Separating Extratropical Zonal Wind Variability and Mean Change

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Abstract
Changes in the naturally occurring modes of extratropical annual mean and zonal mean zonal wind variability are investigated using National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses and Canadian Centre for Climate Modelling and Analysis (CCCma) global climate model simulations. In the Northern Hemisphere, the first and second modes are primarily stratospheric and tropospheric in character, respectively. The surface pressure manifestations of these modes are intimately linked to the Arctic Oscillation (AO), and together suggest separate stratospheric and tropospheric origins for the AO. In the Southern Hemisphere, the first mode describes north–south shifts in the polar front jet accompanied by polar stratospheric jet fluctuations and Antarctic Oscillation (AAO)-like surface pressure anomalies. The second mode is primarily tropospheric and describes interannual changes in the strength and position of the polar front jet. The leading observed modes appear unchanged in strength since the 1950s except in the Northern Hemisphere where the second mode shows some evidence of increasing strength. The leading simulated modes appear unchanged in strength since the beginning of the twentieth century, and are predicted to remain so to the end of the twenty-first century. In all cases the leading modes are superimposed upon significant mean change, which when not properly accounted for can lead to erroneous conclusions.