Chinese Science Bulletin

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Perspectives on the linkage between typhoon activity and global warming from recent research advances in paleotempestology

FAN DaiDu1,3† & LIU Kam-biu2 1 State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China; 2 Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, USA; 3 State Key Laboratory of Coast and Estuarine Science, East China Normal University, Shanghai 200062, China

The recent increase in typhoon (tropical cyclone) activity has attracted great interest and induced heated debates over whether it is linked to global warming or only a return to an active phase of the well-known multi-decadal variability. Due to the short instrumental record, our knowledge is quite scarce on the complex processes and mechanism of typhoon generation, development, and evolution, especially for the rare but highly destructive super-typhoons. It is therefore very important to extend the time span of typhoon activity records. Paleotempestology, a young science that emerged in the early 1990s, studies past typhoon activity spanning several centuries to millennia before the instru- mental era through the use of geological proxies and historical documentary records. This paper pre- sents a brief review and synthesis on the major research advances and findings of paleotempestology with an emphasis on proxy technique development and applications. The methodology has been evolving from single geologic proxy to multi-proxy techniques by integrating microfossils, sedimentary organic elemental ratios, and stable isotopes, together with typical sedimentary textures and structures, for the diagnosis of storm deposits. A newly-developed proxy technique is employing oxygen isotopes preserved in growing laminae of tree rings, cave stalagmites, and reef corals to diagnose typhoon rainwater impacts. Historical documentary records have been systematically compiled and analyzed to reconstruct the history of typhoon activity in some regions. The extracted typhoon-proxy data show that there does not exist a simple linear relationship between typhoon frequency and Holocene climate (temperature) change. Typhoon activity should have a secular and constant linkage with ENSO fluc- tuations, in that more typhoons and hurricanes make landfalls in China, Central and North America during La Niña years than El Niño years. This finding is consistent with that derived from recent in- strumental data. Shifts in positions of subtropical high exert great influence on storm tracks, but their long-term relationship is still not well understood. All these findings are significant in projecting ty- phoon trends under global warming scenarios. Future developments in paleotempestology should strengthen the following research fields: (1) proxy generation mechanism and preservation potential, (2) inter-validation of different proxy data, (3) recognition of storm and tsunami deposits, (4) evaluation of paleo-typhoon intensity, (5) numerical modeling, and (6) regional to global scale comparison studies.

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paleotempestology, typhoon, storm deposit, disaster, return period, proxy, global warming

Received February 16, 2008; accepted May 26, 2008; published online August 30, 2008 doi: 10.1007/s11434-008-0341-2 †Corresponding author (email: ddfan@mail.tongji.edu.cn) Supported jointly by the National Natural Science Foundation of China (Grant Nos. 40476028 and 40576030), Program for New Century Excellent Talents in University (Grant No. NCET-07-0619); the U.S. National Science Foundation (Grant Nos. BCS-0213884 and BCS-0452399), the Inter-American Institute for Global Change Research (CRN2050), and the LSU Council on Research

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Tropical cyclone (TC) has different names around the world, like typhoon in the western North Pacific, hurri- cane in Central and North America, cyclone in South Asia and Australia1). Coastal communities have been exposed to more frequent and intense typhoons leading to gigantic societal and economic disasters in the past few years of the new millennium. In the United States, hurricanes are the costliest events among natural disas- ters. The western Atlantic basin experienced its ex- tremely active hurricane seasons in 2004 and 2005, in- cluding the catastrophic socio-economic impact of Hur- ricane Katrina. The landfall of Katrina in New Orleans caused more than 1600 deaths and over $80 billion damage, topping Hurricane Andrew’s $26.5 billion economic loss to become the costliest hurricane strike in the United States. The shadow left by Katrina is just like another “911” event to the United States[1]. Several new records have recently been set in the rest of the world, including, for example, 10 typhoon landfalls in Japan in 2004, five tropical cyclones affecting the Cook Islands in a five-week period in 2005, super-typhoon Saomai in China, and severe Cyclone Larry in Australia in 2006.

Debate has recently been igniting over whether the recent increases in typhoon activity are attributed di- rectly to the recent warming of the global oceans[2], or merely part of a normal climatic phenomenon as we are returning to an active phase in the mutli-decadal activity cycle[3,4]. The newly published IPCC AR4 concluded that global warming tends to increase typhoon intensity, but there is a large uncertainty in the trends of typhoon frequency under a warmer climate[5]. The recent simula- tion results of higher-resolution climate models indicate that there is no obvious increasing trend in global ty- phoon number or even fewer typhoons occurring in a warmer climate[6]. The major cause of the above debates is our limited understanding of the complex mechanism of typhoon generation, development, and evolution. Moreover, our knowledge on secular variations in ty- phoons, especially intense typhoons, is greatly hampered by very short typhoon instrumental record, which is not longer than 150 years even in the United States. Al- though intense typhoons are rare, their extreme destruc- tiveness can inflict heavy damage to society. It is there- fore vital to extend the length of typhoon activity re- cords to improve our understanding of the variations of

intense typhoon activities. Pre-instrumental typhoon activity history can be re-

constructed by using geological and archival techniques. Paleotempestology, a burgeoning science since the early 1990s, studies past typhoon activity spanning several centuries to millennia (usually 6000 years) before the instrumental record era, mainly through the use of geo- logical proxy techniques. Significant progress has been made in research scope, theory, and methodology since then. Paleotempestology has now become an important component of Quaternary Paleoclimatology[7―9]. Paleo- tempestology is significant not only in its contribution to scientific basic research, but also in its practical applica- tions to society. The return period of intense typhoons for a selected coastal area calculated from proxy data is one of important empirical data for risk assessments by insurance companies, civil planners, and emergency management officials of local governments[9,10].

