Role of the Ocean in Maintaining the North-Atlantic Oscillation

Based on the data on the sea-surface temperature (SST), the heat content of the upper 200-m 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, decadal, 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 subtropical 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.     

On the Statistical Characteristics of the North-Atlantic Oscillation

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 10 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 characteristic of the CAA for time scales of 2–7, 7–15, and more than 15 yr. Each of these variability intervals is analyzed separately. We established a significant correlation between the index 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 pressure 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 intense. We also discuss possible mechanisms of the obtained statistical relations.