Atmospheric conditions during the spring and fall transitions in the coastal ocean off western United States

We examine large-scale atmospheric behavior around the time of the spring and fall transitions in the coastal ocean off the west coast of North America. Records of adjusted sea level (ASL), coastal wind stress, sea level atmospheric pressure (SLP), and 500-mbar heights for the years 1971-1975 and 1980-1983 are analyzed. The records cover periods of 91 days, centered on the dates of the spring and fall transitions as determined from coastal adjusted sea level data. Empirical orthogonal functions are obtained from the ASL, coastal wind stress, and SLP records. The two dominant modes of the ASL and coastal wind stress are similar around the times of both the spring and fall transitions, and the time series for these modes are highly correlated with one another. Transitionlike behavior is evident in the time series of the first modes in both spring and fall, but the spring transition is more pronounced. Principal estimator patterns, formed from the dominant empirical orthogonal functions show the spatial patterns of SLP which force the ASL and coastal wind stress during the transitions. The SLP pattern which coincides with the spring transition is the formation of a high-pressure system centered at 45°N and 140°W along with the development of a low-pressure cell over the southwest continental United States. Inspection of the 500-mbar height composites for 91 days surrounding the spring transition for the 9 years reveals the formation of a ridging pattern and diffluent flow over the western United States at the time of the transition; following the transition, the ridging relaxes but the diffluent flow over the continent remains for the duration of the 45 days examined here. The fall transition is characterized by a rise in ASL, particularly north of 40°N, and a change from southward to northward wind stress. The SLP pattern which coincides with the fall transition involves the appearance of a low-pressure system off western North America centered at 50°N and 140°W, representing the passage of synoptic storms through the region. Prior to the fall transition, the 500-mbar heights are somewhat diffluent and show a trough over the southwestern United States; after the transition the 500-mbar flow over the northeast Pacific and North America is nearly zonal.