Possible Influence Of Selfoscillations In The Oceaniceatmosphere System

A number of scientists assume that some specific components of modern climate change are, in the words of Shuleikin (1953), "within our planet itself, within its liquid and gaseous (and maybe partly within its solid) shells." As early as the 1930s, Shuleikin began to assume that the "ocean-atmosphere-land system is a self-oscillating system."

The self-oscillations of some systems are known to differ in principle from other oscillating processes in that no periodic external forcing is required for their occurrence; they are determined by the properties of the system itself. Self-oscillations require a constant energy source and mechanisms to regulate the input of this energy to the oscillating system, as well as negative feedbacks that tend to return the system to equilibrium.

Shuleikin's self-oscillation scheme concerned the ocean-ice system. Later, it was extended to include the atmosphere because its interaction with the ocean is a significant cause of climate change. In two proposed self-oscillation schemes, the feedback mechanisms are driven by water stratification (Nikiforov and Shpaikher, 1980) and fractures resulting from ice-cover dynamics in the Arctic Basin (Alekseev, 1976) that influence heat exchange between the ocean and the atmosphere, which is known to cause changes in atmospheric circulation. Nikiforov (2006) proposes a theoretical basis for self-oscillations in the Arctic Ocean system: a "chain of mechanisms responsible for the occurrence of the oscillating regime in a definite frequency band'' (p. 107). In this scheme, oscillations with periods of 4-8 years occur in the horizontal plane of interaction between the Arctic and the North European Basins of the Arctic Ocean, while interaction in the vertical plane induces low-frequency oscillations with periods of 20-30 years.

Zakharov (1996) devotes a great deal of attention to self-oscillation in the ocean-ice-atmosphere system as the most probable driver of natural processes in the Arctic. This author considers the ice cover an active climatologic factor determining, in particular, the intensity of the Arctic High and the southerly displacement of the Arctic atmospheric front and the related belt of cyclonic activity. Zakharov (1977) also identifies the layer of low-salinity surface water underlying the ice as the main control on ice cover area (at least in winter). Expansion of this low-salinity water is regulated by the freshwater balance of the Arctic Ocean (inflow of freshwater, its outflow to the Atlantic, and the excess of atmospheric precipitation over evaporation).

Based on these cause-effect relationships, Zakharov suggests a consistent conceptual scheme of self-oscillations in the atmosphere-ocean-ice system that explains current climate change. The following sequence represents this scheme: positive freshwater budget in the Arctic Ocean ^ increasing volume and area of surface Arctic water ^ ice cover expansion and atmospheric cooling ^ a southerly shift of the Arctic climatic front and the precipitation belt ^ decreased freshwater inflow to the Arctic Ocean ^ a negative freshwater budget in the Arctic Ocean ^

decreased volume and area of surface Arctic water and ice expansion, and so forth, reversing the order of these phenomena.

There is, however, one weakness in this scheme. Cooling and expansion of the Arctic High, in some cases, actually leads to the southward shift of the Arctic High and the precipitation belt. But this should result in an increase in river runoff to the Arctic Ocean, rather than its decrease, as precipitation in the relevant river basins increases. There is much less excess of precipitation over evaporation at high latitudes than at more temperate latitudes (Anon. (G)). If this is the case, then instead of a negative feedback, there is a positive feedback (cooling ^ increased runoff, warming ^ decreased runoff), which would not result in self-oscillation. This is confirmed not only by the precipitation decrease in Eastern Europe during the first Arctic warming in the 1930s and the decrease in Caspian Sea level at the same time but also by an analysis of the relationship of Kara Sea ice conditions to river runoff and air temperature by Gudkovich et al. (1981). Their study of 40 years of observations made between 1936 and 1975 indicates that when the relationship between sea ice extent and runoff from the Ob' and Yenisey rivers was reliable, the air temperature decrease and increased runoff operated in tandem, and vice versa (the coherence function is 0.66 to 1.00). However, in the other cases, expansion of the Arctic High signaled its merging with the Siberian High, thus hindering west-to-east air circulation over the Asian continent. Siberian river basin precipitation and runoff to the Arctic Ocean are dependent on the intensity of this air transport (see section 4.7).

During the periods of Arctic warming induced by 60-year cyclic fluctuations of the climate system, as noted above, increased precipitation in temperate latitudes of the Eurasian continent and corresponding increase in river runoff were connected with intensified cyclonic activity over the Arctic Basin, a weakened Arctic High, and progressive increase in atmospheric zonal flows. There was a simultaneous decrease in sea ice extent of the Arctic Ocean seas along with a synchronous positive trend in river runoff, which also does not support the discussed self-oscillation scheme.

