i d e s o f
491
t h e p e r t u r b a t i o n . A s i m i l a r perturbation can a l s o be seen in t h e temperat u r e f i e l d i n ot-space (Fig. 5), i . e . a f t e r removal of t h e v a r i a b i l i t y due t o i n t e r n a l waves.
0
4.0
1
5
10
Distance [ k m ) 15
120-
+
20
25
Figure 5. Temperature and s a l i n i t y f i e l d s in at-space ( f i r s t half o f the s e c t i o n presented i n Fig. 3 ) .
498
Thus,
i n t h e p r e s e n t case, t h e temperature and t h e s a l i n i t y o f t h e
water mass a r e d i f f e r e n t
f r o m those o f t h e neighboring environment.
To
study t h e d e n s i t y c h a r a c t e r i s t i c s o f these water masses, we s h a l l use t h e T-S p r e s e n t a t i o n (Gargett, 1978).
The essence o f t h e method i s as f o l l o w s .
Moving a t a f i x e d depth w i t h i n t h e core of a g i v e n water mass corresponds t o moving up and down on a T-S curve on account o f i n t e r n a l waves.
Crossing
t h e border1 i n e between t w o d i f f e r e n t water masses corresponds t o s w i t c h i n g I f t h e t r a n s l a t i o n from one T-S
t o another T-S curve.
takes p l a c e along an isopycnal,
curve t o another
we may consider t h a t t h e temperature and
s a l i n i t y p e r t u r b a t i o n s a r e d e n s i t y compensated. p l a c e a t an angle t o t h e isopycnals,
I f t h e t r a n s l a t i o n takes
we may draw t h e c o n c l u s i o n t h a t t h e F i g u r e 7 shows t h e T-S p l o t
p e r t u r b a t i o n s are n o t compensated by d e n s i t y .
a t a depth o f 10 m f o r t h e example j u s t discussed.
Despite a considerable
sparseness o f t h e data p o i n t s i n t h e f i r s t h a l f o f t h e p e r t u r b a t i o n , t w o types o f t r a n s l a t i o n s o r "crossings" may be d i s t i n g u i s h e d .
I n the left-hand
p a r t o f t h e p e r t u r b a t i o n , t h e passage over t h e 4 km s u b p e r t u r b a t i o n i s n o t isopycnal ( d o t t e d l i n e s 1 and 2 ) .
However, on t h e r i g h t - h a n d edge o f t h e
p e r t u r b a t i o n ( d o t t e d l i n e 3) t h e c r o s s i n g o f t h e water mass boundary i s isopycnal,
i . e . w i t h o u t any d e n s i t y jump.
Thermohaline p e r t u r b a t i o n s along
t h e r e l a t i v e l y more v a r i a b l e s e c t i o n of t h e 1979 experiment are even harder t o detect.
The f i l t e r i n g o f t h e i n t e r n a l waves g i v e s a p i c t u r e w i t h i r r e g u -
l a r v a r i a b i l i t y (Fig.
6).
Nevertheless,
an 11 km l o n g p e r t u r b a t i o n ( t h e
amplitude o f t h e temperature change i s 1.3OC, change 0 . 1 O/oo)
and t h a t o f t h e s a l i n i t y
can be d i s t i n g u i s h e d ; 10 km f u r t h e r , a c o n s i d e r a b l y l a r g e r
p e r t u r b a t i o n begins.
Two problems i n s t u d y i n g t h i s t y p e o f v a r i a b i l i t y are
t h e f a c t s t h a t p o i n t s on t h e T-S plane a r e g r e a t l y s c a t t e r e d , and t h a t t h e density r e l a t i o n s o f the perturbations are d i f f i c u l t t o elucidate. CONCLUSION F i n a l l y , we would l i k e t o say a few words a b o u t . t h e o r i g i n o f t h e t h e r mohal i n e p e r t u r b a t i o n s . "cold-fresh"
We have observed o n l y two types o f p e r t u r b a t i o n s :
and "warm-salty".
Those types i n d i c a t e w i t h h i g h p r o b a b i l i t y
t h a t d e n s i t y compensation i s achieved.
Since t h e observed patches o f water
masses w i t h d i f f e r e n t T and S c h a r a c t e r i s t i c s a r e d e n s i t y compensated, we may c a l l
7
these patches "macro-intrusions".
"cold-fresh"
B a l t i c , and t h a t t h e "warm-salty" origin.
