Termination of Solar Cycles and Correlated Tropospheric Variability. Robert J. Leamon Scott W. McIntosh Daniel R. Marsh. Earth and Space Science, February 24 2021. https://doi.org/10.1029/2020EA001223
Abstract: The Sun provides the energy required to sustain life on Earth and drive our planet's atmospheric circulation. However, establishing a solid physical connection between solar and tropospheric variability has posed a considerable challenge. The canon of solar variability is derived from the 400 years of observations that demonstrates the waxing and waning number of sunspots over an 11(‐ish) year period. Recent research has demonstrated the significance of the underlying 22 years magnetic polarity cycle in establishing the shorter sunspot cycle. Integral to the manifestation of the latter is the spatiotemporal overlapping and migration of oppositely polarized magnetic bands. We demonstrate the impact of “terminators”—the end of Hale magnetic cycles—on the Sun's radiative output and particulate shielding of our atmosphere through the rapid global reconfiguration of solar magnetism. Using direct observation and proxies of solar activity going back some six decades we can, with high statistical significance, demonstrate a correlation between the occurrence of terminators and the largest swings of Earth's oceanic indices: the transition from El Niño to La Niña states of the central Pacific. This empirical relationship is a potential source of increased predictive skill for the understanding of El Niño climate variations, a high‐stakes societal imperative given that El Niño impacts lives, property, and economic activity around the globe. A forecast of the Sun's global behavior places the next solar cycle termination in mid‐2020; should a major oceanic swing follow, then the challenge becomes: when does correlation become causation and how does the process work?
3 Discussion
In the previous section, we have made use of a modified Superposed Epoch Analysis (mSEA) to investigate the relationships between solar activity measures and variability in a standard measure of the variability in the Earth's largest ocean—the Pacific. We have observed that this mSEA method brackets solar activity and correspondingly systematic transitions from warm‐to‐cool Pacific conditions around abrupt changes in solar activity we have labeled termination points. These termination points mark the transition from one solar activity (sunspot) cycle to the next following the cancellation or annihilation of the previous cycle's magnetic flux at the solar equator—the end of Hale magnetic cycles.
Correlation does not imply causation; however, the recurrent nature of the ONI signal in the terminator fiducial would appear to indicate a strong physical connection between the two systems. Appendix B discusses three statistical Monte Carlo tests that show the chances of these events lining up for five cycles are remote: in summary we may reject the null hypothesis of random cooccurrences with a confidence level p < 3.4 × 10−3. We do not present an exhaustive set of solar activity proxies, but it would appear that the CRF, as the measure displaying the highest variability, as something to be explored in greater detail in coupled climate system models.
There have been many possible explanations postulated for a cosmic‐ray climate connection, including: (i) Forbush decreases inducing increased extratropical storm vorticity (Roberts & Olson, 1973; Tinsley et al., 1989) and atmospheric gravity waves propagating from the auroral ionosphere (Prikryl et al., 2009); (ii) global electric conductivity inducing changes in cloud microphysics (Harrison, 2004; Tinsley, 2000); and (iii) direct formation of ionization particles seeding cloud formation (Svensmark & Friis‐Christensen, 1997; Svensmark et al., 2017). However, the effects of cosmic rays on cloud formation are a matter of hot debate (e.g., Gray et al., 2010; Kristjánsson et al., 2002; Pierce, 2017), with even the sign of the correlation between cosmic rays and climate not agreed on. For all the debate these explanations have generated, they are all irrelevant in terms of the correlations and empirical predictions we discuss here.
Independent of the exact mechanisms of coupling solar modulation to ENSO, which are beyond the scope of this manuscript, the results discussed above and shown in Figure 5 hold for the past five solar cycles, or 60 or so years. The question must be asked, then, why has the regular pattern of Figure 5 occurred and reoccurred regularly since 1966?
