North Atlantic Thermohaline Circulation Research Articles

Optimal nonlinear excitation of decadal variability of the North Atlantic thermohaline circulation

Nonlinear development of salinity perturbations in the Atlantic thermohaline circulation (THC) is investigated with a three-dimensional ocean circulation model, using the conditional nonlinear optimal perturbation method. The results show two types of optimal initial perturbations of sea surface salinity, one associated with freshwater and the other with salinity. Both types of perturbations excite decadal variability of the THC. Under the same amplitude of initial perturbation, the decadal variation induced by the freshwater perturbation is much stronger than that by the salinity perturbation, suggesting that the THC is more sensitive to freshwater than salinity perturbation. As the amplitude of initial perturbation increases, the decadal variations become stronger for both perturbations. For salinity perturbations, recovery time of the THC to return to steady state gradually saturates with increasing amplitude, whereas this recovery time increases remarkably for freshwater perturbations. A nonlinear (advective) feedback between density and velocity anomalies is proposed to explain these characteristics of decadal variability excitation. The results are consistent with previous ones from simple box models, and highlight the importance of nonlinear feedback in decadal THC variability.

The response of the large‐scale ocean circulation to 20th century Asian and non‐Asian aerosols

Multidecadal trends in the large‐scale ocean circulation are influenced by changes in radiative forcings such as long‐lived greenhouse gases, volcanic aerosols, and solar irradiance. Model simulations suggest that anthropogenic aerosols can also force circulation changes, including delaying the weakening of the North Atlantic thermohaline circulation, altering the interhemispheric sea surface temperature gradient, and inducing a pan‐oceanic heat redistribution. The extent to which aerosols from different regions contribute to these oceanic changes is currently unknown. Using specifically designed 20th century coupled climate model experiments that separate Asian and non‐Asian aerosol impacts, it is shown that the non‐Asian aerosol component, predominantly sulfate aerosols, accounts for much of the simulated aerosol‐induced oceanic changes. These include delaying the weakening of the global meridional circulation, increasing the northward heat transport across the equatorial Atlantic, and inducing a subsurface cooling in the subtropical southern Indian Ocean. As global sulfate aerosol levels peaked in the 1980s, these trends may be starting to reverse. This study highlights the importance of Northern Hemisphere non‐Asian anthropogenic aerosols in driving remote changes in Southern Hemisphere subtropical and extratropical oceans.

Simultaneous model spin-up and parameter identification with the one-shot method in a climate model example

We investigate the Oneshot Optimization strategy introduced by Hamdi and Griewank for the applicability and efficiency to identify parameters in models of the earth's climate system. Parameters of a box model of the North Atlantic Thermohaline Circulation are optimized with respect to the fit of model output to data given by another model of intermediate complexity. Since the model is run into a steady state by a pseudo time-stepping, efficient techniques are necessary to avoid extensive recomputations or storing when using gradient-based local optimization algorithms. The Oneshot approach simultaneously updates state, adjoint and parameter values. For the required partial derivatives, the algorithmic/automatic differentiation tool TAF was used. Numerical results are compared to results obtained by the BFGS-quasi-Newton method.

A revised Early Miocene age for the instigation of the Eirik Drift, offshore southern Greenland: Evidence from high-resolution seismic reflection data

The Eirik Drift lies on the continental slope south of Greenland, where it has been formed under the influence of Northern Component Water (NCW). NCW flow is an essential part of the global Thermohaline Circulation (THC), which is closely connected to the world's climate. Changes in pathways and intensity of NCW flow bear information about modifications of the North Atlantic THC in a changing climate. There is some disagreement about when deep-current controlled sedimentation at the Eirik Drift started. While the onset of drift building was previously dated as early Pliocene or late Miocene in age we suggest that the effect of large-scale current deposition had been initiated by at least 19–17 Ma based on the seismostratigraphic analysis of sedimentary structures identified in a set of high-resolution seismic reflection data. This assumption of an early Miocene onset of NCW flow is supported by regional evidence regarding the breaching of the Greenland–Scotland Ridge, which is documented in several erosional unconformities within the North Atlantic. After the onset of deep-current controlled sedimentation at the Eirik Drift, two major changes in the deep-current system are revealed during the Miocene: At the mid- to late Miocene boundary (12–10 Ma) and at 7.5 Ma.

