North Atlantic
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.
Cycles
Through the analysis of a vast collection of observational data, Polyakov et al. (2005) have demonstrated that multidecadal fluctuations on time scales of 50–80 yr are prevalent in the upper 3000 m of the North Atlantic Ocean and adjacent Arctic seas. As an example, Fig. 3 shows Atlantic water inflow to the Barents Sea, as reflected in temperature of the upper 200 m of the Kola Section. Yndestad (1999) identified a cycle with the period of 55.8 years among the dominant cycles in this time series (the other two have periods of 6 and 18 years), although it seems closer to 70-80 years in Fig. 3. Later Yndestad (2006) added the fourth cycle with the period of 74 years to this list. The 74-yr cycle was also found in the time-series of Barents Sea ice extent (Yndestad, 2006).
Multidecadal variations are also apparent in the NAO index. A spectral analysis of the NAO index time series from 1886 to 1994 computed by Yi et al., 1999 (their Fig. 9b) shows significant power at periods of approximately 50 years. Using a wavelet analysis, Yndestad (2006) identified a 74-yr cycle in the winter NAO index time series.
A clear long-term cycle is seen in the winter NAO index since the early 1950s (Fig. 1). The index reached a minimum in the 1960s and a maximum in the late 1980s – early 1990s. Since then it declined, almost completing the full cycle. One can notice the difference in the timing of maxima and minima between the NAO index and Kola section temperature. This phase shift will be discussed in the next section.
The NAO index can be defined not only in terms of atmospheric pressure, but also in terms of temperature. Van Loon and Rogers (1978), for example, used the difference in normalized temperature anomalies in Oslo, Norway, and Jakobshavn, West Greenland. Fig. 4 shows the winter NAO index, defined this way. The correlation between this index and the conventional SLP-based NAO index from CRU is 0.75 for the entire period of observations (1883-2006). As seen in Fig. 4, within the envelope of long-term, secular changes, the NAO index exhibits a shorter, 18-20-yr cycle. Yndestad (2006) links this cycle with the 18.6-yr lunar nodal cycle.
The sum of temperature anomalies in Oslo and Jakobshavn (Fig. 5) identifies the periods of warming and cooling in the entire northern North Atlantic. Two major periods of warming (in the 1920s-1930s and in the past 2 decades) are clearly reflected in this time series. Between these two periods, the index experienced fluctuations with a period of approximately 15 years. As shown by Moron et al. (1998), the life cycle of the 13-15 year oscillation in North Atlantic sea-surface temperature (SST) exhibits a striking seesaw, with the two maxima of opposite signs occurring between Cape Hatteras and the Bermudas and due south of the Denmark Straits, near the North Atlantic Drift, respectively. Another source of climate variability on this time scale may lie in the Arctic. In a conceptual model introduced by Dukhovskoy et al., 2004, the Arctic climate system oscillates with periods from 10 to 15 years.
When SST is averaged over the entire North Atlantic and the upward linear trend is removed, the residual time series exhibits a cycle with an approximate period of 70 years known as the Atlantic Multidecadal Oscillation (AMO). As seen in Fig. 6, a new warm phase of the cycle started in the late 1990s. If the cycle continues, the North Atlantic will remain relatively warm for another 20-30 years.
The NAO and Barents Sea climate
A conventional view of climatic variations in the Barents Sea (as well as the Norwegian Sea and adjacent regions) is that they are driven largely by the NAO: When the NAO is in its positive (negative) phase, the Barents Sea is warmer (colder) than normal (Dickson et al., 2000). It is surprising, therefore, that the correlation coefficient between the winter (DJF) NAO index and winter (DJFM) temperature at the Kola section is only 0.21 for the entire period of observation from 1922-2008. For the time lags of 1 and 2 years (NAO leads), possibly required for Atlantic waters to reach the Barents Sea, the correlation coefficients are 0.24 and 0.14, respectively.
The cross-correlation function between the NAO index and Kola section temperature is presented in Fig. 7. It suggests that both time series have substantial energy of fluctuations at the period of about 80 years. The maximum of the cross-correlation function of 0.47 (significant at the 95% level) is reached when the NAO leads Kola temperature by 16-17 years.
