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White  Paper:  Upper  ocean  layer  impacts  Arctic  sea  ice     Polyakov  et  al.   April  2015  

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Upper  ocean  as  a  regulator  of  atmospheric  and  oceanic  heat  transports  to  the  sea  ice  in  the  Eurasian  Basin  of  the  Arctic  Ocean  

Executive  Summary  

Summary  Processes  that  redistribute  heat  in  the  Arctic  Ocean  are  expected  to  play  an  increasing  role  in  changes  in  sea  ice  cover  as  summer  ice  extent  and  overall  ice  volume  decrease.    Our  existing  conceptual  understanding  of  ocean  heat  flux  processes  and  ocean-­‐ice  coupling  is  inadequate  for  quantifying  the  pathways  for  heat  within  the  modern  Arctic  system.    We  outline  a  research  strategy  to  reduce  uncertainty  in  fluxes,  including  atmosphere-­‐ice-­‐ocean  feedbacks,  to  the  level  required  to  predict  the  transition  of  sea  ice  state  through  projected  changes  in  global  climate.  Our  focus  is  on  the  Eurasian  Basin  (EB),  eastern  Arctic,  which  experiences  multiple  energetic  heat  flux  processes  that  are  distinct  from  the  better-­‐sampled  western  Arctic.  

Eurasian  Basin  regional  focus  

The  principal  sources  of  heat  content  change  in  the  upper  Arctic  Ocean  are  atmospheric  surface  fluxes,  Atlantic  Water  (AW)  and  Pacific  Water  (PW)  inflows,  and  warm  freshwater  from  river  inputs  in  summer.  The  relative  contributions  of  each  differ  between  the  western  and  eastern  Arctic.  Many  factors  including  distinct  regional  stratification  and  circulation  lead  to  large  differences  in  likely  dominant  mechanisms  for  delivering  ocean  heat  to  the  ice  base.  The  balance  of  mechanisms  in  each  region  will  change  as  global  climate  affects  each  of  these  ocean  heat  sources.  

Measured  spatial  variability  of  ocean  heat  content  identifies  the  EB  as  the  region  of  largest  heat  fluxes  from  the  ocean  interior.  AW  heat  content  declines  rapidly  as  the  AW  flows  as  a  boundary  current  along  the  continental  slope  around  the  EB.  The  large  seasonal  pulse  of  warm  freshwater  input  from  the  large  Russian  Arctic  rivers  impacts  stability  over  the  broad  eastern  Arctic  shelf  seas.  Sea  ice  is  predominantly  first-­‐year,  with  a  long  ice-­‐free  season  over  much  of  the  region.  However,  relative  to  the  more  intensively  sampled  western  Arctic,  there  have  been  few  direct  measurements  of  ocean  processes  in  the  EB  that  would  explain  the  inferred  ocean  heat  fluxes.  Our  research  strategy  addresses  this  data  sparseness  by  focusing  on  measurements  within  the  EB.  

Dominant  sources  of  uncertainty  in  ocean  heat  flux  in  the  EB  

The  following  processes  and  unique  EB  features  are  primary  ocean  sources  of  uncertainty  in  understanding  and  modeling  the  current  and  future  Arctic  Ocean  and  sea-­‐ice  state.  

• Mechanisms  for  subsurface  storage  of  summer  insolation  and  sensible  heat  through  open  water  and  thin  ice  to  delay  subsequent  freeze-­‐up  in  fall  and  winter.  

• Size  distributions  of  ice  floes  and  leads  as  controls  on  partitioning  of  heat,  freshwater  and  momentum  exchanges  at  the  ocean  surface  through  development  of  secondary  circulations.  

• The  EB  Cold  Halocline  Layer  (CHL)  should  be  an  effective  barrier  to  vertical  heat  flux  into  the  surface  mixed  layer  (SML)  from  below  but  is  observed  to  be  porous.  

