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<p>The annual cycle of the North Pacific wind stress, Ekman pumping and Sverdrup transport is investigated by means of empirical orthogonal function (EOF) analysis techniques. Fifty-two years of National Centers for Environmental Prediction (NCEP, formerly \"NMC\") and the National Center for Atmospheric Research (NCAR) Reanalysis daily averaged surface wind components covering the extratropical North Pacific are used to calculate daily averaged wind stress components. These wind stress components are averaged to 624 monthly mean fields from which monthly mean Ekman pumping and Sverdrup transport fields are derived. Each data field has the long-term annual mean and linear trend removed at each grid point before EOF analysis. The first three modes are considered to be physically significant based on the plot of the log eigenvalue against its root. The first principal components of each field are highly coherent, have most of their spectral energy at the annual cycle, and are nearly in phase. The second mode of these fields show significant spectral energy at the semiannual cycle. The third mode has more complex spatial variability than the first and second mode. The annual cycle is best described by the first EOF mode which is composed of two seasons (winter and summer) that last for five months each separated by two rapid (approximately one month) transition periods (spring and fall). The annual cycle is complex and certain geographical regions are seasonal \"hot spots\" that add to the complexity. Relationships are derived between the various modes to help explain seasonal movement of the Ekman pumping zero isotach and seasonal variations in Sverdrup transport. The annual variability can reasonably be described using only the first three EOF
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<br>Subjects:</div>
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Moore, Christopher S.</div>
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