Wirbelsturmaktivität in Asien durch El Nino und solare Schwankungen kontrolliert

Wirbelstürme in Asien werden als Taifune bezeichnet. Wie hat sich die Taifun-Aktivität in den letzten Jahrzehnten und Jahrhunderten entwickelt? Gibt es Trends? Wir begeben uns auf einen Streifzug durch die neuere Literatur. Wie alle unsere Beiträge, wird auch dieser Blogartikel in ein paar Wochen in unserem thematischen Inhaltsverzeichnis zu finden sein. Wenn also das nächste Mal ein Taifun in Asien wirbelt und ein Aktivist umgehend den Klimawandel dafür verantwortlich macht, haben Sie den wichtigen klimahistorischen Kontext sofort parat.

Wir beginnen in Japan. Yamashita & Mitsumata 2013 analysierten die Taifunstatistik der letzten 70 Jahre und berichteten eine Abnahme der Taifune, wobei Japan selber aber offenbar von immer mehr Wirbelstürmen getroffen wurde. Seit 1980 gab es keine Veränderungen mehr. Abstract:

Does Global Warming Affect Typhoon Patterns in Japan? Discussion from the Viewpoints of the Numbers of Typhoons That Develop, Approach, and Hit Japan, and Their Intensities
The present paper examines the theory that global warming causes abnormal weather phenomena involving typhoons, focusing on the numbers of typhoons that develop and hit Japan and their intensities. According to records, since 1980, there has been a decrease in the annual number of typhoons that develop during the year, whereas there has been an increase in those that approach and hit Japan. The annual number of typhoons since 1980 is the same as in the period between 1951 and 1960. The numbers of typhoons that approached and hit the country were the highest during the period from 1951 to 1970, and started to decline between 2001 and 2010. When compared to the data on the numbers of typhoons that developed, approached, and hit Japan during the period from 1951 to 1970, there have been no significant changes since 1980. Data on the monthly numbers of typhoons that develop, approach, and hit also suggest that there has been no significant prolongation of the typhoon season or delay in its start. Regarding the intensity of typhoons, there has been no significant change in the mean minimum pressure of typhoons, a decrease in the number of typhoons with a very low minimum pressure, and no significant increase in the number of typhoons with a very large maximum wind velocity.

Weiter in Südkorea und Katsuki et al. 2016. Die Autoren rekonstruierten die Taifunhäufigkeit für die Zeit 1400-1900 n. Chr. und fanden Schwankungen, die eng an den Ozeanzyklus El Niño/Southern Oscillation (ENSO) gekoppelt waren. Zudem entdeckten sie einen Bezug der Taifune zur Variabilität des ostasiatischen Sommermonsuns, der wiederum von der Sonnenaktivität gesteuert wird. Zu Zeiten geringer Sonenaktivität gab es mehr Wirbelstürme, da sich der Monsun verstärkte. Abstract:

Factors controlling typhoons and storm rain on the Korean Peninsula during the Little Ice Age
Documenting multi-decadal typhoon and storm-rain variability is useful to prevent future typhoon and flood disasters. We present the history of typhoon and storm-rain activity in East Asia inferred from multi-proxy analyses of Lagoon Hwajin-po sediments along the eastern coast of Korea. Anthropogenic effects were enhanced in Lagoon Hwajin-po since ca. AD 1900, by increasing farming in the catchment. To avoid these human-induced effects, we reconstructed the history of typhoon and storm-rain activity only for the interval AD 1400–1900. The record indicates that typhoon frequency throughout the Korean Peninsula varied in response to the state of the El Niño/Southern Oscillation. Typhoon variability was likely modulated further by the state of the East Asia summer monsoon (EASM) pattern, associated with variation in the magnitude of solar irradiance. During periods of minimum solar activity, such as the early Maunder Minimum (AD 1650–1675), typhoons struck the east China coast and Korean Peninsula more frequently because of a strengthened EASM.

