6
8/20/2019 Statistical Properties of High-frequency Internal Waves http://slidepdf.com/reader/full/statistical-properties-of-high-frequency-internal-waves 1/6 Chinese Journal of Oceanology and Limnology Vol.20, No.l, P.16-21, 2002 STATISTICAL PROPERTIES OF HIGH FREQUENCY INTERNAL WAVES IN QINGDAO OFFSHORE AREA OF THE YELLOW SEA ~ WANG Tao ~E ~j~) California Institute of Technology, Pasadena, CA 91125, USA ) GAO Tian-fu ~i~) Institute of Acoastics, Chinese Academy of Sciences, Beijing 100080, China ) Received June 1, 1999; revision acceptedJan. 11, 2001 Abstract Densely-sampled hermistor chain data obtained from a shallow-water acoustics exper- iment in the Yellow Sea off the coast of Qingdao were analyzed to examine the statistical properties of the 6 to 520 cpd frequency band internal waves observed. The negative skewness coefficients and the greater- than-3 kurtosis coefficients indicated non-Gaussianity of the internal waves. The probability distributions were negatively skewed and abnormally high peaks. Nonlinear properties, as exemplified by the asymmet- ric waveshapes of the internal waves in the offshore area are described quantitatively. Key words: shallow-water internal wave, skewness, kurtosis, non-Gaussianity, nonlinearity INTRODUCTION Observations revealed the widespread distribution of internal waves on the continental shelf. Much theoretical, numerical, laboratory and field work had been done on their generation and prop- agation. Significant progress has been achieved in shelf internal soliton research. However, statisti- cal properties of internal waves, especially in shallow water, were still little understood Wang and Gao, 2001; Xu, 1999). Are internal waves Ganssian? IWEX temperature, displacement and current data showed that in deep ocean, these waves were mainly Gaussian but sporadically non-Ganssian. The depth and loca- tion dependence of this conclusion is not known Briseoe, 1977). Statistics of high-frequency inter- nal wave signals with periods of less than 300 s were analyzed to examine the correspondence be- tween turbulent mixing and internal waves in the equatorial Pacific, where the skewness of the hori- zontal temperature gradient was significantly different from zero, and was positive in the depth above 40 m. The day to night variability of the signals was illustrated by the change in rms values of iso- therm displacements and their distributions Moum et al., 1992). A long-term prediction of intense internal waves with amplitudes larger than 5 m was observed in the tropical region of the Atlantic us- ing echo sounder measurements, showing that the wave repetition frequency could be described by the Poisson law for extreme events Ivanov et al., 1993). Recent studies addressing the statistics of shallow-water internal waves showed that the conti- nental slope to the south of the Celtic Sea is an area of intense and complicated internal wave activi- ty. To study the propagation of these waves, distributions of time intervals between their occurrence, tidal phases at which they were observed, and their propagation directions, were analysed. More- over, the mean fluctuation speed and skewness of temperature signals showed the different character- isties of wave packets arriving from different generation regions Holt and Thorpe, 1997). Tempera- ture time series were measured from an array of miniloggers in a line at constant depth along the sl- * Project No. 19804013 supported by the NSFC.

Statistical Properties of High-frequency Internal Waves

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Page 1: Statistical Properties of High-frequency Internal Waves

8/20/2019 Statistical Properties of High-frequency Internal Waves

http://slidepdf.com/reader/full/statistical-properties-of-high-frequency-internal-waves 1/6

Chinese Journal of O ceanologyand Limnology

V o l . 2 0 , N o . l , P . 1 6 - 2 1 , 2 0 0 2

S TA T IS TIC A L P R O P E R T I E S O F H I G H F R E Q U E N C Y I N T E R N A L W A V E S

I N Q I N G D A O O F F S H O R E A R E A O F T H E Y E LL O W S E A ~

W A N G T a o ~ E ~ j~ )

Cali fornia Inst i tu te o f Technology, Pa sade na, CA 91125, USA )

G AO T i a n - f u ~ i ~ )

Inst i tu te o f Acoast ics , Chinese Academ y of Sciences, Bei j ing

100080,

China )

Received June 1, 199 9; revision accepted Ja n. 11, 2001

Abst rac t Dense ly-sampledhermistor chain d ata o btained from a shallow -water acoustics exper-

iment in the Y ellow Sea off the co ast of Qingdao were analyzed to ex amine the statistical properties of the

6 to 520 cpd frequency band internal waves observed. The negative skewness coefficients and the greater-

than-3 kurtosis coefficients indicated n on-Gau ssianity of the internal w av es. The probab ility distributions

were negatively skewed and abnormally high peaks. Nonlinear properties, as exemplified by the asymmet-

ric wav eshapes of the internal wav es in the offshore area a re d escribed quantitatively.

