New paper finds another solar amplification mechanism by which the Sun controls climate

A paper published today in Environmental Research Letters finds another potential solar amplification mechanism by which changes in solar UV activity over 11-year solar cycles are amplified to large-scale effects upon climate via modulations of the North Atlantic Oscillation [NAO]. 

The authors model a mechanism whereby large changes (up to 100%) in solar UV over solar cycles affect heating rates of the upper stratosphere, which in turn affect winds and temperature gradients in the troposphere, and heat storage in North Atlantic Ocean. This results in a lagged effect of 3-4 years in the amplitude of the North Atlantic Oscillation, which in turn affects Arctic sea ice extent, other ocean oscillations, the jet stream, and weather patterns around the globe. The paper corroborates several others demonstrating solar influence upon the NAO, as well as other ocean oscillations. 

According to the authors,

Numerous studies have suggested an impact of the 11 year solar cycle on the winter North Atlantic Oscillation (NAO), with an increased tendency for positive [NAO signals to occur at maxima of the solar cycle, and negative NAO signals to occur at minima of the solar cycle]. Climate models have successfully reproduced this solar cycle modulation of the NAO, although the magnitude of the effect is often considerably weaker than implied by observations.  

A leading candidate for the mechanism of solar influence is via the impact of ultraviolet radiation variability on heating rates in the tropical upper stratosphere, and consequently on the meridional temperature gradient and zonal winds...On reaching the troposphere this produces a response similar to the winter NAO. Recent analyses of observations have shown that solar cycle–NAO link becomes clearer approximately three years after solar maximum and minimum. Previous modelling studies have been unable to reproduce a lagged response of the observed magnitude. 

In this study, the impact of solar cycle on the NAO is investigated using an atmosphere–ocean coupled climate model. We show that the model produces significant NAO responses peaking several years after extrema of the solar cycle, persisting even when the solar forcing becomes neutral. This confirms suggestions of a further component to the solar influence on the NAO beyond direct atmospheric heating and its dynamical response. Analysis of simulated upper ocean temperature anomalies confirms that the North Atlantic Ocean provides the memory of the solar forcing required to produce the lagged NAO response. These results have implications for improving skill in decadal predictions of the European and North American winter climate.

A simulated lagged response of the North Atlantic Oscillation to the solar cycle over the period 1960–2009

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M B Andrews 1, J R Knight 1 and L J Gray 2
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M B Andrews et al 2015 Environ. Res. Lett. 10 054022
doi:10.1088/1748-9326/10/5/054022Published 22 May 2015

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Abstract

Numerous studies have suggested an impact of the 11 year solar cycle on the winter North Atlantic Oscillation (NAO), with an increased tendency for positive (negative) NAO signals to occur at maxima (minima) of the solar cycle. Climate models have successfully reproduced this solar cycle modulation of the NAO, although the magnitude of the effect is often considerably weaker than implied by observations. A leading candidate for the mechanism of solar influence is via the impact of ultraviolet radiation variability on heating rates in the tropical upper stratosphere, and consequently on the meridional temperature gradient and zonal winds. Model simulations show a zonal mean wind anomaly that migrates polewards and downwards through wave–mean flow interaction. On reaching the troposphere this produces a response similar to the winter NAO. Recent analyses of observations have shown that solar cycle–NAO link becomes clearer approximately three years after solar maximum and minimum. Previous modelling studies have been unable to reproduce a lagged response of the observed magnitude. In this study, the impact of solar cycle on the NAO is investigated using an atmosphere–ocean coupled climate model. Simulations that include climate forcings are performed over the period 1960–2009 for two solar forcing scenarios: constant solar irradiance, and time-varying solar irradiance. We show that the model produces significant NAO responses peaking several years after extrema of the solar cycle, persisting even when the solar forcing becomes neutral. This confirms suggestions of a further component to the solar influence on the NAO beyond direct atmospheric heating and its dynamical response. Analysis of simulated upper ocean temperature anomalies confirms that the North Atlantic Ocean provides the memory of the solar forcing required to produce the lagged NAO response. These results have implications for improving skill in decadal predictions of the European and North American winter climate.

1. Introduction

The variability of the Sun's output influences the heating of the stratosphere via the absorption of ultraviolet (UV) by ozone (Haigh1994, Gray et al 2009). Observational studies of the influence of the 11 year solar cycle show warm temperature anomalies in the equatorial upper stratosphere at solar maximum compared to solar minimum (Frame and Gray 2010, Mitchell et al 2014). Significant changes in the extratropical atmospheric circulation have been linked to these temperature anomalies (Kodera 1995, Kodera and Kuroda 2002), and this is supported by modelling studies (e.g. Matthes et al 2004, 2006, Ineson et al 2011). One of the mechanisms for 'top-down' solar influence (Gray et al 2010) involves equatorial stratospheric warm anomalies at solar maximum which increases the mean meridional temperature gradient, resulting in an increase in the mean Westerly wind in the mid-latitude stratosphere. This positive zonal wind anomaly is then amplified by forcing from planetary waves propagating upwards from the troposphere. Along with meridional advection, this wave feedback causes the poleward and downward migration and amplification of the wind anomaly to the mid- and high-latitude lower stratosphere, where it is able to influence tropospheric circulation. The resulting surface response involves sea-level pressure changes at solar maximum which are very similar to the positive phase of the Arctic Oscillation (AO), with anomalous low pressure over the North Pole bordered by anomalous high pressure in mid-latitudes (Thompson and Wallace 1998). Conversely, at solar minimum, a negative AO response results from reduced stratospheric meridional temperature gradients and the downward and poleward propagation of negative zonal wind anomalies. This top-down mechanism occurs on seasonal timescales since planetary wave propagation in the stratosphere is limited to the winter half-year.

