Trends of NAO and AO and their associations with stratospheric processes

Trends of NAO and AO are shown, along with trends of various stratospheric fields and dynamical processes, including geopotential height, temperature and zonal wind, as well as EP flux and its divergence, based upon 30 years (1968–1997) of NCEP/NCAR Reanalysis data. Monthly trends are estimated by using linear regression. The NAO trends are similar to the AO trends in February and March, but different in other months, particularly in May and June. Comparing the variation by month of stratospheric trends with that of the NAO and AO index, we find that the largest trends of stratospheric fields and processes are in the same active season (JFM) as that in NAO and AO, but with the timing of peak of the trends somewhat different, the stratospheric trends leading the NAO and AO trends by one month. We also find a significant trend of equatorward refraction of mid‐latitude planetary waves in January.