Bering Sea Bathymetry

Transcription

1 Bering Sea Bathymetry Ice coverage - southeast Bering Sea shelf, See Stabeno et al, 2007 Bering Strait Cold/Cool Period Gulf of Anadyr St. Lawrence Norton Sound Kamchatka Shirshov Ridge Aleutian Bowers Ridge Near Bowers Strait Amchitka St. Matthew Nunivak Pribilof UnAlaska Dutch Harbor Unimak Amukta Decreased ice cover since late 1970s Very little or no ice in early 2000s Return of cold years since ~ 2006 More Variable Regime shift Map courtesy of NASA Cold/Cool Period Courtesy of Sigrid Salo, PMEL Regime shift? Seasonal ice Cover Warm years, and cold years Grebmeier et al Stabeno et al, 2012 DSR 1

2 Long term change in Sea Ice cover? Red = observed Yellow = mean 23 IPCC Blue = mean 4 good IPCC Gray = ensemble runs from 4 good IPCC models = Some reduction in long term, but lots of variability = Hard to predict any year = Not well related to Arctic change Modeled Averaged Jan-March Ice Advection (m/month) Modeled Averaged Jan-March Ice Production (m/month) What determines ice? - cold wind from north - ice advection - ocean flow from south - warmer preceding summer (Stabeno et al., 2007) Stabeno et al, 2012 DSR Model results: Zhang et al, 2010 JPO; Observations: Pease, 1980 Main Currents in the Bering Sea BERING STRAIT THROUGHFLOW Aleutian es a porous boundary for fluxes of water, heat, salt and nutrients Stabeno et al, 2005 ACC~ 2 Sv Shallow Fresh Seasonal If wider than R, can have 2-way flow SAMALGA???? Stabeno et al, 1999 KAMCHATKA ~ 12 Sv (5-15) South but northward deep flow to Bering Sea Deep Water NEAR AMCHITKA BULDIR > 10 Sv (6-12)???? Net ~ 2-3 Sv North North (-4 to + 4 Sv) TOO DEEP TO MIX NUTRIENTS SEGUAM AMUKTA TANGA ~0.4 Sv ~ 4 Sv???? North North GOOD TO MIX NUTRIENTS AKUTAN ~ 0.1 Sv North UNIMAK ~ 0.3 Sv North TOO SHALLOW TO MIX NUTRIENTS 2

3 The Pacific Arctic Gateway The Bering Sea/Bering Strait Relationship Why is Bering Strait important? Bering and Chukchi - indicator of hydrography and change Arctic - Most nutrient-rich, - 1/3 rd of freshwater, - source of heat and halocline waters Globally - influences Atlantic overturning circulation Bering Strait Basics Northward flowing, daily/weekly flow reversals Large seasonal cycles in input to Arctic Ocean, including equilibrium depth Interannual variability, suggesting freshening and warming in recent years ICE 4 ANSF= Aleutian North Slope Current BSC = Bering Slope Current Anadyr waters (colder, saltier, nutrient-rich) Bering Shelf Waters (in between!) Exit route! From Stabeno, Schumacher & Ohtani, 1999 Alaska(n) Coastal Current (warm, fresh, seasonal) By providing an exit, Bering Strait influences flow over the Bering Sea Shelf in volume, BS throughflow can drain BS shelf in ~ 1 year (although the deep Bering Sea may not care) Anadyr and Shpanberg Straits CSR, 1993 et al., JGR, year mean flow field from a model Clement et al, 2005, DSR Generally Northward? Must be comparable to Bering Strait Anadyr Stronger Summer and Winter modes different Strongly related to wind - thus highly variable NOT WELL MEASURED what about models??.. but really need data to verify it (NB simulation of Bering Strait flow ok in some senses, not ok in others) 3

4 Compared to observations? (not many observations) St Lawrence Island (and its polynya) Overland et al, 1996, JGR - drogue-less buoy (thus wind and current driven) - 2 months from Cape Navarin to Bering Strait Weingartner et al, Website Bering Sea Brine NOT mostly from polynyas BUT important habitat?? Danielson et al, JGR, 2006 Bering Slope Current Evidence from drifter tracks, JPO, 1975 Stabeno and Reed, 1994, JPO = could have 2 modes, stable and unstable? Johnson et al, JPO, 2004 Herman et al, 2002 adapted from Stabeno et al, 2001 = fed from Aleutian North Slope Current = 3-5 Sv?? = initially follows slope, then appears to leave it = meandering with eddies = upwelled waters/eddies for productivity Very tricky to get something meaningful if - eddies are important (THEY ARE!) - barotropic is important (IT IS!) 4