We b e l i e v e t h a t t h e observed
i n t r u s i o n was advected from neighboring areas o f t h e n o r t h e r n i n t r u s i o n was probably o f southern B a l t i c
We note t h a t t h e f i r s t type o f p e r t u r b a t i o n s p r e v a i l e d i n t h e
summer o f 1978 and t h e second type i n t h e autumn o f 1979.
499
I
F i g u r e 6. F i g . 4).
Temperature and s a l i n i t y f i e l d s i n ot-space
(same s e c t i o n as i n
500
Salinity (%-)
F i g u r e 7.
T-S p l o t o f s e c t i o n 1 a t a depth o f 10 m.
U n f o r t u n a t e l y , our data a r e s t i l l i n s u f f i c i e n t t o answer some key quest i o n s about t h e thermohaline v a r i a b l i t y i n t h e upper l a y e r s o f t h e B a l t i c . Nevertheless,
t h e d a t a presented i n t h i s paper demonstrate t h a t t h e CTD
towing method i s useful f o r t h e study o f t h e s p a t i a l s t r u c t u r e o f t h e tempera t u r e and s a l i n i t y f i e l d s on meso- and s y n o p t i c scales.
REFERENCES A l l e n , C.H., Simpson, J.H., Carson, R.M., 1980. The s t r u c t u r e and v a r i a b i l i t y o f s h e l f sea f r o n t s as observed by an u n d u l a t i n g CTD system. Oceanologica Acta, 3(1): 59-68. I n t e r n a l waves and Cairns, J.L., 1980. V a F i a b i l i t y i n t h e G u l f o f Cadiz: globs. J. Phys. Oceanogr., g ( 4 ) : 579-595.
501 G a r g e t t , A.E., 1978. M i c r o s t r u c t u r e and f i n e s t r u c t u r e i n an upper ocean f r o n t a l regime. J. Geophys. Res. , g ( C 1 0 ) : 5123-5134. Katz, E.J., 1973. P r o f i l e o f an isopycnal surface i n t h e main thermocline o f t h e Sargasso Sea. J. Pys. Oceanogr., 3: 448-457. Katz, E.J., 1975. Tow s p e c t r a from MODE. J. Geophys. Res., 8019): 11631167. M i l l e r o , F.J., Kremling, K . , 1976. The d e n s i t i e s o f B a l t i c Sea waters. Deep-sea Res., 23: 1129-1138. Pavelson, J., Portsmuth, R . , 1981. A towed system f o r thermohaline f i e l d s measurements. The I n v e s t i g a t i o n and M o d e l l i n g o f t h e Processes i n t h e B a l t i c Sea., T a l l i n n , pp. 16-25. Perkin, R.G., Walker, E.R., 1972. S a l i n i t y c a l c u l a t i o n s from " i n s i t u " measurements. J. Geophys. Res. , 77(33). Woods, J.D., M i n n e t t , P.J., 1979. Analysis o f mesoscale t h e r m o c l i n i c i t y w i t h an example from t h e t r o p i c a l thermocline d u r i n g GATE. Deep-sea Res., 85-96.
z:
THE VARIABILITY OF THE TEMPERATURE, S A L I N I T Y AND DENSITY FIELDS I N THE UPPER LAYERS OF THE BALTIC SEA A. Aitsam,
J. Pavelson
.'
I n s t i t u t e o f Thermophysics and E l e c t r o p h y s i c s , Academy o f Sciences o f t h e Estonian S.S.R. INTRODUCTION I n r e c e n t years, t h e use o f a towed CTD has proved successful i n d e t e r mining t h e s t r u c t u r e o f t h e temperature;' s a l i n i t y and d e n s i t y f i e l d s .
The
s p a t i a l d i s t r i b u t i o n s o f f i e l d s obtained by t h i s method i n a comparatively s h o r t t i m e have an e s s e n t i a l l y h i g h e r r e s o l u t i o n than those obtained from observations a t d i s c r e t e p o i n t s .
Depending on t h e aims o f t h e i n v e s t i g a -
t i o n , t h r e e towing regimes a r e p o s s i b l e : 1. 2.
t h e CTD moves a t a f i x e d depth (Gargett, 1978), t h e CTO performs wavelike motion between two l e v e l s ( A l l e n e t a l . 1980),
3.
t h e CTD moves a l o n g a f i x e d isotherm o r isopycnal (Katz,
1973,
1975). I n most o f t h e s t u d i e s j u s t mentioned,
t h e s p a t i a l s t r u c t u r e o f the
temperature and s a l i n i t y f i e l d s was obtained w i t h o u t s e p a r a t i n g t h e "background" o f i n t e r n a l waves.
