Tree-ring reconstructed rainfall over the southeastern U.S.A. during the medieval warm period and little ice age

A 1053-year reconstruction of spring rainfall (March-June) was developed for the southeastern United States, based on three tree-ring reconstructions of statewide rainfall from North Carolina, South Carolina, and Georgia. This regional reconstruction is highly correlated with the instrumental record of spring rainfall (r = +0.80; 1887–1982), and accurately reproduces the decade-scale departures in spring rainfall amount and variance witnessed over the Southeast during the past century. No large-magnitude centuries-long trends in spring rainfall amounts were reconstructed over the past 1053 years, but large changes in the interannual variability of spring rainfall were reconstructed during portions of the Medieval Warm Period (MWP), Little Ice Age (LIA), and the 20th century. Dry conditions persisted at the end of the 12th century, but appear to have been exceeded by a reconstructed drought in the mid-18th century. High interannual variability, including five extremely wet years were reconstructed for a 20-yr period during the late 16th and early 17th centuries, and may reflect amplified atmospheric circulation over eastern North America during what appears to have been one of the most widespread cold episodes of the Little Ice Age.     

Pacific and Atlantic influences on Mesoamerican climate over the past millennium

A new tree-ring reconstruction of the Palmer Drought Severity Index (PDSI) for Mesoamerica from AD 771 to 2008 identifies megadroughts more severe and sustained than any witnessed during the twentieth century. Correlation analyses indicate strong forcing of instrumental and reconstructed June PDSI over Mesoamerica from the El Ni?o/Southern Oscillation (ENSO). Spectral analyses of the 1,238-year reconstruction indicate significant concentrations of variance at ENSO, sub-decadal, bi-decadal, and multidecadal timescales. Instrumental and model-based analyses indicate that the Atlantic Multidecadal Oscillation is important to warm season climate variability over Mexico. Ocean-atmospheric variability in the Atlantic is not strongly correlated with the June PDSI reconstruction during the instrumental era, but may be responsible for the strong multidecadal variance detected in the reconstruction episodically over the past millennium. June drought indices in Mesoamerica are negatively correlated with gridded June PDSI over the United States from 1950 to 2005, based on both instrumental and reconstructed data. Interannual variability in this latitudinal moisture gradient is due in part to ENSO forcing, where warm events favor wet June PDSI conditions over the southern US and northern Mexico, but dryness over central and southern Mexico (Mesoamerica). Strong anti-phasing between multidecadal regimes of tree-ring reconstructed June PDSI over Mesoamerica and reconstructed summer (JJA) PDSI over the Southwest has also been detected episodically over the past millennium, including the 1950–1960s when La Ni?a and warm Atlantic SSTs prevailed, and the 1980–1990s when El Ni?o and cold Atlantic SSTs prevailed. Several Mesoamerican megadroughts are reconstructed when wetness prevailed over the Southwest, including the early tenth century Terminal Classic Drought, implicating El Ni?o and Atlantic SSTs in this intense and widespread drought that may have contributed to social changes in ancient Mexico.     

Tree-ring-based hydrological records for western Himalaya, India, since a.d. 1560

We analysed 565 increment cores from 325 Himalayan cedar [Cedrus deodara (Roxb.) G. Don] trees growing at 13 moisture-stressed, widely distributed sites in the western Himalayan region. We found a strong positive relationship between our tree-ring width chronologies and spring precipitation which enabled us to reconstruct precipitation back to a.d. 1560. This reconstruction is so far the longest in this region. The calibration model explains 40% variance in the instrumental data (1953–1997). The most striking feature of the reconstruction is the unprecedented increase in precipitation during the late twentieth century relative to the past 438 years. Both wet and dry springs occurred during the Little Ice Age. A 10-year running mean showed that the driest period occurred in the seventeenth century while the wettest period occurred in the twentieth century. Spectral analysis of the reconstructed series indicated a dominant 2-year periodicity.     

