Lehren aus der Klimageschichte: Kälte und schwache Sonne befeuern Sturmtätigkeit in Europa

In unserer kleinen Sturmartikelserie schauen wir heute nach Europa. Wie haben sich die Stürme auf unserem Kontinent entwickelt, welche Trends sind erkennbar bzw. nicht erkennbar? Wir beginnen in Großbritannien. Das britische Metoffice erklärte 2014, dass es derzeit keine Hinweise auf eine Steigerung der Sturmtätigkeit in Großbritannien gebe:

Climate models provide a broad range of projections about changes in storm track and frequency of storms. While there’s currently no evidence to suggest that the UK is increasing in storminess, this is an active area of research under the national climate capability.

The Telegraph brachte im selben Jahr dazu einen treffenden Artikel:

The storms are no different – but we are
It’s not the weather that has got worse, it’s our ability to cope without the creature comforts

[...] This is the worst set of storms for two decades. But two decades is not long. How far back does your memory go? In January 1993 a deep storm (the most intense system of low pressure outside the tropics ever recorded over the north Atlantic) miraculously broke up the oil spilt from the tanker Braer. The Burns Day storm of January 1990 cut off power for half a million. The storm of 1987 blew down 15 million trees. Since history is anything before your own time, history for me includes the storm of 1953 that killed more than 300 in Britain. Who remembers 1928, when 14 drowned in London and piles of Turners wallowed in the Tate?If the effects of the winter storms today seem worse (although they are not), it is partly because power cuts now instantly deprive a generation that has grown dependent on them of technologies that didn’t exist three decades ago: chiefly mobiles and the internet. Their sudden loss brings isolation, alienation, and a desire to blame someone.

Ganzen Artikel in The Telegraph lesen.

Hochinteressant auch die Rekonstruktion der Sturmgeschichte in Wales für die vergangenen 4500 Jahre durch Orme et al . 2015 anhand eines Sedimentkerns. Die Autorne fanden zwölf Phasen erhöhter Sturmtätigkeit, die interessanterweise meist mit dem Beginn und Ende von nordatlantischen Kältephasen zusammenfallen. Die Kältephasen wiederum korrelieren mit solaren Schwächeperioden, wie Bond et al. 2001 zeigen konnten. Hier der Abstract von Orme et al. 2015:

Reconstructed centennial variability of Late Holocene storminess from Cors Fochno, Wales, UK
Future anthropogenic climate forcing is forecast to increase storm intensity and frequency over Northern Europe, due to a northward shift of the storm tracks, and a positive North Atlantic Oscillation. However understanding the significance of such a change is difficult because the natural variability of storminess beyond the range of instrumental data is poorly known. Here we present a decadal-resolution record of storminess covering the Late Holocene, based on a 4-m-long core taken from the peat bog of Cors Fochno in mid-Wales, UK. Storminess is indicated by variations in the minerogenic content as well as bromine deposited from sea spray. Twelve episodes of enhanced storm activity are identified during the last 4.5 cal ka BP. Although the age model gives some uncertainty in the timings, it appears that storminess increased at the onset and close of North Atlantic cold events associated with oceanic changes, with reduced storm activity at their peak. Cors Fochno is strongly influenced by westerly moving storms, so it is suggested that the patterns were due to variations in the intensity of westerly airflow and atmospheric circulation during times when the latitudinal temperature gradient was steepened.

Ein Jahr später legten Orme et al. 2016 nach, diesmal mit einer Sturmstudie zu den Äußeren Hebriden. Die Autoren bestätigten die Hypothese, dass eine positive NAO (Nordatlantische Oszillation) zu erhöhter Sturmtätigkeit in Nordeuropa führt. Insbesondere scheint es während der Mittelalterlichen Wärmeperiode auf den Äußeren Hebriden stürmischer gewesen zu sein als während der nachfolgenden Kleinen Eiszeit. Abstract:

Aeolian sediment reconstructions from the Scottish Outer Hebrides: Late Holocene storminess and the role of the North Atlantic Oscillation
Northern Europe can be strongly influenced by winter storms driven by the North Atlantic Oscillation (NAO), with a positive NAO index associated with greater storminess in northern Europe. However, palaeoclimate reconstructions have suggested that the NAO-storminess relationship observed during the instrumental period is not consistent with the relationship over the last millennium, especially during the Little Ice Age (LIA), when it has been suggested that enhanced storminess occurred during a phase of persistent negative NAO. To assess this relationship over a longer time period, a storminess reconstruction from an NAO-sensitive area (the Outer Hebrides) is compared with Late Holocene NAO reconstructions. The patterns of storminess are inferred from aeolian sand deposits within two ombrotrophic peat bogs, with multiple cores and two locations used to distinguish the storminess signal from intra-site variability and local factors. The results suggest storminess increased after 1000 cal yrs BP, with higher storminess during the Medieval Climate Anomaly (MCA) than the LIA, supporting the hypothesis that the NAO-storminess relationship was consistent with the instrumental period. However the shift from a predominantly negative to positive NAO at c.2000 cal yrs BP preceded the increased storminess by 1000 years. We suggest that the long-term trends in storminess were caused by insolation changes, while oceanic forcing may have influenced millennial variability.

Das Gegenteil scheint in Island der Fall gewesen zu sein. Dort stürmte es während der Kleinen Eiszeit heftiger als während der Mittelalterlichen Wärmeperiode, wie Streeter & Dugmore 2014 dokumentierten. Siehe Abbildung 8 aus dem Paper (via The Hockeyschtick).

Weiter in der Bretagne, Frankreich mit einer Sturmrekonstruktion von Brigitte Van Vliet-Lanoë und Kollegen 2014. Wieder fallen die sturmreichen Phasen mit den kalten (solar-schwachen) nordatlantischen Phasen zusammen. Die Kleine Eiszeit scheint am stürmischten gewesen zu sein. Abstract:

Middle- to late-Holocene storminess in Brittany (NW France): Part I – morphological impact and stratigraphical record
Our study aims to understand the recurring climatic conditions prevailing during the largest storms reaching NW France (Brittany). These storms are responsible for the breaching of coastal barriers and major flooding of lowlands. In a first part of our work, we examine the morphological impact and stratigraphic record of storm events along Western Brittany rocky coasts, with a special focus on the southern coast of the Bay of Audierne, the most exposed coast of the region. In a second paper (‘Middle- to Late-Holocene Storminess in Brittany (NW France): Part II’), we shall focus on the chronology of storm events and their climate forcing conditions. Drilling transects and stratigraphic analyses were first undertaken to constrain chronology, strength and wind direction during the main Holocene storm events. New dates, observations and a relative sea-level (RSL) curve were then used to inform discussion of the necessary climatic and morphologic conditions leading to destructive storm events. Most recorded events appear to be linked with cooling episodes of the Holocene and a RSL close to present. Some storms are clearly responsible for breaching and dune building or remobilisation. We demonstrate that storm frequency and intensity appear to rise in a stepwise manner during the late Holocene. Maximum efficiency is reached during the ‘Little Ice Age’ with clustered events probably lasting several days, but major storms also occurred immediately prior to the ‘Medieval Warm Period’. We suggest that recent coastal dune building from c.ad 1100 until now, despite a sea level close to present and continuously rising, may be a direct consequence of the restoration of beaches after periods of recurrent storminess. This building activity often occurred during dry negative North Atlantic Oscillation (NAO) events, in connection with the available sedimentary supply.

In Teil zwei der Artikelserie gehen Van Vliet-Lanoë und Kollegen auf den Einfluss der Ozeanzyklen ein. Sie identifizieren tyische Konstellationen aus AMO, NAO und solarer Aktivität, die im Zusammenspiel das Sturmgeschehen steuern. Abstract:

Middle- to late-Holocene storminess in Brittany (NW France): Part II – The chronology of events and climate forcing
This study focuses on the recurring climate conditions required for the largest storms occurring in NW France (Brittany). It is based on the analysed records of storm events along Western Brittany coast (see Part I). In this manuscript (Part II), storm recurrence is explored along with forcing mechanisms. Periods of more frequent storm events over the two last centuries are analysed first in order to link these events with possible forcing mechanisms (North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation (AMO) modes) triggering the most destructive storms. Then, palaeostorm events are discussed at the Holocene scale, from 6000 yr BP to present, to verify the forcing mechanisms. Most recorded events appear to be linked with cooling episodes, mostly in winter, a transition to or from a negative winter NAO mode, a positive AMO mode. Extreme storms occur immediately prior to the ‘Medieval Warm Period’ (MWP). Maximum effects are reached prior to the onset of the MWP and during the Maunder and Dalton solar minima. Low storm activity occurred during the Spörer Minimum linked to an acceleration of the Atlantic Meridional Overturning Circulation (AMOC). Main storm triggers seem to correspond to a positive AMO mode with an unstable jetstream configuration driving a negative NAO. In this study, four specific weather configurations were defined to explain each type of recorded storminess. The strongest storms correspond to low AMO and decennial-negative NAO modes (e.g. ‘Little Ice Age’), or high AMO in association with dominant low NAO modes, as during the early Middle Age and present-day period. Fresh or warm oceans in association with a positive NAO mode are stormy but with very low sting storms frequency. Although in agreement with the orbital forcing and the Holocene glacial history, increasing storm frequency and intensity is most probably partly biased by continuous sea-level rise and resulting erosion.

Degeai et al. 2015 präsentierten eine Sturmrekonstruktion aus Südost-Frankreich. Auch hier wieder das gleiche Bild: Die sturmreichen Phasen ereigneten sich während der Kältephasen im Zusammenhang mit solar-schwachen Perioden. Die Autoren fanden eine deutlichen solar-angetriebenen 270-Jahreszyklus im Sturmgeschehen. Abstract:

Major storm periods and climate forcing in the Western Mediterranean during the Late Holocene
Big storm events represent a major risk for populations and infrastructures settled on coastal lowlands. In the Western Mediterranean, where human societies colonized and occupied the coastal areas since the Ancient times, the variability of storm activity for the past three millennia was investigated with a multi-proxy sedimentological and geochemical study from a lagoonal sequence. Mappings of the geochemistry and magnetic susceptibility of detrital sources in the watershed of the lagoon and from the coastal barriers were undertaken in order to track the terrestrial or coastal/marine origin of sediments deposited into the lagoon. The multi-proxy analysis shows that coarser material, low magnetic susceptibility, and high strontium content characterize the sedimentological signature of the paleostorm levels identified in the lagoonal sequence. A comparison with North Atlantic and Western Mediterranean paleoclimate proxies shows that the phases of high storm activity occurred during cold periods, suggesting a climatically-controlled mechanism for the occurrence of these storm periods. Besides, an in-phase storm activity pattern is found between the Western Mediterranean and Northern Europe. Spectral analyses performed on the Sr content revealed a new 270-year solar-driven pattern of storm cyclicity. For the last 3000 years, this 270-year cycle defines a succession of ten major storm periods (SP) with a mean duration of 96 ± 54 yr. Periods of higher storm activity are recorded from >680 to 560 cal yr BC (SP10, end of the Iron Age Cold Period), from 140 to 820 cal yr AD (SP7 to SP5) with a climax of storminess between 400 and 800 cal yr AD (Dark Ages Cold Period), and from 1230 to >1800 cal yr AD (SP3 to SP1, Little Ice Age). Periods of low storm activity occurred from 560 cal yr BC to 140 cal yr AD (SP9 and SP8, Roman Warm Period) and from 820 to 1230 cal yr AD (SP4, Medieval Warm Period).

Nun zu den Stürmen in Spanien. Alejandra Feal-Pérez und Kollegen 2014 analysierten die Sturmgeschichte der letzten 2000 Jahre an der iberischen Nordwestküste. Erneut werden Zusammenhänge mit der NAO beschrieben:

