Lecture 8: Atmospheric Circulation Introductive Physical Oceanography This powerpoint was prepared for purposes of this lecture and course only. It contains graphics from copyrighted books and journals. Please do not use without acknowledgment of these sources.

Reading: Outline: Stewart chapter 4.1 to 4.3, 4.6 Mean large-scale wind and pressure structure Walker and Hadley circulations Westerly winds (Ferrel cell) Surface wind stress Mean precipitation-evaporation fields Temperature structure, potential temperature

Solar Insolation (Kump, Kasting and Crane text)

Seasonality Obliquity (tilt of axis) results in seasons Perihelion - closest to sun Aphelion - farthest from sun

Radiation Balance at the Top of Atmosphere

The Layered Atmosphere From NASA Science site

The Layered Atmosphere

Adiabatic Lapse Rate and Potential Temperature -(dT/dz)dry parcel = g/cp = d Where subscript d = dry. In dry atmosphere, adiabatic lapse rate  = 9.8 C/km In moist atmosphere, adiabatic lapse rate  = 6-7 C/km Potential temperature Reference level is the ground, taken as 1000 mbar.

Troposphere (对流层) Troposphere is vertically well-mixed through convection (heating from below). Temperature decreases with height only because pressure decreases with height. (on average). Sometimes we have an “inversion” when there is colder air on the ground - very stable.

Stratosphere (平流层) Stratosphere is warmest at top: due to absorption of the sun’s UV radiation. (The ozone maximum is in the middle of the stratosphere.)

Sea-Land Breeze Circulation

Monsoon Summer Winter

Convergence and Divergence ITCZ = Intertropical Convergence Zone Actually not exactly on equator: seasonal variations in insolation, so ITCZ sits at maximum heating location. Offset in the mean because of asymmetric distribution of land between hemispheres. ITCZ is well developed north of equator.

ITCZ Seasonal Change

Observed ITCZ Pacific Ocean Atlantic Ocean 9/04

Wind System Surface winds in Hadley cell: deflect to right in NH, deflect to left in SH  TRADE WINDS in both hemispheres

Ocean Surface Wind

Northern Hemisphere Sea Level Pressure (SLP)

Southern Hemisphere Sea Level Pressure

Wind Stress Wind speed is measured above the sea surface. In practice measurements are at different heights. For consistency, adjust all measurements to a 10 meter height. Actual stress on the ocean:  = cDu2 where u is the wind speed at 10 meters,  is the air density 1.3 kg/m3, and cD is the (dimensionless) drag coefficient, which is determined empirically.

as wind speed increases: cD= (0.61 + 0.063u) x 10-3 Wind Stress From Stewart online text Typical value for drag coefficient: Low wind speed: cD= 1.1 x 10-3 as wind speed increases: cD= (0.61 + 0.063u) x 10-3

Surface Winds and Surface Pressure

Hydrologic Cycle (Water Cycle)

Vapor Pressure Warm air holds a lot more water vapor than cold air. In equilibrium, warmer air will free more water molecules from liquid water, and raise the amount of vapor. This eventually falls out as precipitation.

Water Cycle

Mean Cloud Cover

Precipitation-Evaporation Evaporative heat loss

Precipitation-Evaporation Net evaporation-precipitation (red evaporates, blue freshens)

Surface Salinity

Net Heat Exchange

Ocean Surface Temperature

Questions (Due in 2-week) At about what latitude in the northern hemisphere does the switch from trades to westerlies occur? What direction is the prevailing wind at the equator? If the wind speed at 10 meters height is 10 m/sec, what is the wind stress on the ocean? (use drag coefficients from lecture or from Stewart electronic notes) How many main low pressure and high pressure centers are in the world? What are their names? Please identify their latitude and longitude.

Learning about the Ocean, the Climate and the Nature