This paper presents a review of recent developments in proxy techniques, major advances in methodology, theoretical basis, and research achievements in paleo- tempestology, and ends with an outlook of future re- search needs.

1 Proxies used in paleotempestology 1.1 Geological proxies

Typhoon landfall can severely impact coastal hydrody- namics, morphodynamics, and ecosystems (Figure 1), leaving their imprints in geological and biological ar- chives. The history of past typhoon activities can be de- ciphered from these archives by means of reliable proxy techniques, including sedimentological, paleontological, and sedimentary organic geochemical and stable isotopic indicators.

Based on physical environments and typical sedimentary features, the sedimentological proxies can be further classified into: (1) overwash sandy layers in coastal lakes and marshes[11―13], (2) beach ridges and cheniers[14,15], (3) shallow-marine storm sequences with typical hummocky cross stratification[16], (4) storm-related rhythmites in estuarine-deltaic environments[17―19], and (5) storm deposits in atoll lagoons and inner reef flats[20].

Biostratigraphic indicators can be used as proxies or supplementary evidence to distinguish storm-laid layers

1) In this paper we use the word “typhoon” loosely as a generic term for tropical cyclones, unless specified otherwise.

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Figure 1 Sedimentological, paleontological, and geochemical proxies used in paleotempestology (modified from ref. [8]).

in coastal sedimentary sequences. When a typhoon is making landfall, coastal sea and sediment are highly disturbed by storm waves and currents, and marine or-ganism and microfossils can be transported onshore to deposit in nearshore or backbarrier lacustrine/lagoonal environments. The presence of marine microfossils (like foraminifera, dinoflagellates, and diatoms) with high abundance and diversity can be used to interpret the de-positional layers of storm genesis[21―26]. Contents of phytoliths, pollen and even charcoals in the coastal overwash sandy layers are useful indicators of the eco-logical impacts by typhoon strikes[24,26,27].

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Sedimentary organic elemental ratios and isotopic components can also be used as proxies for storm- in-duced deposits[19]. Similar to the transport mechanism of marine microfossils, marine organic matter can be transported onshore and deposited in coastal and estua-rine environments. It is generally known that marine organic matter has heavier δ13C and lower TOC/TN ratio (usually less than 10) than terrestrial organic matter[28]. In the coastal/estuarine strata, storm deposits were found to have heavier δ13C and lower TOC/TN ratio than normal deposits. These geochemical and isotopic signals have been employed as supplementary proxies to identify storm-generated layers in the Yangtze deltaic strata[19].

Stable isotopic proxies for paleotempestology potentially provide much higher resolution than other geological proxies. Typhoon landfall is always accompanied by heavy precipitation. Due to intense

convection and profound isotopic fractionation processes, the δ18O value of typhoon rainwater is markedly lower than low-latitude thunderstorms by as much as 10‰[29,30]. The isotopically depleted signal after typhoon precipitation may persist in surface or soil water for a few weeks before amelioration, and can be incorporated into the growing laminae of tress rings, speleothems, and coral skeletons through biophysical or chemical processes (Figure 1). Through the use of stable isotopic techniques, typhoon signals have been successfully deciphered from oxygen isotopes in tree- tring cellulose[31,32], calcium carbonate of speleo- thems[33], and aragonitic skeletons of reef corals[34,35]. High-resolution δ18O data have been used to reconstruct past typhoon activity history over several decades to hundreds of years, and have been proven to be a promising new proxy for paleotempestology.

1.2 Historical documental proxies

The main hazards related with typhoons include high winds, heavy rain, storm surge, huge waves, and associ- ated tornadoes, inland flooding, landslides, debris flows, etc.[1]. All these hazards could potentially turn into catastrophic disasters to the society. These societal im- pacts tend to be recorded in historical written documents after the initiation of human civilization to have literal history. Documentary records of typhoon activity can be achieved from official histories, semiofficial local ga- zettes (known as Fanzhi in China), newspapers, civilian writings like travel logbooks, diaries, poems, etc. The

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length of these documentary records varies from one region to another in the world, but rarely exceeds a few hundred years. China has the longest documentary re- cord of typhoon landfall in the world, extending back to over a millennium[36,37].

Historical records, albeit shorter in time span than geological proxy records, are outstanding in their high spatial precision and temporal resolution, typically al- lowing reconstruction to the year, month, and even day of the event. Where both geological and documentary records are available, these two sets of data can be used for cross calibration and validation to highlight data re- liability.

2 Paleotempestological studies using sedimentological proxies 2.1 Overwash sandy layers in coastal lacustrine and lagoonal facies

Paleotempestological studies were first carried out along the U.S. Gulf coast[11,38]. Barrier islands are widely dis- tributed along the Gulf coast, where coastal swamps, lagoons, and lakes are also well developed. The back- barrier sedimentary basins receive few terrigenous sediment fluxes, and are dominated by muddy sediments with high organic content deposited during normal weather conditions. During a hurricane landfall, several- meter-high coastal dunes can be overtopped by waves riding over storm tide plus wave set-up, thereby washing coarse-grained sediment from beach and dunes to form overwash fan deposits at the backbarrier environments. Overwash sand layers usually have sharp contacts with the underlying organic-rich dark muddy sediment. It is easy to distinguish visually between storm and normal deposits based on their obvious differences in sediment color (light vs. dark) and grain size (coarse vs. fine)[11,12] (Figure 2(a)). Furthermore, variations of microfossil assemblages in the coastal strata can be used to track coarse-grained sediment source, serving as supplemen- tary indicators to distinguish sandy layers of storm ori- gin[22,23,26,27].

