Solar influences on climate variability are one of the most
controversially discussed topics in climate research. We analyze solar
forcing of flood frequency in central Europe during spring and summer on
interannual to multi-centennial timescales, integrating daily discharge data
of the River Ammer (southern Germany) back to AD 1926 (
Solar forcing of climate variability is one of the most
controversially discussed topics in climate research. On the one hand,
numerous empirical associations between the activity of the Sun and climate
variables like temperature, precipitation, atmospheric circulation and
frequency and intensity of hydrometeorological extremes indicate a solar
influence on climate on regional scales (Adolphi et al., 2014; Bond et
al., 2001; Fleitmann et al., 2003; Gray et al., 2010; Lockwood, 2012; Wirth
et al., 2013). On the other hand, it is assumed that the measured variations
in total solar irradiance (TSI) of about 1.4
In addition to model studies, a way to investigate potential solar–climate
linkages and their underlying mechanisms on short and long timescales and
with high temporal precision is to integrate short instrumental records and
long paleoclimate proxy time series reflecting the same type of data
(Kämpf et al., 2014). Flood layers in the varved Ammersee sediment
record form after major River Ammer floods, transporting eroded detrital
catchment material into the lake (Czymzik et al., 2010, 2013).
The River Ammer has a length of 84
The Ammersee (48
The hydroclimate in the Ammer region, today, is characterized by varying
influences of midlatitude westerly weather regimes transporting moisture
from the North Atlantic and Mediterranean into Europe and continental
high-pressure cells causing atmospheric blocking (Petrow and Merz, 2009).
Mean annual precipitation in the Ammer catchment is
Daily River Ammer discharge data provided by the Bavarian Environmental
Agency were recorded at Gauge Weilheim (550
Detrital layers in the varved Ammersee sediment core AS10
Cross-wavelet analysis reveals regions in two time series with common high spectral power and provides information on the phase relationship (Grinsted et al., 2004). The wavelets were produced using a Morlet mother wavelet. Significance levels were calculated against a red noise spectrum (Grinsted et al., 2004). Before the analyses, all data sets were standardized (zero mean, standard deviation).
Correlation coefficients and significance levels were calculated using a non-parametric random-phase test (Ebisuzaki, 1997). This test is designed for serially correlated time series and, thus, takes into account the effects of smoothing and detrending. It is based on the creation of (here 10 000) random time series that have an identical frequency spectrum to the original data series A but randomly differ in the phase of each frequency. To test the significance of the correlation between A and B, A is then replaced with these random surrogates, and the probability distribution of the correlations that may occur by chance is calculated (Ebisuzaki, 1997).
River Ammer flood frequency in the discharge record and solar
activity.
Flood layer frequency and solar activity.
Cross correlation between the MJJA River Ammer flood frequency composite from the discharge record and total solar irradiance (TSI) (Lean, 2000) indicating significant negative correlations when the River Ammer flood frequency composite lags TSI by 2 to 3 years. Prior to the analysis, the River Ammer flood frequency composite was filtered with a 5-year running mean. Correlations were calculated using a random-phase test (Ebisuzaki, 1997).
The MJJA River Ammer flood frequency indices for discharges between the 90th
and 95th percentile and above the 95th percentile are significantly
correlated from AD 1926 to 2010 (
Flood layers in Ammersee sediment core AS10
Cross-wavelet analysis of MJJA River Ammer flood frequency composite and total solar irradiance (TSI) (Lean, 2000) indicating significant common spectral power (exceeding the 90 % significance level against a red noise spectrum) around 11 years. Arrows pointing down indicate that TSI leads the River Ammer flood frequency composite. Before the analysis the River Ammer flood frequency composite was filtered with a 5-year running mean. Shaded areas indicate the cone of influence where wavelet analysis is affected by edge effects.
Cross-wavelet analysis of the Ammersee flood layer (30-year running window) and reconstructed total solar irradiance records (Steinhilber et al., 2012). Contoured areas exceed the 90 % significance level against a red noise spectrum. Arrows pointing to the left indicate that the time series are anti-phased. Before the analysis, the Ammersee flood layer record was resampled to the resolution of the TSI time series (approximately one data point in 20 years). Shaded areas indicate the cone of influence where wavelet analysis is affected by edge effects.
