Propagating and non-propagating intraseasonal oscillations in the tropical atmosphere: Their vertical and horizontal structures and developing mechanisms

A fixed beamformer is proposed and designed to identify source regions of Intra-Seasonal Oscillations (ISO) in the tropical atmosphere. After tested by simulations of single and complex sources of waves, the fixed beamformer is applied to the ECMWF interpolated data grids to detect and identify source regions of the ISO in the tropical Indian and Pacific Ocean region. Results show that the fixed beamforming technique can uniquely identify the source region of the ISO, and the source regions of all major ISO in the tropical Indian and western equatorial Pacific region during the 29 yr from 1974 to 2002 have been identified.;Examinations of ISO development in the source regions indicate that besides the eastward propagating ISO, there were non-propagating ISO during this 29-year period. To understand why some ISO propagate while others are stationary, statistical analyses are used to examine the vertical and horizontal structures of these two types of ISO. Results show very different structures during the development and evolution of these two different types of ISO. For the propagating ISO, both moisture and temperature processes/disturbances are very important for the development of the ISO. This type of ISO is developed in a relatively warm and wet large-scale environment, and wind enhanced surface evaporation is a major mechanism. For the non-propagating ISO, temperature process is not as important as the moisture process in the development of ISO. Temperature anomalies remained weak before the onset of major convection in the ISO and reached peak intensity afterward peak convection. Both temperature and moisture anomalies developed but were confined within the source region. The non-propagating ISO develop in a relatively cool and dry environment. Although weak low-level easterly anomalies and surface evaporation existed before the onset of major convection in these ISO, radiation-convection interaction mechanism seems to be playing an important role in triggering the nonpropagating ISO. A key support for this notion is that relatively cold temperature anomalies persisted in the middle troposphere during the development of the ISO. This radiative cooling destabilized the troposphere profile and favored convection development.