| description abstract | The Northern Hemispere 500-mb geopotential height analyses for the period 1956?79 have been used to study the existence and persistence of analogues. An expression for the mean-square difference between two 500-mb height fields is formulated using empirical orthogonal functions. This has three advantages. First, Computing time is reduced to about one-tenth of that required by a grid-point expression. This allows us to study a very large amount of data. Second, the part of the mean-square difference due to the climatological seasonal variation of the 500-mb height field can easily be excluded. Third, the analogue index can be easily normalized so that its values in different seasons are comparable. When independent circulation patterns, i.e., those separated by at least 10 days are studied, analogues are found to be very uncommon. Most often they are encountered when the flow Patterns occur in roughly the same time of year. Moreover, the lifetime of analogues is longest when the seasonal time difference is small, i.e., less than 20 days. The degree of similarity of previous evolution between the analogous states seems to be of minor importance for the rate of decline of analogue quality. This result is perhaps not valid when one studies analogues in a limited area only. | |
