Lecture 8 Notes  October 13, 2004

Figure 5-15 in the text shows atmospheric and ocean heat transport and it is not very accurate. More current data indicates that peak transport in the atmosphere is about twice that of the ocean. The figure in the text is

outdated now. The calculation is uncertain because it depends sensitively on measured net radiative flux at the top of the atmosphere and at the atmosphere ocean interface, and these fluxes are difficult to measure accurately themselves. For example, the total heat transport at 75N is computed by summing the net radiation from 75 N to 90 N but weighting the sum by the "area" of each the latitude circle in some kind of incremental fashion (a precise calculation requires using integral calculus). The figures at the right show the state of the art estimates of first total heat transport (top) and the atmosphere and ocean portions of the total heat transport. The curves labeled RT (ERBE) are from satellite measurements and the others are a compilation of modeling and measurements of all types. The figures are from Trenberth and Caron (2001). You can see that the total heat transport varies by about 20% depending on the data source. Even if the curves are off by 20%, the atmosphere would still outpace the ocean by at least 50%. The best guess indicates it is more like 100%.

The next topic for the day was  Land/Ocean contrasts. I mentioned that the albedo of the land is generally higher than that of ocean (even forests tend to reflect more than ocean on average). The
atmospheric heat xport
ocean and atmosphere
thermal inertia of the ocean is much higher than land. Friction is higher over land due to mountains and trees, and also the ocean is free to move somewhat with the winds. Some of the implications of these contrasts are:
  • Sea and land breezes on diurnal (or daily) cycle
  • Cold land - warm ocean in winter (vice versa in summer), this leads to the meaning of the term continentality, which refers to the larger range in seasonal extremes over land. 
  • Long wait to reach a new equilibrium climate due to high inertia of ocean
  • Monsoon circulation
Below you will find more detail about the monsoon, but for now please look at the mean sea level pressure maps in Fig 4-19. Notice that
  • There are highs over land and lows over ocean in the winter hemisphere (i.e., January for the northern hemisphere and June for the southern) 
  • There is so little land in the midlatitudes in the SH that the sea level pressure is relatively uninterrupted, so the flow becomes almost parallel to latitude circles there, what is also called zonal.
  • I drew horizontal circulation maps (see below) that showed the "secondary circulation" associated with the high over cold land (winter) and low over warm land (summer) patters. This is the flow we would expect in the absence of the coriolis force. However the midlatitude coriolis force is so great that flows tend to circle around highs and lows with only a weak flow inwards towards the center of lows due to friction, and similarly a weak flow away from highs. These same figures were used for sea/land breeze and monsoon circulations.
  • Finally remember that Fig 4-19 shows the monthly mean of the sea level pressure, an instaneous map would be much messier.
cold land
 warm land
Figure 4-20 in the book shows a wind field from an instant in time. It is much messier! Notice that the swirls are mostly midlatitude phenomena. Winds in the tropics are smoother because the coriolis force is weak there. Also there is a sense that winds move east to west in the tropics and west to east in the extratropics.

The monsoon  circulations are a seasonal reversal of the winds, but perhaps more importantly they give rise to a predictable rainy season. The circulations are the same the sketches above. They are largest at about 10 deg N, the approximate location of the ITCZ in summer -- the latitude of roughly the peak net solar flux and also the peak surface convergence of the winds. The very larges monsoon is over India owing to the Tibetan Plateau, which provides an effective mechanism to pump heat (through surface evaporation and condensation aloft) and generate convection very high in the troposphere.

The water cycle in Fig 4-24 introduced a new concept -- the advection of water vapor and condensed water droplets in clouds. Advection means horizontal transport, and it applies to temperature and other quantities in addition to water vapor.

The residence time of a reservoir is computed from

Residence time = Reservoir capacity / removal rate (sink)

It is the timescale for change in a system. It is used for the water cycle as well as the carbon cycle. When applied to the ocean in Fig 4-24, RT=1350e13 / 351e12 = 3700 yr.
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Contact the instructor at: atms211@atmos.washington.edu
Last Updated: 9/29/2004