5.7 Meridional Heat Transport
Overall, Earth gains heat at the top of the tropical atmosphere, and it loses heat at the top of the polar atmosphere. The atmospheric and oceanic circulation together must transport heat from low to high latitudes to balance the gains and losses. This north-south transport is called the meridional transport.
How much heat is carried by the ocean and how much by the atmosphere? The sum of the meridional heat transport by the ocean and atmosphere together is calculated accurately from the divergence of the zonal average of the heat budget measured at the top of the atmosphere by satellites. To make the calculation, we assume steady state transports over many years so that any long-term net heat gain or loss through the top of the atmosphere must be balanced by a meridional transport and not by heat storage in the ocean or atmosphere. So let's start at the top of the atmosphere.
Heat Budget at the top of the Atmosphere
Errors arise from calibration of instruments, and from inaccurate information about the angular distribution of reflected and emitted radiation. Satellite instruments tend to measure radiation propagating vertically upward, not radiation at large angles from vertical, and radiation at these angles is usually calculated not measured.
The sum of the meridional heat transported by the atmosphere and the oceans is calculated from the top of the atmosphere budget. First average the satellite observations in the zonal direction, to obtain a zonal average of the heat flux at the top of the atmosphere. Then calculate the meridional derivative of the zonal mean flux to calculate the north-south flux divergence. The divergence must be balanced by the heat transport by the atmosphere and the ocean across each latitude band.
Oceanic Heat Transport
Various calculations of oceanic heat transports, such as those shown in Figure 5.11, tend to be in agreement, and the error bars shown in the figure are realistic.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, firstname.lastname@example.org
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Updated on September 8, 2008