Lec 15: Artic/Southern Ocean

• Arctic Ocean


  Ocean surrounded by land

  
• Contribution of Arctic water masses for the World Ocean differs substantially from the Southern Ocean water masses

  
• Arctic Ocean geography

  • Broad continental shelves → comprise 70% of Arctic's surface area
  • Divided into the Canadian Basin and the Eurasian Basin
  • Basins separated by the Lomonosov Ridge
  • Communicates with Atlantic through Canadian Archipelago and Nordic Seas
  • Communicates with Pacific through Bering Strait

  
• Wind forcing

  High pressure in the vicinity of the north pole → Arctic under the influence of the Polar Easterlies → force anticyclonic surface circulation

  
• "Default" circulation

  1. Cyclonic circulation in the Nordic Seas and Eurasian Basin

  2. Anticyclonic circulation in the Canadian Basin (Beaufort Gyre)

  3. Transpolar Drift (TPD) → major cross-polar circulation between these two systems

  4. Inflows from the Nordic Seas and from the Bering Sea (Pacific)

  5. Outflows through Fram Strait to the Nordic Seas and through the Canadian Archipelago

  
• Thermohaline forcing


  Arctic is a mediterranean sea: has only limited communication with the major ocean basins and its circulation greatly influenced by thermohaline forcing

  
• Since precipitation over the Arctic exceeds evaporation, the region acts as a dilution basin for the Atlantic Ocean

  
• Three-layer Arctic

  • Arctic Surface Water
  • Atlantic Water
  • Arctic Deep/Bottom Water

  
• Arctic Surface Water

  • Occupies the depth range from the surface to 150 - 200 m
  • Has temperatures close to the freezing point
  • Salinity varies strongly
  • Surface layer has low salinity due to continental runoff and water from the North Pacific; seasonal melting of sea ice
  • Sub-surface layer has strong salinity gradient but uniform temperature (cold halocline)
  • Source of sub-surface waters is the cold and saline shelf water produced during winter (sea ice freezing)

  
• Atlantic Water


  • Occupies the depth range between about 150 m and 900 m
  • Enters the Arctic from the North Atlantic
  • Has the same salinity as Bottom Water but is much warmer
  • Warmer than Arctic Surface Water but denser due to high salinity

  
• Arctic Bottom Water


  • Mix of dense waters produced through deep convection and dense water from Arctic shelf
  • Densest water of the world ocean, but not found anywhere outside the Arctic region

  
• Numerous large rivers empty into the Arctic shelf
• Pacific water through Bering Strait is also relatively fresh
• Net of freshwater transport when sea ice forms in one place but melts in another
• Dense water from Arctic shelf

  Low salinity of shelf water, which reflects input of freshwater from rivers, facilitates ice formation. As salt is rejected from the ice, the salinity of the water below the ice is increased. The shelf regions therefore produce a large variety of water bodies through the seasons, some fresh, some salty, some very salty, but all very cold.

  
• Arctic Bottom Water

  The densest water of the world ocean, but it is not found anywhere outside the Arctic region

  
• Arctic Bottom Water


  • Is a mix of different dense waters:
  • Dense waters produced through deep convection
  • Dense water from Arctic shelf

  
• Pacific water through Bering Strait → also relatively fresh
• Freshwater (and sea ice) exported via Fram Strait (between Greenland and Svalbard) and Canadian Arctic Archipelago to North Atlantic
• There is mean ice drift away from the Eurasian coast toward the TPD → ice export through Fram Strait and to the Beaufort Gyre
• This freshwater goes to the Atlantic – directly or released from Beaufort Gyre
• Freshening in the Canadian basin since the 1990s
• Balanced by decrease in Eurasian basin
• Changes were due to shift in pathway of Eurasian runoff forced by strengthening of lower pressure on the Pole
• Implication: saltier ASW on Eurasian basin → halocline weakens → AW gets in contact with sea ice → enhanced melting

  
• The Southern Ocean

  • Communicates with all other oceans
  • The only region where the flow of water can continue all around the globe → ocean circulation comes closest to situation observed in the atmosphere
  • Temperature differences between the sea surface and the ocean floor generally do not exceed 5°C → 20% of the difference found in the tropics
• Density variations with depth in the Southern Ocean

  • Are small and the pressure gradient force is more evenly distributed over the water column → currents are not restricted to the upper few hundred meters of the ocean but extend to great depth
• Antarctic Circumpolar Current (ACC)


  • Has the largest mass transport of all ocean currents → It moves a slab of water more than 2000 meters thick with speeds comparable to other surface currents

  
• Subtropical Front (STF)


  Line where the tropical/temperate dynamics break down → where the permanent thermocline reaches the surface

  
• Since ACC reaches great depths, topography of the ocean floor has a large impact on circulation
• The combined effect of Drake Passage and the Scotia Ridge on the Circumpolar Current is quite dramatic: The current accelerates to squeeze through the gap and hits the obstacle at increased speed. It emerges highly turbulent and shifts sharply northward
• The coastline of Antarctica includes two major indentations: the Weddell and Ross Seas
• These do have western boundaries and thus support regional wind driven gyres with western boundary currents
• Winds and Currents in the Southern Ocean


  • Dominated by:
  • Westerlies from west in the latitude band 40-60°S → drive the ACC (called the West Wind Drift)
  • Easterlies south of 60°S (along the coast) → drive the Antarctic Coastal Current (ACoC), also called East Wind Drift

  
• Those two currents (ACC and ACoC) are the southern hemisphere equivalent of the subpolar gyres
• Cyclonic gyres (clockwise in the SH) in the Weddell and Ross Seas
• No continents to hinder flow of currents around Antarctica, so ACC driven CW, and ACoC drive CCW
• Infinite fetch + large wind speeds + little variation of wind direction = High waves
• Fronts in the Southern Ocean

  • Isopleths of all properties are nearly zonal (east-west) to great depth
  • The isopleths in the ACC are organized into three major fronts separating four broad zones in which isopleths are more widely spaced
  • Within the fronts, the currents are strong and eastward
  • In the zones between the fronts, the flow is dominated by eddies and can be in any direction

  
• Subtropical Front (STF)


  Narrow band around Antarctica where the salinity changes rapidly between 35.0 and 34.5 from north to south and temperatures drop rapidly as well