Documentary reconstruction of monsoon rainfall variability over western India, 1781–1860

Investigations into the climatic forcings that affect the long-term variability of the Indian summer monsoon are constrained by a lack of reliable rainfall data prior to the late nineteenth century. Extensive qualitative and quantitative meteorological information for the pre-instrumental period exists within historical documents, although these materials have been largely unexplored. This paper presents the first reconstruction of monsoon variability using documentary sources, focussing on western India for the period 1781–1860. Three separate reconstructions are generated, for (1) Mumbai, (2) Pune and (3) the area of Gujarat bordering the Gulf of Khambat. A composite chronology is then produced from the three reconstructions, termed the Western India Monsoon Rainfall reconstruction (WIMR). The WIMR exhibits four periods of generally deficient monsoon rainfall (1780–1785, 1799–1806, 1830–1838 and 1845–1857) and three of above-normal rainfall (1788–1794, 1813–1828 and 1839–1844). The WIMR shows good correspondence with a dendroclimatic drought reconstruction for Kerala, although agreement with the western Indian portion of the tree-ring derived Monsoon Asia Drought Atlas is less strong. The reconstruction is used to examine the long-term relationship between the El Nino-Southern Oscillation (ENSO) and monsoon rainfall over western India. This exhibits peaks and troughs in correlation over time, suggesting a regular long-term fluctuation. This may be an internal oscillation in the ENSO-monsoon system or may be related to volcanic aerosol forcings. Further reconstructions of monsoon rainfall are necessary to validate this. The study highlights uncertainties in existing published rainfall records for 1817–1846 for western India.     

The climate of Namaqualand in the nineteenth century

Southern African climatic change research is hampered by a lack of long-term historical data sets. This paper aims to extend the historical climate record for southern Africa to the semi-arid area of Namaqualand in the Northern Cape province of South Africa. This is achieved through extensive archival research, making use of historical documentary sources such as missionary journals and letters, traveller’s writings and government reports and letters. References to precipitation and other climatic conditions have been extracted and categorised, providing a proxy precipitation data set for Namaqualand for the nineteenth century. Notwithstanding problems of data accuracy and interpretation the reconstruction enables the detection of severe and extreme periods. Measured meteorological data, available from the late 1870s, was compared to the data set derived from documentary sources in order to ascertain the accuracy of the data set and monthly rainfall data has been used to identify seasonal anomalies. Confidence ratings on derived dry and wet periods, where appropriate, have been assigned to each year. The study extends the geographical area of existing research and extracts the major periods of drought and climatic stress, from the growing body of historical climate research. The most widespread drought periods affecting the southern and eastern Cape, Namaqualand and the Kalahari were 1820–1821; 1825–1827; 1834; 1861–1862; 1874–1875; 1880–1883 and 1894–1896. Finally, a possible correspondence is suggested between some of the widespread droughts and the El Nino Southern Oscillation (ENSO).     

Long-term hydroclimatic variability in monsoon shadow zone of western Himalaya, India

Tree-ring-width data of Himalayan cedar [Cedrus deodara (Roxb.) G. Don] from 11 homogeneous moisture stressed sites in the monsoon shadow zone of the western Himalaya were used to develop a mean chronology extending back to ad 1353. The chronology developed using Regional Curve Standardization method is the first from the Himalayan region of India showing centennial-scale variations. The calibration of ring-width chronology with instrumental precipitation data available from stations close to the tree ring sampling sites showed strong, direct relationship with March?CApril?CMay?CJune (MAMJ) precipitation. This strong relationship was used to supplement the instrumental precipitation data back to ad 1410. The precipitation reconstruction showed extended period of drought in fifteenth and sixteenth centuries. Increasingly pluvial conditions were recorded since eighteenth century, with the highest precipitation in the early part of the nineteenth century. The decreasing trend in reconstructed precipitation in the last decade of the twentieth century, consistent with the instrumental records, is associated with the decreasing trend in frequency of western disturbances. MAMJ precipitation over the monsoon shadow zone in the western Himalaya is directly associated with the North Atlantic Oscillation (NAO) and NINO3-SST index of El Nino-Southern Oscillation (ENSO), the leading modes of climate variability influencing climate over large parts of the Northern Hemisphere. However, the relationship between ENSO and MAMJ precipitation collapsed completely during 1930?C1960. The breakdown in this relationship is associated with the warm phase of Atlantic Multidecadal Oscillation (AMO). A spectral analysis of reconstructed MAMJ precipitation indicates frequencies in the range of the variability associated with modes of NAO, ENSO and AMO.     