This 'top-down' mechanism cannot explain the recently identified lag of approximately 3 years between solar maximum (minimum) and an increased tendency of a positive (negative) North Atlantic Oscillation (NAO) signal superimposed on the intrinsic year-to-year NAO variability (Gray et al 2013). The ability of the climate system to produce a multi-year lag in the winter NAO response necessitates the persistence of solar signals within the climate system from one winter to the next. Scaife et al (2013) showed that the North Atlantic Ocean is a prime candidate for the source of the lag. Model simulations have demonstrated that the sub-surface North Atlantic Ocean can be influenced by NAO changes related to the internal variability of stratospheric circulation (Reichler et al2012) and changes in multidecadal solar irradiance (Menary and Scaife 2014). On interannual timescales, Scaife et al (2013) presented a mechanism involving coupled atmosphere–ocean feedbacks. The NAO is known to be correlated with a tripole pattern in the North Atlantic sea-surface temperatures (SST), (Visbeck et al 2003), which extends below the surface into the ocean mixed layer. Due to the seasonal cycle in surface heat and turbulent fluxes, the mixed-layer-depth (MLD) is deeper in winter than in summer. This suggests that a winter sub-surface ocean signal, linked to solar variability, could persist by being isolated underneath the shallower summer mixed layer from the modifying influence of surface fluxes from the atmosphere. In autumn, as the summer mixed-layer erodes and the deeper winter mixed layer becomes established, any sub-surface solar signal would reconnect with the surface, giving it the potential to influence the atmosphere. This sequestration and re-emergence of signals from one winter to the next has been shown to operate in other contexts (Alexander et al 1999, Timlin et al 2002, Deser et al 2003, Taws et al 2011), and would give rise to a forcing of the NAO by the ocean (Rodwell and Folland 2002). Hanawa and Sugimoto (2004) identified several regions of re-emergence including areas of the North Atlantic relevant to this study. Scaife et al (2013) argue that a weak solar-related AO/NAO signal could build up over a number of years in the tripole region of the North Atlantic Ocean and feedback onto the atmosphere to produce a peak in the NAO signal after a few years.

Several studies have examined the simulated NAO response to solar forcings. Gray et al (2013) and Mitchell et al (2015) showed that Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations were unable to reproduce the observed NAO response. On the other hand, Ineson et al (2011) were able to simulate a realistic amplitude of the NAO response by imposing a higher level of variability in UV-band irradiance. They reproduced the UV-induced 'top-down' mechanism, connecting the upper-stratosphere and the tropospheric NAO. The simulations from Ineson et al (2011) were further analysed by Scaife et al (2013), who showed that the implied ocean–atmosphere coupling in the model used by Ineson et al (2011) was too weak to produce the observed delay.

In this study we use historical simulations of the period 1960–2009 with CMIP5 evolving forcings to explore the influence of solar variability on the NAO. This is different to the experiments of Ineson et al (2011) which use constant forcings within their solar maximum and solar minimum scenarios. We use two ensembles, the first with solar irradiance held constant and the second with time-varying spectrally resolved solar variability. The difference in response of the ensembles should reveal the influence of the varying solar cycle on the atmosphere and oceans.

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Figure 1. (a) Time-series of imposed TSI anomaly (black line), and UV-band irradiance anomaly (dashed blue line) with respect to the 1960–2009 mean. (b) Composites of upper stratospheric zonal mean temperature (dashed red line) and DJF NAO-index (black line) as a function of lag with respect to solar maximum minus solar minimum. The upper stratospheric temperature is calculated as the annual average of the region bounded by 0.5–5 hPa (approximately 40–55 km), and 30 °S–30 °N. The NAO-index is defined as the DJF surface pressure difference between the Azores and Iceland. The points where the NAO-index is significant at the 95% level are highlighted with squares.

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Summary

We have investigated the NAO response to solar variability using a state-of-the-art atmosphere–ocean coupled model. Historical ensembles for the period 1960–2009 were performed with constant and time-varying solar irradiance. Analysis of the differences between the ensembles was performed to identify solar cycle responses in the atmosphere and ocean. The results demonstrate tropical upper stratospheric heating in response to the imposed UV change at solar maximum compared to solar minimum, and confirm the results of Ineson et al (2011), showing a subsequent surface winter NAO response via a 'top-down' mechanism. The response of the NAO peaks 3–4 years following the extreme phase of the solar cycle. This finding is consistent with a recent re-evaluation of observed responses to the solar cycle (Gray et al 2013) which shows the largest NAO signal at a similar lag. The in-phase response of the Aleutian Low is also in agreement with observational analyses.

