In the Global Climate division, large-scale climate variability is studied, including the effects of climate change. The research activities in the group focus on quantifying decadal to centennial climate variability, studying feedbacks in the earth system, development of global climate scenario’s and development of monthly, seasonal and decadal predictions. The group maintains global climate models of intermediate complexity and develops a state-of-the-art earth system model (EC-EARTH) as research tool and as tool for developing climate scenario’s. The group is internationally recognized in the fields of climate extremes, thermohaline circulation, tropical variability, atmospheric dynamics and land processes.

17-06-2010: On the roles of circulation and aerosols in the decline of mist and dense fog in Europe over the last 30 years

This study only addressed the associations of fog and mist with aerosols, urbanisation and atmospheric circulation. Other factors, such as the availability of moisture due to water table management, the stability of the atmosphere and possible decreased night-time cooling due to greenhouse warming have not yet been addressed. However, as these trends are unlikely to change sign over the next decades, projected further increases in air quality, increases in westerly circulation in winter and maybe drought in summer will likely further decrease the occurrence of fog and mist. (article)

01-06-2010: EC-Earth: a seamless prediction system in action

A publication has been accepted for the Bulletin of the American Meteorological Society (BAMS) describing the EC-Earth model, project and consortium. EC-Earth follows a seamless prediction strategy in which a well tested weather and seasonal forecast model is used for longer time scale predictions and projections. The model is based on the seasonal forecast system of ECMWF. By linking with developments at ECMWF the model stays up to data with the most recent physical parameterizations. The consortium that develops and uses the model consists of 20 partners from 10 countries in Europe. The model provides a platform to the European research community to test new earth system components in a consistent manner. The model appears to perform well on weather time scales (predictions up to a week), interannual time scales (good representation of El Nino) and climate time scales (good representation of mean climate). The model will be used for coordinated model intercomparison projects (CMIP5, CFMIP, PMIP) which are expected to inform the 5th Assessment of IPCC.  

29-04-2010: Arctic sea-ice melt in 2007

During summer 2007 the Arctic sea-ice shrank to the lowest extent ever observed. Unusual atmospheric conditions prevailed in the Arctic that summer, with a persistent high pressure over the Beaufort Sea north of Canada and a low pressure over the northeastern Siberia. These conditions caused atmospheric flow of warm and humid air into the region that suffered severe melt. This led to an increase of the downward longwave radiation at the surface. In the region that experienced unusual ice melt, the enhancement of the surface energy fluxes provided enough extra energy to melt roughly one meter of ice during the melting season. We argue that the positive anomalies of downward longwave radiation and turbulent fluxes at the surface played a key role in initiating the 2007 extreme ice melt.

14-04-2010: What caused Earth’s temperature variations during the last 800,000 years?

Climate fluctuations during the ice ages were caused by changes in insolation, greenhouse gases, ice sheets, atmospheric dust loading, and more. This study addresses the relative importance of these mechanisms and feedbacks on the total glacial-interglacial climate shifts. It strongly suggests that many, and probably all, climate feedbacks are not constant in time, with their strength depending on the climate state. The best estimate, based on paleoclimate data and forcings for the last glacial maximum, for present-day climate sensitivity ranges between 1.4 and 5.2 K, with a best estimate of 2.4 K.

14-04-2010: Response of late Neogene glacial cycles to obliquity forcing

This paper provides constraints for the geometry and time lags of the predominent obliquity-paced glacial stages during the period 2.56-2.40 Myr BP, which marks a major phase in Northern Hemisphere glaciations during the late Pliocene. A high-resolution marine benthic oxygen isotope record was reconstructed from an astronomically-tuned Meditarranean deep ocean core and decomposed into an ice volume and temperature component using an inverse modelling approach. Results suggest that the ~28 kyr fluctuations, which are linked to glacial terminations, are intrinsic to the climate system, and that these thereby govern the geometry of glacial-interglacial variability throughout the Pliocene and Pleistocene.

30-03-2010: Mid-latitude storms and their evolution

Century-long simulations with global circulation models are used to obtain projections for changes in climate, including the effects of stormtracks, but large uncertainties remain. The physical mechanisms behind changes in storm frequency and intensity are not sufficiently well understood from a theoretical perspective to determine whether the results of the simulations are physically reasonable. In a paper that has just been accepted for publication in Journal of the Atmospheric Sciences, H. de Vries et al. 2010 contribute to this problem. They developed a novel theoretical framework to describe the evolution of mid-latitude storms in terms of only a few (Rossby wave) components and their interaction with the background state flow.

29-03-2010: The atmospheric response to a THC collapse

The theory of the tropical Hadley Circulation is extended to describe the response to a collapse of the thermohaline circulation (THC) in the ocean. Drijfhout (2010) contains relations that predict the shift of the Intertropical Convergence Zone, changes in the strength of the Hadley Circulation, changes in its poleward boundary and the latitude of the Subtropical jets. The theory is compared to the response in a coupled climate model. It is argued that a THC collapse and changes in the tropical atmosphere act in concert to explain abrupt climate change.