Comparing the MJJA River Ammer flood frequency composite from the discharge record to an annually resolved TSI reconstruction (Lean, 2000) allows examining solar–flood correspondences at very high temporal resolution based on fixed chronologies. Interestingly, interannual variability in the River Ammer flood frequency composite follows changes in TSI during solar cycles 16–23 (Fig. 2). Both records are broadly anti-phased (Fig. 2). Discrepancies between the MJJA River Ammer flood frequency composite and TSI might be caused by internal climate variability and local climate anomalies. Furthermore, particularly the weak increase in the River Ammer flood frequency composite during the TSI minimum between solar cycles 21 and 22 is likely due to the static nature of the chosen discharge thresholds. Nevertheless, even though no increase in flood frequency is visible during that time for floods with discharges above the 95th percentile, an increase in the frequency of floods with discharges between the 90th and 95th percentile is recorded (Fig. 2). A trend towards lower River Ammer flood frequencies during the more recent years is paralleled by a trend towards higher solar activity (Fig. 2).
Cross correlation indicates significant negative correlations when the River
Ammer flood frequency composite lags TSI by 2–3 years (2-year lag:
Comparing the 5500-year flood layer frequency record to solar activity
indicators from cosmogenic radionuclides enables investigating solar–climate
linkages on long timescales. Comparable to the last 450 years (Czymzik et
al., 2010), the 5500-year flood layer frequency time series (
Significant negative correlations between solar activity and River Ammer flood frequency on interannual to multi-centennial timescales suggest a solar modulation of the frequency of hydrometeorological extremes in the Ammer region (Figs. 2, 3, 4, 5, 6). Further empirical associations between flood frequency and solar activity in records from the Alpine region and central Spain (Moreno et al., 2008; Peña et al., 2015; Vaquero, 2004; Wirth et al., 2013) as well as the agreement with a flood reconstruction from multiple large European rivers of the last 500 years (Glaser et al., 2010) suggest a larger spatial relevance (central Europe) of the flood signal from the Ammer catchment.
One proposed solar–climate linkage is the so-called solar top-down mechanism, expected to modulate the characteristics of the midlatitude storm tracks over the North Atlantic and Europe by model studies (Haigh, 1996; Ineson et al., 2011; Lockwood, 2012). During periods of reduced solar activity, the storm tracks are projected to be on a more southward trajectory. Reduced zonal pressure gradients favor atmospheric blocking and meridional airflow (see the Introduction for details) (Adolphi et al., 2014; Haigh, 1996; Ineson et al., 2011; Lockwood, 2012; Wirth et al., 2013). A similar synoptic-scale configuration of atmospheric circulation is associated with periods of higher River Ammer flood frequency (Rimbu et al., 2016). Periods of higher flood frequency are characterized by a pronounced trough over western Europe intercalated between two ridges south of Greenland and north of the Caspian Sea (Rimbu et al., 2016). Meridional moisture transport mainly from the North Atlantic towards central Europe along the frontal zones of these air-pressure fields increases the flood risk in the Ammer region (Rimbu et al., 2016). These similar atmospheric circulation patterns might suggest that the observed solar activity–flood frequency linkage is related to the so-called solar top-down mechanism. However, we cannot rule out further effects of changes in TSI and/or galactic cosmic rays on River Ammer flood occurrences. The inconsistency that the solar top-down mechanism is active mainly during winter and early spring while River Ammer floods occur during late spring and summer might be reconciled by the effects of cryospheric processes. Ice cover in the Barents Sea and snow in Siberia are expected to transfer the dominant potentially solar-induced winter climate signal into summer (Ogi et al., 2003).
Integrating daily River Ammer discharge data back to AD 1926 and a 5500-year flood layer record from varved sediments of the downstream Ammersee allowed identifying changes in flood frequency in central Europe during spring and summer and their triggering mechanism on interannual to multi-centennial timescales. Flood frequency in both records is significantly correlated to changes in solar activity from the solar Schwabe cycle to multi-centennial oscillations. These significant correlations suggest a solar influence on the frequency of hydroclimate extremes in central Europe. Similar configurations of atmospheric circulation during periods of increased flood frequency and reduced solar activity, as expected to be caused by the so-called solar top-down mechanism by model studies, might indicate that the observed solar activity–flood frequency linkage is related to this feedback. The unexpected direct response of variations in River Ammer flood frequency to changes in solar activity might suggest that the solar top-down mechanism is of particular relevance for hydroclimate extremes. Future climate model studies might help to provide a better mechanistic understanding and test our hypotheses on the linkage between solar activity and flood frequency in central Europe during spring and summer.
Ammersee flood layer data files are archived in the PANGAEA data library
(
This study is a contribution to the Helmholtz Association (HGF) climate initiative REKLIM Topic 8 “Rapid climate change derived from proxy data” and was carried out using TERENO infrastructure financed by the HGF. We thank Florian Adolphi for providing the program for the random-phase test. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: D. Fleitmann