On surface circulation of the Eastern North Pacific

Abstract

A qualitative and statistical study of the trajectory data for 51 satellite-reporting, drogued drifters in the Eastern North Pacific has shown that there are distinct seasonal differences in the patterns of surface flow. The drift observations extend across the region 30-55°N, 170-130°W from June, 1976 through October, 1978, a period characterized by anomalous intensities and locations for the North Pacific high and Aleutian low atmospheric pressure cells. Composite and seasonal trajectory descriptions indicate that the large-scale flow field had a stronger northward component in the winter of 1976-77 than in the following winter of 1977-78. Cluster analyses demonstrate high coherence and north-south symmetry in the flow system. Mesoscale eddy features are prevalent in summer and well-defined anticyclonic meanders are shown to persist at more southern latitudes and to dampen at more northern latitudes. The quantitative study of the time series of trajectory displacement components (latitude and longitude) involves spectral analyses using a new Fourier technique which is a generalization of the standard Fourier method. No significant differences were discerned between ensemble average spectra on the basis of northern and southern regime divisions. The spectral results show that there are characteristic spectral forms evident on a seasonal basis for the mesoscale frequency range of .05 to .5 cpd. The winter spectra have more gentle spectral slopes than the summer spectra. Consistent seasonal variations in intensities occur in the range .1 to .5 cpd for displacement spectra. Year-to-year variations in the spectra are discussed. A slope difference was found between winter longitude spectra for ADS (Anomaly Dynamics Study) II and ADS III drifters. The spectral slopes for the ADS II drifters are more gentle due to lesser intensities in the frequency range .05 to .1 cpd. It is speculated that these differences are related to fundamental variations in the atmospheric forcing and oceanic response for the winter of 1976-77 versus the winter of 1977-78.