The backbarrier sedimentary environments may re- peatedly receive overwash storm deposits through geo- logic time, accumulating vertically to form a series of light-colored sandy layers sandwiched between dark muddy deposits. A chronostratigraphy of overwash storm deposits can be established by means of radiomet- ric dating techniques like 14C, 210Pb and 137Cs dating.

For a selected coast, the return period of typhoons can be calculated by counting sandy layers over a given chronostratigraphic unit[7,11,12,13,39―41].

The paleo-typhoon intensity is much harder to deci- pher from proxy data than the frequency. It is generally assumed that the magnitude of storm surge should have a linear relationship with typhoon intensity, i.e., stronger typhoons tend to induce higher storm surges, resulting in bigger and thicker overwash fans. The paleo-typhoon intensity can therefore be estimated through comparison study of distribution scales with modern overwash fans formed by known-intensity typhoons. For example, the overwash fan deposit by Hurricane Frederic, a category 3 storm that struck Alabama in 1979, was confined to the lake margin of Lake Shelby, suggesting that borehole sandy layers at the lake center must have been deposited by paleo-hurricanes of category 4 or 5 intensity[11]. Fur-thermore, variations in thickness of sandy layers within the same borehole can be used as an indicator of storm intensity, i.e., thicker sandy layers should be laid down by stronger typhoons[12].

2.2 Beach ridges and cheniers

A beach ridge or chenier is an elongated sand body run- ning parallel to a shoreline, composed predominantly of sand, mixed sand and shell, and shell fragments. A che- nier is specially referred to a ridge developed onto mud substrate, and is mainly composed of shell fragments with some sediments of sand to gravel sizes. Extending as uninterrupted coastwise ridges, cheniers can be sev- eral kilometers to tens of kilometers in length, on aver- age 2―2.5 m in thickness and tens of meters to more than 300 m in width[42]. Cheniers are extensively dis- tributed on deltaic plains of large rivers, and are consid- ered to result from the reworking of mudflat deposits by large waves when the sediment supply is decreased and the shoreline is retreating. The development of cheniers is therefore potentially linked with an active era of ty- phoon activities in areas prone to tropical storm impact, and can be used as an important proxy for paleotempe- stology. Cheniers are widely distributed on the mega- deltaic plains of China, and are promising records used for paleotempestological study.

Beach ridges are parallel ridges on the sandy coast, and may be composed predominantly of shell fragments. Low and gentle beach ridges are considered to form by swash waves, while high and cliffy ones with crests 2―

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Figure 2 Sedimentary features of storm overwash layers in coastal lagoonal/lacustrine facies and paleotempestological indication (modified from refs. [8, 12]). (a) Light sandy layers of storm genesis intercalated with dark muddy layers in backbarrier sedimentary environments, radiocarbon dates marked on the right with arrows; (b) chronostratigraphic patterns of overwash sandy layers (horizontal bars) representing catastrophic hurricane strikes over the last 4500 years documented at four sites along the Gulf coast from west to east: PRM/LA, Pearl River Marsh (Louisiana); PM/MS; Pascagoula Marsh (Missis-sippi); LS/AL, Lake Shelby (Alabama); WL/FL, Western Lake (Florida).

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6 m above mean sea level and rich in shell fragments should be produced by storm waves[43]. The latter is widely distributed along the mainland or island coast of Queensland, Australia (Figure 3). The ridges are mainly composed of gravel-sized coral fragments that are pre- dominantly derived from live corals in the Great Barrier Reef

[9,14]. Reef corals are broken apart into fragments by storm waves in cyclone seasons, transported onshore or offshore. Storm-beach ridges are built up by coral detri- tus and coarse lithic sand. Individual ridges contain a number of distinct sedimentary units of beach face, berm, crest, and washover facies[14]. A new storm-beach ridge is deposited seaward of the previously emplaced ridge, separated with distinct sedimentary break from each other

[9,14]. Coral detritus derived from living reef are reliable material to date storm event deposition. The re- currence intervals of intense cyclones on a coastal site can be calculated by high-precision radiocarbon dating of coral fragments from the storm-beach ridges along the transects (Figure 3). The intensity of paleo-cyclones can be estimated through comparison study of the height

of paleo-ridges with modern analogues plus numerical modeling[15,44].

Radiocarbon dates from storm-beach ridges at Cura- coa Island reveal that there have been at least 22 ridge building (storm deposition) events over the last 6000 years, denoting an average return period of intense cy- clones of 280 years. Another ridge transect at Princess Charlotte Bay shows that over 12 intense cyclones should have stricken the study site to build storm ridges over the last 3000 years with a return period of 180 years[14] (Figure 3). Modern analogue study and numeri- cal modeling result show that super-cyclones of category 5 or at least category 4 are capable of building storm ridges of that height. It was therefore estimated that the return period of super-cyclones should be 200―300 years at the Great Barrier Reef over the past 5000 years[14,15]. The estimation is an order of magnitude higher than previous estimates from recent cyclone in- strumental data, which is once every several millen- nia[45]. For example, Cairns has not been stricken by super-cyclones of category 4 and 5 in the 20th century,

Figure 3 Storm-beach ridges along the northeastern Australia coast and their paleotempestological indication (modified from refs. [14, 15]). (a) Location map of study sites; (b)―(c) numbered storm ridges on Curacoa Island (top) and Princess Charlotte Bay (bottom) with corrected radiocarbon age (in a BP) for each ridge marked above the traces.

but geological proxies and numerical modeling results indicate that the area experienced at least two supercy- clones in the 19th century[44]. An active century followed by a quiet century denotes a potential centennial variation in landfalls of super-cyclones in the Cairns region.

