G. van der Schrier

Summer Moisture Variability across Europe

Abstract Maps of monthly self-calibrating Palmer Drought Severity Index (SC-PDSI) have been calculated for the period of 1901–2002 for Europe (35°–70°N, 10°W–60°E) with a spatial resolution of 0.5° × 0.5°. The recently introduced SC-PDSI is a convenient means of describing the spatial and temporal variability of moisture availability and is based on the more common Palmer Drought Severity Index. The SC-PDSI improves upon the PDSI by maintaining consistent behavior of the index over diverse climatological regions. This makes spatial comparisons of SC-PDSI values on continental scales more meaningful. Over the region as a whole, the mid-1940s to early 1950s stand out as a persistent and exceptionally dry period, whereas the mid-1910s and late 1970s to early 1980s were very wet. The driest and wettest summers on record, in terms of the amplitude of the index averaged over Europe, were 1947 and 1915, respectively, while the years 1921 and 1981 saw over 11% and over 7% of Europe suffering from extreme dry or w...

Dansgaard–Oeschger events: bifurcation points in the climate system

Dansgaard-Oeschger events are a prominent mode of variability in the records of the last glacial cycle. Various prototype models have been proposed to explain these rapid climate fluctuations, and no agreement has emerged on which may be the more correct for describing the paleoclimatic signal. In this work, we assess the bimodality of the system reconstructing the topology of the multi--dimensional attractor over which the climate system evolves. We use high-resolution ice core isotope data to investigate the statistical properties of the climate fluctuations in the period before the onset of the abrupt change. We show that Dansgaard-Oeschger events have weak early warning signals if the ensemble of events is considered. We find that the statistics are consistent with the switches between two different climate equilibrium states in response to a changing external forcing (e.g. solar, ice sheets...), either forcing directly the transition or pacing it through stochastic resonance. These findings are most consistent with a model that associates Dansgaard-Oeschger with changing boundary conditions, and with the presence of a bifurcation point.

Sea-level variability in the northwest Atlantic during the past 1500 years: A delayed response to solar forcing?

[1] Numerical experiments with a coupled oceanatmosphere model (ECBilt) have shown that centennial variations in sea level (SL) in the northwest Atlantic may be associated with deep-ocean salinity anomalies generated by solar-forced variations in the North Atlantic overturning circulation. Here we compare simulated SL curves for the Gulf Stream region with reconstructed, late-Holocene SL records from Connecticut (USA). Simulated SL variations lag the solar forcing record by ca. 120 year. This lag is found to be robust over a small number of different experiments. The reconstructed SL curves visually match the solar forcing optimally when lagging it by ca. 125 yr. A quantitative test shows that the correlation is significant, while this result is not sensitive to dating uncertainties. The temporal response pattern of the simulated SL curves compares reasonably well with the reconstructions. INDEX TERMS: 4556 Oceanography: Physical: Sea level variations; 1650 Global Change: Solar variability; 4255 Oceanography: General: Numerical modeling; 4267 Oceanography: General: Paleoceanography; 3030 Marine Geology and Geophysics: Micropaleontology. Citation: van de Plassche, O., G. van der Schrier, S. L. Weber, W. R. Gehrels, and A. J. Wright, Sea-level variability in the northwest Atlantic during the past 1500 years: A delayed response to solar forcing?, Geophys. Res. Lett., 30(18), 1921, doi:10.1029/ 2003GL017558, 2003.

The effects of urbanization on the rise of the European temperature since 1960

The effects of urbanization on the rise of the European daily mean temperature is quantified by comparing European-averaged temperatures based on all meteorological stations in the European Climate Assessment and Dataset with those based on three subsets of stations: from rural areas, from areas with low growth in urbanization, and from areas characterized by relatively low-temperature increase. Land cover information is obtained using the CORINE (Coordination of Information on the Environment) data set, showing that most stations (75%) have a small percentage (up to 10%) of urban area within a 10 km radius and 81% saw no more than 1% change in urbanization between 1990 and 2006. The results show that urbanization explains 0.0026°C/decade of the annual-averaged pan-European temperature trend of 0.179°C/decade. This trend has a strong seasonality, being the largest in summer. Averaged over time, the effects of urbanization on the European-averaged temperature has a strong seasonality as well.

