PP43B-1516
Sea-ice distribution and pressure pattern in the Bering and Okhotsk Seas since the Last Glacial Maximum
The paleo-pressure distribution patterns since the last glacial maximum (LGM) over the Okhotsk and Bering Seas were reconstructed using the historical changes of sea-ice extent based on the distribution of sea-ice related diatom taxa. We compiled the sea-ice distribution record since the LGM using the published data (e.g., Shiga and Koizumi, 2000; Katsuki et al., 2005) and sediment cores MD01-2412 and GH00-1002 in the Okhotsk Sea, and GC-33 and PC-23A in the Bering Sea. The patterns of sea-ice related diatom change were much different in each sediment core site. In the Okhotsk Sea, sea-ice related diatom, Fragilariopsis, increased in the southern areas during the glacial period, in the north central area during the deglaciation, in the east and central areas during the mid-Holocene, and the north and south offshore areas during the late- Holocene. However, in the Bering Sea, this species increased along the paleo-continental shelf during the glacial period, in the north and southeastern areas during deglaciation, in the western area during the mid- Holocene, and in the southeastern area during the late-Holocene. The timings of these changes were generally consistent each other, because the atmospheric circulation mainly controlled the sea-ice distribution, resulting in the possible evolution of the pressure patterns since the LGM: southwestern Aleutian Low mode during the glacial period, intensified high pressure mode during the deglaciation, north Aleutian Low mode during the mid-Holocene, and western Aleutian Low accompanied by secondary low mode during the late Holocene. In addition, fresh water input was another controlling factor to the sea-ice distribution in the Bering Sea. Relative abundances of sea-ice diatom taxa were much higher at the Bølling-Allerød (BA) and early Holocene (EH) than those in the Younger-Dryas and glacial periods, particularly in the northern Bering Sea, as the result of glacier retreated on the Alaska during the BA and EH.
PP43B-1517
VEGETATION AND CLIMATE IN THE PYRENEAN MOUNTAINS (SOUTHERN FRANCE) DURING THE LAST 15000 YEARS BP: A POLLEN-INFERRED QUANTITATIVE RECONSTRUCTION
Climatic linkages between the North Atlantic and west Europe areas have been investigated for the last 15,000 yrs from four high-temporal pollen records located in the western Pyrenees Mountains. Mountain areas are of particular interest because the vegetation is very sensitive to variations in climate and human impact, and also because the vegetation responses to climatic changes are more pronounced at higher altitudes than in the lowlands, and are well recorded in pollen sequences. In the Alps Mountains, the lateglacial and Holocene climate variations are now well known from numerous multi-proxies approaches. However no climate estimates are available in the Pyrenean Mountains (southern France). This study aim to test the sensitivity of the Pyrenean Mountains area to even short term and relatively weak climatic fluctuations recognised in the North Atlantic region. Here, we reconstruct the vegetation and the climate changes along an latitudinal/altitudinal gradient in the western part of the Pyrenees for the last 15,000 yrs BP from 4 high-resolution pollen records located from the oceanic shore to the central Pyrenees with a chronology established on 40 radiocarbon dates : from west to east, the Mouriscot lake record located near the ocean coast at 50m asl (43.45 N/1.55W), the Occabe record in Basque mountains located in the Iraty massif at 1300m (43.03N/1.10W); the Piet record at 1150m in the Ossau valley (42.88N/0.42W) and the Ech record located at lower altitude (600m) in the Lourdes basin (43.7N/0.08W) in the central part of the chain. From the 4 cores, the modern analogues technique has been used to obtain robust and precise quantitative estimates of the annual temperature, the mean temperature of the warmest/coldest month, total annual precipitation, and the ratio of real to potential evapotranspiration. Pollen-inferred climate estimates show that cold and dry conditions prevailed during the Oldest and Younger Dryas while temperate conditions are evidenced during the Lateglacial interstadial and the Holocene. Several cool short-lived events (Older Dryas, Gerzensee/Preboreal Oscillations, 8.2ka) indicate that the climate oscillations associated with the successive steps of the deglaciation in North Atlantic have also been observed in the western Pyrenean areas. This confirms strong climatic linkages between the North Atlantic and west Europe, particularly the western Pyrenees Mountains.