A new method of obtaining high-resolution paleoclimate records from speleothem fluid inclusions

The Greenland ice sheet is accepted as a key factor controlling the Quaternary “glacial scenario“. However, the origin and mechanisms of major Arctic glaciation starting at 3.15 Ma and culminating at 2.74 Ma are still controversial. For this phase of intense cooling Ravelo et al. [1] [A.C. Ravelo, D.H. Andreasen, M. Lyle, A.O. Lyle, M.W. Wara, Regional climate shifts caused by gradual global cooling in the Pliocene epoch. Nature 429 (2004) 263–267.] proposed a complex gradual forcing mechanism. In contrast, our new submillennial-scale paleoceanographic records from the Pliocene North Atlantic suggest a far more precise timing and forcing for the initiation of northern hemisphere glaciation (NHG), since it was linked to a 2–3 °C surface water warming during warm stages from 2.95 to 2.82 Ma. These records support previous models [G.H. Haug, R. Tiedemann, Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation, Nature 393 (1998) 673–676. [2]] claiming that the final closure of the Panama Isthmus (3.0– ∼2.5 Ma [J. Groeneveld S. Steph, R. Tiedemann, D. Nürnberg, D. Garbe-Schönberg, The final closure of the Central American Seaway, Geology, in prep. [3]]) induced an increased poleward salt and heat transport. Associated strengthening of North Atlantic Thermohaline Circulation and in turn, an intensified moisture supply to northern high latitudes resulted in the build-up of NHG, finally culminating in the great, irreversible “climate crash” at marine isotope stage G6 (2.74 Ma). In summary, there was a two-step threshold mechanism that marked the onset of NHG with glacial-to-interglacial cycles quasi-persistent until today.

An east‐west SST anomaly pattern in the midlatitude North Atlantic Ocean associated with winter precipitation variability over eastern China

Winter precipitation tends to vary consistently over most regions of eastern China. The present study reveals that the variation in winter precipitation over eastern China is positively correlated to an east‐west sea surface temperature (SST) anomaly pattern in the midlatitudes and high latitudes of the North Atlantic Ocean. When the SST is higher (lower) in the eastern North Atlantic Ocean and lower (higher) in the western North Atlantic Ocean, winter precipitation over eastern China tends to be above (below) normal. The east‐west SST anomaly pattern in the North Atlantic modifies the atmospheric circulation over the Atlantic Ocean and Europe, and the anomalous circulation extends to downstream East Asia through the propagation of Rossby waves and exerts influence on winter precipitation over eastern China. The formation of the east‐west SST anomaly pattern in the North Atlantic may be associated with anomalies in the North Atlantic thermohaline circulation and in surface winds over the North Atlantic Ocean.

Rapid vegetation response to Lateglacial and early Holocene climatic fluctuation in the South Carpathian Mountains (Romania)

High-resolution pollen, conifer stomata and plant macrofossil analyses of two glacial lake sediments (1740 and 1990 m a.s.l.) are used to reconstruct Lateglacial (LG) and early Holocene (EH) vegetation and tree line changes in the Retezat Mountains. Our results show that during the LG, tree line was between 1750 and 1800 m a.s.l. formed by Larix decidua, Pinus mugo and Picea abies. Early LG spread to high altitudes suggests refugia of these tree species in the mountain. The Younger Dryas cooling resulted in regional steppe-tundra expansion, but tree line position and composition showed little change. The abundance of trees and shrubs decreased at 1740 m a.s.l., but species richness increased with the arrival of Pinus cembra. Our data support climate-model hindcasts for only modest decrease in accumulated growing season heat at mid-high altitudes. Regionally the pollen records suggest enhanced aridity and seasonality. In the EH, tree line reached 2000 m a.s.l. (higher than today) by ∼11,100 cal yr BP. P. mugo, P. cembra, P. abies established around the upper lake suggesting rapid increase in summer temperatures. The EH maximum of L. decidua between 11,200–10,600 cal yr BP was connected to high summer insolation. High altitude expansion of Abies alba between 10,600–10,300 cal yr BP suggested summer mean temperatures ∼2.8 °C higher then today. In comparison with other mountain sites in Europe, LG interstadial tree line was at similar altitude in the S Alps and ∼350 m higher in the Pirin Mountains. LG tree line fluctuation had similar low amplitude in the SE Alps, Retezat and Pirin Mts suggesting relatively weak influence of the North Atlantic thermohaline circulation changes on growing season temperatures.

