The winter of 2009/2010 was characterized by record persistence of the negative phase of the North‐Atlantic Oscillation (NAO) which caused several severe cold spells over Northern and Western Europe. ...This somehow unusual winter with respect to the most recent ones arose concurrently with public debate on climate change, during and after the Copenhagen climate negotiations. We show however that the cold European temperature anomaly of winter 2010 was (i) not extreme relative to winters of the past six decades, and (ii) warmer than expected from its record‐breaking seasonal circulation indices such as NAO or blocking frequency. Daily flow‐analogues of winter 2010, taken in past winters, were associated with much colder temperatures. The winter 2010 thus provides a consistent picture of a regional cold event mitigated by long‐term climate warming.
The skills of isotope‐enabled general circulation models are evaluated against atmospheric water vapor isotopes. We have combined in situ observations of surface water vapor isotopes spanning ...multiple field seasons (2010, 2011, and 2012) from the top of the Greenland Ice Sheet (NEEM site: 77.45°N, 51.05°W, 2484 m above sea level) with observations from the marine boundary layer of the North Atlantic and Arctic Ocean (Bermuda Islands 32.26°N, 64.88°W, year: 2012; south coast of Iceland 63.83°N, 21.47°W, year: 2012; South Greenland 61.21°N, 47.17°W, year: 2012; Svalbard 78.92°N, 11.92°E, year: 2014). This allows us to benchmark the ability to simulate the daily water vapor isotope variations from five different simulations using isotope‐enabled general circulation models. Our model‐data comparison documents clear isotope biases both on top of the Greenland Ice Sheet (1–11‰ for δ18O and 4–19‰ for d‐excess depending on model and season) and in the marine boundary layer (maximum differences for the following: Bermuda δ18O = ~1‰, d‐excess = ~3‰; South coast of Iceland δ18O = ~2‰, d‐excess = ~ 5‰; South Greenland δ18O = ~4‰, d‐excess = ~7‰; Svalbard δ18O = ~2‰, d‐excess = ~7‰). We find that the simulated isotope biases are not just explained by simulated biases in temperature and humidity. Instead, we argue that these isotope biases are related to a poor simulation of the spatial structure of the marine boundary layer water vapor isotopic composition. Furthermore, we specifically show that the marine boundary layer water vapor isotopes of the Baffin Bay region show strong influence on the water vapor isotopes at the NEEM deep ice core‐drilling site in northwest Greenland. Our evaluation of the simulations using isotope‐enabled general circulation models also documents wide intermodel spatial variability in the Arctic. This stresses the importance of a coordinated water vapor isotope‐monitoring network in order to discriminate amongst these model behaviors.
Key Points
We present a metric for comparing model simulations with in situ atmospheric water vapor isotope observations
Model artifacts for temperature and humidity cannot simply explain isotope biases on top of the Greenland Ice Sheet
All models show inability to correctly simulate the spatial structure of marine boundary layer isotopic composition
The isotopic composition of near surface (or planetary boundary layer) water vapor on the south coast of Iceland (63.83°N, 21.47°W) has been monitored in situ between November 2011 and April 2013. ...The calibrated data set documents seasonal variations in the relationship between δ18O and local humidity (ppmv) and between deuterium excess and δ18O. These seasonal variations are attributed to seasonal changes in atmospheric transport. A strong linear relationship is observed between deuterium excess and atmospheric relative humidity calculated at regional sea surface temperature. Surprisingly, we find a similar relationship between deuterium excess and relative humidity as observed in the Bermuda Islands. During days with low amount of isotopic depletion (more enriched values), our data significantly deviate from the global meteoric water line. This feature can be explained by a supply of an evaporative flux into the planetary boundary layer above the ocean, which we show using a 1‐d box model. Based on the close relationship identified between moisture origin and deuterium excess, we combine deuterium excess measurements performed in Iceland and south Greenland with moisture source diagnostics based on back trajectory calculations to establish the distribution of d‐excess moisture uptake values across the North Atlantic. We map high deuterium excess in the Arctic and low deuterium excess for vapor in the subtropics and midlatitudes. This confirms the role of North Atlantic water vapor isotopes as moisture origin tracers.
Key Points
Year‐round monitoring of marine boundary layer water vapor isotopic composition
Intraseasonal to interseasonal variability in North Atlantic water vapor isotopes
Attribution of temporal and spatial variability in water vapor deuterium excess
Since September 2011, a wavelength-scanned cavity ring-down spectroscopy analyser has been remotely operated in Ivittuut, southern Greenland, providing the first record of surface water vapour ...isotopic composition based on continuous measurements in South Greenland and the first record including the winter season in Greenland. The comparison of vapour data with measurements of precipitation isotopic composition suggest an equilibrium between surface vapour and precipitation. δ18O and deuterium excess are generally anti-correlated and show important seasonal variations, with respective amplitudes of ~10 and ~20‰, as well as large synoptic variations. The data depict small summer diurnal variations. At the seasonal scale, δ18O has a minimum in November–December and a maximum in June–July, while deuterium excess has a minimum in May–June and a maximum in November. The approach of low-pressure systems towards South Greenland leads to δ18O increase (typically +5‰) and deuterium excess decrease (typically −15‰). Seasonal and synoptic variations coincide with shifts in the moisture sources, estimated using a quantitative moisture source diagnostic based on a Lagrangian back-trajectory model. The atmospheric general circulation model LMDZiso correctly captures the seasonal and synoptic variability of δ18O, but does not capture the observed magnitude of deuterium excess variability. Covariations of water vapour isotopic composition with local and moisture source meteorological parameters have been evaluated. δ18O is strongly correlated with the logarithm of local surface humidity, consistent with Rayleigh distillation processes, and with local surface air temperature, associated with a slope of ~0.4‰ °C−1. Deuterium excess correlates with local surface relative humidity as well as surface relative humidity from the dominant moisture source area located in the North Atlantic, south of Greenland and Iceland.
