In tropical regions, the atmosphere undergoes marked fluctuations of temperature, pressure and humidity. These fluctuations, which are particularly intense in the Indian Ocean, cause disturbances over differing time-scales. During the southern winter, for example, thermal anomalies result in heavy precipitation followed by dry periods, with the whole system moving from the west of the Indian Ocean as far as the tropical Atlantic. These are the IntraSeasonal Oscillations (MJOs, also known as Madden Julian Oscillations) which influence the summer monsoon and can contribute to triggering an El Niño phenomenon.

Scientists want to assess the significance of the interactions between the atmosphere and the ocean involved in these phenomena.

Two CNRS¹ laboratories have worked with CNES to define the protocol for the Vasco² experiment in the Seychelles. The method involves using low-flying aeroclippers and stratospheric balloons that drift with disturbances associated with MJOs over the oceans and in the atmosphere’s boundary layer, where the circulation of airstreams is influenced by surface relief and by thermal exchanges with the surface, both land and sea. The programme will run for several years in the form of annual campaigns during which balloons will be launched from January until mid-February.

Since MJOs are stronger in the west of the Indian Ocean, the 8 aeroclippers and the 10 BPCLs (for Ballons Pressurisés de Couche Limite, or Pressurised Boundary Layer Balloons) are launched from Mahé Island. The aeroclippers developed for the project can take measurements simultaneously from the surface of the ocean and in the atmospheric boundary layer, a narrow, 150 m band in direct contact with the Earth’s surface. Researchers also want to establish whether the origins of MJOs are related to the vast movements of the atmosphere known as equatorial waves. For this purpose they use BPCLs, flying at an altitude of 1500 m. The balloon and its instrument payload acts like a miniature meteorology station drifting east on the wind.

The experiment will be completed by measurements of the temperature and salinity of the central and western Indian Ocean, performed simultaneously as part of the Cirene oceanography campaign, run by Locean.

The Vasco programme is not only investigating MJOs but also the physical mechanisms governing interactions between climate variations over differing time-scales: seasonal, annual, and interannual. The goal is to improve ways of modelling and forecasting changes in the ocean-atmosphere system in the Indian Ocean.

CNES is providing the researchers with the balloons and aeroclippers for the meteorological component. The CNRS laboratories have developed and built the gondolas that contain the measuring instruments. Ifremer is providing Locean with a vessel, Le Suroît, a sailor’s name for a south-westerly wind, for the Cirene campaign.

¹ LMD: Laboratoire de Météorologie Dynamique (Laboratory for Dynamic Meteorology) Locean: Laboratoire d’Océanographie et du Climat: Expérimentation et Approches Numériques (Oceanography and Climate Laboratory: Experiments and Digital Modelling)

² Variabilité Atmosphérique Intra Saisonnière et Couplage Océanique (Intraseasonal atmospheric variation and air-sea coupling)

_{Last updated : February 2007}