The atmospheric response to extratropical sea surface temperature anomalies
Diploma Thesis
Ute Merkel
Abstract
In the context of decadal and interdecadal climate variability the physical understanding of feedback mechanisms between ocean and atmosphere is of great interest. Whilst the influence of tropical sea surface temperature (SST) anomalies on the general atmospheric circulation has been studied by many authors and is rather well established, there still exist some uncertainties about the atmospheric response to midlatitude SST anomalies.
In order to shed light on some of these uncertainties, we analyse the results of five response experiments that have been performed with the atmospheric circulation model ECHAM3 by imposing SST anomalies of different signs in the North Atlantic, North Pacific or in both oceans simultaneously. The individual anomaly amplitudes have been derived from an EOF analysis of a 320-year control run of the ECHAM3 model coupled to the LSG ocean model. The experiments have been integrated over a period of 30 years and are compared to a 50-year control integration of ECHAM3 forced by climatological SST.
The results suggest that the boundary layer is characterized by a baroclinic response structure with lower (higher) sea level pressure above the positive (negative) SST anomalies. Higher vertical levels and the far field exhibit an equivalent barotropic structure similar to the results of Palmer and Sun (1985). Furthermore, the net surface heat flux exhibits a remote response such that the SST anomaly in one ocean creates an opposite heat flux anomaly in the other ocean in which no anomaly was imposed. In the case of a positive SST anomaly in the Atlantic, for instance, a weakening of the Aleutian low and reduced cold air advection over the Oyashio current are simulated.
The atmospheric response is found to be roughly linear when we compare experiments with SST anomalies of different signs. Warm (cold) SST anomalies in the east Pacific lead to a strong and partly significant remote response over the north-east Atlantic where the response patterns at all tropospheric levels show an intensification (weakening) of the meridional pressure gradient between the Icelandic low and the Azoric high pressure regions.
In a second part of the present study low-pass filtered daily data (periods of 10-90 days) from the experiments mentioned above were analysed to investigate the relationship between SST anomalies in midlatitudes and changes in the probability distributions of weather regimes such as blocking or strong zonal flow. The variance and skewness response patterns suggest that there is an influence of the boundary conditions on the low-frequency variability and predictability of atmospheric circulation patterns and the frequency of circulation anomalies. Possible relationships in the band-pass filtered frequency range will be investigated next.
Reference:
Palmer, T.N., and Z. Sun, 1985: A modelling and observational study of the relationship between sea surface temperature anomalies in the north-west Atlantic and the atmospheric general circulation, Quart. J. Roy. Met. Soc., 111, pp. 947-975