Ein Jahr später legten Katsuki et al. 2017 nach. In einer Studie aus Korea für das mittlere Holozän, also für die Zeit vor einigen tausend Jahren, identifizierten sie wiederum ENSO-Ozeanzyklen sowie solare Schwankungen als Auslöser der Taifun-Variabilität. Die Autoren vermuten, dass die solaren Aktivitätsschwankungen den westlichen Jet Stream beeinflussen, der dann wiederum die Stürme verändert.

Multi-centennial-scale changes in East Asian typhoon frequency during the mid-Holocene
This study reconstructs a record of typhoon frequency over the Korean Peninsula during the mid-Holocene using mineral components and diatom assemblages in deposits of Lagoon Hyangho, located on the east coast of the peninsula. The lagoon deposits confirm the occurrence of cyclical, multi-centennial scale episodes of low salinization induced by typhoon-derived heavy rain. Although shifts in typhoon frequency broadly follow El Niño/Southern Oscillation (ENSO) conditions during the Holocene, evidence from the typhoon-induced deposits in Hyangho Lagoon suggests that the path of the polar westerly jet controls the effect of ENSO on multi-centennial-scale typhoon patterns across the mid-latitude region of East Asia. The influence of ENSO is limited when the westerly jet passes through low latitudes. Fluctuations in solar activity play a key role in regulating movement of the westerly jet. Multi-centennial scale changes in typhoon frequency in mid-latitude East Asia are, therefore, influenced by changes in solar activity and ENSO conditions.

Nun nach Taiwan und Wang et al. 2014. Dort rekonstruierten die Autoren die Taifunhäufigkeit, die währen kalter Phasen wie der Völkerwanderungszeit und der Kleinen Eiszeit zunahm. Abstract:

Late Holocene environmental reconstructions and their implications on flood events, typhoon, and agricultural activities in NE Taiwan
We reconstructed paleoenvironmental changes from a sediment archive of a lake in the floodplain of the Ilan Plain of NE Taiwan on multi-decadal resolution for the last ca. 1900 years. On the basis of pollen and diatom records, we evaluated past floods, typhoons, and agricultural activities in this area which are sensitive to the hydrological conditions in the western Pacific. Considering the high sedimentation rates with low microfossil preservations in our sedimentary record, multiple flood events were. identified during the period AD 100–1400. During the Little Ice Age phase 1 (LIA 1 – AD 1400–1620), the abundant occurrences of wetland plant (Cyperaceae) and diatom frustules imply less flood events under stable climate conditions in this period. Between AD 500 and 700 and the Little Ice Age phase 2 (LIA 2 – AD 1630–1850), the frequent typhoons were inferred by coarse sediments and planktonic diatoms, which represented more dynamical climate conditions than in the LIA 1. By comparing our results with the reconstructed changes in tropical hydrological conditions, we suggested that the local hydrology in NE Taiwan is strongly influenced by typhoon-triggered heavy rainfalls, which could be influenced by the variation of global temperature, the expansion of the Pacific warm pool, and the intensification of El Niño–Southern Oscillation (ENSO) events.

Sommerstürme im Indischen Ozean zeigen für die vergangenen 30 Jahre keinen Trend. Dies fanden Cohen & Boos 2014:

Has the number of Indian summer monsoon depressions decreased over the last 30 years?
Monsoon depressions are cyclonic atmospheric vortices with outer radii near 1000 km that form within the larger-scale monsoon circulations of India and other regions. Recent studies have reported a downward trend in recent decades in the number of Indian summer monsoon depressions. In particular, the years 2002, 2010, and 2012 were noted for having the first summers, in over a century, in which no depressions formed. Here satellite and reanalysis data are used to document the existence of multiple storms in the summers of 2002, 2010, and 2012 that meet traditional criteria for classification as monsoon depressions. Furthermore, the number of extreme synoptic events occurring each summer over the Bay of Bengal is estimated from satellite scatterometers and exhibits no statistically significant trend over the last three decades. These results raise questions about the validity of previously claimed large trends in monsoon depression activity in the Indian summer monsoon.

