SOLAR FORCING OF EL NIŅO AND LA NIŅA

Theodor Landscheidt

Schroeter Institute for Research in Cycles of Solar Activity.

Proceedings of the 1st Solar & Space Weather Euroconference, 'The Solar Cycle and Terrestrial Climate',
Santa Cruz de Tenerife, Tenerife, Spain, 25-29 September 2000 (ESA SP-463, December 2000)

Abstract

Global temperature anomalies are dominated by ENSO events which are viewed to be the most spectacular example of a free internal oscillation of the climate system not subjected to external forcing. It is shown, however, that El Niño, La Niña, and the Southern Oscillation are subjected to strong solar forcing. They are closely connected with special phases in the rise to maximum and the fall to minimum of the 11-year sunspot cycle which coincide with significant accumulations of energetic solar eruptions. This pattern made it possible to forecast the last two El Niños and the extension of La Niña beginning in 1998. On this basis, the next El Niño is to be expected around 2002.9 (+ 0.4). An alternating preponderance of El Niño and La Niña is shown to be linked to the 22-year Hale cycle constituted by 11-year magnetic reversals in sunspot activity. La Niña prevailed in the cycle 1954-1976 and El Niño in the cycle 1976-1996 (P < 10-9). This alternating pattern can be traced back to the Hale cycle beginning in 1889. A predominance of La Niña may be expected in the current 22-year cycle.

Introduction

Anomalous warming (El Niño) or cooling (La Niña) of surface water in the eastern equatorial Pacific occurs at irregular intervals (2 to 7 years) in conjunction with the Southern Oscillation, a massive seesawing of atmospheric pressure between the southeastern and the western tropical Pacific. The coordinated El Niño/Southern Oscillation phenomenon (ENSO), also including La Niña, is thc strongest source o f natural variability in the global climate system. Anomalies in the global temperature – positive or negative deviations from a defined mean temperature – are primarily driven by ENSO events (Peixoto and Oort, 1992). Only when explosive volcanic activity intervenes, global temperature is modulated by its cooling effect.

So it is plausible that there are strong links to weather in other world regions. As this might be the key to long-range seasonal forecasts, there is strong interest in precursors that could make it possible to predict ENSO events. The NOAA tripwire open ocean buoy array including deep ocean moorings and surface drifters gives climatologists an early warning of 3 to 12 months of an impending El Niño. Daily observations of changes in sea surface temperature (SST), surface wind, upper ocean thermal structure, and ocean currents enable researchers to develop models that can be tested by experimental forecasts.

It seems to be very difficult, however, to design models that extend the 12-month limit set by the observation of precursors. Zane and Zebiak of the Lamont-Doherty Earth Observatory made the first successful forecast of an El Niño in early 1986, one year ahead of the event, but their model did not predict the strong El Niño in 1997. At present, there exist no physical or statistical models that can skillfully predict ENSO events at lead times longer than 12 months (Neelin and Latif, 1998). According to Neelin and Latif (1998) weather noise and deterministic chaos, representing the internal variability of the climate system, set the fundamental limits to the lead time. This emphasis on the exclusively internal character of ENSO events is in accordance with the tenet of climatology that ENSO phenomena are the most spectacular example of a free internal oscillation of the climate system not subjected to external forcing (Peixoto and Oort, 1992). If it could be shown that this tenet is not tenable because there is external forcing, this would have far reaching consequences for the global warming debate.

Correlation between ENSO events
and sunspot cycle

If there were external forcing, deterministic chaos, discussed by Neelin and Latif, would not prevent long-range forecasts. Lorenz has emphasized that sensitive dependence on initial conditions and ensuing limited predictability are only valid for processes within the climate system.


Figure 1. Close correlation between extrema in the SOI, indicating ENSO events, and phases a and d within the ascending and declining part of the 11-year sunspot cycle. The phase reversal after 1968 is associated with a predictable perturbation in the Sun's dynamics explained in the text.

External periodic or quasi periodic energy flow can force its rhythm on atmosphere and oceans. Long-term climate effects due to varying solar irradiance, if strong enough, would be a case in point. Investigations into connections between irradiance variations in the course of the 11-year sunspot cycle and changing climate are usually focused on maxima and minima in sunspot activity. It is easy to see that these extrema show no direct relationship with ENSO events. Fig. 1 demonstrates, however, that a close correlaion emerges when other phases of the sunspot cycle are examined.

