Date of this Version
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, the source regions of all major ISO in the tropical Indian and western equatorial Pacific region 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. The non-propagating ISO develop in a relatively cool and dry environment. radiation-convection interaction mechanism seems to be playing an important role in triggering the non-propagating ISO.