An astronomically tuned 8.1 Ma eolian record from the Chinese Loess Plateau and its implication on the evolution of Asian monsoon

The Chinese loess-paleosol sequences have led to a great understanding of the evolution of the East Asian monsoon (EAM) in the Quaternary. However, evolution of the EAM in the late Miocene and Pliocene is quite blurry, hindering our understanding of the mechanisms driving the EAM. Thus, we obtained high-resolution grain size records of 8.1 Ma from the thickest eolian Red Clay sequence at Chaona on the Chinese central Loess Plateau, and astronomically tuned the grain size records to the orbital variation with a time resolution of 1.5 ka to 5.8 ka. The variability of coarse size fraction, a recognized proxy for the East Asian winter monsoon (EAWM), indicates a persistent but stepwise intensification of the EAWM at ca. 5.5 Ma, 3.6 Ma, 2.6 Ma and 1 Ma. Filtering of this proxy record at 100, 41, and 21 ka bands was conducted to trace the evolution of the EAWM in a frequency domain over 8.1 Ma. Temporal variations of these three cycles show a remarkable simultaneous enhancement at ca. 5.5 Ma, indicating a synchronous intensification of high and low latitude signals in the EAWM record. We suggest that the closure of Panama Seaway to form the North Atlantic thermohaline circulation and the high latitude cooling during this time might collectively trigger such a response. Copyright 2011 by the American Geophysical Union.

Representing two centuries of past and future climate for assessing risks to biodiversity in Europe

ABSTRACTAim Environmental changes may have important implications for biodiversity in Europe. This study aimed to provide quantification of changes in regional climate during the 20th century and provide scenarios of future changes in CO2 concentration and climate for the 21st century as an input to models used to study risks to biodiversity in the EU‐funded ALARM project.Location Europe.Methods Monthly time series of eight climate variables (maximum, mean and minimum temperature, precipitation, cloudiness, humidity, growing degree days and evapotranspiration) interpolated to a regular 10′ grid, and of global atmospheric CO2 concentration were constructed for the period 1901–2100. These combine observations from the 20th century with model projections for the 21st century. Projections were selected to capture a range of both climate model and SRES (Special Report on Emissions Scenarios) emissions uncertainties.Results Averaged over Europe, the scenarios describe changes in mean annual temperature by the end of the 21st century relative to 1961–90 that range between 3.0 and 6.1 °C. Annual precipitation is projected to increase in northern Europe (+5 to +13%) and decrease in southern Europe (−17 to −7%). An additional scenario describing the collapse of the North Atlantic thermohaline circulation exhibits a sharp cooling over much of Europe and a drying in most seasons.Main conclusions The climate scenarios presented are consistent with socio‐economic storylines developed in parallel, and include a high‐emissions case, CO2 stabilization and a high‐impact ‘surprise’ scenario, involving the collapse of the North Atlantic thermohaline circulation. They embrace a number of key climate and related variables required for studying the impacts of climate change, offer continental coverage and capture a range of uncertainties in future European climate that are also consistent with the most recent climate projections assessed by the Intergovernmental Panel on Climate Change. The scenarios have already been applied in numerous studies of the impacts of climate change on biodiversity, some reported in this special issue.