Ke y w ord s: shallow-water internal wa ve, skew ness, kurtosis, non-Gaussianity, nonlinearity

I N T R O D U C T I O N

O bse r va t ions r e ve a l e d t he w ide sp r e a d d i s t r i bu t ion o f i n te r na l w a ve s on the c on t ine n ta l she l f .

M u c h the o r e t i c a l , num e r i c a l , l a bo r a to r y a nd f i eld w or k ha d be e n done on the i r ge ne r a t i on a nd p r op -

a ga t ion . S ign i f ic a n t p r og r e s s ha s be e n a c h i e ve d in she l f i n t e r na l so l it on r e se a r c h . H ow e ve r , s t a t is t i -

c a l p r ope r t ie s o f i n t e r na l w a ve s , e spe c i a l ly i n sha l low w a te r , w e r e s t il l l i tt l e unde r s tood W a ng a nd

G a o , 2 0 0 1 ; X u , 1 9 9 9 ) .

A r e i n t e rna l w a ve s G a n ss i a n? I W E X t e m pe r a tu r e , d i sp l a c e m e n t a nd c u r re n t da t a show e d tha t i n

de e p o c e a n , t he se w a ves w e r e m a in ly G a uss i a n bu t spo r a d i c a l l y non - G a nss i a n . T he de p th a nd loc a -

t i on de pe nd e nc e o f t h i s c onc lu s ion i s no t know n B r i s e oe , 19 77 ) . S t a ti s ti c s o f h igh - f r e qu e nc y in t e r -

na l w a ve s igna ls w i th pe r iods o f le s s t ha n 300 s w e r e a na lyz e d to e xa m ine the c o r r e sponde n c e be -

tw e e n tu r bu le n t m ix ing a nd in t e r na l w a ve s i n t he e qua to r i a l P a c i f i c , w he r e t he ske w ne ss o f t he ho r i -

z on ta l t e m pe r a tu r e g r a d i e n t w a s s ign i f i c a n tly d i ff e r en t f r om z e r o , a nd w a s pos it i ve i n t he de p th a bove

40 m . T he da y to n igh t va r ia b i l i ty o f t he s igna ls w a s i l l u s t ra t e d by the c ha nge in r m s va lue s o f i so -

t h e r m d i s p l ac e m e n t s a n d t h e i r d i s tr i b u ti o n s M o u m e t a l . , 1 9 9 2 ) . A l o n g - te r m p r e d i ct i o n o f i n te n s e

in t e r na l w a ve s w ith a m p l i t ude s l a r ge r t ha n 5 m w a s obse r ve d in t he t r op i c a l r e g ion o f the A t l a n t i c u s -

ing e c ho sounde r m e a su r e m e n t s , show ing tha t the w a ve r e pe t it i on f r e que nc y c ou ld be de sc r ibe d by

t h e P o is s o n l a w f o r e x tr e m e e v e n ts I v a n o v et a l . , 1 9 9 3 ) .

R e c e n t s t ud i e s a dd r e s s ing the s t a t i s ti c s o f sha l low - w a te r i n te r na l w a ve s show e d tha t t he c on t i -

ne n ta l s l ope t o the sou th o f t he C e l t ic S e a i s a n a r e a o f i n t e nse a nd c om pl i c a t e d i n t e r na l w a ve a c ti v i -

t y . T o s tudy the p r opa ga t ion o f t he se w a ve s , d i s t r ibu t ions o f t im e in te r va ls be tw e e n the i r oc c u r r e nc e ,

t i da l pha se s a t w h ic h the y w e r e obse r ve d , a nd the i r p ropa ga t ion d i r e c t i ons , w e r e a na ly se d . M or e -

ove r , t he m e a n f l uc tua t ion spe e d a nd ske w ne ss o f t e m pe r a tu r e s igna l s show e d the d i f f e r en t c ha r a c t e r -

i s t ie s o f w a ve pa c ke t s a r ri v ing f r om d i f f er e n t ge ne r a t i on r e g ions H o l t a nd T hor p e , 19 97 ) . T e m pe r a -

tu r e t im e se r i es w e r e m e a su r e d f r om a n a r r a y o f m in i logge rs i n a l i ne a t c ons t a n t de p th a long the s l -

