Modelling the concentration of atmospheric CO2 during the Younger Dryas climate event

 The Younger Dryas (YD, dated between 12.7–11.6 ky BP in the GRIP ice core, Central Greenland) is a distinct cold period in the North Atlantic region during the last deglaciation. A popular, but controversial hypothesis to explain the cooling is a reduction of the Atlantic thermohaline circulation (THC) and associated northward heat flux as triggered by glacial meltwater. Recently, a CH₄-based synchronization of GRIP δ¹⁸O and Byrd CO₂ records (West Antarctica) indicated that the concentration of atmospheric CO₂ (CO^atm ₂) rose steadily during the YD, suggesting a minor influence of the THC on CO^atm ₂ at that time. Here we show that the CO^atm ₂ change in a zonally averaged, circulation-biogeochemistry ocean model when THC is collapsed by freshwater flux anomaly is consistent with the Byrd record. Cooling in the North Atlantic has a small effect on CO^atm ₂ in this model, because it is spatially limited and compensated by far-field changes such as a warming in the Southern Ocean. The modelled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP (Northern Hemisphere) and from Byrd and Vostok (East Antarctica) during the YD. δ¹³C depletion and PO₄ enrichment are predicted at depth in the North Atlantic, but not in the Southern Ocean. This could explain a part of the controversy about the intensity of the THC during the YD. Potential weaknesses in our interpretation of the Byrd CO₂ record in terms of THC changes are discussed. Received: 27 May 1998 / Accepted: 5 November 1998     

Decadal time scale variability recorded in the Quelccaya summit ice core δ18O isotopic ratio series and its relation with the sea surface temperature

 The spectral characteristics of the δ¹⁸O isotopic ratio time series of the Quelccaya ice cap summit core are investigated with the multi taper method (MTM), the singular spectrum analysis (SSA) and the wavelet transform (WT) techniques for the 500 y long 1485–1984 period. The most significant (at the 99.8% level) cycle according to the MTM F-test has a period centered at 14.4 y while the largest variance explaining oscillation according to the SSA technique has a period centered at 12.9 y. The stability over time of these periods is investigated by performing evolutive MTM and SSA on the 500 y long δ¹⁸O series with a 100 y wide moving window. It is shown that the cycles with largest amplitude and that the oscillations with largest extracting variance have corresponding periods aggregated around 13.5 y that are very stable over the period between 1485 and 1984. The WT of the same isotopic time series reveals the existence of a main oscillation around 12 y which are also very stable in time. The relation between the isotopic data at Quelccaya and the annual sea surface temperature (SST) field anomalies is then evaluated for the overlapping 1919–1984 period. Significant global correlation and significant coherency at 12.1 y are found between the isotopic series and the annual global sea surface temperature (GSST) series. Moreover, the correlation between the low (over 8 y) frequency component of the isotopic time series and the annual SST field point out significant values in the tropical North Atlantic. This region is characterized by a main SST variability at 12.8 y. The Quelccaya δ¹⁸O isotopic ratio series may therefore be considered as a good recorder of the tropical North Atlantic SSTs. This may be explained by the following mechanism: the water vapor amount evaporated by the tropical North Atlantic is function of the SST. So is the water vapor δ¹⁸O isotopic ratio. This water vapor is advected during the rainy season by northeast winds and precipitates at the Quelccaya summit with its tropical North Atlantic isotopic signature. It is also suggested from this described stability of the decadal time scale variability observed in the Quelccaya isotopic series, that the decadal time scale GSST variability was also stable during the last five centuries. Received: 12 February 1997 / Accepted: 9 September 1997     

Characterizing atmospheric circulation signals in Greenland ice cores: insights from a weather regime approach

