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The Propagation of Tropical Disturbances over the Indian Ocean during the Summer Monsoon

Daniel CadetLaboratoire de Météorologie Dynamique du CNRS, Ecole Polytechnique, 91128 Palaiseau, Cedex, France

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Paul Olory‐TogbéLaboratoire de Météorologie Dynamique du CNRS, Ecole Polytechnique, 91128 Palaiseau, Cedex, France

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Abstract

Daily nephanalysis of NOAA 4 satellite visible images over the Indian Ocean during the 1975 summer monsoon was used to construct time-longitude sections of cloud cover during a month and a half. Tropical disturbances traveled over the Indian Ocean during the summer monsoon—eastward moving to the north of the equator and westward moving to the south. Conventional meteorological data recorded as island stations agree well with these results; the passage of the disturbances over these stations was associated with a decrease of pressure and sometimes a noticeable change in surface wind direction. Using low-level superpressure balloon data, balloon trajectories and cloud brightness were compared. The influence of the tropical disturbances on the low-level air flow circulation over the Indian Ocean is shown. The possible link between these traveling disturbances and pulsations in the Indian summer monsoon circulation is also investigated.

Abstract

Daily nephanalysis of NOAA 4 satellite visible images over the Indian Ocean during the 1975 summer monsoon was used to construct time-longitude sections of cloud cover during a month and a half. Tropical disturbances traveled over the Indian Ocean during the summer monsoon—eastward moving to the north of the equator and westward moving to the south. Conventional meteorological data recorded as island stations agree well with these results; the passage of the disturbances over these stations was associated with a decrease of pressure and sometimes a noticeable change in surface wind direction. Using low-level superpressure balloon data, balloon trajectories and cloud brightness were compared. The influence of the tropical disturbances on the low-level air flow circulation over the Indian Ocean is shown. The possible link between these traveling disturbances and pulsations in the Indian summer monsoon circulation is also investigated.

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