Spectral analysis of synoptic time scale disturbances over the tropical eastern Pacific during summer 1989, 1990 and 1991

Abstract

In this study we used data analyzed by the Navy Operational Global Analysis and Prediction System (NOGAPS) to study the 4 - 10 day disturbances over the eastern tropical Pacific. The period of study is May - September of 1989, 1990 and 1991. Spectral and cross-spectral analyses were used to determine the structure of the disturbances. The results show zonal wavelengths of 3000 to 8000 kilometers and a tendency for northeast-southwest tilt in the meridional direction. The most active year appears to be 1991, with the highest relative variance in the 4 - 10 day window and the highest coherence between parameters. The vertical tilt was found to be westward with height above 300 - 400 hpa in 1989 and above 200 hpa in 1990 and 1991. This interannual variation was consistent with the change in vertical shear of the mean zonal wind. Below 400 hpa in 1989 and 200 hpa in 1990 and 1991, the vertical tilt was eastward with height. The thermal structure was consistent with the hydrostatic relationship in all three years. The waves transport heat poleward above 200 hpa and equatorward below 200 hpa.