However, when s t u d y i n g t h e v a r i a b i l i t y o f these
v a r i a b l e s , t h e r e l a t i v e p a r t due t o i n t e r n a l waves should be determined. This i s i m p o r t a n t f o r a c o r r e c t e v a l u a t i o n o f t h e c h a r a c t e r i s t i c s o f t h e non-wave
perturbations.
As a f i r s t approximation,
an isopycnal a n a l y s i s
m i g h t be used as i n r e c e n t s t u d i e s by Woods and M i n n e t t (1979), and Cairns (1980).
Despite i t s shortcomings,
t h i s method i s t h e b e s t one a t present.
The aim of t h e p r e s e n t study i s t o determine t h e main c h a r a c t e r i s t i c s o f t h e temperature, s a l i n i t y and d e n s i t y f i e l d s on h o r i z o n t a l scales l a r g e r than 1 km i n t h e upper l a y e r s o f t h e open p a r t o f t h e B a l t i c Sea.
F i r s t , we
s h a l l describe t h e experiments and t h e processing o f t h e data.
Then, the
most t y p i c a l r e s u l t s w i l l be presented and discussed.
F i n a l l y , some hypothe-
ses about t h e o r i g i n o f t h e observed s t r u c t u r e w i l l be formulated.
EXPERIMENTS AND DATA PROCESSING
All
the
experimental
data were obtained u s i n g t h e towed measuring
device c o n s t r u c t e d a t t h e I n s t i t u t e o f Thermophysics and E l e c t r o p h y s i c s o f t h e Academy o f Sciences o f t h e Estonian S.S.R. 1981).
(Pavelson and Portsmuth,
The i n s t r u m e n t c o n s i s t s o f an underwater u n i t , t h e "FISH",
onboard system ( F i g u r e 1).
The " F I S H "
and an
i s equipped w i t h a CTD MARK I11
490
2
MINI -
INTERFACE
z
PL 0 T TER .''
. COMPUTER H P 9825A
HP 9862A
i
r - - ---
-- 1
I
I
CABLE
%
CT D
b
DECK UNIT
TAPE RECORDER KENNEDY 9832
1
U N D E R WATER UNIT
'FISH'
F i g u r e 1.
Block diagram o f t h e towed measuring device.
( N B I S ) and guided by means o f small wings a c t i v a t e d by a m i n i a t u r e e l e c t r i c motor.
The data are t r a n s m i t t e d t o t h e onboard t e r m i n a l and s t o r e d on a
tape r e c o r d e r KENNEDY-9832 w i t h t h e h e l p o f s p e c i a l i n t e r f a c e s . o f t h e "FISH"
The c o n t r o l
motion and t h e p r e l i m i n a r y p l o t t i n g o f temperature s e c t i o n s
a r e performed by an HP-9825-A computer. For t h e experiments r e p o r t e d i n t h i s paper, t h e computer was programmed t o give the "FISH"
a wavelike motion.
The t o w i n g speed was between 5 and 7
knots, t h e CTD was lowered t o a depth o f 40 m, and t h e l e n g t h o f t h e h o r i z o n t a l c y c l e v a r i e d from 370 t o 500 m. hertz,
the v e r t i c a l
Using a measuring frequency o f 30
r e s o l u t i o n was b e t t e r than 5 cm,
since the v e r t i c a l
v e l o c i t y d i d n o t exceed 1.5 m s-I. Experiments u s i n g t h e towed CTD were performed d u r i n g t h e 9 t h and 1 5 t h c r u i s e s o f t h e R/V "Ayu-Dag" Sea.
i n t h e c e n t r a l and southern p a r t s o f t h e B a l t i c
The l o c a t i o n s o f t h e CTD s e c t i o n s a r e shown i n F i g u r e 2.
9 t h c r u i s e (1978),
t i o n s 1, 2 and 3; t h e l e n g t h s o f these s e c t i o n s were 90, respectively.
During t h e
we worked i n t h e extended BOSEX area and o b t a i n e d sec-
D u r i n g t h e 1 5 t h c r u i s e (1979),
85 and 120 km
measurements were made along
s e c t i o n 4 (200 km), extending from t h e BOSEX area t o t h e I s l a n d o f Bornholm. Hydrometeorological c o n d i t i o n s d u r i n g these two c r u i s e s were various.