PP43B-1518
Role of the Bering Sea on the glacial strength of North Pacific Intermediate Water in the North Pacific
Various kinds of geochemical and micropaleontological data were obtained from an 18-m long piston core (PC23A) collected from the northern slope area (60°09.52'N, 179°27.82'W, 1002 m deep) in the Bering Sea. The age of the upper part of core was determined by planktonic foraminifera AMS 14C dates and the presence of L. nipponica sakaii and absence of A. setosa constrained the age of lowermost of core to be younger than 70 kyr. Our paleoceanographic data demonstrate that the glacial period in the Bering Sea was clearly characterized by low production in the surface water and oxic bottom-water condition, maintaining relatively fast ventilation. In contrast, during the deglacial period, the Bering Sea shows high surface-water production and anoxic bottom-water condition, featuring the slow ventilation. Our results confirm that the Bering Sea played an important source region for North Pacific Intermediate Water (NPIW) during the glacial period. In addition, during stadials in the Bering Sea, strong intermediate water formation with weak Alaskan Current corresponds to strong cold California Current in the California margin of NE Pacific, resulting in the oxic bottom-water condition because of the full influence of NPIW when southward North Pacific Current in the warm NW Pacific was strong due to weakened subarctic gyre system. In contrast, during interstadials in the Bering Sea, weak intermediate water formation with strong Alaskan Current was associated with strong warm counter California Current in the NE Pacific margin due to weak cold California Current, resulting in anoxic or dysoxic bottom condition because of little influence of NPIW, when nouthward Subarctic Current in the cold NW Pacific was strong due to strengthened subarctic gyre system.
PP43B-1519
Investigating multi-decadal scale changes in ENSO variability using a coupled A/OGCM
We simulate multi-decadal and multi-centennial ENSO variability using GENMOM, a non flux-corrected A/OGCM comprised of the GENESIS V3.0 atmospheric model and the MOM2 ocean model. The model produces realistic ENSO variability comparable with similar models used in the 2007 IPCC assessment. Long integrations of the model yield significant changes in the frequency and amplitude of ENSO on decadal and centennial time scales. A suite of equilibrium simulations in which atmospheric CO2 levels were prescribed at LGM (180 ppmV), 6Ka (280 ppmV with 6Ka solar forcing), present (355 ppmV), doubled and quadrupled values display differing characteristics of ENSO variability. Wavelet analysis of the present simulation shows significances at both 12 – 30 years multi-decadal range and 70 – 100 year centennial scale range. Initial results show that only slight changes in ENSO variability and amplitude exist between simulations calculated over the entire length of the runs. However, subsampling at decadal and centennial scales shows large changes, indicating a low frequency pattern in the ENSO signal.
PP43B-1520
North Atlantic Climate Variability: Preliminary Analysis of Historical Weather Data and Stable Isotope Time Series of Cave Dripwater and Holocene-Age Stalagmites from Bermuda
Bermuda, located in the center of the Sargasso Sea of the North Atlantic Ocean, is a unique laboratory for analyzing Holocene climate change. This subtropical area lacks a strong seasonal cycle, making it an ideal place to look for low-frequency cycles identified in observational records of North Atlantic climate. A preliminary statistical analysis of historical climate data, provided by the Bermuda Weather Service and spanning a period from 1852 to 2006, will be presented. Emphasis will be placed on the identification of cycles corresponding to such climate modes as the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO). To extend the record back farther than the observational period, it is necessary to identify and analyze paleoclimate proxies. Carbonate caves on the island of Bermuda contain numerous speleothems, which have the potential to serve as extremely high-resolution (sub-decadal) recorders of climate change via the stable oxygen and carbon isotope concentrations contained in their crystalline form. Stalagmite stable isotope data is reflective of changes in stable isotopes of the cave dripwater feeding the stalagmite. Approximately monthly time series of δ18O data from three dripwater sites in each of two Bermuda caves, Fantasy Cave and Leamington Cave, will be presented, covering the period from April 2006 to March 2008. Values from Fantasy Cave range from -4.7 permil to -3.0 permil. Interestingly, a positive excursion of ~0.8 permil in the dripwater δ18O is associated with the passage of Hurricane Florence over the island in September 2006; possible causes of this spike include a change in vapor source or hurricane wind-induced sea spray. The dripwater data will help us to calibrate and interpret the stable isotope data from stalagmite calcite in our samples. Several stalagmites retrieved from caves in the Hamilton Parish region of Bermuda were dated by U-series methods and found to cover a period from 4.7 thousand years ago to the present. Time series of stable isotope data (δ18O and δ13C) from these stalagmites will be presented. Initial results from one stalagmite, covering approximately 4150-2600 years ago, show a range of -4.0 permil to -1.5 permil in δ18O, and -9.0 permil to -3.5 permil in δ13C. Analysis and discussion of this data will include implications for the dominant modes of North Atlantic climate variability over the last 4.7 thousand years.
