- A feature connected with the SO is the El Nino, a warm ocean current that flows past the Peruvian Coast
- In place of the cold Peruvian current, every 2 to 5 years.
ENSO (El Nino Southern Oscillations)
- The changes in pressure conditions are connected to the El Nino. Hence, the phenomenon is referred to as ENSO
- This is a name given to the periodic development of a warm ocean current along the coast of Peru
- It occurs when easterly trade winds in the tropical Pacific relax- even reverse- to allow a vast pool of warm water piled up in of central and South America, leading to higher- than- normal sea- surface temperatures across the equatorial Pacific.
- As a temporary replacement of the cold Peruvian current
- El Nino’ is a Spanish word meaning ‘the child’,
- Refers to the baby Christ, as this current starts flowing during Christmas
- As the ocean releases its heat and moisture to the atmosphere, intense thunderstorms once cooped up over the western Pacific spread along the equator as well.
- The presence of the El Nino leads to an increase in sea-surface temperatures and weakening of the trade winds in the region
- The cumulative effect of this activity changes large-scale circulation patterns at higher latitudes, altering storm tracks that change the typical distribution of rain and snowfall, as well as seasonal temperatures.
THE ONSET OF THE MONSOON AND WITHDRAWAL
- The Monsoon, unlike the trades, are not steady winds but are pulsating in nature, affected by different atmospheric conditions encountered by it
- The duration of the monsoon is between 100-120 days from early June to mid-September.
‘Burst ‘of the monsoon,
- Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days. This is known as the ‘burst ‘of the monsoon
- The monsoon arrives at the southern tip of the Indian peninsula generally by the first week of June.
- Subsequently, it divides into two – the Arabian Sea branch and the Bay of Bengal branch.
- The Arabian Sea branch reaches Mumbai about ten days later on approximately the 10 Th of June.
- The Bay of Bengal branch also advances rapidly and arrives in Assam in the first week of June.
- The lofty mountains cause the monsoon winds to deflect towards the west
- Over the Ganga plains
- By mid-June, the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country.
- The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the north-western part of the Ganga plains.
- Delhi generally receives the monsoon showers from the Bay of Bengal branch by the end of June (tentative date is 29 Th of June).
- By the first week of July, western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the monsoon.
- By mid-July, the monsoon reaches Himachal Pradesh and the rest of the country
- The withdrawal of the monsoon begins in north-western states of India by early September.
- By mid-October, it withdraws completely from the northern half of the peninsula.
- The withdrawal from the southern half of the peninsula is fairly rapid
- By early December, the monsoon has withdrawn from the rest of the country.
- The islands receive the very first monsoon showers, progressively from south to north,
- From the first week of April to the first week of May.
- The withdrawal takes place progressively from north to south from the first week of December to the first week of January.
- The monsoon type of climate is characterised by a distinct seasonal pattern.
- The weather conditions greatly change from one season to the other.
- Are any seasons experienced in your place? Four main seasons can be identified in India –
- The cold weather season,
- The hot weather season
- The advancing monsoon
- The retreating monsoon
The Cold Weather Season (winter)
- The cold weather season begins from mid-November in northern India and stays till February.
- December and January are the coldest months in the northern part of India.
- The northeast trade winds prevail over the country.
- They blow from land to sea and hence, for the most part of the country, it is a dry season.
- Some amount of rainfall occurs on the Tamil Nadu coast from these winds as here they blow from sea to land.
- In the northern part of the country, a feeble high-pressure region develops, with light winds moving outwards from this area.
- Influenced by the relief, these winds blow through the Ganga valley from the west and the northwest.
- The weather is normally marked by clear sky, low temperatures and low humidity and feeble, variable winds.
- A characteristic feature of the cold weather season over the northern plains is the inflow of cyclonic disturbances from the west and the northwest.
- These low-pressure systems, originate over the Mediterranean Sea and western Asia and move into India, Along with the westerly flow.
They cause the much-needed winter rains over the plains and snowfall in the mountains. Although the total amount of winter rainfall locally known as ‘mahawat ’is small, they are of immense importance for the cultivation of ‘rabi ’crops