Jet Stream Variability: Creating a Jet Index

A Jet Stream Index (JSI) has been created as a way to quantify the variance of the Northern Hemisphere mid-latitude jet stream. Sixty years of 300 mb geopotential heights from the NCEP/NCAR Reanalysis were evaluated through Principal Component Analysis (PCA). The variance of those Principal Components (PC) representing the long wave pattern was summed to create the JSI. We show that the variance of the jet stream is strongly seasonal. In addition, our analysis of the index shows that the frequency of extreme long wave patterns and associated synoptic events may be increasing over the last 60 years. We classify this trend in terms of the overall trend as well as the seasonal trends. Finally, we use output from the Coupled Model Intercomparison Project phase 5 (CMIP5) to look at long-term statistics and potential future changes in the JSI.