2.3 Storm sequences in the estuarine-deltaic sed- imentary facies

Shelf storm deposits (tempestites) have been studied since the 1960s, and flourished in the 1980s. The storm current theory has been recognized as a benchmark in the field of sedimentology as much as the turbidity cur-rent theory having revolutionized the study of clastic sediments[46,47]. However, the sedimentological study of tempestites is not directly linked to the development of the burgeoning field of paleotempestology in the 1990s. The low preservation rate of tempestites, which are sus-ceptible to reworking by normal-weather waves and in-tense bioturbation, therefore make it unlikely to contain a high-resolution record of past storm events. The latest study results show that formation and preservation of shelf tempestites are highly dependent on sediment sup-ply and sedimentation rates. In the mega- delta regions where sedimentation rates are high due to abundant sediment supply, event deposits attributed to intense ty-phoons can survive from wave rewor- king[18,48―50] and can be used as a potential proxy for high-resolution pa-leotempestological study.

The main physical processes during a storm event on a muddy coast or shelf begin with the scouring of the muddy substrate during the rising phase of the storm. The resuspended sediments are transported out of the site to produce a maximum scour face during storm peaking. Storm waning results in deposition. Coarser sediment is first settled out of the agitated water and a fining-upward sand-dominated layer is laid down over the erosion surface with some lithic gravels and shell fragments near the bottom. Fine-grained deposits laid down before storm vanishing transit gradually into nor-mal-weather sedimentation, consisting of mud-domi- nated layer (Figure 4). Small sequence of alternating sand-dominated and mud-dominated layers with ero-sional base are typical storm-generated sedimentary feature on muddy coasts and shelves[18,48―56].

Hydrodynamic variations in a river flood event or a spring-neap tidal cycle have the potential to produce sedimentary structures similar to those caused by storms. In practice, small sequences are ascertained to be storm- generated through the use of supplementary proxies like geochemical indicators. It is generally known that ter- restrial organic matter (OM) has lighter δ13C values and higher atomic TOC/TN ratios than marine OM[28]. Car- bon isotope and TOC/TN ratios have been widely and successfully used as indicators of the source and fate of organic matter, as well as the physical and biochemical processes for the cycling of organic matter in estuaries

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Figure 4 Sedimentological and organic geochemical indicators for storm deposits in the Yangtze delta.

and adjacent shelf seas[57―59]. They are also employed as supplementary indicators of the source and distribution of sand and organic matter during storms[60]. In the Holocene strata of the Yangtze River delta, sand- domi-nated layers have heavier δ13C values and lower TOC/ TN ratios than the overlying mud-dominated layers (Figure 4). It was deduced that sand-dominated layers of storm genesis have higher marine OM component of storm-tide input than the overlying post-storm muddy deposits of normal estuarine facies.

Foraminiferal assemblage can be used as another proxy for identifying storm deposits in the estuarine- deltaic strata. In the Pearl River estuary, foraminifera in the storm deposits are characterized by having higher diversity and abundance of offshore benthic and plank- tonic species with small tests. By contrast, foraminifera in the normal deposits are characterized by lower diver-

sity and abundance, dominance by native species with malformed tests, and absence of planktonic species[21]. Therefore, great abundance of exotic foraminiferal spe- cies can be used as an indicator of storm deposition. This criterion has been employed to identify storm lay- ers in four cores collected in the subaqueous Pearl River delta. A maximum of 17 storm layers was determined in a borehole Holocene strata, and radiocarbon dates sug- gested that intense typhoons (category 4 and above) struck the site once in ~350-year intervals[17].

3 Speleothem and tree-ring stable isotope records of recent tropical cyclone events 3.1 Speleothems

Speleothem microbands—including their thickness and brightness, stable isotopic ratios, and trace elemental

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compositions—can provide useful proxy records of past climate changes, and have been widely employed in pa- leoclimatic (including precipitation and temperature) and paleoecological reconstructions spanning decades to hundreds of millennia. Only in the recent decade, how- ever, have the strongly depleted oxygen isotopic signal in stalagmites been explored for use as a proxy for ty- phoon activity[61,62]. Due to low sampling frequency (approximately annual) and low analytical precision (larger sample requirement), early attempts to develop typhoon speleothem proxy can only detect the signals of decadal and/or multidecadal variations in typhoon fre- quency but fail to determine single typhoon events[33].

