SIO 210 Talley Topic 5: Atlantic circulation and thermohaline driving

Lynne Talley, 2000
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Reading and study questions

Reading (on reserve):

Read one text carefully and skim the other(s).

Pickard and Emery, read sections 7.34, 7.35; (skim 7.41, 7.44, 7.45)

Tomczak and Godfrey, pp. 253-259, 263-272, and chapter 15

Other reading:

Ocean Circulation, sections 4.1 and 4.3 (very nice figures, good historical information, measurement methods)

Reid, J.L., 1994. On the total geostrophic circulation of the North Atlantic Ocean: flow patterns, tracers and transports. Progr. in Oceanogr., 33, 1-92. (source of many figures for class)

Schmitz, W.J. and M. S. McCartney, 1992. On the North Atlantic circulation. Rev. of Geophysics, 31, 29-50. (source of many figures for class)

Talley, L. D., 1996. North Atlantic circulation, reviewed for the CNLS conference. Physica D, 98, 625-646. (read the review sections in lieu of class notes on the thermohaline circulation; skip the Bermuda time series portion at the end)

Atlases (skim through to see what they look like):

Geosecs atlas for the Atlantic Ocean

Fuglister, F. C., 1960. Atlantic Ocean Atlas of temperature and salinity profiles and data from the IGY of 1957-1958. Woods Hole Oceanographic Institution Atlas Series, 1, 209 pp.

Worthington, L.V. and W.R. Wright, 1970. North Atlantic Ocean Atlas of Potential temperature and salinity in the deep water... Woods Hole Oceanographic Institution Atlas Series, 2, 24 pp. and 58 plates.

Wright, W.R. and L.V. Worthington, 1970. The water masses of the North Atlantic Ocean: a volumetric census of temperature and salinity. Serial atlas of the marine environment, folio 19. New York: American Geographical Society. 9 pp.

Informal collection of vertical sections for the Atlantic Ocean on Talley homepage (CTD quantities only).

Study questions

Water masses and thermohaline forcing

1. What is a water mass?

2. What are the major water masses of the North Atlantic? What are their sources and what distinguishes them? How is each one formed?

3. What are the five components of North Atlantic Deep Water? Where does each originate? Describe the way each enters the deep North Atlantic. Where does each become a part of the "NADW"? Is the NADW a completely undifferentiated, homogeneous water mass?

4. What is the main characteristic of Subtropical Mode Water? Locate Subtropical Mode Waters in several oceans using vertical sections in different atlases (for instance Geosecs sections in the western part of each ocean, or Lynn and Reid or Wust meridional sections) and note their temperatures and the western boundary currents which they abut.

5. Locate the Antarctic Intermediate Water in all oceans using its most common characteristic. How far north is it easily discernible in the N. Atlantic and N. Pacific?

6. Locate deeper waters of Antarctic origin on meridional sections extending to the north. What properties are used to differentiate these from Deep Waters moving south?

7. Where are the saltiest and the freshest intermediate and deep waters found? (e.g. look at different isopycnals and note the extremes)

8. Contrast the average mixed layer depths in the northern N. Pacific with those in the northern N. Atlantic. What does this tell you about the relative importance of wind and thermohaline forcing for the surface layers in the two oceans?

9. Where do low salinity intermediate waters form in the northern hemisphere? Contrast the formation of the N. Atlantic and N. Pacific intermediate waters.

10. Where is there active and relatively deep (>1000m) convection in the North Atlantic and adjacent seas? Where is shallower convection (200-1000m) found in the North Atlantic?

11. Where does the densest water form in the northern hemisphere?

12. Why is the Mediterranean Sea important even though the net exchange of water with the N. Atlantic is quite small?

13. What do we mean by "age" of water? What is the approximate difference in age between deep waters of the N. Pacific and deep waters of the N. Atlantic?

14. Explain the 36N meridional transport diagram in Roemmich and Wunsch in terms of water masses and the direction they are moving on average.

