We will see briefly about monsoon and then we will move on to the facts about South West Monsoon. Some of us might be thinking what is unique about monsoon, here comes the answer: Monsoon is the seasonal change in the prevailing wind that occurs mainly due to difference in heating of land and ocean. The monsoon season that are found in different regions of the world (NAM, SAM - North American Monsoon and South American Monsoon; NAF, SAF - North African Monsoon and South African Monsoon; IND - Indian Monsoon, EAS - East Asian Monsoon, WNP - Western North Pacific Monsoon and AUS -Australian Monsoon) are shown in the map below:
Figure 1. Monsoon regions of the world. (Credit: Prof. Bin Wang. WCRP report)
(Red color dots denotes the region where, monsoon rainfall contributes more than 55% of annual total rainfall)
These monsoon regions are mostly having agriculture dependent economy. India gets most (80%) of its annual rainfall from South West Monsoon season (SWM) between June to September.
Winds generally move from high pressure area to low pressure area. In the same way, the SWM occurs due to the wind movement from high pressure area which we call as Mascarene High in south Indian Ocean to low pressure over north west Rajasthan and adjoining Pakistan (Fig. 2).
Figure 2. Southwest Monsoon flow (H-High pressure and L - Low pressure).
The south east trade winds from southern hemisphere crosses the equator and turns its direction to south west due to the rotation of earth (coriolis force) and enters the Arabian Sea and Bay of Bengal. These winds travels thousands of kilometres and picks up lots of moisture as they traveled over ocean and when they move towards the low pressure area they give copious rainfall. The SWM reaches Andaman before hitting kerala. When these moisture ladden south west winds hits western ghats in Kerala, that is considered as the onset of monsoon over India. Normally June 1 is considered as the normal onset date. The the Arabian sea branch covers the country from south to north and Bay of Bengal branch covers from East to West (influence of the Himalayas) and covers the entire country by second week of July.
We will be thinking that monsoon has started, so for the next four months we are going to get evenly distributed rainfall, but that does not happen here. There will be weeks with very light or no rainfall (break in monsoon) and period with more rainfall (active monsoon). This variation that happens within the season is known as intraseasonal variation.
The normal monsoon starts to withdraw from the country starting from west and northwest India from 15 September and will completely recede the country by 15 October. The withdrawal period is characterized by dry weather for consequent five days and change in south westerly wind direction. The normal onset and withdrawal dates of SWM over India is given below (Fig. 3.)
Figure. 3. Normal Onset and Withdrawal of SWM
Credit: India Meteorological Department
To know all about India's South West Monsoon-normal dates, observed scenario and forecasts. Kindly visit India Meteorological Department (IMD) website.
The SWM forecast is issued by India Meteorological Department in India in two phases. First phase is in April and second phase in last week of May or first week of June.
How the monsoon is going to impact agriculture
The crop selection and crop production depends on monsoon rainfall in SWM season. If there is delay in monsoon then resowing or refilling is needed. If break in monsoon is prolonged or if there is more rainfall and flood conditions crop will give less produce, or total loss of crops in later condition. Seasonal forecast is of prime important in selecting the crop and for further management activities (intercultural operations) medium range forecast is needed.
Lets take a case: Late onset of South West Monsoon forecast and rice crop.
In this scenario we can delay the transplanting of rice from nursery to main field and while transplanting, due to the aged seedling -number of plants per hill has to be increased to maintain the optimum plant population in the main field.