The
measurements o f t h e 9 t h c r u i s e t o o k p l a c e a t t h e beginning o f August, i . e . ,
491 a t a t i m e o f weak winds and s t r o n g thermal s t r a t i f i c a t i o n .
During t h e 1 5 t h
c r u i s e ( a t t h e end o f September), s t r o n g v a r i a b l e winds and a negative heat f l u x p r e v a i l e d and combined t o d e s t r o y t h e s t r a t i f i c a t i o n o f t h e upper l a y ers.
F i g u r e 2.
Locations o f t h e "FISH" tow sections.
A c h a r a c t e r i s t i c f e a t u r e o f measurements made w i t h a towed h i g h f r e quency device, when s t u d y i n g s y n o p t i c scale phenomena, i s t h e accumulation For example, some o f o u r s e r i e s have up
o f a g r e a t amount o f i n f o r m a t i o n .
6 t o 16 x 10 data p o i n t s . i n essential
losses
It i s e v i d e n t t h a t subsampling t h e s e r i e s r e s u l t s
i n the determination o f isolines.
method i s n o t used here.
this
The d a t a processing c y c l e includes a p r e l i m i n a r y
and t h e d e t e r m i n a t i o n o f c h a r a c t e r i s t i c p e r t u r b a t i o n s o f t h e
processing
temperature and s a l i n i t y f i e l d s .
First,
syncroerrors a r e e l i m i n a t e d from
t h e CTD data; t h e r e a r e about 300 syncroerrors p e r hour. i n c o r r e c t values o f pressure P,
A t t h e same stage,
temperature T and c o n d u c t i v i t y C a r e r e Those Pi,
moved, u s i n g t h e f o l l o w i n g c r i t e r i a :
Ti and Ci which do n o t meet
the conditions
-
Pi-l < 1 dbar, a r e considered i n c o r r e c t . Pi
Therefore,
Ti
-
Ti-l
< l0C,
Ci
-
Ci-l
-1 < 1 mmho cm
I n t h e second step, a l l t h e s e r i e s a r e smoothed, t o lessen t h e i n f l u ence o f t h e v e r t i c a l m i c r o s t r u c t u r e and t h e noise l e v e l .
Taking a running
average o f t h e d a t a w i t h a 1 m f i l t e r appears t o be t h e b e s t way t o e l i m i nate t h e m i c r o s t r u c t u r e , a l t h o u g h i t r e s u l t s i n a c e r t a i n deformztion o f t h e s y n o p t i c s c a l e thermohaline s t r u c t u r e .
From t h e smoothed s e r i e s , s a l i n i t y
492
and s p e c i f i c d e n s i t y ,
at,
are c a l c u l a t e d based on known formulae f o r t h e
B a l t i c Sea ( P e r k i n and Walker, 1971; M i l l e r o and Kremling, 1976). I n t h e f i n a l step, a l l p o s s i b l e s p a t i a l i s o l i n e s a r e determined.
Time
i s o l i n e s are transformed i n t o s p a t i a l i s o l i n e s w i t h t h e h e l p o f t h e c o r r e s ponding n a v i g a t i o n data.
This i s f o l l o w e d by t h e p l o t t i n g o f s e c t i o n s o f
t h e T, S , and ut f i e l d s i n space P ; f i n a l l y , s e c t i o n s o f t h e T and S f i e l d s a r e p l o t t e d i n space at, i n t e r n a l waves.
i n o r d e r t o e l i m i n a t e t h e kinematic e f f e c t o f
The comparison o f the s e c t i o n s o f b o t h types a l l o w s us t o
d i v i d e t h e f i e l d s i n t o components, t o separate t h e v a r i o u s p e r t u r b a t i o n s and t o evaluate t h e i r c h a r a c t e r i s t i c s .