Late-Holocene storm imprint in a coastal sedimentary sequence (Northwest Iberian coast)
A combination of sedimentological, geomorphological, and pedological methods has been used to study a late-Holocene sedimentary sequence in a rock coast sector from NW Spain, with the aim of relating it to storm events and their morphodynamic effects. The sequence contains two coarse beach layers at an elevation of 2.8–3.5 m above the present highest astronomical tide (HAT). Radiocarbon dating revealed that their deposition had begun during 1735–1590 cal. yr BP and has continued until the present. The entire beach system experienced considerable morphological change after 1320–1230 cal. yr BP, with a westward displacement of the beach and a retreat of the sedimentary cliff in the eastern section. The two beach layers seem to have been formed by vertical aggradation of clasts and sand during high-energy storm events, highlighting the role of these events in the formation of sedimentary sequences on the rocky coasts of mid-Atlantic Europe. The effects of a recent storm event, recorded in March 2008, and the results of wave calculations suggest that long swell waves were needed for the accretion of the clasts. Using a hindcast model of wave data, we found a positive correlation between the winter North Atlantic Oscillation (WNAO) index and the winter monthly mean wave height and peak period. While the 14C chronology of beach sedimentation coincides with known climatic periods dominated by a positive NAO index, these results point to the importance of high-energy events and the synergies between past and present processes in the recent evolution and the morphodynamics of rock coast environments.

Springen wir nun in den Südosten Spaniens. Dezileau et al. 2016 haben hier die Sturmgeschichte der letzten 6500 Jahre rekonstruiert. Wieder fallen die sturmreichen Phasen in die solar-schwachen nordatlantischen Kälteperioden. Die Autoren stellen explizit einen Zusammenhang mit den Bond-Zyklen her. Abstract:

Extreme storms during the last 6500 years from lagoonal sedimentary archives in the Mar Menor (SE Spain)
Storms and tsunamis, which may seriously endanger human society, are amongst the most devastating marine catastrophes that can occur in coastal areas. Many such events are known and have been reported for the Mediterranean, a region where high-frequency occurrences of these extreme events coincides with some of the most densely populated coastal areas in the world. In a sediment core from the Mar Menor (SE Spain), we discovered eight coarse-grained layers which document marine incursions during periods of intense storm activity or tsunami events. Based on radiocarbon dating, these extreme events occurred around 5250, 4000, 3600, 3010, 2300, 1350, 650, and 80 years cal BP. No comparable events have been observed during the 20th and 21st centuries. The results indicate little likelihood of a tsunami origin for these coarse-grained layers, although historical tsunami events are recorded in this region. These periods of surge events seem to coincide with the coldest periods in Europe during the late Holocene, suggesting a control by a climatic mechanism for periods of increased storm activity. Spectral analyses performed on the sand percentage revealed four major periodicities of 1228 ± 327, 732 ± 80, 562 ± 58, and 319 ± 16 years. Amongst the well-known proxies that have revealed a millennial-scale climate variability during the Holocene, the ice-rafted debris (IRD) indices in the North Atlantic developed by Bond et al. (1997, 2001) present a cyclicity of 1470 ± 500 years, which matches the 1228 ± 327-year periodicity evidenced in the Mar Menor, considering the respective uncertainties in the periodicities. Thus, an in-phase storm activity in the western Mediterranean is found with the coldest periods in Europe and with the North Atlantic thermohaline circulation. However, further investigations, such as additional coring and high-resolution coastal imagery, are needed to better constrain the main cause of these multiple events.

Fazit: Die Sturmtätigkeit in Europa hat sich stets während Kältephasen verstärkt. Kälte und Stürme ereigneten sich im Zuge von solaren Schwächeperioden, die sich als Auslöser der Variabilität anbieten. Der übegeordnete Zyklus beträgt hier 1000 Jahre (Eddy-Zyklus), der wohl den Wechsel zwischen Römischer, Mittelalterlicher und Moderner Wärmeperiode und den dazwischengeschalteten Kältephasen gebracht hat. Ozeanzyklen modulieren das Geschehen im Jahrzehntmaßstab, mit einer Zyklendauer von 60 Jahren. Die Sturmrekonstruktionen aus Europe zeigen ein einheitliches Bild, das die Klimamodellierer und Attributions-Forscher nun aufgreifen, erklären und in ihre Simulationen aufnehmen müssen. Angesichts der starken und systematischen solaren Signatur wird es schwer werden, den verschwindend gering angenommenen Strahlungsantrieb für solare Schwankungen in der Klimagleichung aufrechtzuerhalten.