final_program_abstracts[1]

11 IMSC Session Program
Different definitions of the North Atlantic oscillation index: A
comparison of their effects on surface climate elements in
Europe
Monday - Poster Session 4
Lucie Pokorná and Radan Huth
Institute of Atmospheric Physics, Academy of Sciences of the Czech Republic, Prague,
Czech Republic
The aim of this work is to compare various definitions of the North Atlantic
Oscillation (NAO) index that have frequently been employed in recent studies. The
period used for the comparison is 1958-1998 and only winter and summer seasons are
analyzed. We examine seven definitions of the monthly NAO indices and scores
(scores were derived from rotated solutions of principal component analysis):
• Index based on normalized sea level pressure (SLP) anomaly between Ponta
Delgada, Azores, and Stykkisholmur, Iceland,
• Index based on SLP anomaly between Gibraltar and Reykjavik, Iceland,
• Index based on SLP anomaly between the centres of the Azores High (AH)
and Icelandic Low (IL),
• The first principal component of 4 characteristics of the NAO centres: SLP
anomalies in the centres of the AH and IL and their latitudes,
• Rotated principal component score of the NAO mode for the field of the 500
hPa geopotential heights in the CDAS dataset (produced by the National
Weather Service, NOAA),
• Rotated principal component score of the NAO mode for the 500 hPa height
field in the NCEP/NCAR reanalysis,
• Rotated principal component score of the NAO mode for the SLP field in the
NCEP/NCAR reanalysis.
The correlations of indices with four climatic elements over Europe are used for
comparison: mean monthly maximum and minimum surface temperature, the number
of days with precipitation ≥0.1 mm, and monthly precipitation amounts. The climate
elements are defined at 82 stations across whole Europe and are taken from the
ECA&D database.
Correlation among the indices and scores range from 0.64 to 0.86 in winter, while
from 0.50 to 0.87 in summer; correlations of all pairs of indices and scores are
statistically significant. The NAO is clearly defined in winter, the location of the
centres varying only little among different definitions. The differences in the surface
climate response between the indices and scores in winter are small and statistically
insignificant for the major part of Europe. The character of the subtropical centre is
sensitive to the definition of the NAO in summer. It shifts westward and northward
for the definition based on SLP, the western, central, and northern Europe being
influenced by a weak positive anomaly during the positive phase of NAO. For the
definition based on 500 hPa heights, the southern centre of the NAO splits into two
cells, all Europe except the Mediterranean being influenced by a strong positive
anomaly. The correlations of the NAO indices with surface temperature and
Abstracts 41