Girishkumar et al. 2014 heben die bedeutende Rolle der ENSO-Ozeanzklik für die Wirbelstürme im Golf von Bengalen hervor. In der warmen (positiven) PDO-Phase treten Wirbelstürme häufiger in La Nina- als in El Nino-Jahren auf. Abstract:

Influence of Pacific Decadal Oscillation on the relationship between ENSO and tropical cyclone activity in the Bay of Bengal during October–December
The relationship between ENSO and tropical cyclones (TCs) activity in the Bay of Bengal (BoB) during October–December under cold (1950–1974) and warm (1975–2006) phase of Pacific Decadal Oscillation (PDO) is investigated. A statistically significant difference in the formation of total number of TCs and intense TCs (Category-1 and above) between El Niño and La Niña years is observed when the PDO was in warm phase. Our analysis shows that, there is a tendency to form more number of TCs during La Niña years (2.62 TCs per season) than during El Niño years (1.6 TCs per season) under warm phase of PDO. Moreover, the difference is quite high for intense TCs cases, such as, relatively more number of intense TCs forms in the BoB during La Niña years (1.4 TCs per season) compared to El Niño years (0.10 TCs per season) under warm phase of PDO. However, the difference in the formation of total number of TCs and intense TCs between La Niña and El Niño years is not significant under cold phase of PDO. Significant enhancement in low level cyclonic vorticity and mid-troposphere humidity during La Niña years compared to El Niño years when the PDO was in warm phase, rather than the PDO was in cold phase leads to this difference. Our analysis further shows that how the ENSO related teleconnection to the Indian Ocean region differ under warm and cold phase of PDO.

Eine Modellierung von Parth Sarthi et al. 2015 prognostiziert im weiteren Verlauf der globalen Erwärmung eine Abnahme der Wirbelstürme im Indischen Ozean:

Possible future changes in cyclonic storms in the Bay of Bengal, India under warmer climate
The aim of this paper is to study the model’s simulated frequency, track, intensity and location of cyclonic storms (CSs) and severe cyclonic storms (SCSs) in the Bay of Bengal (BoB), India. For the purpose, the PRECIS (Providing REgional Climates for Impacts Studies) a regional climate modelling system, of UK Met Office, is used. This model is integrated for the period 1961–1990 (baseline) and the future time period 2071–2100 (High emission scenario, A2). To run the model, the initial and lateral boundary conditions are supplied by UK Met Office. The analysis of frequency, track, intensity and location are carried out for May, June, September and October for the period 1961–1990 and 2071–2100. To evaluate the model’s performance in simulating storms frequency during 1961–1990, chi square test is carried out with observed storms for the same period. The model’s simulated frequency of storms is an overestimation of observations although the frequency of model’s simulated storms during 2071–2100 is less than that of during 1961–1990. In general, model’s simulated storms are found moving in the northwest direction from their initial location in all months during 1961–1990 and 2071–2100. In model’s simulations, the drop in central pressure is relatively more in the months of May, June, September and October during 2071–2100 compared with that during 1961–1990. During 2071–2100, more intense storms may be possible in the months of May and June compared with that of September and October. It is observed that the model is able to simulate the initial locations of storms during 1961–1990 and 2071–2100 close to observations for the months of June and September, especially.

Die nächste Studie kommt aus China. Liu et al. 2017 analysierten die Entwicklung der tropischen Wirbelstürme in China während der letzten 1000 Jahre anhand von historischen Dokumenten über Schiffsunglücke. Sie fanden unterschiedliche regionale Schwerpunkte der Wirbelsturmaktivität die sich in El Nino- und La Nina-Jahren unterschieden.

