Stratosphere impact on tropical climate – STT-CLIM

Until the 1990s, the scientific community that was studying climate, its variability and evolution, had no specific interest for the middle-atmosphere. However, lately, several institutes have intensifed their efforts in order to study the influence of the stratosphere on the climate. Traditionnally, from a dynamical point of view, the tropospehere was considered as the driver of the stratosphere mainly through the propagation of atmospheric waves from below up to the stratosphere, which leads for example to the Brewer-Dobson circulation, the Quasi-Biennial Oscillation (QBO) in the Tropics, and Sudden Stratospheric Warmings during arctic winter. But today, there is growing evidence that stratospheric circulation also affects weather and climate. Many exchanges take place in the UTLS (Upper Troposphere Lower Stratosphere) region, with upward transport in the Tropics and downward transport in mid-latitudes, but there are also exchanges both ways in mid-latitudes. Recent studies have also shown that low-frequency stratospheric circulaton influences tropospheric variability patterns. This leads to the conclusion that there is a strong dynamical coupling at several time scales. Since the stratosphere influences the troposphere, a way of improving medium- and long-term predictions is to extend vertically GCM to the stratosphere in order to be able to describe properly the interactions between these two layers. The influence of the stratosphere on the climatic system is a major issue of the SPARC program (Stratospheric processes and their role in Climate »; www.atmosp.physics.utoronto.ca/SPARC/index.html). Within this particular framework, we propose to study the influence of the stratosphere on tropical climate and more specifically on on the Indo-Pacific sector, characterized in particular by the Asian and Australian monsoons.As a matter of fact, the monsoon and its associated precipitations are important in ecological, economical and demographic terms. The monsoon concerns a large part of the world population who is very dependent of its quality and fluctuations. Monsoon failure can lead to drought, famine and shortages of water. On the other hand, abnomally intense monsoon precipitation may induce flooding, crop loss, and disease epidemics. From a public policy perspective, reliable predictions of monsoon intensity before wet-monsoon onset could facilitate the implementation of policies designed to mitigate the negative impacts of adverse monsoon conditions. Thus, the medium- to long- range previsibility of the monsoon is crucial in the sub-tropical area because of the vulnerability of the environment. However, the monsoon system is highly variable from both a temporal and spatial point of view, with variability modes ranging from the intra-seasonal to the decadal sacle. Monsoon forecasting therefore implies the understanding of a variety of complex mechanisms involved in its variability and associated patterns (Webster et al., 1998). In this project, we will first describe the observed response of the tropical troposphere, and more specifically over the Indo-Pacific sector, to stratospheric forcing. Use will be made of reanalyses datasets (NCEP and ERA-40) and several observation files (REYNOLDS, CMAP) to document the links between the stratosphere and the tropsphere. In addition, to aid in the interpretation of the observational results, we will then investigate, using a CCM (Climate-Chemistry Model), how the stratosphere influences the circulation in the Tropics. There are several variability modes through which the tropical troposphere and the stratosphere can be connected ; these include : - The Quasi-Biennial Oscillation The existence of a biennial component in tropospheric climatic variables and, in particular, in the Asian summer monsoon rainfall and various El-Niño Southern Oscillation (ENSO) indices is well known (e.g. Rasmusson et al., 1990). More specifically, the presence of a biennial signal in the...