Quasi-periodic Cyclogenesis in Relation to the General Circulation

A striking case of recurring cyclogenesis at ten day intervals during February 1953 with a tendency to continue until mid-May has been studied against the background of the planetary circulation averaged over periods up to thirty days. The data suggest a periodicity-producing mechanism whose period may be defined by the underlying long period (e.g., 30-day) mid-tropospheric wave pattern. In this particular case the patterns of waves observed on 5-day mean charts oscillated about certain more stable centers of action in a manner consistent with vorticity considerations. These oscillations recurrently deployed continental polar and maritime tropical air masses into juxtaposition and set the stage for cyclone growth. The movement and development of these cyclones strengthened the westerlies, thereby temporarily permitting increase of the planetary wave length. After the energy of thermal contrast was tapped and the cyclones moved off or filled, the regional westerlies declined leading to long wave retrogression and the initiation of the cycle anew. The annoying ≥change in phase≤ which has beset workers in periodicity for years is shown to be associated with abrupt though logical rearrangements of the mean planetary circulation incident to change of season.