TROPICAL-MIDDLE LATITUDE INTERACTIONS VIEWED VIA WAVE ENERGY FLUX IN THE FREQUENCY-DOMAIN

Over the upper troposphere of the polar latitudes the zonal flows exhi bit a large variance on the time scale of the Madden-Julian oscillatio n, i.e. roughly 30-50 days. The other prominent regions for these intr aseasonal oscillations are the Asian and Australian monsoon belts. The se two regions are separated by the so-called critical latitude, to th e south of which easterlies generally prevail and westerlies are preva lent to the north. A perplexing issue is that of possible tropical-mid dle latitude interactions across the critical latitude. The notion of the critical latitude emerged from the linear theories for the wave en ergy flux which assume a constancy in time for the zonal flows. This s ame problem, viewed in its full non-linear context, can be cast in a f requency domain. Such a formulation does not assume a constancy of the zonal flows in time but does permit the intraseasonal variations of t he zonal flows to be present. The computation of the wave energy flux, from the more complete non-linear system in the frequency domain, req uires the handling of linear, quadratic and triple product terms via u se of Hayashi's co-spectral method. These results of the present study , based on 6 years of daily global data sets, show that wave energy fl ux clearly passes from the latitudes of the monsoon to the polar latit udes. A strong convergence of wave energy flux in the polar latitudes suggests the tropical-middle latitude convergence interactions across the so-called critical latitude-when the problem is viewed in the freq uency domain. (C) 1997 Elsevier Science B.V.