CLIMATE
VARIABILITY
RESEARCH
Heat Fluxes in the Atmosphere and Ocean
Over long timescales the Earth absorbs more solar radiation in tropical regions than it does at higher latitudes. Similarly, more longwave radiation is emitted to space in the tropics than at high latitudes. However, due to the relative magnitudes of these two processes the net effect is that energy is absorbed at low latitudes and emitted to space at high latitudes. Assuming that over long timescales the heat storage in the Earth's atmosphere and oceans does not change, then the atmosphere and ocean must flux heat from the tropical regions to high latitudes.
The meridional flux of heat in the atmosphere and oceans should balance that required by the top of the atmosphere (TOA) radiative fluxes. An intercomparison of the three heat fluxes allows an estimation of the internal consistency of data to be made. This gives an indication of how reliable the results are.
Heat fluxes in the atmosphere and oceans are being investigated using the CGAM ERA dataset covering the period January 1979 through to December 1993. This dataset was derived from the ERA dataset at full horizontal resolution and upper air fields were interpolated onto 17 pressure levels. Heat fluxes for the atmosphere, oceans and TOA have been computed for the fifteen year climatology.
Previous heat flux studies (Trenberth and Solomon, 1994, Gleckler, 1994) estimate that the TOA fluxes peak at 4 to 6 PW near 30N and -3 to -6 PW near 30S. Here the values are near the lower bounds of the range. The atmospheric fluxes are in the middle of the range for the previous studies: 3 to 6 PW near 30N and -3 to -6 PW near 30S. The oceanic fluxes in the subtropical northern hemisphere are in reasonable agreement with the previous studies: 0.5 to 1.5 PW near 15N. However, at higher northern latitudes and in the southern hemisphere the agreement is not so good. Indeed, in the southern hemisphere from 30S to 60S the oceanic heat fluxes are equatorward. The largest residuals with values of about 0.5 PW are to be found from 50N to 70N and 40S to 80S. The problem of unrealistic oceanic heat fluxes, notably in the southern hemisphere, has been noted in the previous studies and is thought to be due to the poor representation of the radiative effects of clouds. Previous authors used ERBE TOA radiation data to improve their results. We will follow suit.
Future work will be to compute the heat fluxes for the individual years in the ERA period and the results will be examined for interannual variability. Heat fluxes will also be calculated from the AMIPII dataset and compared with the results from the ERA.
References
Trenberth, K.E. and Solomon, A., 1994. The global heat balance: heat transports in the atmosphere and ocean. Climate Dynamics, 10, 107-134.
Gleckler, P.J. et al., 1994. Cloud-radiative effects on implied oceanic energy transports as simulated by atmospheric general circulation models. PCMDI Report, 15.
Paul Berrisford and Alan O'Neill
CGAM, University of Reading
P.Berrisford@reading.ac.uk
