Evolution of the Distribution of Upper-Tropospheric Humidity over the Indian Ocean: Connection with Large-Scale Advection and Local Cloudiness

Abstract : The spatial and temporal distribution of Upper-Tropospheric Humidity (UTH) observed by the SAPHIR/Megha-Tropiques radiometer is analyzed over two sub-regions of the Indian Ocean during October-November-December over 2011-2014. The properties of the distribution of UTH were studied regarding the phase of the Madden-Julian Oscillation (active or suppressed) and the large-scale advection vs. local production of moisture. To address these topics, first a lagrangian back-trajectory transport model was used to assess the role of the large-scale transport of air masses in the intraseasonal variability of UTH. Second, the temporal evolution of the distribution of UTH is analyzed using the computation of the higher moments of its probability distribution function (PDF) defined for each time step over the domain. Results highlight significant differences in the PDF of UTH depending on the phase of the MJO. The modeled trajectories ending in the considered domain originate from an area that strongly varies depending on the phases of the MJO: during the active phases, the air masses are spatially constrained within the tropical Indian Ocean domain, while a distinct upper tropospheric (200-150hPa) westerly flow guides the intraseasonal variability of UTH during the suppressed phases. Statistical relationships between the cloud fractions and the moments of the PDFs of UTH are found to be quite similar regardless of the convective activity. However, the occurrence of thin cirrus clouds is associated to a drying of the upper troposphere (enhanced during suppressed phases) while the occurrence of thick cirrus-anvil clouds appears to be significantly related to a moistening of the upper troposphere.