Showing papers on "Climate oscillation published in 2001"

Rapid changes of glacial climate simulated in a coupled climate model.

Abstract: Abrupt changes in climate, termed Dansgaard-Oeschger and Heinrich events, have punctuated the last glacial period (approximately 100-10 kyr ago) but not the Holocene (the past 10 kyr). Here we use an intermediate-complexity climate model to investigate the stability of glacial climate, and we find that only one mode of Atlantic Ocean circulation is stable: a cold mode with deep water formation in the Atlantic Ocean south of Iceland. However, a 'warm' circulation mode similar to the present-day Atlantic Ocean is only marginally unstable, and temporary transitions to this warm mode can easily be triggered. This leads to abrupt warm events in the model which share many characteristics of the observed Dansgaard-Oeschger events. For a large freshwater input (such as a large release of icebergs), the model's deep water formation is temporarily switched off, causing no strong cooling in Greenland but warming in Antarctica, as is observed for Heinrich events. Our stability analysis provides an explanation why glacial climate is much more variable than Holocene climate.

Antarctic sea ice and the control of Pleistocene climate instability

Abstract: A hypothesis is presented for the origin of Pleistocene climate instability, based on expansion of Antarctic sea ice and associated changes in the oceans' salinity structure. The hypothesis assumes that thermohaline overturning is dominated by the reconfigured conveyor of Toggweiler and Samuels [1993b], in which deepwater upwelling is restricted to high southern latitudes. The reconfigured conveyor is shown to be potentially stabilized in an “on” mode by precipitation at high southern latitudes and potentially destabilized into “on” and “off” modes by the counteracting influence of Antarctic sea ice. The mechanism is clarified by the use of a hydraulic analogue. We hypothesize that this mechanism accounts for dominant patterns of thermohaline overturning and climate instability between Pleistocene warm and cold periods. The hypothesis is shown to be consistent with a range of paleoceanographic evidence and to potentially account for details of observed rapid climate changes during glacial and interglacial periods, including aspects of interhemispheric timing.

Interhemispheric climate links revealed by a late-glacial cooling episode in southern Chile

Abstract: Understanding the relative timings of climate events in the Northern and Southern hemispheres is a prerequisite for determining the causes of abrupt climate changes. But climate records from the Patagonian Andes1,2,3,4 and New Zealand5,6,7,8 for the period of transition from glacial to interglacial conditions—about 14.6–10 kyr before present, as determined by radiocarbon dating—show varying degrees of correlation with similar records from the Northern Hemisphere. It is necessary to resolve these apparent discrepancies in order to be able to assess the relative roles of Northern Hemisphere ice sheets and oceanic, atmospheric and astronomical influences in initiating climate change in the late-glacial period. Here we report pollen records from three sites in the Lake District of southern Chile (41° S) from which we infer conditions similar to modern climate between about 13 and 12.2 14C kyr before present (bp), followed by cooling events at about 12.2 and 11.4 14C kyr bp, and then by a warming at about 9.8 14C kyr bp. These events were nearly synchronous with important palaeoclimate changes recorded in the North Atlantic region9, supporting the idea that interhemispheric linkage through the atmosphere was the primary control on climate during the last deglaciation. In other regions of the Southern Hemisphere, where climate events are not in phase with those in the Northern Hemisphere, local oceanic influences may have counteracted the effects that propagated through the atmosphere.

Covariation of carbon dioxide and temperature from the Vostok ice core after deuterium-excess correction

Abstract: Ice-core measurements of carbon dioxide and the deuterium palaeothermometer reveal significant covariation of temperature and atmospheric CO2 concentrations throughout the climate cycles of the past ice ages. This covariation provides compelling evidence that CO2 is an important forcing factor for climate. But this interpretation is challenged by some substantial mismatches of the CO2 and deuterium records, especially during the onset of the last glaciation, about 120 kyr ago. Here we incorporate measurements of deuterium excess from Vostok in the temperature reconstruction and show that much of the mismatch is an artefact caused by variations of climate in the water vapour source regions. Using a model that corrects for this effect, we derive a new estimate for the covariation of CO2 and temperature, of r2 = 0.89 for the past 150 kyr and r2 = 0.84 for the period 350-150 kyr ago. Given the complexity of the biogeochemical systems involved, this close relationship strongly supports the importance of carbon dioxide as a forcing factor of climate. Our results also suggest that the mechanisms responsible for the drawdown of CO2 may be more responsive to temperature than previously thought.

