On the structure of the 30 to 50 day mode over the globe during FGGE

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Abstract
6 vertical levels of the entire 365-day global FGGE Illb analysis for 5 variables are subjected to a time-series analysis and a frequency filter to investigate the structure of the 30- to 50-day mode. This study isolates regions where the amplitude of these low-frequency oscillations are large; the vertical structures across these large amplitude regions are also presented. The seasonal variation of the maximum 30–50 day filtered wind in the lower troposphere is highlighted. The major results for the FGGE year show that the 30- to 50-day mode has its largest amplitude in the upper troposphere of polar latitudes and in the summer monsoon region. During the northern winter, active regions are also located over the equatorial belt of the central Pacific ocean. The phase propagation on the pressure surfaces are examined simply from an analysis of a time sequence of low-frequency weather maps. The vertical phase propagation is illustrated by pressure-time plots of the low-frequency data sets at individual locations. This analysis suggests vertical propagation over convective areas and a lack of it over most other regions. A highlight of this study is a phenomenon we have labelled as “low-frequency storms”. Here we illustrate long-lasting, low-frequency, weather systems that propagate meridionally (1) over the summer monsoon regions from the equator to the Himalayas and (2) over the eastern Pacific ocean from the equator northwards during the northern winter season. An example of such a long-lasting system is traced to 60° N; subsequently, it appears to move zonally from the gulf of Alaska across the Canadian Arctic, north Atlantic, and Europe prior to its dissipation over Siberia. The potential for interactions between these low-frequency systems and polar front cyclones is another interesting aspect of this investigation. The low-frequency oscillations and motions are considered important since their amplitude in the troposphere is large (8 ms-1) and they are well-defined within their scales of motion (on the order of 4000 km or larger). DOI: 10.1111/j.1600-0870.1985.tb00432.x

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