Abrupt Holocene climate change and potential response to solar forcing in western Canada

Several abrupt climate events during the Holocene, including the widely documented oscillation at 8.2 thousand years before present (ka), are attributed to changes in the North Atlantic thermohaline circulation. Additional mechanisms, such as interactions between atmospheric circulation, ice-sheet dynamics, and the influence of solar irradiance, also have been proposed to explain abrupt climatic events, but evidence remains elusive. This study presents evidence from multi-proxy analyses on the Holocene sediments of Eleanor Lake, interior British Columbia. Climatic inferences from our decadal-resolution record of biogenic silica (BSi) abundance are supported by changes in diatom and pollen assemblages from the same core and correlations with existing regional climate records. The BSi record reveals abrupt and persistent climatic shifts at 10.2, 9.3, and 8.5 ka, the latter two of which are coeval with major collapses of the Laurentide Ice Sheet. The record also reveals a short-term cooling at 8.2 ka that is distinct from the 8.5 ka event and similar in magnitude to several other late-Holocene coolings. BSi is correlated with solar-irradiance indices ( r = 0.43–0.61), but the correlation is opposite in sign to that expected from direct solar forcing and weakens after 8 ka. Possible mechanisms causing the abrupt and persistent climate changes of the early Holocene include 1) sudden losses of ice and proglacial lake extent, causing a shift in the meridional structure of atmospheric circulation, 2) a possible link between solar minima and El Niño-like conditions that are correlated with warm spring temperature in interior British Columbia, and 3) the influence of solar irradiance variability on the position of the polar jet, possibly via effects on the strength of the glacial anticyclone.

Tropical Pacific influences on the North Atlantic Thermohaline Circulation

Most global climate models simulate a weakening of the North Atlantic thermohaline circulation (THC) in response to enhanced greenhouse warming. Both surface warming and freshening in high latitudes, the so-called sinking region, contribute to the weakening of the THC. Some models simulate even a complete breakdown of the THC at sufficiently strong forcing. Here results from a state-of-the-art global climate model are presented that does not simulate a weakening of the THC in response to greenhouse warming. Large-scale air-sea interactions in the tropics, similar to those operating during present-day El Ninos, lead to anomalously high salinities in the tropical Atlantic. These are advected into the sinking region, thereby increasing the surface density and compensating the effects of the local warming and freshening. The results of the model study are corroborated by the analysis of observations.

North Atlantic Current variability through marine isotope stage M2 (circa 3.3 Ma) during the mid‐Pliocene

The mid‐Pliocene was an episode of prolonged global warmth and strong North Atlantic thermohaline circulation, interrupted briefly at circa 3.30 Ma by a global cooling event corresponding to marine isotope stage (MIS) M2. Paleoceanographic changes in the eastern North Atlantic have been reconstructed between circa 3.35 and 3.24 Ma at Deep Sea Drilling Project Site 610 and Integrated Ocean Drilling Program Site 1308. Mg/Ca ratios andδ18O fromGlobigerina bulloidesare used to reconstruct the temperature and relative salinity of surface waters, and dinoflagellate cyst assemblages are used to assess variability in the North Atlantic Current (NAC). Our sea surface temperature data indicate warm waters at both sites before and after MIS M2 but a cooling of ∼2–3°C during MIS M2. A dinoflagellate cyst assemblage overturn marked by a decline inOperculodinium centrocarpumreflects a southward shift or slowdown of the NAC between circa 3.330 and 3.283 Ma, reducing northward heat transport 23–35 ka before the global ice volume maximum of MIS M2. This will have established conditions that ultimately allowed the Greenland ice sheet to expand, leading to the global cooling event at MIS M2. Comparison with an ice‐rafted debris record excludes fresh water input via icebergs in the northeast Atlantic as a cause of NAC decline. The mechanism causing the temporary disruption of the NAC may be related to a brief reopening of the Panamanian Gateway at about this time.

Massive perturbation in terrestrial ecosystems of the Eastern Mediterranean region associated with the 8.2 kyr B.P. climatic event

The climatic perturbation at ca. 8.2 kyr B.P. is the strongest short-term climate anomaly within the Holocene. It is generally attributed to a meltwater-induced slowdown of the thermohaline circulation in the North Atlantic. Model simulations and available proxy data suggest that it was strongest in the high to middle latitudes around the North Atlantic. Based on new pollen data from Tenaghi Philippon, northeastern Greece, we provide evidence for a massive climate-induced turnover in terrestrial ecosystems of the Aegean region associated with the 8.2 kyr B.P. event. The reconstructed winter temperature decline of >4 °C is much stronger than suggested by model simulations and proxy data from more northern latitudes of Europe, although the latter provide a direct downstream response to a North Atlantic thermohaline circulation slowdown. We attribute this discrepancy to mesoclimatic effects; a stronger influence of the Siberian High during the 8.2 kyr B.P. event may have enhanced the katabatic air flow from the mountains bordering the study site via a larger, longer persisting snow cover. Our data demonstrate that high-amplitude temperature anomalies and increased seasonality connected to the 8.2 kyr B.P. event may also have occurred in the lower mid-latitudes, much farther south than previously thought. The magnitudes of these anomalies appear to have been strong enough to have seriously affected Neolithic settlers in the northeastern Mediterranean region.