Greenland ice cores offer seasonal to annual records of δ¹⁸O, a proxy for precipitation-weighted temperature, over the last few centuries to millennia. Here, we investigate the regional footprints of the North Atlantic weather regimes on Greenland isotope and climate variability, using a compilation of 22 different shallow ice-cores and the atmospheric pressure conditions from the twentieth century reanalysis (20CR). As a first step we have verified that the leading modes of winter and annual δ¹⁸O are well correlated with oceanic (Atlantic multidecadal oscillation) and atmospheric [North Atlantic oscillation (NAO)] indices respectively, and also marginally with external forcings, thus confirming earlier studies. The link between weather regimes and Greenland precipitation, precipitation-weighted temperature and δ¹⁸O is further explored by using an isotope simulation from the LMDZ-iso model, where the 3-dimensional wind fields are nudged to those of 20CR. In winter, the NAO+ and NAO? regimes in LMDZ-iso produce the largest isotopic changes over the entire Greenland region, with maximum anomalies in the South. Likewise, the Scandinavian blocking and the Atlantic ridge also show remarkable imprints on isotopic composition over the region. To assess the robustness and model dependency of our findings, a second isotope simulation from the isotopic model is also explored. The percentage of Greenland δ¹⁸O variance explained by the ensemble of weather regimes is increased by a factor near two in both LMDZ-iso and IsoGSM when compared to the contribution of the NAO index only. Similarly, weather regimes provide a net gain in the δ¹⁸O variance explained of similar magnitude for the whole set of ice core records. Greenland δ¹⁸O also appears to be locally affected by the low-frequency variations in the centres of action of the weather regimes, with clearer imprints in the LMDZ-iso simulation. This study opens the possibility for reconstructing past changes in the frequencies of occurrence of the weather regimes, which would rely on the sensitive regions identified here, and the use of additional proxies over the North Atlantic region.     

GCM simulations of the Last Glacial Maximum surface climate of Greenland and Antarctica

 The LMDz variable grid GCM was used to simulate the Last Glacial Maximum (LGM, 21 ky Bp.) climate of Greenland and Antarctica at a spatial resolution of about 100 km.The high spatial resolution allows to investigate the spatial variability of surface climate change signals, and thus to address the question whether the sparse ice core data can be viewed as representative for the regional scale climate change. This study addresses primarily surface climate parameters because these can be checked against the, limited, ice core record. The changes are generally stronger for Greenland than for Antarctica, as the imposed changes of the forcing boundary conditions (e.g., sea surface temperatures) are more important in the vicinity of Greenland. Over Greenland, and to a limited extent also in Antarctica, the climate shows stronger changes in winter than in summer. The model suggests that the linear relationship between the surface temperature and inversion strength is modified during the LGM. The temperature dependency of the moisture holding capacity of the atmosphere alone cannot explain the strong reduction in snowfall over central Greenland; atmospheric circulation changes also play a crucial role. Changes in the high frequency variability of snowfall, atmospheric pressure and temperature are investigated and possible consequences for the interpretation of ice core records are discussed. Using an objective cyclone tracking scheme, the importance of changes of the atmospheric dynamics off the coasts of the ice sheets, especially for the high frequency variability of surface climate parameters, is illustrated. The importance of the choice of the LGM ice sheet topography is illustrated for Greenland, where two different topographies have been used, yielding results that differ quite strongly in certain nontrivial respects. This means that the paleo-topography is a significant source of uncertainty for the modelled paleoclimate. The sensitivity of the Greenland LGM climate to the prescribed sea surface conditions is examined by using two different LGM North Atlantic data sets. Received: 23 October 1997 / Accepted: 17 March 1998     

An investigation of the controls on Irish precipitation δ18O values on monthly and event timescales

This two-year study investigates the relative influence of meteorological variables (precipitation amount and temperature), atmospheric circulation, air mass history, and moisture source region on Irish precipitation oxygen isotopes (δ¹⁸O_p) on event and monthly timescales. Single predictor correlations reveal that on the event scale, 20% of δ¹⁸O_p variability is attributable to the amount effect and 7% to the temperature effect while on the monthly timescale the North Atlantic Oscillation accounts for up to 20% of δ¹⁸O_p variability and the amount and temperature effects are not significant. In comparison, multivariate linear regression reveals that the interaction of temperature and precipitation amount explains up to 40% of δ¹⁸O_p variance at event and monthly timescales. Five-day kinematic back trajectories suggest that the amount-weighted mean δ¹⁸O_p value of southerly- and northerly-derived events are lower by 2‰ relative to events derived from the west. Because air mass history and atmospheric circulation appear to influence δ¹⁸O_p in Ireland, Irish paleo-δ¹⁸O_p proxy records are best interpreted as reflecting a combination of parameters, not just paleotemperature or paleorainfall.     

