Zonal index and its variability

Abstract

The zonal index, defined as the difference in hemisphere-wide averaged mean sea level pressures between 35◦ N and 55◦ N, is a tool that allows the study of variations in circulation patterns at mid-latitudes. Apart from an annual periodicity, its variation has a cyclical component called index cycle. We have determined that its period is around 5 weeks, based on data from the last four decades. We have also analysed the long-term trend of this index, obtaining a slight growth, in line with other indices such as the NAO. We have computed correlations with several meteorological variables, and we have corroborated its low predictive capacity, which was one of the reasons why it practically disappeared in modern meteorology