• Upward  fluxes  in  the  pycnocline  above  the  AW  layer  in  the  EB  due  to  double  diffusion  is  predicted  to  be  roughly  an  order  of  magnitude  larger  than  in  the  western  Arctic;  however,  flux  estimates  rely  on  lab-­‐based  parameterizations  that  do  not  consider  interactions  of  DD  with  external  sources  of  shear.  

White  Paper:  Upper  ocean  layer  impacts  Arctic  sea  ice     Polyakov  et  al.   April  2015  

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• Barotropic  and  baroclinic  tides  along  the  path  of  the  AW  boundary  current  around  the  EB  cause  energetic  mixing  at  the  seabed,  sea-­‐ice  base,  and  in  the  pycnocline;  however,  distribution  of  mixing  is  poorly  mapped  and  is  sensitive  to  poorly  known  benthic  and  surface  boundary  layer  structure.  

• The  energy  of  wind-­‐forced  inertial  waves  is  sensitive  to  SML  depth  and  density  contrast  relative  to  the  deep  ocean,  and  to  sea-­‐ice  state.    

• Many  of  the  above  processes  are  sensitive  to  SML  properties  and  ice  dynamics,  leading  to  complex  atmosphere-­‐ice-­‐ocean  feedbacks  as  each  medium  affects  fluxes  of  heat,  freshwater  and  momentum  within  the  coupled  Arctic  system.  

Recommended  research  actions  

• Detailed  process  studies  using  drifting  ice  camps  in  the  EB  for  different  seasons:  Late-­‐summer  and  late-­‐winter  campaigns  targeting  the  unique  mechanisms  of  oceanic  heat  exchange  through  the  CHL  and  surface  mixed  layer  in  the  eastern  EB  (Fig.  1).  

• Improve  ocean  monitoring:  required  at  critical  locations  within  the  EB  where  major  water  mass  transports  and  transformations  take  place.  These  sites  include  the  outflow  from  the  St.  Anna  Trough  (where  the  Barents  Sea  branch  of  AW  meets  the  Fram  Strait  branch),  and  sites  in  the  central  deep  basins  to  monitor  changes  in  processes  influencing  upward  heat  fluxes  from  the  spreading  AW  layer.  

• Develop  new  technologies  and  interdisciplinary  programs:    Synergistic  combinations  of  different  types  of  observations  and  technologies  (e.g.,  microstructure  vertical  profiles  coordinated  with  spatial  surveys  using  autonomous  underwater  vehicles,  multidisciplinary  buoys,  and  high-­‐resolution  aircraft  and  satellite  observations)  is  essential  to  avoid  potential  ambiguity  in  data  analyses.    

• Coordinate  US  activities  with  existing  Arctic  Observational  Network  (AON)  and  international  programs:  Link  the  EB  observations  with  another  elements  of  the  international  AON,  thus  providing  the  large-­‐scale  spatial  and  long-­‐term  temporal  coverage  required  for  optimized  interpretation  of  new  data  sets.  

• Integrate  observations  with  models:  Use  existing  high-­‐resolution  pan-­‐Arctic  models  to  optimize  fieldwork  sampling  strategies,  and  prioritize  fieldwork  sampling  towards  reducing  uncertainty  from  largest  modeled  sources  of  error  in  explicit  and  parameterized  heat  fluxes.  Develop  new  process  models  aimed  at  better  understanding  and  more  accurate  parameterization  of  sub-­‐grid-­‐scale  heat  fluxes  in  pan-­‐Arctic  and  larger-­‐domain  ocean  and  coupled  climate  models.  

 Figure   1.   A   suite   of   coordinated  late   summer   –   early   fall  (September-­‐October)   and   winter  (March-­‐April)  field  campaigns  that  would  contribute   to  developing  a  comprehensive,   quantitative  understanding   of   heat   transports  within   the   upper   Eurasian   Basin.  Circulation   of   the   surface   water  and   intermediate   Atlantic   Water  of   the   Arctic   Ocean   is   shown   by  blue  and  red  arrows,  respectively.