Oxygen and Hydrogen Isotopic Signatures of Large Atmospheric Ice Conglomerations

Specific studies about the stable isotope composition (¹⁸O/¹⁶O and D/H) of atmospheric icy conglomerations are still scarce. The present work offers, for the first time, a very detailed analysis of oxygen and hydrogen isotopic signatures of unusually large ice conglomerations, or “megacryometeors”, that fell to the ground in Spain during January 2000. The hydrochemical analysis is based on the bulk isotopic composition and systematic selective sampling (deuterium isotopic mapping) of eleven selected specimens. δ¹⁸O and δD (V-SMOW) of all samples fall into the Meteoric Water Line matching well with typical tropospheric values. The distribution of the samples on Craig's line suggests either a variation in condensation temperature and/or different residual fractions of water vapour (Rayleigh processes). Three of the largest megacryometeors exhibited unequivocally distinctive negative values (δ¹⁸O = −17.2%₀ and δD = −127 %₀ V-SMOW), (δ¹⁸O = −15.6%₀ and δD = −112%₀ V-SMOW) and (δ¹⁸O = −14.4%₀ and δD = −100%₀ V-SMOW), suggesting an atmospheric origin typical of the upper troposphere. Theoretical calculations indicate that the vertical trajectory of growth was lower than 3.2 km. During the period in which the fall of megacryometeors occurred, anomalous atmospheric conditions were observed to exist: a substantial lowering of the tropopause with a deep layer of saturated air below, ozone depression and strong wind shear. Moreover, these large ice conglomerations occurred during non-thunderstorm conditions, suggesting an alternative process of ice growth was responsible for their formation.     

Atmospheric CO along the Trans-Siberian Railroad and River Ob: source identification using isotope analysis

The concentration, radiocarbon (¹⁴C) and stable isotope (¹³C and ¹⁸O) content of CO have been determined in air samples collected across Russia (about 8,500 km) and along the Ob river during the summer of 1999 to study the CO sources and sinks. An instrumented carriage on the Trans-Siberian railway and a boat on the river Ob were used as atmospheric measurement platforms. In general, CO mixing ratios, CO stable isotope ratios, as well as the abundances of ¹⁴CO over West Siberia were similar to those found at remote northern hemispheric baseline monitoring stations. Identified sources of CO along the Ob appear to be connected to methane oxidation based on an inferred δ¹³C_source = −36.8 ± 0.6‰, while the value for δ¹⁸O_source = 9.0 ± 1.6‰ identifies it as burning. Thus flaring in the oil and gas production can be supposed to be a source. The extreme ¹³C depletion and concomitant ¹⁸O enrichment for two of the boat samples unambiguously indicates contamination by CO from combustion of natural gas (inferred values δ¹³C_source = −40.3‰ and δ¹⁸O_source = 17.5‰). For these two samples, that have strongly elevated ¹⁴CO concentrations, the industrial area near Tomsk is identified as a source area using meteorological calculations. Along the Trans-Siberian Railroad background CO was to various degrees contaminated with CO from methane combustion (δ¹³C_source = −35.7 ± 6.2‰ and δ¹⁸O_source = 10.3 ± 1.8‰). The impact of industrial burning was discernable in the vicinity of Perm-Kungur.     

Glacial meltwater cooling of the Gulf of Mexico: GCM implications for Holocene and present-day climates

An atmospheric general circulation model, the NCAR CCM, has been used to investigate the possible effects that reduced Gulf of Mexico sea surface temperatures (SST) could have on regional and hemispheric climates. ¹⁸O records and terrestrial evidence indicate at least two major glacial meltwater discharges into the Gulf of Mexico subsequent to the last glacial maximum. It is probable that these discharges reduced Gulf of Mexico SST. We have conducted three numerical experiments, with imposed gulf-wide SST coolings of 3°C, 6°C, and 12°C, and find in all three experiments significant reductions in the North Atlantic storm-track intensity, along with a strong decrease in transient eddy water vapor transport out of the Gulf of Mexico. Surface pressures are higher over the North Atlantic, indicating a reduction of the climatological Icelandic low. The region is generally cooler and drier, with a reduction in precipitation that agrees well with evidence from Greenland ice cores. Other statistically significant changes occur across the Northern Hemisphere, but vary between the three experiments. In particular, warmer, wetter conditions are found over Europe for both the 6°C and 12°C SST reductions, but cooler conditions are found for the 3°C reduction. This indicates a dependence, in both the sign and magnitude of the model response, on the magnitude of the imposed SST anomaly. The results suggest that the present-day North Atlantic storm track is dependent on warm Gulf of Mexico SST for much of its intensity. They also suggest that meltwater-induced coolings may help account, in part, for some of the climatic oscillations that occurred during the last glacial/interglacial transition.     

