Notes_General circulation of the southern hemisphere. (1) (1).pdf

For ENVS module

1. 27/2-3/3: General circulation of the southern
hemisphere
2023/03/02 ENV210 Lecture Notes: LT Dube 1

2. Time and space scales of atmospheric disturbances
Horizontal distance scale
•Microscale= 10
-2
to 10
3
m, e.g.
small scale turbulence (s/min)
•Local scale = 10
2
to 5x10
4
m, e.g.
small cumulus (min/hr)
•Mesoscale= 10
4
to 2x10
5
m, e.g.
tornado, local winds (d/wk)
•Macroscale= 10
5
to 10
8
m, e.g.
hurricane, anticyclone, jet
(w/mo/yr)
Fig.11.1
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ENV210 Lecture Notes: lTd 2

3. 1. Energy balance
2023/03/02 ENV210 Lecture Notes: lTd 3

4. Zonally averaged fluxes of solar and terrestrial radiation absorbed and emitted by
the Earth-Atmosphere system
•Energy surplusbetween 0
O
and
40
O
S. Higher latitudes have
deficit.
•Balance between incoming and
outgoing radiant energy is
constant over long periods 
mechanisms to transfer energy
between equator and poles.
Fig.11.2
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ENV210 Lecture Notes: lTd 4

5. Average annual latitudinal distribution of the components of the poleward energy
flux
•Total energy transfer is greatest between
30
O
and 40
O
N/S.
•Energy transfer components = sensible
heat, latent heat and ocean currents.
•Sensible heat transport has double
maximum. Larger max. at 50
O
-60
O
due to
travelling low pressure disturbances
(eddies).
•Latent heat transport max. at the tropics.
Same with oceans transfer.
•This energy imbalance causes global
circulation.
•Mean transfer breaks down into transient
and standing disturbances.
Fig.11.2
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ENV210 Lecture Notes: lTd 5

6. •Thermohaline circulation
•In the thermohaline circulation,
dense, cold, salty surface water
sinks in the northern Atlantic
Ocean, generating slow bottom
currents that in turn cause slow
upwelling in the Indian Ocean,
western Pacific Ocean, and along
the coast of Antarctica.

7. Wind field
2023/03/02 ENV210 Lecture Notes: lTd 7

8. • Mean wind resolved
into zonal and
meridional
components.
• Zonal component
larger than meridional.
• Stratospheric tropical
easterlies occur
through the year.
• Due to cooling of
polar stratosphere in
winter, polar night
easterlies develop.
• Jet cores located at 30
km with speeds >45 m
s
-1
in SH.
Tropopause elevated
over the equator than
poles.
Mean zonaltropospheric and stratospheric winds (m/s) in summer and winter
Fig.11.8
2023/03/02 ENV210 Lecture Notes: lTd 8

9. Mean summer and winter zonal wind and mass flux in the southern hemisphere
Fig.11.10
2023/03/02 ENV210 Lecture Notes: lTd 9

10. Meridionalwind field
•(Fig. 11.10).
•Hadley cells are thermally-direct(warm air rising, cold air sinking OR ascending
in equatorial regions, subsidence on poleward side).
•Ferrelcells are thermally-indirect(cold air rises, warm air sinks/characterised by
subsidence on equatorward side and ascent on poleward side).
•in austral/southern summer, equatorial trough and northern Hadley cell are in
the southern hemisphere. Southern Hadley cell is compressed and region of
max surface convection occurs at 10
O
-15
O
S. Ascending limb promotes deep
convection.
•Divergence at 30
O
S where descending limbs of Hadley cell and Ferrel cell meet.
•At surface, air moves equatorward to complete Hadley cell circulation, and
poleward to complete Ferrelcell.
•Meridional pressure profile yields Equatorial Trough near the equator (in
summer), Subtropical Ridge at 30
O
S, and Antarctic Trough at 60
O
S. Equatorial
Trough is in the northern hemisphere in austral winter meridional pressure
profile change.
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ENV210 Lecture Notes: lTd 10

11. Mean meridional wind components in DJF (summer) at 200 hPaand 700 hPa
Fig.11.11
2023/03/02 ENV210 Lecture Notes: lTd 11

12. Mean meridional wind components in DJF (summer) at 200 hPaand 700 hPa(Contd)
•Summer mean meridional flow is poleward over easternparts of
landmasses and equatorwardover westernparts due to location of
oceanic subtropical HP cells
•South Indian Anticyclone produces an northerly component of wind along
the east coast of southern Africa which strengthens the manifestation of the
Ferrelcell to the south and weaken the Hadley cell to the north.
•South Atlantic Anticyclone produces an southerlycomponent of wind
along the west coast of southern Africa which weakens the manifestation of
the Ferrelcell to the south and strengthenthe Hadley cell to the north.
2023/03/02
ENV210 Lecture Notes: lTd 12

13. Streamlines of near-sfcflow in global tropics in Jan and Jul showing ITCZ.
A=Anticycl., C=Cyclone
Fig.11.12
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14. •Throughout the yrITCZ is in the NH in the eastern
Pacific and Atlantic regions.
•Over Africa, Indian Ocean and western Pacific regions,
ITCZ migratesfrom SH to NH between Jan and Jul.
•Where streamlines with easterly component cross the
equator, direction of CF changes from left to Right or
vice versa  recurvatureof flow to become westerly
(over north Africa and Indian Ocean in July (Satellite -
11.13).
•Westerly winds play role in monsoon circulation .
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ENV210 Lecture Notes: lTd 14

15. Meteosatimage of ITCZ over tropical North Atlantic Ocean and West Africa in
July
Fig.11.13
2023/03/02 ENV210 Lecture Notes: lTd 15

16. Mean location of ITCZ and ZAB: (a) and (b) Jan and Feb, (c) and (d) Jul and Aug.
■Single arrows = Low level flow, double arrows = 3 km flow
■Linked circles = location of major sfctroughs
■Crosses = LP and HP
Fig.11.15
2023/03/02 ENV210 Lecture Notes: lTd 16

17. •North of 20
O
S, 3 surface airstreams affect tropical
southern Africa in summer, (1) recurved south
Atlantic air moving into Africa from 12
O
S, (2) north-
east monsoon air over east Africa; (3) tropical
easterlies from Indian Ocean.
•convergence at the ITCZ(north-easterlies and
south easterlies)
•convergence at the Congo Air Boundary (CAB)
(Indian Ocean easterly and Atlantic air masses)
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ENV210 Lecture Notes: lTd 17