Insight into tropical cyclone behaviour through examining maritime disasters over the past 1000 years based on the dynastic histories of China – A dedication to Ocean Researcher V
This paper uses China’s historical records to gather information on maritime disasters caused by tropical cyclones. Official records show that there were a total of thirteen major maritime disasters possibly caused by tropical cyclones. Maritime disaster variation trends indicate that tropical cyclones tend to strike the southern coast of China during a La Niña-like stage whereas an El Niño-like stage leads to tropical cyclones being guided northward toward the Korean Peninsula and Japan. During China’s Yuan Dynasty (AD 1271–1368) Kublai Khan tried to invade Japan twice during an El Niño-like stage and encountered intense tropical cyclones, which the Japanese called Kamikaze. During a La Niña-like stage, multiple maritime disasters occurred in the East China Sea, the Taiwan Strait and the South China Sea in the Qing Dynasty (AD 1636–1912). Indeed, the historical record of the Qing Dynasty details maritime disasters at least 9 times. These events were also caused by tropical cyclones. In addition, statistics on the seasons of tropical cyclone genesis and their landfall locations from AD 1945–2013 in the Northwest Pacific Ocean region reveal that in summer, tropical cyclones tend to make landfall along coastal regions north of Fujian Province, eastern China; however, in autumn and winter, cyclones tend to be guided further southward to make landfall in southern China (Hainan), Vietnam, and the Philippines. This phenomenon in seasonal variation is reflected in the maritime disaster events recorded for coastal regions of southeastern China and landfall locations of tropical cyclones during the Qing Dynasty.

Ni et al. 2015 analysierten die tropischen Wirbelstürme auf dem chinesischen Festland für die vergangenen 55 Jahre. Sie fanden ab 1980 eine starke Abnahme der Häufigkeit und Stärke der Stürme.

Climatology and trends of tropical cyclone high wind in mainland China: 1959–2011
The climatology and trends of tropical cyclone (TC) high wind (TCHW; wind speeds ≥10.8 m s−1) in mainland China during 1959–2011 were studied based on a comprehensive series of surface wind observations from 574 stations. Of these stations, 41 were affected by TCHW more than once per year and were mainly located along the southeast coastline or at some inland mountain locations. The contribution of TCHW frequency to high wind (HW; wind speeds ≥10.8 m s−1) frequency was highest along the southeast coastline of China and decreased rapidly at inland locations. Both the TCHW frequency (intensity) and HW frequency (intensity) displayed significant downward (weakened) trends after the 1980s. The proportion of HW events accounted for by TCHW decreased, although the trend was not significant. The changes in TCHW frequency and intensity were associated with variations in the number of influencing TCs (ITC: a TC that causes at least one TCHW event), ITCTs (ITC time: an observation time with ITCs), the range of ITC influence, and ITC intensity and track. The annual number of ITCs and ITCTs declined consistently during the study period. The range of ITC influence as related to the TC size in 1983–2011 was smaller than in 1959–1982. The variation of the percentiles of ITC intensity showed an increasing (decreasing) trend of weaker (stronger) ITCs. Years with the maximum TCHW frequency had more ITCs and ITCTs than those with the minimum TCHW frequency.

Dazu passt die folgende Pressemitteilung der Penn State University vom 17. Februar 2017. Die Forscher fanden für China seit 1960 eine Halbierung der Sturmtätigkeit:

Study shows China’s severe weather patterns changing drastically since 1960
In one of the most comprehensive studies on trends in local severe weather patterns to date, an international team of researchers found that the frequency of hail storms, thunderstorms and high wind events has decreased by nearly 50 percent on average throughout China since 1960.

The team analyzed data from the most robust meteorological database known, the Chinese National Meteorology Information Center, a network of 983 weather observatories stationed throughout China’s 3.7 million square miles. Meteorologists have been collecting surface weather data through the network since 1951 or earlier, which provided the researchers an unprecedented look at local severe weather occurrences. “Most of the data published on trends in severe weather has been incomplete or collected for a limited short period,” said Fuqing Zhang, professor of meteorology and atmospheric science and director, Center for Advanced Data Assimilation and Predictability Techniques, Penn State. “The record we used is, to the best of our knowledge, the largest, both in time scale and area of land covered.”

