Active and break events of Indian summer monsoon during 1901–2014

Abstract

The study lists active and break monsoon events over India over a very long period (1901–2014) identified using criteria based on a rainfall index derived over a critical high rainfall region called core monsoon zone. The break and active spells identified in this study were mostly comparable with that identified in the earlier studies based on similar rainfall criteria during the common data period (1951–2007). However, some noticeable differences were observed in the rainfall anomaly pattern associated with the break monsoon spells identified in this study and that identified based on the synoptic criteria in the earlier studies. The stringent rainfall criteria used in this study seems to be better criteria for identifying the breaks. During the study period, both the active and break spells of short duration were more frequent than the long duration with about 63.4 % of the break spells and 94.3 % of the active spells falling in the range of 3–6 days. There were no active spells of duration ≥13 days. Whereas, about 8 % of the break spells were of duration ≥13 days. During both the halves of the data period (1901–1957 and 1958–2014), there was no change in the distribution of the break events. However, the number of active spells showed an increase of about 12 % in the in the second half, which was mainly in the short duration (3–6 days) spells. During the data period, decadal variations of break days showed an out phase of relationship with the number of days of monsoon depression (MD). Relatively stronger in phase relationship was observed between the decadal variation of MD days and that of the active days till around early 1980s which reversed later due to sudden decrease in the MD days. During the same period, both the active and break days were in the increasing phase. This was also coincided with the sudden and significant increase in the number of days of monsoon lows (LOW). The LOWs, which generally have short life helped in the occurrence of active spells of short duration. Thus, post early 1980s, the increase in the active days covering short duration active spells caused by the significant increase in the LOW days compensated the decrease in the active days covering relatively long duration active spells caused by the MD days. This lead to the out of phase relationship between MD days and the active days post early 1980s.