Relationship of Late-Winter Temperatures in Europeto North Atlantic Surface Winds: A Correlation Analysis

Summary  The relationship between European surface temperature and winds over the eastern North Atlantic are investigated for the years 1988 to 1997. Daily Special Sensor Microwave Imager SSM/I observations are used to evaluate a monthly surface wind index that quantifies the influence of southwesterly flow. Our wind index and the monthly-mean surface-air temperatures in late winter and early spring over France and northern-latitude Europe are highly correlated. In February, the year-to-year increases/decreases match every year for France (correlation of 0.82 with the Index); and every year with just one exception for Europe (correlation with the Index of 0.76 for a longitudinal strip through Europe 45–50° N, and 0.73 for the 50–60° N strip). In March, the increases/decreases of the wind Index and of the temperatures for France also match, but the correlation with the Index is lower, 0.65. The high correlation between our Index and the large interannual fluctuations in the monthly temperature in late winter and early spring indicate that the onset of the spring conditions in Europe is significantly influenced by the wind patterns over the eastern North Atlantic. Coinciding with the fluctuations from warm-Europe/high-Index winter to the opposite conditions, we observe “seesaw” effects, fluctuations over the North Atlantic, in opposite directions in the east (25–5° W), and the west (65–45° W). In the low-Index years we find that: (a) the surface-air temperatures in the west are appreciably higher than in the east (but slightly lower in the high-Index year), and (b) the difference between the 500 mb meridional wind in the west and that in the east is positive and large, exceeding 10 m s⁻¹ (but it becomes negative and small in the high-Index years). The “seesaw” effects suggest that a positive feedback exits between these cross-Atlantic temperature differences and the surface winds. Received August 7, 1998 Revised April 23, 1999     

A degradation approach to accelerate simulations to steady-state in a 3-D tracer transport model of the global ocean

 We have developed a new method to accelerate tracer simulations to steady-state in a 3-D global ocean model, run off-line. Using this technique, our simulations for natural ¹⁴C ran 17 times faster when compared to those made with the standard non-accelerated approach. For maximum acceleration we wish to initialize the model with tracer fields that are as close as possible to the final equilibrium solution. Our initial tracer fields were derived by judiciously constructing a much faster, lower-resolution (degraded), off-line model from advective and turbulent fields predicted from the parent on-line model, an ocean general circulation model (OGCM). No on-line version of the degraded model exists; it is based entirely on results from the parent OGCM. Degradation was made horizontally over sets of four adjacent grid-cell squares for each vertical layer of the parent model. However, final resolution did not suffer because as a second step, after allowing the degraded model to reach equilibrium, we used its tracer output to re-initialize the parent model (at the original resolution). After re-initialization, the parent model must then be integrated only to a few hundred years before reaching equilibrium. To validate our degradation-integration technique (DEGINT), we compared ¹⁴C results from runs with and without this approach. Differences are less than 10‰ throughout 98.5% of the ocean volume. Predicted natural ¹⁴C appears reasonable over most of the ocean. In the Atlantic, modeled Δ¹⁴C indicates that as observed, the North Atlantic Deep Water (NADW) fills the deep North Atlantic, and Antartic Intermediate Water (AAIW) infiltrates northward; conversely, simulated Antarctic Bottom Water (AABW) does not penetrate northward beyond the equator as it should. In the Pacific, in surface eastern equatorial waters, the model produces a north–south assymetry similar to that observed; other global ocean models do not, because their resolution is inadequate to resolve equatorial dynamics properly, particularly the intense equatorial undercurrent. The model’s oldest water in the deep Pacific (at −239‰) is close to that observed (−248‰), but is too deep. Surface waters in the Southern Ocean are too rich in natural ¹⁴C due to inadequacies in the OGCM’s thermohaline forcing. Received: 18 March 1997 / Accepted: 27 July 1997     

Link between the North and South Atlantic during the Heinrich events of the last glacial period

