final_program_abstracts[1]

11 IMSC Session Program
Spatial characteristics of gridded Swiss temperature trends:
Local and large-scale influences
Friday - Parallel Session 6
Paulo Ceppi, Simon C. Scherrer and C. Appenzeller
Climate Services, Federal Office of Meteorology and Climatology MeteoSwiss,
Zürich, Switzerland
Temperature is an essential variable in monitoring the impact of global climate
change. Here we perform a detailed regional trend analysis of Swiss Alpine
temperatures in the period 1959 to 2008 using a newly available gridded data set
based on homogenised observations from 91 MeteoSwiss stations. The aim is to
quantify possible large-scale and local-scale contributions to the local trends. It is
shown that the yearly trends are all positive and highly significant, with an average
warming rate of 0.35 °C/decade, consistent with results from earlier studies. The
values show fairly little spatial variability, with 90% of all gridpoint trends between
0.30 and 0.39 °C/decade. This indicates that the warming in Switzerland has exceeded
the NH extratropical mean trend by a factor of 1.6 over the last 50 years. On a
seasonal scale, the analysis also reveals overall positive temperature trends, but
seasonal and spatial variability are pronounced. The weakest trends (mostly
insignificant at the 5% level) are observed in autumn (SON, 0.17 °C/decade on
average) and early summer features the strongest warming rates, peaking at 0.48
°C/decade in May-June-July. A pronounced altitude dependence is found from late
summer to early winter (ASO to NDJ), where the trends decrease with elevation.
We investigate the impact of large-scale versus local-scale influences on seasonallyaveraged
temperatures using a regression model with atmospheric circulation patterns
and northern hemispheric temperatures as explanatory variables. The analysis reveals
that 45 (summer) to 80% (winter) of the interannual variability of Swiss gridpoint
temperatures can be explained by largescale influences. In spring, summer and
autumn, a fraction of up to 20% of the temperature trends remains unexplained by the
model. In spring, a positive trend anomaly close to the zero-degree isotherm is not
detected by our model, while the largest unexplained trend magnitudes are found at
low elevations in autumn and winter.
Our results suggest that snow-albedo feedback effects might be responsible for the
unexplained 10% higher spring trends near the altitude of the zero degree isotherm
and snow line. In autumn, the observed decrease in mist and fog frequency may be a
key process explaining the 20% higher Swiss autumn temperature trends at low
elevations. Changes in soil-atmosphere interactions could explain part of the
difference between observed and modelled trends in summer. For the unexplained
lower than modelled temperature trends in late winter at low altitudes the physical
mechanisms remain to be determined.
Abstracts 354