A new Lagrangian moisture source diagnostic is applied to identify the atmospheric conditions relevant for the fractionation of stable water isotopes during evaporation over the ocean and subsequent ...transport to Greenland. Northern Hemisphere winter months with positive and negative North Atlantic Oscillation (NAO) index are studied on the basis of ERA‐40 reanalysis data. Diagnosed moisture transport conditions are supplied to a Rayleigh‐type isotope fractionation model to derive estimates for the isotopic composition of stable isotopes in winter precipitation on the Greenland plateau for the two NAO phases. Because of changes in atmospheric circulation, moisture source locations for precipitation in Greenland vary strongly for different phases of the NAO. The mean source SST is ∼5.0 K warmer during negative NAO months compared to the positive phase. This signal is considerably stronger than what would result from interannual SST variability at a spatially fixed moisture source. Furthermore, moisture transport takes place at warmer temperatures during NAO negative conditions. Simulated average isotopic depletion of Greenland precipitation is less negative by 3.8 ± 6.8‰ for δ18O during the negative compared to the positive NAO phase. Comparison with ice core data from central Greenland for three winters shows good agreement between observed and simulated variability. The synoptic interplay of the initial conditions at the moisture sources and of the atmospheric transport conditions leads to enhanced NAO‐related interannual variability of stable isotopes. This could be important for understanding rapid shifts in stable isotopes during past climates. The isotope modeling applied here, however, considerably underestimates the absolute level of isotopic depletion. The offset is attributed to approximations in the model and uncertainties in the comparison with observational data. The high spatial resolution of the Lagrangian method reveals the nonhomogeneous structure of isotope NAO variability over the Greenland ice sheet. The results are therefore potentially useful for selecting new ice core drilling sites with maximum NAO variability.
The causes of the recent increase in Antarctic sea ice extent, characterised by large regional contrasts and decadal variations, remain unclear. In the Ross Sea, where such a sea ice increase is ...reported, 50% of the sea ice is produced within wind-sustained latent-heat polynyas. Combining information from marine diatom records and sea salt sodium and water isotope ice core records, we here document contrasting patterns in sea ice variations between coastal and open sea areas in Western Ross Sea over the current interglacial period. Since about 3600 years before present, an increase in the efficiency of regional latent-heat polynyas resulted in more coastal sea ice, while sea ice extent decreased overall. These past changes coincide with remarkable optima or minima in the abundances of penguins, silverfish and seal remains, confirming the high sensitivity of marine ecosystems to environmental and especially coastal sea ice conditions.
Choosing the future of Antarctica Rintoul, S R; Chown, S L; DeConto, R M ...
Nature (London),
06/2018, Letnik:
558, Številka:
7709
Journal Article
Recenzirano
Odprti dostop
We present two narratives on the future of Antarctica and the Southern Ocean, from the perspective of an observer looking back from 2070. In the first scenario, greenhouse gas emissions remained ...unchecked, the climate continued to warm, and the policy response was ineffective; this had large ramifications in Antarctica and the Southern Ocean, with worldwide impacts. In the second scenario, ambitious action was taken to limit greenhouse gas emissions and to establish policies that reduced anthropogenic pressure on the environment, slowing the rate of change in Antarctica. Choices made in the next decade will determine what trajectory is realized.
A program of individual precipitation events and river water sampling and of water isotopic measurements (δD, δ18O) was carried out during summer 1996 along a northeast/southwest transect of the ...Tibetan Plateau. The spatial distribution of both δ18O and deuterium excess (d = δD‐8*δ18O) of the precipitation reveals three distinct regions. Simulations with a simple isotopic model and seasonal isotopic variations measured at two extreme south and north locations support our interpretation in terms of different summer moisture origins: (1) South of the Himalayan mountains, the moisture provided by the Indian monsoon has been recycled over the Indian peninsula. (2) Between the Himalayas and the Tanggula mountains the oceanic moisture is directly transported from the Bay of Bengal along the Brahmaptra River valley. (3) North of the Tanggula mountains, the moisture is not provided by the monsoon anymore but by continental water recycling.
The conventional interpretation of ice core deuterium and oxygen 18 isotope profiles based on the use of present‐day observations (spatial slope) underestimates glacial‐interglacial surface ...temperature changes in Central Greenland by up to a factor of two. This likely results from changes in the seasonality of the precipitation due to the particular location of the Greenland ice sheet next to the highly variable northern polar front. In this regard the situation is much simpler for central Antarctica and this should be reflected in the temperature interpretation of ice core isotopic records. With this in mind, we closely examine all relevant information, focusing on the East Antarctic Plateau where both model and empirical isotope‐temperature estimates are available. We point to the fact that correctly accounting for the influence of ocean isotopic change is important when interpreting deuterium profiles from ice cores in this region. The evidence presently available indicates that, unlike for Greenland, the present‐day spatial‐slope can probably be taken as a surrogate of the temporal slope to interpret glacial‐interglacial isotopic changes at sites such as Vostok and EPICA Dome C. Corresponding temperature changes are within −10% to +30% of those obtained from the conventional interpretation based on the use of the spatial slope.
A high-resolution deuterium profile is now available along the entire European Project for Ice Coring in Antarctica Dome C ice core, extending this climate record back to marine isotope stage 20.2, ...∼800,000 years ago. Experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation. We assessed the general correspondence between Dansgaard-Oeschger events and their smoothed Antarctic counterparts for this Dome C record, which reveals the presence of such features with similar amplitudes during previous glacial periods. We suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records.
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