EVAT 554OCEAN-ATMOSPHERE

DYNAMICS

LARGE-SCALE ATMOSPHERIC CIRCULATION (CONT)

LECTURE 9

(Reference: Peixoto & Oort, Chapter 3,7)

THERMALLY-DRIVEN CIRCULATION

Pole Equator

Approximate circulation as steady, linear, two-dimensional (meridional-vertical plane)

THERMALLY-DIRECT CIRCULATION FOR AN

IDEALIZED NON-ROTATING EARTH

HADLEY CELL CIRCULATION

THERMALLY-DRIVEN CIRCULATION

Pole Equator

Approximate circulation as steady, linear, two-dimensional (meridional-vertical plane)

THERMALLY-DIRECT CIRCULATION FOR AN

IDEALIZED NON-ROTATING EARTH

WHAT ABOUT EXTRATROPICS?

Descending Air

HighPressure

Ascending Air

LowPressure

THERMALLY-DIRECT

CIRCULATIONCOLD WARM

Descending Air

HighPressure

Ascending Air

LowPressure

THERMALLY INDIRECT

CIRCULATIONWARM COLD

‘FERREL CELLS’ ARE THERMALLY-INDIRECT CIRCULATIONS

HADLEY CELLS ARE THERMALLY DIRECT CIRCULATIONS…

As are the (weak) polar cells

030 3060 60N S

LatitudeHighHigh LowLow

Surface Pressure Systems

Low

MERIDIONAL CIRCULATION PATTERNS

HORIZONTAL ATMOSPHERIC CIRCULATION

Cloud and Precipitation Occur in Areas of Surface Convergence

and Rising Motion

JulyJanuary

There are important seasonal changes in the semi-permanent features of the surface atmospheric

circulationITCZ migrates with Sun, but lags it...

GLOBAL ATMOSPHERIC CIRCULATION

JulyJanuary

There are corresponding seasonal changes in the upper air (e.g. 500 mb) circulation

GLOBAL ATMOSPHERIC CIRCULATION

Vertical Atmospheric Circulation

030 3060 60N S

LatitudeHighHigh LowLow

Surface Pressure SystemsLow

GLOBAL ATMOSPHERIC CIRCULATION

Zonally-averaged ps

Surface Pressure Systems

GLOBAL ATMOSPHERIC CIRCULATION

Zonally-averaged ps

030 3060 60N S

Westerly

Easterly Surface Wind Systems

Why should there be any easterlies here??

east-west component of wind at surface consistent with Geostrophic Balance

(westerlies)

(easterlies)

/ˆ1 paf

uG

0/ p 0Gu

0/ p 0Gu

0/ p0f

L’Hopital’s rule

22 /ˆ'

1

paf

uG

cos2' f

GLOBAL ATMOSPHERIC CIRCULATION

Geostrophic Balance

GLOBAL ATMOSPHERIC CIRCULATION

gzp /

/ˆ1 pa

fu

/cosˆ1v pa

fpk

f ˆ

ˆ1

G V

Hydrostatic Balance

Combine these

zpk

fz

ˆˆ

1G

V

kfg ˆˆ

fag

zu

cosv

fag

z

T

Tfag

zu

T

Tfag

z cosv

THERMAL WIND

p

ap

ap 1,

cos1

THERMAL WIND

T

Tfag

zu

THERMAL WIND

MERIDIONAL-VERTICAL CIRCULATION

Streamfunctionz

a /

cos1v

/1wa

)( v t

0)( v

0)cosv(

zw

Continuity

Steady state