AOS 101 Weather and Climate Lisha M. Roubert University of Wisconsin-Madison Department of Atmospheric & Oceanic Sciences The Jet Stream Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere (troposphere) that generally blow from west to east at an altitude of 20,000 - 50,000 feet . Surface temperatures determine where the jet stream will form. The greater the difference in temperature, the faster the wind velocity inside the jet stream. This abrupt change in temperature causes a large pressure difference, which forces the air to move. Jet streams can flow up to 200 mph (322 km/h), are 1000's of miles long, 100's of miles wide, and a few miles thick. Jet streams follow the boundaries between hot and cold air. Since these hot and cold air boundaries are most pronounced in winter, jet streams are the strongest for both the northern and southern hemisphere winters. Clouds along a jet stream over Canada. Why is it important to know about Jet Streams? Meteorologists use the location of some of the jet streams as an aid in weather forecasting. The main commercial relevance of the jet streams is in air travel, as flight time can be dramatically affected by either flying with the flow or against the flow of a jet stream. Clear-air turbulence is often is found in a jet stream's vicinity. Jet streams flow from west to east in the upper portion of the troposphere. What causes Jet Streams? Jet streams are caused by a combination of a planet's rotation on its axis and atmospheric heating (by solar radiation on Earth and on other planets by internal heat). The jet stream results from latitudinally and vertically large gradients of temperature and pressure at the intersection of a colder air mass from the north with a warmer air mass from the south. The jet tends to occur at about the height of the tropopause, the transition from the troposphere (where temperature decreases with height) to the stratosphere (where temperature increases with height). The combination of these differences in air mass, plus some help from the coriolis force (coriolis acceleration) creates the acceleration of winds into the jet stream. The Polar and Subtropical Jet Stream 1. Polar Jet Stream Cold polar air flowing down from the north meets the warmer air mass over the United States causing the polar jet stream to form. 2. Subtropical Jet Stream (stronger in the winter) The ascending air in the equator flows towards the poles after reaching the tropopause. It is deflected by coriolis force during it’s path towards the poles and becomes a westerly wind. To conserve angular momentum the air blows rapidly from west to east creating the subtropical jet stream. The Subtropical Jet stream is formed primarily because of conservation of angular momentum. The Polar and Subtropical Jet Stream Wind Speeds in the Jet Stream Within the jet stream, currents travel at varying speeds but are greatest at the core. Jet streaks are areas inside the jet stream where the velocity is higher than the rest of the stream. Jet streaks cause air to rise, which lowers pressure at the surface. When surface low pressures form the rising air can cause clouds, precipitation and storms. The jet stream can also contain wind shear, a violent and sudden change in wind direction and speed. Wind shear can occur outside the jet stream as well, even at the surface. When vertical winds blast downward it can cause an airplane that is in the process of take off to suddenly lose altitude and potentially crash. For this reason all commercial planes in the U.S. since 1996 have been equipped with windshear warning systems. Jet Streams on Weather Maps The color filled regions indicate wind speed in knots. The shades of blue indicate winds less than 60 knots, while winds greater than 120 knots are given in shades of red. Displacement of the Jet Stream Areas of high and low pressure act like a moving riverbed, buckling and snaking the path of the jetstream as it flows to the east. Winds flow clockwise around areas of High Pressure H Winds flow counter-clockwise around areas of Low Pressure L Displacement of the Jet Stream At times, the polar jet stream may dip further south into the U.S., bringing cold weather with it. At other times it retreats into Canada, leaving milder weather in the U.S. Fig. 1. Average jet stream winds (250 mb, ~25,000 feet) during Monsoon Season 2009. Fig. 2. Average jet stream winds (250 mb, ~25,000 feet) difference from climatology during Monsoon Season 2009. The jet stream was displaced to the south from its typical location due to El Niño development. Displacement of the Jet Stream El Niño and La Niña cause changes in the distribution of Low and High Pressure regions. For this reason El Niño and La Niña cause shifts of the Jet Stream. The Pacific/North American Teleconnection Pattern (PNA) and the Jet Stream The Pacific/ North American teleconnection pattern (PNA) is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extratropics. The PNA pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. Positive phase →associated with an enhanced East Asian jet stream and with an eastward shift in the jet exit region toward the western United States. Negative phase→associated with a westward retraction of that jet stream toward eastern Asia, blocking activity over the high latitudes of the North pacific, and a strong split-flow configuration over the central North Pacific. PNA Negative and Positive Phases Negative Phase Positive Phase Higher pressure Lower pressure •Stronger high / low pressure systems •Jet stream and region of storm formation shift eastward Winds •Weaker high / low pressure systems •Jet stream and region of storm formation shifts west toward central Pacific. Surface Air Pressure and Jet Stream Showing Blocking During Negative Phase of PNA Pattern H L COLD H Negative phase of PNA pattern favors blocking and strong cold-air outbreaks into western North America. Homework 1) 2) 3) Go to the following website http://www.nytimes.com/2010/04/16/opinion/16winches ter.html?_r=1 and read the article : “A Tale of Two Volcanoes”. Answer the following questions based on the article: What impact did the eruption of Krakatoa have in the field of atmospheric sciences? Explain how the eruption of Krakatoa contributed to this discovery. If Krakatoa had never erupted, do you think we would have been able to make the discovery of what you indicated in question 1 eventually? Through which methods and/or instrumentation do you think we would have been able to prove it’s existence?