Modulation of gravity waves by planetary waves (2 and 16 d): observations with the North American-Pacific MLT-MFR radar network

In an earlier study based upon medium frequency radar (MFR) data from Saskatoon (52°N) the variability of time-sequences of gravity wave (GW) variances was linked to tidal (12,24 h) and planetary wave (2 d) oscillations of the wind. Fifty days of data were chosen from each of winter, spring and autumn seasons for this most comprehensive assessment of wave interactions. While modulations of the GW variances were observed, the results indicated considerable intermittency in the strength and direction of waves from the GW sources. Here we extend the study to other sites in the MLT-MFR (mesosphere, lower-thermosphere) network, and focus upon the planetary waves (PWs) and their modulating influences upon the GW variances. The PW events include the 2 d waves as seen at Saskatoon, London, Hawaii and Christmas Island during the summer of 1994; and 16 d wave activity evidenced at Saskatoon and London throughout 1994, and also during 12 and 7 years of observations, respectively. The modulations of the sequences of GW variances (10– 100/150 min , 2– 6 h periods) are significant, and the phase-differences between the PW (2 d,16 d) oscillations in the time-sequences of the winds and of the GW variances allow the propagation directions for the GW fluxes to be inferred. These are eastward in summer months and westward in winter at MLT (60– 90 km) altitudes.