Lecture 17: Atmospheric circulation & pressure distrib’ns (Ch 8) In the context of map discussions, already we have touched on a few of the concepts of Ch8. Today we’ll cover: 3-cell model of dominant planetary scale wind motions, or “general circulation” observed general circulation jet streams Rossby waves

Fig. 8-2a We spoke earlier (slide 2, Lec. 9) of the vast and continuous range of scales of motion in the atmosphere… “The largest-scale patterns, called the general circulation, can be considered the background against which unusual events occur, such as” (droughts etc.)… p 213 Hadley’s (1735) Single-cell Model ocean-covered planet sub-solar point perpetually over equator equatorial heating produces lift and symmetric poleward motion aloft; sink at poles return equator-ward motion at surface reasoned earth’s rotation must deflect the wind

Fig. 8-2a Single-cell Model** is consistent with the observed (and important) surface equatorial “trade winds”, “the most persistent on earth” though polar surface easterlies emerge only as a long term average, not a prevailing feature BUT: surface easterlies everywhere would slow down rotation rate of earth! ** Hadley is honoured by a “Hadley Centre for Climate Prediction & Research” in UK Air aloft “diverges” out of the air column, air at surface “converges” into column

Ferrel’s (1865) Three-cell Model the low lat. “Hadley cell”is thermally driven by powerful ITCZ convection the mid lat. Ferrel cell is indirect (forced by the other two) the high lat. Polar cell is thermally driven by sink at the poles, but, we do not get vigorous overturning modern view is that only the Hadley cell is considered “real” System still conceived as an ocean-covered earth with the subsolar point on the equator Fig. 8-2b

Three-cell Model Equatorial lows + ITCZ Fig. 8-2b

Convective cloud in the ITCZ (“the doldrums”). The ITCZ migrates N-S seasonally with changing solar declination Fig. 8-3

Three-cell model – winds aloft in the Hadley cell equator Air rising at equator feeds into poleward upper streams, which are deflected by the Coriolis force (to right in N. hemisphere) to produce a zonal component of motion… result: westerly upper currents in both hemispheres About 30 o N Arrows denote rate of movement of the ground surface towards the east… angular mtm of a stationary parcel is largest at the equator Alternative view (Sec. 8-1) each parcel with fixed mass m tends to conserve its “angular momentum” m v r. Constancy of m v r demands an increase of the zonal component (v) of the parcels velocity as it moves north, where the circumference of a latitude line about the earth (2  r ) is shorter. (Friction intervenes so conservation not quite perfect.) “zonal” component (of wind velocity) “meridional” component Rising parcel

Three-cell Model This poleward moving upper current cools, and around o latitude (ie. in the “horse latitudes”) it sinks, with consequent adiabatic warming mitigating against cloud at that latitude Fig. 8-2b

Surface westerlies (realistic) + upper easterlies (unrealistic) realistic polar surface easterlies do emerge as a long term average Three-cell Model Land/ocean mix + topography make true climate rather different from 3-cell model Fig. 8-2b

What’s observed: semipermanent surface pressure cells & winds Gone in summer Migrates W and weakens in summer (Sea-level isobars averaged over 30 Januaries) note strong influence of continents! Fig. 8-4a

Fig. 8-4b

What’s observed: mid-tropospheric winds heights largest over tropics lowest heights h and strongest gradient  h /  x (thus, strongest winds) in winter zonal component generally dominant Fig. 8-6

Polar front jet Recall P falls more slowly with increasing height in warm air, where density lower so wherever there are strong horiz temperature gradients (fronts) there are strong height gradients strong height gradient implies strong wind perpendicular to the height gradient (see Geostrophic law)… strongest in winter (stronger T-grdnt) jet here shown as a climatological feature as weather feature, irregular - meander & branch Fig. 8-8

Long (Planetary/Rossby) Waves — wavelength  1000’s kilometers typically around globe (fewer, longer, stronger in winter) not always unambiguously identifiable - but can be shown mathematically to exist in ideal atmos. due to N-S variation of Coriolis force (usually) move slowly eastward Waves in mid-latitude mid/upper troposphere causing convergence and divergence aloft mb con div con divL

Fig Example of high amplitude Rossby wave over N. America Sept. 22, 1995 Edmonton low +4 o C, high +20 o C Winnipeg low -2 o C, high +11 o C

Fig Rossby wave moving eastward

Notice the “short wave” here; covered later – are associated with storms Rossby wave moving eastward