MET 10 Lecture 9 Global Winds, Air Masses, and Fronts Chapter 8 & 9 Dr. Craig Clements San Jose State University

Westerly winds and the Jet Stream Jet streams : rivers of fast-moving air 100’s of miles long, several hundred miles wide, less than a mile thick. Upper-level winds above the middle latitudes in both hemispheres blow in a wavy west-to-east direction. Aloft, we generally find higher pressure over equatorial regions and lower pressures over polar regions. Jet streams are usually found at the tropopause, km. Wind speeds in jet streams are typically 100 knots, sometimes 200 knots. There are two jet streams in the northern hemisphere: The polar jet and the subtropical jet.

Westerly winds and the Jet Stream

During the summer, the Pacific high moves northward. Sinking air along its eastern margin (over California) produces a strong subsidence inversion, which causes relatively dry weather to prevail. Along the western margin of the Bermuda high, southerly winds bring in humid air, which rises, condenses, and produces abundant rainfall.

Polar and subtropical jet streams

Global Wind Patterns and Ocean Currents As the wind blows along the ocean, it causes the surface water to drift along with it. The moving water gradually piles up, creating pressure differences within the water itself. This leads to further motion of the water at greater depths. Because of the larger frictional drag in water, ocean currents move more slowly than wind.

Ocean Temperatures

As winds blow parallel to the west coast of North America, surface water is transported to the right (out to sea). Cold water moves up from below (upwells) to replace the surface water. Winds and Upwelling

El Nino and the Southern Oscillation Typically, over the eastern pacific off the Peruvian coast, southerly winds promote upwelling of cold, nutrient rich water. This is followed by a few weeks of warm water moving south and replacing the cold water. This reversal is called El Nino meaning boy child (Christ child) because it coincides with Christmas. At times this warming of the ocean waters off of Peru lasts for many months. When this occurs, this is considered a major El Nino event. Why does the ocean become so warm over the eastern Pacific?

Normally, in the tropical Pacific, the trade winds are persistent and blow westward from a region of higher pressure over the eastern Pacific toward a region of lower pressure centered near Indonesia. The easterly trades create upwelling, moving the surface water to the west. As the water moves westward, it is heated by the sun. In the Pacific Ocean, surface water along the equator is usually cool in the east and warm in the west. A break down in surface pressure patterns occurs every few years. The pressure rises over the western Pacific and falls over the eastern Pacific producing east winds that moves the warmer waters to the east towards South America.

Why does the ocean become so warm over the eastern Pacific? Toward the end of this warming, the atmospheric pressure over the eastern Pacific now begins to rise while it lowers in the western Pacific. This see-saw pattern of reversing surface air pressure at opposite ends of the Pacific Ocean is called the Southern Oscillation. Because the pressure reversals and ocean warming are more or less simultaneous, scientists call this phenomenon: The El Nino/Southern Oscillation or ENSO. Now if cooling occurs (a cold-water episode) this is termed: La Nina

(a) Average sea surface temperature departures from normal as measured by satellite. During El Niño conditions upwelling is greatly diminished and warmer than normal water (deep red color) extends from the coast of South America westward, across the Pacific. (b) During La Niña conditions, strong trade winds promote upwelling, and cooler than normal water (dark blue color) extends over the eastern and central Pacific. (NOAA/PHEL/TAO)

During El Niño conditions, a persistent trough of low pressure forms over the north Pacific and, to the south of the low, the jet stream (from off the Pacific) steers wet weather and storms into California and the southern part of the United States. During La Niña conditions, a persistent high-pressure area forms south of Alaska forcing the polar jet stream and accompanying cold air over much of western North America. The southern branch of the polar jet stream directs moist air from the ocean into the Pacific Northwest, producing a wet winter for that region.

Air Masses and Fronts

An infrared satellite image that shows maritime tropical air (heavy yellow arrow) moving into northern California on January 1, The warm, humid airflow (sometimes called “the pineapple express”) produced heavy rain and extensive flooding in northern and central California.

A front is the transition zone between two air masses of different densities. Thus, they separate air of different temperatures and humidities too. Upward extend of a front is referred to as a frontal surface or frontal zone. Stationary front- has essentially no movement. Cold front- represents a zone where cold, dry, stable polar air meets warm, moist, unstable subtropical air. Fronts

Criteria used to locate a front on a surface weather map. 1.Sharp temperature changes over relatively short distance. 2.Changes in the air’s moisture content (indicated by changes in the dew point). 3.Shifts in wind direction. 4.Pressure and pressure changes 5.Clouds and precipitation patterns. Fronts

Surface weather map

Vertical view of clouds, precipitation, winds across the warm front

Drylines are not warm fronts or cold fronts, but represent a narrow boundary where there is a steep horizontal change in moisture as indicated by a rapid change in dew-point temperature. A dryline separates warm, moist maritime tropical (mT) on its eastern side from hot, dry continental tropical air (cT) on its western side.