As Figure 4 shows, we only have continuous cosmic‐ray observations from 1964, just before the cycle 19 terminator. The F10.7 record extends back to 1947 (near the peak of cycle 17), but Pacific Sea Surface temperatures and sunspot areas extend to the 1870s. Other than the 1915 termination of solar cycle 14, there is little evidence for such a correlation prior to the events discussed here. (Cycle 14 was notably the weakest cycle of the twentieth Century, and thus almost certainly had the highest CRF prior to the continuous observation record.) However, there certainly have been changes in the Earth's atmosphere over the twentieth Century…
3.1 Atmospheric Changes
It is probably not a coincidence that the period of terminator‐ENSO correlation corresponds to the close‐to‐monotonic rise in global sea surface temperatures over the same time period as Figure 4 (the “hockey stick” graph). Tropospheric warming leads to stratospheric cooling (Ramaswamy et al., 2006); do the effects of a colder stratosphere on the physical and chemical processes in it make it more susceptible to amplifying transient changes in solar input? Further, since about 1945, the Pacific Decadal Oscillation (Mantua et al., 1997) has been in a predominantly negative phase, the feedback to net irradiance from clouds has been increasingly negative (Zhou et al., 2016), and a ∼4%–6% decrease in cloud cover over the western Pacific (∼140–160°E) has been reported from ship‐borne observations since 1954, with a comparable increase over the mid‐Pacific (∼150–120°W; Bellomo et al., 2014). Note that 160°E is the “balance point” about which warm and cold SSTs flip in an El Niño‐La Niña transition (Pinker et al., 2017).
Thus, over the past several decades the cloud pattern in the western Pacific has adopted an almost El Niño‐like default state, consistent with an observed eastward shift in precipitation in the tropical Pacific and weakening of the Walker circulation over the last century (Deser et al., 2004; Vecchi & Soden, 2007a), and which has been tied, via simple thermodynamics, to a warmer atmosphere. Over the same past four decades timeframe, evidence for a changing Brewer‐Dobson circulation—the mass exchange between troposphere and stratosphere characterized by persistent upwelling of air in the tropics—comes from satellite and radiosonde data, which indicate a reduction in temperatures and ozone and water vapor concentrations, particularly in the tropical lower stratosphere at all longitudes (Thompson & Solomon, 2005), pointing to an accelerated tropical upwelling (Rosenlof & Reid, 2008). Domeisen et al. (2019) discuss the teleconnection of ENSO both vertically, to the stratosphere, and thence latitudinally affecting the strength and variability of the stratospheric polar vortex in the high latitudes of both hemispheres: El Niño events are associated warming and weakening of the polar vortex in the polar stratosphere of both hemispheres, while a cooling can be observed in the tropical lower stratosphere. These impacts are linked by a strengthened Brewer‐Dobson circulation, with planetary waves generated by latent heat release from tropical thunderstorms being the likely modulation mechanism (Deckert & Dameris, 2008; Domeisen et al., 2019).
Thus, it is entirely plausible that since changes in the (upper) atmosphere brought on by a strengthened Brewer‐Dobson circulation, weakened Pacific Walker circulation, and less cloudy Western Pacific, enables the relatively constant terminator‐driven changes to have sufficient “impact” to flip the system from El Niño to La Niña, independent of the actual mechanism that couples solar changes to clouds and ENSO. Such circulation changes are only likely to intensify in a future with higher tropical heat and moisture at the sea surface, affecting not only tropospheric climate but also stratospheric dynamics.
3.2 Socioeconomic Implications
Forecasting ENSO and its related climate variations is a high‐stakes societal imperative given that El Niño impacts “lives, property, and economic activity around the globe” (McPhaden, 2015). For instance, as an example of not necessarily being concerned about why or how an empirical relationship works from a forecast standpoint, Smith et al. (2016) noted “An ability to forecast the time‐averaged NAO months to years ahead would be of great societal benefit, but current operational seasonal forecasts show little skill.” Dunstone et al. (2016) thus improved the skill of 12 months ahead NAO forecasts when an 11 years (solar cycle) parameterized solar irradiance forcing term was added. Smith et al. and Dunstone et al. focused on the severity of the British winter, primarily from the viewpoint of the energy and insurance sectors.
Directly focused on catastrophic ENSO impacts, flooding in Australia during the 2010–2012 La Niñas and the ensuing economic cleanup costs (A$5–10 billion (US$4.9–9.8 billion) in Queensland alone) led to the commissioning of a Government Report on “two of the most significant events in Australia's recorded meteorological history” (Bureau of Meteorology, 2012). Similarly, the Peruvian government estimated the very strong 1997–1998 El Niño event cost about US$3.5 billion, or about 5% of their gross domestic product (GDP). Globally, United Nations estimates of El Niño‐related damage from the same event ranged from US$32 to US$96 billion. In the United States, NOAA assessed direct economic losses from that event likely exceeded US$10 billion (Weiher, 1999).
