Asian Summer Monsoon Heat Sources and Moisture Sinks in Global Reanalyses: Consistent Representation of Variability and Trends?

A comprehensive description of the water and energy cycles associated with the Asian summer monsoon has not been reached yet. In this context, it is important to analyze differences and similarities among current and commonly-used Reanalyses in depicting the main features of heating and moistening of the monsoon. NCEP/NCAR, NCEP/DOE and ERA40 data are used to study the three-dimensional variability of the atmospheric heat sources Q1 and moisture sinks Q2 of the Asian monsoon during the last decades. Vertically-integrated estimates and vertical profiles are analyzed, and temporal variations at sub-monthly scale are also considered in order to investigate in detail the annual cycle of the monsoon. The intercomparison of the three Reanalyses reveals several significant differences in terms of location, timing, intensity and (horizontal and vertical) variability of heating and convection, as the models do not necessarily depict the development and occurrence of the monsoon in the same way. A comparison with TRMM data is also carried out. The occurrence of some physical processes at regional scale (such as, land-surface heating over the Tibetan Plateau and land-atmosphere forcing; predominance of weak vs. deep convection and its spatial propagation during the season) is analyzed in the three datasets. At interdecadal scale, a trend of the diagnosed diabatic heating is found over the northern regions of the Indian Subcontinent, where an increase of springtime mid-to-upper Tropospheric heating is noticed. Correspondingly, the spatial distribution of observed precipitation, representative of the vertically-integrated heating in the Tropics, exhibits a consistent variability. Analysis is currently in progress to ascertain if these changes can be attributed to the increased concentration of absorbing aerosols as deduced from long-term available satellite observations and to study and understand the mechanisms by which the aerosol loading can affect the monsoon water cycle dynamics and variability. The need and usefulness of high-resolution Reanalysis data is finally discussed.