NATS 101 Lecture 20 Global Circulation

Supplemental References for Today’s Lecture Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2nd Ed. 505 pp. Prentice Hall. (ISBN 0-13-027394-5) Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Introduction to the Atmosphere, 8th Ed. 484 pp. Prentice Hall. (ISBN 0-13-087957-6)

Review Global Energy Balance (from long ago!) Thermally Direct Circulations AGAIN!

Annual Energy Balance Heat transfer done by winds and ocean currents Radiative Warming Radiative Cooling Radiative Cooling NH SH Heat transfer done by winds and ocean currents Differential heating drives winds and currents

Global Energy Budget Averaged over entire earth, incoming solar radiation is equal to outgoing IR Tropics absorb more solar radiation than they emit IR to space Surplus of radiant energy in tropics Poles absorb less solar radiation than they emit IR to space Deficit of radiant energy in poles

Global Circulation To balance the inequalities in the global energy budget, energy must be transported from the tropics to the poles. 40% of transport is done by oceans 60% of transport is done by atmosphere

Thermally Direct Circulation DIV CON H L Warm air toward poles Heat Heat Warm Rising Sinking Cold L H Cold air toward equator CON DIV Heat Heat Equator Poles

Global Circulation Winds throughout the world are averaged over a long period of time (over many winters) Local wind patterns vanish Distinct patterns in the prevailing winds emerge Driven by the unequal heating of the earth’s surface

Consider Waterworld A Simple Model Earth uniformly covered by water Land-Sea heating difference isn’t factor Sun is always directly over the equator No seasons Earth doesn’t rotate Use average daily sun No diurnal cycle and … ? Equatorial region Greatest heating Polar region Least heating Polar region Least heating

Waterworld Single Equator to Pole Cell Cold air Sinking @ Poles Hadley Cell Divergence Aloft Warm air Rising @ Equator Convergence @ Surface Ahrens Fig. 7.14 Divergence @ Surface Convergence Aloft

Consider a Rotating Waterworld Triple Cell Consider a Rotating Waterworld Equator-to-Pole temperature difference and rotation of Earth produce 3 circulation cells Hadley Cell (Strong Thermally Direct) Ferrel Cell (Indirect: Forced by Hadley & Polar) Polar Cell (Weak Thermally Direct) Hadley Ferrel Polar Equator Pole

Rotating Waterworld Prevailing Winds Ahrens Fig. 7.16

Major Surface Pressure Zones Ahrens Fig 7-17

ITCZ Inter-Tropical Convergence Zone Near equator Northeast Trades (N.H.) Converge with Southeast Trades (S.H.) along this zone. Is not evident as a continuous band around the globe on a day-to-day basis.

Jet Streams Swiftly flowing air currents, generally near tropopause. Subtropical Jet Stream: On polar side of Hadley Cell; westerly wind Polar Front Jet Stream: On Equator side of Polar Front; westerly wind Ahrens Fig 7-22

Why Jet Streams in Mid-Latitudes? Strong Thermal Contrast Temperature gradient produces increasing PGF with altitude & thus increasing wind speeds. Aguado & Burt Fig 8-6

Mid-Latitude Westerlies Slopes of isobaric surfaces become steeper with altitude Lutgens & Tarbuck Fig 8-6 Cold Warm

Real World Circulation Land-Ocean heating difference, along with the difference between tropics and poles, and rotation of earth. Sun not always directly over the Equator (cause of the seasons). Expect high pressure over cold land in the winter. Expect low pressure over warm land in the summer.

Ahrens Fig 7.17a

Ahrens Fig 7.17b

Pacific High, Bermuda High Ahrens Fig 7-19 Subtropical Highs follow the sun!

Pacific High, Bermuda High Ahrens Fig 7-20

Global Circulation - Precipitation Ahrens Fig 13-3

Summary Global Circulation Differential Heating Between Tropics and Poles Three Cells Mid-Latitude Westerlies Patterns shift slightly with seasons Precipitation Major Deserts occur under Sub-Tropical High Mid-latitude storms occur along Polar Front

Next Lecture Topic- Atmosphere-Ocean Interactions El Nino and La Nina Reading - Ahrens pg 189-197 Problems - 7.17, 7.18