Millennial‐scale depositional cycles related to British Ice Sheet variability and North Atlantic paleocirculation since 45 kyr B.P., Barra Fan, U.K. margin

Abstract: Lithology, lithic petrology, planktonic foraminiferal abundances, and clastic grain sizes have been determined in a 30 m-long core recovered from the Barra Fan off northwest Scotland. The record extends back to around 45 kyr B.P., with sedimentation rates ranging between 50 and 200 cm/kyr. The abundance of ice-rafted debris indicates 16 glacimarine events, including temporal equivalents to Heinrich events 1–4. Enhanced concentrations of basaltic material derived from the British Tertiary Province suggest that the glacimarine sediments record variations in a glacial source on the Hebrides shelf margin. Glacimarine zones are separated by silty intervals with high planktonic foraminifera concentrations that reflect an interstadial circulation regime in the Rockall Trough. The results suggest that the last British Ice Sheet fluctuated with a periodicity of 2000–3000 years, in common with the Dansgaard-Oeschger climate cycle.

A sea ice climate switch mechanism for the 100‐kyr glacial cycles

Abstract: A box model of the ocean-atmosphere-sea ice-land ice climate system is used to study a novel mechanism for self-sustained oscillations of the climate system on a time scale of 100,000 years, without external forcing. The oscillation in land ice volume has the familiar sawtooth shape of climate proxy records. The most novel aspect of the climate oscillations analyzed here is the crucial role played by the sea ice. The sea ice acts as a “switch” of the climate system, switching it from a growing land glaciers mode to a retreating land glaciers mode and shaping the oscillation's sawtooth structure. A simple explanation of the 100-kyr timescale is formulated on the basis of the mechanism seen in the model. Finally, rapid sea ice changes such as those seen in our model, and their drastic effects on the climate system, may provide an explanation to some of the rapid climate changes observed to be a part of the variability at all timescales in the paleorecord.

Orbit-related long-term climate cycles revealed in a 12-Myr continental record from Lake Baikal

Abstract: Quaternary records of climate change from terrestrial sources, such as lake sediments1,2 and aeolian sediments3,4, in general agree well with marine records5,6. But continuous records that cover more than the past one million years were essentially unavailable until recently, when the high-sedimentation-rate site of Lake Baikal was exploited1,2,7,8. Because of its location in the middle latitudes, Lake Baikal is highly sensitive to insolation changes9 and the entire lake remained uncovered by ice sheets throughout the Pleistocene epoch, making it a valuable archive for past climate. Here we examine long sediment cores from Lake Baikal that cover the past 12 million years. Our record reveals a gradual cooling of the Asian continental interior, with some fluctuations. Spectral analyses reveal periods of about 400 kyr, 600 kyr and 1,000 kyr, which may correspond to Milankovitch periods (reflecting orbital cycles). Our results indicate that changes in insolation were closely related to long-term environmental variations in the deep continental interior, over the past 12 million years.

