On the trends in global circulation in 50 years of NCEP/NCAR reanalyses

Abstract

To represent objective characteristics of circulation patterns, the spectral structure of both stratospheric and tropospheric fields is analysed in terms of spherical harmonic coefficients. The aim of this study is to compare the long-term behaviour of circulation patterns and their connections to some “extra-terrestrial” influence and circulation indices as well. A huge database of reanalyses from NCEP is used with appropriate spectral coefficients available for period 1948-now four times per day in 28 levels for relative vorticity, divergence, temperature, etc. Temporal analysis of significant spherical harmonics is introduced as well as the comparison of their changes with respect to the changes of solar activity and the phase of QBO. Generally, these wavenumbers mark no or a little positive trends, but for some distinctive wavenumbers representing individual patterns of polar vortex shape negative trend can be seen as well at the first part of time series. Changes with respect to solar activity cycle and QBO phase are presented for whole spectral domain not too well expressed, quite significant results are analysed for a few distinct wavenumbers, mainly in case of separation of time series for 1948–1972 and 1973–1997 periods. Moreover, long-term natural variability connected to the extra-terrestrial influence is studied as well as interannual variability with the emphasis to the QBO. Cross-correlation analysis shows significant difference for selected wavenumbers against solar flux F10.7 in separated periods with some correlation for the first one and rather no significant correlation for the second period. Long-term changes in the variability of the circulation patterns are analysed by means of wavelet analysis access to the series of selected wavenumbers displaying development of some periodicities in time. The connection with the tropospheric circulation is also discussed in terms of coefficients of spherical harmonics mostly for all cases in this study. The paper does not provide definite evidence for connections of atmospheric circulation with decadal solar variability, finally, this could be hardly achieved by statistical analysis only, but the aim of this study is to show possible connections between some helio-geophysical or extra-terrestrial parameters and circulation parameters and their long-term changes with emphasis to some distinctive circulation patterns or shapes of polar vortex.