The thickness of stalagmite laminae is generally tens of microns and seldom exceeds 100 μm. Short-term ty- phoon-induced excursions can be detected only when the microsampling resolution is much finer than the thickness of growing rings. Choosing fast-growing sta- lagmites is another way to improve temporal resolution

of proxy records. Frappier et al.[33] analyzed oxygen isotopes in an actively growing stalagmite from a cave in central Belize, with microsampling resolution of 20 μm, yielding a record of monthly to weekly temporal resolution (Figure 5). A binary logistic regression model was developed to identify negative δ

18O excursions at- tributable to hurricane effects, and the correctness of discrimination is high in comparison with instrumental data of hurricanes passing close to the cave. Two excur- sions by two consecutive hurricanes occurring over two weeks were successfully identified. The results also showed an obvious linear relationship between δ

18O excursion amplitude and hurricane intensity[33] (Figure 5). High-resolution stalagmite proxy data can be a pow- erful tool to reconstruct paleo-typhoon frequency and intensity.

3.2 Tree rings

Oxygen isotopes in tree-ring cellulose have been studied

Figure 5 Hurricane frequency and intensity registered in negative oxygen isotopic excursions of a cave stalagmite in central Belize (modified from ref. [33]). (a) Photograph of stalagmite section; (b) cyclogenic excursion (A-K) highlighted in the δ

18O curve; (c) records of distance and intensity of hurri-canes affecting Belize; (d) the magnitude of cyclogentic δ

18O negative excursion determined by a logistic regression model.

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since the 1970s, and widely used to study paleoclimatic parameters including paleotermperature and paleopre- cipitation. It was only in recent years that attempts have been made to use oxygen isotopes in tree rings as ty- phoon proxy for paleotempestological studies[31,32]. Miller et al.[31,32] reconstructed 200-year (AD1770―1990) hurricane activity history based on high-resolution oxy- gen isotopic data of latewood cellulose in the tree rings of longleaf pines in Georgia, USA (Figure 6). Detailed instrumental records of hurricane intensity, precipitation and tracks were used to validate the reliability of the proxy-derived data for the period of AD1940―1990. In the validation study only one false positive spike was identified (that is, a hurricane was determined by proxy data but lacking instrumental evidence), and only three storms were falsely discriminated by proxy data for the period of AD1855 to 1990 (Figure 6(c)). Consequently, oxygen isotopes in tree-ring cellulose can be used as a useful proxy tool for reconstructing hurricane activity history extending back several centuries to millennia[31].

Oxygen isotopic values of tree-ring cellulose are not only closely related to source water (rainwater) compo- sition, but are also influenced by other factors like tem- perature, soil composition and moisture, and the amount of storm precipitation. Therefore, the isotopic proxy can be a reliable record of paleo-typhoon frequency but not the intensity[30,31]. Limited by sampling resolution, oxy- gen isotope values are usually measured over single latewood segments, so the averaged seasonal isotopic signals are not sufficient to further discriminate between single or multiple storm impacts over a typhoon season. Typhoon-related isotope depletion in soil water can only persist for a few weeks before amelioration, so the depletion signals can be highly dampened or completely missed in an average seasonal growing lamina sample, unable to decipher paleo-typhoon events. Higher sam- pling resolution is therefore expected to address the ephemeral nature of typhoon-related isotope depletion within a small portion of latewood cellulose, improving the precision of proxy typhoon record[31].

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Figure 6 Variations of oxygen isotopes in tree-ring cellulose and used as proxy record of hurricanes (modified from refs. [31, 32]). (a) Location of study site; (b) photograph of tree-ring section; (c) proxy record of typhoon events determined by negative latewood δ 18O residuals (<−0.5), the residual equal to observed δ 18O value minus model predicted δ 18O value. The reliability of derived data was compared to instrumental records of hurricane occurrence in the period of AD1940―1990.

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4 Paleotempestological studies using historical documentary data

Based on their length of recorded history, typhoon-prone countries can be classified into New World countries like Australia and the United States, and Old World countries like China and India[37]. In Australia, no record of cyclones exists before the arrival of Europeans in the 18th century. Neither does any written record of hurri- canes exist before the Columbian contact in North America including USA, Mexico, and other Caribbean countries in the late 15th century. The earliest descrip- tion of hurricanes in North America was presumably written by Christopher Columbus himself nearly 500 years ago during one of his voyages to the Caribbean. Since then, documentary records of hurricanes have been available in local newspapers, ship logs, plantation diaries, and chronicles. These documentary records have been laboriously scouted to compile inventories of At- lantic hurricanes[63,64]. The written records are charac- terized by narrative description and uneven spatial dis- tribution, and can only be used to compile the hurricane frequency for selected regions. For example, a compila- tion of a 223-year time series of hurricane landfalls for South Carolina from AD1778 to 2000 shows that there are two active periods with high frequency of hurricane landfalls in the 1830s and from AD1880 to 1910[65]. Docu- mentary evidence for Caribbean hurricane activ-ity span- ning the period of AD1500―1900 shows two active periods during the 16th and the 18th centuries with a spike in AD1766―1780, separated by a relatively inactive period during the 17th century[66].

