Model Experiments on the Impact of Pacific Sea Surface Temperature Anomalies on Blocking Frequency

Abstract

Sensitivity experiments with a perpetual January version of a low-resolution general circulation model are conducted to investigate the influence of Pacific sea surface temperature (SST) anomalies on blocking in the Northern Hemisphere. Six 1200-day integrations, each of which contains a different specified SST anomaly superimposed upon a fixed climatological SST distribution, are compared with a 1200-day control simulation to examine the impact of both equatorial Pacific and midlatitude North Pacific anomalous forcing. An ensemble of eight independent 90-day realizations is extracted from each simulation. Distributions of the ensemble-mean differences in the total number of blocking days from the control simulation are presented, along with estimates of the statistic significance of these differences. Pacific SST anomalies do not strongly affect the total number of days in which a block is present anywhere over the North Pacific basin, but they apparently can affect the preferred regions over the North Pacific basin where model blocking occurs as follows: A warm tropical Pacific SST anomaly tends to enhance blocking activity along the west coast of North America and suppress it near the Aleutian Islands, while a cool midlatitude North Pacific SST anomaly near the dateline tends only to suppress blocking activity near the Aleutian Islands. The combination of a warm tropical SST anomaly and a cool extratropical SST anomaly is more effective in modifying blocking activity over the midlatitude North Pacific than either SST anomaly acting alone. The inclusion of either a tropical or extratropical Pacific SST anomaly does not produce a change in the blocking activity deemed significant over other regions of the Northern Hemisphere. Based on the results of this paper and previous studies, it is concluded that certain Pacific SST anomaly patterns having realistic amplitudes can favor the onset of the recurrence of blocking in a particular geographical region in the model, but they do not appear to affect strongly either the persistence of an individual blocking event once initiated or the dynamical processes important to its maintenance as reflected in local balances of heat and vorticity.