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

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

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

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

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

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

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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.