5 Bering Sea Greenbelt 18 th May th June th June th May th June 2001 SeaWiFs false color image of Chlorophyll-A 23 rd May th June 2001 Upwelling Eddies Shumacher and Stabeno, Okkonen et al, DSR, 2004 Regimes and Fronts of the Bering Sea Shelf COASTAL DOMAIN - inshore of 50m depth contour summer = well mixed/weakly stratified Inner or Structural Front (5-30km wide) MIDDLE SHELF DOMAIN - between 50m and 100m depth contours summer = strongly stratified 2 layer - top wind mixed - bottom tidal mixed T diff > 8 deg, density set by T winter = fully mixed to 100m Mean Circulation ~ 5 cm/s Tidal circulation ~ 30 cm/s Rectification and trapping Tides important! e.g. Kowalik and Stabeno, 1999, JGR = Mix bottom 40m = x-slope transport? Middle Transition Zone (~ 50 km wide) Best Science Plan, from Kachel **Schumacher and Stabeno 1998** Stabeno et al, 2001 Kachel et al, 2002 Hunt et al, 2002 OUTER SHELF DOMAIN - between 100m and 200m depth contours summer = 3 layers - top well mixed (wind) - middle increasing density - bottom well mixed (tides) Slope Front SLOPE WATERS - in water depths > 200m over the slope 5

6 Tides over the whole shelf Many SST fronts Belkin and Cornillon, 2005 = complex = fronts align with topography = mostly over shelf The Cold Pool Stabeno et al, 2002 WINTER - ice melt - ice formation SPRING/SUMMER - surface warming - stratification Northern, Intermediate & Southern Shelves Cold Pool M4 M2 Freshwater Pool (unmixed ice melt) Best Science Plan, from Kachel COLD POOL =Cold (< 2 deg C) Water at depth, covered by warmer surface waters = Variable year to year = Important for fish Midshelf advection weak, but stratified insulates bottom from surface. Advection more important in south? N Bloom subsurface S Bloom surface (tides weaker in N, so thinner bottom mixed layer, thicker pycnocline) Bottom layer nutrient rich 1 st -5 th Sept 2008 Stabeno et al, 2012 DSR 6

7 Warm and Cold Regimes Stabeno et al, 2012 DSR; (also Overland et al, 2012) Used to flip between regimes Recently, run of warm or cold years (increased ecosystem stress) Stabeno et al, 2012 Warm years: = less ice = variable currents, with northward flow Dec-Feb, rest weak = lack of large copepods and euphausiids = poor cod and pollock = fewer whales Cold = ice related bloom early, then nothing till fall Cold years: - more ice - westward currents - rebound of large copepods, euphausiids, cod, pollock No obvious match - vertical stratification (both layers warmer/colder) (NB here T stratifies more than S) - crustaceans, flounder Stabeno et al, 2012 Warm = warm water bloom late then nothing till fall Stabeno et al,

8 WARM PHASE COLD PHASE Typical wintertime Sea Surface Temperature (colors), Sea Level Pressure (contours) and surface windstress (arrows) anomaly patterns during warm and cool phases of PDO PDO Pacific Decadal Oscillation Mantua et al, 1997 SST = PC1(t)* EOF1(x,y) + PC2(t)* EOF2(x,y) +.. PDO Victoria Pattern.. recently also need Victoria Pattern Bond et al, 2003 PDO important = different pattern of SST variability leading principal component of North Pacific monthly sea surface temperature variability (poleward of 20N for the period) VP important = now second EOF more important Bering Sea as an oceanographer would see it Kamchatka Shirshov Ridge Near Strait Bowers - polynyas? Gulf of Anadyr Bowers Ridge COASTS - riverine input? Aleutian DEEP BASIN ~ 3500m - nutrient source? Amchitka St. Matthew PACIFIC INPUT through a leaky Ridge OUTFLOW TO ARCTIC Bering Strait St. Lawrence Pribilof Amukta Norton Sound SHALLOW SHELF < 200m - atmosphere driven Nunivak - mixing - canyons for upwelling UnAlaska Dutch Harbor Unimak Map courtesy of NASA Sea-ice - Univ. Illinois Productive Sub-Arctic Sea - fishing - communities - ecosystem Eider Ducks, Bering Sea - USGS Bering Crabbing - C.Arnold, 2006 Nutrient Cycling Upwelling and Eddies Polynyas Tidal/Wind mixing over shelf NASA Visible Earth Green belt Cold Pool Climate Change? SST patterns - Pacific Decadal Oscillation - Victoria Pattern 8