The VASCO-CIRENE experiment

Recent studies based on Tropical Rainfall Measuring Mission's (TRMM) Microwave Imager (TMI) data shows that tropical intraseasonal perturbations may be associated to Sea Surface Temperature (SST) variations of several degrees, especially south of the equator in the western Indian Ocean (55°E-80°E, Eq-15°S) during boreal winter. While this variability is partly reproduced by forced or coupled Ocean models, the relative role of different physical processes (warm layer formation, Ekman pumping, sub-surface cooling due to vertical mixing, surface fluxes) in these intraseasonal SST perturbations still has to be established. Since there are very few in situ observations in this region, an experimental campaign is needed to confirm the hypotheses that can be built using numerical modeling.

The objective of the VASCO-CIRENE campaign is to measure the impact of the different physical processes listed above on SST perturbations from diurnal (warm layer) to intraseasonal time-scales. This aims to better explain (i) the mechanisms of the intraseasonal variability of the SST and (ii) the feedback of these SST variations on the atmosphere. The CIRENE ship campaign, within the West Indian Ocean will be synchronized with the VASCO observing system (Aeroclippers and pressurized balloons launched from the Seychelles). During CIRENE, physical oceanography, air-sea fluxes and atmospheric measurements will be collected following a quasi-Lagrangian approach (i.e. the ship will follow the ocean surface flow). A special care will be taken in measuring the diurnal cycle in the surface layer (B. Ward's ASIP instrument) since it is believed to play an important role in intraseasonal SST variability. Biogeochemical measurements (nutrients, pigments) will also be collected because they can provide useful information on the physical processes at work. These measurements will be combined with those from VASCO and from PROVOR floats deployed in 2004. Both the scientific objectives and the geographical location of this campaign fit into CLIVAR IOP objectives and the CIRENE region is now recognized as a key region for the development of intraseasonal events.