The mild winters and greater than average rainfall to the Southwest in El Niño years save US energy consumers US$2.2 billion less in fuel heating costs (Teisberg, 1999); however, the savings is lost in La Niña years with more sever winters. Agriculture is the most climate sensitive industry and climate is the primary determinant of agricultural productivity. Estimates of the impacts on U.S. agriculture of the 1997–1998 El Niño and the 1998–1999 La Niña; those losses range from US$1.5–$1.7 billion from El Niño and US$2.2–$6.5 billion from La Niña (Chen et al., 2001; Weiher & Kite‐Powell, 1999). That assessment comes with the important caveat that losses associated with El Niño‐related floods or droughts in some areas can be offset by gains elsewhere, for instance through reduced North Atlantic hurricane activity, lower winter heating bills or better harvests for certain crops—Argentinian wheat yields are strongly increased in El Niño years, for example, whereas US (and moreso Canadian) yields fall (Gutierrez, 2017). Nevertheless, combining the costs of natural disaster recovery with the costs associated with yields of major commodity crops (Gutierrez, 2017; Iizumi et al., 2014), the need to be able to predict ENSO events beyond a seasonal forecast (e.g., https://community.wmo.int/activity‐areas/climate/wmo‐el‐ninola‐nina‐updates (World Meteorological Organization) is high.
That crop yields in North and South America, Australia, and Eurasia vary, along with regional temperature and precipitation changes, makes it clear that ENSO influences, through “teleconnections,” (e.g., Bjerknes, 1969; Domeisen et al., 2019) the global dynamics of seasonal winds, rainfall, and temperature. These teleconnections imply, indeed require, coupling throughout the atmosphere, and despite the mention of troposphere in the title of this paper, manifestations of ENSO are observed throughout the neutral atmosphere and higher.
One final economic impact consideration, also tied to global teleconnections, is the strength of Atlantic hurricane season, which is relatively strong in the first year of La Niña after an El Niño, when waters are still warm but upper‐level wind shears are favorable for cyclone genesis (Vecchi & Soden, 2007b). As such, we may expect a particularly active season in 2021, and maybe even 2020, depending on exactly when the terminator and ENSO transition occurs.
4 Conclusion
As discussed in M2014, the band‐o‐gram developed therein could be extrapolated linearly out in time. The linear extrapolation of the solar activity bands outward in time was verified in McIntosh et al. (2017) by updating the original observational analysis and comparing to the earlier band‐o‐gram. M2014 projected that sunspot cycle 25 spots would start to appear in 2019 and swell in number following the terminator in mid‐2020. Six years later, we are seeing these predictions come true with the first numbered active regions and low level (C‐class) flaring activity. Based on the mSEA of the past 60 years, an enduring warm pool in the central and western Pacific at solar minimum (ONI has been consistently positive since early 2018, even though it never got so warm to become a fully fledged strong El Niño event) was not unexpected, and we expect a rapid transition into La Niña conditions later in 2020 following the sunspot cycle 24 terminator. Given the warm waters, we project a particularly active Atlantic hurricane season in 2021, and maybe even 2020, depending on exactly when the terminator and ENSO transition occurs this year.
In conclusion, we have presented clear evidence in Figure 5 of a recurring empirical relationship between ENSO and the end of solar cycles. We have tried to avoid discussion of causation, which, due to its controversial nature could lead to dismissal of the empirical relationship, and we want open a broader scientific discussion of solar coupling to the Earth and its environment. Nevertheless, independent of the exact coupling mechanisms, the question must be asked, why has the pattern occurred and reoccurred regularly for the past five solar cycles, or 60 years? We have only a few months at most to wait to see if this Terminator‐ENSO relation continues at the onset of the coming solar cycle 25. Should this next terminator be associated with a swing to La Niña then we must seriously consider the capability of coupled global terrestrial modeling efforts to capture “step‐function” events, and assess how complex the Sun‐Earth connection is, with particular attention to the relationship between incoming cosmic rays and clouds and precipitation over our oceans. ENSO is the largest mode of atmospheric variability driving extreme weather events with large costs and so any improvement in prediction of that would be of societal benefit.
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