Introduction:
Rainfall in India varies seasonally, and the total monthly or seasonal rainfall is calculated based on the amount received across the country during specific months or seasons.
The monsoon, which starts in June and moves from the southern peninsula to central India, significantly influences this pattern. For instance, in June, rainfall was recorded as 11% below normal across the country.
Body:
Definition and Calculation of Rainfall Categories:
- Rainfall can be classified as below-normal, normal, or above-normal using statistical methods.
- The mean (m) and standard deviation (d) of historical rainfall data are used for this.
If the rainfall falls within:
m – d to m + d: Normal rainfall
Less than m – d: Below-normal rainfall
More than m + d: Above-normal rainfall
Factors Contributing to Distorted Rainfall Distribution:
Monsoon Trough Movement:
- The monsoon trough, a low-pressure zone between Pakistan and the Bay of Bengal, oscillates during the season.
- When the trough moves southward, regions like central and southern India receive more rainfall.
- When it shifts north, the Himalayan foothills receive excess rain, while other regions face a deficit.
- Example: In 2023, southern and central India experienced excess rainfall due to the southward shift of the monsoon trough, while parts of northern India faced drought-like conditions.
Offshore Trough and Other Weather Systems:
- An offshore trough, which forms along the coast, can also result in excess rainfall in certain regions.
- Example: In the monsoon season of 2019, coastal Maharashtra saw record rainfall due to an active offshore trough.
Insufficient Rainfall in Northern and Northeastern India:
- Regions like the northwest and northeast of India received below-normal rainfall because of fewer rain-bearing systems, with some systems failing to reach these areas.
- Example: In the 2022 monsoon, parts of Uttar Pradesh and Bihar received significantly less rainfall due to weak systems in the area.
Conclusion:
Although the monsoon season started with below-normal rainfall in June, improved rainfall distribution in July, driven by low-pressure systems in the Bay of Bengal, enhanced overall seasonal performance.
Example: From July 19 to July 25, India saw 41% excess rainfall (80.5 mm compared to the usual 57.2 mm), benefiting southern and central regions, which experienced a surplus. However, northern and northeastern regions still lagged due to inconsistent rain-bearing systems.