The North Atlantic Oscillation  (NAO) refers to swings in the atmospheric sea level pressure differences between the Arctic and  the subtropical Atlantic. It exerts a strong control on winter climate in Europe, North America, and Northern Asia.

The NAO index is defined as the normalized pressure difference between measurements of stations on the Azores and Iceland. A positive NAO index indicates a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low. The increased pressure difference results in more and stronger winter storms crossing the Atlantic Ocean on a more northerly track. This results in warm and wet winters in Europe and cold and dry winters in Greenland and Northern Canada, while the eastern Unites States experience mild and wet winter conditions. A negative NAO index points to a weak subtropical high and a weak Icelandic low. The reduced pressure gradient results in fewer and weaker winter storms crossing mostly on west-east paths bringing moist air into the Mediterranean and cold air to Northern Europe. The east coast of the United States gets more cold air and snow while Greenland enjoys mild winters (Hurrell, 1995).

Despite these significant impacts of the NAO, it is not yet known which climate processes govern NAO variability, how the phenomenon has varied in the historical past, and to what extent it is predictable. Hurrel (2003) holds that the variations in the NAO are largely unpredictable as they arise from internal stochastic interactions between atmospheric storms and the mean atmospheric flow producing random fluctuations. He seems to take it for granted that the NAO is a free internal oscillation of the climate system not subjected to external forcing. I have shown however (Landscheidt, 2001a) that  the NAO is closely related to energetic solar eruptions. This external forcing is corroborated by evidence that other dominant modes of global climate variability like the El Niño/Southern Oscillation (ENSO) (Landscheidt, 2000a) and the Pacific Decadal Oscillation (PDO) (Landscheidt, 2001b) are so closely linked to the sun’s eruptional activity and  special phases in solar cycles that long-range forecasts can be based on this relationship. The last three El Niños and the course of the last La Niña  were correctly predicted on this basis years ahead of the respective events (Landscheidt, 2002). Moreover, it has been shown that the coolest phase of the current cold PDO regime is to be expected around 2007 and  the next regime shift from cold to warm around 2016 (Landscheidt, 2001b).

The inter-annual forecast of ENSO events has meanwhile been completed by a model that predicts El Niño and La Niña activity on a decadal scale (Landscheidt, 2003c). A similar model is presented here for the NAO. The forecasts cover the first half of this century.

2.  Analysis of yearly NAO index and forecast of NAO trend 

The blue curve in Fig. 1 shows yearly means of the NAO index covering the period 1825 to 2002. Jones et al. (1997) used early instrumental data to extend this index  back to 1825. Data with lots of  missing values go back to 1821, but were excluded here. The index is available at the Climate Research Unit of the University of East Anglia (2003). To see the trend, the time series was subjected to 30-year moving window Gaussian kernel smoothing (Lorzcak).

The cyclic pattern of the curve is closely linked to a well-investigated solar motion cycle. I have shown that the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO), El Niño and La Niña, extrema in global temperature anomalies, drought in Africa and U.S.A., as well as European floods are linked to cycles in the sun’s irregular orbital motion around the center of mass of the solar system (Landscheidt, 1983-2003). The rate of change of the sun’s orbital angular momentum L  - the rotary force dL/dt driving the sun’s orbital motion (torque) - forms a torque cycle with a mean length of 16 years (Landscheidt, 2001a,b). Perturbations in the sinusoidal course of this cycle recur at quasi-periodical intervals and mark zero phases of a perturbation cycle (PC) with a mean length of 35.8 years. These zero phases are called instances of greatest perturbation in the torque cycle (GPTC). As to details, I refer to Figure 2 of my on-line paper “Solar eruptions linked to North Atlantic Oscillation” (Landscheidt, 2001 a).

The GPTC phases play an important role in the long-range forecast of diverse climate phenomena. They indicate, for instance, the peaks of warm PDO regimes and the coolest phases of cold PDO regimes (Landscheidt, 2001b) and are closely linked to extended dry and wet spells measured by the U.S. drought index (Landscheidt, 2003 a). As to the details and physical implications of the Sun’s irregular orbital motion I refer to my papers “New Little Ice Age instead of global warming?” (Landscheidt, 2003b) and “Extrema in Sunspot Cycle Linked to Sun’s Motion" (Landscheidt, 1999).