ERRORS The e s t i m a t i o n o f e r r o r s interpretation. 1.
i s a l s o i m p o r t a n t i n d a t a processing and
The main sources o f e r r o r s are:
t h e thermal i n e r t i a o f t h e pressure sensor, p a r t i c u l a r l y i n case
o f h i g h temperature g r a d i e n t s ;
2. lated;
3.
t h e accuracy w i t h which P,T,C
a r e measured, and S and ut calcu-
t h e use o f T and ut i n s t e a d o f t h e p o t e n t i a l temperature, 8 , and
t h e p o t e n t i a l d e n s i t y ue. The a n a l y s i s o f our c a l c u l a t i o n s leads t o t h e f o l l o w i n g conclusion. When t h e s t r a t i f i c a t i o n i s s t r o n g (e.g.
summer o f 1978), t h e r e c o r d i n g o f a
change i n t h e v e r t i c a l d i r e c t i o n o f t h e CTD motion i s s i g n i f i c a n t l y delayed due t o t h e thermal i n e r t i a o f t h e pressure sensor.
T h i s causes apparent
s h i f t s i n t h e depths o f t h e v a r i o u s i s o l i n e s w i t h i n t h e f o l l o w i n g ranges: up t o 1, 5 and 2 m f o r isotherms, i s o h a l i n e s and isopycnals, r e s p e c t i v e l y . Therefore, we chose t o use o n l y CTD d a t a c o l l e c t e d d u r i n g t h e upward motion o f t h e FISH.
I n doing so,
t h e h o r i z o n t a l r e s o l u t i o n i s decreased by a
f a c t o r two b u t t h i s i s n o t c r i t i c a l i n view o f t h e scales o f t h e phenomena under study. The second source o f e r r o r s i s t h e absolute accuracy o f t h e i n s t r u ments.
I n view o f t h e l o n g term s t a b i l i t y o f t h e p r o b e ' s o p e r a t i o n and
p a r t i a l c a l i b r a t i o n , AT = k 0.005°C,
AC =
*
0.005 mmho cm-'
and AP = k 3 m
may be taken as e r r o r bounds.
The l a t t e r r e f l e c t s mainly t h e thermal i n e r -
t i a o f t h e pressure sensor.
Since we a r e mainly i n t e r e s t e d i n r e l a t i v e
i s o l i n e changes and d a t a a r e c o l l e c t e d . i n one v e r t i c a l d i r e c t i o n o n l y , t h e pressure e r r o r may be considered systematic.
Based on t h e formulae used i n
t h e c a l c u l a t i o n s , i t can be determined t h a t AS = t 0.010 O/oo and Ao,
= t
0.0085 ut u n i t s . Finally,
l e t us consider t h e problem o f t h e deformation o f t h e i s o -
l i n e s , which i s r e l a t e d t o t h e nonconservative nature o f t h e temperature and
493
density.
under t h e i n f l u e n c e o f
Indeed,
p a r c e l moves up and down. same water
parcel
Cairns (1980),
i n t e r n a l waves,
a given water
A t c e r t a i n depths, because o f compression, t h e
has d i f f e r e n t temperature and d e n s i t y .
According t o
t h e use o f d e n s i t y i n s t e a d o f p o t e n t i a l d e n s i t y leads t o the
f o l l o w i n g e r r o r s i n temperature and s a l i n i t y : AT = AS =
where
ds(K/g)t dz
= c o e f f i c i e n t o f a d i a b a t i c compredsion,
K
5=
displacement o f water p a r c e l due t o i n t e r n a l waves.
I t can be seen t h a t t h e s m a l l e r t h e v e r t i c a l d e n s i t y g r a d i e n t and t h e l a r g e r
t h e amplitude o f t h e i n t e r n a l waves, the g r e a t e r are t h e e r r o r s i n t h e i s o pycnal a n a l y s i s .
Taking values t y p i c a l o f t h e upper l a y e r o f t h e B a l t i c Sea
= 10-1 O C m-', dS/dz = 5 x O/oo m-', and u n i t s m - l ) and f o r t h e amplitude o f t h e i n t e r n a l waves,
f o r t h e g r a d i e n t s (dT/dz dut/dz
u 2 x lo-'
= 2 x lo-'
we g e t AT
-1
and AS -1
OC
O/oo.
We s h a l l add t h e e r r o r s made i n
t h e d e t e r m i n a t i o n o f isotherms and isopycnals i n space P w i t h o u t c o n s i d e r i n g c o m p r e s s i b i l i t y AT
=
2 x
OC
and Aut E 5 x
ut u n i t s .
a r e s m a l l e r than t h e corresponding absolute accuracies.