In the Old World, historical records are poorly pre- served in India, leaving few evidence for paleo-cyclone activity studies[37]. China is famous for the extraordinary length of its documentary history. The earliest written record is the inscription on oracle bones of the Shang Dynasty (16th―11th century B.C.). The documentary archives of paleoclimatic information can be traced back to 2000 years ago, and the time series of climatic de- scriptions were almost uninterrupted in the most recent millennium[67]. Various names had been used for tropical cyclones in Chinese historical documents, including jufeng, ju, and dafeng, before the modern term taifeng, i.e., typhoon, became popular during the past century. The documentary descriptions also include some impor- tant accompanying weather phenomena and consequent

disasters, such as heavy rain, wind tide, sea flooding, seawall damage, etc. Based on present available data, the earliest written record of “dafeng and sea flooding” event was dated back to AD 228[36]. According to a sur- vey of Chinese historical literature, the term Ju first ap- peared in a book entitled Nanyue Zhi that was written by Shen Huaiyuan in the Liu-Song era of the Nan Dynasty (AD 420―479)[37]. The earliest unequivocal record of a typhoon landfall in China is the typhoon that struck the coastal city of Mizhou in Shandong Province (now Gaomi City) in AD 816[37]. Since the Song Dynasty ty- phoon landfalls have been systematically registered as a natural disaster in the chapter “Wu Xing Zhi” (Records of Five Elements or Disasters) of the official history books. It is worth noting that “Wu Xing Zhi” only com- piles those typhoon landfalls that had severe societal impacts or economic losses. The most important and most voluminous typhoon data come from fang zhi (semi-official local gazettes), especially those for coastal cities and counties in East and South China. These data have partly been compiled as an important chapter in various books of natural disaster or meteorological re- cord collections[36,68―73]. A 1000-year time series of ty- phoon landfalls for Guangdong Province has been pro- duced from fang zhi sources[74,75] Statistical analyses on the historical typhoon data from Guangdong reveal dis- tinct multi-decadal (~50 years) and higher-frequency variations in typhoon landfall that may be linked to the Pacific Decadal Oscillation (PDO) and ENSO[74,75]. It is also inferred that there exists an obvious linear relation- ship between the frequency of typhoon landfall in Guangdong and ENSO phases, i.e., more frequent ty- phoon landfalls in La Niña years than in El Niño years. The finding is consistent with the result from the recent 50-year instrumental data[69,82].

5 Linkage between paleo-typhoon activ- ity and climate changes 5.1 Relationship of paleo-typhoon activity with Holocene climatic (temperature) changes

Several episodes of Holocene climatic fluctuations have been recognized, like as the Holocene Climatic Opti- mum, Medieval Warm Period (MWP) and Little Ice Age (LIA). It is important to study changes in paleo-typhoon activity in relation to Holocene cold and warm phases, which will improve our understanding of future trends in

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typhoon activity under global warming scenarios. Since the early 1990s, only a few paleotempestological studies have been produced that contain paleo-typhoon records back to 5000―6000 years ago. Dated storm-generated beach ridge series along the Queensland coast in Austra- lia reveal that the frequency of super-cyclone landfalls at the site was statistically constant over the last 6000 years with a return period of 200―300 years (Figure 3). It is therefore indicated that warmer climate in the Holocene Optimum may not have induced more frequent landfalls of super-cyclones[14,15]. The U.S. Gulf coast is a region where much paleotempestological study has been con- ducted extensively since the early 1990s. Several proxy records from overwash depositional archives have re- vealed a pattern of millennial fluctuation in hurricane activity: two relatively inactive periods during the past 1000 years and 5000―3800 a BP, separated by a hyper- active period of 3800―1000 a BP[7,12]. Typhoon millen- nial fluctuation is not found to link with presumed Holocene temperature changes. For example, the hyper- active period of 3800―1000 a BP did not coincide with a period of markedly warmer climate in the region or globally, as commonly assumed. It is proposed that the millennial fluctuation of intense hurricane landfall along the Gulf coast may be controlled by the long-term shifts in the position of the Bermuda High, rather than directly linked to Holocene temperature changes[12].

In the last millennium, paleo-typhoon activity ar- chives can be extracted from both geological and docu- mentary proxy data. Yu et al.[20], based on U-series dat- ing of the storm-relocated coral blocks in an atoll lagoon of Yongshu Reef, South China Sea, discussed six intense typhoon strikes occurring in the last 1000 years, with four of them occurring in the MWP (AD 1000―1500) and the other two in the LIA (AD 1500―1900). A posi- tive linear relationship between the frequency of intense typhoons with climate changes was inferred. However, other paleotempestological studies reveal an equivocal linkage between typhoon frequency and climate changes in the past 1000 years. Time series analyses of the fre- quency of typhoon landfall in the Southeast China coast from historical documentary data (for example, see http://data.typhoon.gov.cn/TYDATA500/index.htm) have revealed that colder periods in the 17th and 19th centu- ries had more typhoon landfalls than other periods dur- ing the LIA[75]. A similar phenomenon has been reported from the geological proxy data in the Caribbean and U.S.

Atlantic coast, in that the period from AD 950 to 1700 was a relatively inactive regime of hurricane landfalls, and abruptly changing to a relatively active regime since about AD1700[76,77]. Documentary records from Puerto Rico also suggest an increase in severe hurricane dam- age during the latter half of the LIA, in that severe dam-age occurred only 3 times in the period of AD1550―

1700 while at least 16 events were recorded between AD1700 and 1850[78]. It is inferred that the frequency of intense typhoons should have a complex relationship with a multitude of environmental factors instead of re- sponding linearly to temperature alone in a changing climate.

5.2 Linkage of paleo-typhoon activity with ENSO phases

Donnelly and Woodruff[76] examined in detail the rela-tionship between a proxy hurricane record in Puerto Rico and a proxy record of El Niño events from Laguna Pallcacocha, Ecuador[79]. They found that weak El Niño periods (for example, 4400―3600 a BP, ~2500―1000 a BP, and 250 a BP to present) had more intense hurricane strikes than strong El Niño periods (for example, ~3600―2500 a BP, and 1000―250 a BP). The fre-quency of typhoon landfalls in the South China coast (including Guangdong, Fujian, and Zhejiang provinces) from documentary records in the period of AD1600―1911 was statistically analyzed under two separate phases of La Niña and El Niño years by Liang and Zhang[69]. The results show that the frequency of ty-phoon landfalls is much greater during La Niña years than during El Niño years. The extrapolations are in good accordance with the conclusions from recent in-strumental data in the North Atlantic[80] and the western North Pacific basins[81,82]. It is therefore inferred that the relationship between typhoon activity and the El Niño/Southern Oscillation fluctuations should be secular and constant in these two regions.