We diagnose the source of the NAO lag in the model by examining its surface and sub-surface solar responses in the North Atlantic Ocean. We find evidence for amplification of 'top-down' solar-related NAO changes via an ocean feedback over a period of several years, as suggested by Scaife et al (2013). This feedback is analysed by examining solar cycle responses in the different nodes of the North Atlantic tripole SST pattern, as this pattern reflects NAO–ocean coupling. The Northern and Middle nodes of the tripole show temperature responses in the surface and sub-subsurface ocean with a similar lag to the NAO. The Southern node, however, does not show any lag. In the Middle node we find re-emergence of solar signals imprinted on the ocean from the previous winter. By remaining intact below the shallow ocean mixed-layer that forms in summer, these signals can re-emerge in winter and reinforce the 'top-down' forcing of the NAO via coupling with the atmosphere. This mechanism is not evident in the Northern and Southern nodes. The simulated re-emergence in the North Atlantic Ocean causes an accumulation of the solar signal, allowing the NAO to grow over several years. This growth is limited by the reversal of the solar cycle, resulting in a lag approximately equal to one quarter of its period. Although we do not explicitly demonstrate here that the growth in the NAO response arises through feedback from the solar SST signal in the Middle node the existence of this feedback is supported by previous studies (Rodwell and Folland 2002, Timlin et al 2002) that show the influence of tripole SSTs on the NAO.

The NAO (Hurrell et al 2003) is a key mode of regional climate variability that strongly influences the wintertime weather of Northern Europe and Eastern North America. The ability to reproduce the lagged NAO response to solar forcing in atmosphere–ocean coupled models offers the possibility of increased NAO predictability and hence skill in seasonal forecasts (Scaife et al 2014) and decadal forecasts up to a few years ahead (Smith et al 2012).

New paper shows yet another way the Sun controls climate - via ocean oscillations

A paper recently published in the Journal of Geophysical Research finds a strong positive relationship between solar activity and the North Atlantic Oscillation (NAO) over the past 30 years of the 20th century. The study finds a lagged relationship with changes in solar activity followed by changes in the trend of the NAO a few years later. The NAO in turn has profound effects upon the climate of the Northern Hemisphere, including Arctic sea ice. The IPCC dismisses the role of the Sun on climate by only looking at a single variable - the Total Solar Irradiance (TSI), while ignoring large changes in solar UV and secondary effects such as on cloud formation and ocean oscillations.

Graph (a) is the North Atlantic Oscillation (NAO), smoothed version to right. Graph (b) is the Solar Activity aa index, smoothed version to right. Note the significant increase in solar activity over the 20th century.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D16109, 15 PP., 2011

doi:10.1029/2011JD015822

Nonlinear and nonstationary influences of geomagnetic activity on the winter North Atlantic Oscillation

Key Points

• The nonlinear relationship between the geomagnetic aa index and the winter NAO
• Multidecadal variation of solar activity and trend changes in the winter NAO
• The aa-NAO relationship is in the declining phase of even-numbered solar cycles

Yun Li

CSIRO Climate Adaptation Flagship, CSIRO Mathematics, Informatics and Statistics, Wembley, Western Australia, Australia

Hua Lu

British Antarctic Survey, Cambridge, UK

Martin J. Jarvis

British Antarctic Survey, Cambridge, UK

Mark A. Clilverd

British Antarctic Survey, Cambridge, UK

Bryson Bates

CSIRO Climate Adaptation Flagship, CSIRO Marine and Atmospheric Research, Wembley, Western Australia, Australia

The relationship between the geomagnetic aa index and the winter North Atlantic Oscillation (NAO) has previously been found to be nonstationary, being weakly negative during the early 20th century and significantly positive since the 1970s. The study reported here applies a statistical method called the generalized additive modeling (GAM) to elucidate the underlying physical reasons. We find that the relationship between aa index and the NAO during the Northern Hemispheric winter is generally nonlinear and can be described by a concave shape with a negative relation for small to medium aa and a positive relation for medium to large aa. The nonstationary character of the aa-NAO relationship may be ascribed to two factors. First, it is modulated by the multidecadal variation of solar activity. This solar modulation is indicated by significant change points of the trends of solar indices around the beginning of solar cycle 14, 20, and 22 (i.e., ∼1902/1903, ∼1962/1963, and ∼1995/1996). Coherent changes of the trend in the winter time NAO followed the solar trend changes a few years later. Second, the aa-NAO relationship is dominated by the aa data from the declining phase of even-numbered solar cycles, implying that the 27 day recurrent solar wind streams may be responsible for the observed aa-NAO relationship. It is possible that an increase of long-duration recurrent solar wind streams from high-latitude coronal holes during solar cycles 20 and 22 may partially account for the significant positive aa-NAO relationship during the last 30 years of the 20th century.