PP43B-1521
Local and Global Sea Level During the Last Interglacial: A Gaussian Process Approach
Although its warmth was caused primarily by variations in Earth's orbital forcing, the Last Interglacial (LIG) stage (ca. 130-115 ka), with polar temperatures ~3-5 C warmer than today, serves as a partial analog for low-end future warming scenarios. Multiple indicators suggest LIG global sea level was higher than at present; based upon a small set of local sea level indicators, the IPCC Fourth Assessment Report inferred an elevation of ~4-6 m. Sea level is often mistakenly viewed as a simple function of changing global ice volume. This viewpoint neglects local variability, which arises from several factors, including the distortion of the geoid and the elastic and isostatic effects of lithospheric deformation by shifting ice masses. Accurate reconstruction of past global and local sea levels therefore requires integrating global data sets of local sea level indicators. Such attempts have rarely been made for periods predating the Last Glacial Maximum. To assess the robustness of the IPCC's global estimate and search for patterns in local sea level reflective of meltwater source, we have compiled a more comprehensive database that includes a variety of local sea level indicators, among them coral terraces, raised beaches, and sedimentological facies transitions, from sixty-five localities, as well as a global sea level record derived from oxygen isotopes. We generate a global synthesis from these data using a novel statistical approach based upon Gaussian process modeling. In Gaussian process modeling, as in the closely related geospatial technique of kriging interpolation, an estimate of a field value -- in this case, sea level -- at any arbitrary point of interest is derived from the data using a covariance function. We find an appropriate covariance function through a training process that employs pseudo-proxy data generated from a simple gravitational sea-level model under many plausible ice sheet histories. We use a Markov Chain Monte Carlo approach to account for geochronological errors. Analysis of synthetic histories suggest that the current data set is capable of resolving LIG global sea level with a 95% confidence interval of about ±2 m. Due to uncertainties in dating, it is unable to resolve robustly a short-lived mid-LIG lowstand like that proposed by several authors. We will present estimates of global and local sea level history during the LIG derived from this statistical approach.
PP43B-1522
Glacial Abrupt Cold Events: No Impact on the ITCZ Summer Position in the West Atlantic
Palaeoclimate records and numerical model simulations indicate that changes in tropical and subtropical sea surface temperatures and in the annual average position of the intertropical convergence zone (ITCZ) are linked to high-latitude climate changes on millennial to glacial-interglacial timescales. It has recently been suggested that cooling in the high latitudes associated with abrupt climate-change events is evident primarily during the northern hemisphere winter, implying increased seasonality at these times. However, it is unclear whether such a seasonal bias also exists for the low latitudes. Here we present foraminiferal Mg/Ca based sea surface temperatures in the northeastern Gulf of Mexico for the past 300,000 years. Our results suggest that sea surface temperatures are controlled by the migration of the northern boundary of the Atlantic Warm Pool, and hence are linked to the position of the ITCZ during boreal summer, and are relatively insensitive to winter conditions. Our results suggest that summer Atlantic Warm Pool expansion is primarily affected by glacial-interglacial variability and low-latitude summer insolation. Because a clear signature of rapid climate- change events, such as the Younger Dryas cold event, is lacking in our record, we conclude that high- latitude events seem to influence only the winter Caribbean climate conditions, consistent with the hypothesis of extreme northern-hemisphere seasonality during abrupt cooling events.
