Dry air layers observed over the central equatorial Indian Ocean

Radiosonde sounding with high vertical resolution data at Gan Island, Maldives (0.7S, 73.2E) detected frequent appearance of dry air layers at various heights. In particular, mid-tropospheric dry layers result in significant decrease of total precipitable water vapor. While such mid-tropospheric dry layers often appeared after the passage of deep convections associated with the Madden-Julian oscillation (MJO), almost half of them appeared apart from the passage of MJO-convection. Backward trajectory analysis showed a common feature for those mid-tropospheric dry air layers that they originated from the subtropics in both hemispheres (mostly were located in the range of 40E - 60E) about a weak ago, when and where equatorward flow was accelerated mainly due to anti-cyclonic flow associated with Rossby wave breaking or cyclonic flow associated with extra-tropical baroclinic systems.

Currently, an international field campaign CINDY2011 (Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011), which aims to collect in-situ data for the study of initiation process of MJO-convection, is planned to be taken place in and around the central tropical Indian Ocean from October 2011 through January 2012. Above result suggest that the influence of the subtropics of the western Indian Ocean should be taken into account for the study of moisture variability over the central Indian Ocean, which is thought to be one key element for the initiation process.