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5.3 Impacts of shifts in positions of subtropical high on paleo-storm tracks

Based on a comparison study of proxy paleo-hurricane data over a regional scale, an anti-phase relationship was found between the U.S. Gulf coast and the Atlantic coast[7,12,25]. In the Gulf coast, the period 3800―1000 a BP was a hurricane hyperactive period, separating the two relatively quiet periods of ~5000―3800 a BP and the last 1000 years[7]. Proxy data from the Atlantic coast

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in the northeastern U.S. suggest that the recent mil- len-nium was an active period but was preceded by a rela-tively quiet period of 1000―3000 a BP[83]. Such millen-nial variations in hurricane frequency have been pro-posed to link with long-term shifts in the position of the Bermuda High[7,12]. According to the “Bermuda High hypothesis”, a southwestward shift of the Bermuda High would direct more hurricanes to make landfall on the Gulf coast, whereas a northeastward shift would steer more hurricanes towards the Atlantic coast (Figure 7). According to paleoclimatic proxy data, the North Atlan- tic atmospheric circulation system underwent a signifi- cant change over a large scale during 4000 to 3000 a BP, resulting in a southwestward shift of the Bermuda High. The shift in the steering mechanism probably caused more hurricanes to hit the Gulf coast, hence the hyperac- tive period[7,12]. The anti-phase relationship in hurricane landfalls between the Gulf coast and Atlantic coast has also been demonstrated by a statistical analysis of his- torical hurricane data in relation to the North Atlantic Oscillation (NAO), which is in turn related to the posi- tion of the Bermuda High[84]. It is worth noting that the hypothesis has been examined in the light of some new evidence from various locations including, for example, proxy records from South Carolina[25] and from Long Island in New York[77]. The Long Island proxy data show a similar pattern of paleo-hurricane activity with those of Puerto Rico over the past 2500 years, in which the interval from 2500―1000 a BP was a relatively ac- tive period and the recent 1000 years were relatively quiescent. Proxy data from the Gulf coast also indicate a relatively quiescent interval beginning about 1000 years ago[7]. The seemingly synchronous transition from active

to quiescent hurricane periods in these three regions suggests that more work is needed to untangle the rela- tive roles of the various climate factors (Bermuda High, ENSO, sea surface temperature) in modulating hurricane activity[76].

Time-series analyses on the frequency of typhoon landfalls in Guangdong Province of southern China since AD975 reveal two active periods (AD1660―1680, AD1850―1880) coinciding with two of the coldest and driest intervals of the LIA in northern and central Chi-na[75]. It was hypothesized that an intensification of the westerlies and a southward shift of the subtropical high-pressure system during these cold intervals caused a southward shift of the typhoon tracks, resulting in more typhoons striking Gungdong but fewer landfalls in the east-central Chinese coast[75]. However, historical typhoon data summaries (http://data.typhoon.gov.cn/ TYDATA500/index.htm) from three east-central Chi- nese coastal provinces (Zhejiang, Shanghai, Jiangsu) seem to show multiple peaks in the frequency of ty- phoon landfalls in the three provinces including some- what higher counts during the two cold intervals of the LIA in the 17th and the 19th centuries. Consequently, the linkage of typhoon tracks to shifts in the position of subtropical high in the western North Pacific should be more complex and still needs further study.

6 Conclusions and outlook

An increasing trend of intense tropical cyclones has been observed in the Atlantic and western North Pacific basins in recent decades. The issue of whether this in- crease could be directly attributed to anthropogenically-

Figure 7 Long-term shifts in the position of the Bermuda High and their impacts on hurricane tracks (modified from ref. [12]).

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induced global warming has been hotly debated[2―4]. Skeptics of the global warming connection think that we are just at an active phase of multi-decadal variations in typhoons[3,4], while supporters point out the strong posi-tive relationship between increasing sea surface tem-peratures and increased typhoon activity[2]. The debates are mainly attributed to the short intervals of instrumen-tal records, greatly hampering our full understanding of the complex mechanism of typhoon generation, devel-opment, and evolution, especially with regard to the most destructive intense typhoons. Paleotempestology is a burgeoning science that can extend the interval of ty-phoon records beyond the instrumental era, thereby con-tributing to a better understanding of the periodicities of (intense) typhoons, through the use of geological and historical proxy records.

