The document discusses Hadley circulation, an atmospheric pattern in the tropics characterized by rising air at the equator and sinking air in the subtropics, influencing global climate and weather patterns. It highlights evidence of the Hadley cell's expansion and its relationship with global warming, leading to changes in precipitation and increasing subtropical aridity. Future impacts of these changes on ecosystems and climate patterns are also explored.

2. Hadley Circulation,
Evidences and Impacts
M.phil Environmental Sciences
Fatima Jinnah Women University

3. Contents
• Hadley circulation
• Evidences
• Future impacts
– Role of Sea-level pressure
– Outgoing long waves
– Sea-surface temperatures(SSTs)

4. Definition
• An atmospheric circulation pattern occurring
in the tropics and circulation is intimately
related to the trade winds, tropical rainbelts,
subtropical deserts and the jetstreams.

5. Process
• Air rises up into the atmosphere at or near the
equator, flows toward the poles above the
surface of the Earth, returns to the Earth’s surface
in the subtropics, and flows back towards the
equator converging with its counterpart from
northern or southern hemisphere.
• The Hadley cells show seasonal variation in their
intensity, geographical extent and latitudinal
position.

6. Hadley cell

7. History
• Need of mapping circulation?
• Early Ideas on Trade Winds
• British Debate
• Hadley Principle’s birth
– George Hadley (1735)

8. Why this happens?
• Energy Balance
– 30 N and 30 S there is a surplus of radiation
– Net deficit at all greater latitudes
– mechanisms to transport the surplus energy
towards the poles

9. Cont’d
Net surplus used to evaporate water as most
of surface covered by ocean or vegetation

10. Cont’d
Convective uplift of warm wet equatorial air creates a
thermal low pressure zone, the “ Equatorial trough” .
Rising air hits tropopause and turns north and south
Poleward travelling air cools and converges making air sink
Sinking air deflected towards equator makes Hadley cell.

11. Mechanism Explained!
• Flow of air occurs because the Sun heats air at the Earth’s
surface near the equator.
• warm air rises, creating a band of low pressure at the
equator.
• Rising air reaches the top of the troposphere (10-15 kms)
above Earth’s surface, air flows towards north and south
poles.
• The Hadley cell eventually returns air to the surface of the
Earth in the subtropics, near 30 degrees north or south
latitude.

12. Cont’d
The area of low pressure and converging winds
(air flowing together) on equator is called the
Intertropical Convergence Zone (ITCZ).
These winds are turned toward the west by the
Coriolis effect and become the trade winds or
the tropical easterlies.

13. ITCZ (Hadley cell)

14. 1) ITCZ
2)mid-latitude precipitation zone

15. ITCZ
• ITCZ and Hadley cells are not stationary
• Move north and south with the position of
sun
• Presence of ITCZ produces rain in over that
area

16. Evidences
• Evidence of poleward expansion
• Evidence of Solar Influence
– UV irradiance and ozone heating in the upper
stratosphere.
– cyclic variations in relationship invisible in the
long-term average.

17. Experimental observations?
• Expansion of about 2 to 4.5 degrees of latitude since
1979.
• The expansion of the Hadley circulation implies a
poleward expansion of the band of subtropical
subsidence, leading to enhanced mid-latitude
tropospheric warming and poleward shifts of the
subtropical dry zone.
• This would contribute to an increased frequency of
midlatitude droughts in both hemispheres.

18. Poleward expansion
• Both observational outgoing longwave
radiation and precipitation datasets show an
annual average total poleward expansion of
the Hadley cells of about 3.6o latitude.
• A widening of the Hadley cell has also been
seen in recent satellite observations

19. Cont’d
• Analysis of upper tropospheric humidity, cloud
amount, surface air temperature, and vertical
velocity confirm that changes are associated
with a decadal-time-scale strengthening of the
tropical Hadley circulations.
• Equatorial convective regions have intensified
in up-ward motion and moistened, while both
the equatorial and subtropical subsidence
regions have become drier and less cloudy.

