Introduction

A large number of observational and numerical studies have been carried out in recent years to investigate the general circulation of the tropical atmosphere. The observational studies were greatly affected by the recent development of a wide variety of global observational systems (satellite, aircraft, etc.). Most of these studies have dealt with regional or global monthly/seasonal mean analyses of the tropical flow fields (e.g., Aspliden et al., 1965; Flohn, 1971 and Newell et al., 1972) and provided characteristic mean features of the tropospheric motion fields. However, not many long sequences of daily analyses required for a quantitative investigation have ever been performed. Only two such studies are available (Krishnamurti and Rogers, 1970 and Krishnamurti et al., 1975). These studies provided daily analyses of the upper-tropospheric motion fields during northern summer seasons. More recently, it has been recognized that the analyses of the tropical motion fields prepared at the US National Meteorological Center (NMC) have been improved as a result of extensive data-gathering efforts and have become useful data sources.