The sensitivity of the climate response to the magnitude and location of freshwater forcing: last glacial maximum experiments

Proxy records indicate that the locations and magnitudes of freshwater forcing to the Atlantic Ocean basin as iceberg discharges into the high-latitude North Atlantic, Laurentide meltwater input to the Gulf of Mexico, or meltwater diversion to the North Atlantic via the St. Lawrence River and other eastern outlets may have influenced the North Atlantic thermohaline circulation and global climate. We have performed Last Glacial Maximum (LGM) simulations with the NCAR Community Climate System Model (CCSM3) in which the magnitude of the freshwater forcing has been varied from 0.1 to 1 Sv and inserted either into the subpolar North Atlantic Ocean or the Gulf of Mexico. In these glacial freshening experiments, the less dense freshwater provides a lid on the ocean water below, suppressing ocean convection and interaction with the atmosphere above and reducing the Atlantic Meridional Overturning Circulation (AMOC). This is the case whether the freshwater is added directly to the area of convection south of Greenland or transported there by the subtropical and subpolar gyres when added to the Gulf of Mexico. The AMOC reduction is less for the smaller freshwater forcings, but is not linear with the size of the freshwater perturbation. The recovery of the AMOC from a “slow” state is ∼200 years for the 0.1 Sv experiment and ∼500 years for the 1 Sv experiment. For glacial climates, with large Northern Hemisphere ice sheets and reduced greenhouse gases, the cold subpolar North Atlantic is primed to respond rapidly and dramatically to freshwater that is either directly dumped into this region or after being advected from the Gulf of Mexico. Greenland temperatures cool by 6–8 °C in all the experiments, with little sensitivity to the magnitude, location or duration of the freshwater forcing, but exhibiting large seasonality. Sea ice is important for explaining the responses. The Northern Hemisphere high latitudes are slow to recover. Antarctica and the Southern Ocean show a bipolar response, with warming and reduced sea ice. This warming continues after the cessation of the freshwater forcing and shows a dependence on the duration of the freshwater forcing. Equatorward of the expanded sea ice, the simulated temperature and salinity anomalies are sensitive to the amount of colder and fresher waters that are advected out of the subpolar North Atlantic. In the tropical Atlantic, the recovery of the Intertropical Convergence Zone (ITCZ) from its more southerly position during the freshwater forcing is much more rapid than the recovery of the AMOC, and is more related to the recovery of low-latitude surface temperatures than Greenland temperature or sea ice. These results have implications for using proxy records as indirect measures of the AMOC.

The 1500-year climate oscillation in the midlatitude North Pacific during the Holocene

Suborbital climate variability during the last glacial period is suggested to have involved a 1500-year pacing cycle, but the expression and spatial distribution of the ~1500-year oscillation during interglacials remains unclear. We generated a multidecade resolution record of alkenone sea surface temperature (SST) in the northwestern Pacific off central Japan during the Holocene. The SST record showed centennial and millennial variability with an amplitude of ~1 °C throughout the entire Holocene. Spectral analysis for SST variation revealed a statistically significant peak with 1470-year periodicity. The SST variation partly correlated with the variations of ice-rafted hematite-stained grain content in North Atlantic sediments. These findings indicate that the mean latitude of the Kuroshio Extension has varied on a 1500-year cycle, and suggest that a climatic link exists between the North Pacific gyre system and the high-latitude North Atlantic thermohaline circulation. The regular pacing at 1500-year intervals seen throughout both the Holocene and the last glacial period suggests that the oscillation was a response to external forcing.