The curve shows slightly smoothed standardized monthly data of the SOI, the Southern Oscillation Index (Climate Prediction Center, 1998). It measures the pressure gradient across the tropical Pacific which, in turn, is an indicator of equatorial wind variations. Low negative SOI values, indicating EI Niños, go along with weaker than normal trade winds over the central Pacific, warmer than normal sea surface temperatures (SST) over the eastern equatorial Pacific, and a reduced westward pressure gradient with changing wind stress values. High positive SOI values indicate La Niña conditions, just the opposite of the El Niño scenario. In Fig. 1 the data are reversed so that strong positive peaks point to El Niños and negative deviations to La Niñas.

After 1970, on the right of the two arrows marked by PTC, all filled triangles point to El Niños and all empty triangles to negative deviations. All of these triangles mark special phases in the 11-year sunspot cycle. This cycle is not symmetric. Reliable observations available since 1750 show that the mean rise to the sunspot maximum (4.3 years) is considerably steeper than the decline to the sunspot mini- mum (6.7 years). The mean ratio of the rising part to the whole 11-year cycle is 0.39. Nature often repeats patterns on different scales. The phases indicated by triangles form such a fractal. The filled triangles mark points a and d which divide the ascending and the descending part of the sunspot cycle such that the ratio 0.39, found in the whole cycle, is again established in the respective parts.

A maximum entropy frequency analysis of monthly SOI data (1950-1998) shows that the ratio 0.39 stands out in the frequency pattern. Phase d falls at 2.6 years after the zero phase of the declining part. This period nearly coincides with a sharp outstanding frequency peak at 2.5 years which is significant beyond the 1% level. This was confirmed by a Panofsky and Brier
X2-test applied to the Blackman-Tukey power spectrum of the SOI data. Phase a is also emphasized by these tests.

Midpoints between phases a and d (a/d and d/a), marked by empty triangles, are farthest away from points a and d. So it is consistent that they indicate the opposite effect, La Niña instead of El Niño in the range after 1970. Before 1970 everything is reversed. Empty triangles, indicating a/d and d/a, consistently point to El Niños, and filled triangles, marking a and d, to La Niñas. As this conspicuous pattern is linked to the Sun's activity, which again is based on the Sun's dynamics, an explanation of the phase reversal, too, should be found in the Sun's dynamics.

Perturbations in Sun's motion and phase reversals

Babcock's solar dynamo model links the Sun's varying activity to its differential rotation on its axis. It takes into account the Sun's spin momentum, but not its orbital angular momentum related to its irregular oscillation about the centre of mass of the solar system, first described by Newton. This orbital momentum can reach 25% of the spin momentum and varies forty-fold within a few years (Landscheidt, 1999). If there were transfer of angular momentum from the Sun's orbit to the spin on its axis, this would make a difference of up to 7% in its equatorial rotational velocity (Blizard, 1982). Such acceleration or deceleration has actually been observed.

This seems to be indicative of a case of spin-orbit coupling of the spinning Sun and the Sun revolving about the centre of mass involving transfer of angular momentum. Coupling could result from the Sun's motion through its own ejected plasma (Landscheidt, 1999). The low corona can act as a brake on the Sun's surface (Dicke, 1964). Change in the Sun's rotation on its axis could have a crucial effect on the unstable tachocline where the turbulent convective zone meets the more stable radiative zone and variations in rotation are observed to be a regular phenomenon (Howe et al., 2000). This is just the region where the Sun's varying activity is supposed to have its roots.

It has been shown that there are cycles in the Sun's motion which are associated with solar activity and climate change (Landscheidt, 1983, 19S7, 1988, 1990, 1998 a, 199Sc). On this basis, I correctly predicted, for example, energetic solar eruptions, strong geomagnetic storms, the end of the Sahelian drought, and drought conditions around 1999 in the United States 1 to 4 years before the events. The forecast of solar eruptions and geomagnetic storms, checked by astronomers and the Space Environment Center, Boulder, covered six years and reached a hit rate of 90%. Fig. 2 shows how the rate of change of the Sun's orbital angular momentum (L) – the torque dL/dt driving the Sun's motion about the centre of mass of the solar system – constitutes a torque cycle of varying length.


Figure 2. Time rate of change dL/dt of the orbital angular momentum in the Sun's irregular motion about the centre of mass of the solar system. Filled circles mark the initial phases of a cycle in dLldt. Arrows indicate perturbations in the sinusoidal course of dL/dt that are associated with phase reversals in related solar-terrestrial cycles.

