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