* Project No. 19804013supported by the NSFC.

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No. 1 WAN G et al . : STATISTICALPROPERTIES OF INTERNAL WAVES 1 7

o p i n g b o u n d a r y o f L a k e G e n e v a s o u t h e a s t o f O u c h y . D i s t r ib u t i o n s o f t e m p e r a t u r e t i m e d e r i v a ti v e s

w e r e f o u n d t o h a v e a s m a l l n e g a t i v e s k e w n e s s , w h i c h i s c o n s i s t e n t w i t h t h e i n t e r n a l w a v e d y n a m i c s

i n t h e s u rf z o n e ( T h o r p e a n d L e m m i n , 1 9 9 9 ) .

T h i s w o r k i s a i m e d a t i n v e st i g a ti n g t h e s e c o n d - , t h i r d - a n d f o u r t h - o r d e r m o m e n t s a n d p r o b a b i l i -

t y d i s t ri b u t io n s o f i n t e r n a l w a v e s i n s h a l l o w w a t e r , a n d a p p l y i n g t h e d a t a o b t a i n e d t o d e s c r i b e s t a t i s -

t i c a l ly t h e i r n o n l i n e a r p r o p e r t i e s . R e s u l t s o f a s t u d y o n i n t e r n a l w a v e s i n a s t r o n g s e a s o n a l t h e r m o -

c l i n e o f t h e Y e l l o w S e a a r e p r e s e n t e d a n d a n a l y z e d .

D E F I N I T I O N S

C o n s i d e r a s a m p l e fu n c t i o n 7 / ( t ) f r o m a n e r g o d ic r a n d o m p r o c e s s . H e r e , r / ( t ) i s t h e t i m e

h i s to r y o f i n t e r n a l w a v e e l e v a t i o n s . T h e r t h m o m e n t s / z r o f 7?( t ) a r e d e f i ne d b y

,Ur = r1 - ~ ) r P r l ) d 7 ] , r ; 1 , 2 , 3 , 4 , ' ( 1 )

w h e r e P ( 7 1 ) i s t h e p r o b a b i l i t y d e n s i t y f u n c t i o n , a n d ~ is t h e m e a n v a l u e

= f i ~ ' I P ( ' j ) d ' l ( 2 )

A s s u m e r / ( t ) t a k e s o n d i s c r e t e v a l u e s ~ 1 , r ]2 , , r /N , t h e n

N

/~ , = ~ , , ~ ( ' ] , - ~ ? )~ , r = 1 , 2 , 3 , 4 '

10

2 1 5

2 0

25

30

14 I 6

t i

1 8 20 22 24 26 28

Temperature (~

Fig . 1 M ea n emperature profi le (August 23 14:5 0 -

August 25 16:3 9 , 1992)

t h e r m i s t o r c h a i n w i t h 3 2 c h a n n e l s s p a c e d O . 4 m

a p a r t . T h e s a m p l i n g t i m e i n t e r v a l w a s 6 . 4 s , a n d t h e l e n g t h 4 9 h 4 9 r a i n . I n th e w a t e r o f 3 3 m

d e p t h , a m e a n w e l l - m i x e d s u r f a c e l a y e r e x t e n d e d t o 9 . 5 m , t h e t e m p e r a t u r e t h e n d r o p p e d fr o m

