Processes That Affect Stratification in Shelf Waters

I n t r o d u c t i o n S t r a t i f i c a t i o n o f s h e l f water s r e f l e c t s th e ba lanc e betwee n buoyan t f o r c e s t h a t r e s u l t fro m h e a t i n g an d c o o l i n g , e v a p o r a t i o n an d p r e c i p i t a t i o n , an d r u n o f f , an d m i x i n g f o r c e s suc h a s win d s t r e s s a t th e s u r f a c e an d c u r r e n t s t r e s s a t th e bottom . S inc e mos t o f thes e f o r c e s var y a t a v a r i e t y o f f r e q u e n c i e s fro m t i d a l t o s e a s o n a l , an d eve n g e o l o g i c a l f o r t h a t m a t t e r , we ca n expec t v a r i a t i o n s i n s t r a t i f i c a t i o n t o occu r a t s i m i l a r f r e q u e n c i e s . I n t h i s pape r we w i l l examin e s e a s o n a l v a r i a t i o n s i n observe d s t r a t i f i c a t i o n an d i n th e processe s t h a t a f f e c t s t r a t i f i c a t i o n . Mos t o f ou r d i s c u s s i o n w i l l b e base d o n dat a fro m th e s o u t h e a s t e r n U n i t e d S t a t e s c o n t i n e n t a l s h e l f , th e Sout h A t l a n t i c B i g h t (SAB ) ( F i g u r e 1) . Th e c y c l e o f s t r a t i f i c a t i o n i n th e SA B i s dramat i c becaus e o f th e extrem e s e a s o n a l an d l a t i t u d i n a l v a r i a t i o n s t h a t occu r t h e r e an d thu s i s a goo d l a b o r a t o r y f o r examinin g s t r a t i f i c a t i o n p r o c e s s e s . I n t h i s pape r we w i l l sho w t h a t w h i l e som e o f th e processe s t h a t a f f e c t s t r a t i f i c a t i o n ar e w e l l known, som e o f th e mos t importan t terms , suc h a s r u n o f f an d e v a p o r a t i o n , ar e d i f f i c u l t t o parame t e r i z e , an d thu s a t r u e d e t e r m i n a t i o n o f s e a s o n a l v a r i a t i o n s i n buoyanc y f l u x canno t b e determine d by summin g terms . We w i l l a l s o sho w t h a t th e a d v e c t i v e t r a n s p o r t o f hea t i n t o th e r e g i o n may a t t ime s excee d o t h e r form s o f buoyanc y f l u x . A v a i l a b l e oceanographi c da t a fro m th e SA B wer e use d b y A t k i n s o n e t a l . [1983 ] t o c r e a t e c h a r t s showin g mea n oceanographi c c o n d i t i o n s a t monthl y increments . Thes e da t a wer e use d t o c a l c u l a t e th e monthl y mea n bu l k s t r a t i f i c a t i o n f o r area s o f th e SAB d e f i n e d b y 1 ° o f l a t i t u d e an d th e 0, 20, 40 and 60m i s o b a t h s ( F i g u r e 2 ) . Th e mea n bul k s t r a t i f i c a t i o n i s th e s i m p l e d i f f e r e n c e betwee n near botto m an d n e a r s u r f a c e d e n s i t y . Dur in g th e w i n t e r , bu l k s t r a t i f i c a t i o n i s lo w becaus e o f h i g h win d s t r e s s , lo w r u n o f f , an d n e g a t i v e hea t f l u x . I n e a r l y s p r i n g , b u l k s t r a t i f i c a t i o n i n c r e a s e s f i r s t i n th e souther n SA B becaus e o f decrease d winds , i n c r e a s e d h e a t i n g , an d r u n o f f . I n th e summer, bu l k s t r a t i f i c a t i o n i n c r e a s e s throughou t th e SA B p a r t l y becaus e o f p o s i t i v e hea t f l u x an d decrease d win d s t r e s s bu t a l s o becaus e o f subsur f a c e G u l f Strea m i n t r u s i o n s . B y October , bu l k s t r a t i f i c a t i o n g e n e r a l l y decrease s becaus e o f i n c r e a s e d win d s t r e s s ; however , a n are a o f i n crease d bul k s t r a t i f i c a t i o n occur s o f f southeas t F l o r i d a becaus e o f southwar d a d v e c t i o n o f f r e s h e r c o a s t a l w a t e r s . Thes e mea n monthl y d i s t r i b u t i o n s l e a d t o s e v e r a l c o n j e c t u r e s r e g a r d i n g s t r a t i f i c a t i o n i n th e SAB: 1. S t r a t i f i c a t i o n i n th e s p r i n g an d summer i n c r e a s e s becaus e o f (1 ) i n c r e a s e d i n s o l a t i o n , (2 ) reduce d winds , an d (3 ) s u b s u r f a c e i n t r u s i o n s o f dense r G u l f Strea m water . 2. S t r a t i f i c a t i o n i n th e f a l l an d w i n t e r gen e r a l l y decrease s becaus e o f (1 ) decrease d i n s o l a t i o n an d (2 ) i n c r e a s e d winds . 3. S t r a t i f i c a t i o n ca n i n c r e a s e l o c a l l y becaus e o f th e t r a p p i n g o f l o w s a l i n i t y wate r a lon g th e c o a s t . S t r a t i f i c a t i o n i s th e ne t r e s u l t o f energ y i n p u t s c r e a t i n g buoyanc y f o r c e s p l u s mix in g f o r c e s t h a t d e s t r o y s t r a t i f i c a t i o n . We no w d i s c u s s th e v a r i o u s form s o f buoyan t energy . F l u x o f Buoyan t Energ y Buoyanc y r e p r e s e n t s a f o r c e t h a t tend s t o s t a b i l i z e th e wate r column . C o n s e r v a t i o n o f buoyancy , 117 Coastal and Estuarine Sciences Baroclinic Processes on Continental Shelves Vol. 3 Copyright 1986 by the American Geophysical Union