Unprecedented low twentieth century winter sea ice extent in the Western Nordic Seas since A.D. 1200

We reconstructed decadal to centennial variability of maximum sea ice extent in the Western Nordic Seas for A.D. 1200–1997 using a combination of a regional tree-ring chronology from the timberline area in Fennoscandia and δ¹⁸O from the Lomonosovfonna ice core in Svalbard. The reconstruction successfully explained 59% of the variance in sea ice extent based on the calibration period 1864–1997. The significance of the reconstruction statistics (reduction of error, coefficient of efficiency) is computed for the first time against a realistic noise background. The twentieth century sustained the lowest sea ice extent values since A.D. 1200: low sea ice extent also occurred before (mid-seventeenth and mid-eighteenth centuries, early fifteenth and late thirteenth centuries), but these periods were in no case as persistent as in the twentieth century. Largest sea ice extent values occurred from the seventeenth to the nineteenth centuries, during the Little Ice Age (LIA), with relatively smaller sea ice-covered area during the sixteenth century. Moderate sea ice extent occurred during thirteenth–fifteenth centuries. Reconstructed sea ice extent variability is dominated by decadal oscillations, frequently associated with decadal components of the North Atlantic Oscillation/Arctic Oscillation (NAO/AO), and multi-decadal lower frequency oscillations operating at ~50–120 year. Sea ice extent and NAO showed a non-stationary relationship during the observational period. The present low sea ice extent is unique over the last 800 years, and results from a decline started in late-nineteenth century after the LIA.     

On the long-term context of the 1997–2009 ‘Big Dry’ in South-Eastern Australia: insights from a 206-year multi-proxy rainfall reconstruction

This study presents the first multi-proxy reconstruction of rainfall variability from the mid-latitude region of south-eastern Australia (SEA). A skilful rainfall reconstruction for the 1783–1988 period was possible using twelve annually-resolved palaeoclimate records from the Australasian region. An innovative Monte Carlo calibration and verification technique is introduced to provide the robust uncertainty estimates needed for reliable climate reconstructions. Our ensemble median reconstruction captures 33% of inter-annual and 72% of decadal variations in instrumental SEA rainfall observations. We investigate the stability of regional SEA rainfall with large-scale circulation associated with El Niño–Southern Oscillation (ENSO) and the Inter-decadal Pacific Oscillation (IPO) over the past 206 years. We find evidence for a robust relationship with high SEA rainfall, ENSO and the IPO over the 1840–1988 period. These relationships break down in the late 18th–early 19th century, coinciding with a known period of equatorial Pacific Sea Surface Temperature (SST) cooling during one of the most severe periods of the Little Ice Age. In comparison to a markedly wetter late 18th/early 19th century containing 75% of sustained wet years, 70% of all reconstructed sustained dry years in SEA occur during the 20th century. In the context of the rainfall estimates introduced here, there is a 97.1% probability that the decadal rainfall anomaly recorded during the 1998–2008 ‘Big Dry’ is the worst experienced since the first European settlement of Australia.     

North Atlantic oscillation control of droughts in north-east Spain: evaluation since 1600 a.d.

This paper analyses the role played by the North Atlantic Oscillation (NAO) in the creation of drought conditions in a semi-arid region of north-east Spain (the middle Ebro valley), from 1600 to the year 2000. The study used documents from ecclesiastical archives for the seventeenth, eighteenth and nineteenth centuries. For the twentieth century, instrumental precipitation records were used as well. A December–August drought index from 1600 to 1900 was compiled from the historical documentary sources (rogation ceremonies). The index was validated by means of precipitation records between 1858 and 1900 and independent precipitation data from 1600 reconstructed by means of dendrochronological records. Using instrumental data a drought index was also calculated (Standardized Precipitation Index, SPI) for the 1958–2000 period. We found that the NAO was important in explaining the droughts identified in the study area from documents and instrumental data. Positive values of the winter NAO index are prone to cause droughts in the middle Ebro valley. This finding has been verified since 1600 by means of two independent reconstructions of the winter NAO index. The same behaviour has been observed during the nineteenth and twentieth centuries by means of instrumental records. The climatic and geographic factors that explain the high influence of North Atlantic Oscillation on droughts in this region are discussed in depth.     

