Phase A of the NASA Pacific Exploratory Mission in the western North Pacific (PEM-West A) region was conducted during September-October 1991. The background meteorology of eastern Asia and the western North Pacific region during the PEM-West A study is described. Mean large-scale flow patterns are discussed along with transient synoptic scale features. Synoptic summaries are given, together with selected examples of meteorological processes that gave rise to some of the changes observed in the measured trace gases. Examples of large-scale ozone features are also related to meteorological processes.