On the potential impact of the stratosphere upon seasonal dynamical hindcasts of the North Atlantic Oscillation: a pilot study

We have investigated the importance of the stratosphere?Ctroposphere linkage on the seasonal predictability of the North Atlantic Oscillation in a pilot study using a high horizontal resolution atmospheric general circulation model, and covering the 14 winters from 1979/1980 to 1992/1993. We made an ensemble of simulations with the Meteo-France ??Arpege Climat?? model (V3.0) with a well-resolved stratosphere, and a broad comparison is drawn with hindcasts from previously published experiments using low-top and lower horizontal resolution models, but covering the same winters with the same ensemble size and verification method. For the January?CFebruary?CMarch North Atlantic Oscillation index, the deterministic hindcast skill score is 0.59, using re-analyses as verification. It is comparable to the reported multi-model skill score (0.57). The largest improvement originates from the winter 1986/1987 characterised by a major stratospheric sudden warming. We demonstrate that there is then a high-latitude zonal-mean zonal wind decrease in the stratosphere?Ctroposphere hindcasts over a broad pressure range. This is consistent with a composite analysis showing that model anomalous vortex events, either weak or strong, lead to a North Atlantic Oscillation index anomaly in the troposphere, which persists, on average, for 1?month after the anomaly peaked in the stratosphere.