North Atlantic Forcing Research Articles

The 4.2 ka event: A review of palaeoclimate literature and directions for future research

In recent years, much evidence has been presented on the 4.2 ka event. A review of 317 palaeoclimate papers shows that dry conditions were common during the event, especially from Eastern Mediterranean to India. The 4.2 ka event was not, however, a global drought event. Wet conditions were reported especially for central/northern Europe and sub-Saharan Africa. The 4.2 ka event is typically characterized either as short (4.2–4.0 ka) or long (4.4–3.8 ka) episode, possibly developing over an extended interval of time, in keeping with the North Atlantic forcing and correlating with the Bond 3 event of ice-rafted debris. This forcing is understood to drive a southward migration of the Inter-Tropical Convergence Zone (ITCZ), resulting in decreased rainfall over most of the Asian monsoon region, with possibility that an interplay of El Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) has modulated the global circulation. Cold conditions were also reported but less frequently, in comparison to other Bond events such as the 8.2 ka event, Dark Ages Cold Period and Little Ice Age. Some high-resolution records show a double peak structure of which two anomalies are tree-ring dated to 4.14–4.05 ka and 3.97 ka. Accurately and precisely dated high-resolution records indicative of various climatic variables, especially outside of the traditional study region (Mediterranean–Middle East–India–China), including reconstructions of the ENSO and NAO histories and ITCZ migrations, are crucially needed for rigorous examination of the global scale characteristics of the 4.2 ka event and its forcings. Such research seems to be just beginning.

North Atlantic forcing of Indian Winter Monsoon intensification: Evidence from Holocene sediments from the tropical Indian Ocean Island of Sri Lanka

The teleconnection between the Asian monsoon system and North Atlantic forcing is an enduring prospect of the Earth’s climate. During the Holocene interstadial, the Indian summer monsoon showed asynchronous weakening links to ice rafting events documented in the North Atlantic region. However, the sensitivity of the Indian Winter Monsoon in response to North Atlantic cold spells is unclear due to a lack of compelling evidence. This study aims to extract the deglacial Indian Winter Monsoon signals using lithogenic tracers in coastal sediments and explore its association with the North Atlantic cooling episodes. A 5.1 m sediment core was retrieved from Pottuvil Lagoon in the southeastern coast of Sri Lanka, and the concentrations of K, Rb, Mg, Al, and Ti in 101 sub-sections were analysed using ICP-MS. The core- chronology was established by Bacon 2.2 age-depth modelling based on calibrated AMS 14C dates. The monsoon signal was reconstructed using element proxies and compared with the drift ice indices from the North Atlantic deep-sea sediments. Results revealed distinct phases of intense monsoon activity at 2553–2984 years BP, 3899–5021 years BP, and 5244–5507 years BP intervals with intermittent weak phases during 2253–2553, 2984–3899, and 5021–5244 years BP. The episodes of the intensified Indian Winter Monsoon coincided with Bond Events 2, 3, and 4, showing a strong coherence with the North Atlantic’s deglacial climate. Thus, on a millennial scale, North-Atlantic cooling has triggered intense winter monsoon conditions over the tropical Indian Ocean region from the mid to late Holocene. In comparison with regional monsoon archives, the Pottuvil winter monsoon record exhibits an anti-phase association with the Indian Summer Monsoon during Holocene ice-rafted debris events. The geochemical approach executed in this study could provide new insight into the millennial-scale pacing of the winter counterpart of the Indian monsoon links to climate extremes of high northern latitudes.

