SIO 210 Talley Topic 3: Dynamical quantities, transports and forcing

Lynne Talley, 2000
Back to lecture notes

Reading and study questions

Dynamic height: Pickard and Emery, chapter 6.255 Forcing: review mechanisms, especially geostrophy, Ekman transport and Sverdrup transport, discussed by Hendershott. (Supplementary reading: see Tomczak and Godfrey, chapters 3 and 4)

Study questions:

Geostrophic method, transports and dynamic height

1. What are the common methods for measuring currents in the ocean?

2. Why do we use a method based on temperature and salinity instead of direct current measurements for most of the ocean?

3. How are temperature and salinity information used to calculate currents? What is the biggest drawback to this method?

4. Consider various distributions of circulation that might accomplish the same transport of, say, heat. That is, there can be a circulation that is mostly horizontal - say, a subtropical gyre. How is heat transported poleward in such a horizontal gyre? There can also be circulation in which poleward surface flow is balanced by equatorward deep flow. How is heat transported (and what sign is the heat transport) in this kind of circulation?

Forcing

1. What are the two principle driving forces for the general circulation?

2. How is wind forcing communicated to the general circulation: What is the direct response of the ocean to the imposed surface stress? How deep does this response extend? How is the forcing communicated deeper in the water column?

3. How does the other forcing (from #1) drive the general circulation? Is the associated circulation stronger or weaker than the part of the wind-driven circulation near the sea surface?

Study calculations

1. What is the volume transport in m3 /sec of a current with velocity 10 cm/sec over a width of 200 km and a depth of 5000m?

2. What dynamic height difference over 100 km is associated with a current speed of 100 cm/sec?

3. What dynamic height difference is associated with a transport of 100 Sverdrups? (1 Sverdrup = 10**6 m**3/sec)

4. For a wind stress of 1 dyne/cm**2, what is the vertically-integrated Ekman transport at 40N? If the wind stress changes from eastward at 1 dyne/cm**2 at 40N to westward at 1 dyne/cm**2 at 20N, what is the order of magnitude of the vertical velocity? Is the vertical velocity up or down? (In calculating this, don't worry about the effect of the changing latitude on the Ekman transport, and assume the change in stress is linear with distance.)

Other sources for viewgraphs

Barnier, B., L. Siefridt and P. Marchesiello, 1994. Thermal forcing for a global ocean circulation model using a three-year climatology of ECMWF analyses. J. Marine Systems, 6, 363-380. (net surface heat flux; gif plot data)

Chereskin, T. K., 1995. Direct evidence for an Ekman balance in the California Current. J. Geophys. Res., 100, 18261-18269.

Gordon, A.L., 1986. Interocean exchange of thermocline water. J. Geophys. Res., 91, 5037-5046. 339-359.

Hellerman, S. and M. Rosenstein, 1983. J. Phys. Oceanogr., 13, 1093-1104. (wind stress maps)

Hsiung, J., 1985. Estimates of global oceanic meridional heat transport. J. Phys. Oceanogr., 15, 1405-1413. (net heat flux maps)

Levitus, S., 1982. Climatological atlas of the world ocean. NOAA Prof. Paper 13, 173 pp. (climatological data used for average property maps; source of data for gif plots)

Lynn, R.J. and J.L. Reid, 1968. Characteristics and circulation of deep and abyssal waters. Deep-Sea Res., 15, 577-598. (good reference with meridional vertical sections)

Ralph, E. A. and P. P. Niiler, 1999. Wind-driven currents in the tropical Pacific. J. Phys. Oceanogr., 29, 2121-2129.

Reid, J.L., 1969. Sea-surface temperature, salinity, and density of the Pacific Ocean in summer and in winter. Deep-Sea Res., 16 (suppl), 214-224. (surface property maps)

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.

Schmitt, R., P. Bogden, C. Dorman, 1989. Evaporation minus precipitation and density fluxes for the North Atlantic. J. Phys. Oceanogr., 19, 1208-1221.

Schmitz, W. J., 1995. On the interbasin-scale thermohaline circulation. Rev. Geophys., 33, 151-173.

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., 1984. Meridional heat transport in the Pacific Ocean. J. Phys. Oceanogr., 14, 231-241.

Wijffels, S. E., R. W. Schmitt, H. L. Bryden and A. Stigebrandt, 1992. Transport of freshwater by the oceans. J. Phys. Oceanogr., 22, 155-162.

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.

Wyrtki, K., 1975. Fluctuations of the dynamic topography in the Pacific Ocean. J. Phys. Oceanogr., 5, 450-459.