High resolution benthic oxygen isotope records combined with radiocarbon datings, from cores retrieved in the North, Equatorial, and South Atlantic are used to establish a reliable cronostratigraphy for the last 60 ky. This common temporal framework enables us to study the timing of the sub-Milankovitch climate variability in the entire surface Atlantic during this period, as reflected in planktonic oxygen isotope records. Variations in sea surface temperatures in the Equatorial and South Atlantic reveal two warm periods during the mid-stage 3 which are correlated to the warming observed in the North Atlantic after Heinrich events (HL) 5 and 4. However, the records show that the warming started about 1500 y earlier in the South Atlantic. A zonally averaged ocean circulation model simulates a similar north-south thermal antiphasing between the latitudes of our coring sites, when pertubated by a freshwater flux anomaly. We infer that the observed phase relationship between the northern and the southern Atlantic is related to periods of reduced NADW production in the North Atlantic, such as during HL5 and HL4. Received: 24 November 1998 / Accepted: 16 May 1999     

Glacial termination: sensitivity to orbital and CO2 forcing in a coupled climate system model

 To study glacial termination and related feedback mechanisms, a continental ice dynamics model is globally and asynchronously coupled to a physical climate (atmosphere-ocean-sea ice) model. The model performs well under present-day, 11 kaBP (thousand years before present) and 21 kaBP perpetual forcing. To address the ice-sheet response under the effects of both perpetual orbital and CO₂ forcing, sensitivity experiments are conducted with two different orbital configurations (11 kaBP and 21 kaBP) and two different atmospheric CO₂ concentrations (200 ppmv and 280 ppmv). This study reveals that, although both orbital and CO₂ forcing have an impact on ice-sheet maintenance and deglacial processes, and although neither acting alone is sufficient to lead to complete deglaciation, orbital forcing seems to be more important. The CO₂ forcing has a large impact on climate, not uniformly or zonally over the globe, but concentrated over the continents adjacent to the North Atlantic. The effect of increased CO₂ (from 200 ppmv to 280 ppmv) on surface air temperature has its peak there in winter associated with a reduction in sea-ice extent in the northern North Atlantic. These changes are accompanied by an enhancement in the intensity of the meridional overturning and poleward ocean heat transport in the North Atlantic. On the other hand, the effect of orbital forcing (from 21 kaBP to 11 kaBP) has its peak in summer. Since the summer temperature, rather than winter temperature, is found to be dominant for the ice-sheet mass balance, orbital forcing has a larger effect than CO₂ forcing in deglaciation. Warm winter sea surface temperature arising from increased CO₂ during the deglaciation contributes to ice-sheet nourishment (negative feedback for ice-sheet retreat) through slightly enhanced precipitation. However, the precipitation effect is totally overwhelmed by the temperature effect. Our results suggest that the last deglaciation was initiated through increasing summer insolation with CO₂ providing a powerful feedback. Received: 22 February 2000 / Accepted: 17 September 2000     

Humidity Effect and Its Influence on the Seasonal Distribution of Precipitation δ18O in Monsoon Regions

The humidity effect, namely the markedly positive correlation between the stable isotopic ratio in precipitation and the dew-point deficit △Td in the atmosphere, is put forward firstly and the relationships between the δ18O in precipitation and △Td are analyzed for the Urumqi and Kunming stations, which have completely different climatic characteristics. Although the seasonal variations in δ18O and △Td exhibit differences between the two stations, their humidity effect is notable. The correlation coefficient and its confidence level of the humidity effect are higher than those of the amount effect at Kunming, showing the marked influence of the humidity conditions in the atmosphere on stable isotopes in precipitation.Using a kinetic model for stable isotopic fractionation, and according to the seasonal distribution of meanmonthly temperature at 500 hPa at Kunming, the variations of the δ18O in condensate in cloud aresimulated. A very good agreement between the seasonal variations of the simulated mean δ18O and themean monthly temperature at 500 hPa is obtained, showing that the oxygen stable isotope in condensateof cloud experiences a temperature effect. Such a result is markedly different from the amount effect atthe ground. Based on the simulations of seasonal variations of δ18O in falling raindrops, it can be foundthat, in the dry season from November to April, the increasing trend with falling distance of δ18O in fallingraindrops corresponds remarkably to the great ATd, showing a strong evaporation enrichment function infalling raindrops; however, in the wet season from May to October, the δ18O in falling raindrops displaysan unapparent increase corresponding to the small ATd, except in May. By comparing the simulated meanδ18O at the ground with the actual monthly δ18O in precipitation, we see distinctly that the two monthlyδ18O variations agree very well. On average, the δ18O values are relatively lower because of the highlymoist air, heavy rainfall, small △Td and weak evaporation enrichment function of stable isotopes in thefalling raindrops, under the influence of vapor from the oceans; but they are relatively higher because of the dry air, light rainfall, great △Td and strong evaporation enrichment function in falling raindrops, under the control of the continental air mass. Therefore, the δ18O in precipitation at Kunming can be used to indicate the humidity situation in the atmosphere to a certain degree, and thus indicate the intensity of the precipitation and the strength of the monsoon indirectly. The humidity effect changes not only the magnitude of the stable isotopic ratio in precipitation but also its seasonal distribution due to its influence on the strength of the evaporation enrichment of stable isotopes in falling raindrops and the direction of the net mass transfer of stable isotopes between the atmosphere and the raindrops. Consequently, it is inferred that the humidity effect is probably one of the foremost causes generating the amount effect.     