Stronger North Atlantic than Tropical Pacific Effects on North Pacific Decadal Prediction

Abstract Sea surface temperature (SST) variability in the North Pacific (NP) has broad impacts on climate, ecology, and fishing, but its decadal prediction remains challenging. Some studies have highlighted the importance of tropical Pacific forcing on NP SST variability, while others suggested a non-negligible role of North Atlantic forcing. The contribution of each region to NP SST decadal prediction needs to be carefully evaluated. Here, we apply linear inverse models (LIMs) to assess decadal prediction skill of observed NP SST (based on predictions averaged over 2–5 and 6–9 years in advance). By using model experiments that restore to observed SST variability, we quantify tropical Pacific and North Atlantic forcing, and find that the latter exhibits stronger contribution than the former to NP SST decadal prediction. Specifically, by removing the North Atlantic forcing (tropical Pacific forcing), the area ratio of the NP region with significant prediction skill reduces from 49% to 29% (43%) for years 2–5 and from 48% to 26% (44%) for years 6–9. Further analyses reveal the possible reason: NP SST decadal prediction is strongly modulated by two leading non-orthogonal patterns of the LIMs with long decay time (although higher-order patterns also play a role for years 2–5); the North Atlantic forcing largely and robustly contributes to the long decay time, while the tropical Pacific forcing plays a secondary role. Our study calls for more efforts on improving the simulation of the Atlantic trans-basin effect for better predicting NP climate.

North Atlantic forcing of autumn drought in Southwest China

Drought often lasts long and is thus closely related to slowly varying external forcing such as sea surface temperature (SST). Here, based on observed precipitation and SST data along with NCEP–DOE reanalysis data, the possible impacts of North Atlantic SST on drought formation in Southwest China are investigated. Results show that northeast–southwest-orientated dipole SST anomalies in the mid–high latitudes of the North Atlantic are closely related to autumn drought in Southwest China; the linear correlation coefficient between them reaches 0.48 during 1979–2020, significant at the 0.001 level. The dipole SST anomalies trigger southeastward-propagating Rossby waves and induce barotropic cyclonic circulation anomalies over India and the western Tibetan Plateau. This enhances the upward motion in northern India and the western Tibetan Plateau and causes a compensating downdraft, reduced precipitation, and consequent drought formation in Southwest China.摘要干旱持续时间长, 因此往往和缓慢变化的海温异常相关. 本文利用观测降水和NCEP-DOE再分析资料, 探讨了北大西洋中高纬地区东北-西南走向的偶极子海温异常对我国西南地区秋季干旱的影响. 结果显示该偶极子海温异常可激发向东南方向传播的Rossby波, 在印度北部和青藏高原西部引起正压气旋性环流和上升运动异常, 并在我国西南地区引起补偿性下沉运动, 导致降水减少, 引发干旱.

Exact dating of the Meghalayan lower boundary based on high-latitude tree-ring isotope chronology

Subdivision of the Holocene Series/Epoch into the Greenlandian, Northgrippian and Meghalayan Stages/Ages has recently been ratified based on stable isotope records from ice-core and speleothem archives. The base of the most recent chronostratigraphic unit, corresponding to the Northgrippian−Meghalayan boundary, coincides with the ‘4.2 ka event’ recognised as a low-latitude drought anomaly. The Global Stratotype Section and Point (GSSP) for the boundary, the oxygen isotope record from the Mawmluh Cave speleothem (India), demonstrates this event with markedly weakened Asian summer monsoon. Here we contribute to the assessments of the geological time scale and the global characteristics of this event by detailing an isotopic excursion in tree-ring carbon isotopes from high-latitude/subarctic Europe. The δ13C chronology demonstrates extremely overcast (wet) conditions, especially between 2190 and 2100 BCE, with anomalous conditions sustaining until 1990 BCE. In addition to demonstrating its exact dating and duration, the δ13C data also illustrate the two-stage nature of the event and highlight the greater magnitude of the earlier stage. This reinforces the characterisation of this ‘Meghalayan anomaly’ in the context of other proxy-sites around the world. The North Atlantic forcing, previously associated with weakened Asian summer monsoon, accords with the suggested roles of North Atlantic Oscillation and/or southwards shift of Inter-Tropical Convergence Zone in producing the hydroclimatic anomaly. In fact, it would be clarifying to separate the hydroclimatic ‘Meghalayan anomaly’ from the broadly cited and potentially longer lasting ‘4.2 ka event’, to analyse their respective forcing mechanisms. We conclude by dating the Meghalayan lower boundary to 2190 BCE.