The initial phases of this cycle are marked by filled circles. Perturba- tions in the sinusoidal course of the torque cycle, indicated by arrows, occurred around 1933 and 1968. Such events recur at quasi-periodic intervals and mark initial phases of a perturbation cycle with a mean length of 35.8 years. Obser- vation shows that such perturbations, which are predictable, release phase reversals in cycles of climate phenomena connected with the Sun's activity.

Forecast of ENSO events

The perturbation in the torque cycle that occurred in 1968 falls just at the phase reversal in the correlation pattern presented in Fig. l. It is marked by arrows and the acronym PTC (Perturbation in torque cycle). The next PTC will occur in 2007. The pattern in Fig. 1 made it possible to predict the last two El Niños (Landscheidt, 1995) and the extension of La Niña beginning in 1998. In the first week of January 1999 I predicted that La Niña should prevail till 2000.1 and beyond. This proved correct though the lead time was as long as 13 months. End of March 1999 I extended the lead time to 15 months by predicting that La Niña would last till 2000.5. This again turned out correct. The successful forecast of the last El Niño was made more than two years before the event. At present there exist no physical or statistical models that can skillfully predict ENSO events at lead times longer than 12 months (Neelin and Latif, 1998), though daily precursor observations are continuously taken into account. In many cases the forecasts of specialized institutes change every few months. As I predicted in January 1999, the maximum phase of the next El Niño is to be expected around 2002.9 (+ 0.4). This is an even longer lead time. If such long-range forecasts of ENSO events in the tropical Pacific could be shown to be dependable, this might be the key to long-range forecasts of seasonal weather in other world regions linked to ENSO.

Energetic solar eruptions and ENSO events

Why is it that just phases a and d within the ascending and descending part of the 11-year sunspot cycle are related to ENSO events? Fig. 3 gives a potential answer. It shows the distribution of highly energetic X-ray flares within the respective ascending and descending part of the sunspot cycle. The sample covers all flares X >6 observed by satellites between 1970 and 1998. These data are available at the National Geophysical Data Center, Boulder. The rising and falling parts of different length were normalized to have equal length 1. Then they were superimposed to make it easy to recognize identical phases. Intense X-ray flares, nearly always accompanied by heavy coronal mass ejections, are geophysically more effective than flares categorized into classes of optical brightness (Joselyn, 1986). As many as 19 of the 34 investigated X-ray flares concentrate on the short interval of 0.23 on the unit scale, marked by a horizontal bar at the top left. Only 15 of the flares fall at the remaining large interval covering a range of 0.77 on the unit scale.



Figure 3.
Distribution of intense X-ray flares (X >6), observed 1970- 1998, in relation to phases a and d in the ascending and declining part of the 11-year sunspot cycle. The climatological effect, linked to phases a and d, lags the significant accumulation of flares, the conceivable cause.

The normalized position of points a and d is marked by a filled triangle. The climate effect, observed at a and d lags the solar eruptions, the conceivable cause, though only by 8 months on average. Statistically, the flare accumulation is highly significant. Even when compared with the distribution of mean counts of grouped optical flares, bootstrap resampling and randomization tests show that the probability o f a false rejection of the sceptic null hypothesis is much smaller than 0.01. Highly energetic cosmic ray flares observed between 1942 and 1970 corroborate this result. All events listed by Sakurai (1974) were included in the sample except the weakest events with a cosmic ray increase < 2%. The distribution shows a strong accumulation in the same range.

Potential physical background

At present there are no strict physical arguments that could explain in detail how solar activity causes ENSO events. It is possible, however, to develop working hypotheses that suggest potential mechanisms. Intense X-ray flares are nearly always accompanied by strong proton events and coronal mass ejections which cause the highest velocities in the solar wind and create shock waves that compress and intensify the magnetic fields which modulate the intensity of galactic cosmic rays. There are indications that this has a strong effect on cloud cover over the oceans. The overall effect of clouds is that they cool the planet more than they heat it. The global mean long-wave and short-wave cloud forcing are both larger than the trace gas forcing by a factor of 15 to 20.

The short-wave effect (albedo) shifts the system to a cooler climate, while the long-wave effect (absorption) causes a warmer climate. The Earth Radiation Budget Experiment (ERBE) data show that in the present climate the short-wave forcing is stronger and generates a net cooling of at least – 17 W/m2 (Ramanathan et al., 1989). Svensmark and Friis-Christensen (1997) have shown that global cloud cover over the oceans, observed by satellites, is linked to variations in the flux of galactic cosmic rays modulated by the solar wind (r = 0.95). This effect, attributed to cloud seeding by ionized secondary particles, was observed to induce a change in cloud cover by more than 3% within 3 % years. The corresponding change in radiative forcing is in the range 0.8 to1.7 W m2. This is significant, as the total radiative forcing by CO2 accumulated in the atmosphere since pre-industrial times has been estimated to be 1.5 W m2. According to Svensmark (1998) cosmic ray forcing explains nearly all of the temperature change in the period 1980-1995.

