Effect of solar activity
A substantial portion of the climate variability in the Atlantic sector is associated with the North Atlantic Oscillation (NAO), with variations occurring on a wide range of scales. The influence of solar activity (expressed by various indices) on the NAO has been studied by a number of authors. Having reviewed the information available to him, Lamb (1972, p. 252) noticed several tendencies in the surface parameters in relation to solar activity. These tendencies included:
- Strong development of the mid-latitude westerlies, Icelandic low and Azores high around the middle of the declining phase of solar activity;
- Strong meridional and cellular circulation systems at some stage during the more rapid rises of solar disturbance and greatest frequency of very strong anomalies of pressure and temperature (intense systems);
- Also (perhaps later) during the ascent, a phase of strong middle latitudes westerlies, strong Icelandic low and Azores high, seems particularly liable to occur.
More recently, Bucha and Bucha (1998) found a correlation between geomagnetic activity and sea level pressure variations similar to the NAO for the period 1970 to 1996. They suggested a mechanism based on winds generated in the polar thermosphere following geomagnetic storms.
Bochnicek and Hejda (2005) demonstrated that during the winter periods (January–March) of the years 1963–2001 high geomagnetic activity was nearly always associated with a positive phase of the NAO, whereas low geomagnetic activity tended to couple with the negative phase. Palamara and Bryant (2004) and Fujita and Tanaka (2007) found a similar relationship with the Northern Annular Mode (NAM). According to Thejll et al. (2003), who studied the relationship between the geomagnetic index Ap and the NAO for the period 1949-2000, the correlation was high and significant only since about 1972. However, for the period 1949–1972 no significant correlations were found at the surface while significant correlations still existed in the stratosphere. This might indicate that the solar forcing, primarily acting in the stratosphere, is propagating its influence downward in the later period but not in the earlier.
A robust relationship between solar cycle variations, proxied by the 10.7 cm solar radio flux, and the NAM has been found by Ruzmaikin and Feynman (2002). In particular, the NAM index was found to be systematically more negative (corresponding to a weaker polar jet) during low solar activity (Ruzmaikin et al., 2004).
Kodera (2002, 2003) showed that the spatial structure of the NAO varies significantly according to the phase of the solar cycle. During solar maximum phases, the NAO covers the Northern Hemisphere and extends into the stratosphere, which is similar to the Arctic Oscillation (AO) (Thompson and Wallace, 1998), except for the Pacific sector. By contrast, for minimum solar phases, the NAO is confined to the Atlantic sector and to the troposphere.
Boberg and Lundstedt (2002, 2003) showed that variations of the NAO index could be correlated with the electric field strength of the solar wind. Using geopotential height data they found a strong correlation between the electric field strength of the solar wind and pressure variations in the stratosphere and troposphere. For the tropospheric pressure the influence is confined to the North Atlantic and resembles the action of the NAO.
On a secular time scale, Kirov and Georgieva (2002) found a negative correlation between the NAO index and sunspot activity: the index had a maximum during the period of low solar activity in the late 19th and early 20th century and a minimum during the period of high solar activity in the 1950s and 1960s. However, since the data covers only one secular cycle, their conclusion is not statistically sound. In their later work, addressing the issue of instability in solar terrestrial relationships, Georgieva et al. (2007) underscored the importance of asymmetry between sunspot numbers in the northern and southern solar hemispheres. They hypothesize that when the southern solar hemisphere is more active, increasing solar activity in the secular solar cycle leads to strengthening of the zonal atmospheric circulation, and when the northern solar hemisphere is more active, increasing solar activity in the secular solar cycle leads to weakening of the zonal circulation.
There are also a number of works that have examined the effect of solar activity on climatic variables other than the NAO (but often closely related to the NAO). Here, for brevity, we will mention just one of those works, because it underscores the importance of the 22-year Hale cycle, which manifests itself in reversal of polarity of sunspots from one 11-year cycle to another. According to Bochkov (1978), during even cycles of solar activity and on its ascending branch, the Barents Sea is characterized by suppressed cyclonic activity, negative anomalies of sea and air temperature and increased ice cover. In contrast, during the decreasing branch of solar activity (2-5 years after its maximum), the Barents Sea tends to be warmer than normal. The situation during the odd cycles of solar activity is less clear.
Despite the complexity of solar effect on North Atlantic climate, most of the authors seem to agree that negative (positive) NAO phases tend to occur during low (high) levels of solar activity. This simplified relationship refers to both the 11-year and secular solar cycles.
Currently, the solar activity is at the beginning of its 24th cycle. Also, it seems to be on a declining phase of the secular cycle, but still remains relatively high. The behaviour of the NAO (Fig. 1) and AO (Fig. 2) indices in recent decades seem to be consistent with the above relationship: both indices reached their maximum values in the early 1990s and now tend to stay close to their average values.
As the 24th solar cycle progresses, entering into its ascending phase, one can expect a weakening of the subpolar low and developing of a meridional type of atmospheric circulation, with an increasing frequency of the negative NAO. Closer to the maximum of solar activity, which is expected in 2011-2012 (see solar activity), and on the descending branch of the cycle, zonal atmospheric circulation (positive NAO) may become prevalent again. Much will depend on whether the 24th cycle will be weak or strong. If it is going to be a weak cycle (which is somewhat more likely), the NAO may become strongly negative, resulting in a substantial cooling in the Northeast Atlantic, Norwegian and Barents Seas.