Related: The Medieval Climate Anomaly and NAO

New paper finds multiple solar amplification mechanisms which modulate winter surface temperatures

A paper published today in the Journal of Geophysical Research Atmospheres finds "significant differences in the temperature patterns" of the Northern Hemisphere winter dependent upon the "four [phases of the solar cycle], which indicates a solar cycle modulation of winter surface temperatures." Thus, the paper describes 4 potential solar amplification mechanisms in relation to 4 phases of each solar cycle, and possibly a fifth related to solar modulation of the North Atlantic Oscillation [NAO] as has been found by prior papers. 

According to the authors,

"Several recent studies have found variability in the Northern Hemisphere winter climate related to different parameters of solar activity. While these results consistently indicate some kind of solar modulation of tropospheric and stratospheric circulation and surface temperature, opinions on the exact mechanism and the solar driver differ. Proposed drivers include, e.g., total solar irradiance (TSI), solar UV radiation, galactic cosmic rays and magnetospheric energetic particles. 

While some of these drivers are difficult to distinguish because of their closely similar variation over the solar cycle, other suggested drivers have clear differences in their solar cycle evolution. For example, geomagnetic activity and magnetospheric particle fluxes peak in the declining phase of the sunspot cycle, in difference to TSI and UV radiation which more closely follow sunspots. Using 13 solar cycles (1869–2009) we study winter surface temperatures and North Atlantic oscillation (NAO) during four different phases of the sunspot cycle: minimum, ascending, maximum and declining phase. 

We find significant differences in the temperature patterns between the four cycle phases, which indicates a solar cycle modulation of winter surface temperatures. However, the clearest pattern of the temperature anomalies is not found during sunspot maximum or minimum, but during the declining phase, when the temperature pattern closely resembles the pattern found during positive NAO. Moreover, we find the same pattern during the low sunspot activity cycles of 100 years ago, suggesting that the pattern is largely independent of the overall level of solar activity."

The authors find this "pattern [similar to the positive NAO] is largely independent of the overall level of solar activity," therefore climate scientists trying to correlate Total Solar Irradiance [TSI], which is considered by many in climate science to represent "the overall level of solar activity," will not see such patterns or even look for them or simulate them with climate models. 

By solely focusing on TSI and ignoring e.g. large changes solar UV of up to 100% over a single solar cycle, geomagnetic changes, ignoring all potential solar amplification mechanisms, climate models dismiss the role of the Sun in climate change. 


Spatial distribution of Northern Hemisphere winter temperatures during different phases of the solar cycle

V. Maliniemi*, T. Asikainen and K. Mursula

Several recent studies have found variability in the Northern Hemisphere winter climate related to different parameters of solar activity. While these results consistently indicate some kind of solar modulation of tropospheric and stratospheric circulation and surface temperature, opinions on the exact mechanism and the solar driver differ. Proposed drivers include, e.g., total solar irradiance (TSI), solar UV radiation, galactic cosmic rays and magnetospheric energetic particles. While some of these drivers are difficult to distinguish because of their closely similar variation over the solar cycle, other suggested drivers have clear differences in their solar cycle evolution. For example, geomagnetic activity and magnetospheric particle fluxes peak in the declining phase of the sunspot cycle, in difference to TSI and UV radiation which more closely follow sunspots. Using 13 solar cycles (1869–2009) we study winter surface temperatures and North Atlantic oscillation (NAO) during four different phases of the sunspot cycle: minimum, ascending, maximum and declining phase. We find significant differences in the temperature patterns between the four cycle phases, which indicates a solar cycle modulation of winter surface temperatures. However, the clearest pattern of the temperature anomalies is not found during sunspot maximum or minimum, but during the declining phase, when the temperature pattern closely resembles the pattern found during positive NAO. Moreover, we find the same pattern during the low sunspot activity cycles of 100 years ago, suggesting that the pattern is largely independent of the overall level of solar activity.

New paper finds another amplification mechanism by which the Sun controls climate

A paper published today in Quaternary Science Reviews reconstructs climate of the central Alps over the past 10,000 years and finds precipitation and floods were driven by changes in solar activity. The authors propose variations in solar activity and insolation cause widening and shrinking of the Hadley cell, and influence on the North Atlantic Oscillation [NAO] and Intertropical Convergence Zone [ITCZ]. The paper adds to many other peer-reviewed publications finding solar amplification mechanisms by which small changes in solar activity have large effects on climate.

The authors also find floods and heavy precipitation were more common during cold periods such as the Little Ice Age than during warm periods such as the Medieval Warm Period, the opposite of claims that warming increases precipitation and floods from increased atmospheric water vapor.