The l a t t e r
Consequently, i t i s
necessary t o use t h e p o t e n t i a l temperature and d e n s i t y i n isopycnal a n a l y s i s o n l y i n t h e presence o f extremely small d e n s i t y gradients. RESULTS I n t h i s s e c t i o n , we p r e s e n t and discuss t h e r e s u l t s o b t a i n e d along t h e s e c t i o n s described e a r l i e r . experiments i s s t r i k i n g .
The d i f f e r e n c e between t h e 1978 and t h e 1979
During t h e summer experiment (1978), t h e s t r u c t u r e
o f t h e i s o l i n e s o f t h e temperature and s a l i n i t y f i e l d s i s r a t h e r smooth.
In
t h e thermocline r e g i o n , l o n g w a v e l i k e p e r t u r b a t i o n s w i t h l e n g t h s o f 30-40 km and amplitudes o f up t o 3 m can be d i s t i n g u i s h e d .
The r e s u l t s o f t h e sec-
t i o n made along t h e a x i s o f t h e B a l t i c Sea i n autumn (1979) are q u i t e d i f f e r e n t i n character.
We f i n d l a r g e f l u c t u a t i o n s (up t o 10 m) o f t h e i s o -
l i n e s a t a l l depths where measurements were made. p e r t u r b a t i o n s i s 20-25 km, i . e .
The l e n g t h scale o f these
s m a l l e r than i n summer.
Wavelike perturba-
t i o n s o f small scales were n o t studied, s i n c e they are d i s t o r t e d because o f t h e Doppler e f f e c t d u r i n g f i e l d measurements.
T y p i c a l examples o f s e c t i o n s
o b t a i n e d d u r i n g these experiments a r e presented i n Figures 3 and 4.
494
Distance ( km 1
F i g u r e 3.
Temperature,
(summer 1978). density:
from tow s e c t i o n 1
temperature: l0C; s a l i n i t y : 0.05 O/oo;
0.2 at u n i t s .
F i g u r e 4 ( f a c i n g page). s e c t i o n 4 (autumn 1979).
0.05 O/oo;
s a l i n i t y and d e n s i t y f i e l d s
Contour i n t e r v a l s :
density:
Temperature, s a l i n i t y and d e n s i t y f i e l d s f r o m tow Contour i n t e r v a l s :
0 . 1 at u n i t s .
temperature:
l0C; s a l i n i t y :
495
I
10
I
Distance I k m ) 20 30 40 50 Isotherms
I
I
I
1
Isdhalines
I
I s 0pycnals
496
On t h e b a s i s o f temperature s e c t i o n s , i t i s p r a c t i c a l l y impossible t o i d e n t i f y p e r t u r b a t i o n s o f non-wave salinity field, only.
Therefore,
method,
origin.
However,
on s e c t i o n s o f t h e
such p e r t u r b a t i o n s can be d i s t i n g u i s h e d , b u t q u a l i t a t i v e l y
however,
we
shall
use t h e isopycnal
has a grave shortcoming.
analysis
hereafter.
This
I f non-wave p e r t u r b a t i o n s o f
temperature and s a l i n i t y do n o t f u l l y compensate each o t h e r , d e n s i t y sect i o n s w i l l n o t r e f l e c t a " p u r e l y " wavelike p i c t u r e .
When t h i s i s t h e case,
t h e study o f t h e temperature and s a l i n i t y f i e l d s i n at-space
may n o t y i e l d
r e l i a b l e estimates o f t h e dimensions and amplitudes o f t h e p e r t u r b a t i o n s . As a f i r s t approximation,
l e t us assume t h a t t h e observed p e r t u r b a t i o n s
o f temperature and s a l i n i t y a r e d e n s i t y compensated.
Then, we may i d e n t i f y
p e r t u r b a t i o n s w i t h t h e f o l l o w i n g s t a t i s t i c s f o r t h e 1978 summer experiment (Table 1). Table 1 C h a r a c t e r i s t i c s o f t h e temperature f i e l d p e r t u r b a t i o n s , summer 1978
Maximum length (h)
Mean distance (km)
8
10
19
28
0.6
13
4
8
21
0.8
No.
Upper l a y e r (16°C) Intermediate l a y e r (4°C)
Mean temperature change ("C)
Mean length (h)
The f o l l o w i n g conclusions can be drawn from these r e s u l t s . and l o n g p e r t u r b a t i o n s dominate i n t h e upper l a y e r .
Relatively rare
I n the intermediate
l a y e r , below t h e thermocline, t h e r e are more p e r t u r b a t i o n s , b u t t h e i r dimensions a r e c o n s i d e r a b l y smaller.
