SIO 210 Talley Topic 5: North Atlantic circulation and water masses. Thermohaline forcing.

Lynne Talley, 1997
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Reading,references and study questions for topic 5 - click here

Some online figures.

A large number of vertical CTD sections can be accessed using the vertical section atlas for the Atlantic .

North Atlantic meridional section at 35W

Potential Temperature,
Salinity,
Potential Density (0 dbar),
Potential Density (4000 dbar)
A reference is:
McCartney, M.S., 1992. Recirculating components to the deep boundary current of the northern North Atlantic. Prog. Oceanogr., 29, 283-383.

North Atlantic zonal section at 24N

These sections show conditions in the central subtropical gyre, showing the cold, lower oxygen Antarctic Bottom Water west of the mid-Atlantic ridge, the high oxygen North Atlantic Deep Water along the western boundary with recirculation into the western basin, the high salinity Mediterranean Water. Potential Temperature,
Salinity,
Oxygen

Roemmich, D. and C. Wunsch, 1985. Two transatlantic sections: meridional circulation and heat flux in the subtropical North Atlantic Ocean, Deep-Sea Res., 32, 619-664.

North Atlantic zonal section at 47N

These sections show conditions in the subpolar gyre, including the cold deep western boundary current, the salinity minimum of the Labrador Sea Water, and northward penetration of the Mediterranean Water salinity maximum. Potential Temperature,
Salinity,
Potential vorticity
Salinity as a function of density rather than depth

Outline

1. Large-scale circulation. Cartoon from Tomczak and Godfrey. Reid (1994) adjusted steric height at the sea surface. Worthington (1976) "two gyres" (Gulf Stream and North Atlantic Current as two western boundary currents for the subtropical gyre) with transports. Wind stress curl pattern showing relation to the Gulf Stream and North Atlantic Current (McCartney, 1982).

2. Gulf Stream. Davis (1991) and Owens (1992) floats at surface, 700 m and 2000 m.a Schmitz (1980) current meters at 55W. Showing penetration of Gulf Stream to bottom, meandering, deep recirculations (westward flow on either side of the eastward Gulf Stream).

3. Subduction. Central Water T/S relation. Creation of shallow salinity maximum (Subtropical Underwater) in the southern subtropical gyre through subduction (Worthington, 1976).

Salinity along 25W showing subducted high salinity
Wind stress curl pattern from McCartney with cartoon of subduction in the northern subtropical gyre. Penetration of high salinity along isopycnals from Sarmiento, and merging into Mediterranean influence at greater depth.

4. Subtropical Mode Water (Eighteen Degree Water). Observations from Challenger expedition in 1873 (Worthington, 1976). Relation of formation and properties to Gulf Stream (Talley and Raymer, 1982).

Potential temperature section at 64W showing Eighteen Degree Water
from the Atlantic vertical section atlas. .

5. North Atlantic Deep Water formation. (For much more complete Talley text, see Physical D reference listed in study questions.) Five sources. Three northern sources: Greenland Sea overflows into North Atlantic (deepest), Labrador Sea (intermediate depth), Mediterranean outflow (shallowest part). Two southern sources: Antarctic Intermediate Water and Antarctic Bottom Water.

  • Mediterreanean outflow. (Ochoa and Bray; Zenk). Reid (1994) salinity distribution at about 1000 m.
  • Subpolar Mode Water as antecedent for Norwegian Sea inflow and Labrador Sea inflow (McCartney and Talley; McCartney).
  • Labrador Sea Water. Deep convection in the Labrador Sea. 47N salinity section showing Labrador Sea Water. Maps showing spreading through North Atlantic subpolar regions and southward under the Gulf Stream in the upper portion of the Deep Western Boundary Current. (Talley and McCartney, 1982; Reid, 1994).
  • Norwegian-Greenland Sea. (Greenland/Iceland/Norwegian or GIN Sea overflows). Greenland Sea deep convection region offshore of ice edge (Morawitz et al., 1996). Outflow through Denmark Strait along Greenland coast (Dickson and Brown, 1994.) Southward spread in deeper part of DWBC (Jenkins, 1980). Deep recirculation gyres (McCartney cartoon). Reid maps.
  • Antarctic Intermediate Water. Influence from South Atlantic near same densities as Mediterranean Water. Entry into N. Atlantic through western boundary current across equator and zonal flows in the tropics. Tracing northward using its high silica signal (Tsuchiya).
  • Antarctic Bottom Water. Reid (1994) deep maps showing intrusion of bottommost waters into western North Atlantic, crossing equator in a bottom western boundary current.

    Transport diagrams for North Atlantic Deep Water formation, mainly from Schmitz and McCartney.