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<>/F 4/A<>/StructParent 3/Contents( A r c t i c O s c i l l a t i o n - i s t e c h n i c a l l y d e f i n e d a s t h e f i r s t e m p i r i c a l o r t h o g o n a l f u n c t i o n \( E O F \) o f s e a l e v e l p r e s s u r e . E O F i s a s t a t i s t i c a l t e c h n i q u e t o i d e n t i f y c o m m o n m o d e s o r p a t t e r n s i n a g i v e n d a t a s e t . T h e A O i s t h e m o s t c o m m o n m o d e o r p a t t e r n o f v a r i a b i l i t y i n t h e N o r t h e r n H e m i s p h e r e s e a l e v e l p r e s s u r e f i e l d . T h e A O i s f o u n d i n a l l m o n t h s o f t h e c a l e n d a r y e a r b u t i s m o s t a c t i v e / s t r o n g e s t i n t h e w i n t e r m o n t h s . E O F i s l i n e a r s t a t i s t i c a l t e c h n i q u e s o t h e i d e n t i f i e d p a t t e r n e x i s t s i n b o t h t h e n e g a t i v e o r p o s i t i v e p h a s e . T h e p o s i t i v e A O p h a s e i s c h a r a c t e r i z e d b y a l o w - p r e s s u r e c e n t e r a n o m a l y o v e r t h e e n t i r e A r c t i c b a s i n o r p o l a r c a p \( t h e r e g i o n p o l e w a r d o f 6 0 N \) a n d h i g h - p r e s s u r e c e n t e r a n o m a l i e s i n e a c h o c e a n b a s i n i n t h e m i d - l a t i t u d e s . T h e p o s i t i v e A O i s o f t e n a s s o c i a t e d w i t h c o l d w e a t h e r i n t h e A r c t i c b u t m i l d w e a t h e r a c r o s s t h e m i d - l a t i t u d e s i n c l u d i n g t h e E a s t e r n U n i t e d S t a t e s , E u r o p e a n d E a s t A s i a . T h e n e g a t i v e A O p h a s e i s c h a r a c t e r i z e d b y a h i g h - p r e s s u r e c e n t e r a n o m a l y o v e r t h e e n t i r e A r c t i c b a s i n o r p o l a r c a p a n d l o w - p r e s s u r e c e n t e r a n o m a l i e s i n e a c h o c e a n b a s i n i n t h e m i d - l a t i t u d e s . T h e n e g a t i v e A O i s o f t e n a s s o c i a t e d w i t h m i l d e r w e a t h e r i n t h e A r c t i c b u t s e v e r e w i n t e r w e a t h e r , i n c l u d i n g c o l d - a i r o r A r c t i c o u t b r e a k s a n d s n o w s t o r m s a c r o s s t h e m i d - l a t i t u d e s i n c l u d i n g t h e E a s t e r n U n i t e d S t a t e s , E u r o p e a n d E a s t A s i a . T h e A O c a n b e c o n s i d e r e d a n a t m o s p h e r i c m i x i n g i n d e x . D u r i n g t h e p o s i t i v e A O p h a s e t h e r e i s l i t t l e m i x i n g o f a t m o s p h e r i c m a s s e s b e t w e e n t h e A r c t i c a n d l o w e r l a t i t u d e s \( m i d - l a t i t u d e s a n d s u b t r o p i c s \) s o t h e c o l d a i r s t a y s b o t t l e d u p i n t h e A r c t i c a n d m i l d a i r d o m i n a t e s t h e m i d - l a t i t u d e s . D u r i n g t h e n e g a t i v e A O p h a s e t h e r e i s i n c r e a s e d m i x i n g o f a t m o s p h e r i c m a s s e s b e t w e e n t h e A r c t i c a n d l o w e r l a t i t u d e s \( m i d - l a t i t u d e s a n d s u b t r o p i c s \) s o t h e c o l d a i r i s r e l e a s e f r o m t h e A r c t i c a n d m i l d a i r i n t h e m i d - l a t i t u d e s a n d s u b t r o p i c s f l o w s m o r e e a s i l y i n t o t h e A r c t i c r e g i o n .) >>
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