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  • Author or Editor: Jean Philippe Duvel x
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Laurent Goulet
Jean-Philippe Duvel


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.

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Jean Philippe Duvel
Rémy Roca
, and
Jérôme Vialard


In situ and satellite observations reveal that the tropical intraseasonal oscillation is occasionally associated with large variations in sea surface temperature (SST). The purpose of this paper is to find the physical origin of such strong SST perturbations (up to 3 K) over the Indian Ocean by examining two intraseasonal events in January and March 1999. Analysis of SST data from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and from drifting buoys reveals that these two intraseasonal events deeply modify the SST field between the equator and 10°S, while the surface flux perturbation extends over a wide area of the tropical Indian Ocean. Forced ocean general circulation model (OGCM) simulations are successful in reproducing the spatial patterns of this intraseasonal SST variability albeit with a weaker amplitude. The weaker amplitude given by the OGCM is partly related to the absence of warm-layer formation in the model. The model simulation reveals that the background oceanic subsurface structure explains the observed latitudinal distribution of the SST perturbations. For the Indian Ocean, the Ekman pumping (reinforced in 1999 due to La Niña conditions) gives a thermocline close to the surface between 5° and 10°S that inhibits the deepening of the mixed layer during strong wind episodes and thus gives a mixed layer temperature more reactive to surface forcing. Other factors like the Ekman dynamics associated with the wind burst and the precipitation perturbation south of the equator also contribute toward preventing the deepening of the mixed layer. For these regions, as is found over the western Pacific, the intraseasonal variability of the SST is mainly driven by the surface fluxes perturbation, and not by advection or exchanges with the subsurface. As a consequence, the phasing and the magnitude of convective and large-scale dynamical perturbations of the surface fluxes, which are regionally dependent, are also determinant factors for the local amplitude of the SST perturbation. Finally, results show a relation at interannual time scales between the thermocline structure and the mixed layer depth south of the equator that may have consequences on interannual changes in the intraseasonal activity over the Indian Ocean.

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