The methodology of paleotempestology has been greatly advanced and diversified since its early devel-oping stage in the 1990s. Single proxy method com-monly based on sedimentary features was employed at the early stage, evolving into using multi-proxy methods including microfossils (e.g. foraminifera, diatoms, phy-toliths, dinoflagellates, pollen), sedimentary organic isotopes, and elemental ratios to identify storm deposi-tional events in coastal sedimentary archives. Storm- generated overwash fan deposits in coastal barrier sys-tems have been the most widely used geological proxy in paleotempestological studies. Other geological prox-ies such as beach ridges can also provide useful tools for reconstructing paleo-typhoon activity history. Although typical storm sequences are susceptible to reworking by subsequent waves and bioturbation, they can be well preserved in some deltaic and muddy shelf sedimentary environments with rapid sedimentation, thus providing a potential proxy for high-resolution paleo-typhoon activ-ity reconstruction. Another new and promising proxy is the stable-isotopic signal preserved in the growing laminae of tree rings, reef corals, and cave stalagmites, where the strongly depleted δ18O signal linked to ty-phoon rainwaters can be used to reconstruct high-reso- lution typhoon activity history. Historical documentary records are outstanding in their high temporal resolution and spatial precision. The temporal length and com- pleteness of the historical documentary records vary greatly among the typhoon-prone countries. From China a 1000-year documentary record of typhoon landfalls has been published. Hurricane documentary records can

be traced back to about 500 years ago in the United States and the Caribbean region, and have been partly compiled and published. Time-series of paleo-typhoon documentary records are very important to provide in-sight into the periodicity of long-term variations in ty-phoon frequency. Whenever there is a time overlap be-tween historical documentary and instrumental records for a selected region, a detailed comparison between the two datasets should be conducted to evaluate the reli-ability of the historical documentary data and to deci-pher quantitatively the intensity of typhoons from dif-ferent narratives in the written records. If both docu-mentary and geological proxy records are contemporar-ily available in a selected region, these two sets of data should be comparatively studied to validate data inter-pretation.

Data validation, including the adoption of a mod-ern-analog approach, is important in paleotempestologi-cal studies. Different geological events, such as hurri-cane and tsunami, may result in similar sedimentary or geochemical signals preserved in the proxy record. A case in point is that some coastal areas around the Pa-cific Ocean and the Indian Ocean are prone to both tsu-namis and typhoon strikes, where the discrimination of storm deposits from tsunami sedimentation should be carefully carried out. Sixteen papers have been compiled into a special issue of the journal “Sedimentary Geol-ogy” in 2007 to discuss genesis mechanism and diagnos-tic features of tsunami deposits[85]. Two of these papers focus on how to discriminate tsunami and storm deposits based on physical criteria like sediment composition, textures and grading, types and organization of stratifi-cation, features and thickness of depositional sequences, geometry, and landscape conformity[86,87]. It is worth noting that storm and tsunami deposits share many common criteria, making them difficult to discriminate from each other especially in a single core. A collection of different data from multiple cores together with trenches is proposed to effectively discriminate the two depositional units[87].

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A number of important conclusions can be obtained from recent paleotempestological studies, shedding sig- nificant light on possible future changes in global ty- phoon activity under a warmer climate. A few case stud- ies based on geological proxy records show that a warmer climate alone during the Holocene Optimum may not have increased the frequency of intense ty- phoons. In the last millennium, the frequency of typhoon

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activity was not found to fluctuate linearly with climatic change over the centennial timescale. On the contrary, typhoon frequency seemed to have increased at least regionally during the coldest phases of the LIA, which begs adequate explanation. Up to now, our understand- ing of the long-term trends of typhoon activity in the Holocene is still very limited. Both the temporal and spatial resolution of the paleotempestology database is still very low. Knowledge and time resolution of SST (sea surface temperature) fluctuations in tropical oceans during the Holocene are still inadequate for understand- ing the interaction between typhoon generation and tropical ocean SST variations. New findings show that climatic change in the Holocene was far from synchro- nous in different regions of the world[5]. As the knowl- edge on SST variations in different tropical oceans dur- ing the Holocene is gradually increasing together with higher resolution of temporal and spatial coverage[88―90], it will undoubtedly improve our understanding of cyc- logenesis mechanism and periodic variations in different tropical cyclogenetic basins.

The frequency of intense typhoons is not only related to air and sea surface temperatures, but is also modu- lated by other environmental factors. For example, the frequency has a close relationship with ENSO phases, i.e., more frequent typhoon landfalls during La Niña years than during El Niño years. The finding from proxy data is consistent with that from recent instrumental data, indicating a consistent relationship between typhoon activity and ENSO fluctuations, which is important for us to predict typhoon activity change. Moreover, the frequency of typhoon landfalls in a selected region is potentially modulated by long-term shifts in the position

of the subtropical high. That is, a southward shift of subtropical high during colder phases would steer more typhoon landfalls in lower-latitude regions, whereas a northward shift of the position of the subtropical high would direct more typhoons toward the higher-latitude regions during warmer phases. Modern climatological studies support the hypothesis that typhoon tracks are modulated by the position of the subtropical high, but their long-term relationship needs more evidence to validate.

The frequency of typhoon landfalls over a selected coast varies as a function of a complex set of climatic factors. While global warming is projected to continue in the near future, a consensus has not been reached concerning the future trends of typhoon activity. Future study should put emphasis not only on improving mod- ern field observations but also on the retrieval of more paleo-typhoon proxy records from Holocene geological archives. Paleotempestological studies should highlight the following research needs in the near future: (1) genesis mechanism and preservation potential of ty- phoon proxies in geological and biological archives, (2) comparative studies among different proxy techniques, (3) recognition of tropical storm deposits from other high-energy event deposits like tsunami deposits, (4) developing proxy methods to determine the intensity of past typhoons, (5) using numerical models to simulate storm surges by past typhoons and their impacts, and (6) comparative studies of past typhoon activities on re-gional and global scales to improve our understanding of typhoon periodicities, teleconnection, and cyclogensis mechanisms.

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