Another approach to the 35.8-year cycle has been presented in Fig. 3 of my paper “Trends in Pacific Decadal Oscillation Subjected to Solar Forcing” (Landscheidt, 2001b).  It has been shown that absolute values of the torque cycle (|dL/dt|) form a shorter cycle that plays, e. g., a major role in solar forcing of the North Atlantic Oscillation (Landscheidt, 2001a) and discharges in river catchment areas (Landscheidt, 2000c,d). When a Gaussian low-pass filter suppressing wavelengths shorter than 9 years is applied to |dL/dt|, new oscillations emerge as shown in Fig. 3 of the quoted paper for 1721 - 2077. Minima in the smoothed |dL/dt|-curve are identical with initial phases GPTC in the perturbation cycle. So it is easy to compute the precise dates of these phases for any period. Within the range of the investigated NAO index, GPTCs fall at 1829.5, 1867.2, 1901.8, 1933.6, 1968.9, and beyond that range at 2007.2,  2044.9., and 2080.7.

In nearly all of my papers I could show that there are phase reversals in the climate time series related to solar motion cycles (Landscheidt 1983-2003). These are not ad hoc inventions, but computable phases of instability that occur when the zero phase of a longer solar motion cycle coincides with a zero phase of a shorter solar motion cycle. The arrow in Fig. 1 indicates a zero phase of the 179-year cycle, described in my paper “Decadal-scale variations in El Niño intensity” (Landscheidt, 2003c), which coincides with the GPTC phase 1901.8.

After the phase reversal around 1902, all deep minima in the NAO curve coincide with GPTCs indicated by red triangles. Before 1902 the relationship is reversed.  GPTCs go along with outstanding maxima in the curve.  Only GPTC 1829.5 does not fit.  This could be an effect of the deteriorating quality of the earliest data in the index reconstruction. The green triangles point to zero phases of the second harmonic of the perturbation cycle (SHPC) in between GPTCs. After the phase reversal they consistently coincide with maxima in the NAO curve and before 1902 with minima.

The extended maximum between 1890 and 1920 can be explained by  the phase reversal. After the GPTC 1901.8, going along with a maximum, a minimum was to be expected in the regular course of the oscillation.  Instead, another maximum appeared because of the phase reversal.  The situation is comparable to the Medieval Maximum of solar activity that can also be explained by such a phase reversal (Landscheidt, 2003b). Another extended NAO maximum of this kind is not to be expected in the foreseeable future as the next phase reversal related to a zero phase in the 179-year cycle will not occur before 2080.

Accordingly, the oscillatary pattern established after the phase reversal should stay stable. A forecast of the NAO trend can be read from Fig. 1. Deep minima in the trend curve are to be expected around 2007 and 2044 and  an outstanding maximum around 2026.

3.  Analysis and forecast of NAO winter season 

The effects of the NAO are most noticeable in the winter months December to March (Jones et al, 1997). The blue curve in Fig 2 shows these seasonal values. They were subjected to Gaussian kernel smoothing (Lorzcak) with a narrower 15-year moving window to get a more detailed  trend perspective. As can be seen from the figure, the pattern after the phase reversal is nearly the same as in Fig. 1 so that there is no need to formulate a more differentiated trend forecast. There is, however, some change in the period before the phase reversal. The SHPCs (green triangles) are related to maxima,  as after 1902, and more frequent minima go along with the fourth harmonic of the 35.8-year perturbation cycle (FHPC) indicated by smaller triangles in cyan colour. Theoretically, this is interesting, but it has no effect on the development in the foreseeable future.

4.  Link between NAO and solar eruptions

I have shown in several papers that  energetic solar eruptions (coronal mass ejections, flares, and eruptive prominences) have a strong effect on diverse climate phenomena including El Niño and La Niña (Landscheidt, 1983-2003). So it suggests itself to investigate whether energetic solar eruptions are connected with NAO variations, too. Not all strong solar eruptions have an impact on the near-Earth environment. The effect at Earth depends on the heliographic position of the eruption and conditions in interplanetary space. Indices of geomagnetic activity measure the response to those eruptions that actually affect the Earth. Mayaud’s aa index (Mayaud, 1973; Coffey, 1958-1999) is homogeneous and covers a long period back to 1868. So I compared the aa index with the NAO data of this period.

