MONSOON( Long Period Average)

GEOGRAPHY

ADHYAYAN IAS ACADEMY

For the 2021 Southwest Monsoon Season Rainfall

Summary of the Forecast for the 2021 southwest monsoon Rainfall

a. Southwest monsoon seasonal (June to September) rainfall over the country as a whole is

most likely to be normal (96 to 104 % of Long Period Average (LPA)).

b. Quantitatively, the monsoon seasonal (June to September) rainfall is likely to be 98% of the

Long Period Average (LPA) with a model error of ± 5%. The LPA of the season rainfall

over the country as a whole for the period 1961-2010 is 88 cm.

c. Neutral ENSO conditions are prevailing over the Pacific Ocean and Neutral Indian Ocean

Dipole (IOD) conditions are prevailing over the Indian Ocean. The latest global model

forecast indicates neutral ENSO conditions are likely to continue over the equatorial Pacific

and negative IOD conditions are likely to develop over the Indian Ocean during the ensuing

monsoon season.

As sea surface temperature (SST) conditions over the Pacific and the Indian Oceans are known to

have strong influence on Indian monsoon, IMD is carefully monitoring the evolution of sea

surface conditions over these Ocean basins.

IMD will issue the updated forecasts in the last week of May2021. In addition to update for the

April forecast, forecasts for monsoon season (June-September) rainfall for four homogenous

regions and forecast for the month of June also will be issued.

1. Background

Since 2003, India Meteorological Department (IMD) has been issuing the operational long-range

forecast (LRF) for the southwest monsoon season (June-September) rainfall averaged over the

country as a whole in two stages. The first stage forecast is issued in April and the second stage or

update forecast is issued by the end of May. These forecasts are prepared using the state-of-the-art

Statistical Ensemble Forecasting system (SEFS) that was developed indigenously by IMD. In the

second stage, in addition to an update for April forecast, additional forecasts for monthly rainfall

for July and August for the country as a whole and forecasts for seasonal rainfall (June to

September Category Rainfall Range.

under the Monsoon Mission of the Ministry of Earth Sciences (MoES).

2. New Forecasting Strategy

There have been demands from different users and government authorities for forecasts of spatial

distribution of seasonal rainfall along with the regionally averaged rainfall forecasts for better

regional level planning of activities. For this specific purpose, Office of Climate Research &

Services, IMD, Pune has now developed a Multi-Model Ensemble (MME) forecasting system

based on coupled global climate models (CGCMs) from different global climate prediction and

research centers including the Monsoon Mission CFS (MMCFS) being used by IMD. Multi-Model

Ensemble (MME) is a universally accepted technique, which is used to improve skill of forecasts

and reduce forecast errors when compared to a single model-based approach. The performance

improvements are completely attributed to the collective information of all models used in the

MME forecasting system.

For the first stage forecast, the existing statistical forecasting system and the new MME based

forecasting system are used to generate forecasts. The MME based forecasting system also will be

used in the second stage forecasts in May for generating probabilistic forecasts for the country as a

whole and four homogenous regions of India.

For generating monthly forecasts, IMD will now use a dynamical MME framework instead of

present statistical models. Monthly probabilistic forecast for all four months (June to September)

will be prepared during the last week of previous month using the MME forecasting system.

IMD has also been making efforts to develop a separate forecast for the Monsoon Core Zone

(MCZ), which represents most of the rainfed agriculture region in the country. A separate forecast

for the MCZ will be more useful for agricultural planning and crop yield estimation etc. In the

second stage forecast in May, IMD will issue a separate probabilistic forecast for the MCZ, based

on MME system and a new statistical model.

3. Forecast for the 2021 Southwest monsoon Season (June–September) rainfall over the

country as a whole

3a.Forecast Based on Operational Statistical Ensemble Forecasting System (SEFS)

The forecast based on SEFS suggests that quantitatively, the monsoon seasonal rainfall is likely to

be 98% of the Long Period Average (LPA) with a model error of ± 5%. The LPA of the season

rainfall over the country as a whole for the period 1961-2010 is 88 cm.

The 5 category probability forecasts for the Seasonal (June to September) rainfall over the country

as a whole based on the SEFS forecast are given below, which suggests maximum probability for

monsoon seasonal rainfall to be

Below Normal 90 – 96 25 17

Normal 96 -104 40 33

Above Normal 104 -110 16 16

Excess > 110 5 17

3. b. Forecast based on the Multi Model Ensemble Forecasting System

For generating the MME forecast for 2021 southwest Monsoon season rainfall, March initial

conditions have been used. Climate models with the highest forecast skills over the Indian

monsoon region including MMCFS have been used to generate MME forecasts.

The MME forecast also suggests that the monsoon rainfall during the 2021 monsoon season (June

to September) averaged over the country as a whole is likely to be normal (96-104% of LPA).

The spatial distribution of probabilistic forecasts for tercile categories (above normal,

normal and below normal) for the seasonal rainfall (June to September) is shown in Fig.1.The

spatial distribution suggests either normal or above normal probability is likely over most parts of

the country.

4. Sea Surface Temperature (SST) Conditions in the equatorial Pacific & Indian Oceans

La Niña conditions (colder than normal SST over the equatorial Pacific) developed during second

part of the previous year, which peaked in November. However, La Nina conditions over the

equatorial Pacific started weakening in the early part of 2021 and conditions are now moving

towards ENSO neutral conditions. Subsurface temperatures over the equatorial Pacific have

substantially warmed and atmospheric patterns reflect neutral ENSO conditions. The latest

MMCFS and other global model forecast indicate further warming trends and retuning of ENSO

neutral conditions in the early part of the upcoming monsoon season.

At present, neutral Indian Ocean Dipole (IOD) conditions are prevailing over Indian Ocean. The

latest forecast from the MMCFS and other global models together indicate that neutral IOD

conditions are likely to turn into negative IOD conditions during the ensuing monsoon season.

seasonal rainfall over India during the 2021 southwest monsoon season (June-September). The

figure illustrates the most likely categories as well as their probabilities. The white shaded areas

represent climatological probabilities. The probabilities were derived using the MME forecast

prepared from a group of best coupled climate models. (Tercile categories have equal

climatological probabilities, of 33.33% each).

source : PIB