The team, who report their findings today (Feb. 17, 2017) in Scientific Reports, found that the strength of the East Asian Summer Monsoon decreased at a rate strongly correlated to that of severe weather throughout the same time period. The monsoon is an annually recurring, long-term weather phenomenon that brings warm, moist air from the south to China in the summer, and cooler air from the north to China in the winter. A monsoon’s strength is measured by calculating the average meridian wind speed in this area. “We believe that changes in monsoon intensity are affecting severe weather in the area because of the strong correlation we found, but we cannot say the monsoon is the exclusive cause,” said Zhang. “A monsoon is one of the major drivers of severe weather because it affects the three necessary ‘ingredients’ for severe weather, which are wind shear, instability and triggering.”

Wind shear is the difference between the wind speed and direction at different altitudes. Because a monsoon brings southerly winds into China, a weaker summer monsoon would decrease the overall low tropospheric wind shear. The weaker monsoons would also bring less warm, moist air from the south — one of the most common sources of instability in the atmosphere. A common triggering mechanism for severe convective weather is lifting by the front, a high temperature gradient across the monsoon, and this would also be reduced in a weaker summer monsoon.

Some studies suggest that climate change may be one of the reasons that the Asian Summer Monsoon weakened. One factor in monsoon formation is the difference between the temperature above land and the temperature above adjacent ocean or sea. A warming climate would affect the difference between these two and, as a result, simulations show that this could continue decreasing the monsoon’s strength. However, the team noted that other major changes in the area — such as an increase in industrialization and air pollution in China in the 1980s — might have played a significant role in the region’s atmospheric changes and could affect the severe weather.

While a decrease in severe weather might sound beneficial, it may not always be a good thing. “There are many natural cycles that rely on severe weather and the precipitation it brings,” said Qinghong Zhang, professor of atmospheric and oceanic sciences, Peking University, lead author of the study, who conducted this research while on sabbatical at Penn State. “A decrease in storms could potentially lead to an increase in droughts. Also, some theorize that while the frequency of severe weather decreases, their intensity could potentially increase. We cannot say if this is true yet, but it is something we will analyze in the future.”

This was the first study in its level of detail because of the amount of data collected by the Chinese National Meteorology Information Center. The study also showed that occurrences of hail remained relatively steady from 1961 through the 1980s before plummeting. “The frequency of thunderstorms and high winds decreased gradually over the time period we studied, but not hail,” said Qinghong Zhang. “This is something we don’t fully understand at this point but plan to investigate more.” Xiang Ni, doctoral student, Peking University, contributed to this study. The Chinese National Science Foundation, the National Basic Research Program of China and the U.S. National Science Foundation supported this research.

Während die Stürme in China und anderen Orten Asiens abnehmen, ist auf den Philippinen in den letzten 60 Jahren eine Zunahme verzeichnet worden, wie die Unversität Sheffield 2016 mitteilte.

Fazit: Der klimahistorische Kontext ist erneut entscheidend, um zunächst die Sturmvariabilität und ihren Antrieb zu verstehen. Ozeanzyklen und Sonne sind wichtige Treiber, die in den Modellen noch nicht korrekt erfasst sind. Die Lage und Intensität der Jetstreams könnte ein Schlüssel zur solaren Klimabeeinflussung werden. Ist es wirklich die Kälte, die die Taifune in Gebieten wie Taiwan häufiger werden lässt, oder eher die schwache Sonnenaktivität, die den Jetstream verändert? Food for Thought.

 

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.

 

Hurrikan-Gefahr an der US-Atlantikküste war während der Mittelalterlichen Wärmephase besonders hoch

Bis zum 14. Januar 2017 kann man sich auf zdf.de noch den Dokumentarfilm”Der Hurrikan von Galveston” anschauen, einem Supersturm der vor mehr als hundert Jahren die Küstenregion von Houston zerstörte. Aus der Film-Beschreibung:

Die Insel-Stadt Galveston wird am 8. September 1900 durch einen Hurrikan ausgelöscht. Nachdem die einzige Brücke zum Festland zerstört war, saßen 37 000 Bewohner in einer tödlichen Falle.

