As monsoon finally arrives, key technical terms you need to know to follow IMD updates

The onset of the Southwest monsoon marks the beginning of the four-month long (June-September) monsoon season and this year the southwest monsoon hit the Kerala coast on June 8 after a delay of 7 days. The onset of monsoon in Kerala typically occurs around June 1 every year.

This year, it was the longest delay in arrival of monsoon in the last seven years since 2016 when the monsoon onset was marked on June 8.

Here are some of the key terms used by the IMD in the monsoon forecast which may help you understand the weather predictions better:  

Onset, advancement, withdrawal

The advancement of the monsoon refers to the northward progression of the rain-bearing winds across the Indian subcontinent. After the onset on Kerala coast the monsoon advances northwards, usually in surges, covering different regions of the country in the next couple of weeks by July 15, according to the India Meteorological Department (IMD).   

The weather forecaster on Friday, June 9 said that conditions are favourable for further advance of Southwest monsoon into some more parts of Central Arabian Sea, remaining parts of Kerala, Tamil Nadu and Karnataka in the next 24 hours. The IMD also predicted that monsoon could progress to some more parts of southwest, Central and northeast Bay of Bengal and some parts of northeastern states.

Update of Onset of Southwest Monsoon.#Monsoon #WeatherUpdate #India #IMD@DDNewslive @moesgoi @ndmaindia @airnewsalerts pic.twitter.com/MOX3B1AxJM

— India Meteorological Department (@Indiametdept) June 9, 2023

Variability/periodicity of monsoon

The interannual variability of monsoon rainfall refers to the year-to-year variation observed over a large number of years. It is influenced by global oceanic and atmospheric phenomena such as the El Nino Southern Oscillation (ENSO). The current transition from La Nino to neutral conditions in the equatorial Pacific region has raised concerns, as climate models suggest a possible development of El Nino conditions during the monsoon season.

Northern Limit of Monsoon

The northernmost limit of the monsoon's advance on any given day is referred to as the Northern Limit of Monsoon (NLM). According to IMD, the predicted range for the NLM extends from 55ºE to 80ºE. Additionally, the zonal wind speed over the region is bounded by latitude 5-10ºN.

Long period average

The Long Period Average (LPA) of rainfall is the rainfall observed across a certain location for a specified interval such as month or season averaged over a long time. LPA of seasonal monsoon rainfall from June to September is expected to be 96 per cent of the LPA, with a model error of ± 5 per cent. However, the possibility of El Nino conditions developing during the monsoon season introduces uncertainties into this forecast.

Low pressure area

A low-pressure area is defined as a region in which the atmospheric pressure at sea level is lower than that of the surrounding areas. In recent weather updates, the IMD highlighted the formation of a low-pressure area in the southwest Arabian Sea, which may evolve into a cyclonic storm named Cyclone Biparjoy.

Synoptic mode of variation

The synoptic-scale weather systems, including monsoon lows, monsoon depressions, mid-tropospheric cyclones, tropical cyclones, and western disturbances, significantly influence weather conditions over the Indian subcontinent. These systems, varying in size and duration, play a vital role in shaping the annual mean Indian rainfall and can bring extreme events such as floods, snowstorms, and avalanches to the region.

Monsoon depression

According to IMD, the depressions which form in the monsoon season are called the monsoon depressions. These are low pressure areas with two or three closed isobars and cause most of the monsoon rains. The monsoon depression could be of bay origin, land origin or Arabian Sea origin.