Westward extension of North Pacific subtropical high in summer

The basic features of the North Pacific subtropical anticyclone in summer are studied using the European Centre for Medium Range Weather Forecasts (ECMWF) Re-Analysis data for the period (1979-1993). It has been shown that the North Pacific subtropical high displays substantial interannual variability at its western edge. Further analysis indicates a significant relationship between the zonal variation of the subtropical high, and the variation in the intensity of atmospheric convection over the warm pool. The stronger (weaker) convection over the warm pool is associated with more eastward (westward) position of the western Pacific subtropical high. The UK Universities(2) Global Atmospheric Modelling Programme General Circulation Model (UGAMP GCM) is used to examine the influence of the local SST anomalies, and the associated anomalous convection over the tropical western Pacific warm pool on the zonal displacement of the North Pacific subtropical high. The model results show that the suppressed atmospheric convection caused by lower SSTs in the warm pool results in anomalous anticyclonic circulation in lower level over the subtropical western Pacific, and westward extension of the subtropical high. The results are consistent with the composite results based on the reanalysis data, and imply the SST anomalies off-equator over the western Pacific play an important role in the zonal shift of the North Pacific subtropical high in summer. Despite the fact of good agreements in many aspects between the model simulations and composite analyses based on observation, there are some differences between the two especially in terms of upper-level extratropical circulation anomalies over East Asia.