Analysing and combining atmospheric general circulation model simulations forced by prescribed SST: northern extratropical response

The ECHAM 3.2 (T21), ECHAM 4 (T30) and LMD (version 6, grid-point resolution with 96 longitudes × 72 latitudes) atmospheric general circulation models were integrated through the period 1961 to 1993 forced with the same observed Sea Surface Temperatures (SSTs) as compiled at the Hadley Centre. Three runs were made for each model starting from different initial conditions. The mid-latitude circulation pattern which maximises the covariance between the simulation and the observations, i.e. the most skilful mode, and the one which maximises the covariance amongst the runs, i.e. the most reproducible mode, is calculated as the leading mode of a Singular Value Decomposition (SVD) analysis of observed and simulated Sea Level Pressure (SLP) and geopotential height at 500 hPa (Z500) seasonal anomalies. A common response amongst the different models, having different resolution and parametrization should be considered as a more robust atmospheric response to SST than the same response obtained with only one model. A robust skilful mode is found mainly in December-February (DJF), and in June-August (JJA). In DJF, this mode is close to the SST-forced pattern found by Straus and Shukla (2000) over the North Pacific and North America with a wavy out-of-phase between the NE Pacific and the SE US on the one hand and the NE North America on the other. This pattern evolves in a NAO-like pattern over the North Atlantic and Europe (SLP) and in a more N-S tripole on the Atlantic and European sector with an out-of-phase between the middle Europe on the one hand and the northern and southern parts on the other (Z500). There are almost no spatial shifts between either field around North America (just a slight eastward shift of the highest absolute heterogeneous correlations for SLP relative to the Z500 ones). The time evolution of the SST-forced mode is moderatly to strongly related to the ENSO/LNSO events but the spread amongst the ensemble of runs is not systematically related at all to the intensity of Niño3.4 SST anomalies. The leading reproducible mode in JJA is clearer and more skilful for SLP than for Z500 and also seems related to the SST time evolution of tropical Pacific. It is characterised by an out-of-phase between the whole North Pacific and a horseshoe shaped area from Eastern Siberia and Gulf of Mexico. The leading OM mode found in MAM and SON, are quite close to the DJF one (at least for the modelled anomalies), but they are less skilful than in DJF. The most skilful mode (i.e. SLP-Z500 mode in DJF and SLP mode in JJA) is almost similar to the most reproducible one during these particular seasons. In MAM and SON, the SST-forced pattern is very close to the wintertime one. The warm episodes in the central and eastern tropical Pacific are then associated with negative pressure anomalies at the sea level and also at 500 hPa over the whole North Pacifkc and from SE US Coast to Western Europe (from SE US Coast to Scandinavia for Z500) and positive pressure anomalies on Central Canada, north of 55°-60°N across the North Atlantic and also over Northern Siberia (in MAM). The variance forced by SST are lower in MAM and SON than in DJF and, as suggested above, the skill of this SST-forced mode is weak in MAM and almost close to zero in SON.

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