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Atmospheric General Circulation and Climate (Chap 6) Atmospheric General Circulation • Also: Global or planetary circulation. • The general circulation (GC) of the atmosphere is the totality of motions that characterize the large scale flow of the atmosphere (e.g., Hadley cells, jet stream) • Statistical description of wind, temperature, humidity, precipitation, etc. The mean is an example. • The GC can be simulated by numerically solving equations of atmosphere (i.e., climate modeling) Atmospheric General Circulation • The energy for the GC comes from the uneven distribution of solar radiation with respect to latitude • The motion of the GC is modified by Earth’s rotation (Coriolis force) and other physical effects (e.g., heating, friction, turbulence) • Effects of the GC: – Rapid movement and exchange of properties (compared to ocean or land) – Redistribution of heat from equator to poles; redistribution of moisture from oceans to land. If Earth rotated very slowly • The sun would heat the equatorial regions more than the poles. • In response, a vertically oriented circulation cell would develop in each hemisphere • Warm air rising at equator, then radiatively cooling and sinking at pole. Lutgens and Tarbuck (2001) Rotating Earth • Coriolis force deflects the flow to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. • Amount of deflection is smallest at equator and largest at poles. Rotating Earth • Idealized circulation on rotating Earth has three cells in each hemisphere. • Coriolis turns the upper branch of the Hadley cells to the east, meaning air does not reach pole. • Sinking air reaching surface spreads out following Corilois. Lutgens and Tarbuck (2001) Idealized Winds and Pressure Lutgens and Tarbuck (2001) “Mean Meridional Circulation” (MMC) MMC Ferrell Cell Hadley Cell Polar Cell Hadley and polar cells are “thermally direct”, meaning that warm is is rising and cool air is sinking. Ferrell cells are “thermally indirect”, weaker, and less well defined. Two types of atmospheric jet streams Eddy driven jet (EJ) (Polar front jet) More poleward jet located at polar front between polar and Ferrel cells (driven by temperature gradients) Subtropical jet (SJ) • At the poleward boundary of the Hadley cell Subtropical jet: angular Momentum • Parcels of air moving from equator toward pole in upper branch of Hadley cell speed up to conserve angular momentum (like ice skater bringing arms toward body). • Accounts for subtropical jet streams uϕ (m/s) ϕ (°N) Subtropical jet: angular Momentum • Theoretical wind speeds in the subtropical jet stream (135 m/s) are much higher than the observed ones (40 m/s). • This is because northward motion in the Hadley cells is slow (~1 m/s). It takes ~1 month for air parcels to travel from equator to 30ºN. • Energy losses by friction and other processes have enough time to reduce the angular momentum of the air parcels. Atmospheric Jets Subtropical jet (SJ) Eddy driven jet (EJ) (Polar front jet). Often hard to distinguish Seasonal changes in pressure and wind Subpolar low Polar Highs Equatorial Lows Subtropical Highs • Land-Sea contrast and seasonal effects lead to longitudinal variations. Subpolar low(s) January Seasonal changes in pressure and wind July • Remember: This is a climatology. A daily weather map may show a quite different picture. Seasonal changes in pressure and wind Notice that ITCZ shifts about 15 degrees into summer hemisphere. As ITCZ arrives, it brings the rainy “monsoon” season. Seasonal changes in pressure and wind ITCZ migration is associated with changes in position and strength of Hadley Cells (cell descending in winter hemisphere is stronger). July 30°S Annual mean (also Fall and Spring) 15°S 0° 15°N 30°N 0° 15°N 30°N January 30°S 15°S Energy balance “change = in – out” Energy in atmosphere Types of energy (J/kg) Percent of total energy Internal energy due to T 70% Potential energy due to z 27% Latent energy due to phase 2.9% Kinetic energy due to motion 0.1% Energy in atmosphere • Internal energy (IE) and potential energy (PE) constitute about 97% of the total energy. • Although KE is small, it is still very important because it is responsible for motions and transports. Energy is converted between types by circulation. For example, warm moist air rising in Hadley cell is conversion from internal and latent energy to potential energy. Atmospheric Energy Balance Ea=energy of atmospheric column of unit area (1 m2) from the surface to TOA. RTOA≈ 0 Atmosphere Rs LP Ea SH RTOA : Net radiation into the top of the atmosphere Rs : Net radiation into the surface Ra = RTOA - Rs: net radiative heating of atmosphere LP : latent heat release during precipitation SH : sensible heat transfer from the surface ΔFa : energy leaving column horizontally Atmospheric Energy Balance Change in Ea over time is DEa = RTOA - Rs + LP + SH - DFa Dt = Ra + LP + SH - DFa In annual mean, DFa = Ra + LP + SH , so RTOA≈ 0 Atmosphere Rs Ea LP SH TS Atmospheric General Circulation • Atmosphere moves energy from surplus around equator toward energy deficit poleward of 60°N and 60°S. Annual mean zonal mean energy budget from: Hartmann 1994 Atmospheric General Circulation • Atmosphere moves energy from surplus around equator toward energy deficit poleward of 60°N and 60°S. Annual mean zonal mean energy budget LP pattern resembles P pattern from: Hartmann 1994 Atmospheric General Circulation • Atmosphere moves energy from surplus around equator toward energy deficit poleward of 60°N and 60°S. • Energy exported from equatorial region (positive ΔFa) • Energy imported into polar regions (negative ΔFa) Annual mean zonal mean energy budget from: Hartmann 1994 Heat transport • Northward atmospheric energy transport is by eddies (areas of high and low pressure) near 45°N and S that move warm air poleward and cool air equatorward. • Hadley cells export energy from tropics. Heat transport • Poleward energy transport peaks where eddies are most vigorous (~40°N and S) from: Hartmann 1994 1 2 3 4 5 Practice with energy balance 1. Verify the energy balance 2. Why are radiative and turbulent heat fluxes balanced or imbalanced at this latitude? 3. Draw a diagram illustrating what the value of ΔFa means physically. How is this flow maintained by the general circulation (eddies/MMC)? Walker circulation and monsoons Walker circulation • Intense convection over southeast Asian warm pool, Africa, and South America generates eastwest oriented circulations in the tropics. • Visible in annual means • Weakening of Walker cell associated with El Nino Walker cell Monsoon circulation • Thermally direct circulation (warm air rising) • Summer: rising motion and moisture convergence over land = heavy precipitation • Winter: sinking motion and moisture divergence over land = dry conditions Monsoon circulations • Monsoons visible over India, Africa, and the southwestern US. JAN JUL