Abstract
This study presents the time development of two major modes of atmospheric circulation variability in the Northern Hemisphere, the North Atlantic Oscillation (NAO) and the Pacific/North American Pattern (PNA), during the 20th century. We employ 500 hPa winter monthly means of the long-term reanalysis 20CR, which cover the 1871-2011 period.
We use a moving Principal Component Analysis calculated for 40-year periods with a one-year step. The spatial structure of NAO is well developed and stable during the whole 20th century using the ensemble mean.
However, substantial changes in PNA pattern occur during the late 19th century. Its centres are weaker and smaller, except the centre over the northern Pacific Ocean, which is stronger and larger in the early period.
However, these changes are not visible when using ensemble members. Therefore, we suggest that these shifts do not reflect real changes in the atmosphere but are produced by the reanalysis and statistical method itself.