PP43B-1523
Firn processes and N-15 of N2: potential for a gas-phase climate proxy
Ice cores provide unique records of past atmospheric composition, but these must be corrected for the age difference between ice and gas at a given depth in order to compare gas concentration changes with climate changes recorded in the ice. This gas age-ice age difference depends on the age of the ice at the bottom of the firn layer, where the bubbles are closed-off. Firn densification models are used to calculate this age difference in the past, but have an uncertainty on the order of 1000 years for Central Antarctic sites. A proxy for climate in the gas phase would remove the need for this correction. δ 15N of N2 records physical fractionation processes in the firn column. Here we present the δ 15N record from the EPICA Dome C (EDC) ice core covering the last three glacial terminations and 5 glacial-interglacial cycles between 300 and 800 ka. We show that δ 15N is positively correlated with the ice deuterium content, a proxy for temperature, over the entire available EDC record. Thermal fractionation is an unlikely explanation for this correlation; instead, we propose an accumulation-permeability-convection mechanism. Comparison of inferred convection zone depth and accumulation rate over individual glacial-interglacial cycles reveals hysteresis-like behavior. The tightest correlation is observed over glacial terminations, supporting the idea that δ 15N is a property in the gas phase that records deglacial warming.
PP43B-1524
Coupled Warming and Drought in the American Southwest During Long mid-Pleistocene Interglacials (MIS 11 and 13)
An 82-m deep lacustrine sediment core from the Valles Caldera, northern New Mexico reveals details of climate change over two glacial cycles in the middle Pleistocene. Core VC-3, taken from the Valle Grande, has a basal 40Ar/39Ar date of 552 kyr from a tephra associated with the eruption of the South Mountain rhyolite which formed the lake. A variety of proxies including core sedimentology, organic carbon and carbon isotopic ratios, pollen, scanning XRF analysis and a new paleotemperature proxy, MBT (methylated branched tetraether) content of soil bacteria reveal two major warm periods above the basal tephra which we correlate with interglacials MIS 13 and MIS 11. This chronology is corroborated by the identification of two geomagnetic field "events" which are correlated with globally recognized events (14α and 11α). The lacustrine record terminates at ~350 ka when the lake filled its available accommodation space behind the dam of rhyolite lava. MBT temperature estimates show average glacial temperatures in core VC-3 of ~ -4°C, and average interglacial temperatures of ~ +4°C, and the general trends are well corroborated by multiple proxies including pollen and lacustrine organic productivity estimates. A temperature increase of ~9°C occurs during Termination V, the largest glacial termination in the Pleistocene. Multiple proxies from VC-3 show significant structure during the two interglacials present in the core (MIS 13 and 11). Three warm substages (~ 2°C warmer) are recognized within MIS 11 based on organic productivity (Corg, Si/Ti ratios), pollen taxa, elevated charcoal from fires, and the MBT temperature estimates. These warm substages appear to be a strong response to precessional forcing in the SW continental interior even though the amplitude of eccentricity-modulated precession was at a minimum during MIS 11. These results suggest that future climate change in the SW may be characterized by similar natural temperature variability on precessional timescales, superimposed on future anthropogenic warming. Intervals of mudcrack facies representing significant drought conditions occur during or just after the warmest phases of the two interglacials. This past coupling between warm temperatures and extended drought in the SW as a natural feature of long interglacials is consistent with recent predictions of extended Dust-Bowl-like conditions in the SW as a response to global warming.