Tree-Ring Reconstructed Winter Precipitation and Tropical Teleconnections in Durango, Mexico

Earlywood width chronologies from Douglas-fir tree rings were used to reconstruct winter (November–March) precipitation for more than 600 years over Durango, Mexico. The tree-ring data were obtained from two sites of long-lived Douglas-fir in northern and southern Durango and the seasonal climatic precipitation data were regionally averaged from five weather stations well distributed across the state. The averaged earlywood chronology accounted for 56% of the variance in instrumental November–March precipitation 1942–1983. We validated the reconstruction against independent precipitation records. The worst winter drought of the 20th century in Durango occurred 1950–1965. However, the reconstruction indicates droughts more severe than any witnessed in the 20th century, e.g., the 1850s–1860s, and the megadrought in the mid- to late-16th century. Reconstructed winter precipitation 1540–1579 shows 33 of 40 years were dry. Persistent drought may be linked to extended La Niña episodes. The Tropical Rainfall Index (TRI) correlates well with instrumental and reconstructed winter precipitation (r = 0.49 and 0.55, respectively), reflecting the strong ENSO modulation of cool season climate over northern Mexico. The ENSO teleconnection varies through time, with TRI-reconstructed precipitation correlations ranging from 0.78 to 0.27 in five periods 1895–1993. The 1942–1983 winter observed and reconstructed Durango data correlate well with the corresponding seasonalization of the All-Mexico Rainfall Index (AMRI; r=0.68, P<0.0001 and r=0.70, P<0.001, respectively), indicating that both the observed and the reconstructed precipitation often reflect broad-scale precipitation anomalies across Mexico. New long Douglas-fir and baldcypress tree-ring chronologies are now available for central and southern Mexico near major population centers, allowing the exploration of relationships between drought, food scarcity, and social and political upheaval in Mexican history.     

Multi-decadal scale variability in autumn-winter rainfall in south-western Australia since 1655 AD as reconstructed from tree rings of Callitris columellaris

We present the first tree-ring based reconstruction of rainfall for the Lake Tay region of southern Western Australia. We examined the response of Callitris columellaris to rainfall, the southern oscillation index (SOI), the southern annular mode (SAM) and surface sea temperature (SST) anomalies in the southern Indian Ocean. The 350-year chronology was most strongly correlated with rainfall averaged over the autumn-winter period (March–September; r = ?0.70, P < 0.05) and SOI values averaged over June–August (r = 0.25, P < 0.05). The chronology was not correlated with SAM or SSTs. We reconstructed autumn-winter rainfall back to 1655, where current and previous year tree-ring indices explained 54% of variation in rainfall over the 1902–2005 calibration period. Some variability in rainfall was lost during the reconstruction: variability of actual rainfall (expressed as normalized values) over the calibration period was 0.78, while variability of the normalized reconstructed values over the same period was 0.44. Nevertheless, the reconstruction, combined with spectral analysis, revealed that rainfall naturally varies from relatively dry periods lasting to 20–30 years to 15-year long periods of above average rainfall. This variability in rainfall may reflect low-frequency variation in the El Niño-Southern Oscillation rather than the effect of SAM or SSTs.     

500-yr. precipitation variability in Southern Taihang Mountains,China, and its linkages to ENSO and PDO

An annually resolved and absolutely dated ring-width chronology spanning 657 yrs. is constructed with Whitebark pine (Pinus bungeana Zucc.) samples from the southern Taihang Mountains, Eastern China. On the basis of a significant correlation between the tree-ring width index and observed instrumental data, precipitation in current May is reconstructed for the region since AD 1510, with predictor variables accounting for 37.9 % of the variance in precipitation data. In agreement with other drought reconstructions, notable dry spells occur in the 1630s–1650s, 1680s–1700s, and 1770s–1800s, whereas wet periods prevail in the 1530s–1570s, 1840s–1870s, and 1950s-present. Wavelet analysis reveals clear 2–8, 20–40, and 80–130 yrs cycles at the 95 % confidence level for the reconstructed series over the past 500 yrs, suggesting possible linkages with the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). Correlation analysis between the tree-ring series, ENSO, and PDO index further demonstrates that precipitation is negatively correlated with PDO and ENSO in the long term.     