The two-step shape and timing of the last deglaciation in Antarctica

The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record.     

Influence of Low-frequency Solar Forcing on the East Asian Winter Monsoon Based on HadCM3 and Observations

In this study, we investigate the influence of low-frequency solar forcing on the East Asian winter monsoon(EAWM)by analyzing a four-member ensemble of 600-year simulations performed with Had CM3(Hadley Centre Coupled Model,version 3). We find that the EAWM is strengthened when total solar irradiance(TSI) increases on the multidecadal time scale. The model results indicate that positive TSI anomalies can result in the weakening of Atlantic meridional overturning circulation, causing negative sea surface temperature(SST) anomalies in the North Atlantic. Especially for the subtropical North Atlantic, the negative SST anomalies can excite an anomalous Rossby wave train that moves from the subtropical North Atlantic to the Greenland Sea and finally to Siberia. In this process, the positive sea-ice feedback over the Greenland Sea further enhances the Rossby wave. The wave train can reach the Siberian region, and strengthen the Siberian high. As a result, low-level East Asian winter circulation is strengthened and the surface air temperature in East Asia decreases. Overall,when solar forcing is stronger on the multidecadal time scale, the EAWM is typically stronger than normal. Finally, a similar linkage can be observed between the EAWM and solar forcing during the period 1850–1970.     

Tropical cooling and the isotopic composition of precipitation in general circulation model simulations of the ice age climate

 We test the climate effects of changes in the tropical ocean by imposing three different patterns of tropical SSTs in ice age general circulation model simulations that include water source tracers and water isotope tracers. The continental air temperature and hydrological cycle response in these simulations is substantial and should be directly comparable to the paleoclimatic record. With tropical cooling imposed, there is a strong temperature response in mid- to high-latitudes resulting from changes in sea ice and disturbance of the planetary waves; the results suggest that tropical/subtropical ocean cooling leads to significant dynamical and radiative feedbacks that might amplify ice age cycles. The isotopes in precipitation generally follow the temperature response at higher latitudes, but regional δ¹⁸O/air temperature scaling factors differ greatly among the experiments. In low-latitudes, continental surface temperatures decrease congruently with the adjacent SSTs in the cooling experiments. Assuming CLIMAP SSTs, ¹⁸O/¹⁶O ratios in low-latitude precipitation show no change from modern values. However, the experiments with additional cooling of SSTs produce much lower tropical continental δ¹⁸O values, and these low values result primarily from an enhanced recycling of continental moisture (as marine evaporation is reduced). The water isotopes are especially sensitive to continental aridity, suggesting that they represent an effective tracer of the extent of tropical cooling and drying. Only one of the tropical cooling simulations produces generalized low-latitude aridity. These results demonstrate that the geographic pattern of cooling is most critical for promoting much drier continents, and they underscore the need for accurate reconstructions of SST gradients in the ice age ocean. Received: 26 July 1999 / Accepted: 10 July 2000     

Decadal‐to‐interdecadal fluctuations of arctic sea‐ice cover and the atmospheric circulation during 1954–1994

Abstract

The relationship between the Arctic and subarctic sea‐ice concentration (SIC) anomalies, particularly those associated with the decadal‐scale Greenland and Labrador Seas “Ice and Salinity Anomalies (ISAs) “, and the overlying atmospheric circulation fluctuations is investigated using the singular value decomposition (SVD) and composite map analysis methods. The data analyzed are monthly SIC and sea level pressure (SLP) anomalies, which cover the northern hemisphere poleward of 45°N and extend over the 41‐year period 1954–1994.