Atlantic Ocean heat piracy and the bipolar climate see-saw during Heinrich and Dansgaard-Oeschger events

Abstract: The millennial-scale asynchrony of Antarctic and Greenland climate records during the last glacial period implies that the global climate system acts as a bipolar see-saw driven by either high-latitudinal and/or near-equatorial sea-surface perturbations. Based on the results of recent modelling of generic Heinrich and Dansgaard-Oeschger scenarios, we discuss the possibility that oscillations of the deep-ocean conveyor may have been sufficient to cause this bipolar see-saw. The bipolar climate asynchrony in our scenarios is caused by the toggle between North Atlantic heat piracy and South Atlantic counter heat piracy. Ocean circulation has an enhanced sensitivity to the northern deep-water source as the North Atlantic Deep Water (NADW) cannot enter the Southern Ocean at depths shallower than the bottom of the Drake Passage. Any shoaling of the NADW can, therefore, increase the northward incursion of Antarctic Bottom Water (AABW), and trigger an interhemispheric climate oscillation. As hundreds of years are required to warm the respective high latitudes, the observed climate lead and lags between the two hemispheres can be explained entirely by the variability of the meridional overturning and by the corresponding change in the oceanic heat transport. Accordingly, it is entirely feasible for the global climate to work like a pendulum, which theoretically could be controlled by pushing at either of the deep-water sources. Our model scenarios suggest that it is entirely feasible for the bipolar climate see-saw to be controlled solely by variations in NADW formation. Copyright (C) 2001 John Wiley & Sons, Ltd.

Snow–Albedo Feedback and Seasonal Climate Variability over North America

Abstract: Interannual variations in the tropical Pacific sea surface temperatures related to El Nino-Southern Oscillation (ENSO) are known to influence wintertime surface climate anomalies over North America. However, the role of local land surface processes in this phenomenon is not well understood. Here, using a suite of atmospheric general circulation model simulations, it is demonstrated that the North American surface climate anomalies related to ENSO are greatly enhanced by a local snow-albedo feedback. Implications of this feedback mechanism on seasonal climate predictions and greenhouse gas-induced climate changes are discussed.

Ice sheet-thermohaline circulation interactions in a climate model of intermediate complexity

Abstract: A vertically integrated dynamic ice sheet model is coupled to the atmosphere-ocean-sea ice-land surface climate model recently developed by Wang and Mysak (2000). The background lateral (east-west) ice sheet discharge rate used by Gallee et al. (1992) is reduced and the planetary emissivity is increased (to parameterize the cooling effect of a decrease of the atmospheric CO2 concentration), in order to build up substantial ice sheets during a glacial period and hence set the stage for ice sheet-thermohaline circulation (THC) interactions. The following iceberg calving scheme is then introduced: when the maximum model height of the North American ice sheet reaches a critical value (2400 m), a prescribed lateral discharged rate is imposed on top of the background discharge rate for a finite time. Per a small prescribed discharge rate, repeated small iceberg calving events occur, which lead to millennial-scale climate cycles with small amplitudes. These are a crude representation of Dansgaard-Oeschger oscillations. Over one such cycle, the zonally averaged January surface air temperature (SAT) drops about 1.5°C at 72.5°N. However, a large prescribed lateral discharge rate leads to the shut down of the THC. In this case, the January SAT drops about 5°C at 72.5°N, the sea ice extent advances equatorward from 57.5° to 47.5°N and the net ice accumulation rate at the grid of maximum ice sheet height is reduced from 0.24 to 0.15 m/y. Since data strongly suggest that a collapsed THC was not a steady state during the last glacial, we restore the THC by increasing the vertical diffusivity in the North Atlantic Ocean for a finite time. The resulting climate cycles associated with conveyor-on and conveyor-off phases have much larger amplitudes; furthermore, the strong iceberg calving events lead to a larger loss of ice sheet mass and hence the period of the oscillations is longer (several thousand years).