Cold Water in a Warm World: Investigating the Origin of the Nordic Seas' Unique Surface Properties During MIS 11

Surface waters in the Nordic Seas were colder and fresher throughout the marine isotope stage (MIS) 11 interglacial compared to present day. This has been previously attributed to the continuous delivery of freshwater sourced from large ice structures characteristic of the preceding glacial interval, MIS 12. While it is conventionally believed that high-latitude surface freshening can trigger a reduction of the Atlantic Meridional Overturning Circulation (AMOC), multiple lines of evidence suggest a vigorous AMOC despite elevated freshwater forcing in the Nordic Seas. Here, we review and reanalyze evidence for sea surface properties throughout the Nordic Seas and North Atlantic. We find that surface waters in the Nordic Seas experienced an unusually variable inception of interglacial temperature conditions with multiple high-magnitude cold excursions. While cold events in the North Atlantic were frequently associated with in situ meltwater deposition as reconstructed by ice-rafted debris, this proxy was virtually uncorrelated with cold events in the Nordic Seas. Additionally, stable nitrogen analysis revealed variable levels of nutrient utilization in the Nordic Seas’ surface layer throughout MIS 11. This may reflect a dynamic structure of the upper ocean concomitant with an intermittent rate of freshwater delivery. Based on this combination of evidence, we suggest that the colder and fresher surface layer in the Nordic Seas was supplied from higher latitudes, rather than from locally-sourced iceberg meltwater as is characteristic of North Atlantic forcing. Pairing proxy-based evidence with recent numerical simulations further decouples surface freshening in the Nordic Seas and Greenland meltwater input, discrediting Greenland as a source of freshwater to this region during the later phase of MIS 11. Because the origin of freshwater has implications for its rate of delivery, our study might help to explain the active AMOC despite surface freshening during MIS 11 and should be recognized when considering this interglacial as an analogue for near-future climate change scenarios.

Reconstruction of late Holocene autumn/winter precipitation variability in SW Romania from a high-resolution speleothem trace element record

We present the first high-resolution trace element (Mg/Ca, Sr/Ca, Ba/Ca) record from a stalagmite in southwestern Romania covering the last 3.6 ka, which provides the potential for quantitative climate reconstruction. Precise age control is based on three independent dating methods, in particular for the last 250 yr, where chemical lamina counting is combined with the identification of the 20th century radiocarbon bomb peak and 230Th/U dating. Long-term cave monitoring and model simulations of drip water and speleothem elemental variability indicate that precipitation-related processes are the main drivers of speleothem Mg/Ca ratios. Calibration against instrumental climate data shows a significant anti-correlation of speleothem Mg/Ca ratios with autumn/winter (October to March) precipitation (r = −0.61, p < 0.01), which is statistically robust when considering age uncertainties and auto-correlation. This relationship is used to develop a quantitative reconstruction of autumn/winter precipitation.During the late Holocene, our data suggest a heterogeneous pattern of past regional winter hydroclimate in the Carpathian/Balkan realm, along with intermittent weakening of the dominant influence of North Atlantic forcing. In agreement with other regional paleo-hydrological reconstructions, the observed variability reveals periodically occurring strong NW–SE hydro-climate gradients. We hypothesize, that this pattern is caused by shifts of the eastern edge of the area of influence of the NAO across central-eastern Europe due to the confluence of North Atlantic forcing, and other climatic features such as the East Atlantic/Western Russia (EAWR) pattern.

Tropical Indo-Pacific hydroclimate response to North Atlantic forcing during the last deglaciation as recorded by a speleothem from Sumatra, Indonesia

