Chapter 7: Atmospheric Circulations Scales of atmospheric motions Eddies - big and small Local wind systems Global winds Global wind patterns and the oceans

Scales of Atmospheric Motions scales of motion microscale mesoscale synoptic scale planetary scale Fig 7.1 Lots of important weather events occur on microscales, like evaporation of liquid water molecules from the earth’s surface.

Figure 7.2: The scales of atmospheric motion with the phenomena’s average size and life span. (Because the actual size of certain features may vary, some of the features fall into more than one category.) Fig. 7-2, p. 171

Eddies - Big and Small eddy rotor wind shear turbulence Wind shear can sometimes be observed by watching the movement of clouds at different altitudes.

Kelvin Helmholtz waves Clear-air turbulence Billow clouds Figure 1: The formation of clear air turbulence (CAT) along a boundary of increasing wind speed shear. The view of wind is from the side, with a top layer of air moving over a layer below. Figure 1, p. 173

Local Wind Systems Thermal Circulations isobars and density differences thermal circulations

Sea and Land Breezes sea breeze land breeze sea breeze front Florida sea breezes Sea and land breezes also occur near the shores of large lakes, such as the Great Lakes.

Figure 7.6: Typically, during the summer over Florida, converging sea breezes in the afternoon produce uplift that enhances thunderstorm development and rainfall. However, when westerly surface winds dominate and a ridge of high pressure forms over the area, thunderstorm activity diminishes, and dry conditions prevail. Fig. 7-6, p. 175

Seasonally Changing Winds - the Monsoon Monsoon wind system India and eastern Asian monsoon other monsoons

Figure 7.10: Enhanced infrared satellite image with heavy arrow showing strong monsoonal circulation. Moist, southernly winds are causing showers and thunderstorms (yellow and red areas) to form over the southwestern section of the United States during July, 2001. Fig. 7-10, p. 178

Mountain and Valley Breezes mountain breeze The nighttime mountain breeze is sometimes called gravity winds or drainage winds, because gravity causes the cold air to ‘drain’ downhill.

Katabatic Winds Strong drainage winds: steep slope Katabatic winds are quite fierce in parts of Antarctica, with hurricane-force wind speeds. Bora: a cold, gusty northeasterly wind along the Adriatic coast in the former Yugoslavia

Chinook (Foehn) Winds Chinook winds—warm and dry compressional heating chinook wall cloud In Boulder, Colorado, along the eastern flank of the Rocky Mountains, chinook winds are so common that many houses have sliding wooden shutters to protect their windows from windblown debris. It is called a Foehn along the leeward slopes of Alps.

Figure 7.14: A chinook wind can be enhanced when clouds form on the mountain’s windward side. Heat added and moisture lost on the upwind side produce warmer and drier air on the downwind side. Fig. 7-14, p. 180

Santa Ana Winds Santa Ana wind compressional heating wildfires Many Southern California residents regularly hose down their roofs to prevent fires during Santa Ana wind season. Difference between this and Chinook wind Santa Ana: from elevated desert plateau; Chinook: from cold plateau

Desert Winds dust and sand storms dust devils – from surface usually with a diameter of a few meters and a height of <100 m

General Circulation of the Atmosphere cause: unequal heating of the earth’s surface effect: atmospheric heat transport Ocean currents also transport heat from the equator to the poles and back.

Single-cell Model basic assumptions: no rotation Hadley cell why is the single-cell model wrong? One of the world’s premier atmospheric science research facilities,the Hadley Centre for Climate Research, is named after George Hadley.

Three-cell Model model for a rotating earth Hadley cell doldrums subtropical highs trade winds intertropical convergence zone westerlies polar front polar easterlies Many global circulation terms, including ‘trade winds’ and ‘doldrums’, were named by mariners who were well acquainted with wind patterns. Upper troposphere easterly is inconsistent with the observed westerly

Figure 7.21: The idealized wind and surface-pressure distribution over a uniformly water-covered rotating earth. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Fig. 7-21, p. 185

Average Surface Winds and Pressure: The Real World semipermanent highs and lows Bermuda high & Pacific high Icelandic low & Aleutian low Siberian high The Bermuda High frequently brings hot, muggy weather to the eastern US. in summer The ITCZ shifts toward the north in July (from January)

Figure 7.22: Average sea-level pressure distribution and surface wind-flow patterns for January (a) and for July (b). The heavy dashed line represents the position of the ITCZ. Fig. 7-22a, p. 188

Fig. 7-22b, p. 189

The General Circulation and Precipitation Patterns ITCZ, midlatitude storms, polar front Most of the world’s thunderstorms are found along the ITCZ. Low rainfall over the subtropical regions

Westerly Winds and the Jet Stream jet streams subtropical jet stream polar front jet stream Low-level jet stream over the Central plains of the U.S. (within 2 km above surface), bringing moist and warm air to form nighttime thunderstorms

Global wind-driven ocean current Figure 7.29: Average position and extent of the major surface ocean currents. Cold currents are shown in blue; warm currents are shown in red. Fig. 7-29, p. 193

Winds and Upwelling Upwelling is strongest when wind is parallel to the coastline

El Niño and the Southern Oscillation El Niño events Southern Oscillation La Niña teleconnections ENSO is an example of a global-scale weather phenomenon. Often Arizona has a wetter winter during El Nino and a drier winter during La Nina; Usually northwestern U.S. has a drier winter during El Nino and a wetter winter during La Nina.

Figure 7.32: In diagram (a), under ordinary conditions higher pressure over the southeastern Pacific and lower pressure near Indonesia produce easterly trade winds along the equator. These winds promote upwelling and cooler ocean water in the eastern Pacific, while warmer water prevails in the western Pacific. The trades are part of a circulation (called the Walker circulation) that typically finds rising air and heavy rain over the western Pacific and sinking air and generally dry weather over the eastern Pacific. When the trades are exceptionally strong, water along the equator in the eastern Pacific becomes quite cool. This cool event is called La Niña. During El Niño conditions—diagram (b)—atmospheric pressure decreases over the eastern Pacific and rises over the western Pacific. This change in pressure causes the trades to weaken or reverse direction. This situation enhances the countercurrent that carries warm water from the west over a vast region of the eastern tropical Pacific. The thermocline, which separates the warm water of the upper ocean from the cold water below, changes as the ocean conditions change from non-El Niño to El Niño. Fig. 7-32, p. 196

Other Atmosphere-Ocean Interactions North Atlantic Oscillation Arctic Oscillation: pressure difference between Arctic and regions to its south Pacific Decadal Oscillation

Figure 7.36: Typical winter sea surface temperature departure from normal in °C during the Pacific Decadal Oscillation’s warm phase (a) and cool phase (b). (Source: JISAO, University of Washington, obtained via http://jisao.washington.edu/pdo. Used with permission of N. Mantua.) Fig. 7-36, p. 199