According to the authors, "We found that flood frequency was higher during cool periods, coinciding with lows in solar activity. In addition, flood occurrence shows periodicities that are also observed in reconstructions of solar activity from 14C and 10Be records (2500–3000, 900–1200, as well as of about 710, 500, 350, 208 (Suess cycle), 150, 104 and 87 (Gleissberg cycle) years). As atmospheric mechanism, we propose an expansion/shrinking of the Hadley cell with increasing/decreasing air temperature, causing dry/wet conditions in Central Europe during phases of high/low solar activity. Furthermore, differences between the flood patterns from the Northern Alps and the Southern Alps indicate changes in North Atlantic circulation."

Fig. 6. Stacked flood records for the N- and S-Alps (100-year low-pass filtered) spanning (a) the past 10 kyr and (b) the past 2 kyr. Both representations show strong decadal- to millennial-scale fluctuations in flood activity. In a), gray areas and gray arrows mark periods with increased flood activity. In b), important historic and climatic periods characterized by rather high/low flood occurrence are marked with dark/light areas. LIA: Little Ice Age; MCA: Medieval Climate Anomaly; MP: Migration Period; RE: Roman Empire.

Fig. 8. Comparison of the Alpine flood reconstruction to records reflecting solar forcing, as well as to other climate proxy records and reconstructions: a) 30°N summer insolation (Berger and Loutre, 1991); Holocene cold events reported by (b) Wanner et al. (2011) (short gray bars) and (c) Bond et al. (1997) (with numbers 0–6); d) variations in TSI (Steinhilber et al., 2009) (50-year running mean with 100-year smooth); flood activity in the (e) N-Alps and (f) S-Alps (100-year low-pass filtered), on the right: arrow indicates state of the NAO based on S-Alpine flood frequency; g) global glacier advances (Denton and Karlén, 1973); h) NAO reconstruction from Greenland (Olsen et al., 2012); i) precipitation record from the Cariaco Basin (Haug et al., 2001); j) NAO reconstructions covering the past 1000 years (Trouet et al., 2009); k) ssNa concentrations from the GISP2 ice core (56 and 57) (100-year low-pass filtered); l) storminess (0–1) record from the NE United States (Noren et al., 2002). Gray shaded areas and gray arrows mark periods with enhanced flood activity in the Alpine realm. Blue arrows mark periods in the N-Alps that show an opposite flood activity than the S-Alps. Elevated flood activity in the S-Alps is an indicator for a more southerly positioned Atlantic circulation system and a tendency towards lower NAO indices.

Holocene flood frequency across the Central Alps – solar forcing and evidence for variations in North Atlantic atmospheric circulation

• Stefanie B. Wirth^{a, , 1, 2,} , 
• Lukas Glur^b, 2, 
• Adrian Gilli^a, 
• Flavio S. Anselmetti^b, c

• ^a Geological Institute, ETH Zurich, Zurich, Switzerland
• ^b Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland
• ^c Institute of Geological Sciences and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland

Highlights

•

Lake sediments are a valuable terrestrial archive of past flood events.

•

High flood frequency in the Alps is driven by low solar activity.

•

Widening/shrinking of the Hadley cell brings dry/wet conditions to the Alps.

•

South-Alpine flood frequency indicates changes in a paleo-NAO pattern.

•

Frequent S-Alpine floods suggest a southerly position of the N-Atlantic circulation.

---

Abstract

The frequency of large-scale heavy precipitation events in the European Alps is expected to undergo substantial changes with current climate change. Hence, knowledge about the past natural variability of floods caused by heavy precipitation constitutes important input for climate projections. We present a comprehensive Holocene (10,000 years) reconstruction of the flood frequency in the Central European Alps combining 15 lacustrine sediment records. These records provide an extensive catalog of flood deposits, which were generated by flood-induced underflows delivering terrestrial material to the lake floors. The multi-archive approach allows suppressing local weather patterns, such as thunderstorms, from the obtained climate signal. We reconstructed mainly late spring to fall events since ice cover and precipitation in form of snow in winter at high-altitude study sites do inhibit the generation of flood layers. We found that flood frequency was higher during cool periods, coinciding with lows in solar activity. In addition, flood occurrence shows periodicities that are also observed in reconstructions of solar activity from ¹⁴C and ¹⁰Be records (2500–3000, 900–1200, as well as of about 710, 500, 350, 208 (Suess cycle), 150, 104 and 87 (Gleissberg cycle) years). As atmospheric mechanism, we propose an expansion/shrinking of the Hadley cell with increasing/decreasing air temperature, causing dry/wet conditions in Central Europe during phases of high/low solar activity. Furthermore, differences between the flood patterns from the Northern Alps and the Southern Alps indicate changes in North Atlantic circulation. Enhanced flood occurrence in the South compared to the North suggests a pronounced southward position of the Westerlies and/or blocking over the northern North Atlantic, hence resembling a negative NAO state (most distinct from 4.2 to 2.4 kyr BP and during the Little Ice Age). South-Alpine flood activity therefore provides a qualitative record of variations in a paleo-NAO pattern during the Holocene. Additionally, increased South Alpine flood activity contrasts to low precipitation in tropical Central America (Cariaco Basin) on the Holocene and centennial time scale. This observation is consistent with a Holocene southward migration of the Atlantic circulation system, and hence of the ITCZ [Intertropical Convergence Zone], driven by decreasing summer insolation in the Northern hemisphere, as well as with shorter-term fluctuations probably driven by solar activity.