Wieder einmal ist der Kontext wichtig. Die American Geophysical Union (AGU) dokumentierte am 11. Februar 2015 die Hurrikangeschichte der letzten 2000 Jahre an der Nordostküste der USA. Unerwartet: In den Zeiten 150-1150 n.Chr. und 1400-1675 n.Chr. ereigneten sich Hurrikan-reiche Phasen, in denen Stürme auftraten, die jene der letzten 150 Jahre bei weitem übertrafen:

Monster hurricanes reached U.S. Northeast during prehistoric periods of ocean warming

Intense hurricanes possibly more powerful than any storms New England has experienced in recorded history frequently pounded the region during the first millennium, from the peak of the Roman Empire into the height of the Middle Ages, according to a new study. The findings could have implications for the intensity and frequency of hurricanes that the U.S. East and Gulf coasts could experience as ocean temperatures increase as a result of climate change, according to the study’s authors.

A new record of sediment deposits from Cape Cod, Mass., show evidence that 23 severe hurricanes hit New England between the years 250 and 1150, the equivalent of a severe storm about once every 40 years on average. Many of these hurricanes were likely more intense than any that have hit the area in recorded history, according to the study. The prehistoric hurricanes were likely category 3 storms – like Hurricane Katrina — or category 4 storms – like Hurricane Hugo — that would be catastrophic if they hit the region today, according to Jeff Donnelly, a scientist at Woods Hole Oceanographic Institution in Massachusetts and lead author of the new paper accepted for publication in Earth’s Future, a journal of the American Geophysical Union.

The study is the first to find evidence of historically unprecedented hurricane activity along the northern East Coast of the United States, Donnelly said. It also extends the hurricane record for the region by hundreds of years, back to the first century, he said. “These records suggest that the pre-historical interval was unlike what we’ve seen in the last few hundred years,” said Donnelly. The most powerful storm to ever hit Cape Cod in recent history was Hurricane Bob in 1991, a category 2 storm that was one of the costliest in New England history. Storms of that intensity have only reached the region three times since the 1600s, according to Donnelly.

The intense prehistoric hurricanes were fueled in part by warmer sea surface temperatures in the Atlantic Ocean during the ancient period investigated than have been the norm off the U.S. East Coast over the last few hundred years, according to the study. However, as oceans temperatures have slowly inched upward in recent decades, the tropical North Atlantic sea surface has surpassed the warmth of prehistoric levels and is expected to warm further over the next century as the climate heats up, Donnelly said. He said the new study could help scientists better predict the frequency and intensity of hurricanes that could hit the U.S. East and Gulf coasts in the future. “We hope this study broadens our sense of what is possible and what we should expect in a warmer climate,” Donnelly said. “We may need to begin planning for a category 3 hurricane landfall every decade or so rather than every 100 or 200 years.” “The risk may be much greater than we anticipated,” he added.

Donnelly and his colleagues examined sediment deposits from Salt Pond near Falmouth on Cape Cod. The pond is separated from the ocean by a 1.3- to 1.8-meter (4.3- to 5.9-foot) high sand barrier. Over hundreds of years, strong hurricanes have deposited sediment over the barrier and into the pond where it has remained undisturbed. The researchers extracted nine-meter (30-foot) deep sediment cores that they then analyzed in a laboratory. Similar to reading a tree ring to tell the age of a tree and the climate conditions that existed in a given year, scientists can read the sediment cores to tell when intense hurricanes occurred. The study’s authors found evidence of 32 prehistoric hurricanes, along with the remains of three documented storms that occurred in 1991, 1675 and 1635.

