Hadley Circulation, Evidences and Impacts

Xarrin

Sindhu

M.phil Environmental SciencesFatima Jinnah Women University

Contents

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

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.

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.

Hadley cell

History

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

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

Cont’d

Net surplus used to evaporate water as most of surface covered by ocean or vegetation

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.

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.

Cont’dThe 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.

ITCZ (Hadley cell)

1) ITCZ 2)mid-latitude precipitation zone

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

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.

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.

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

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.

Mean monthly ITCZ structure

Comparison indicates that winter hemisphere cell is stronger than

summer hemisphere cell

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

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

Seasonal change w.r.t. Jet stream

(wind amplitude)

Hadley circulation dynamics-evidence for cell widening

• Winter cell increased in intensity since 1950• Variation related to variation in SST

HOW

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

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.

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

ITCZ w.r.t season

Role of sea level pressure

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

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

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.

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.

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.

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

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.

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.

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.