2 6 ~ t o 1 6~ a t a d e p t h o f 1 6 . 5 m , a n d d r o p p e d sl o w l y t o a b o u t 1 5~ a t t h e s e a b e d ( F i g . l ) . A

m a x i m u m B ru nt -V a is ~il ~l f r e q u e n c y o f a b o u t 9 3 0 c p d ( p e r i o d 1 . 5 5 r a i n ) w a s s u p p o r t e d . T h e c h a i n

s p a n n e d t h e w a t e r c o lu m n f r o m 6 . 3 m t o 1 8 . 7 m w h e r e t h e st r o n g s e a s o n a l t h e r m o c l i n e o c c u r re d

3 )

w h e r e

= N , = , ( 4 )

T h e f i r s t m o m e n t i s z e r o f o r a n y r a n d o m p r o -

c e s s; t h e se c o n d m o m e n t is th e v a r i a nc e a 2 ( a ,

s t a n d a r d d e v i a t i o n ) ; t h e t h ir d m o m e n t , w h i c h i s a

m e a s u r e o f t h e a s y m m e t r y o f t h e w a v e s h a p e a b o u t

t h e h o r iz o n t a l a x i s , i s c a l l e d t h e s k e w n e s s ; a n d

t h e f o u r t h m o m e n t , w h i c h g i v e s a n i n d ic a t i o n o f

t h e p e a k e d n e s s o f t h e s t a t i s ti c a l d i s t r i b u t i o n , i s

c a l l e d t h e k u r t o s i s . T h e c o e f f ic i e n t s o f s k e w n e s s

a n d k u r t o s i s a r e

23 ,u3 ,u4

- 3 , 2 , ~ 4 - 2 5 )

~L~2 / /2

O B S E R V A T I O N S A N D D A T A P R O C E S S I N G

W e h a v e c o n d u c t e d s e v e r a l s h a l l o w - w a t e r

a c o u s t i c s e x p e r i m e n t s i n t h e Q i n g d a o o f f sh o r e a r e a

o f t h e Y e l l o w S e a s i n c e 1 9 7 9 . H i g h - q u a l i t y i n t e r -

n a l w a v e d a t a w e r e g a t h e r e d A u g u s t 1 9 9 2 f r o m a

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18 CHIN . J. OCEANOL. LIMNOL. 20 1) , 2002 Vol.20

W an g e t a l . , 1999; 20 00 ) . The tempera ture s igna ls of every channel were grouped and averaged over

13 data points. The num ber of data points N of every channel totalled 2155, and the N yquist f requency

was 520 cpd. This averaging removes the fluctuations in thermistor depth due to surface gravity waves and

ship motions, while still keepin g the high-frequency information in the sign al. T he obtained ten-g~erature

time ser ies was then l inearly interpolated , f rom the top channel to the bottom on e, to f ind the d epth t ime

series for a given temperature T, that is the isotherm in the t im e-dep th coordinate sy stem.

The i sotherms conta ined mot ions in three dis t inc t f requency bands , the synopt ic band inc luding

synopt ic weather pa t te rn and iner t ia l wav es , the tida l band inc luding diurna l and s emidiurna l t ides ,

and the high- f requen cy band con ta ining inte rna l wave s . The low-f requency component was of inte res t

in a previous pape r by W ang e t a l . 20 00 ) ; the present investiga t ion was concent ra ted on the high-

f requenc y wave com pon ent . A h ighpass But te rwor th f i l te r was f ir s t used to remove the mear i depth

of every i sotherm af te r wh ich, > 6 cp d mot ions were then removed f rom the t ida l and synopt ic

band s ; and the poss ible e f fec ts of surface t ide on tempe ra ture var ia tions were removed as well Ap el

e t a l . , 199 7; Pr ingle , 19 99 ) . In coas ta l r egions away f rom f jords and es tuar ies , var ia t ions in sea

water densi ty a re de te rmined m ainly by temp era ture , so the e f fec t of sa l ini ty can b e neglec ted

Zh ao , 19 92 ) . Therefore , the resul t ing isotherms w ere regarded as e leva t ions of the inte rna l waves

Sa gg io a nd I m be r ge r , 1998 ; Sm a l l e t a l . , 19 99 ) . I t shou ld be po in t ed out tha t the ve r ti c a l a x i s o f

Fig . 1 , extending downw ards , r epresents the depth from the s t il l water sur fac e . To be consis tent

wi th t radi tiona l def ini t ions , the u pward inte rna l wave displacemen ts f rom the i r mea n leve ls were tak-

en as posi t ive and vice versa . Ver t ica l displacements of the high pass- f i l te red 2 5 .0 - 17 .5 ~C iso-

therms w i th wave he ights of up to 5 m are shown in Fig . 2 .

