Abstract
The behaviour of the stationary and travelling planetary-scale waves during northern hemisphere blocking has been investigated. Data from 29 winter seasons from 1950 to 1979 were used. A winter season was defined as all days from 1 December through 28 February. The data were decomposed in a quasi-stationary part, computed as a running time average, and a travelling part, being the difference between the observed value and the time average. For each latitude line, the geopotential height of the first three waves (quasi-stationary as well as travelling) were computed as a function of longitude and time. We have focused our interest on the total contribution from the planetary-scale (the first three) quasi-stationary versus travelling waves.
The behaviour of the planetary-scale waves during two major blocking situations is documented. Characteristics of these waves during blocked flow have been computed for the 29 winter seasons. It is found that the stationary component of the height field for the first three waves is almost always greater than the corresponding climatological value in areas with blocked flow, and that the resultant of the height field of the travelling ultra long waves is mostly greater than zero in the same area. These two conditions are almost necessary for the formation of blocks, but they are not sufficient.
