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T H E F U T U R E O F O C E A N O G R A P H Y

NUM ERICAL SIM ULATION OF ARC TICSE A-ICE AND O C E A N CIRCULATIONBy David M. Hol land

R R E N T L Y T HE RE IS c o n s i d e r a b l e i n -in improving our understanding

Our understanding of the interac-

The Arctic Ocean is essentially con-

the Arctic Ocean. The

D.M . Ho l l a nd , De p a r t me n t o f A t mo sp h e r i c a n d

sor: L awr ence A. Mysak).

rated by the Lomonosov Ridge, whichextends from Siberia across the ArcticOcean to Greenland. The waters of thebasin communicate with the PacificOcean through the narrow and shallowBering Strait and with the AtlanticOcean via the relatively wider anddeeper Fram Strait as well as the wideopening between Spitsbergen and Nor-way. There is also a flow from thewestern Arctic through the CanadianArctic Archipelago into Baffin Bay.Freshwater is received along the periph-ery of the basin from numerous rivers.The Arctic Ocean is well stratifiedbecause of the annual cycle of sea-icegrowth and melt and also because ofriver input. There is a strong pycnoclineat -300 m depth, which separates thecold, fresh surface waters from the rela-tively warm, salty deeper waters. Thegeneral circulation of the waters withinthis basin is not well known. It is as-sumed that the surface mixed layer isdragged along by the moving ice sur-face and thus follows an anticyclonicpath in the Canadian basin and thenexits the basin through Fram Strait. Thedeeper waters are thought to flow in theopposite (i.e., cyclonic) sense. Thedeeper waters may be strongly con-strained by topography and thus flowalong the contours of bottom topogra-phy.

The present modeling work was moti-vated in part by two previous modelingstudies of the Arctic Ocean, namely thoseof Hibler and Bryan (1987) and Semtner(1987). Both of these studies used a cou-pled sea-ice-ocean model with specifiedatmospheric forcing to simulate the gen-eral circulation of the Arctic sea-ice andocean. Hibler and Bryan were not able toinvestigate the deeper circulation becausethey had imposed an artificial damping o fthe salinity and temperature fields backto long-term averages (i.e., a relaxationconstraint) at all depths below the toplevel of their ocean model. Semtner

(1987) removed the relaxation constrainof Hibler and Bryan and simulated an anticyclonic circulation at all depths in thCanadian Basin while obtaining a cyclonic flow below 200 m depth in thEurasian Basin. This anticyclonic flowpattern in the Canadian Basin is inconsistent with observations that suggest a cyclonic flow in that basin.

The Oberhuber model used here is general circulation model based on thfollowing ideas: isopycnals are used aLagrangian vertical coordinates, a realistic equation of state is included, thprimitive equations together with thhydrostatic approximation are appliedand a surface mixed layer and a snowand sea-ice model are coupled to thinterior ocean. The sea-ice model incorporates both thermodynamics andynamics. A fundamental differencfrom other sea-ice models is that Oberhuber uses a different numericascheme (i.e., implicit) and also solvethe sea-ice equations written in spherical coordinates. The work presentedhere is the first application of the Oberhuber model to the Arctic Ocean; Oberhuber (1993) has previously applied thmodel to the North Atlantic.

The so-called Atlantic layer of theArctic Ocean is the result of warmsaline water penetrating into the ArctiOcean via the Fram Strait (and alsopartly from the Barents Sea). Thiswater is heavier than the surface Arctiwaters and subducts as it travels northof Fram Strait , occupy ing the watecolumn between 300 and 1,000 mbelow the surface. The model simulation of the velocities and temperaturesat a depth of 500 m are shown in Figure 1. The velocity pattern indicatesthat the waters there are basically constrained to follow the topography. Themodeled circulation consists of threeprincipal cyclonic gyres (see the numbers 1, 2, and 3 in Fig. 1), which simply mimic the pattern of the ocean

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t o p o g r a p h y . T h i s c i r c u l a t i o n c a n b ec o m p a r e d w i t h t h e c i r c u l a ti o n i n f e r r e df r o m e a r l i e r t e m p e r a t u r e a n d s a l i n i t ym e a s u r e m e n t s , w h i c h s h o w a s i n g leb a s i n - w i d e g y r e a s o p p o s e d t o t h em u l t i g y r e s t r u c t u r e s h o w n h e r e . H o w -e v e r , a n a n a l y s i s o f r e c e n t d a t a c o l -l e c t e d d u r i n g t h e 1 99 1 O d e n c r u i s es u g g e s t s t h e e x i s t e n c e o f a m u l t i g y r es t r u c t u r e ( R u d e l s e t a l . , 1 9 9 3 ) .