15. Why does the deep western boundary current in the North Atlantic go the opposite direction from the Gulf Stream?

16. What are the basic dynamics assumed to drive much of the deep circulation? What direction would you expect the interior deep flow to go if the ocean were flat-bottomed (vertical sides) with isolated sources of deep water?

Further reading and figure sources

Davis, R.E., 1991. Lagrangian ocean studies. Ann. Rev. Fluid Mech., 23, 43-64.

Dickson, R.R., and J.Brown, 1994, The production of North Atlantic Deep Water: sources, rates and pathways, J.Geophys.Res. 99, 12319-12341.

Owens, W.B., 1991. A statistical description of the mean circulation and eddy variability in the northwestern Atlantic using SOFAR floats. Prog. in Oceanogr., , 257-303.

Iselin, C.O., 1939. The influence of vertical and lateral turbulence on the characteristics of the waters at mid-depths. Trans. of the Amer. Geophys. Union, 1939, 414-417.

Jenkins, W. J. and P.B. Rhines, 1980. Tritium in the deep North Atlantic Ocean. Nature, 286, 877-880.

Lee, A. and D. Ellett, 1965. On the contribution of overflow water from the Norwegian Sea to the hydrographic structure of the North Atlantic Ocean. Deep-Sea Res., 12, 129-142.

McCartney, M.S., 1992. Recirculating components to the deep boundary current of the northern North Atlantic. Prog. Oceanogr., 29, 283-383.

McCartney, M.S. and L.D. Talley, 1982. The subpolar mode water of the North Atlantic Ocean. J. Phys. Oceanogr., 12, 1169-1188.

Mantyla, A. and J.L. Reid, 1983. Abyssal characteristics of the World Ocean waters. Deep-Sea Res., 30, 805-833.

Ochoa, J. and N. A. Bray, 1991. Water mass exchange in the Gulf of Cadiz. Deep-Sea Res., 38, S465-504. (Med. water sections)

Reid, J.L., 1981. On the mid-depth circulation of the World Ocean. In Evolution of Physical Oceanography, MIT Press, 70-111.

Reid, J.L., 1989. On the total geostrophic circulation of the South Atlantic Ocean: flow patterns, tracers and transports. Prog. in Oceanogr., 23, 149-244.

Reid, J.L. and R. J. Lynn, 1971. On the influence of the Norwegian-Greenland and Weddell seas upon the bottom waters of the Indian and Pacific oceans. Deep-Sea Res., 18, 1063-1088.

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.

Sarmiento, J.L., C.G.H.Rooth and W. Roether, 1982. The North Atlantic tritium distribution in 1972. J. Geophys. Res., 87, 8047-8056.

Schmitt, R.W., 1981. Form of the temperature-salinity relationship in the Central Water: evidence for double-diffusive mixing. J. Phys. Oceanogr., 11, 1015-1026.

Stommel, H. P. Niiler, and D. Anati, 1978. Dynamic topography and recirculation of the North Atlantic. J. Mar. Res., 36, 450-468.

Talley, L.D. and M.S. McCartney, 1982. Distribution and circulation of Labrador Sea Water., J. Phys. oceanogr., 12, 1189-1205.

Tsuchiya,M., 1989. Circulation of the Antarctic Intermediate Water in the North Atlantic Ocean. J. Mar. Res., 47, 747-755.

Tsuchiya, M., L.D. Talley and M.S. McCartney, 1992. An eastern Atlantic section from Iceland southward across the equator. Deep-Sea Res., 39, 1885-1917.

Worthington, L.V., 1976. On the North Atlantic circulation. Johns Hopkins Oceanographic studies, 6. (Look at ch. 13 - 18 Water, ch. 14 - salinity maximum water, ch. 3 - NADW. Skim other chapters for further reading if you wish.)

Worthington, L.V., 1981. The water masses of the world ocean: some results of a fine-scale census. In Evolution of Physical Oceanography, MIT Press, 42-69.

Zenk, W., 1975. On the Mediterranean outflow west of Gibraltar. Meteor Forsch.- Ergebnisse, A (16), 23-24.