Extended EOF Analysis of Tropical Disturbances: TOGA COARE

Abstract

Empirical orthogonal function (EOF) analyses are performed of time‐height series of zonal and meridional winds and of cumulonimbus heating and drying in the Tropics. The data are from a rawinsonde array in the western Pacific located between the equator and 108S during the intensive observation period of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). The EOF analyses are performed by applying a window of 20 days to the data and thus calculating the EOFs of the time development of the vertical structure. The wind time series is found to be well represented by two pairs of EOFs, each representing an oscillation. The first oscillation has a period of approximately 40 days, is predominantly in the zonal wind component, and has a first internal mode vertical structure with westerly anomalies in the lower troposphere corresponding to easterly perturbations in the upper troposphere. This pair describes 48.3% of the variance. A second EOF pair in the wind is a zonal variation that occurs predominantly in the upper troposphere. It has a period of approximately 24 days and describes 13.9% of the variance. The heating‐drying series is described by a dominant oscillation of period 40 days representing 41% of the variance. The structure is maximum in the middle troposphere and is associated with the same physical phenomeon as the dominant (u, y) oscillation. The second EOF pair for heating‐drying has a period of 13 days, so there is a large time separation in periodiocities for heating‐drying compared to that for winds. The second (13 day) oscillation in heating‐drying has the same vertical structure as the dominant (40 day) oscillation.