Note t h a t t h e mean temperature change i s
l a r g e r i n t h e i n t e r m e d i a t e l a y e r t h a n i n t h e upper l a y e r . We were unable t o c a l c u l a t e analogous s t a t i s t i c s f o r t h e 1979 autumn experiment because t h e isotherms v a r i e d g r e a t l y ( t h e thermocline i s a t t h e bottom o f t h e l a y e r under study over p a r t o f t h e s e c t i o n ) .
I n spite o f
t h a t , temperature p e r t u r b a t i o n s w i t h l e n g t h scales o f 4 t o 12 km and average amplitudes o f
l0C
may be i d e n t i f i e d a g a i n s t a background o f s m a l l - s c a l e
"noise" ( p e r t u r b a t i o n s s h o r t e r t h a n 4 km w i t h amplitudes o f 0.3"C).
A considerable non-wave v a r i a b i l i t y o f s a l i n i t y i s observed m a i n l y i n t h e upper l a y e r . temperature.
I n most cases, t h e p e r t u r b a t i o n s a r e s i m i l a r t o those o f
For example i n F i g u r e 3, we see a s a l i n i t y p e r t u r b a t i o n which
i s about 18 km long.
There i s a sharp s a l i n i t y g r a d i e n t on b o t h s i d e s o f
491
t h e p e r t u r b a t i o n . A s i m i l a r perturbation can a l s o be seen in t h e temperat u r e f i e l d i n ot-space (Fig. 5), i . e . a f t e r removal of t h e v a r i a b i l i t y due t o i n t e r n a l waves.
0
4.0
1
5
10
Distance [ k m ) 15
120-
+
20
25
Figure 5. Temperature and s a l i n i t y f i e l d s in at-space ( f i r s t half o f the s e c t i o n presented i n Fig. 3 ) .
498
Thus,
i n t h e p r e s e n t case, t h e temperature and t h e s a l i n i t y o f t h e
water mass a r e d i f f e r e n t
f r o m those o f t h e neighboring environment.
To
study t h e d e n s i t y c h a r a c t e r i s t i c s o f these water masses, we s h a l l use t h e T-S p r e s e n t a t i o n (Gargett, 1978).
The essence o f t h e method i s as f o l l o w s .
Moving a t a f i x e d depth w i t h i n t h e core of a g i v e n water mass corresponds t o moving up and down on a T-S curve on account o f i n t e r n a l waves.
Crossing
t h e border1 i n e between t w o d i f f e r e n t water masses corresponds t o s w i t c h i n g I f t h e t r a n s l a t i o n from one T-S
t o another T-S curve.
takes p l a c e along an isopycnal,
curve t o another
we may consider t h a t t h e temperature and
s a l i n i t y p e r t u r b a t i o n s a r e d e n s i t y compensated. p l a c e a t an angle t o t h e isopycnals,
I f t h e t r a n s l a t i o n takes
we may draw t h e c o n c l u s i o n t h a t t h e F i g u r e 7 shows t h e T-S p l o t
p e r t u r b a t i o n s are n o t compensated by d e n s i t y .
a t a depth o f 10 m f o r t h e example j u s t discussed.
Despite a considerable
sparseness o f t h e data p o i n t s i n t h e f i r s t h a l f o f t h e p e r t u r b a t i o n , t w o types o f t r a n s l a t i o n s o r "crossings" may be d i s t i n g u i s h e d .
I n the left-hand
p a r t o f t h e p e r t u r b a t i o n , t h e passage over t h e 4 km s u b p e r t u r b a t i o n i s n o t isopycnal ( d o t t e d l i n e s 1 and 2 ) .
However, on t h e r i g h t - h a n d edge o f t h e
p e r t u r b a t i o n ( d o t t e d l i n e 3) t h e c r o s s i n g o f t h e water mass boundary i s isopycnal,
i . e . w i t h o u t any d e n s i t y jump.
Thermohaline p e r t u r b a t i o n s along
t h e r e l a t i v e l y more v a r i a b l e s e c t i o n of t h e 1979 experiment are even harder t o detect.
The f i l t e r i n g o f t h e i n t e r n a l waves g i v e s a p i c t u r e w i t h i r r e g u -
l a r v a r i a b i l i t y (Fig.
6).
