4st PART










The Jet-Stream

The Jet Stream is a very fast stream of air a few hundred kilometers wide, and only a few kilometers thick. It is usually located at the tropopause between 6-9 miles (10-15 kilometers) above the Earth's surface. These currents blowing from west to east according to the rotation of the Earth. Winds speed inside these currents is about 124 to 186 mph (200 to 300 km/h) but can exceed 248 mph (400 km/h).

There are two main jet streams :

- the subtropical jet stream at about 30 degrees latitude ;

- the polar jet stream at about 60 degrees latitude.

Location of the Polar and Subtropical Jet Stream.

A jet stream is formed when a warm current from the tropics meets a cold current from the poles. The strong thermal contrast forces the air to flow horizontally and as the Earth rotates, this fast-moving air picks up speed and produces a jet stream.

The location of the jet stream denotes the place of the strongest contrasts of temperatures between different latitudes on the Earth's surface, thus sharing, or bounding, the cold air (north) of hot air (south) of jet stream in the Northern Hemisphere. For example, when the winter is mild in southern Quebec, it is because the jet stream retreats northward in Canada, so it is not uncommon, during a severe winter of see the jet stream plunging down on the Gulf of Mexico and bringing cold air into the center of the United States. Often, even within the jet stream, there are areas where the wind is stronger than around. These regions play an important role in the formation of precipitation and depressions. Moreover, we note that systems tend to borrow their trajectory. The jet stream can take directions towards the north according to the movement of air masses. Consequently, the strongest jet streams usually occur during the winter months, when large differences of temperature exist between low and high latitudes.

During El Niño, events, the jet streams are increasing, because the rise of temperature in the central Pacific increases the difference of pressure between the equator and mid latitudes. The NAO has the same effect, but much smaller. When the NAO+ jet stream is more located to the north than in the NAO-.

Click here to see an animation showing the speed of the jet stream during the
winter 1982-1983 (November-March) marked by an el nino very intense. The
strongest winds are blowing in the white areas surrounded by red (over 220 km/h)
and in red areas (over 150 km/h) and blue-green areas (less than 100 km/h)

A video explaining the formation of Jet-Stream

The subtropical jet streams that circulate in parallel with latitudes (green arrows) furnish the trade winds, returning
toward the equator (blue arrows), completing the Hadley cell. They also feed the winds that are moving towards
the poles and are part of the Ferrel cell. Global atmospheric circulation includes several "cells" or loops juxtaposed
wind. In an event El Niño, which modifies the Walker cell (parallel to the equator) and the Hadley cell, neighboring
cells are also modified, as well as their neighbors ... which is why El Niño has implications across the globe.

Click here to see an animation of the Jet Stream over France : Jet stream animation :

Horizontal map at 210 hPa. Loop over time.

Click here to see an animation in 3D of the Jet Stream :

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