Retrieving a common accumulation record from Greenland ice cores for the past 1800 years

Abstract. In the accumulation zone of the Greenland ice sheet the annual accumulation rate may be determined through identification of the annual cy-cle in the isotopic climate signal and other parameters that exhibit seasonal vari-ations. On an annual basis the accumulation rate in different Greenland ice cores is highly variable, and the degree of correlation between accumulation series from different ice cores is low. However, when using multi year averages of the dif-ferent accumulation records the correlation increases significantly. A statistical model has been developed to estimate the common climate signal in the differ-ent accumulation records through optimization of the ratio between the variance of the common signal and of the residual. Using this model a common Green-land accumulation record with five years resolution for the past 1800 years has been extracted. The record establishes a climatic record which implies that very dry conditions during the 13th century together with dry and cold spells dur-ing the 14th century may have put extra strain on the Norse population in Green-land and have contributed to their extinction

Tipping points:Early warning and wishful thinking

[1] The causes for and possible predictions of rapid climate changes are poorly understood. The most pronounced changes observed, beside the glacial terminations, are the Dansgaard‐Oeschger events. Present day general circulation climate models simulating glacial conditions are not capable of reproducing these rapid shifts. It is thus not known if they are due to bifurcations in the structural stability of the cli-mate or if they are induced by stochastic fluctuations. By analyzing a high resolution ice core record we exclude the bifurcation scenario, which strongly suggests that they are noise induced and thus have very limited predictability

A minimum thermodynamic model for the bipolar seesaw

[1] The simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of Antarctic ice cores. The model results from the classic bipolar seesaw by coupling it to a heat reservoir. In this ‘‘thermal bipolar seesaw’ ’ the heat reservoir convolves northern time signals with a characteristic timescale. Applying the model to the data of GRIP and Byrd, we demonstrate that maximum correlation can be obtained using a timescale of about 1000–1500 years. Higher correlations are obtained by first filtering out the long-term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw. The model resolves the apparent confusion whether northern and southern climate records are in or out of phase, synchronous, or time lagged. INDEX TERMS: 1620 Global Change

Attenuation of drill cables

The frequency characteristics of a drill cable determines both the type of communication that can be used between a drill or logger and the surface, as well as the maximum rate of information that can be transferred. In order to revise the design, the attenuation of the cable was measured. The result was far from the characteristics of a coaxial cable. At medium audio frequencies, the attenuation was quite low. However at both low and high frequencies, the attenuation increased significantly, and the high frequency cut off could be approximated by a 3-order low pass filter with a cut off frequency of 8kHz. For comparison, the attenuation of a similar size coaxial type cable will be in order of 1dB/300m at 150kHz increasing to 2dB/300m at 1MHz. If a bandwidth higher than 10kHz is required for a 4km long oceanographic type electromechanical steel armoured cable, this has to be of coaxial construction with a shield separated from the armour