Multi-century variability in the Pacific North American circulation pattern reconstructed from tree rings

We here present a reconstruction (1725–1999) of the winter Pacific North American (PNA) pattern based on three winter climate sensitive tree ring records from the western USA. Positive PNA phases in our record are associated with warm phases of ENSO and PDO and the reorganization of the PNA pattern towards a positive mode is strongest when ENSO and PDO are in phase. Regime shifts in our PNA record correspond to climatic shifts in other proxies of Pacific climate variability, including two well-documented shifts in the instrumental period (1976 and 1923). The correspondence breaks down in the early 19th century, when our record shows a prolonged period of positive PNA, with a peak in 1800–1820. This period corresponds to a period of low solar activity (Dalton Minimum), suggesting a ‘positive PNA like’ response to decreased solar irradiance. The distinct 30-year periodicity that dominates the PNA reconstruction in the 18th century and again from 1875 onwards is disrupted during this period.     

A Semi-Quantitative, Regional Precipitation Data Set for Studying African Climates of the Nineteenth Century, Part I. Overview of the Data Set

This article describes a historical archive of proxy and actual precipitation data that extends the African climate record back to the early nineteenth century. The `proxy' archive includes verbal, documentary references which contain information related to rainfall conditions, such as references to famine, drought, agriculture or the nature of the rainy season. The precipitation archive includes all observations made in Africa during the nineteenth century. It consists of records for 60 stations in Algeria, 87 stations in South Africa and 304 stations scattered over the rest of Africa. Information is particularly plentiful for the 1880s and 1890s. The two parts have been be combined into a semi-quantitative regional data set indicating annual rainfall conditions in terms of anomaly classes (e.g., normal, dry, wet). This data set extends from the early nineteenth century to 1900 and distinguishes seven anomaly classes, using numbers ranging from –3 to +3 to represent very wet, wet, good rains, normal, dry, drought, and severe drought. The regionalization is based on 90 geographical regions shown via studies of the modern precipitation record to be climatically homogeneous with respect to the interannual variability of rainfall. The regional aggregation allows the voluminous fragmentary information available in historical sources to be used systematically to produce multi-year time series that can be directly integrated into the modern record for each region. The resultant time series can also be subjected to statistical analysis, in order to investigate nineteenth century climate over Africa. Spatial detail is added to the data set by utilizing a unique methodology based on climatic teleconnections established from studies of rainfall variability over Africa. The historical information and station records have been combined into a file containing a regional anomaly value for up to 90 geographic regions and the years 1801–1900. Gaps necessarily remain in the matrix, but as early as the 1820s over 40 regions are represented. By the 1880s generally around 70 regions or more are represented.     

Three centuries of Slovakian drought dynamics

Tree-ring data from Slovakia are used to reconstruct decadal-scale fluctuations of the self-calibrated Palmer Drought Severity Index (scPDSI) over 1744–2006. The ring width chronology correlates at 0.58 (annual) and 0.88 (decadal) with regional-scale (48–50°N and 18–20°E) summer (June–August) scPDSI variations (1901–2002). Driest and wettest years common to the tree-ring and target data are 1947, 1948, 1964, and 1916, 1927, 1938, 1941, respectively. The model indicates decadal-scale drought ~1780–1810, 1850–1870, 1940–1960, and during the late twentieth century. The wettest period occurred ~1745–1775. Instrumental measurements and documentary evidence allow the reconstructed drought extremes to be verified and also provide additional insights on associated synoptic drivers and socioeconomic impacts. Comparison of anomalous dry conditions with European-scale fields of 500 hPa geopotential height retains positive pressure anomalies centered over Central Europe leading to atmospheric stability, subsidence and dry conditions. Negative mid-tropospheric geopotential height anomalies over Western Europe are connected with anomalous wet conditions over Slovakia. Nine existing, annually resolved hydro-climatic reconstructions from Central Europe, which were herein considered for comparison with the Slovakian findings, reveal significant high- to low-frequency coherency among the majority of records. Differences between the Slovakian and the other reconstructions are most evident at the end of the nineteenth century.     

