Wednesday, 10 May 2000
At many locations in the United States the time series of daily temperature departure from normal (TDFN) sometimes exhibit intraseasonal oscillations on timescales of 2070 days, which persist for several cycles. Amplitudes may be quite large (36 ºC) even though an entire season averages close to normal. Successful long-range forecasting of these oscillations would have considerable economic value, particularly during the colder months. In this study TDFN records of a number of locations in the United States for the period 19531995 are Fourier analyzed. Amplitude and power spectra of successive 128-day segments starting at the beginning of each month are computed, and the monthly and frequency distributions of significant peaks in the spectra are plotted for the three ENSO phases, viz. El Niño, La Niña and neutral. The results indicate that the oscillations are clearest on the West Coast and that they are associated with the 3060-day oscillations of convective activity which often occur in the Indian and Pacific oceans and which probably modulate the location of the mid-latitude jet stream in a quasi-periodic fashion. Both the monthly and frequency distributions of significant spectral peaks are affected by ENSO phase. Long-range forecasting of the intraseasonal temperature oscillations will apparently require running global circulation models ahead in time for many weeks but may be feasible since the principal interest here is in the overall position of the general circulation pattern and the jet stream and not in the occurrence of individual storms.
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