A New Approach to Detect and Characterize Intermittent Atmospheric Oscillations: Application to the Intraseasonal Oscillation

Abstract

This paper presents a method, the local mode analysis (LMA), that makes it possible to extract the most persistent oscillations present in the time evolution of an atmospheric field. This method is particularly suitable to analyze intermittent tropospheric oscillations related to dynamic or thermodynamic instabilities such as the intraseasonal oscillation (ISO). These intermittent oscillations generally exhibit various spatial structures that succeed one another in time and that are difficult to isolate in a simple and comprehensive manner using conventional approaches such as empirical orthogonal functions or composite analyses. The main objective of the LMA approach is to identify the different structures of a given oscillation in order to better understand its physical origin and to test the applicability of different theoretical hypotheses. The LMA also makes it possible to test the representativity of a mean structure in regard to actual modes that succeed one another in time. The LMA is applied to the National Oceanic and Atmosphere Administration?Advanced Very High Resolution Radiometer outgoing longwave radiation time series in order to study the variability of the convective perturbation at the intraseasonal timescale (30?60 days). The LMA depicts the most intense and persistent modes of the ISO very well and shows the strong variability of the spatial organization of the convective perturbation at this timescale. Results exhibit interannual and seasonal variations of the mean period and amplitude of the ISO with a tendency to have less persistent modes and smaller periods of the oscillation during El Niño years and during summer. The maximum perturbation of the convection by the ISO is not located on the equator but rather around 10°?15° in the summer hemisphere. Several persistent modes exhibit neither the phase opposition between the Indian and Pacific Oceans nor the eastward equatorial propagation that characterize the average mode of Northern Hemisphere winter. Inspecting the ensemble of ISO modes, this eastward propagation of the convective perturbation is well defined only over the Indian Ocean. The convective perturbation over the Maritime Continent is basically stationary, and the eastward propagation over the Pacific Ocean appears only for the strongest convective perturbations.