Physical Oceanography, Vol.
ANALYSIS OF OBSERVATIONS AND METHODS FOR CALCULATING
HYDROPHYSICAL FIELDS IN THE OCEAN
ROLE OF THE OCEAN IN MAINTAINING THE NORTH-ATLANTIC OSCILLATION
A. B. Polonskii, E. P. Semiletova, and G. F. Dzhiganshin
UDC 551.465-551.5 (261.1)
Based on the data on the sea-surface temperature (SST), the heat content of the upper
layer, and the sea-level pressure, we analyze the low-frequency variability of the SST and heat
content in the North Atlantic in 1950–1992 and the index of North-Atlantic Oscillation (NAO) in
1940–1995. It is confirmed that the role of the ocean and various mechanisms controlling the
variability of SST changes for processes corresponding to different time scales (interannual, de-
cadal, and interdecadal). It is shown that the interaction of tropical and subtropical latitudes is of
principal importance on the interannual scale, the processes regulating the variability of subtropi-
cal gyre are important on the decadal scale, and the variations of the NAO with lower
frequencies are controlled by the oceanic variability at high latitudes. We discuss possible
feedbacks in the ocean–atmosphere system maintaining the NAO.
The North-Atlantic Oscillation is one of the main manifestations of climatically valuable signals in the
Northern Hemisphere. It can be described as quasisynchronous strengthening and weakening of the centers of
action in the atmosphere of the North Atlantic, namely, of the Azores High and Iceland Low.
The NAO was first
described in . Later, its statistical characteristics were analyzed in numerous works [2– 10]. It was shown
that the spectra of the NAO index are characterized by pronounced peaks on the interannual and decadal scales.
50–100-yr oscillations of the pressure field are also observed but cannot be reliably separated and analyzed
with the help of traditional statistical methods on the basis of the available (relatively short-term) instrumental
data. Similar oscillations are also observed in the field of SST. At the same time, the statistical characteristics of
the North Atlantic Oscillation are unstable and vary with time. As one of the causes of this instability, one can
mention the variability of the efficiency of feedbacks in the ocean–atmosphere system [3, 5–7].
The role of feedbacks in the ocean–atmosphere system in maintaining the NAO has been little studied. In
numerous works, it is stated that the ocean is a fairly passive element of this system and simply integrates rela-
tively high-frequency atmospheric perturbations. This concept goes back to Hasselmann’s ideas . In other
works, it is stated that the large-scale interaction of the ocean and atmosphere is a very important element sup-
porting low-frequency variations of the atmospheric circulation in the North Atlantic [3, 7, 12]. It should be
emphasized that the advection of thermal anomalies plays an especially important role in this process. The anal-
ysis of the low-frequency variability of temperature of the active layer of the ocean and its heat content carried
out on the basis of the experimental data [13, 14] and according to the results of numerical experiments within
the framework of the model of general circulation of the ocean  confirms this conclusion.
Marine Hydrophysical Institute, Ukrainian Academy of Sciences, Sevastopol. Translated from Morskoi Gidrofizicheskii Zhurnal,
47–55, September–October, 2002. Original article submitted April 9, 2001; revision submitted April 24, 2001.
282 0928-5105/02/1205–0282 $27.00 © 2002 Plenum Publishing Corporation