The 8.2 ka event: Evidence for seasonal differences and the rate of climate change in western Europe

Recent studies have drawn attention to differences in the seasonal impact of the 8.2 ka event, with longer cooler summers and shorter cooler/drier winters. However, there are no data available on the simultaneity or the rate of onset of the seasonal changes in Europe. Based on the microfacies and geochemical analyses of seasonally laminated varved sediments from Holzmaar, we present evidence of differences in duration and onset time of changes in summer temperature and winter rainfall during the 8.2 ka event. Since both summer and winter climate signals are co-registered within a single varve, there can be no ambiguity about the phasing and duration of the signals. Our data show that the onset and withdrawal of the 8.2 ka summer cooling occurred within a year, and that summer rains were reduced or absent during the investigated period. The onset of cooler summers preceded the onset of winter dryness by ca. 28 years. In view of the differences in nature and duration of the impact of the 8.2 ka event we suggest that a clearer definition of the 8.2 ka event (summer cooling or winter cooling/dryness) needs to be developed. Based on regional comparison and available modelling studies we also discuss the roles of solar variability, changes in North Atlantic Thermohaline circulation, and North Atlantic Circulation (NAO) during the period under consideration. Wavelet analyses of seasonal laminae indicates that the longer NAO cycles, linked to changes in the N. Atlantic temperatures, were more frequent during the drier periods.

Climatic implications of correlated Upper Pleistocene glacial and fluvial deposits on the Cinca and Gállego Rivers (NE Spain) based on OSL dating and soil stratigraphy

We correlate Upper Pleistocene glacial and fluvial deposits of the Cinca and Gállego River valleys (south central Pyrenees and Ebro basin, Spain) using geomorphic position, luminescence dates, and time-related trends in soil development. The ages obtained from glacial deposits indicate glacial periods at 85 ± 5 ka, 64 ± 11 ka, and 36 ± 3 ka (from glacial till) and 20 ± 3 ka (from loess). The fluvial drainage system, fed by glaciers in the headwaters, developed extensive terrace systems in the Cinca River valley at 178 ± 21 ka, 97 ± 16 ka, 61 ± 4 ka, 47 ± 4 ka, and 11 ± 1 ka, and in the Gállego River valley at 151 ± 11 ka, 68 ± 7 ka, and 45 ± 3 ka. The times of maximum geomorphic activity related to cold phases coincide with Late Pleistocene marine isotope stages and Heinrich events. The maximum extent of glaciers during the last glacial occurred at 64 ± 11 ka, and the terraces correlated with this glacial phase are the most extensive in both the Cinca (61 ± 4 ka) and Gállego (68 ± 7 ka) valleys, indicating a strong increase in fluvial discharge and availability of sediments related to the transition to deglaciation. The global Last Glacial Maximum is scarcely represented in the south central Pyrenees owing to dominantly dry conditions at that time. Precipitation must be controlled by the position of the Iberian Peninsula with respect to the North Atlantic atmospheric circulation system. The glacial systems and the associated fluvial dynamic seem sensitive to 1) global climate changes controlled by insolation, 2) North Atlantic thermohaline circulation influenced by freshwater pulses into the North Atlantic, and 3) anomalies in atmospheric circulation in the North Atlantic controlling precipitation on the Iberian Peninsula. Our scenario of glacial and fluvial evolution during the Late Pleistocene in northern Spain could be extrapolated to other glaciated mountainous areas in southern Europe.

Flow‐weighted values of runoff tracers (δ18O, DOC, Ba, alkalinity) from the six largest Arctic rivers

We present new flow‐weighted data for δ18OH2O, dissolved organic carbon (DOC), dissolved barium and total alkalinity from the six largest Arctic rivers: the Ob', Yenisey, Lena, Kolyma, Yukon and Mackenzie. These data, which can be used to trace runoff, are based upon coordinated collections between 2003 and 2006 that were temporally distributed to capture linked seasonal dynamics of river flow and tracer values. Individual samples indicate significant variation in the contributions each river makes to the Arctic Ocean. Use of these new flow‐weighted estimates should reduce uncertainties in the analysis of freshwater transport and fate in the upper Arctic Ocean, including the links to North Atlantic thermohaline circulation, as well as regional water mass analysis. Additional improvements should also be possible for assessing the mineralization rate of the globally significant flux of terrigenous DOC contributed to the Arctic Ocean by these major rivers.