4 ̂

2

~2

0 5 113 15 20 25 30 35 40 45

T i me h )

F i g . 2 V e r t i c a l d i s p l a c e m e n t s o f h i g h p a s s - f i l t e r e d is o t h e r m s a t 0 . 5 ~ C i n t e r v a l s w i t h t h e l o w e s t i s o t h e r m a t 1 7 . 5 ~ C .

T h e c u r v e s w e r e s e q u e n t i a l l y o f f se t b y 2 . 5 m

STATISTICS

The s tandard devia t ion, and skewness and kur tos is coef f ic ients of each i sotherm are given in

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No. I WA NG et al. : STATISTICAL ROPER TIES OF INTERNA LWAVES 19

Ta b le 1 . The m e a n de p th o f t he i so the r m be f o re t he h ighpa s s f i l te r ing i s show n in t he t a b l e a s w e l l .

The in t e r na l w a ve e le va t ion m a y be r e ga r de d a s a c om bina t ion o f a s t a t i c o r t im e - inva r i a n t c om -

pone n t a nd a dyna m ic o r f l uc tua t ing c om pon e n t . The s t a t i c c om pone n t i s de sc r ibe d by the m e a n va l -

ue 7 /, a nd the dy na m ic c om pone n t by the s t a nda r d de v ia t i on 6 . The inc r e a se o f ~ w a s 3 . 2 1 m w he n

T d e c r e as e d f ro m 2 5 . 0 ~ t o 1 7 . 5 ~ T h e m e a n g r a d ie n t w a s - 0 . 4 3 m / ~ T h e s t a n d a rd d e v i a -

t i o ns o f t h e 2 5 . 0 ~ - 1 7 . 5 ~ i s ot h er m s r a n g e d f r om 0 . 4 3 m t o 0 . 5 8 m , i n d i c at i n g t h a t t h e w a v e

e ne r gy d id no t va r y g r e a t ly w i th de p th .

Al tho ugh s tandard de via t ions of d i f fe rent i so the rm s va r ied l i t t le , the skew ness coef f ic ients va r -

i e d s ign i f ic a n t ly f r om - 0 . 0 6 to - 0 . 5 0 . I t c a n be exp la ine d tha t t he w a ve e ne r gy w a s no t un i f o r m ly

d i s t r ibu t e d a long t im e a nd c on c e n t r a t e d on the l a r ge - a m p l i t ude w a ve s be tw e e n 3 h a nd 8 h a nd those

be tw e e n 28 h a n d 33 h . The i r ve r t ic a l l y a sym m e t r i c w a ve fo r m s de t e r m ine d m a in ly t he ske w ne ss va l -

ue o f t he c o r r e spond ing i so the r m F i g . 2 ) . The ne ga t ive ske w ne ss va lue s sugge s t e d t ha t t he c re s t

w i th pos i t i ve d i sp l a c e m e n t ) he igh t s o f t he i n t e rna l w a ve s w e re le s s tha n the i r t r ough w i th ne ga t ive

d i sp l a c e m e n t ) de p th s , t ha t i s t o s a y , t he t r oughs w e r e sha r p a nd de e p w he r e a s t he c r e s t s w e r e r a the r

low a nd b r oa d .

F u r the r m or e , t he non - z e r o ske w ne ss va lue s i nd i c a t e d t ha t t he i n t e rna l w a ve s i n t he Q ingda o of f -

sho r e a r e a w e r e non - G a uss i a n , s i nc e a G a uss i a n no r m a l ) r a ndom p r oc e s s ha s ~3 = 0 . The l ine a r

the o r y o f r a ndom se a i n t e r na l w a ve s w a s ba se d on the supe r pos i ti on o f in f in i te s im a l a m p l i t ude w a ve

c om pone n t s w i th un i f o rm ly a nd r a ndom ly d i s t r ibu t e d pha se a ng le s . H ow e ve r , i f t he p r oc e s s is non -

l i ne a r , som e o f t he pha se a ng le s a r e no t i nde p e nde n t , bu t ha ve a f ixe d r e l a t ionsh ip t o e a c h o the r .