In Fig. 8 both time series are overlapped, with the NAO series shifted forward by 17 years. The match between the series is closer since the late 1970s, when the correlation coefficient reaches 0.71 (data for 1979-2008). Obviously, this correlation coefficient is somewhat inflated due to the strong upward trend during this period. When the linear trend is removed the correlation coefficient is reduced to 0.49, but still remains statistically significant at the 95% level.
A significant portion of NAO variability occurs at the interannual time scale. At this time scale, an advection of warm Atlantic air and water into the Norwegian Sea during a positive NAO phase is not enough to increase SST in the region substantially enough to exert any feedback effect on the Icelandic low. If, for some reason, a positive phase of the NAO continues for a number of years, it may lead to an increasingly intense and widespread influence of Atlantic waters in the Nordic Seas (Dickson et al., 2000), although the exact mechanisms linking the anomalous wind field to the inflow are not clear (Furevik and Nilsen, 2005). The increased presence of warm Atlantic waters causes an enhanced upward ocean heat flux creating a situation conducive to a more eastward positioning of the Icelandic low (Bengtsson et al., 2004). A strong, single-centered and shifted eastward Icelandic low implies further warming in the Norwegian and Barents Seas. When the Icelandic low is shifted eastward far enough, it also intensifies an advection of polar water and sea ice transport through Fram Strait, and hence cooling in the waters off the east coast of Greenland (Hilmer and Jung, 2000). A sharpening contrast between the cold west an warm east in the Nordic Seas establishes favorable conditions for further diabatic contributions to cyclonic development and, hence, strengthening of the Icelandic low (Rogers et al., 2004). Thus, a positive feedback loop is created that leads to an amplified warming in the Norwegian and Barents Seas and the entire Arctic.
Since the Icelandic low is shifted eastward, SLP near Iceland increases. The advection of cold arctic air west of Greenland is also reduced and the area warms up. As a result, the NAO index (computed using either SLP or temperature data) decreases. One may say that the decrease of the NAO index in this case is strictly technical, simply because of the way the index is calculated. Meantime, the Icelandic low remains strong, with its central pressure being below normal.
As the Arctic becomes anomalously warm, the equator-pole gradient diminishes and the zonal hemispheric circulation weakens. Being part of this circulation, the Icelandic low also starts weakening, and the advection of warm air along its eastern periphery northward diminishes. This starts the cooling trend in the Barents Sea. Another mechanism that leads to a weakening of the Icelandic low is associated with an intense freshwater flux through Fram Strait by a strong and eastward shifted Icelandic low. Due to anomalous freshwater inflow, water stratification increases, deep convection ceases, ice appears in the Greenland Gyre, the atmosphere starts cooling and SLP increases (Dukhovskoy et al., 2004).
After the Arctic cools down, the equator-pole gradient increases again and the hemispheric zonal circulation intensifies. The strong zonal flow acts as a barrier to warmer air reaching the high latitudes and the Arctic continues to cool, which creates a positive feedback. In the Southern hemisphere, which has significantly more ocean and less land than the Northern hemisphere, this effect is expressed even stronger (Thompson and Solomon, 2002). The break of the loop occurs when the westerlies accelerate to the point when it becomes unstable.
As the westerlies slow down and the Icelandic low retreats to its more normal position between Iceland and the southern tip of Greenland, the fresh water inflow to the Nordic Sea though Fram Strait decreases. The reduction of fresh, buoyant surface water combined with overall colder temperature in the Arctic decreases water column stability and favors deep convection in the Greenland Sea. This thermohaline ventilation generates overflows to the North Atlantic, which also require a compensating inflow of Atlantic water to the Nordic Seas (Hansen and Osterhus, 2000). This starts a new cycle of warming, deepening of the Icelandic low and its shift eastward.
