Papers by Aksel Walløe Hansen
EAEJA, Apr 1, 2003
The winter d 18 O signal is extracted from 7 Greenland ice cores covering the past $700 years. To... more The winter d 18 O signal is extracted from 7 Greenland ice cores covering the past $700 years. To filter out noise and local variations in the 7 isotope records a principal component analysis is carried out on the ice core data. A comparison between the time series of the first principal component (PC1) with 67 years of winter (December to March) temperature measurements from 3 southern Greenland synoptic stations shows highly significant correlations. Southern Greenland winter temperatures are known to be greatly influenced by the North Atlantic Oscillation (NAO). A good proxy for southern Greenland temperatures is therefore expected to reveal at least parts of the NAO signal. It is shown that the PC1 time series indeed is significantly correlated to the NAO during the winter months. The inclusion of ice core winter season d 18 O time series in future multiproxy NAO reconstructions is therefore recommended.
Physics and Chemistry of the Earth, Part B: Hydrology, Oceans and Atmosphere, 2001
As part of the Danish NEAREX project a three-dimensional Eulerian hemispheric air pollution model... more As part of the Danish NEAREX project a three-dimensional Eulerian hemispheric air pollution model is used to study the transport and concentrations of atmospheric C02 in the North East Atlantic region. The model domain covers the major part of the Northern Hemisphere and currently the model includes simple parameterizations of the main sinks and sources for atmospheric C02. One of the objectives of the project is to study and maybe quantify the relative importance of the various sinks and source types and areas for this region. In order to do so the model has been run with differentiated source types. Here the model setup and the used parameterizations will be described. The model is validated by comparing the results with atmospheric measurements from four monitoring stations in or close to the northern part of the North Atlantic. Some preliminary model results will be shown and shortly discussed.
Environmental Monitoring and Assessment, Jun 1, 2006
As part of the Danish NEAREX project the origin and variability of anthropogenic atmospheric CO 2... more As part of the Danish NEAREX project the origin and variability of anthropogenic atmospheric CO 2 over the Northeast Atlantic Region (NEAR) has been studied. The project consisted of a combination of experimental and modelling activities. Local volunteers operated CO 2 sampling stations, built at University of Copenhagen, for 14 C analysis at four locations (East Denmark, Shetland Isles, Faroe Isles and Iceland). The samples were only collected during winter periods of south-easterly winds in an attempt to trace air enriched in fossil-fuel derived CO 2 due to combustion of fossil fuels within European countries. In order to study the transport and concentration fields over the region in detail, a three-dimensional Eulerian hemispheric air pollution model has been extended to include the main anthropogenic sources for atmospheric CO 2. During the project period (1998-2001) only a few episodes of transport from Central Europe towards NEAR arose, which makes the data set for the evaluation of the method sparse. The analysed samples indicate that the signal for fossil CO 2 , as expected, is largest (up to 3.7 ± 0.4% fossil CO 2) at the Danish location closest to the European emissions areas and much weaker (up to ∼1.5 ± 0.6% fossil CO 2) at the most remote location. As the anthropogenic signal is weak in the clean atmosphere over NEAR these numbers will, however, be very sensitive to the assumed background 14 CO 2 activity and the precision of the measurements. The model simulations include the interplay between the driving processes from the emission into the boundary layer and the following horizontal/vertical mixing and atmospheric transport and are used to analyse the meteorological conditions leading to the observed events of high fossil CO 2 over NEAR. This information about the history of the air masses is essential if an observed signal is to be utilised for identifying and quantifying sources for fossil CO 2 .
Journal of Geophysical Research: Oceans, 2012
ABSTRACT To monitor the pan-Arctic seasonal freeze-thaw transitions of the land surface and sea i... more ABSTRACT To monitor the pan-Arctic seasonal freeze-thaw transitions of the land surface and sea ice, we analyze daily backscatter data from satellite scatterometry to examine the time series on an annual basis by applying an optimal edge detection scheme, and iterate against an internal median climatology to mitigate unreasonable outliers. By applying this novel algorithm to resolution-enhanced QuikSCAT data from 1999 to 2009, we have mapped a decade of seasonal freeze-thaw transitions across the landmass and sea ice north of 60°N at a spatial resolution better than 5 km. The data set has been validated against surface air temperature measurements and snow depth obtained from a distributed network of weather stations and drift buoys. Most retrieved timings from surface and QuikSCAT measurements agree to less than a week at thaw transition for both land and sea ice and at freeze transition for sea ice, indicating successful retrieval over a range of surface covers. While the spatial pattern of freeze-thaw transition changes substantially from year to year, the interannual variability of the mean transition timing over a particular surface is small.
Journal of Geophysical Research, 1997
A series of idealized ozone perturbations has been studied with the French community general circ... more A series of idealized ozone perturbations has been studied with the French community general circulation model ArpSge. The perturbations are uniform 50% and 75% reductions and an ozone hole type reduction concentrated in the lower part of the stratosphere and the upper part of the troposphere. We compare the radiative drive, the temperature response under the fixed dynamical heating approximation, and the full dynamical response. In all experiments the main effect of the dynamics is • weakening of the di•ba. tic meridion•l circulation •nd •n associated latitudinal smoothing of the temperature response. The weakening of the diabatic meridional circulation is in agreement with a reduction of the wave forcing, associated with an extended tongue of low refractive index close to the tropopause. While the fixed dynamical heating approximation in general represents the pattern of the temperature response well, the strength of the response is altered when allowing dynamical effects. In the lower stratosphere this adjustment is 25%-50% of the fixed dynamical heating response. The mean zonal wind is very stable to uniform reductions, but the strength of the winter stratospheric jet increases drastically in the hole type experiment. The ozone perturbations decrease the variability in the winter hemisphere, which in the control experiment and the uniform reduction experiments is of oscillatory nature with a timescale of 50 to 100 days. 1995]. Of special concern in a climate change context are the well-established downward trends in the stratospheric ozone content and the somewhat more controversial increase in the upper tropospheric ozone content. The reason for this is that the climate is particularly sensitive to ozone changes occurring in the vicinity of the tropopause. A very clear demonstration of this fact can be found in a paper by Lacis et al. [1990] that describes the results of a series of one-dimensional
ABSTRACT The North Atlantic Oscillation (NAO) is known to be associated with specific patterns of... more ABSTRACT The North Atlantic Oscillation (NAO) is known to be associated with specific patterns of northern hemisphere winter temperature and precipitation anomalies. The months of January and February 1984 had a clear positively phased NAO signal. However, compared to the expected NAO temperature anomaly pattern, both months showed patterns which were clearly distorted. The so called NAO temperature seesaw between western Europe and the Greenland west coast area broke down during the period. Extremely cold conditions in western Greenland were associated with below normal temperatures over the British Isles. This situation is found to be unique during the last 135 years. It does nevertheless cause great concern, as all efforts to reconstruct past NAO variability beyond the era of instrumental pressure observations, are dependent on the stability of the NAO temperature anomaly pattern.
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Papers by Aksel Walløe Hansen