Abrupt changes in Atlantic Meridional Overturning Circulation are known to have affected the strength of the Indian and Asian Monsoons during glacial and deglacial climate states. However, there is still much uncertainty around the hydroclimate response of the Indo-Pacific Warm Pool (IPWP) region to abrupt climate changes in the North Atlantic. Many studies suggest a mean southward shift in the intertropical convergence zone (ITCZ) in the IPWP region during phases of reduced Atlantic meridional overturning, however, existing proxies have seasonal biases and conflicting responses, making it difficult to determine the true extent of North Atlantic forcing in this climatically important region.Here we present a precisely-dated, high-resolution record of eastern Indian Ocean hydroclimate variability spanning the last 16 ky (thousand years) from δ18O measurements in an aragonite–calcite speleothem from central Sumatra. This represents the western-most speleothem record from the IPWP region. Precipitation arrives year-round at this site, with the majority sourced from the local tropical eastern Indian Ocean and two additional long-range seasonal sources associated with the boreal and austral summer monsoons. The Sumatran speleothem demonstrates a clear deglacial structure that includes 18O enrichment during the Younger Dryas and 18O depletion during the Bølling–Allerød, similar to the pattern seen in speleothems of the Asian and Indian monsoon realms. The speleothem δ18O changes at this site are best explained by changes in rainfall amount and changes in the contributions from different moisture pathways. Reduced rainfall in Sumatra during the Younger Dryas is most likely driven by reductions in moisture transport along the northern or southern monsoon transport pathways to Sumatra. Considered with other regional proxies, the record from Sumatra suggests the response of the IPWP to North Atlantic freshwater forcing is not solely driven by southward shifts of the ITCZ, but also a reduction in moisture transport along both monsoon pathways.

North Atlantic forcing of moisture delivery to Europe throughout the Holocene.

Century-to-millennial scale fluctuations in precipitation and temperature are an established feature of European Holocene climates. Changes in moisture delivery are driven by complex interactions between ocean moisture sources and atmospheric circulation modes, making it difficult to resolve the drivers behind millennial scale variability in European precipitation. Here, we present two overlapping decadal resolution speleothem oxygen isotope (δ18O) records from a cave on the Atlantic coastline of northern Iberia, covering the period 12.1–0 ka. Speleothem δ18O reveals nine quasi-cyclical events of relatively wet-to-dry climatic conditions during the Holocene. Dynamic Harmonic Regression modelling indicates that changes in precipitation occurred with a ~1500 year frequency during the late Holocene and at a shorter length during the early Holocene. The timing of these cycles coincides with changes in North Atlantic Ocean conditions, indicating a connectivity between ocean conditions and Holocene moisture delivery. Early Holocene climate is potentially dominated by freshwater outburst events, whilst ~1500 year cycles in the late Holocene are more likely driven by changes internal to the ocean system. This is the first continental record of its type that clearly demonstrates millennial scale connectivity between the pulse of the ocean and precipitation over Europe through the entirety of the Holocene.

Late Holocene precipitation and temperature changes in Northern Europe linked with North Atlantic forcing

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 66:37-48 (2015) - DOI: https://doi.org/10.3354/cr01331 Late Holocene precipitation and temperature changes in Northern Europe linked with North Atlantic forcing Tomi P. Luoto1,2,*, Liisa Nevalainen2 1Division of Geology and Geochemistry, Department of Geosciences and Geography, University of Helsinki, PO Box 64 FI-00014, Finland 2Division of Aquatic Sciences, Department of Biological and Environmental Science, University of Jyvaskyla, PO Box 35 FI-40014, Finland *Corresponding author: tomi.luoto@helsinki.fi ABSTRACT: Long-term paleoclimatic records are needed for understanding natural variability in the context of climate change. Two lake sediment cores were collected from eastern Finland from sites considered sensitive for precipitation and temperature, respectively, over the last ~3000 yr. The paleoclimatic reconstructions based on fossil Chironomidae (Insecta: Diptera) assemblages were considered against a North Atlantic Oscillation (NAO) reconstruction to disentangle possible forcing mechanisms. Generally low effective precipitation was reconstructed between ~3000 and 1500 cal yr BP (calibrated years before present). A distinct stream flow event occurred between ~2400 and 2200 cal yr BP, and the highest effective precipitation occurred ~1300 cal yr BP. The Medieval Climate Anomaly (MCA) between ~1200 and 800 cal yr BP was dry, but the Little Ice Age (LIA) between ~700 and 200 cal yr BP had fluctuating precipitation. The temperature reconstruction depicted a late Holocene cooling trend, which ended at ~1300 cal yr BP when the warm MCA began. The MCA was followed by the cold LIA and the last 100 yr have been characterized by rapid warming. Present temperatures are higher than during any time period of the late Holocene. A general trend of positive NAO mode with elevated summer temperatures and dry hydroclimatic conditions prevailed during the MCA and a negative mode with reduced temperatures during the LIA. However, the NAO-hydroclimate relationship over eastern Scandinavia was not straightforward during the LIA, which varied in moisture dynamics. The records from this present study provide a unique climate archive from the crossing point of southern and northern and eastern and western air masses. KEY WORDS: Chironomidae · Climate change · Eastern Finland · North Atlantic Oscillation · Paleoclimatology Full text in pdf format PreviousNextCite this article as: Luoto TP, Nevalainen L (2015) Late Holocene precipitation and temperature changes in Northern Europe linked with North Atlantic forcing. Clim Res 66:37-48. https://doi.org/10.3354/cr01331 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 66, No. 1. Online publication date: October 06, 2015 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2015 Inter-Research.