Global climate models: Past, present, and future

Abstract: Global climate is a result of the complex interactions between the atmosphere, cryosphere (ice), hydrosphere (oceans), lithosphere (land), and biosphere (life), fueled by the nonuniform spatial distribution of incoming solar radiation. We know from climate reconstructions using recorders such as ice cores, ocean and lake sediment cores, tree rings, corals, cave deposits, and ground water that the Earth's climate has seen major changes over its history. An analysis of the temperature variations patched together from all these data reveals that climate change occurs in cycles with characteristic periods, for example 200 million, 100,000, or 4–7 years. For some of these cycles, particular mechanisms have been identified, for example forcing by changes in the Earth's orbital parameters or internal oscillations of the coupled ocean-atmosphere system. However, major uncertainties remain in our understanding of the interplay of the components of the climate system. Paleoclimate reconstructions, in particular from ice cores (1) also have shown that climate can change (e.g., ΔT = 5°C) over extremely short periods of time such as a few years. Over the last century, humans have altered the Earth's surface and the composition of its atmosphere to the extent that these factors measurably affect current climate conditions. There is concern that perhaps during one human generation we will gradually change climate conditions or even trigger a rapid and much more dramatic shift. We might be “poking an angry beast” (2).

Climatic Fluctuations in Lithuania Against a Background of Global Warming

Abstract: An analysis of the climatic fluctuations recorded in Lithuania over the 19–20th centuries suggests that, against a background of global warming, trends of climatic elements changeability have been varying with different seasons of the year: winters and springs have warmed up, precipitation in the cold period of the year has increased, whereas summer and autumn temperatures have changed just insignificantly. Thus, the continental character of the climate could be treated to have fallen into a general decline, which is observed throughout Europe and not only in Lithuania. As projected by forecast climate models, by the middle of the 21st century the Lithuanian climate will have warmed up 1.5–1.7°C. Yet, climate change tendencies could be altered by reduced emissions of greenhouse gases and the related worldwide control, as well as by the distribution of temperature anomalies in the Arctic Region and the adjacent latitudes, and by the associated changes in the ocean and atmospheric circulation.

Astronomical forcing in continental sediments. An integrated stratigraphic study of Miocene deposits from the Calatayud and Teruel basins, NE Spain

Abstract: During the last decades, there is an increasing concern about global climate change as a consequence of anthropogenic influences. Recently, a report presented by the working group of the Intergovernmental Panel on Climate Change (IPCC) concluded that there is convincing evidence that most of the warming observed over the last 50 years is the result of human activities. The working group also predicts that climate will continue to change throughout the 21st century, due to human influence, such as the emission of greenhouse gasses. Clearly, it is essential to understand the natural variability of global and regional climate change to discriminate and quantifY man-induced changes. Long records ofpast changes in climate offer a key role in identifYing the effects of anthropogenic influences and climate models used for predicting global climate change in the future should also be able to reconstruct and 'predict' past climate changes (Barron et al., 1995). The conclusions put forward by the IPCC are based on measurements of temperature and on climate proxy data for the Northern Hemisphere over the last 1,000 years inferred from e.g. tree rings, corals and ice cores. Of course, longer-term natural climate variations also occur as demonstrated in studies of Pleistocene records. Studies of Pleistocene climate records from ice cores, corals, varves and deep-sea sediments show that the Earth's climate has varied on a millennial scale with periods of several thousand years (Pisias et ai., 1973; Dansgaard et ai., 1984; Pestiaux et ai., 1988; Bond et ai., 1997). Millennial-scale climate cycles are also present in the older part of the Pleistocene (Oppo et ai., 1998; Raymo et ai., 1998; McManus et ai., 1999) and in the Pliocene (Steenbrink, 2001). The origin of these cycles, however, is not well understood and may be attributed to internal forcing mechanisms of the climate system (e.g. ice-sheet dynamics and atmosphere-ocean interactions) or to external mechanisms (e.g. solar variability, long-term tidal variations and harmonics of primary orbital frequencies). Longer-term climate variability, on time scales of 10-103 kiloyears (kyr), is found in Pleistocene as well as in Tertiary and older records and is manifested by changes in sediment properties (e.g. lithology, colour or grain size variations), in fossil communities and in geochemical and isotope composition of the sediments. Numerous studies have demonstrated that cyclic changes observed in these records are related to orbitally induced variations in climate (e.g. Shackleton and Opdyke, 1973; Hilgen, 1991a, b; Tiedemann et al., 1994; Shackleton et al., 1995; Olsen et al., 1996; Van Vugt et al. 1998; Steenbrink et ai., 2000). Such climate proxy data may be incorporated as constraints in climate models, which should result in more accurate model predictions offuture (and past) climate changes on orbital time scales. The subject of this thesis is to study such orbitally forced climate variations and to establish well dated climate proxy records for part of the continental Neogene using a multidisciplinary and integrated stratigraphic approach.

