Intraseasonal Oscillations in Sea Surface Temperature, Wind Stress and Sea Level off the Central California Coast
The wavelet transform is used to conduct spectral and cross-spectral\nanalysis of daily time series of sea surface temperature (SST), surface\nwind stress, and sea level offthe central California coast for an\n18-year period from 1974 through 1991. The spectral band of primary\ninterest is given by intraseasonal time scales ranging from 30 to\n70 days. Using the wavelet transform, we examine the evolutionary\nbehavior of the frequently observed 40–50 day oscillation originally\ndiscovered in the tropics by Madden and Julian, and explore the relative\nimportance of atmospheric vs oceanic forcing for a range of periods\nwhere both could be important. Wavelet power spectra of each variable\nreveal the event-like, nonstationary nature of the intraseasonal\nband. Peaks in wavelet power typically last for 3–4 months and occur,\non average, approximately once every 18 months. Thus, their occurrence\nand/or duration offcentra l California is somewhat reduced in comparison\nto their presence in the tropics. Although peaks in wind stress often\ncoincide with peaks in SST and/or sea level, no consistent relationships\nbetween the variables was initially apparent. The spectra suggest,\nhowever, that relationships between the variables, if and where they\ndo exist, are event-dependent and thus have time scales of the same\norder. Cross-wavelet spectra between wind stress and SST indicate\nthat periods of high coherence (>0.90) occur on at least six occasions\nover the 18-year period of record. Phase differences tend to be positive,\nconsistent with wind forcing. For wind stress vs sea level, the cross-wavelet\nspectra indicate that periods of high coherence, which tend to correlate\nwith lags\n\nclose to zero, also occur, but are less frequent. As with SST, the\nperiods of high coherence usually coincide with events in the wavelet\npower spectra. The somewhat weaker relationship between wind stress\nand sea level may be due to an independent contribution to sea level\nthrough remote forcing by the ocean originating in the tropics. Finally,\nsimple dynamical arguments regarding the lag relationships between\nthe variables appear to be consistent with the cross-wavelet results.