An Arctic Expert's View of the Great Ice Melt of 2012

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James Morison of the University of Washington has been studying Arctic sea ice for decades and runs an annual expedition to the North Pole to drop instruments through the ice into the ocean below. Here he lays out one scenario for the forces driving recent dramatic changes in summer sea ice on the Arctic Ocean.
Read more from Morison on Arctic change here: http://j.mp/dotmorison
And more on sea ice in a warming climate here: http://j.mp/DotArcIce
Here's the Web home for the North Pole Environmental Observatory: http://psc.apl.washington.edu/northpole/

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An Arctic Expert's View of the Great Ice Melt of 2012

  1. 1. A  Highly  Specula/ve  Discussion  of  Bathymetry-­‐Ocean  Controls  on  Ice   Extent   Jamie  Morison   8/29/2012     Acknowledgments:  Florence  FeMerer  for  the  MASIE  image   in  kmz  format,  Son  Ngheim  for  first  poin/ng  out  the  idea.  
  2. 2. Claim:  At  minimum  ice  extent,  the  ice  edge  mainly  corresponds  the  deep-­‐ water  /  shallow-­‐water  boundary  (e.g.,  500  m  isobath.  The  sense  of   bathymetry-­‐ocean  controls  on  ice  edge  posi/on  come  in  two  flavors.  MASIE  Ice  Extent  for  Aug  28,  2012  Overlaid  in  Semi-­‐transparent  White  on  Google  Bathymetry  
  3. 3. Ice  Survives  in  the  Shallower  Region:  The  prime  example  is  the  East  Greenland  shelf,  which  almost  always  has  a   sea  ice  cover  and    where  the  the  ice  edge  almost  always  lies  along  the  500  m  isobath.  The  reason  for  this  is  that   Arc/c  Surface  Water  near  the  freezing  point  flows  out  of  the  Arc/c  Ocean  over  the  East  Greenland  con/nental   slope  and  thermodynamically  buffers  the  oulowing  sea  ice  against  the  effects  of  the  atmosphere.  The  front   associated  with  the  East  Greenland  Current  lies  roughly  over  the  shelf  break,  and  any  ice  that  dri]s  east  across   this  front  encounters  much  warmer  water  of  the  Greenland  Sea  cyclonic  gyre  circula/on  and  rapidly  melts.  We   saw  this  up  close  in  the  MIZEX  cruises  of  the  early  1980s  and  it  looks  like  it  s/ll  happens  today.   Greenland   Sea  Gyre  MASIE  Ice  Extent  for  Aug  28,  2012  Overlaid  in  Semi-­‐transparent  White  on  Google  Bathymetry  
  4. 4. Ice  Survives  in  the  Deeper  Region:  This  is  shown  along  most  of  the  Eurasian  con/nental  shelf  break.  Here  the  inflow  of   warm  Atlan/c  Water  via  the  Barents  Sea  Branch  and  Eurasian  runoff  input  heat  to  the  ocean  over  the  shelf.  This  and  the   fact  that  radia/ve  hea/ng  is  trapped  in  the  shallow  water  provides  a  thermodynamic  environment  that  is  rela/vely   hos/le  to  the  overlying  sea  ice    compared  to  deeper  water  off  the  shelf,  which  has  the  chance  to  develop  a  freezing   point  mixed  layer  and  even  a  cold  halocline  that  provide  an  ice  bath  buffer  for  the  sea  ice.  Consequently  the  ice   disappears  first  off  the  con/nental  shelves  and  only  disappears  from  the  deep  water  areas  if  there  is  a  major   disturbance  in  the  upper  ocean.  The  examples  of  this  are  where  we  are  seeing  important  ice  losses.   War m  Ba rent s  Sea  Bra nch   Greenland   Sea  Gyre  MASIE  Ice  Extent  for  Aug  28,  2012  Overlaid  in  Semi-­‐transparent  White  on  Google  Bathymetry  
  5. 5. Example  1  off  the  Laptev  Sea:  The  ice  edge  has  retreated  away  from  the  Laptev  Sea  shelf  break  and  we  even   see  reduced  ice  out  over  the  eastern  Amundsen  Basin.    This  is  specula/ve,  but  over  the  past  couple  of  decades   cyclonic  circula/on  aligned  along  the  Russian  shelf  break  has  tended  to  shi]  Eurasian  runoff    eastward  to   Canada  Basin,  which  tends  to  weaken  the  cold  halocline  layer  that  insulates  sea  ice  from  Atlan/c  Water  heat  in   the  Eurasian  Basin.  Hence  ice  disappears  more  easily  over  deep  water.   War m  Ba rent s  Sea  Bra nch   Greenland   Sea  Gyre  MASIE  Ice  Extent  for  Aug  28,  2012  Overlaid  in  Semi-­‐transparent  White  on  Google  Bathymetry  
  6. 6. Example  2,  Beaufort  Sea:    As  recently  as  a  couple  of  years  ago,  major  chunks  of  the  ice  edge  would  align  with   the  Beaufort  shelf  break.  This  year’s  storm  event  separated  the  ice  into  two  divergent  chunks,  one  ice  edge   moving  north  and  one  south.  The  southern  por/ons  melted  most  quickly  over  the  shallowest  regions  of  the   Chukchi  shelf,  likely  for  the  reasons  men/oned  earlier  with  respect  to  the  typical  “ice-­‐remains-­‐over-­‐deep-­‐ water”  case.  It  is  noteworthy  that  some  of  this  ice  remains  over  the  deeper  parts  of  the  shelf  associated  with   the  bathymetry  around  Herald  Canyon  (unfortunately  the  google  bathymetry  doesn’t  resolve  this  bathymetry).   War m  Ba rent s  Sea  Bra nch   Greenland   Sea  Gyre  MASIE  Ice  Extent  for  Aug  28,  2012  Overlaid  in  Semi-­‐transparent  White  on  Google  Bathymetry  
  7. 7. Implica/ons  for  Future  Ice  Extent:  I  think  that  in  the  past  even  under  warm  atmospheric  condi/ons,  the  ocean  has   provided  refuge  areas  for  summer  sea  ice.  Major  reduc/ons  of  ice  extent  into  these  refuge  areas  s/ll  require  special   circumstances.  For  the  sake  of  argument,  I’ll  speculate  that  we  won’t  have  an  ice  free  Arc/c  Ocean  un/l  winter  freezing   is  radically  decreased  or  the  Arc/c  Ocean  circula/on  changes  significantly.  As  long  as  freezing  occurs  in  the  winter,  the   ocean  will  tend  to  build  up  a  buffer  that  can  see  major  areas  of  sea  ice  through  the  summer.  Substan/al  increases  in   ocean  heat  fluxes  through  Fram  and  Bering  straits  could  change  this.  More  subtle,  if  changes  in  atmospheric  circula/on   cause  changes  in  upper  ocean  circula/on,  structure,  and  mixing  that  allow  more  ocean  heat  to  get  to  the  ice  (e.g.,   weakening  of  the  cold  halocline)  the  ocean  might  lose  this  buffering  capacity  and    summer  sea  ice  will  mostly  disappear.    MASIE  Ice  Extent  for  Aug  28,  2012  Overlaid  in  Semi-­‐transparent  White  on  Google  Bathymetry  

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