Evidence and cause of climate cycles in polar motion

Abstract: The long-term variation of polar motion contains a number of periods similar to climate cycles. Two possible causes for these long-term variations are mass redistributions produced by variations of atmospheric and oceanic circulation, and mass exchanges between the cryosphere and hydrosphere. Inner-core wobble, which can be inferred from the observed motion of the geomagnetic pole, is another phenomenon with periods similar to climate cycles. Only observations relating to mass redistributions caused by atmosphere dynamics and inner-core wobble are available for sufficiently long periods of time to investigate their influence on climate cycles in polar motion. Both processes contribute to climate cycles in polar motion, but they cannot completely explain these cycles. Possible sources of climate cycles are discussed.

Large-Scale Temperature Patterns in Past Centuries: Implications for North American Climate Change

Abstract: Recent climate reconstructions are analyzed specifically for insights into those patterns of climate variability in past centuries with greatest impact on the North American region. Regional variability, largely associated with the El Nino/Southern Oscillation (ENSO) phenomenon, the North Atlantic Oscillation (NAO), and multidecadal patterns of natural variability, are found to mask the emergence of an anthropogenic temperature signal in North America. Substantial recent temperature anomalies may however indicate a possible recent emergence of this signal in the region. Multidecadal North Atlantic variability is likely to positively reinforce any anthropogenic warming over substantial parts of North America in coming decades. The recent magnitudes of El Nino events appear to be unprecedented over the past several centuries. These recent changes, if anthropogenic in nature, may outweigh the projection of larger-scale climate change patterns onto the region in a climate change scenario. The implications of such changes for North America, however, are not yet clear. These observations suggest caution in assessing regional climate change scenarios in North America without a detailed consideration of possible anthropogenic changes in climate patterns influencing the region.

North Atlantic climate variability in early Palaeogene time: a climate modelling sensitivity study

Abstract: Abstract Understanding the nature and causes of the variability associated with past warm, high pCO2 climates presents a significant challenge to palaeoclimate research. In this paper we investigate the early Eocene climatic response in the North Atlantic region to forcing from an indirect effect of atmospheric methane (via polar stratospheric clouds (PSCs)), and we investigate the response of the climate system to forcing from a combination of orbital insolation changes and high atmospheric pCO2 concentration. We find that sea surface temperatures (SSTs), sea ice extent, net surface moisture, continental runoff and upwelling in the North Atlantic Ocean are all sensitive to those forcing factors, and that the degree of sensitivity is a function of location and season. Our results suggest that high-latitude SST values can vary by as much as 20 °C during the winter season in response to precessional and polar cloud forcing, whereas in contrast summer temperature varies by 4 °C or less. Model predictions of net surface moisture balance also vary substantially with our prescribed forcing. There is a large difference in variability between the localized net surface moisture results and the mean North Atlantic Ocean results, which suggests that large-scale assumptions about past surface ocean salinities and seawater δ18O may need to be reassessed. According to model results, the influx of terrigenous material via continental runoff to the North Atlantic Ocean should be highly seasonal, with greatest runoff occurring in spring. Our model results also indicate that changes in wind-driven upwelling and in continental runoff on a precessional time scale should be seen in regions of the central North Atlantic.