The in t e r na l w a ve c om pone n t s be c om e c oup le d to e a c h o the r a nd non l ine a r c om pone n t s a r e p r e se n t a s

ind i c a t e d by the ske w e d w a ve sha pe s .

T a b l e S t a t i s ti c s o f i n t e r n a l w a v e s in Qingdao o f f sh o r e a r e a o f t h e Y e l l o w Sea

T oC ) ~ m) a m ) 3,3 X4

25.0 11 .84 0 .46 - 0 .36 4 .12

24 .5 12 .11 0 .45 - 0 .37 4 .33

24 .0 12 .33 0 .44 - 0 .25 4 .27

2 3 .5 1 2 .5 1 0.4 3 - 0 .2 5 4 .4 4

23 .0 12 .68 0 .44 - 0 .30 4 .55

22 .5 12 .82 0 .44 - 0 .34 4 .66

22 .0 12 .95 0 .45 - 0 .39 4 .83

21 .5 13 .07 0 .45 - 0 .44 5 .0 l

21 .0 13 .19 0 .46 - 0 .46 5 .05

20 .5 13 .30 0 .46 - 0 .48 5 .0 l

20 .0 13 .43 0 .47 - 0 .50 4 .92

19 .5 13 .56 0 .48 - 0 .50 4 .85

19 .0 13 .72 0 .50 - 0 .46 4 .71

18 .5 13 .91 0 .52 - 0 .38 4 .78

18 .0 14 .24 0 .58 - 0 .25 5 .29

17 .5 15 .05 0 .54 - 0 .06 3 .24

The ku r to s i s c oe f f i c ie n t s o f t he i n t e r na l w a ves r a nge d fr om 3 . 2 4 to 5 . 2 9 , g r e a t e r t ha n the no r -

r e al va lue 3 , a nd w a s a no the r i nd i c a t i on o f t he non - G a uss i a n i ty o f t he w a v e s . I t is e xpe c t e d t ha t t he

s t a ti s t ic a l d i s t r i bu t ions o f t he i n t e r na l w a ve s ha ve h igh e r t ha n no r m a l pe a ks .

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20 CH IN. J . O CEANO L. LIMNO L. , 20 (1 ) , 2002 Vol . 20

P R O B A B I L I T Y D I ST R I B U T IO N S

T h e p r o b a b i l i ty d i s t r ib u t i o n s o f t h e i s o t h e r m d i s p l a c e m e n t s , n o r m a l i z e d w i t h r e s p e c t t o z , a r e

c o m p a r e d w i t h t h e c o r r e s p o n d i n g G a u s s i a n d is t r i b u t io n s in F i g . 3 .

0 4

0 2

0

2 4 5 ~

C

C

0

6

(

]

2 4 O ~

2

C

4 6 f

0 2

O

C

C

C

0 .6

o

0 4

22 C

- 4 -2 0 2 4

C

4 2 0 2

C

4 2 0 2

q / a

1 7 5 ~

4 4 2 0 2

Fig. 3 Com parisonbetw een internal w av e (histogram ) and no rm al (solid fine) distributions

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d i s t r ib u t i o n o f t h e 1 7 . 5 ~ i s o t h e r m is s l ig h t l y d e v i a t e d f ro m t h e n o r m a l , a n d t h e d e v i a t i o n s b e c o m e

s i g n if i ca n t f o r t h e 2 0 . 0 a n d 1 9 . 5 i s o t h e r m s . M o r e o v e r , p e a k s o f t h e w a v e d i s p l a ce m e n t d i st r i-

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s a t i o n . T h e r e s u l t s w e r e c o n s i s t e n t w i t h th o s e f r o m a n a l y s i s o f t h e i n t e r n a l w a v e s t a t i s t i c s .

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t h a t t h e k u r t o s i s c o e f f i c i e n t s w e r e gr e at er t h a n 3 , i n d i c a t e d s k e w e d w a v e s h a p e s a n d n o n - G a u s s i a n

d i s t r i b u t i o n s o f t h e i n t e r n a l w a v e e l e v a t i o n s .

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