Lecture 4 Atmospheric Circulation

THE ZONALLY-AVERAGED

CIRCULATION

part I, zonal winds

Schematic of wind intensity in the west to eastdirection near the Earth’s Surface

Winds blowing from the west are called westerlies,Winds blowing from the east are called easterlies.

POLAR

EASTERLIES

WESTERLIES

TRADES

At the Earth’ssurface, winds areprimarily from thewest in mid-latitudesand from the east inlow and highlatitudes.

EquatorSummerPole

WinterPole

10 Km

30 Km

50 Km

70 Km

ZONAL WIND

SUBTROPICA L JET(~35 m/s)

SUMMER JET(~50 m/s)

POLAR-NIGHT JET(~80 - 100 m/s)

EW

W W

0

0

0

30

Schematic showing outstanding features of the zonallyaveraged zonal wind distribution

THE ZONALLY-AVERAGED

CIRCULATION

part II, meridional winds

Annual-average atmospheric mass circulation in the latitude pressureplane (meridional plane). The arrows depict the direction of airmovement in the meridional plane. The contour interval is 2x1010 Kg/sec -this is the amount of mass that is circulating between every twocontours. The total amount of mass circulating around each "cell" isgiven by the largest value in that cell. Data based on the NCEP-NCARreanalysis project 1958-1998.

DJF and JJA meridional overturningcirculation. Units are 1010 kg s-1

TheHadleyCells

Ocean heat transport

Dynamicalcooling byadvection

Dynamicalwarming bysubsidence

Radiationsolar downinfrared up

Moisturetransport

Moisturetransport

Evaporation

Hadley circulation and heat budget in subtropics

Latentheating inconvective

rain

warm

Heat transportby transients

NORTH-SOUTH VERTICAL SECTION TROUGHTROPOSPHERE

Three-Cell Circulation ModelThe cells tops are around the tropopause

Here we point outan apparent

contradiction:The meridional

overturning cellsweaken in

midlatitudes, butthe energytransport

deduced fromradiative balancepeaks. What’s

going on?

THE ROLE OFEDDIES

The role of eddies: some background

We wish to examine in detail climatological transport in the meridional

direction in a zonal-mean sense. Transport is the product of two quantities,

the meridional wind and the variable being transported. We might naively

think that we could calculate it by simply multiplying zonal-mean, time-mean

wind by zonal-mean, time-mean of the variable being transported.

However, to calculate it accurately, it turns out we have to take into account

the fact that wind and the transported variable may be correlated along a

latitude circle in the time-mean, and may also be correlated in time. In other

words, we must consider the role of transient and stationary eddies.

Stationary and Transient EddiesStationary eddies are characterized by persistent zonalasymmetries in atmospheric variables. These zonalasymmetries average to zero along a latitude band (bydefinition), and so are completely absent from zonal-mean climatologies. They generally arise from zonalasymmetries in the land/sea configuration, and thereforeplay a larger role in the climate of the northernhemisphere.

Transient eddies are characterized by departures fromthe time mean flow at a particular location. They arealso absent from climatologies (by definition), and so areabsent from zonal-mean characterizations of theatmosphere. Transient eddies play a larger role in theclimate of the southern hemisphere.

An example of the impact of an eddy on mean transport

The role of eddies: a mathematical framework

In general, instantaneous point observations of a scalar quantity A can be broken down

into 3 components: (1) zonal-mean, time-mean, (2) time-mean departure from the

zonal mean, and (3) instantaneous departure from the sum of the first two

components :

!

A = A

_"

# $ %

& ' + A

*

_

+ A'

Here the bracket denotes zonal-mean, the overbar denotes time-mean, the asterisk

denotes a departure from the zonal mean, and the prime denotes departures from time

mean.

The role of eddies: a mathematical framework

If we have another scalar quantity B, it may be of interest to examine the zonal-

mean, time-mean product of A and B. Our decomposition gives us a means of

breaking this down into three different components:

!

[AB] = [A ][B ] + [A *B *]+ [ " A " B ]

(1) (2) (3)

The first component is the contribution from the time-mean, zonal-mean product

(mean meridional component), the second is from the zonal-mean covariance

of the departures from zonal mean (stationary eddies), and the third is from the

time-mean, zonal-mean covariance of the transient eddies.

mean meridional circulation transient and stationary eddies

ANGULARMOMENTUM

BALANCE

The atmosphericcirculation

averaged over alllongitudes andpresented as alatitude-heightcross-section.Zonal flows areshown with the

colors. What doyou notice aboutthe association

between theHadley cell and the

subtropical jet?

If we calculate thespeed of the zonal

flow in thesubtropical jet

stream based onconservation of

angularmomentum of a

parcel at rest at theequator which

rises and makesits way poleward inthe Hadley Cell, we

find it is about 4times larger than

observed. What isdissipating the

angularmomentum?

LARGE-SCALEFLOWS ACROSS

LONGITUDES

(i.e. zonalasymmetries)

THE WALKER CIRCULATION VERTICALSECTION ALONG EQUATOR

The Walker Circulation