Interannual variability of the northwestern Iberia deep ocean: Response to large‐scale North Atlantic forcing

AbstractThe oceanic hydrography of the north‐easternmost region of the North Atlantic subtropical gyre has been monitored since 2003 by three sections extending between 100 and 200 nautical miles from the Spanish NW and N coast into the Atlantic and the Bay of Biscay. The sections were occupied twice a year from 2003 to 2010, annually after that, and measure the whole water column (&gt;5000 m). Correlation of series in the vertical and among sections, autocorrelation and estimates of the effect of the noise induced by the mesoscale field, all indicate that observed signatures are robust changes of water masses at the regional scale. The hydrographic time series are not characterized by smooth trends but instead by shifts that persist through consecutive cruises. The most notable features include a shift to more saline central waters around 2005 after which they remained stable, and a decrease in thermohaline properties of the Labrador Sea Water from autumn 2008 to 2010. Years with a strong winter North Atlantic Oscillation (NAO) index are characterized by shifts in thermohaline properties across most of the intermediate levels, with the most notable event being the warming and increasing salinity that followed the large NAO index drop of 2010. The observations are consistent with current understanding of the large‐scale functioning of the North Atlantic, which predicts a northeastward expansion of subtropical temperate waters in the eastern boundary as a response to NAO forcing. The observed variability is discussed in relationship to large‐scale circulation.

Quaternary climate change and Heinrich events in the southern Balkans: Lake Prespa diatom palaeolimnology from the last interglacial to present

Lake Prespa, in the Balkans, contains an important palaeo-archive in a key location for understanding Quaternary climate variability in the transition between Mediterranean and central European climate zones. Previous palaeoenvironmental research on sediment cores indicates that the lake is highly sensitive to climate change and that diatoms are likely to be strong palaeohydrological proxies. Here, we present new results from diatom analysis of a ca. 91 ka sequence, core Co1215, which spans the time from the end of the last interglacial to the present. Fluctuations in the diatom data were driven primarily by changes in lake level, as a function of shifts in moisture availability. Warmer interglacial (MIS 5, MIS 1) and interstadial (MIS 3) phases exhibit higher lake levels in spite of enhanced evaporative concentration, underlining the importance of changes in precipitation regimes over time. Low lake levels during glacial phases indicate extreme aridity, common to all Mediterranean lakes. Evidence for fluctuations in trophic status is linked, in part, to lake-level change, but also reflects nutrient enrichment from catchment processes. MIS 5a is characterized by the highest lake productivity in the sequence, but low lake levels, which are ascribed primarily to very low precipitation. On a suborbital timescale, the diatoms provide evidence for correlation to the millennial-scale variability recorded in the Greenland oxygen isotope records and clearly reflect the impact of the Heinrich H6, H5 and H3–1 ice-rafting events, suggesting the dominant influence of North Atlantic forcing in this region. Although the highest-amplitude shift in the diatom assemblages occurs during the time of H4 (40–38 ka), it may be superimposed upon the impact of the 39.28 cal ka BP Campanian Ignimbrite volcanic eruption. Diatoms from Lake Prespa core Co1215 display the first strong evidence for the impact of Italian volcanic activity on lacustrine biota in this region. Results emphasize the complexity of diatom response thresholds in different studies across the region. In the case of Lake Prespa, the results highlight the important role of precipitation for maintaining the hydrological balance of the lake, and indirectly, its biodiversity.