‘Splendid rains have fallen’: links between El Niño and rainfall variability in the Kalahari, 1840–1900

This study uses a range of published and unpublished historical documentary sources to explore the nature of rainfall variability in the Kalahari Desert and adjacent hardveld regions of central southern Africa during the seventeen Pacific El Niño–Southern Oscillation (ENSO) episodes that occurred between 1840 and 1900. Documentary data are used in two ways. First, maps of relative annual rainfall levels are presented for each of the 12 single-year and five protracted ENSO episodes during the period, in order to identify the associated inter-annual rainfall variations. These suggest that the relationship between ENSO episodes and rainfall variability identified for the twentieth century, whereby warm events are frequently preceded by wetter conditions during the austral summer prior to the event year and succeeded by drought in the following summer, has broadly held for much of the last 160 years. This is despite the long-term fluctuations in precipitation and temperature which are known to have occurred over this period. Droughts are identified following at least thirteen of the 17 single-year and protracted ENSO episodes. Pre-ENSO wetter periods are less common, with only nine of the ENSO episodes preceded by above-normal rainfall. Second, the documentary data are analyzed in detail in order to reveal any evidence for high resolution intra-annual variations in the seasonal distribution of rainfall during ENSO events. Seasonal sequences of rainfall/drought appear to have closely followed contemporary patterns, with heavy rainfall commonly occurring late in the pre-ENSO year or early in the ENSO year(s), and drought at the start of the post-ENSO year. This relationship can be seen to hold most strongly for single-year ENSO warm events and for the first year of protracted events, but rainfall conditions were more variable during the later years of protracted events.     

Tree-ring reconstructed megadroughts over North America since a.d. 1300

Tree-ring reconstructed summer Palmer Drought Severity Indices (PDSI) are used to identify decadal droughts more severe and prolonged than any witnessed during the instrumental period. These “megadroughts” are identified at two spatial scales, the North American continental scale (exclusive of Alaska and boreal Canada) and at the sub-continental scale over western North America. Intense decadal droughts have had significant environmental and socioeconomic impacts, as is illustrated with historical information. Only one prolonged continent-wide megadrought during the past 500 years exceeded the decadal droughts witnessed during the instrumental period, but three megadroughts occurred over the western sector of North America from a.d. 1300 to 1900. The early 20th century pluvial appears to have been unmatched at either the continental or sub-continental scale during the past 500 to 700 years. The decadal droughts of the 20th century, and the reconstructed megadroughts during the six previous centuries, all covered large sectors of western North America and in some cases extended into the eastern United States. All of these persistent decadal droughts included shorter duration cells of regional drought (sub-decadal  ≈  6 years), most of which resemble the regional patterns of drought identified with monthly and annual data during the 20th century. These well-known regional drought patterns are also characterized by unique monthly precipitation climatologies. Intense sub-decadal drought shifted among these drought regions during the modern and reconstructed multi-year droughts, which prolonged large-scale drought and resulted in the regimes of megadrought.     

Tree-ring based hydroclimate reconstruction over northern Vietnam from Fokienia hodginsii: eighteenth century mega-drought and tropical Pacific influence

We present here the first statistically calibrated and verified tree-ring reconstruction of climate from continental Southeast Asia. The reconstructed variable is March–May (MAM) Palmer Drought Severity Index (PDSI) based on ring widths from 22 trees (42 radial cores) of rare and long-lived conifer, Fokienia hodginsii (Po Mu as locally called) from northern Vietnam. This is the first published tree ring chronology from Vietnam as well as the first for this species. Spanning 535 years, this is the longest cross-dated tree-ring series yet produced from continental Southeast Asia. Response analysis revealed that the annual growth of Fokienia at this site was mostly governed by soil moisture in the pre-monsoon season. The reconstruction passed the calibration-verification tests commonly used in dendroclimatology, and revealed two prominent periods of drought in the mid-eighteenth and late-nineteenth centuries. The former lasted nearly 30 years and was concurrent with a similar drought over northwestern Thailand inferred from teak rings, suggesting a “mega-drought” extending across Indochina in the eighteenth century. Both of our reconstructed droughts are consistent with the periods of warm sea surface temperature (SST) anomalies in the tropical Pacific. Spatial correlation analyses with global SST indicated that ENSO-like anomalies might play a role in modulating droughts over the region, with El Niño (warm) phases resulting in reduced rainfall. However, significant correlation was also seen with SST over the Indian Ocean and the north Pacific, suggesting that ENSO is not the only factor affecting the climate of the area. Spectral analyses revealed significant peaks in the range of 53.9–78.8 years as well as in the ENSO-variability range of 2.0 to 3.2 years.     