Climatologist explains halt of global warming via natural North Atlantic Oscillation, solar activity

Climatologist Dr. Eduardo Zorita, one of the authors of the recent paper rejecting the climate models at a confidence level >98% over the past 15 years, has a new post in which he states that the model vs. real-world discrepancy is even greater during the winter months [Dec-Feb], with only 0.2% of 6,104 climate model runs projecting the observed negative trend in winter temperatures [-0.10 C/decade] over the past 15 years. Climate models instead predicted that the most warming would occur during the winter months, the opposite of observations. 

According to Dr. Zorita, the observed global warming stagnation "does not seem to dovetail with an increased heat uptake by the ocean nor with a driving role of the Tropical Pacific, as suggested by the recently published paper by Kosaka et al." Dr. Zorita instead proposes that the natural North Atlantic Oscillation [NAO] explains the temperature stagnation, possibly via effects on cloud cover. 

Dr. Zorita also notes the NAO may be driven by solar activity: "Shindell et al. found in climate simulations conducted with the GISS model a few years ago that the NAO could also be affected by solar forcing - and this is the interesting link I would like to put forward here... They found that low solar irradiance would nudge the NAO toward a negative state, and this could explain the extreme European cold temperatures during the Late Maunder Minimum. Is this happening now again, albeit in a lesser magnitude? If it is, then a weaker sun would explain a global tendency to weaker temperature trends, with a stronger slow down over Eurasia."

The intriguing stagnation

By Dr. Eduardo Zorita, Die Klimazwiebel, September 15, 2013

We are not lacking hypothesis about the recent hiatus - or stagnation- in the global mean temperature: the ocean is taking up more heat, stratospheric water vapour has decreased, the sun has recently weakened and volcanic activity has also gathered a quicker click. A preliminary look at the structure of the stagnation may, or may not, offer some clues about how likely which of these hypothesis, or which combination, may end up being the correct one.

The global mean temperatures observed in the last 15 years are indeed at the very lower edge of the ensemble of trends simulated by climate models. This is the main message of our manuscript and the paper by Fyfe et al.. It has been argued that the starting year in the 15-year period used to compute the trends is misleading because 1998 experienced a very strong El Niño, but by now almost everyone recognizes that, whereas in reality the causes of the hiatus may still very well compatible with the effect of greenhouse gas forcing, current climate models have serious difficulties in simulating such low temperature trends as observed in this period when they are forced by the present external forcings and reasonable extrapolations of this forcing into the future. One important caveat here is that climate projections into the future assume constant solar activity and no volcanic activity, which arguably is not totally realistic.

One intriguing aspect of the stagnation is that it has not been equally distribution across all 12 months of the annual cycle. It has mainly occurred during the months of December, January and February, with the trends in June, July and August over the last 15 years being more similar to the corresponding trends in the period 1980 to 1997.

Figure 1

This means that temperature trends have decelerated much more during the boreal winter months. Actually the trend over the last 15 years in these months is remarkably negative, as can be seen in the following diagrams:

Figure 2

Form the CMIP5 ensemble of climate models that is being used in the 5th IPCC Assessment Report we can derive an ensemble of 15-year periods chosen from the simulations driven by the RCP4.5 scenario from 2005 until 2060 (from a total of 109 simulations, 6104 overlapping 15-year segments) Only 2 per thousand of these 6104 15-year trends are lower than the trends observed during December-to-February in 1998-2012 (HadCRUT4). This occurs in 5% of the trends of the boreal summer season (June-to-August), and in 2 % of the annual means.

What is the spatial 'fingerprint' of the stagnation, or in other words, in which regions are the recent temperature trends more strongly subdued - or even become negative- and in which regions the trends continue unabated ? The following figure tries to give a visual impression of this spatial structure for the December-to-February case. For each grid cell in the HadCRUT4 data set we have computed the linear trend in two periods: 1998-2012 and 1980-1997, and then taken the difference. Negative values indicate the grid-cells where temperature trends have recently slowed ; positive values, where they have recently increased relative to the 'base period' 1980-1997.
Figure 3

This has been also found by Cohen et al. applying slightly different methods. It is obvious that, although trends have recently become smaller in general, the region and the season bearing the brunt of the temperature stagnation is Eurasia in wintertime. We will later call this pattern the DJF [Dec-Jan-Feb] stagnation fingerprint. At first sight this does not seem to dovetail with an increased heat uptake by the ocean nor with a driving role of the Tropical Pacific, as suggested by the recently published paper by Kosaka et al. . It looks to us rather like the effect of the North Atlantic Oscillation on surface temperatures with some additional global contribution. The NAO has shown up in its negative phase in the recent Northern Hemisphere winters, favouring a more meridional circulation and causing polar air intrusions in Eurasia. It is, however, not totally clear how an atmospheric circulation mode, which in principle just shuffles air masses around, can influence global mean temperatures. Wallace et al suggested long ago that the NAO would modify the heat fluxes to the ocean by advecting cold/warm air masses from the continents to the ocean surface. The NAO might also modulate cloud cover that, in turn, could modify the global radiation balance, but this is not widely accepted and, to our knowledge, these lines of research has not be followed up.