Nevertheless,
an 11 km l o n g p e r t u r b a t i o n ( t h e
amplitude o f t h e temperature change i s 1.3OC, change 0 . 1 O/oo)
and t h a t o f t h e s a l i n i t y
can be d i s t i n g u i s h e d ; 10 km f u r t h e r , a c o n s i d e r a b l y l a r g e r
p e r t u r b a t i o n begins.
Two problems i n s t u d y i n g t h i s t y p e o f v a r i a b i l i t y are
t h e f a c t s t h a t p o i n t s on t h e T-S plane a r e g r e a t l y s c a t t e r e d , and t h a t t h e density r e l a t i o n s o f the perturbations are d i f f i c u l t t o elucidate. CONCLUSION F i n a l l y , we would l i k e t o say a few words a b o u t . t h e o r i g i n o f t h e t h e r mohal i n e p e r t u r b a t i o n s . "cold-fresh"
We have observed o n l y two types o f p e r t u r b a t i o n s :
and "warm-salty".
Those types i n d i c a t e w i t h h i g h p r o b a b i l i t y
t h a t d e n s i t y compensation i s achieved.
Since t h e observed patches o f water
masses w i t h d i f f e r e n t T and S c h a r a c t e r i s t i c s a r e d e n s i t y compensated, we may c a l l
7
these patches "macro-intrusions".
"cold-fresh"
B a l t i c , and t h a t t h e "warm-salty" origin.
We b e l i e v e t h a t t h e observed
i n t r u s i o n was advected from neighboring areas o f t h e n o r t h e r n i n t r u s i o n was probably o f southern B a l t i c
We note t h a t t h e f i r s t type o f p e r t u r b a t i o n s p r e v a i l e d i n t h e
summer o f 1978 and t h e second type i n t h e autumn o f 1979.
499
I
F i g u r e 6. F i g . 4).
Temperature and s a l i n i t y f i e l d s i n ot-space
(same s e c t i o n as i n
500
Salinity (%-)
F i g u r e 7.
T-S p l o t o f s e c t i o n 1 a t a depth o f 10 m.
U n f o r t u n a t e l y , our data a r e s t i l l i n s u f f i c i e n t t o answer some key quest i o n s about t h e thermohaline v a r i a b l i t y i n t h e upper l a y e r s o f t h e B a l t i c . Nevertheless,
t h e d a t a presented i n t h i s paper demonstrate t h a t t h e CTD
towing method i s useful f o r t h e study o f t h e s p a t i a l s t r u c t u r e o f t h e tempera t u r e and s a l i n i t y f i e l d s on meso- and s y n o p t i c scales.
REFERENCES A l l e n , C.H., Simpson, J.H., Carson, R.M., 1980. The s t r u c t u r e and v a r i a b i l i t y o f s h e l f sea f r o n t s as observed by an u n d u l a t i n g CTD system. Oceanologica Acta, 3(1): 59-68. I n t e r n a l waves and Cairns, J.L., 1980. V a F i a b i l i t y i n t h e G u l f o f Cadiz: globs. J. Phys. Oceanogr., g ( 4 ) : 579-595.
501 G a r g e t t , A.E., 1978. M i c r o s t r u c t u r e and f i n e s t r u c t u r e i n an upper ocean f r o n t a l regime. J. Geophys. Res. , g ( C 1 0 ) : 5123-5134. Katz, E.J., 1973. P r o f i l e o f an isopycnal surface i n t h e main thermocline o f t h e Sargasso Sea. J. Pys. Oceanogr., 3: 448-457. Katz, E.J., 1975. Tow s p e c t r a from MODE. J. Geophys. Res., 8019): 11631167. M i l l e r o , F.J., Kremling, K . , 1976. The d e n s i t i e s o f B a l t i c Sea waters. Deep-sea Res., 23: 1129-1138. Pavelson, J., Portsmuth, R . , 1981. A towed system f o r thermohaline f i e l d s measurements. The I n v e s t i g a t i o n and M o d e l l i n g o f t h e Processes i n t h e B a l t i c Sea., T a l l i n n , pp. 16-25. Perkin, R.G., Walker, E.R., 1972. S a l i n i t y c a l c u l a t i o n s from " i n s i t u " measurements. J. Geophys. Res. , 77(33). Woods, J.D., M i n n e t t , P.J., 1979. Analysis o f mesoscale t h e r m o c l i n i c i t y w i t h an example from t h e t r o p i c a l thermocline d u r i n g GATE. Deep-sea Res., 85-96.
z: