Index cycle is a complete circular motion of Jet stream. Typically it blows from west to east, but change in direction is a common characteristics of this wind. This wind system takes 1 to 2 months to make an index cycle. Four necessary steps are required for this index cycle.
First stage: In this stage the wavy pattern of this wind is not prominent and wind blow from west to east. Existence of the jet stream in this phase is being found along 40°N latitude. Here also to be mentioned that the velocity of the wind is so high that’s why this stage is also called as High Zonal Index.
Second stage: wavy pattern of this phase is getting prominent and wind moves towards the equator gradually and velocity also getting declined.
Third stage: wavy wind in this phase extend upto 15°-20° latitude and near about 60° longitude. Cold polar wind and warm tropical wind move towards tropical region and polar region respectively. The wavy pattern of the wind is so large that only three to six Rossby wave can cover the whole earth.
Fourth stage: This last stage is characterized by extreme meandering of the jet stream, resulting into cutting off meanders and given birth independent circular pattern In the form of cyclonic and anticyclonic circulation. velocity of the wind in this phase declined rapidly. This phase is also called as low zonal index.
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