T h e m o d e l e d t e m p e r a t u r e s h o w s ap a t t e r n c o n s i s t e n t w i t h t h e c i r c u l a t i o n .I t s h o w s t h e w a r m , s a l i n e A t l a n t i cw a t e r e n t e r i n g v i a F r a m S t r a i t. T h et e m p e r a t u r e d e c r e a s e s a s t h e w a t e r c i r -c u l a t e s c y c l o n i c a l l y i n t h e E u r a s i a nB a s i n . T h i s c o o l i n g o f t h e A t l a n t i cl a y e r w a t e r i s d u e t o m i x i n g a n d d i f f u -s i o n o f t h e A t l a n t i c l a y e r w a t e r w i t ht h e w a t e r s a b o v e a n d b e l o w i t . S i m i -l a r l y , t h e t e m p e r a t u r e p a t t e r n s u g g e s t sa c y c l o n i c c i r c u l a t i o n i n t h e C a n a d i a nB a s i n . T h e A t l a n t i c l a y e r w a t e r e n t e r st h e C a n a d i a n B a s i n b y f lo w i n g o v e r

t he L o m o n o s o v R i d g e n e a r w h e r e t h er i d g e i n t e r s e c t s t h e c o n t i n e n t a l s l o p e .F u r t h e r m o r e , t h e m o d e l e d t e m p e r a t u r ei s c o n s i s t e n t w i t h t h e p a t t e r n o f t h e o b -s e r v e d f i e l d ; h o w e v e r , t h e a b s o l u t et e m p e r a t u r e s i n t h e s i m u l a t i o n a r ee v e r y w h e r e - 0 . 5 C t o o w a r m .

O v e r a l l , t h e s t u d y h a s v a l i d a t e d t h eu s e f u l n e s s o f t h e m o d e l f o r s i m u l a t i n gt h e g e n e r a l c i r c u l a t i o n o f th e s e a - i c ea n d o c e a n i n t h e A r c t i c . A l t h o u g h n o td i s c u s s e d h e r e , t h e s i m u l a t i o n o f t h es e a - i c e t h i c k n e s s , c o m p a c t n e s s , a n d v e -l o c i t i e s i s i n g o o d a g r e e m e n t w i t h o b -s e r v a t i o n s ( H o l l a n d , 1 9 9 3 ) . T h e r e a rel i m i t a t i o n s o f t h e m o d e l t h a t p r e v e n t ad i r e c t a p p l i c a t i o n o f t h e r e s u lt s p r e -s e n t e d h e r e t o t h a t o f t h e g l o b a l c l i m a t es y s t e m . F i r s t , a g l o b a l m o d e l d o m a i nw a s n o t u s e d ; r a t h e r , t h e s i m u l a t i o nw a s p e r f o r m e d u s i n g o n l y t he A r c t i cO c e a n a n d t h e N o r t h A t l a n t i c O c e a n . I nr e a l i t y , t h e A r c t i c O c e a n i s i n t r i c a t e l yc o n n e c t e d t o t h e g l o b a l o c e a n . V a r i a -

1 .1 1 . 2 1 . 3 1 , 4 1 . 5 1 . 6 1 . 7 1 . 8 1 . 9 2 . 0 2 . 1 2 . 2 2 . 3 2 . 4U N I T ! Ce, !# us

Fi g . 1 : S i mu l a t e d t e m p e r a t u r e a n d c i r c u l a t i o n o f th e A t l a n t i c l a y e r ( i. e. , d e p t h o f 5 0 0m) i n th e Ar c t i c Oc e a n Or b r J a n u a r y ) . Th e u n d e r l y i n g c o n t o u r s g i v e t h e t e mp e r a t u r ef i el d , w h i c h r a n g e s f r o m h o t t e m p e r a t u r e s i n e x c e s s o f 2 . 4 C i n F r a m S t r a i t t o c o l dt e mp e r a t u r e s l e s s th a n 1 C i n th e c e n t e r o f t h e Ca n a d i a n Ba s i n . Th e o v e r l y i n g v e c t o r si l lu s t r a te a mu l t i g y r e c y c l o n i c c ir c u l a t i o n w i t h ma x i m u m s p e e d s o f - 3 c m/ s . Th e s o l i dg r e e n c o l o r i n d i c a t e s l a n d . Th e t h i n wh i t e l i n e s r e p r e s e n t l in e s o f c o n s t a n t l o n g i t u d es p a n n i n g o u t . f r o m t h e No r t h Po l e , wh i c h i s l o c a t e d n e a r th e c e n t e r o f t h e ima g e . Th en u mb e r s ' 1, 2 , a n d 3 i n t h e f i g u r e s h o w t h e c e n t e r s o f t h r e e c y c l o n i c g y r e s .