****Covariation of carbon dioxide and temperature from the Vostok ice core after deuterium-excess

Abstract: Ice-core measurements of carbon dioxide and the deuterium palaeothermometer reveal significant covariation of temperature and atmospheric CO2 concentrations throughout the climate cycles of the past ice ages. This covariation provides compelling evidence that CO2 is an important forcing factor for climate. But this interpretation is challenged by some substantial mismatches of the CO2 and deuterium records, especially during the onset of the last glaciation, about 120 kyr ago. Here we incorporate measurements of deuterium excess from Vostok in the temperature reconstruction and show that much of the mismatch is an artefact caused by variations of climate in the water vapour source regions. Using a model that corrects for this effect, we derive a new estimate for the covariation of CO2 and temperature, of r 2 = 0.89 for the past 150 kyr and r 2 = 0.84 for the period 350–150 kyr ago. Given the complexity of the biogeochemical systems involved, this close relationship strongly supports the importance of carbon dioxide as a forcing factor of climate. Our results also suggest that the mechanisms responsible for the drawdown of CO2 may be more responsive to temperature than previously thought.

A Numerical Modeling Study of the Coupled Variability of Lake Victoria in Eastern Africa and the Regional Climate

Abstract: The objective of this investigation was to investigate and study the coupled atmosphere-lake climate system over the Lake Victoria basin, and determine the corresponding physical mechanisms that are involved. The primary research vehicle for the investigation is a fully coupled model of the regional climate of Eastern Africa and Lake Victoria which has been developed and applied in this study. The atmospheric component of the model is the NCAR Regional Climate Model (RegCM2). The lake component of the model is based on the Princeton Ocean Model (POM) configured for Lake Victoria by replacing the open boundaries in the standard version of the model with a closed coastline and adopting the bythemetry of Lake Victoria. The horizontal resolution is 20 km for both the atmosphere and lake model components.The results show that the bythemetry and geometry of the lake play a fundamental role in determining the climatology of Lake Victoria. There exists Kelvin-like waves in the thermocline trapped along the coast and they propagate clockwise around Lake Victoria with periodicity of about 30 days. The oscillations entirely disappear in the case of the isothermal conditions. The 3-dimensional model produces a surface temperature pattern indicative of horizontal lake water mixing associated with the horizontal spiral pattern that is not present in the 1-dimensional model. Preliminary comparison of the coupled RegCM2-POM model simulation results with the observations indicates that the model produces more realistic lake surface temperatures (LST) and rainfall over and around the lake than the standard version of RegCM2 in which a simple one dimensional thermal diffusion lake model is used. Over Eastern Africa, the regional climate variability is significantly influenced by the circulation over the Lake Victoria basin. Moisture advection contribution is important but secondary to evaporation in explaining the heavy rainfall over the lake. The interaction between the lake-land breeze and the prevailing northeasterly flow accounts for the asymmetry in the distribution of the diurnal rainfall variations and the southwestward movement of the dominant bands of divergence/convergence. During the 1982 El Nino when the averaged LST over the lake was higher than that during the normal year, the LST gradient was weakened along the SW-NE axis over the lake by the strong lake circulation. This results in LST distribution whereby the southwestern region of the lake is cooled while the region of maximum LST moves to the central-eastern region of the lake from the southwestern region of the lake. The net change in rainfall distribution over the lake during the 1982 El Nino is a combination of the effect associated with the large-scale convergence pattern and the meso-scale climate changes associated with the shift of the region of maximum rainfall toward the central-eastern part of the lake from the western part of the lake in response to the LST redistribution. Conversely, the weaker lake circulation enhances the LST gradient over…

Short-Term Abrupt Climate Events of Last Glaciation: The Evidences from Stalagmite Records in Southwest China

Abstract: Based on the δ 18O and δ 13C records from 3 stalagmites taken from Guangxi and Guizhou Provinces, several short-term climate events with 10 2-10 3a scale have been revealed in last glciation, at 114～124ka BP, 145 ka BP, 20 ka BP, 27 ka BP, 36～37 ka BP and 41 ka BP respectively These records can be compared with the YD and H1-H4 records from North Atlantic sediments and Greenland ice, but there are slight age difference among the short-period climate events caused by the different test methods and test objects (including test error), more evidences are needed to confirm their simultaneity If so, the driving force of the short-term transitional events may be attributed to the astronomical factors as long-term climate cycle Furthermore research should be carried out