At times the 30–60 day filtered outgoing longwave radiation (OLR) perturbations exhibited a systematic eastward propagation across the equatorial Indian Ocean–western Pacific during the five summers (1 May–30 September) of 1979–83. Such occasions are defined as “E” phase, while periods of irregular movement are designated as “NE” phase. Global-scale behavior of the 30–60 day filtered streamfunction and velocity potential fields differs significantly from E to NE phase.
During E phase at 200 mb, a series of time-clustered, space-overlapping disturbances develop over the northern as well as the southern subtropics. Although individual disturbances are nearly stationary, a wave packet clearly propagates eastward with an approximate phase speed of 8° longitude per day and a space scale of wavenumber 1. Sandwiched between the Southern and Northern hemisphere wave packets are relatively weak equatorial zonal wind perturbations which also move eastward. At 850 mb, E phase behavior is characterized by strong 30–60 day southerly surges originating over the cold midlatitude Indian Ocean. These southerly surges coincide with widespread convection over the monsoon region (0°–20°N, 40°–160°E), where a strong continent–Ocean heat contrast exists. The 30–60 day southerly surges are much less pronounced over relatively orographic free regions of the western hemisphere tropics.
During NE phase, no systematic eastward propagation occurs in the streamfunction and velocity potential fields. Compared with the E phase, the streamfunction patterns are much less organized, with dominant wave-numbers higher than 2. At 850 mb, southerly surges are limited to the immediate vicinity of the African east coast.