Even if the temperature stagnation is indeed related to the NAO, the main question mark in this puzzle still remains : is the stagnation due to internal [natural] stochastic variations or to the external forcings? The NAO is known to be an internal model of climate variability, but it is clearly influenced by the external forcing as well. Future climate projections participating in the CMIP3 project showed that the NAO would tend to shift to a more positive model under greenhouse has warming. Shindell et al. found in climate simulations conducted with the GISS model a few years ago that the NAO could also be affected by solar forcing - and this is the interesting link I would like to put forward here - provided that the climate model includes a well-resolved stratosphere with the corresponding ozone-related chemistry. They found that low solar irradiance would nudge the NAO toward a negative state, and this could explain the extreme European cold temperatures during the Late Maunder Minimum. Is this happening now again, albeit in a lesser magnitude? If it is, then a weaker sun would explain a global tendency to weaker temperature trends, with a stronger slow down over Eurasia. The following figure shows two time series: one describes the strength with which DJF stagnation fingerprint appears in each boreal winter, denoted here stagnation index (in the winters in which the index is strongly positive the spatial fingerprint of the stagnation is particularly strongly represented in the data of that boreal winter); the second time series displays the Total Solar Irradiance. The correlation between both is just suggestive of a weak link, but it is clearly not a definitive proof. Clearly, this fingerprint is expressing itself very strongly in the last few years, but without a corresponding drop in solar activity. Something else is happening that is not captured by this quite preliminary explanation.


Figure 4

Finally, another question mark. Over the past millennium, solar activity and volcanic activity appear statistically anti-correlated. In fact, this is one of the difficulties to disentangle the effect of each of these forcings on surface temperatures in the last centuries. And, intriguingly enough, we see the same phenomenon again in the last 15 years: a weaker sun and more volcanic aerosols. Does anyone dare an explanation ?

New paper finds solar activity explains abrupt slowdown of Atlantic Meridional Overturning Circulation [AMOC]

A paper under review for Climate of the Past finds low solar activity explains an abrupt slowdown of the Atlantic Meridional Overturning Circulation [AMOC] during the period 1915-1935.

According to the authors, the modeled mechanism is

"The weakened AMOC can be explained in the following. The weak total solar irradiance (TIS) during early twentieth century decreases pole-to-equator temperature gradient in the upper stratosphere. The North polar vortex is weakened, which forces a negative North Atlantic Oscillation (NAO) phase during 1905–1914. The negative phase of NAO induces anomalous easterly winds in 50–70° N belts, which decrease the release of heat fluxes from ocean to atmosphere and induce surface warming over these regions. Through the surface ice–albedo feedback, the warming may lead to continuously melting sea ice in Baffin Bay and Davis Strait, which results in freshwater accumulation. This can lead to salinity and density reductions and then an abrupt slowdown of AMOC."

The AMOC and NAO ocean oscillations in turn have profound effects on other ocean and atmospheric oscillations and the global climate. The paper joins many other peer-reviewed publications linking solar activity to lagged effects on ocean and atmospheric oscillations and may represent yet another solar amplification mechanism. 

Clim. Past Discuss., 10, 2519-2546, 2014 www.clim-past-discuss.net/10/2519/2014/ doi:10.5194/cpd-10-2519-2014 An abrupt slowdown of Atlantic Meridional Overturning Circulation during 1915–1935 induced by solar forcing in a coupled GCM

P. Lin¹, Y. Song^1,2, Y. Yu¹, and H. Liu^1
1State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, Beijing 100029, China
²College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China
Abstract. In this study, we explore an abrupt change of Atlantic Meridional Overturning Circulation (AMOC) apparent in the historical run simulated by the second version of the Flexible Global Ocean–Atmosphere–Land System model – Spectral Version 2 (FGOALS-s2). The abrupt change is noted during the period from 1915 to 1935, in which the maximal AMOC value is weakened beyond 6 Sv (1 Sv = 10⁶ m³ s⁻¹). The abrupt signal first occurs at high latitudes (north of 46° N), then shifts gradually to middle latitudes (∼35° N) three to seven years later. The weakened AMOC can be explained in the following. The weak total solar irradiance (TIS) during early twentieth century decreases pole-to-equator temperature gradient in the upper stratosphere. The North polar vortex is weakened, which forces a negative North Atlantic Oscillation (NAO) phase during 1905–1914. The negative phase of NAO induces anomalous easterly winds in 50–70° N belts, which decrease the release of heat fluxes from ocean to atmosphere and induce surface warming over these regions. Through the surface ice–albedo feedback, the warming may lead to continuously melting sea ice in Baffin Bay and Davis Strait, which results in freshwater accumulation. This can lead to salinity and density reductions and then an abrupt slowdown of AMOC. Moreover, due to increased TIS after 1914, the enhanced Atlantic northward ocean heat transport from low to high latitudes induces an abrupt warming of sea surface temperature or upper ocean temperature in mid–high latitudes, which can also weaken the AMOC. The abrupt change of AMOC also appears in the PiControl run, which is associated with the lasting negative NAO phases due to natural variability.

