Climate Modelling Group
School of Earth and Ocean Sciences

Regional Climate Change Detection

Zwiers, together with Dr. Xuebin Zhang of the Climate Monitoring and Data Interpretation Division of MSC, has initiated a study to evaluate the potential for attribution and detection studies on the regional scale. This is motivated by the strong global scale detection results reported in the IPCC TAR, which implicitly suggest that it may now be appropriate to undertake detection on a smaller (e.g., continental) scale, and by a desire to obtain more policy relevant detection results. Our very preliminary results suggest that the greenhouse gas plus sulphate aerosol signal, as simulated by the CCCma model, is detectable in the climates of both North American and Eurasia. There has been discussion in the literature of the extent to which the Northern Hemisphere warming of the past several decades is related to the trend in the NAO. Consequently, we repeated the detection exercise after removing the NAO signal from the surface temperature data, and find that the detection result continues to hold.

Multi-Model Ensembles

To date several models have been separately used to detect the influence of anthropogenic greenhouse gases and sulphate aerosol on climate, each giving somewhat different results for the estimated influence of each forcing and its associated uncertainties. First we wish to synthesize results from different models and secondly we wish to make use of the multi-model ensemble to further constrain uncertainties. We have successfully combined output from five general circulation models (HadCM2, HadCM3, ECHAM3, CGCM1 and CGCM2) to reduce the uncertainties in a joint detection of greenhouse gas and sulphate aerosol over the past fifty years, both of whose influences were clearly detectable in the observations.

The Arctic and Antarctic Oscillations

Over recent decades there has been an observed trend in winter sea level pressure, corresponding to an increase in the Arctic Oscillation and Antarctic Oscillation indices, which has been shown to be inconsistent with natural variability. Most general circulation models show changes of this sign when forced with increases in greenhouse gases. We apply statistical detection techniques, which have previously only been applied to temperature, and detect the influence of greenhouse gases and sulphate aerosol over the past fifty years using CGCM1. These techniques do however indicate that decadal variability in sea level pressure may be too low in this model, suggesting that caution be applied in interpreting these results.

Recent studies suggest that important changes in atmospheric circulation at high southern latitudes will occur under enhanced greenhouse forcing. Dáithí Stone, a Ph.D. student, is currently investigating the mechanisms responsible for and the seasonal nature of these changes.

Daily maximum and minimum temperature trends in the CCC coupled model

The recent observed global warming trend over land has been characterised by a faster warming at night, leading to a considerable decrease in the diurnal temperature range (DTR). Analysis of simulations of a climate model including observed increases in greenhouse gases and sulphate aerosols revealed a similar trend in the DTR of -0.2°C per century, albeit of smaller magnitude than the observed -0.8 °C per century. This trend in the model simulations is related to changes in cloud cover and soil moisture. These results indicate that the observed decrease in the DTR could be a signal of anthropogenic forcing of recent climate change. Published in: Stone, D.A., and A.J. Weaver, 2002: Daily maximum and minimum temperature trends in a climate model. Geophysical Research Letters, in press.

Diurnal temperature range trends in 20th and 21st century simulations of the CCCma coupled model.

Trends in the diurnal temperature range (DTR) were examined in the late 20th and 21st centuries in a coupled climate model representing the atmosphere, ocean, sea ice, and land surface systems. Consistent with past studies, the DTR decreased during this time. These decreases were concentrated in middle latitudes, with much smaller changes occurring in the low latitudes. Strong seasonal characteristics to this pattern existed, although these were different in either hemisphere. In the model integrations, variations in the DTR were much more sensitive to changes in feedbacks than in direct forcing. The DTR was found to be rather insensitive to the scattering of sunlight by sulphate aerosols and the increased mean temperature. Instead, variations in the DTR arose mostly from changes in clouds and in soil moisture. Consequently, the decreases arose from increases in the reflection of solar radiation by clouds moderated by decreases in the ground heat capacity due to decreasing soil moisture. Both factors contributed about equally to the DTR trend. The exception to this relation occurred in the middle latitudes during winter, when snow cover reduced the influence of changes in solar radiation and soil moisture. Decreases during this season were a consequence of the tendency in CGCM1 for the DTR to be very small when the mean temperature is near the freezing point. The importance of soil moisture implies that changes in the physiological response of vegetation and in land use could have important effects on the DTR. Published in: Stone, D.A. and A.J. Weaver, 2002: Diurnal temperature range trends in 20th and 21st century simulations of the CCCma coupled model. Climate Dynamics, submitted.

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