Measurements of cosmic ray flux registering myons instead of neutrons go back to 1937. When Svensmark (1998) compared these data with temperature in the Northern Hemisphere, his former results were corroborated. Short-term observations show the same response. According to Pudovkin and Veretenenko (1995), Forbush decreases – sudden deep drops in cosmic ray flux within 2 days after energetic flares – coincide with local shrinking of cloud cover by 3%. It is assumed that the secondary ions produced by cosmic rays serve as condensation nuclei with hygroscopic properties that enhance the formation of clouds. Pruppacher and Klett (1997) have provided evidence that this is occurring in thunderstorms, but it is not clear to which extent cloud development is affected. The underlying microphysical processes are not yet understood in detail.

They should be analysed by laboratory experiments. Svensmark is planning such experiments in cooperation with CERN. Unexpected support for a link between cosmic ray flux and cloud cover comes from the observation that Neptune's whitish methane clouds increase in surface coverage at intervals of about 11 years when the cosmic ray flux is intense because the sunspot cycle is in a minimum (Baliunas and Soon, 1998). As the regions of the tropical Pacific around Indonesia and northern Australia, where ENSO events are triggered by instabilities in the sea surface pressure, have dense and extensive cloud layers reaching altitudes of 22 km at geomagnetic latitudes close to -20', allowing relatively strong cosmic ray effects, it is not unimaginable that there is a physical link between energetic solar eruptions and variations in surface pressure gradients that release instabilities in the ENSO regions. Energetic solar eruptions could indirectly shrink the dense cloud layers around the centre of the large low pressure cell in the western tropical Pacific via a weaker cloud seeding effect of cosmic rays. The increasing insolation would disturb sea surface pressure enough to trigger instability in the pessure balance of the Southern Oscillation that could be amplified by concomitant feedback processes in the atmosphere-ocean system.

This would explain a close correlation between cosmic ray intensity, cloud cover, and Southern Oscillation Index found by Kuang, Jiang, and Yung (1998) for the period 1983 - 1991. Revealingly, in their graphical presentation (Fig. 2a) the SOI lags cloud cover which again lags cosmic rays. This points to a causal relationship. Isolated Forbush decreases, mentioned already, are associated with immediate decreases in cloudiness by 3% that last a week and longer. Even such short-term effects could be sufficient to release or sustain conditions for the development of El Niños, especially when they trigger tropical cyclones (Ramage, 1986). I do not pretend that the proposed hypothetical mechanism is actually working. Others are imaginable. Flares increase the Sun's UV radiation by at least 16%. Ozone in the stratosphere absorbs this excess energy which causes local warming and circulation disturbances. General circulation models developed by Haigh (1996), Shindell et al. (1999), and Balachandran et al. (1999) confirm that circulation changes initially induced in the stratosphere can penetrate into the troposphere and influence temperature, air pressure, Hadley circulations and storm tracks by changing the distribution of large amounts of energy already present.



Figure 4.
Global temperature anomalies based on satellite, balloon sonde, and surface observations. Filled triangles mark initial phases of the torque cycle in the Sun's motion and empty triangles asymmetric phases in between explained in the text. Plus signs indicate midpoints between the respective phases. The correlation pattern made it possible to predict the last two extrema in the departures. The next positive extremum is to be expected around 2002.9. A minimum in the departures should be reached around 2000.7.

As ENSO events are linked to trade winds and trade winds to Hadley cells that may be affected by flare induced circulation change in the stratosphere, it seems plausible that energetic solar eruptions may be an essential link in the causal chain triggering ENSO events, especially if there is a barrage of solar eruptions covering weeks. It may be objected that these working hypotheses lack detail and precision. However, we do not even know exactly how individual ENSO events come into existence.


Figure 5. Monthly-mean temperature anomalies in °C averaged over the Northern Hemispheric mass between surface and 25-km height (After Peixoto and Oort, 1992). Filled triangles mark initial phases of the torque cycle in the Sun's motion about the centre of mass of the solar system. Empty triangles indicate phases that reflect the ratio 0.61 of the declining part of the sunspot cycle to the whole 11-year cycle. Maxima of the anomalies closely follow the strong variations in the width of intervals between consecutive crucial phases in the torque cycle.

