Chapter 5 - The Oceanic Heat Budget

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
Heat flux through the top of the atmosphere is measured by radiometers on satellites.

1. Insolation is calculated from the solar constant and observations of reflected sunlight made by meteorological satellites and by special satellites such as the Earth Radiation Budget Experiment Satellite.
2. Back radiation is measured by infrared radiometers on the satellites.
3. The difference between insolation and net infrared radiation is the net heat flux across 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
Oceanic heat transport are calculated three ways:

1. Surface Flux Method calculates the heat flux through the sea surface from measurements of wind, insolation, air, and sea temperature, and cloudiness using bulk formulas. The fluxes are integrated to obtain the zonal average of the heat flux (Figure 5.7). Finally, the meridional derivative of the net flux gives the flux divergence, which must be balanced by heat transport in the ocean.
2. Direct Method calculates the heat transport from values of current velocity and temperature measured from top to bottom along a zonal section spanning an ocean basin. The flux is the product of northward velocity and heat content derived from the temperature measurement.
3. Residual Method first calculates the atmospheric heat transport from atmospheric measurements or the output of numerical weather models. This is the direct method applied to the atmosphere. The atmospheric trans-port is subtracted from the total meridional transport calculated from the top-of-the-atmosphere heat flux to obtain the oceanic contribution as a residual (Figure 5.11).

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.

Figure 5.11 Northward heat transport for 1988 in each ocean and the total transport summed over all oceans calculated by the residual method using atmospheric heat transport from ECMWF and top of the atmosphere heat fluxes from the Earth Radiation Budget Experiment satellite. From Houghton et al., (1996: 212), which used data from Trenberth and Solomon (1994).

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