Decadal variability in the midlatitudes has been investigated with ECBILT1. We have focused on decadal variability in the North Atlantic and in the Southern ocean. In those regions the dominant patterns of covariability between sea surface temperature and 800 hPa geopotential height and their preferred time scales show good qualitative agreement with observations. The physical mechanisms of the simulated decadal variability have been investigated with a number of idealised experiments in which various feedback processes have been switched on and off. This analysis revealed that the midlatitude decadal variability is strongly associated with subsurface oceanic modes. These modes are generated by dominant, internal modes of the atmospheric circulation. The ocean sets the time scale. The feedback from the ocean to the atmosphere is not crucial for either the structure or the time scale of these decadal modes. This feedback, however, enhances the intensity of the decadal variability. The generation of the dominant time scale in the subsurface oceanic mode is different for the North Atlantic decadal mode and the Antarctic Circumpolar Wave (ACW). For the ACW the preferred time scale appears to be generated by the advective resonance mechanism. In this mechanism the ocean temperature can be approximated by a passive tracer, which is advected by an ocean with a fixed velocity. The preferred time scale is set by the advection velocity of the ocean and the horizontal scale of the atmospheric mode. For the decadal variability in the North Atlantic the dynamical response of the ocean circulation to temperature anomalies and related anomalous salt advection appears to be crucial for the generation of the decadal time scale.

With ECBILT1 we have investigated the effect of variable solar forcing on climate variability. For realistic amplitudes solar forcing dominates over internal variability in global mean surface air temperature beyond decadal time scales. On the regional scale the internal variability dominates on all time scales.

With ECBILT1 a 40,000 year integration has been performed to study internal low-frequency behaviour. The model displays quasi-periodical behaviour on a time scale of about 10,000 years, characterised by rapid switches between two quasi-equilibrium solutions on decadal time scale. The mechanism is similar to the deep-decoupling oscillations observed in ocean models with mixed boundary conditions. The effect of sea-ice on the heat and freshwater fluxes appears to be crucial to generate this type of oscillation in a coupled atmosphere/ocean/sea-ice model. This experiment suggests the possibility of rapid transitions in the climate system.

In collaboration with Dr. Hugues Goose of the University of Louvain-La-Neuve decadal variability over the Arctic ocean has been investigated with ECBILT2/CLIO. A decadal oscillation in the Arctic ocean with a period of about 18 year is simulated. The geographical pattern and time scale are supported by the available observational evidence.

With ECBILT2/CLIO, an ensemble of integrations has been performed for future concentrations of trace gases in the atmosphere up to the year 2100. The response of surface air temperature to the increasing concentrations of green house gases qualitatively resembles the results of similar runs performed with more comprehensive climate models. The simulated increase of surface air temperature falls in the lower range of published estimates. The thermohaline circulation decreases, the sea-ice cover in the Greenland-Iceland-Norwegian sea increases. The ensemble of integrations is being analysed to estimate changes in the occurrence of extreme events due to the change in the mean climate.

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Greet de Graaf / Brigitta Kamphuis

Updated on July 10, 2002