Atmosphere and Ocean Origins of North American Droughts*

Abstract The atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada. Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. Oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of internal atmospheric variability. Up to half of the soil moisture deficits during severe droughts in the southeast United States in 2000, Texas in 2011, and the central Great Plains in 2012 were related to SST forcing, although SST forcing was an insignificant factor for northern Great Plains drought in 1988. During the early twenty-first century, natural decadal swings in tropical Pacific and North Atlantic SSTs have contributed to a dry regime for the United States. Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts.

North Atlantic forcing of tropical Indian Ocean climate.

The response of the tropical climate in the Indian Ocean realm to abrupt climate change events in the North Atlantic Ocean is contentious. Repositioning of the intertropical convergence zone is thought to have been responsible for changes in tropical hydroclimate during North Atlantic cold spells, but the dearth of high-resolution records outside the monsoon realm in the Indian Ocean precludes a full understanding of this remote relationship and its underlying mechanisms. Here we show that slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials and the Younger Dryas stadial affected the tropical Indian Ocean hydroclimate through changes to the Hadley circulation including a southward shift in the rising branch (the intertropical convergence zone) and an overall weakening over the southern Indian Ocean. Our results are based on new, high-resolution sea surface temperature and seawater oxygen isotope records of well-dated sedimentary archives from the tropical eastern Indian Ocean for the past 45,000 years, combined with climate model simulations of Atlantic circulation slowdown under Marine Isotope Stages 2 and 3 boundary conditions. Similar conditions in the east and west of the basin rule out a zonal dipole structure as the dominant forcing of the tropical Indian Ocean hydroclimate of millennial-scale events. Results from our simulations and proxy data suggest dry conditions in the northern Indian Ocean realm and wet and warm conditions in the southern realm during North Atlantic cold spells.

Late Glacial to Holocene climate change and human impact in the Mediterranean: The last ca. 17 ka diatom record of Lake Prespa (Macedonia/Albania/Greece)

Lake Prespa (Macedonia/Albania/Greece) occupies an important location between Mediterranean and central European climate zones. Although previous multi-proxy research on the Late Glacial to Holocene sequence, core Co1215 (320cm; ca. 17cal ka BP to present), has demonstrated its great value as an archive of Quaternary palaeoclimate data, some uncertainty remains in the interpretation of climate change. With the exception of oxygen stable isotope data, previous palaeolimnological interpretation has relied largely on proxies for productivity. Here, existing interpretation is strengthened by the addition of diatom data. Results demonstrate that shifts in diatom assemblage composition are driven primarily by lake-level changes and thus permit more confident interpretation of shifts in moisture availability over time, while corroborating previous interpretation of catchment- and climate-induced productivity shifts. An inferred cold, arid shallow lake phase between ca. 17.1 and 15.7cal ka BP is not only followed by a high-productivity phase from ca. 15.7cal ka BP with Late Glacial warming, but also is the first evidence for a gradual increase in lake level, in line with other regional records. Clear evidence for a Younger Dryas climate reversal between ca. 13.1 and 12.3cal ka BP is followed by an unusually gradual transition to the Holocene and deeper, oligotrophic–mesotrophic lake conditions are reached by ca. 11.0cal ka BP. In contrast to the arid episode from ca. 10.0 to 8.0ka inferred from positive 18δOcalcite values, rapid diatom-inferred lake-level increase after the start of the Holocene suggests high moisture availability, in line with palynological evidence, but with only very subtle evidence for the impact of an 8.2ka cold event. The maintenance of high lake levels until 1.9cal ka BP, and the peak of inferred humidity from ca. 7.9 to 6.0cal ka BP, matches the oxygen stable isotope profile and confirms that the latter is driven primarily by evaporative concentration rather than reflecting regional shifts in precipitation sources over time. During the Late Holocene progressive eutrophication is inferred between 1.9 and present. Two shallow phases at ca. 1.0cal ka BP and at ca. 100years ago probably represent an aridity response which is added to increase human impact in the catchment. Overall, the study is important in confirming previous tentative inferences that Late Glacial to Holocene moisture availability has strong affinity with other sites in the Eastern Mediterranean. It also tracks the pattern of North Atlantic forcing.