The SVD₁ (first) mode of the coupled variability, which accounts for 57% of the square covariance, is for the most part an atmosphere‐to‐ice forcing mode characterized by the decadal timescale. The aforementioned ISA anomalies are clearly captured by this mode whose SIC anomalies are dominated by a strong dipole across Greenland. However, as part of the same mode, there is also a weaker SIC dipole in the northern North Pacific which has opposite‐signed anomalies in the Sea of Okhotsk and the Bering Sea. It is also shown that there exists a significant negative correlation between the decadal SIC variability in the Greenland‐Barents Seas region associated with this mode and the North Atlantic Oscillation, whose spectrum also exhibits a quasi‐decadal signal.

The SVD₂ mode accounts for 12% of the square covariance and shows no evidence of a dominant forcing field of either SIC or SLP. This SVD mode exhibits very low frequency (interdecadal) variability, and its co‐variability is mainly concentrated in the northern North Pacific. It appears to be a high‐latitude extension of the recently investigated interdecadal North Pacific Oscillation. The spatial structure of the second mode complements the case of the first SVD mode whose co‐variability mainly occurs in the northern North Atlantic.     

Closure of the Panama Seaway during the Pliocene: implications for climate and Northern Hemisphere glaciation

The “Panama Hypothesis” states that the gradual closure of the Panama Seaway, between 13 million years ago (13 Ma) and 2.6 Ma, led to decreased mixing of Atlantic and Pacific water Masses, the formation of North Atlantic Deep water and strengthening of the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere (NH) high latitudes, culminating in the intensification of Northern Hemisphere Glaciation (NHG) during the Pliocene, 3.2–2.7 Ma. Here we test this hypothesis using a fully coupled, fully dynamic ocean-atmosphere general circulation model (GCM) with boundary conditions specific to the Pliocene, and a high resolution dynamic ice sheet model. We carry out two GCM simulations with “closed” and “open” Panama Seaways, and use the simulated climatologies to force the ice sheet model. We find that the models support the “Panama Hypothesis” in as much as the closure of the seaway results in a more intense Atlantic thermohaline circulation, enhanced precipitation over Greenland and North America, and ultimately larger ice sheets. However, the volume difference between the ice sheets in the “closed” and “open” configurations is small, equivalent to about 5 cm of sea level. We conclude that although the closure of the Panama Seaway may have slightly enhanced or advanced the onset of NHG, it was not a major forcing mechanism. Future work must fully couple the ice sheet model and GCM, and investigate the role of orbital and CO₂ effects in controlling NHG.     

A shift of the NAO and increasing storm track activity over Europe due to anthropogenic greenhouse gas forcing

 In accordance with a number of other general circulation model experiments, the coupled atmosphere-ocean-GCM ECHAM4+OPYC3 simulates increasing upper air storm track activity over the east Atlantic and Western Europe with rising greenhouse gas forcing. This paper addresses the question to what extent this change is attributable to the variable north Atlantic Oscillation (NAO), which is closely related to the intensity of the Atlantic storm track’s extension into Europe. The NAO index, which is based on sea level pressure fluctuations over the north Atlantic in the 300-y control run of this model, only shows a moderate increase within the 240-y scenario run, so that its long-term trend does not exceed the variability of the control climate before the end of the simulation. In contrast, the steadily growing storm track activity over northwestern Europe already surpasses the standard deviation defined from the control run after about 160 y. This effect is associated with a change of the NAO pattern. A determination of the centres of action for subsequent 10-y periods based on empirical orthogonal functions shows a systematic northeastward shift of the NAO’s northern variability centre from a position close to the east coast of Greenland, where it is also located in the control run, to the Norwegian Sea. Received: 10 September / Accepted: 15 January 1999     

CO2 and Northern Hemisphere ice volume variations over the middle and late Quaternary

 The atmospheric CO₂ concentrations have been reconstructed over the past 600 ka based on regression between the Vostok CO₂ data and the SPECMAP oxygen isotope values. A lag of 4.5 ka (CO₂ preceding δ¹⁸O) gives the best results. A polynomial of order 5 explains 66% of the Vostok CO₂ variance over the last 220 ka. The Northern Hemisphere ice-sheet volume was simulated over the past 575 ka using the LLN 2-D model, forced by insolation and these statistically reconstructed atmospheric CO₂ concentrations. The simulated ice volume fluctuations resemble the deep-sea oxygen isotope variations. CO₂ of interglacial level is necessary for explaining both the interglacial at oxygen isotopic stage 11 and our present-day interglacial.