The lack of elaborate theory does not impair the heuristic importance of the results, especially as they were corroborated by successful forecast experiments. Epistemologically, the stages of gathering data, finding patterns, and setting up working hypotheses necessarily precede the development of precise theories.

Torque cycle and temperature anomalies

The torque cycle in the Sun's motion, presented in Fig. 2, adds to the dependability of ENSO forecasts as it is closely connected with extrema in global temperature anomalies which are primarily driven by ENSO events. Fig. 4 from the World Climate Report presents a case in point. It shows global temperature anomalies based on satellite, balloon sonde, and surface observations. They are referenced to a common zero point in 1979, the beginning of satellite measurements. Black triangles mark initial phases of the torque cycle that have been shown to coincide with accumulations of energetic solar eruptions (Landscheidt, 1976). Empty triangles indicate the mean ratio 0.61 of the declining part of the sunspot cycle to the whole 11-year cycle. Phases 0.61 and 0.39 are mirror images of each other in relation to the symmetry centre of the cycle. Plus signs mark midpoints between the respective phases.

Temperature consistently lags the crucial phases in the torque cycle by a few months, thus pointing to a causal relationship. The close correlation made it possible to correctly forecast a negative extremum for 1997.0 and a positive one for 1998.6 (Landscheidt, 1998b). The next minimum is to be expected around 2000.7 and the following maximum around 2002.9. Forecasts of maxima should turn out to be more precise than forecasts of minima. Figure 5 after Peixoto and Oort (1992) shows Northern Hemisphere temperature anomalies for the troposphere and the low stratosphere. It extends the correlation back to 195S so that it covers four decades. It is conspicuous how closely maxima in the anomalies follow the strong variations in the width of intervals between consecutive active phases in the torque cycle. Southern Hemisphere temperatures corroborate this connection.

Alternating preponderance of El Niño and La Niña

The true sunspot cycle is the magnetic Hale cycle with a mean length of 22 years. The magnetic polarities o f preced- ing and following sunspots in each hemisphere reverse in each 11-year cycle so that their return to the original magnetic state is linked to the initial phase of the Hale cycle. Fig. 6 shows a close relationship between the Multivariate ENSO Index (MEI) based on the main observed variables over the tropical Pacific (Wolter and Timlin, 1998), and the Hale cycle.



Figure 6. Preponderance of La Niña in the Hale cycle 1954-1976 and of El Niño in the following magnetic sunspot cycle ending 1996. The ENSO data are based on the MEI (Wolter and Timlin,1998). The highly significant alternating pattern can be traced back to 1900. If the pattern holds, a predominance of La Niña may be expected in the current Hale cycle.

The preponderance of La Niña in the Hale cycle 1954 - 1976 and of El Niño in the following cycle is obvious. This alternating pattern can be traced back to 1900, as far as monthly SOI data are avail- able. The connection can be evaluated quantitatively by investigating to which degree the SOI means in consecutive Hale cycles deviate from each other and whether their positive and negative signs form a consistent alternating pattern. A bootstrap analysis based on the t-test yields P < 10-9 for the two Hale cycles shown in Fig. 6. The distributions in the preceding Hale cycles back to the initial phase 1S89, too, yield highly significant results, though on a lower level (P < 10 '). This could be due to deteriorating quality of observational data. If the pattern holds, a preponderance of La Niña is to be expected during the Hale cycle that began in 1996. So far, there were two La Niñas and one El Niño. A predominance of La Niñas, lasting 22 years, would have a strong effect on global temperature comparable to the cool period in the sixties and early seventies when temperatures were falling in spite of a steep increase in anthropogenic CO2.

Outlook

The results presented here, though tested by forecast experiments, are only first tentative steps in a new direction. There are many unsolved problems, and not only theoretical ones. Phase reversals linked to perturbations in the torque cycle, as presented in Fig. 1, can be traced back to the beginning of instrumental records. They are a heuristic actuality. Yet how can it be explained that there is a complete reversal in the effect though we are dealing with the same phases in the sunspot cycle and unchanged conditions in the climate system? Obviously we need to understand first what happens in the Sun's convection zone when perturbations in the torque cycle occur. Is there a bistable oscillator that switches to another mode when a perturbation in the torque cycle disturbs the established equilibrium? Another haunting observation is that not only strong solar eruptions,-but also extended lulls in flare activity can affect ENSO events. I think that these problems can only be solved by a joint interdisciplinary effort of open-minded scientists.

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