North Atlantic forcing of millennial-scale Indo-Australian monsoon dynamics during the Last Glacial period

Recent studies of the Last Glacial period Indo-Australian summer monsoon (IASM) have revealed links to both northern and southern hemisphere high latitude climate as well as to regional ocean conditions. Particular interest has been paid to the monsoon response to Heinrich events, with variability explained by meridional shifts in positioning of the intertropical convergence zone (ITCZ), but this model has not been adequately tested. In addition, the shorter-lived Dansgaard/Oeschger (D/O) events have not been detected (beyond D/O-1, the Bølling/Allerød) in land-based records from the Indo-Pacific, despite their prominent expression in stalagmites from southern Asia, raising questions about the sensitivity of the IASM to these events. Here we present a Southern Hemisphere stalagmite oxygen isotopic time series from Ball Gown Cave (BGC), tropical northern Australia, located on the margins of the modern austral summer ITCZ, that spans 40–31 and 27–8 ka. Elevated IASM rainfall coincides with Heinrich stadials and the Younger Dryas, while decreased rainfall characterizes D/O interstadials, a response that is anti-phased with sites spanning the Indo-Pacific Warm Pool and with Chinese records of the East Asian summer monsoon. The BGC time series thus reveals a precipitation dipole consistent with a southward (northward) migration of the ITCZ during periods of high northern latitude cooling (warming) as the primary driver of millennial-scale IASM variability during the Last Glacial period. Our record indicates a strengthening of the IASM after the Younger Dryas period, likely as a result of rising sea level and sea surface temperatures, breaking the link with the high latitudes.

North Atlantic forcing of Amazonian precipitation during the last ice age

The last glacial period was marked by rapid reorganizations of oceanic and atmospheric circulation. Speleothem records from the Amazon Basin suggest that precipitation variability was linked to these events. The last glacial period was marked by multiple, abrupt reorganizations of ocean and atmosphere circulation1. On thousand-year timescales, slowing of the Atlantic meridional overturning circulation was associated with cooling in the high northern latitudes, whereas strengthened circulation was linked to northern warming1,2. In the tropics, these millennial-scale events were primarily reflected in altered patterns of precipitation3. These hydrologic fluctuations induced ecological changes in the Atlantic seaboard and the high Andes2, but less is known about the Amazon Basin. Here we reconstruct precipitation over Amazonian Ecuador over the past 94,000 years using a δ18O record from speleothems collected in Santiago Cave in western Amazonia. We interpret the variability of the δ18O record as changes in the source and amount of precipitation. With the exception of the period between 40,000 and 17,000 years ago, abrupt, high-frequency changes coincide with shifts in North Atlantic circulation, indicating a high-latitude influence on Amazonian precipitation over millennial timescales. On longer timescales, the record shows a relationship to precessional changes in the Earth’s orbit. In light of the lack of extreme aridity in our records, we conclude that ecosystems in western Amazonia have not experienced prolonged drying over the past 94,000 years.