Another disadvantage of Zakharov's (1996, 1997) concept is that the author considers only a horizontal advective mechanism for the fluctuation in low salinity surface water distribution. Meanwhile, as Section 4.6 shows, vertical circulation driven by baric-field vorticity (salinification of surface water during strong cyclonic activity and freshening at its weakening) is also important.

The large-scale interaction of the ocean and the atmosphere is most pronounced in the so-called energy-active zones such as the Norwegian energy-active zone (NEAZO) in the North European Basin, where relatively warm and saline Atlantic Ocean waters meet cold and low salinity water exported from the Arctic Basin. The atmosphere-ocean interaction is regulated here by the contrasts of the underlying surface temperature, on which the intensity of cyclogenesis depends. Baric field vorticity influences vertical circulation of the water and its stratification, which affects the process of convection that brings heat from deeper layers to the surface of the ocean. Gudkovich and Kovalev (2002) express this interdependent chain of self-oscillations and the time lags between them as:

. ,.L+(3)^ AP+(2) ^ S +(6) ^ L-(3) ^ AP"(2) ^ S"(6)^ L+ ..., where L, AP, and S denote the sea ice extent, mean annual vorticity of the wind field, and surface water layer salinity, respectively; signs (+) and (—) express the maxima and minima of the indicated values, and figures in brackets show the corresponding average values of lags (in years). The period of these self-oscillations is, on average, 22 years. This is a typical series of self-oscillations: a 10-year increase in sea ice extent results in its decrease, and vice versa (a negative feedback). The components of this process include: dependence of the intensity of cyclonic activity on sea ice extent and the related temperature gradients of the water and the air, dependence of surface-layer salinification on cyclonic activity, and inverse dependence of sea ice extent on surface water salinity and air temperature in the region. The time lags between these processes are probably determined by their "inertia", because changes in some characteristics of the ocean or the ice cover "can result from prolonged accumulation of stochastic forcing by the atmosphere'' (Alekseev, 1995, p. 195).

The prevailing period of about 20 years suggests the possible influence of solar activity whose interannual variations exhibit a pronounced 22-year (Hale) cycle. Further research is needed on two processes with similar time cycles: the solar activity-influenced formation of baric anomalies (see section 5.2) and the lunar long-period tide with a cycle of about 19 years (e.g., Maksimov, 1970; Sleptsov-Shevlevich, 1991). These factors may have a stabilizing influence on the cyclic processes in the ocean-ice cover-atmosphere system.

A hypothesis set forth by Dukhovskoy et al. (2004) proposes another example of self-oscillation in this system. This hypothesis attributes the fluctuations in decadal-scale /NAO indices to self-oscillation that results from heat and freshwater exchange between the Arctic Basin and the Nordic Seas. According to this hypothesis, during periods of weak interaction between these regions, low salinity surface water accumulates in the Beaufort Gyre area and causes the sea level to rise. At this time, a freshwater deficit in the Nordic Seas reduces the ocean vertical stratification leading to an increase of heat flux from the ocean to atmosphere, which causes intensified cyclonic activity in the atmosphere and decreased sea level. The growing sea level gradient between the two regions then leads to strong interaction as freshwater flows from the Arctic Basin to the North European Basin, and warm Atlantic water flows northward. The Arctic High then weakens, the air temperature increases, and both the sea level gradient between the regions and freshwater runoff decrease. As a result, the system gradually returns to a state of weak interaction. The model developed by Dukhovskoy et al. (2004) showed the period of this self-oscillation to be 10-12 years.

A weak point in this hypothesis is the absence in nature of a critical level gradient value at which the convergence of full flows in the Ekman layer of anticyclonic circulation attains an opposite sign that is inherent in the divergence conditions. After all, the accumulation of water with decreased density in the anticyclonic circulations and a corresponding sea level increase are restricted by the vertical circulation (downwelling and water outflow at depth). This is illustrated by such global ocean currents as the Gulf Stream and the Kuroshio, where baroclinic current speeds and sea level gradients are two orders of magnitude greater than those typical of the Arctic Ocean. No data suggest that vertical cross-circulation in such currents attains an opposite direction.

Was this article helpful?

0 0

Post a comment