East Asia and Western Pacific METEOROLOGY AND CLIMATE

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first one. In his work, the long-term weather process is divided into 3 kinds
of time scales, in which rhythm is the one with a scale of 3—6 months. Zhao
et al. (1982) (15) pointed out that there are certain rules on the spatial distribution
of the rhythm index. The significant indices mainly locate in three
regions: the North Pacific, the North Atlantic and the region from the
South Asia to the West Pacific. In contrast, few high rhythm indices are
found in the continent of middle— high latitudes. Most works related to the
rhythm phenomenon are based on the calculation of lag correlation
coefficients. An restriction here is that the relation must be linear. However,
if the analogous method is used, the restriction will disappear and the complicated
properties and nonlinear interactions of the real system can be studied.
This analogy was originally used for assessing predictability by Lorenz
(1969) (4) in his investigations of middle-range weather forecasting. It has
also been found that this approach can be successfully applied to researching
the rhythm in long-term circulation variations (Wang, 1984) (12) .
Using monthly averaged surface pressure field, SOOhpa geopotential hight
field over the Northern Hemisphere and the maps of SST anomalies over
pacific and Atlantic during 1951-1975, Wang (1984) (12) examined their similarities
between any two of them. It is found that the similarities do not decrease
monotonicsely with time, but increased in some specific interval of
time, for example, from five to seven months after the initial month. It
means that there is rhythm phenomenon in the atmospheric circulation variation
as well as in the oceanic variations. However, in Wang's paper, the
category of similarity is a sign correlation r
where, N is the total number of points in the two anomaly maps used;
n + and n_ the number of points of the same and opposite signs,
respectively.
The analogous evolution of 500hpa monthly mean circulation is analysed
with strict statistical method by Huang, Gao and Chou (unpublished), recently.
The similarity index is
•R^-dnqVyl-yW) (2)
where c= 16 /In2, 0^ is the similarity coefficient between two anomaly
fields in different years, The average value of 0y is given by
cT = l-r(l-^) (3)
(1)