Figure 3 shows the result. The red curve represents yearly means of the aa index, normalized to the standard deviation and subjected to 30-year moving window Gaussian kernel smoothing (Lorzcak). The blue curve shows the yearly NAO index treated in the same way. Between 1940 and present the two time series show a clear positive correlation. The correlation coefficient is as high as r = 0.81 and explains 66 percent of the variance. Also from 1868 to 1890 the correlation is positive and strong: r = 0.80. Between 1890 and 1940, however, the correlation is negative and reaches r = - 0.83. Bootstrap re-sampling, making use of 500,000 samples drawn at random from the observed set, shows that there is less than 1 chance in 50,000 to falsely reject the sceptic null hypothesis of  no correlation.

The change in the sign of correlation is not as strange as it seems at first sight. It is a first indication that the quality of the solar effect on climate depends on the level of solar activity. The red curve in Fig. 3 shows clearly that the sun’s eruptional activity was much weaker before 1940 than afterwards. It will be rather difficult to explain the different effect of high and low solar activity in strict physical terms, but there are at least indications now where to search for explanations.

Revealingly, the correlation between NAO and sunspot numbers R  is much weaker than between NAO and aa. Between 1868 and 1890 and 1940 to present it is smaller than r = 0.5. This corroborates the hypothesis put forward in nearly all of my papers that the sun’s eruptional activity is the most potent driving force behind climatic change, much stronger than the relatively weak variations in the sun’s irradiance in the course of the 11-year sunspot cycle. As GCMs do not take the effect of solar eruptions into account, they do not reflect reality.

5.  Background and Outlook

It is to be expected that the presented results will be dismissed as a statistical artifact as there is no detailed causal explanation of the relationship between NAO and solar eruptions in strict terms of physics.  Yet how could this be done as long as climatologists have no physical explanation of the NAO.  The positive and negative modes of this phenomenon establish covariations, but do not explain them (Leroux, 2003). Only a few years ago Wanner (1999) commented: “How and why does the NAO see-saw from one mode to another? … Despite many studies this question remains open and the mechanism of the flip flop quite mysterious.”  Quite recently Hurrell (2003), a specialist at NAO research, conceded that “many open issues remain about which climate processes govern NAO variability…” Hopefully, Mobile Polar High (MPH) dynamics as described by Leroux (1993, 2003) will contribute to a solution of the problem. 

IPCC proponents prayer-wheel-wise repeat the mantra that in recent decades the effect of solar activity on climate has marvellously disappeared. Figures 1 to 3 and the statistical analysis of the correlation between the aa index and NAO up to the present show clearly that with regard to the North Atlantic Oscillation this is not true. Just  in the decades 1970 to present the correlation between aa and NAO is closest and reaches r = 0.97. Earlier investigations have shown that  in recent decades the other dominant modes of climate variability, ENSO and PDO, have been subjected to such strong solar forcing that forecasts can be based on this relationship (Landscheidt, 2001b, 2002). So the textbook tenet that NAO, ENSO, and PDO are free internal oscillations of the climate system not subjected to external forcing is no longer tenable and the claim that the solar effect has not been observed for decades is inconsistent with facts.

Fig. 2 shows that there have been strong variations in the NAO index in recent decades. Hurrell (2003) thinks that they provide “relatively strong evidence that … increases in greenhouse gas concentrations are influencing the recent behaviour of the NAO.”  Here he seems to suppose that solar forcing is negligible. The presented results show that this conclusion is not justified.

IPCC proponents continue to contend that there are no professional physical models that could explain the effect of solar eruptions on climate. In Chapter 4 of my paper “Long-range forecast of U.S. drought based on solar activity” I have given an overview of such models (Landscheidt, 2003a). Meanwhile, Benestad (2002) has written a book on “Solar Activity and Earth’s Climate“ which reviews the rich literature on physical explanations of the widely reported correlations between magnetic activity in the outer layers of the sun and changes in weather and climate on planet Earth up to 2001 (Tinsley, 2003). It is  a valuable update of the comprehensive review by Herman and Goldberg (1978) propagated by NASA before the beginning of the global warming debate. I am not optimistic enough to assume that IPCC adherents will read this book, but I am convinced that it will stimulate research by unprejudiced independent scientists so that, some day, a detailed physical explanation of the relationship between solar eruptions and variations in the NAO will be found.

References