PP43B-1525
Solar Forcing of Holocene Droughts in a Stalagmite Record From West Virginia in East- Central North America
Elevated Sr/Ca ratios and δ13C values in Holocene-age stalagmite BCC-002 from east-central North America record six centennial-scale droughts during the last five North Atlantic Ocean ice-rafted debris (IRD) episodes, previously ascribed to solar irradiance minima. Spectral and cross-spectral analyses of the multi-decadal resolution Sr/Ca and δ13C time series yield coherent ~200 and ~500 years periodicities. The former is consistent with the de Vries solar irradiance cycle. Cross-spectral analysis of the Sr/Ca and IRD time series yields coherent periodicities of 715- and 455-years, which are harmonics of the 1,450±500 year IRD periodicity. These coherencies corroborate strong visual correlations and provide convincing evidence for solar forcing of east-central North American droughts and strengthen the case for solar modulation of mid-continent climates. Moisture transport across North America may have lessened during droughts because of weakened north-south temperature and pressure gradients caused by cooling of the tropical Pacific and Atlantic Oceans.
PP43B-1526
Sub-millennial atmospheric CO2 variations at around Antarctic warming event 1
How atmospheric CO2 varies and is controlled on various time scales is an important question for understanding how the carbon cycle and climate change are linked. Past CO2 variations on orbital to millennial timescales are relatively well studied, but variations on decadal to centennial timescales are not. Here we show preliminary submillennial-scale CO2 variations from the Siple Dome, Antarctica ice core. So far we analyzed 40 samples from 37.3-40.5 ka (on GISP2 time scale). The average time resolution is 80 years, but with a better resolution at some rapid changes of CO2. The time interval is of great interest because it covers or partly includes Antarctic warming event 1, Dansgaard-Oeschger event 8 (DO-8) and Heinrich event 4 (H-4). In general, the Siple Dome high-resolution records confirm the previous millennial records from Taylor Dome and Byrd ice cores. A striking feature of the Siple Dome record is rapid CO2 changes of 5-18 ppm in decades or a century, compared to 20-40 ppm variations on millennial timescales. One of the CO2 jumps occurred at the time of the abrupt warming in Greenland associated with DO-8 event. The true CO2 change rate could have been larger because our data resolution is similar to time intervals of the abrupt changes, and gas exchange before bubble close-off and gas diffusion in ice after bubble close-off smooth the rapid changes. Precise comparison of CO2 and the timing of H-4 is difficult due to chronological uncertainties. We will continue the CO2 analysis for the Siple Dome ice core and the updated results will be presented at the meeting.
PP43B-1527
Millennial-Scale Response and Impact of Climate Variability in the Gulf of California Across the LGM/Holocene Transition
Within the modern climate system, the interannual variability in the tropical Pacific ocean-atmosphere circulation intensity, defined as the El Niño/Southern Oscillation (ENSO), has significant impacts both within and beyond the tropics. Despite evidence for past "ENSO-like" climate variability at a variety of timescales, the behaviour of the ENSO system under different climate boundary conditions continues to be debated. Here, we investigate the millennial-scale signature of ENSO-like variability across the transition from the Last Glacial Maximum to the early Holocene, testing the ENSO system response to changes in insolation, atmospheric CO2 and global ice volume. We have analysed a remarkable high resolution sediment core from the Gulf of California, MD02-2515 (IMAGES MONA expedition). The mm-scale (annually) laminated diatomaceous muds reflect seasonal variations in ocean and atmospheric circulation, driven in part by a monsoon climate, and interannual variability driven by ENSO. We address both the marine and terrestrial records of past ENSO-like variability using a variety of geochemical proxies. The UK37' and TEX86 indices for sea-surface temperature are combined with accumulation rates of chlorins, alkenones and diatom-specific sterols, alongside opal and organic carbon contents, to investigate the intensity of the upwelling system. We show that the transition from the LGM to the Holocene is marked by a slight warming (~ 2°C) and a reduction in the amplitude of millennial-scale SST variability (from ~ 5°C to ~ 3°C). SSTs differ between the UK37' and TEX86 proxies, reflecting production by different source organisms in different seasons and/or water depths. The difference between the two SST values is smallest when other proxies indicate reduced upwelling, consistent with a reduction in seasonal SST variability. Alkenone and chlorin concentrations detail highly variable coccolithophore and total primary production, but suggest reduced productivity and a weakened upwelling system during the Younger Dryas event that punctuates the transition. Terrigenous inputs are dominated by aeolian sources, given high n-alkane concentrations that contrast with very low (<0.1) BIT index values that are most likely derived from fluvial sources. Both the concentrations and average chain lengths of the n- alkanes indicate weaker wind systems and/or cool conditions during the Younger Dryas. Thus, whilst SSTs reveal a smooth transition from the glacial to interglacial, events during the transition itself appear more significant for marine production and conditions onshore.
