Australia: The Land Where Time Began

A biography of the Australian continent 

The Jet Stream

There are Polar Jet Streams and subtropical jet streams in both the Northern Hemisphere and the Southern Hemisphere. The Subtropical Jet Stream is located in the upper poleward end of the Hadley Cell and the Polar Jet Stream is in upper polar end of the Ferrel Cell, to the north and south of the Equator.

Jet streams are rivers of air flowing at high speed at an altitude of somewhere between 10-15 km in the atmosphere, near the tropopause, the boundary between the troposphere and the stratosphere, meandering in the same manner as rivers on land. The area of the strongest temperature contrast on the surface between the northern colder air and the southern warmer air is indicated by the position of this upper-level Jet Stream. A stronger demarcation is present during winter.

The north polar Arctic Jet Stream encircles the Earth in northern latitudes separating the cold air to the north from the warm air on its southern side. There is less sea ice forming as the Arctic region warms, with the result that more heat energy is transferred from the ocean to the atmosphere. There is a reducing difference in temperature between the air to the north of the jet stream and the air to the south of it, the velocity of the air in the jet stream is declining and its meanders are becoming larger. There are larger loops along the course of the jet steam; a larger northern loop brings warmer air to the north and larger southern loops bring cold Arctic air to the south. This strong current or currents of air form the Jet Stream.

Changes in the Arctic Polar Front Jet Stream are being caused by global warming, and weather patterns are being affected by these changes, and there is increasing weather uncertainty.

The future effects of the northern Polar Jet Stream on weather in the Northern Hemisphere are a major concern of climate scientists. As the northern polar Jet Stream meanders across the continents and ocean basins in the Northern Hemisphere it has a major influence on climate.

As global warming progresses the extent of the northern ice cap is rapidly decreasing, and as the ice retreats a greater area of sea surface is being exposed to the sunlight and more heat energy is absorbed by the ocean which affects the Jet Stream. The speed of the Jet Stream decreases and it develops a widely meandering path as the temperature of the Arctic rises, resulting in more warm air moving north and more cold air moving south. When combined with increased energy in the atmosphere there is increased instability in the Northern Hemisphere with increased chances of severe storm activity.

In the middle latitudes weather systems move rapidly across the surface as they are driven by fast-moving air currents such as the Jet Stream are not good breeding grounds for air masses.

In March 2012 there was a very early spring with a heat wave and unusually warm weather in the U.S. No single weather event can be attributed directly to global warming, though all current climate events result from a new average global temperature which continues to rise steadily.

The ice sheets that covered much of North America disrupted the Jet Stream and affected weather, delivering increased rainfall (pluvial events) to the southwestern U.S. that is presently dry. In Europe and Siberia the ice sheets had a similar effect, increasing rainfall to Iran and Afghanistan that are now dry areas.

According to Frakes, Frances & Syktus² the boundary conditions for the Last Glacial Maximum (LGM) that included expanded ice sheets and reduced CO₂ levels, atmospheric general circulation (AGCM) coupled with a static mixed layer ocean, produce significant cooling of the Southern Hemisphere.

Reduced CO₂ levels in the high southern latitudes (Broccoli & Manabe, 1987) caused the cooling at high southern latitudes. When these boundary conditions were used for the LGM the tropospheric circulation is modified significantly, particularly in the Northern Hemisphere during winter. The jet stream was split by the ice sheet covering North America which sent a southern branch, which was stronger than that of the present, over the southern part of the U.S. The Jet Stream, being the boundary separating the cold, dry high-latitude air masses from the warm, wet low latitude air masses, as well as displacement to the south of the Jet Stream, led to conditions in which most of the U.S. was dominated by cold air during the LGM (Kutzbach, 1985).

Links

NOAA National Weather Service 

Sources & Further reading

1. Farmer, G. Thomas & Cook, John, 2013, Climate Change Science: A modern Synthesis, The Physical Climate Vol.1, Springer Dordrecht

2. Frakes, Lawrence A., Francis, Jane E. & Syktus, Jozef L., 2005, Climate Modes of the Phanerozoic, Cambridge University Press