Jet Streams and Indian Monsoons
The jet streams are high altitude (9000-12000 m) Westerly winds between middle latitudes (summer 35°N, winter 20°N-35°N) in the Northern hemisphere. Recent researches have shown that these winds exert considerable impact on surface weather conditions. The tropical Easterly jet stream extends far to the North of Tibet of the Tibet and the air flow is roughly along the Kolkata- Bangalore axis. These upper air Easterlies descend into the permanent high pressure area formed over the southern Indian Ocean, This naturally intensifies the high pressure already present there.
It is a form this high pressure cell that the onshore winds start blowing towards the thermally induced low pressure area, developed in the Northern part of the Indian sub-continent. After crossing the equator such winds become South-Westerly and are known as the South-Westerly Summer Monsoons.
These surface winds have vast potentiality for South-Westerly summer, monsoon and precipitation. It is therefore, clear that the strength of the Easterly jet stream is directly related to the intensification of permanent high formed over Southern Indian Ocean. Since this high pressure makes the pressure gradient steeper, so it is the main causative factor for determining the vigour of the summer monsoon.
The word El-Nino means child Christ because this current appears around Christmas in December. December is summer month in Peru (south hemisphere). It is complex weather system that appears once every 3 to 7 years, bringing drought, floods and other weather extremes to different parts of the world.
The system involves oceanic and atmosphere phenomena with the appearance of warm currents off the Cost of Peru in the Eastern Pacific. El-Nino is merely an extension of the warm equatorial current Humboldt Current. This current increases the temperature of water on the Peruvian coast by 10°C which result in
• Distortion of equatorial atmospheric circulation.
• Irregularities in the evaporation of sea water.
• Reduction in the amount of Planktons which further reduces the number of fish in the sea.
Effect of El-Nino on Indian Monsoon
It has been noticed that changes in the pressure conditions over the southern Ocean also affect the monsoons. Normally, when the tropical Eastern South Pacific Ocean experiences high pressure (El-Nino phenomena), the tropical Eastern Indian Ocean experiences low pressure. But in certain years, there is a reversal in the pressure conditions and the Eastern Pacific has lower pressure in comparison to the Eastern Indian Ocean.
The difference in pressure over Tahiti (Pacific Ocean, 18°S/149°W) and Darwin in Northern Australia (Indian Ocean, 12°30’/131°E) is computed to predict the intensity of the monsoons. If the Pressure difference were negative, it would mean below average and late monsoon.