Southwest Monsoon – How Critical is July?

Today’s Business Line has carried an article on How Critical is July to the performance of Southwest Monsoon, based on a study done by us http://www.thehindubusinessline.com/industry-and-economy/agri-biz/poor-july-rain-may-lead-to-below-normal-monsoon-overall-says-study/article7395871.ece. The complete study is given below

Is July the most critical month for Southwest Monsoon? July is considered the most critical month for Southwest Monsoon as by this time Monsoon onset has covered most parts of the country and the monsoon trough is well established thereby making conditions conducive for the development of Land depressions over Gangetic Plains. With IMD expecting July to be worse than June this study aims to understand the linkage between the rains for the month of July and the corresponding Southwest Monsoon season on the whole.

Why is July the most critical month of Southwest Monsoon?

  • Nearly one third of the country’s rains for the Southwest Monsoon period falls during the month of, July.
  • The country receives almost 20% more rains in July than August, similarly it receives about 40% more rains in July compared to June and almost 70% more rains than September.
  • 21 out of 30 Meteorological Sub Divisions of the country receive the highest monthly rainfall of Southwest Monsoon during the month of July
  • Saurashtra sees almost 40% of its SWM seasonal rainfall fall during the month of July while at the other extreme Tamil Nadu sees only 21% of its SWM seasonal rainfall fall during July.
  • July & August together contribute as high as 2/3rd of the SWM seasonal rainfall in about 14 meteorological sub divisions. Composite Madhya Pradesh sees more than 70% of its SWM seasonal rainfall fall during July & August.
  • The statistical probability of a meteorological sub division receiving less than 90% rainfall during the month of July ranges between 35% for sub divisions like Jharkhand, West Madhya Pradesh to almost 50% for sub divisions like Haryana Saurashtra & Bihar.
Divisions with lowest statistical probability for failure in July Probability (Normal Years)
Jharkhand 35%
West Madhya Pradesh 35%
Chhattisgarh 37%
Konkan & Goa 37%
Sub Himalayan West Bengal 37%
Divisions with highest statistical probability for failure in July Probability (Normal Years)
Bihar 49%
Haryana 49%
Saurashtra 49%
Assam & Meghalaya 48%
Madhya Maharashtra 48%

What happens during El Nino Years? Considering 2015 is an El Nino Year how does the probability stack up for the divisions to receive less than 90% rainfall.

  • Except for Odisha, Konkan & Goa and Chhatisgarh for all other sub divisions El Nino years sees an increase in probability of receiving less than 90% rainfall.
Divisions with lowest statistical probability for failure in July (El Nino Years) Probability (Normal Years) Probability (El Nino Years)
Chhattisgarh 37% 35%
Konkan & Goa 37% 35%
Odisha 40% 35%
Jharkhand 35% 40%
Sub Himalayan West Bengal 37% 40%
  • The probability at the top end of the scale is at a critical 70% for Haryana, Punjab & West Rajasthan. In other words 7 out of 10 El Nino years has seen these sub divisions receive less than 90% rainfall.
  • The divisions where almost 2 out of 3 El Nino years has seen less than 90% rains during July
Divisions with highest statistical probability for failure in July during El Nino Years Probability (Normal Years) Probability (El Nino Years)
Punjab 43% 70%
West Rajasthan 48% 70%
Haryana 49% 70%
West Uttar Pradesh 40% 65%
Tamil Nadu 46% 65%

Relationship between July & Seasonal Southwest Monson rainfall. How does a Bad July possibly influence the SWM of the season? If one were to isolate the year both July & Southwest Monsoon seasonal rainfall has received less than 90% rainfall the synopsis are as follows.

  • In 20 out of 30 Divisions if July has received less than 90% rainfall 1 in 2 years the seasonal Southwest Monsoon rainfall has been less than 90% establishing as well a very strong relationship between the performance of July & the seasonal Southwest Monsoon rainfall. The relationship is very strong in certain divisions like North Interior Karnataka, West Uttar Pradesh
Divisions with highest probability of SWM deficit the same years of July Deficit Probability of SWM deficit if July fails
North Interior Karnataka 70%
West Uttar Pradesh 68%
Punjab 67%
Saurashtra 65%
Marathwada 64%
  • On the other hand in divisions like Odisha the probability of SWM failing the year July rainfall has failed is only 37% Despite July failing the seasonal SWM rainfall more often than not has not been deficit.
Divisions with highest probability of SWM deficit the same years of July Deficit Probability of SWM deficit if July fails
Odisha 37%
Sub Himalayan West Bengal 39%
Coastal Karnataka 42%
East Uttar Pradesh 43%
Chhattisgarh 43%

During El Nino Years how does a failed July influence the seasonal SWM rainfall?

  • Critically During El Nino Years 21 out of 30 divisions see seasonal SWM fail 2 out of 3 times July has failed. This is as high as 90% for divisions like Bihar. Overall the trend is during El Nino Years failure during the month of July results in failure of SWM for the season.  The only exceptions to this are Odisha, Sub Himalayan West Bengal and Assam & Meghalaya where the ratio of failed July resulting in failed Monsoon is less than 50%.  In some divisions like Bihar 9 out 10 El Nino years when July has failed the SWM season has failed as well.  7 divisions in all have a probability of more than 80% for a failed SWM season in case of a failed July.
Divisions with highest probability of SWM deficit the same years of July Deficit Probability of SWM deficit if July fails Probability of SWM deficit if July fails (El Nino Years)
Bihar 52% 90%
North Interior Karnataka 70% 89%
Marathwada 64% 88%
Konkan & Goa 48% 86%
West Rajasthan 57% 86%
West Uttar Pradesh 68% 85%
Saurashtra 65% 83%

The Big Picture. The importance of July across all divisions of the country. (Please click image for full resolution)

The complete picture on which division is under threat of a derailed monsoon because of a bad July under El Nino conditions.

swm_july_3