Intraseasonal Variability and Physical Characteristics of the Break and Active Phases of the Mainland Indochina Southwest Monsoon

The term ‘break’ is traditionally meant only for dry spells occurring after monsoon onset in the region. Simply put, the daily rainfall of the monsoon pauses across the region for a few days, which is called a ‘break spell’, and the revised pattern is called ‘active’. Researchers have suggested that standardised anomalies of three consecutive days of rainfall prevail when categorising active and break spells. This study examined break spells and active spells at interannual, intraseasonal, and decadal scales by examining the frequency and spatial distribution of three consecutive days of rainfall occurrences of different intensities linked to break and active events over the mainland Indochina region. The difference in the surface-to-upper wind circulation between active spells with excessive moist convection and intense break events with less rainfall was explained by various atmospheric parameters. During active phases, the easterly jet migrated south, while subtropical westerly entered lower latitudes during the break. During break spellings, the upper and lower troposphere jet winds will be weaker and dislocated over the study area but stronger during active time. La Niña encourages more break days than active days and distinguishing between vertical meridional circulation and intense break events with a heat trough-type circulation and active spells with moist convection is crucial for developing suitable prediction tools.