PP43B-1528
The North Atlantic Oscillation since 1500 A.D.: Climate Anomalies and Human Affairs
The North Atlantic Oscillation (NAO) is related to the near-surface climate of the Baltic Sea area in winter,
although the correlations undergo significant non-stationarities on longer time scales. For example the
Maximum Ice extent of the Baltic Sea (MIB) from 1720 to 1999 correlates well (30%) with the reconstructed
winter-NAO, but shows 30 year running correlations between 16% and 64%. These non-stationarities can
be found for the NAO and monthly mean sea-level of Baltic gauges and historical station temperatures for
shorter time scales in observations, and at longer time scales in different GCM simulations with and without
external forcing. This poses an additional problem for reconstructions of circulation indices from near-surface
proxies. For historical time scales, synoptical evidences derived from historical documents can reveal
important additional information besides the sole statistical approaches.
A detailed study of the long-term vs. short-term climate variability of the so called "Little Ice Age" (LIA, 1550-
1900 A.D.) reveals a very high year-to-year variability of the NAO and the near-surface climate of the Baltic
with some pronounced clustering in time. As human affairs have been strongly influenced by short-term
climate variability in these periods, we can show a strong relationship between societal developments and
high short-term climate variability.
The unpredictability and strong fluctuations of the seasonal conditions exert a high pressure on the
agricultural society. Combining new evidences from historical documents with reconstructions of the NAO and
its near-surface impacts, the high short-term variability is found to be a typical feature of the LIA, and seems
to be mainly caused by the dynamics of a mostly negative mode of the NAO compared to the 20th century,
which leads to more meridional circulation with more extreme fluctuations. This is also supported by other
studies on Andalusian winter rainfall or drought severity-indices in Morocco since 1000 A.D., which have
been significantly wetter than the 20th century due to a weak Azores High and negative values of the NAO
index.
http://coast.gkss.de/staff/schenk/research.html
PP43B-1529
Novel Insights to the Factors Controlling Monsoon Precipitation in Southwestern US From a mid-Pleistocene Lake Sediment Record, Valles Caldera, Northern New Mexico
Monsoon precipitation is vital for the arid regions of Southwestern United States, but currently there are insufficient quantitative paleoclimate data for parameterizing climate models. We studied an 82-m lacustrine sediment core from Valles Caldera, Northern New Mexico. The basal age of the core is 552 ±3 kyr determined by dating of a tephra layer. Multiple proxies including, TOC, δ13C, C/N ratios, magnetic susceptibility and pollen taxa revealed three glacial and two interglacial periods, spanning Marine Isotope Stages (MIS) 10 through 14. In order to quantitatively elucidate the hydrological history of the region during these stages, we measured the concentrations and δD values of higher plant leaf waxes throughout the core. δD values of leaf waxes show pronounced glacial-interglacial variations throughout the core, with interglacials (MIS11 & 13) showing 40-60 permil higher values than glacials. We interpret such systematic hydrogen isotopic changes as changes in δD values of summer precipitation. The higher values during the interglacials indicate reduced summer monsoon intensity as a result of precipitation amount effect. Given that winter precipitation would also have been lower during interglacials as a result of the retreat of the jet stream northward, interglacial periods must be characterized by significantly drier conditions than glacial periods throughout the seasonal cycles. Superimposed on the glacial-interglacial hydrogen isotopic changes are distinctive ~40 permil cyclic fluctuations of δD values, with a periodicity similar to the precessional cycles, suggesting solar insolation as an important driver for summer monsoon intensity in the Southwestern United States.