20. Mean monthly ITCZ structure
Comparison indicates that winter
hemisphere cell is stronger than
summer hemisphere cell

21. Seasonal changes in Hadley cell w.r.t.
temp.

22. Seasonal changes in Hadley cell w.r.t.
pressure

23. Seasonal change
w.r.t. Jet stream
(wind amplitude)

24. Hadley circulation dynamics-evidence
for cell widening
• Winter cell increased in intensity since 1950
• Variation related to variation in SST
HOW

25. How
Hadley
cell Responds strongly to El-
Nina(warming)
Responds weakly
to La-Nina(cooling)
Linked to inter annual
variation of ENSO
Non linear change in statistical
properties increased frequency and
amplitude of winter Hadley cell
Warming of tropical
Indo-west Pacific
warm pool
accelearted winter
Hadly cell

26. Sea Surface Temperatures
• Hadley circulation is changing in response to a
warming in the tropical Indian Ocean and
Pacific Ocean during the past 50 years.

27. Result of SST in GCM
• widening of the Hadley cell in response to
increases in global mean temperature (perhaps
by 2 degrees latitude over the 21st century
• lead to large changes in precipitation in the
latitudes at the edge of the cells.
• Scientists fear that the ongoing presence of
global warming might bring drastic changes to
the ecosystems in the deep tropics and that the
deserts will become drier and expand

28. ITCZ w.r.t
season

29. Role of sea level pressure
• Sea level pressure from observational and
reanalysis datasets show smaller magnitudes
of poleward expansion, of about 1.2o latitude.

30. Future Impacts
• The Hadley circulation spans half of the
surface area of the globe and variability within
this system affects the lives of billions of
people.
• Hadley cells produce tropical rainforests and
worlds major subtropical deserts
• It is assumed to effect the global climate
pattern

31. Cont’d
• Dessicating Winds from Hadley Cells (desert
formation)
• driest locales on Earth are situated in similar
latitude bands, between 15-30 degrees
latitude in the Northern and Southern
Hemispheres
• motion from the subsiding branch of the
Hadley cell fluxes moisture away from these
locations and into the moist deep tropics.

32. Cont’d
• regions bordering these deserts are often
among the most tenuous of ecosystems, there
are important implications for possible
changes with global warming.
• Climate models show a general drying of the
subtropics (and moistening of the deep
tropics) in simulations of global warming.

33. Increase in OLRs
• Increase in subtropical OLR is indicative of an
intensification of the Hadley circulation
• an intensified Hadley circulation would lead to
stronger descending motion in the subtropics
in both hemispheres, and thus cause greater
emissions of infrared radiation into space.

34. So…
• Low values of OLR=cold cloud tops=deep
convective cloud
• High values of OLR=clear skies resulting from
subsidence in subtropical highs
• Which means decreased cloud formation and
less rain

35. Conclusion
• The Hadley Circulations are fundamental
regulators of the Earth’s energy budget.
• Hadley Cell intensity is associated with the
gradient in latent heat release from the tropics
to the subtropics, driven in the model by the
gradient in sea surface temperature.
• It is not related to the absolute warmth of the
climate, or of the tropical sea surface
temperatures.

36. Cont’d
• The poleward extent of the Hadley Cell is affected by
numerous processes, including the influence of
topography in the extratropics.
• It also does not vary systematically with global mean
temperature. Only the strongest Hadley Cell changes
are longitudinally homogeneous
• Pacific Ocean is the most important basin for the zonal
average Hadley Cell response. Although the latitudinal-
average precipitation does respond to Hadley Cell
intensity and extent, the soil moisture variations are
less correlated, due to differing seasonal effects and
the influence of temperature/evaporation changes.

37. References
• Anders Persson (2006). Hadley's Principle: Understanding and
Misunderstanding the Trade Winds.History of Metereology.3:17–42
• Xiao-Wei Quan, Henry F. Diaz, and Martin P. Hoerling (2004). Changes in
the Tropical Hadley Cell since 1950
• Henry F. Diaz and Raymond S. Bradley. The Hadley Circulation: Present,
Past, and Future. Advances in Global Change Research. 21. Springer
Netherlands.pp. 85–120.
• Dargan M.W. Frierson, Jian Lu, and Gang Chen (2007). Width of the Hadley
cell in simple and comprehensive general circulation models.Geophysical
Research Letters 34 (18)
• Dian J. Seidel, Qian Fu, William J. Randel, and Thomas J. Reichler (2007).
"Widening of the tropical belt in a changing climate".Nature Geoscience 1
(1):21–4.
• Celeste M. Johanson and Qiang Fu (2009). Hadley Cell Widening: Model
Simulations versus Observations.Journal of Climate. 22 (10):2713–25.