“A sky of brass and burning winds”: documentary evidence of rainfall variability in the Kingdom of Lesotho, Southern Africa, 1824–1900

Forecasting austral summer rainfall in southern Africa is hampered by a lack of long-term instrumental data. This paper extends the historical record for the subcontinent by presenting the first extensive 19th century climate history for Lesotho derived from documentary evidence. The data sources included unpublished English-, French- and Sesotho-language materials archived in Lesotho, South Africa and the UK. These included letters, journals and reports written by missionaries and colonial authorities, which were supplemented by newspapers, diaries, travelogues and other historical sources. Each source was read in chronological order, with any climate information recorded verbatim. Observations were classified into five categories (Very Wet, Relatively Wet, ‘Normal’, Relatively Dry, and Very Dry) based upon the predominant documented climate during each ‘rain-year’ (July to June). The latter portion of the chronology was then compared for accuracy against available instrumental precipitation records from Maseru (1886–1900). The results yield a semi-continuous record of climate information from 1824 to 1900. Data are restricted to lowland areas, but reveal drought episodes in 1833–34, 1841–42, 1845–47, 1848–51, 1858–63, 1865–69, 1876–80, 1882–85 and 1895–99 (the most severe drought years being 1850–51 and 1862–63) and wet periods or floods in 1835–36, 1838–41, 1847–48, 1854–56, 1863–65, 1873–75, 1880–81, 1885–86 and 1890–94. The rainfall chronology is compared with similar records for South Africa, Botswana and Zimbabwe. Linkages to possible forcing mechanisms, including ENSO teleconnections and historical coral-derived southwest Indian Ocean sea surface temperature variations are also explored.     

Reconstructions of spring/summer precipitation for the Eastern Mediterranean from tree-ring widths and its connection to large-scale atmospheric circulation

This study represents the first large-scale systematic dendroclimatic sampling focused on developing chronologies from different species in the eastern Mediterranean region. Six reconstructions were developed from chronologies ranging in length from 115 years to 600 years. The first reconstruction (1885–2000) was derived from principal components (PCs) of 36 combined chronologies. The remaining five, 1800–2000, 1700–2000, 1600–2000, 1500–2000 and 1400–2000 were developed from PCs of 32, 18, 14, 9, and 7 chronologies, respectively. Calibration and verification statistics for the period 1931–2000 show good levels of skill for all reconstructions. The longest period of consecutive dry years, defined as those with less than 90% of the mean of the observed May–August precipitation, was 5 years (1591–1595) and occurred only once during the last 600 years. The longest reconstructed wet period was 5 years (1601–1605 and 1751–1755). No long term trends were found in May–August precipitation during the last few centuries. Regression maps are used to identify the influence of large-scale atmospheric circulation on regional precipitation. In general, tree-ring indices are influenced by May–August precipitation, which is driven by anomalous below (above) normal pressure at all atmospheric levels and by convection (subsidence) and small pressure gradients at sea level. These atmospheric conditions also control the anomaly surface air temperature distribution which indicates below (above) normal values in the southern regions and warmer (cooler) conditions north of around 40°N. A compositing technique is used to extract information on large-scale climate signals from extreme wet and dry summers for the second half of the twentieth century and an independent reconstruction over the last 237 years. Similar main modes of atmospheric patterns and surface air temperature distribution related to extreme dry and wet summers were identified both for the most recent 50 years and the last 237 years. Except for the last few decades, running correlation analyses between the major European-scale circulation patterns and eastern Mediteranean spring/summer precipitation over the last 237 years are non-stationary and insignificant, suggesting that local and/or sub-regional geographic factors and processes are important influences on tree-ring variability over the last few centuries.