PP43B-1530
Characterizing the Statistical Properties of Dansgaard-Oeschger Events
Ice core records from Greenland have shown times of rapid warming during the most recent glacial period. These Dansgaard-Oeschger (D-O) events are important to our understanding of both past climate systems and modern climate volatility. In this presentation, we present new approaches for statistically evaluating whether the timing of D-O events is cyclical or random. Specifically, we consider permutation testing and bootstrapping methodologies for assessing the significance of cyclicity manifested in the periodogram. We discuss implications for understanding the past glacial period.
PP43B-1531
Atlantic sea surface temperature trends and variability since 1552
North Atlantic sea-surface temperature (SST) variability can have a near global impact on climate. Observed variability has been described as a natural multidecadal (65-100 year) oscillation superimposed upon a linearly-increasing, low frequency background warming. The multidecadal portion of this variability may be persistent, suggesting useful decadal climate predictions may soon be possible. However, our understanding of multidecadal Atlantic SST variability prior to the brief instrumental record relies almost exclusively on high latitude tree-ring proxies. No proxy SST reconstruction from the Atlantic itself has the resolution, dating accuracy and length needed to assess the behavior of multidecadal variability. We present the first absolutely dated and annually-resolved multi-centennial record of Atlantic sea surface temperature. Our 439- year coral-based reconstruction suggests western low-latitude Atlantic SSTs were nearly as warm as today from 1552-1570, cooled by more than 1 degree C from 1650-1730 and generally warmed to the present. Estimates of background variability suggest anthropogenic forcing can account for most of the warming since 1850. Multidecadal variations superimposed upon this background disappear prior to about 1730 in favor of interdecadal (15-20 year) variability. This suggests observed multidecadal variability is not persistent and may be difficult to predict.
PP43B-1532
Lake Level Changes Under a Constant Climate
Lake-level changes are important indicators of climate change, providing particular insight into the nature of hydrological balances (e.g., Benson et al., 1989; Cross et al., 2000). Indeed, evidence from paleo lake changes is one of the primary reasons the hydrological cycle in the subtropics have come to be seen as potential 'tipping points' of the climate system (Lenton et al., 2007). Our understanding of future lake-level variations is contingent on a more sophisticated understanding of the drivers of past lake-level variations. Two fundamental properties of geophysical systems are that there exists both some source of inertia (or 'memory'), and also a stochastic climate forcing – natural year-to-year fluctuations in climate. The effect of the inertia is to integrate the short time-scale variability and produce persistent anomalies on longer timescales, even in a constant climate (e.g., Hasselman, 1975; Wunsch, 1999; Roe, 2008). It is in the context of this natural variability that climate change must be evaluated. In the case of a lake, the volume of water plays the role of the inertia, and the random stochastic forcing is due to interannual variations in evaporation and precipitation. We have developed a simple, versatile lake-level model based on mass conservation, catchment area, surface evaporation, and lake-reservoir geometry, from which one can determine the dynamic response time of a lake. The model predicts lake-level changes in response to climate changes, and lake-level variability in response to stochastic, natural climate variability. The results clearly show that large and persistent lake- level anomalies can arise from stochastic climatic variability, even in the absence of a trend. In addition, we use this model to characterize the long-term variability of lake levels from threshold crossing statistics in order to determine (1) the lake-level variability expected from stochastic climate variability, (2) the rapidity and magnitude of lake-level changes required to be considered outside of that expected from stochastic variability, and (3) the length of the lake-level record necessary to be able to confidently attribute a trend in lake level to a trend in climate.
PP43B-1533
Regionally heterogeneous paleoenvironmental responses in the West African and South American monsoon systems on glacial to millennial timescales
Although millennial-scale paleoenvironmental changes have been well characterized for high latitude sites, short-term climate variability in the tropics is less well understood. While the Intertropical Convergence Zone may act as an integrator of tropical climate changes, regional factors also play an important role in controlling the tropical response to climate forcing. Understanding these influences, and how they modulate the response to global climate forcing under different mean climate states is thus important for assessing how the tropics may respond to future climate change. Here, we examine new centennial-resolution records of paleoenvironmental change from isotopic and relative abundance data from molecular biomarkers in sediment cores from Lake Bosumtwi and Lake Titicaca. We assess the relative response of the West African and South American monsoon systems to millennial and suborbital-scale climate variability over the last ca. 30,000 years. While there is evidence for synchronous climate variability in the two systems, the dominant paleoenvironmental changes appear largely decoupled, highlighting the importance of regional climatology in controlling the response to climate forcing in tropical regions.
