Greatbatch, Richard John, Gollan, Gereon, Jung, T. and Kunz, Torben (2012) Factors influencing Northern Hemisphere winter mean atmospheric circulation anomalies during the period 1960/61 to 2001/02 Quarterly Journal of the Royal Meteorological Society, 138 . pp. 1970-1982. DOI 10.1002/qj.1947.

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Abstract

Influences from the Tropics, the stratosphere and the specification of observed sea
surface temperature and sea-ice (SSTSI) on Northern Hemisphere winter mean
circulation anomalies during the period 1960/61 to 2001/02 are studied using a
relaxation technique applied to the ECMWF model. On interannual time-scales,
the Tropics strongly influence the Pacific sector but also the North Atlantic sector,
although weakly. The stratosphere is found to be influential on the North Atlantic
Oscillation (NAO) on interannual time-scales but is less important over the Pacific
sector. Adding the observed SSTSI to the tropical relaxation runs generally improves
the model performance on interannual time-scales but degrades/enhances the
model’s ability to capture the 42-year trend over the Pacific/Atlantic sector. While
relaxing the stratosphere to the reanalysis fails to capture the trend over the whole
42-year period, the stratosphere is shown to be influential on the upward trend of the
NAO index from 1965 to 1995, but with reduced amplitude compared to previous
studies. Influence from the Tropics is found to be important for the trend over both
time periods and over both sectors although, across all experiments, we can account
for only 30% of the amplitude of the hemispheric trend. Copyright c� 2012 Royal
Meteorological Society

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