The prehistoric sediments showed that there were two periods of elevated intense hurricane activity on Cape Cod – from 150 to 1150 and 1400 to 1675. The earlier period of powerful hurricane activity matched previous studies that found evidence of high hurricane activity during the same period in more southerly areas of the western North Atlantic Ocean basin – from the Caribbean to the Gulf Coast. The new study suggests that many powerful storms spawned in the tropical Atlantic Ocean between 250 and 1150 also battered the U.S. East Coast. The deposits revealed that these early storms were more frequent, and in some cases were likely more intense, than the most severe hurricanes Cape Cod has seen in historical times, including Hurricane Bob in 1991 and a 1635 hurricane that generated a 20-foot storm surge, according to Donnelly.

High hurricane activity continued in the Caribbean and Gulf of Mexico until 1400, although there was a lull in hurricane activity during this time in New England, according to the new study. A shift in hurricane activity in the North Atlantic occurred around 1400 when activity picked up from the Bahamas to New England until about 1675. The periods of intense hurricanes uncovered by the new research were driven in part by intervals of warm sea surface temperatures that previous research has shown occurred during these time periods, according to the new study.  Previous research has also shown that warmer ocean surface temperatures fuel more powerful storms.

The sediment coring and analysis by Donnelly and his colleagues “is really nice work because it gives us a much longer period perspective on hurricanes,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge. “It gives you something that you otherwise wouldn’t have any knowledge of.” The new research was funded in part by the National Science Foundation’s Division of Ocean Sciences.

Das entsprechende Paper “Climate forcing of unprecedented intense-hurricane activity in the last 2000 years” von Donnelly et al. 2015 kann als pdf kostenlos im Open-Access-Verfahren heruntergeladen werden.

Die Hurrikangeschichte Floridas war Thema einer Studie von Christian Ercolani und Kollegen, die im Oktober 2015 in den Quaternary Science Reviews herauskam. Das Forscherteam fand, dass Hurrikane während der Mittelalterlichen Wärmeperiode häufiger auftraten und in der nachfolgenden Kleinen Eiszeit deutlich weniger wurden. Dies deutet daraufhin, dass das Hurrikangeschehen in Florida an die Oberflächentemperatur des Atlantikwassers gekoppelt ist. Hier der Abstract:

Intense Southwest Florida hurricane landfalls over the past 1000 years
Recent research has proposed that human-induced sea surface temperature (SST) warming has led to an increase in the intensity of hurricanes over the past 30 years. However, this notion has been challenged on the basis that the instrumental record is too short and unreliable to reveal long-term trends in hurricane activity. This study addresses this limitation by investigating hurricane-induced overwash deposits (paleotempestites) behind a barrier island in Naples, FL, USA. Paleotempestologic proxies including grain size, percent calcium carbonate, and fossil shells species were used to distinguish overwash events in two sediment cores spanning the last one thousand years. Two prominent paleotempestites were observed in the top 20 cm of both cores: the first identified as Hurricane Donna in 1960 whereas an older paleotempestite (1900–1930) could represent one of three documented storms in the early 1900s. An active period of hurricane overwash from 1000 to 500 yrs. BP and an inactive period from 500 to 150 yrs. BP correlate with reconstructed SSTs from the Main Development Region (MDR) of the North Atlantic Ocean. We observe an increased number of paleotempestites when MDR SSTs are warmer, coinciding with the Medieval Warm Period, and very few paleotempestites when MDR SSTs are cooler, coinciding with the Little Ice Age. Results from this initial Southwest Florida study indicate that MDR SSTs have been a key long-term climate driver of intense Southwest Florida hurricane strikes.

Highlights:

  • MDR SSTs have been a key long-term climate driver of Southwest Florida hurricanes.

  • We observe an increased number of paleotempestites when MDR SSTs are warmer.

  • An active period of hurricane overwash was observed from 1000 to 500 yrs. BP.

  • An inactive period of hurricane overwash was observed from 500 to 150 yrs. BP.

  • Correlations exist between the Little Ice Age, Medieval Warm Period, and our record.

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