Variations of hemispheric zonal index since 1899 and its relationships with air temperature

Abstract Mean monthly values of the zonal index of surface pressure for the zone 35°–65°N during the period 1899–1990 are studied. The extreme values – 11·5 hPa in February 1947 and 16·5 hPa in January 1978 define the range of zonal index variability. The highest annual value occurred in 1990. On average, the maximum of zonal index occurs in October (mean value 7·5 hPa), the minimum in May (2·5 hPa). Spectral variance analysis shows 2‐month, 3‐month, 6‐month, annual, quasi‐biennial, and 13·2‐year cyclicity in time series of monthly values. The quasi‐biennial and 13‐year cyclicity appear also in seasonal values of the zonal index. Instability of the zonal index spectra and autocorrelation for monthly values has been noted. The period under study is divided into three subperiods characterized by high (1899–1938 and 1972–1990) and low (1939–1971) zonal indexes. The correlation and correspondence between zonal index and air temperature changes of the Northern Hemisphere and over Europe has been also examined.