In light of this conceptual model, the sequence of climatic events in the past 100 years can be interpreted as follows:
| 1900s - mid-1920s: | This period was characterized by a strong zonal circulation as evidenced by the frequency of Vangenheim-Girs’ zonal atmospheric circulation type W (Girs, 1971) or atmosphere’s angular momentum (Salstein and Rosen, 1986). The latter reached the record maximum (for the period 1860-1985) in the early 1910s. The general westerly type (W) of Lamb (1972), which corresponds mostly with SW'ly winds at the surface, especially in southern Britain, was most frequent in the early 1920s. Overall, this period was the coldest in the Arctic in the last 100+ years (Polyakov et al., 2003). |
| Mid 1920s - 1940s: | A sharp increase of the Arctic temperature in the 1920s was one of the most fascinating climatic events of the 20th century. This Arctic warming was associated with an eastward shift of the Icelandic low, which can be seen in Fig. 6 in van Loon and Rogers, 1978. This is also consistent with (Bengtsson et al., 2004) who found a steadily increasing transport of warm water into the Barents Sea during this time driven by increasingly southwesterly to westerly winds between Spitsbergen and the northernmost Norwegian coast. The warming in the Barents Sea (Fig. 3) and the Arctic (Polyakov et al., 2003) peaked in the late 1930s. At that time, the NAO index (both SLP-based and SAT-based) was already declining. |
| 1950s - 1970s: | The NAO index continued to decline and reached extremely negative values in the 1960s (Fig. 4). The Icelandic low was weak (and often split into two centers) and unable to pump much warm air into the Nordic Seas. Temperature in the Barents Sea and the Arctic overall cooled down substantially. In the 1950s and 1960s, water stratification in the Greenland Sea was weak, suggesting an intensification of deep-water convection (Vinje et al., 2002). In the early 1970, the mid-latitude westerly winds over the northern hemisphere sharply increased, perhaps due to a strong temperature gradient between the pole and the equator. However, the warming in the Barents Sea was not enough to cause an eastward shift of the Icelandic low. Therefore, when the Icelandic low weakened again in the late 1970s (probably in association with the regime shift in the Pacific climate), the Arctic was still too cold and temperature in the Barents Sea plunged to the lowest values on record (Fig. 3). |
| 1980s-2008: | Strong and sustained zonal circulation has established over the Northern hemisphere during this period. Its characteristic feature is a northward shift of the jet streams, accompanied by a widening of the tropical belt (Seidel et al., 2008). Both the inflow of the Atlantic waters and the upward oceanic heat flux in the Barents Sea has increased substantially (Zhang et al., 2004). As a result, the Icelandic low was consistently shifted eastward since the 1990s (Jung et al., 2003; Hurrell and Dickson, 2004) and the Arctic warming accelerated. Years 2005-2008 were particularly warm in the Barents Sea rivaling those in the late 1930s (Fig. 3). The NAO index, however, after reaching the maximum in the early 1990s, has started to decline (Fig. 1). The discrepancy between the NAO/AO indices and Arctic temperature variability in recent years is termed the “Arctic paradox” (Overland and Wang, 2005). |
| 2009-2023: | If the 17-yr time lag between the NAO index and Kola Section temperature remains in the future, one can expect that the Barents Sea will remain anomalously warm until about 2012 (Fig. 8). After that, a long term cooling trend until about 2023 is likely. The cooling may start as a sharp decline of winter temperatures. |
Conclusion
It is expected that the NAO index, due to its 18-yr cycle, will remain positive in the next 2-3 years. It will not reach, however, the level observed in the early 1990s. The maximum of the secular cycle in the NAO index appears to have passed, and the index has started its long-term decline that may last for several decades.
The warming trend in the Norwegian and Barents Seas (as well as the Arctic overall) is close to its end. The temperature there will level off in the next few years and then start to decline. The transition to a colder climate may be quite sharp.
Since the Icelandic low will remain strong in the next few years and return to its more normal position in Davis Strait, temperature in the Northwest Atlantic will decrease. However, due to a long-term weakening trend of the Icelandic low, the region’s climate will be warmer than normal.
The North Atlantic overall will remain warmer than normal, largely due to the tropical and subtropical regions. The positive phase of the AMO will continue for another 20-30 years.
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