Physical Oceanography, Vol.
ANALYSIS OF OBSERVATIONS AND METHODS FOR CALCULATING
HYDROPHYSICAL FIELDS IN THE OCEAN
ON THE STATISTICAL CHARACTERISTICS
OF THE NORTH-ATLANTIC OSCILLATION
A. B. Polonskii and E. P. Semiletova
By using archival monthly data for 100
yr, we analyze the statistical structure of time series
characterizing the variability of the Azores High and Iceland Low. We show that there exists a
long-term tendency towards approach of the centers of action of the atmosphere (CAA) in the
North Atlantic and their strengthening. At the same time, quasiperiodic decadal intensification
of the CAA is accompanied by an increase in the distance between them. In the spectra of sea-
level pressure in the Azores High and Iceland Low, significant peaks for periods of 2–5 and
yr are strongly pronounced. Furthermore, oscillations with a period ∼50 yr are clearly seen.
The main contribution to the interannual changes in the index of the North-Atlantic Oscillation
(NAO) is made by pressure variability in the Iceland Low. Different ways of behavior are char-
acteristic of the CAA for time scales of 2–7, 7–15, and more than 15 yr. Each of these vari-
ability intervals is analyzed separately. We established a significant correlation between the in-
dex of the Southern Oscillation (SO) and characteristics of the CAA of the North Atlantic only
for time scales of 7–15 yr. It demonstrates that, as the SO index increases in autumn, the pres-
sure at the center of the Azores High grows and the latitudinal distance between the Azores High
and Iceland Low decreases, i.e., the zonal circulation in the North Atlantic becomes more in-
tense. We also discuss possible mechanisms of the obtained statistical relations.
The centers of action of the atmosphere in the North Atlantic, i.e., the Azores High and Iceland Low, are of
major importance in the formation of climatic variability of the European region. Walker and Bliss  were the
first to describe the quasisynchronous low-frequency variability of the CAA of the North Atlantic in 1932. After
that, the NAO was considered in numerous works [2
11]. The NAO index (defined as the normalized differ-
ence between surface pressures at one of the stations in the Azores or in Lisbon and at one of the stations in Ice-
land) or the Rossby index (the pressure difference between the Azores High and Iceland Low
) is usually used
as a quantitative characteristic. The coefficient of correlation between the monthly values of NAO and Rossby
indices, calculated by centenary series, varies within the range 0.54
0.77, reaching its maximum in winter .
In the spectra of NAO indices, significant peaks for periods of 2
3 and 8
10 yr are clearly pronounced. The
time series of Rossby indices, determined by monthly surface-pressure charts, are noisier as compared with the
series of NAO indices because marine meteorological data (especially before the middle of the 20th century) are
limited and of low quality. At the same time, monthly pressure charts enable us to analyze, in addition, the
It may be more appropriate to call this difference an analog of the Rossby index , but we here use a shorter term.
Marine Hydrophysical Institute, Ukrainian Academy of Sciences, Sevastopol. Translated from Morskoi Gidrofizicheskii Zhurnal,
42, May–June, 2002. Original article submitted October 24, 2000; revision submitted December 11, 2000.
142 0928-5105/02/1203–0142 $27.00 © 2002 Plenum Publishing Corporation