GEU 0027: Meteorology Lecture 10 Wind: Global Systems

Global Circulation In the absence of rotation, air would tend to flow from the equator toward the poles. Hot, less dense air rising at the equator, becomes denser as it cools and descends at the poles, traveling back to tropical areas to heat up again.

Global Circulation Because of earth’s rotation we have several circulation cells not just one per hemisphere

Three-cell Model With the 3 cell structure of wind circulation and the combination of PGF and Coriolis, semi-permanent circulation patterns are established.

Semi-permanent Pressure and Winds Semi-permanent structures in the atmosphere provide consistent wind patterns and breeding grounds for air masses.

Semi-Permanent Pressure (January)

500-mb streamline and isotherms in January Figure 10.8 Average 500-mb chart for the month of January (a) and for July (b). Solid lines are contour lines in meters above sea level. Dashed lines are isotherms in °C. Arrowheads illustrate wind direction.

Stronger Winter PGF

Semi-Permanent Pressure (July)

500-mb streamline and isotherms in July Figure 10.8 Average 500-mb chart for the month of January (a) and for July (b). Solid lines are contour lines in meters above sea level. Dashed lines are isotherms in °C. Arrowheads illustrate wind direction.

Figure 10.6 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.

Figure 10.7 Average annual precipitation for Los Angeles, California, and Atlanta, Georgia.

Weaker Summer PGF

Intertropical Convergence Zone ITCZ Hot equatorial air rises in convection. Air moves away from the equator toward the poles. Low Pressure results around the equator. A band of convective thunderstorms circles the tropical areas of the globe.

ITCZ Equatorial Cumulus and Thunderstorms

India Monsoon Precipitation

Monsoons (dry)

Monsoons (wet)

Subtropical High and the ITCZ (Sahara)

Sahara Desert and the Sahel of Africa

Figure 10.11 (b) Satellite image showing clouds and positions of the jet streams for the same day.

Figure 10.11 (a) Position of the polar jet stream and the subtropical jet stream at the 300-mb level on March 9, 2005. Solid lines are lines of equal wind speed (isotachs) in knots. Heavy lines show the position of the jet stream core.

Jet Locations Jet stream locations greatly affect local and global climate.

Jet Formation Sharply varying pressure and temperature differences create the exaggerated situation shown. Tightly packed isobars create stronger winds aloft in the frontal region.

Polar Jet Winds are Westerly and parallel to the frontal boundary. This creates the polar jet stream. It is strongest in the winter and weakest in the summer.

Seasonal Polar Jet Changes Location and Velocity variations

What cause the jets? L = m v r r = distance from rotational axis m = mass v = velocity Figure 10.13 Air flowing poleward at the tropopause moves closer to the rotational axis of the earth (r2 is less than r1). This decrease in radius is compensated for by an increase in velocity and the formation of a jet stream.

Figure 4 A well-developed surface storm usually shows up as a wave with a tilted trough (dashed line) on a 500-mb chart. The wave transports westerly momentum poleward because the winds east of the trough have a greater westerly component than do the winds west of the trough. Fig. 4, p. 270

Jupiter’s Bands Higher Angular Momentum, Yields more zones?

Wind Jets Other jet formation mechanisms are less well known.

The Dishpan Experiment Uneven heating of the equator and poles of the earth. Rotation. Viscosity and turbulence.

Rossby Waves Kinking in the jet stream occurs on a cyclic basis. Weather patterns are also somewhat cyclic.

Rossby Cycle A complete Rossby cycle observed over ~ 6 weeks.

Figure 10.10 A jet stream is a swiftly flowing current of air that moves in a wavy west-to-east direction. The figure shows the position of the polar jet stream and subtropical jet stream in winter. Although jet streams are shown as one continuous river of air, in reality they are discontinuous, with their position varying from one day to the next.

Ocean Currents

Gulf Stream

Coastal Upwelling

Ekman spiral, Ekman layer, and Ekman transport Figure 10.17 The Ekman Spiral. Winds move the water, and the Coriolis force deflects the water to the right (Northern Hemisphere). Below the surface each successive layer of water moves more slowly and is deflected to the right of the layer above. The average transport of surface water in the Ekman layer is at right angles to the prevailing winds.

Normal South Pacific Condition Easterly “trade-winds” usually prevail and upwelling occurs When exceptionally strong this cooling is called a La Nina.

El Nino During an El Nino, pressure conditions (and winds) reverse. Extremely warm water and wind reversal affects weather.

Sea Surface Temperatures A warm water wave migrates eastward during and El Nino as upwelling and cooling is severely diminished along the western coast of S. America.

Figure 10.21 (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) Fig. 10-21b, p. 276

El Nino and La Nina events, and ENSO Cycle La Nina versus El Nino conditions over the past 60 years. The Y-axis is a parameter calculated from a combination of: air temperature water temperature air pressure (sea-level) wind speed and direction cloud cover

Weather pattern changes during El Nino condition

Weather pattern changes during La Nina condition Figure 10.23 Typical winter weather patterns across North America during an El Niño warm event (a) and during a La Niña cold event (b).

Global hydrological impacts of El Nino Figure 10.24 Regions of climatic abnormalities associated with El Niño–Southern Oscillation conditions. A strong ENSO event may trigger a response in nearly all indicated areas, whereas a weak event will likely play a role in only some areas. Note that the months in black type indicate months during the same years the major warming began; months in red type indicate the following year. (After NOAA Climatic Prediction Center.)

Pacific Decadal Oscillation (PDO) Figure 10.25 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://www.tao.atmos.washington.edu/pdo. Used with permission of N. Mantua.)

Figure 10.25 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://www.tao.atmos.washington.edu/pdo. Used with permission of N. Mantua.)

Figure 10.25 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://www.tao.atmos.washington.edu/pdo. Used with permission of N. Mantua.)

North Atlantic Oscillation (NAO) Figure 10.26 Change in surface atmospheric pressure and typical winter weather patterns associated with the (a) positive phase and (b) negative phase of the North Atlantic Oscillation.

Figure 10.26 Change in surface atmospheric pressure and typical winter weather patterns associated with the (a) positive phase and (b) negative phase of the North Atlantic Oscillation. Fig. 10-26a, p. 280

Figure 10.26 Change in surface atmospheric pressure and typical winter weather patterns associated with the (a) positive phase and (b) negative phase of the North Atlantic Oscillation. Fig. 10-26b, p. 280