Submesoscale	  stirring	  of	  the	  Kuroshio	  in	  the	  East	  China	  Sea	  	  Yusuke	  Uchiyama	  (Dept.	  Civil	  Eng.,	  Kobe	  University)	  Key	  words:	  JCOPE2-­‐ROMS	  downscaling,	  Kuroshio,	  submesoscale	  eddies,	  Ryukyu	  Islands	  	  Abstract	  the	  islands.	  	   	   A	  synoptic	  regional	  downscaling	  experiment	  on	  Kuroshio	  off	   Ryukyu	   Islands,	   Japan,	   exhibits	   the	   evident	  predominance	   of	   submesoscale	   anticyclonic	   eddies	   over	  cyclones	   in	   the	   narrow	   strip	   between	   Kuroshio	   and	   the	  islands	  (Uchiyama	  et	  al.,	  2013).	  In	  order	  to	  investigate	  the	  mechanism	   of	   the	   anticyclone	   dominance	   over	   cyclones	  with	   an	   attention	   to	   topographic	   effects	   of	   the	   islands,	   we	  carry	   out	   a	   detailed	   oceanic	   downscaling	   experiment	   in	   a	  double	   nested	   configuration	   forced	   by	   JCOPE2	   and	  GPV-­‐MSM	  with	  a	  horizontal	  resolution	  of	  1	  km	  (Fig.	  1).	   	   A	  to	   directly	   investigate	   the	   island	   topography	   effects	   around	  Figure	   1:	   Geographical	   configuration	   of	   the	   double	   nested	  JCOPE2-­‐ROMS	   downscaling	   model	   for	   the	   Ryukyu	   Islands	  fringing	  the	  East	  China	  Sea.	  

the	   Ryukyu	   Islands	   by	   removing	   all	   the	   island	   topography	  	  preliminary,	   accompanying	   experiment	   is	   then	   conducted	  above	   z	   >	   -­‐1000	   m.	   	   If	   the	   islands	   are	   removed,	   the	  submesoscale	   negative	   vorticity	   on	   the	   eastern	   side	   of	   the	  Kuroshio	   is	   markedly	   attenuated	   as	   compared	   to	   the	  baseline	  case	  with	  islands	  (Kamidaira	  et	  al.,	  2014).	  	   	   An	   energy	   conversion	   analysis	   relevant	   to	   eddy	   kinetic	  energy	   demonstrates	   that	   the	   dominance	   of	   negative	  vorticity	   between	   Kuroshio	   and	   the	   Ryukyu	   Islands	   is	  induced	   by	   a	   combination	   of	   the	   lateral	   shear	   instability	  and	  the	  baroclinic	  instability	  in	  the	  upper	  ocean,	  leading	  to	  eddy	   shedding	   on	   the	   shelf	   break	   and	   subsequent	  development	   of	   eddies	   confined	   in	   the	   narrow	   strip	  between	   the	   continental	   shelf	   slope	   of	   the	   East	   China	   Sea	  and	  the	  Okinawan	  ridge	  (Fig.	   2).	   	   We	  further	  examine	  the	  heat	   transport	   due	   to	   eddying	   currents	   and	   focus	   on	   the	  divergent	   component	   of	   the	   eddy	   heat	   flux	   (EHF).	   	   The	  diagnostic	  EHF	  analysis	  suggests	  that	  the	  dominance	  of	  the	  negative	   vorticity	   is	   substantial	   in	   emergence	   of	   more	  energetic	   submesoscale	   anti-­‐cyclonic	   eddies	   that	   lead	   to	  promoting	  lateral	  material	  exchange	  between	  Kuroshio	  and	  	  Figure	   2:	   Cross-­‐sectional	   plots	   of	   (a)	   barotropic	   conversion	  rate,	   Km Ke,	   (b)	   baroclinic	   conversion	   rate,	   PeKe,	   and	   (c)	   eddy	  kinetic	   energy,	   EKE,	   along	   the	   SSE-­‐NNW	   transect	   off	   the	  Okinawa	  main	  island	  normal	  to	  the	  Kuroshio.	  The	  abscissa	  is	  the	  distance	   from	   the	   main	   island,	   contours	   are	   the	   stream-­‐wise	  velocity	  in	  m/s	  of	  the	  Kuroshio.