New paper explains how the Sun controls ocean oscillations

A paper published today in the Journal of Atmospheric and Solar-Terrestrial Physics finds that solar poloidal and toroidal fields have different long-term variations and opposite effects upon the North Atlantic Oscillation [NAO]. Several other papers have shown that solar changes are correlated with ocean oscillations. IPCC climate models do not incorporate the link between solar activity and ocean oscillations and thus are critically flawed and cannot be relied upon to forecast climate.

Fig. 2. Long-term variations of the sunspot-related (full circles, solid line) and non-sunspot-related (open circles, dashed line) geomagnetic activity, moving averages over 30 years with a step of 10 years (climatic normals).

Solar influences on atmospheric circulation

• K. Georgieva^a, , , 
• B. Kirov^a, 
• P. Koucká-Knížová^b, 
• Z. Mošna^b, 
• D. Kouba^b, 
• Y. Asenovska^a

• ^a Space and Solar-Terrestrial Research Institute, Bulgarian Academy of Sciences
• ^b Institute of Atmospheric Physics, Czech Academy of Sciences

http://dx.doi.org/10.1016/j.jastp.2012.05.010, How to Cite or Link Using DOI

• Abstract

Various atmospheric parameters are in some periods positively and in others negatively correlated with solar activity. Solar activity is a result of the action of solar dynamo transforming solar poloidal field into toroidal field and back. The poloidal and toroidal fields are the two faces of solar magnetism, so they are not independent, but we demonstrate that their long-term variations are not identical, and the periods in which solar activity agents affecting the Earth are predominantly related to solar toroidal or poloidal fields are the periods in which the North Atlantic Oscillation is negatively or positively correlated with solar activity, respectively. We find further that solar poloidal field-related activity increases the NAM index, while solar toroidal field-related activity decreases it. This is a possible explanation of the changing correlation between the North Atlantic Oscillation and solar activity.

Highlights

► Solar poloidal and toroidal fields have different long-term variations. ► The correlation NAO/solar activity changes with the ratio poloidal/toroidal field. ► Solar poloidal field-related activity increases the NAM index. ► Solar toroidal field-related activity decreases the NAM index

New paper finds the Sun controls European & North Atlantic weather via ocean oscillations

A paper published today in the Journal of Geophysical Research Atmospheres finds changes in solar activity over the 11-year solar cycle had a significant, lagged effect on North Atlantic & European weather patterns over the 140 year period from 1870-2010. According to the authors, "The analysis reveals a statistically significant 11-year solar signal over Europe and the North Atlantic provided the data are lagged by a few years. The delayed signal resembles the positive phase of the North Atlantic Oscillation (NAO) following a solar maximum. The corresponding sea surface temperature response is consistent with this." 

The paper suggests a solar amplification mechanism by which tiny 0.1% changes in solar irradiance during solar cycles have a lagged effect on the natural North Atlantic Oscillation (NAO), which in turn has large effects upon North Atlantic & European weather patterns. The paper adds to many other peer-reviewed publications finding solar amplification mechanisms including ocean oscillations such as ENSO and the NAO, atmospheric oscillations such as the Madden-Julian Oscillation, Quasi-biennial Oscillation, Aleutian Low, Eurasian pattern, & Asian monsoon, and via stratospheric ozone, and sunshine hours/clouds.

A Lagged Response to the 11-year Solar Cycle in Observed Winter Atlantic/European Weather Patterns

Lesley J. Gray, Adam A. Scaife, Daniel M. Mitchell, Scott Osprey, Sarah Ineson, Steven Hardiman, Neal Butchart, Jeff Knight, Rowan Sutton, Kunihiko Kodera

DOI: 10.1002/2013JD020062

The surface response to 11-year solar cycle variations is investigated by analysing the long-term mean sea level pressure and sea surface temperature observations for the period 1870–2010. The analysis reveals a statistically significant 11-year solar signal over Europe and the North Atlantic provided the data are lagged by a few years. The delayed signal resembles the positive phase of the North Atlantic Oscillation (NAO) following a solar maximum. The corresponding sea surface temperature response is consistent with this. A similar analysis is performed on long-term climate simulations from a coupled ocean–atmosphere version of the Hadley Centre model that has an extended upper lid so that influences of solar variability via the stratosphere are well resolved. The model reproduces the positive NAO signal over the Atlantic / European sector but the lag of the surface response is not well reproduced. Possible mechanisms for the lagged nature of the observed response are discussed.