On Atmospheric Energy Conversions Between the Zonal Flow and the Eddies

Abstract

Energy transformations between the zonal mean and the eddies have been computed from observed data for a single winter month, January 1962. The available data were the height fields of the 850, 700, 500, 300 and 200 mb surfaces. Our calculations are therefore based on the geostrophic or balanced approximations. The horizontal fields of the streamfunction and the thickness are written as one-dimensional Fourier series, and the energy transformation between the zonal mean and the different longitudinal wave-numbers are evaluated. In order to evaluate the energy transformations in the wave number regime, it is necessary to compute the meridional transports of momentum and heat as a function of the wave number. These results appear therefore as useful by-products of the calculations. The main conclusions which can be made on the basis of the calculations are:

1 The maximum transport of sensible heat occurs in the middle latitudes and at the lower elevations. More than 50% of the transport of sensible heat is carried out by wave numbers 1 to 4, while the next four wave numbers (5 to 8) transport half this amount.

2 The transport of momentum is toward the north south of about 55° N and toward the south north of this latitude. The maximum northward transport of momentum occurs at 35° N at higher elevations, while the maximum southward transport takes place at 65° N also at higher elevations. About 50% of the northward transport of momentum is carried out by wave numbers 1 to 4 and about 30% by the next four wave numbers (5 to 8).

3 The conversion of zonal available potential energy to eddy available potential energy is found to be positive for almost all wave numbers with a single maximum around the wave numbers 2 and 3.

4 The conversion of eddy kinetic energy to zonal kinetic energy is found to be positive when summed over all wave numbers. The conversion is much smaller than the conversion mentioned in 3. The spectrum is very irregular showing positive conversions for the small wave numbers, but negative conversions for some of the larger wave numbers.

The conclusions mentioned above are tentative, since they are based on data from a single winter month. Computations for other winter months and for other seasons are being planned.