Environmental response to the cold climate event 8200 years ago as recorded at Højby Sø, Denmark

The need for accurate predictions of future environmental change under conditions of global warming has led to a great interest in the most pronounced climate change known from the Holocene: an abrupt cooling event around 8200 years before present (present = A.D. 1950), also known as the ‘8.2 ka cooling event’ (ka = kilo-annum = 1000 years). This event has been recorded as a negative δ18O excursion in the central Greenland ice cores (lasting 160 years with the lowest temperature at 8150 B.P.; Johnsen et al. 1992; Dansgaard 1993; Alley et al. 1997; Thomas et al. 2007) and in a variety of other palaeoclimatic archives including lake sediments, ocean cores, speleothems, tree rings, and glacier oscillations from most of the Northern Hemisphere (e.g. Alley & Ágústsdóttir 2005; Rohling & Pälike 2005). In Greenland the maximum cooling was estimated to be 6 ± 2°C (Alley et al. 1997) while in southern Fennoscandia and the Baltic countries pollenbased quantitative temperature reconstructions indicate a maximum annual mean temperature decrease of around 1.5°C (e.g. Seppä et al. 2007). Today there is a general consensus that the primary cause of the cooling event was the final collapse of the Laurentide ice sheet near Hudson Bay and the associated sudden drainage of the proglacial Lake Agassiz into the North Atlantic Ocean around 8400 B.P. (Fig. 1; Barber et al. 1999; Kleiven et al. 2008). This freshwater outflow, estimated to amount to c. 164,000 km3 of water, reduced the strength of the North Atlantic thermohaline circulation and thereby the heat transported to the North Atlantic region, resulting in an atmospheric cooling (Barber et al. 1999; Clark et al. 2001; Teller et al. 2002). The climatic consequences of this meltwater flood are assumed to be a good geological analogue for future climate-change scenarios, as a freshening of the North Atlantic is projected by almost all global-warming models (e.g. Wood et al. 2003; IPCC 2007) and is also currently being registered in the region (Curry et al. 2003). In an ongoing project, the influence of the 8.2 ka cooling event on a Danish terrestrial and lake ecosystem is being investigated using a variety of biological and geochemical proxy data from a sediment core extracted from Højby Sø, north-west Sjælland (Fig. 2). Here we present data on changes in lake hydrology and terrestrial vegetation in response to climate change, inferred from macrofossil data and pollen analysis, respectively.

Oceanic link between abrupt changes in the North Atlantic Ocean and the African monsoon

Abrupt changes in the African Monsoon, which have been recorded throughout the late Pleistocene and early Holocene epochs, tend to coincide with changes in North Atlantic thermohaline circulation. A numerical simulation shows that the interaction between thermohaline circulation in the North Atlantic Ocean and wind-driven currents in the topical Atlantic Ocean contributes to the rapidity of African Monsoon transitions during abrupt climate change events. Abrupt changes in the African monsoon can have pronounced socioeconomic impacts on many West African countries. Evidence for both prolonged humid periods and monsoon failures have been identified throughout the late Pleistocene and early Holocene epochs1,2. In particular, drought conditions in West Africa have occurred during periods of reduced North Atlantic thermohaline circulation, such as the Younger Dryas cold event1. Here, we use an ocean–atmosphere general circulation model to examine the link between oceanographic changes in the North Atlantic Ocean and changes in the strength of the African monsoon. Our simulations show that when North Atlantic thermohaline circulation is substantially weakened, the flow of the subsurface North Brazil Current reverses. This leads to decreased upper tropical ocean stratification and warmer sea surface temperatures in the equatorial South Atlantic Ocean, and consequently reduces African summer monsoonal winds and rainfall over West Africa. This mechanism is in agreement with reconstructions of past climate. We therefore suggest that the interaction between thermohaline circulation in the North Atlantic Ocean and wind-driven currents in the tropical Atlantic Ocean contributes to the rapidity of African monsoon transitions during abrupt climate change events.