Deglacial paleoclimate in the southwestern United States: an abrupt 18.6 ka cold event and evidence for a North Atlantic forcing of Termination I

We present a new U-series dated speleothem record (PC-1) from the Great Basin that documents deglacial climate variability between ca 20.1 and 15.6 ka. Our data show an abrupt 18.6 ka cold event preceding Heinrich event 1 that is consistent with expansion of the Laurentide Ice sheet during the ‘binge’ phase of ice growth. This event coincided with dessication of pluvial Lake Mojave suggesting cold and dry conditions in the southern Great Basin at this time. PC-1 δ 18O values before and during Heinrich event 1 are similar, but an increase in stalagmite growth rates suggests wetter conditions that coincided with deposition of spring deposits in southern Nevada. The time interval of our record is consistent with the timing of pluvial conditions in the Great Basin as evident from a comparison to regional wetness proxies. Our new speleothem record, recovered from the recharge area for Devils Hole, does not show a δ 18O increase coincident with the abrupt increase in Devils Hole δ 18O at c. 18 ka, challenging the view that the Great Basin experienced an early Termination I. This hypothesis is supported by two other southwest speleothem records that demonstrate deglaciation was synchronous with forcing from the North Atlantic Ocean. We suggest that Devils Hole speleothem δ 18O values may partly reflect source water changes in the regional aquifer. Further, Devils Hole δ 13C minima coincide with peak global glacial conditions and weak Asian monsoon periods, suggesting that they constrain better the timing of pluvial conditions in the Great Basin.

Winter daily precipitation variability over Cumbria, Northwest England

Daily precipitation totals at 55 sites were used to investigate geographic variability in winter (DJF) rainfall over Cumbria, NW England, over an 11-year period. Winter is the wettest season (>800 mm in the mountainous Lake District), with rainfall mechanisms closely linked to North Atlantic forcing. The Lamb weather type catalogue was used to identify rainfall distributions under different wind directions. Precipitation magnitude over Cumbria is much more sensitive to a change in wind direction than the geographic pattern in rainfall, with southwesterly (easterly) winds producing the highest (lowest) spatially averaged daily rainfall totals of 8.2 mm (0.6 mm). S-mode principal components analysis was used to identify the main patterns of precipitation variability. Three principal components (PCs) were retained as being statistically significant (cumulative explained variance for unrotated PCs = 84.3%), with a correlated PC structure (direct oblimin rotation) best describing the spatial variance in rainfall. PC 1 has a very high index of strength (variance measure = 40.9), indicating that there is one dominant rainfall pattern. PC 1 shows a gradient between wetter conditions in southwest Cumbria and over the central Lake District and drier conditions in NE Cumbria, and is usually caused by active zonal west to southwest flows. Almost of equal importance to PC 1 is PC 3 (variance measure = 39.3), which has a more uniform rainfall distribution than PC 1 and is usually caused by fronts stalling over the region. PC 2, which shows an east to west decline in rainfall totals, is much less important than PCs 1 and 3 (variance measure = 18.6). PC 2’s rainfall pattern can be caused by easterly flows with high pressure over Scandinavia and low pressure over the Continent, or by strong southwesterly flows, with depressions often centred over Scotland. Finally, cluster analysis was carried out to identify precipitation regions for all days and for each wind direction. Clusters were found to be largely stable to changes in wind direction, with stations in the central Lake District often clustered together, thus highlighting the importance of orographic enhancement of rainfall in this region.

Influence of the North Atlantic on simulated atmospheric variability

An atmospheric general circulation model is used to investigate the influence of the North Atlantic Ocean on atmospheric variability. The study covers the period from 1950 to 1994. The observed sea surface temperature and sea ice extension are used to force the atmospheric model. Several configurations of the oceanic boundary conditions were made to isolate the role of the North Atlantic and to study its non-linear interaction with forcings from other oceanic basins. The multi-realization character of the experiments distinguishes between the internal random part and the external forced part of the total variability. The potential predictability can thus be evaluated. The response of the atmosphere is also studied with a modal approach in terms of hemispheric teleconnection patterns. The North Atlantic Ocean has a direct influence on both the Northern Hemisphere annular mode and the Pacific-North-America pattern, leading to a weak predictability. However the direct response is largely modulated by forcings from other oceanic basins. The non-linearity of the system compensates the predictable component of the annular mode induced by the North Atlantic forcing. Furthermore it reduces the forced component of the Pacific-North-America pattern, increasing its chaoticity.