Precipitation and surface wind data have been analyzed to search for variability and interannual variations of signals in the period range of tropical intraseasonal oscillations (or TIO's) in central Panama. In this study this period range is taken to be roughly from 40 to 130 days. The data consist of daily precipitation totals from 24 stations in the former Panama Canal Zone from 1965 through 1997 (for a subset, from 1905-1971). These stations provide a reasonably uniform coverage of the territory of the Panama Canal. Daily wind data are available at present from a small number of stations from 1985 through 1997; newly available data will go back to at least 1965. All precipitation data have been qualified using a number of methods to ensure accuracy.
Spatial variability of precipitation over the 24 stations has been analyzed using empirical orthogonal eigenfunctions. The first three eigenmodes account for over 85% of the signal variance. The first mode represents an overall increase or decrease in precipitation, the second mode represents a "see-saw" between the Pacific and Caribbean, and the third mode is a more geographically localized increase over the higher regions on the Caribbean side of Panama. The third mode is of particular interest since on several occasions during 1965-1997 it shows a short-period burst of higher rainfall in the time before a major ENSO event.
In order to elucidate the spectral energy content of the precipitation signal an energy spectrum of the total rainfall summed over the 24 stations was computed. This spectrum shows, in addition to the expected annual and semiannual peaks, a broad peak at roughly 80-120 days and an apparently white spectrum at higher frequencies. Although the 80-120-day peak is statistically significant this mode of analysis was not found to be particularly unambiguous since the TIO-period signals are known to be highly intermittent. In order to obtain a clearer picture of the transient TIO signals a wavelet transform of the total precipitation data was computed. It was found that the results are in general not strongly sensitive to the type of wavelet used. The precipitation analysis indicates that in the TIO period range a number of years show bursts of energetic signals, including 1965/1966, 1970/1971, 1981, 1986 and 1996. These bursts typically precede the dry periods associated with ENSO events in this region of the world. The wind data (particularly the eastward component) shows corresponding strengthening in these components (suggesting that part of the signal in precipitation may be linked to orographic influences in the eastern part of the isthmus), although at present this can be verified only back to 1985. There is also an interesting suppression of annual and semiannual variability in this component in the early 1990's. The relation between variations in the TIO signals and the larger-scale ENSO forcing is now being actively investigated.