t i o n s i n t h e g l o b a l t h e r m o h a l i n e c i r c ul a t i o n c o u l d h a v e i m p a c t s o n t h e A r c t iO c e a n a n d v i c e v e r s a . S e c o n d l y , m a j o r l i m i t a t i o n o f t h e m o d e l i s th a t c o n s i s t s o n l y o f a c o u p l e d i c e - o c e am o d e l w i t h t h e a t m o s p h e r e s p e c i f i e d . Ir e a l i t y , t h e p o l a r c l i m a t e s y s t e m i nc l u d e s t h r e e f r e e l y i n t e r a c t i n g s y s t e m sn a m e l y t h e a t m o s p h e r e , s e a - i c e , a no c e a n , w i t h v a r i o u s f e e d b a c k p r o c e s s e sO n l y t h e s e a - i c e a n d o c e a n a r e c o u p l et o g e t h e r h e re ; c o n s e q u e n t l y , t h e r e a rn o f e e d b a c k s o p e r a t i n g b e t w e e n t h e a tm o s p h e r e a n d t h e s e a - i c e o r o c e a n .

T h e r e s u l t s o f t h is w o r k , w h i c h a rf u r t h e r d e s c r i b e d i n H o l l a n d ( 1 9 9 3 )i n d i c a t e t h a t f u r t h e r s t u d i e s o f t hp o l a r r e g i o n s c a n b e p r o f i t a b l y c a r r i eo u t u s i n g t h e O b e r h u b e r m o d e l . F u t u rw o r k w i l l i n v o l v e s t u d y i n g i n t e r a n n u av a r i a b i l i t y o f t h e A r c t i c s e a - i c e c o v ea n d s i m u l a t i n g t h e g e n e r a l c i r c u l a t i o no f t h e A r c t i c O c e a n a n d i t s c o n n e c t i ot o t h e g l o b a l c i r c u l a t i o n . T h e m o d ew i l l l i k e w i s e b e u s e d t o i n v e s t i g a t e t hc i r c u l a t i o n i n t h e A n t a r c t i c t o m a kc o m p a r i s o n s w i t h o b s e r v a t i o n s a no t h e r m o d e l i n g s t u d i e s .Acknowledgments

T h i s p r o j e c t w a s c a r r i e d o u t w i t h t ha s s i s t an c e o f P ro f . L a w r e n c e A . M y s aa n d D r . J o s e f M . O b e r h u b e r . C o m p u t i nre s o u rc e s w e re p ro v id e d b y C ra y R e s e a rcI n c . a n d t h e A r c t i c R e g i o n S u p e r c o m p u tin g C e n te r o f t h e U n iv e r s i ty o f A la s k a .R e f e r e n c e sCa r m a c k , E .C . , 1 9 90 : L a r g e - s c a l e p h y s i c a l o c e a n

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O b e r h u b e r , J .M. , 1 9 93 : S i mu l a t i o n o f t h e A t l a n tic i r c u l a t i o n w i t h a c o u p l e d s e a i c e - mi x el a y e r - i s o p y c n a l g e n e r a l c i r c u l a t i o n mo d e lPar t I : model descr ip t ion . J. Phys. Oceanogr., 23, 8 0 8 - 8 2 9 .

Ru d e l s , B . , E .P . J o n e s , L .G . A n d e r s o n , a n d GKat tner , 1994 : On the o r ig in and c i rcu la t iono f t h e A t l a n t i c L a y e r a n d t h e i n t e r me d i a td e p t h w a t e r s o f t h e A r c t i c O c e a n . I n : ThR ol e o f t he P o l ar O c e ans i n Shap i ng t hGlobal Environment , R . M u e n c h a n d O .MJ o h a n n e s e n , e d s . Amer. Geophys. U n i o nW a s h i n g t o n D .C . I n P r e s s .

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