In the first stage the jet stream lies quite close to the Polar Regions and flows from due west to east. In the northern hemisphere, the cold air mass is found to the north of the upper-level westerlies.
To the south of the jet lies the mild air of the multitudes. The westerlies in this stage have shifted towards higher latitudes where there is a lot of cyclonic activity. Pressure systems in this stage have got east-west orientation.
North-south pressure gradient is relatively steeper. The air mass exchange between the temperate and tropical regions is at its minimum. The first stage thus represents the high zonal index.
In the second stage, the amplitude of jet stream waves increases. The whole of the jet moves towards the equator as a result of which there is an inroad of cold polar air southward. The warm air masses from lower latitudes move towards higher latitudes.
In the third stage, the bends in the jet stream become sharper and the amplitude of waves registers a further increase. At this stage the tropical air masses move farther north, and the cold polar air moves farther south.
Now, the jet stream is positioned nearer the equator and the exchange of tropical and polar air masses takes place on a much larger scale. The temperature gradient is directed from east to west. In the last stage of index cycle, the giant size meanders of the jet are cut off from the main stream.
The result is that an immense pool of cold and dense polar air is isolated in the upper troposphere of the lower latitudes where it is encircled by entirely different air masses. In the upper atmosphere of higher latitudes the tropical air masses are entrapped by the colder air.
This is called the low zonal index of the jet stream. The zonal character of the upper-level westerlies is no longer in existence. They are fragmented into a number of cells.