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Abstract:

A series of 120‐day ensemble integrations of a general circulation model, designed to assess the impact of geographically localized sea‐surface‐temperature (SST) anomalies in both the tropics and extratropics, are described. These experiments contribute firstly to an appraisal of the relative roles of tropical and extratropical SST anomalies on interannual variability of the large‐scale circulation in the northern extratropics, and secondly to an assessment of the role of quasi‐stationary diabatic‐heating anomalies on model systematic error, including blocking activity. Overall it is found that SST anomalies associated with El Niño and La Niña have a larger and more reproducible impact on the extratropics than the chosen extratropical SST anomalies. These extratropical anomalies were localized to the north‐west Pacific, and north‐west Atlantic, with realistic amplitude. However, unlike earlier studies, a response to the extratropical North Pacific SST anomalies has been obtained over the North Pacific which is correlated with the sign of the imposed SST anomaly. The response to extratropical SST anomalies in the north‐west Atlantic are similar to the results obtained from an earlier study. The downstream responses to the extratropical Pacific and Atlantic SST anomalies are qualitatively similar to one another. Overall it is concluded that the northern large‐scale flow is influenced by such extratropical SST anomalies. The response to idealized tropical SST anomalies was also studied. In particular, a localized anomaly over Indonesia had a very substantial impact on the Hadley circulation, on zonal flow, and on blocking frequency over the North Pacific and Europe. This response was such as to reduce model systematic error: locally in the vicinity of the SST anomaly, remotely around the tropics, and remotely in the extratropics. A similar, though weaker, impact on Euro‐Atlantic blocking was obtained with an idealized Caribbean SST anomaly. Further statistical and dynamical analyses suggested that the extratropical response to the Indonesian SST anomaly occurs through the creation of two distinct planetary‐scale regimes, in one of which the formation of blocks is much favoured by increased ridges on the north‐eastern side of the oceans. Copyright © 1994 Royal Meteorological Society