Empirical and dynamic modes in the CCS
Smith, Jerome A
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CTD data from the California Current System (CCS) are analysed in terms of both empirical orthogonal functions (EOFs) and quasigeostrophic vertical modes (QGMs). A single EOF accounts for about 97% of the total variance in hydrostatic pressure perturbations to 3000m. The first four pressure EOFs correspond quite well to the first four QGMs. The QGMs are orthogonal with respect to integration " over the total water depth, while the EOFs are orthogonal over the depth interval where there are data; thus, as subsets of increasingly deeper cast data are used, the EOFs and QGMs become more alike. The EOF amplitudes must also be uncorrected over the sample of profiles used, while those of the QGMs need not be; yet, the EOF analysis roughly selects the first four QGMs, even from just eight casts to 3000m. Apparently, the variance contained in each successive QGM decreases precipitously; thus, the variance in QGM 1 overwhelms any correlated part of QGM 2. Likewise, the variance left over in QGM 2 is still sufficient to overwhelm QGM 3 (and so on). (Note also that low-frequency internal waves, e.g. internal tides, have virtually identical modal structure to the QGMs; these may also contribute to the EOFs) . !r; the upper 100 meters, the EOFs vary markedly from survey to survey. The density analysis can produce an EOF which doesn't fit into the normal' QGM sequence, but rather is strongly surface trapped, being confined roughly to the top 100 meters. This mode, which may represent mixed or Ekman layer variability, can account for up to 20% of the total density variance.
NPS Report NumberNPS68-84-003
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