For the first time, a study published recently has provided the statistics of dry and wetter than normal monsoon season conditions during July and August over India for the longest period (1901-2014) based on a single criterion — rainfall.
The dry condition, also called known as break, occurs when large-scale rainfall over the monsoon trough zone (the zone between which the monsoon fluctuates north and south wards) is interrupted for several days. On the other hand, the intervals between spells of dry monsoon conditions when the rainfall is higher than normal are known as active spells.
The study was published in the journal Climate Dynamics.
The study found that the rainfall anomaly during the break and active spells was such that there was above normal rainfall during the active (wet) spell and scanty or below normal rainfall during the dry spell.
During the active spell, rainfall was “significantly” above normal particularly over the following areas — monsoon trough zone, along the west coast, along the foothills of Himalayas and over southeast Peninsula.
The impact of active/break monsoon conditions on the rainfall and low level wind circulation patterns is not limited to the Indian region. It is felt over a larger spatial scale extending up to west Pacific as well as other areas of the Asian monsoon region.
During the study period, both the active and break spells of short duration of 3-6 days were more frequent than the long duration (of more than 13 days). In the case of break spells it was 63.4 per cent, while it was 94.3 per cent in the case of active spells.
During both the halves of the data period (1901-57 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 per cent during the second half; the increase was mainly in the short duration (3-6 days) spells.
In an email to this Correspondent, the lead author of the paper, Dr. D.S. Pai, who is the Head of Climate Division, India Meteorological Department, Pune, notes: “It was found that during the data period, both break and active days show significant variation in rainfall between each decade of the study period (1901-2014) for the months of July and August. This is natural.”
According to him, till around early 1980s there was more number of monsoon depression days leading to long duration active spells. Post early 1980s, this condition reversed leading to shorter active spells due to a sudden decrease in the monsoon depression days.
Post early 1980s, there was an increase in the short duration active spells. This was caused by the significant increase in periods of less intense low pressure than the low pressure on monsoon depression days. An increase in periods of less intense low pressure compensated for the decrease in longer duration active spells caused by the monsoon depression days.