PP43B-1534
Southern Hemisphere PDO?: Interhemispheric symmetry suggests tropical forcing of Pacific decadal variability
There is considerable debate in the community over whether the Pacific Decadal Oscillation (PDO) is an independent mode of variability centered in the North Pacific or simply a reddened response to El Niño- Southern Oscillation (ENSO) forcing from the tropical Pacific. The PDO is defined as the leading Empirical Orthogonal Function of detrended monthly sea surface temperature (SST) anomalies in the North Pacific north of 20°N. We applied an identical procedure to the South Pacific south of 20°S to compute what is effectively a Southern Hemisphere PDO index from 1901-2007. The correlation between the resulting North and South Pacific leading principal components (PCs) is 0.38 and regression of basin-wide Pacific SST anomalies onto these two PCs yields patterns with high spatial correlation (r = 0.75). Thus, the spatial structure associated with the PDO in the North Pacific can be well explained by the leading mode of variability for the South Pacific. Low pass filtering (>7 years) of the PCs results in even greater pattern correlation (r = 0.87) indicating Pacific decadal variability is very similar on both sides of the equator. We modeled these North and South Pacific-defined PDO indices using a first-order autoregressive model, as done by Newman et al. (2003). The model is driven only by the current ENSO index and reemergence of the previous year's model PDO index. This simple model does a remarkable job of reproducing the observed PCs (r = 0.62 for the North Pacific; r = 0.75 for the South Pacific) suggesting an extratropical oscillation independent of ENSO is largely unnecessary to explain Pacific decadal variability. The above results highlight the strong similarity in decadal climate variability in the North and South Pacific, which suggests a common forcing from the tropics. Thus, as several researchers have noted previously, the PDO may best be viewed as a reddened response to ENSO.
PP43B-1535
The seasonal cycle as template for climate variability on astronomical time scales
On long time scales, changes in insolation are one main driver for climate variability. However, the question how insolation forcing is transferred into the climate system is largely unknown. Here, we propose the new concept that the climate response on astronomical time scales can be approximated by the local climate response to local seasonal insolation. Therefore we globally estimate empirical transfer functions between the observed daily temperature and the daily insolation and apply the transfer function on the long-term insolation. The concept is validated using a transient coupled climate model simulation for the Holocene. Further the temperature evolution of the last 750ky, predicted by our approach is analyzed and compared to proxy datasets. Our model predicts significant amplitudes in the eccentricity and semi-precession frequency band in the tropics caused by nonlinear responses which are already present in the seasonal cycle. Mid-latitudes are dominated by precession, high latitudes by obliquity. Further, it is found that the expected frequency response and the phasing highly depend on the location. Our local time-independent approach complements the global Milankovitch hypothesis (climate variations are driven by 65°N summer insolation) in explaining observed climate variability and offers new insights in interpreting paleoclimate records.
PP43B-1536
A High-Resolution Speleothem Record From Florida of Atmospheric Teleconnections Since 1,500 Years Ago
Understanding atmospheric teleconnections between tropical, subtropical, and higher-latitude regions of the North Atlantic Ocean is necessary to better evaluate the anthropogenic contribution to climate change. Here, we present a precisely dated, high- resolution speleothem record of stable isotopes and trace elements from Florida spanning the last 1,500 years. By using a multi-proxy approach, the different climatic influences were deconvolved, including the NAO, ENSO, PDO, and ITCZ, which all can affect our region. Further comparison using time-series analysis between our data and other high-resolution records covering this same period reveal differing influences of these teleconnections on geographic regions. Our record shows both the influence of changing rainfall above the cave and the influence of sea surface temperatures on atmospheric convection caused by atmospheric-oceanic variability over time.