Exploring an ensemble approach to estimating skill in multiproxy palaeoclimate reconstructions

This paper describes an approach that gives an estimate of the reconstructive skill of a proxy-based palaeoclimatological reconstruction. Uncertainties in proxy data are likely to be reflected in variations in reconstructive skill of the proxy-based climatic reconstructions. The method is based on making an ensemble of reconstructions, providing a probability distribution of each reconstruction estimate. The relative breadth of the distribution for a particular reconstructed value should give an indication of the reconstructive skill. The ensemble reconstruction approach draws on the ensemble prediction system as used for operational weather forecasting. The ensemble reconstruction approach is tested using a recent multiproxy reconstruction of the winter North Atlantic Oscillation (NAO) index. It is shown that the ensemble approach provides a representation of the degree of certainty associated with the reconstructed NAO-index values.

Trends in high-precipitation events in Jakarta made the flooding of January 2014 more likely

The January 2014 floods paralyzed nearly all of Jakarta, Indonesia. The precipitation events that lead to these floods were not very unusual but show positive trends in the observed record.

Future equivalent of 2010 Russian heatwave intensified by weakening soil moisture constraints

The 2010 heatwave in eastern Europe and Russia ranks among the hottest events ever recorded in the region1,2. The excessive summer warmth was related to an anomalously widespread and intense quasi-stationary anticyclonic circulation anomaly over western Russia, reinforced by depletion of spring soil moisture1,3–5. At present, high soil moisture levels and strong surface evaporation generally tend to cap maximum summer temperatures6–8, but these constraints may weaken under future warming9,10. Here, we use a data assimilation technique in which future climate model simulations are nudged to realistically represent the persistence and strength of the 2010 blocked atmospheric flow. In the future, synoptically driven extreme warming under favourable large-scale atmospheric conditions will no longer be suppressed by abundant soil moisture, leading to a disproportional intensification of future heatwaves. This implies that future mid-latitude heatwaves analogous to the 2010 event will become even more extreme than previously thought, with temperature extremes increasing by 8.4 °C over western Russia. Thus, the socioeconomic impacts of future heatwaves will probably be amplified beyond current estimates.The 2010 Russia heatwave had devastating impacts, including loss of life, wildfire and drought. Model simulations reveal similar heatwaves may be amplified by up to 8 °C in the future as soil moisture becomes less able to suppress maximum temperatures.

Simulating the extreme 2013/2014 winter in a future climate

How would the spell of extreme weather observed over North America and western Europe during the 2013/2014 winter manifest itself in a warmer climate? Here a forced sensitivity method is used to calculate optimal model tendency perturbations which result in a simulation which has its upper atmospheric circulation shifted in the direction of the January 2014 jet stream pattern. When applied to a simulation run under present-day conditions the main features of the observations of this event are reproduced, such as the more zonal position and increased strength of the midlatitude North Atlantic storm track, the anomalous temperature pattern over North America, and the excessive precipitation in parts of Europe. When this method is applied to a future warmer climate, the North Atlantic storm track changes to a more zonal orientation, but its strength does not see the significant increase that is obtained in the present-day simulations. Despite southward advection of polar air into northeastern America, which has also occurred during the 2013/2014 winter, the associated drop in temperature is less in the future climate compared to the present-day climate because of Arctic amplification. The less steep drop in temperatures over northeastern America leads to a smaller land-sea temperature contrast, less baroclinic instability, and a reduced increase of midlatitude storminess. While one may expect the future increased atmospheric moisture content to augment the net precipitation, for this specific event, this effect is counterbalanced by the dynamics, leading to a similar net increase in precipitation in the future simulations compared to the present day.

The 2010 Pakistan floods in a future climate

The summer 2010 floods hitting Pakistan were the severest on record. Coinciding with these events was the 2010 heatwave over eastern Europe and Russia, which also ranks among the severest ever recorded in the region. Both events were related to an anomalously widespread and intense quasi-stationary anticyclonic circulation anomaly over western Russia which provided favourable conditions, in combination with monsoonal forcing factors, for the Pakistan precipitation events. Here, a data assimilation technique is used which results in a climate model simulation which has its mean upper atmospheric circulation shifted in the direction of the anomalous anticyclonic circulation of summer 2010. This primes the climate model to reproduce, much more frequently than in a climate simulation without this technique, to simulate the conditions which led to the Pakistan 2010 floodings. These experiments are conducted under both present-day and future climatic conditions. In the present-day climate, the main features of the 2010 Pakistan precipitation events are modeled realistically, although the amplitude of the extreme precipitation is underestimated. The simulated future equivalent of the observed extreme precipitation events shows a stronger precipitation over the Bay of Bengal to Kashmir in northern India and northern Pakistan, and from the Arabian Sea to northern Pakistan. In the model context, these precipitation increases are substantial with 50–100% increases in rainfall rates. This implies that the future equivalent of the 2010 Pakistan floodings may have even stronger socio-economic impacts.