The Tropics: Climatology and Large-Scale Circulations

Tropical M. D. Eastin

The Tropics: Climatology and Large-Scale Circulations


Outline

Climatology

• Radiation• Land / Ocean• Temperature• Winds• Moisture• Clouds and Precipitation

Large-Scale Circulations

• Hadley Cell• Walker Circulation


The Tropics: Climatology

Outgoing Longwave Radiation OLR

Incoming Shortwave

Radiation Sun

Earth

The need for radiation balance:





The need for radiation balance:

• Keep the global temperature constant (The earth acts like a black body)• The primary role of weather is to redistribute the solar energy (heating)

such that the Earth can most effectively re-radiate the energy back to space

Annual Mean Solar Radiation Observed at Surface (W m -2)



Land / Ocean Forcing:

Land

• Major elevation features deflect air over (clouds and precipitation) and around (cyclonic / anticyclonic flow)• Differences in elevation create

thermal gradients due to surface heating (e.g. Indian Monsoon)

Ocean

• Oceans are a heat and moisture reservoir that the atmosphere “taps” (ocean has a large heat capacity)• Differential solar heating leads to thermal gradients and ocean currents

Land-Ocean

• Large heat / moisture gradients often help force atmospheric circulations (nor-easterlies and land-sea breezes)

Sea-Surface Temperatures (C)



Air Temperature (C):

• Near-surface air temperatures are

dominated by surface type (e.g. desert, snow, mountains, water) and cloud cover

• Weak temperature gradients within

the Tropics

• Strong temperature gradients between the Tropics and Polar

JUL

JAN



Air Temperature (C):

• Tropical tropopause is much higher than

its extra-tropical counterpart (caused by deep convection of ITCZ)

• In general, the mean Tropics are more unstable than the extra-tropics

• Temperature maximum in NH summer is

located at 20º-30ºN (Continent effects)

• Maximum temperature gradient is NH is farther north during their summer

JAN

JUL

Meridional Cross Sections:

• Constructed from zonal means (averages around latitude circles)

2

02

1xdx

longitude

variable in question(e.g. temperature)

zonalmean



JULJAN

JULJAN

200 mb

850 mb 850 mb

200 mb



Zonal (east-west) Wind (m/s):

• Jet Streams are maximum in the winter when the N-S temperature gradients are strongest

• Southern Hemisphere has two jets in the

winter (in troposphere and stratosphere) due to the lack of land in SH and a very cold Antarctic

• Easterlies slope from a low-level maximum in the winter hemisphere to an upper-level maximum in summer hemisphere - related to ITCZ and the Hadley Cell

• Weak westerlies evident at 10N in the mean zonal wind - Why?

JAN

JUL

E

E

W

W

WW



Meridional (north-south) Wind (m/s):

• Much weaker than zonal wind

• Dominated by Hadley Cell circulation

• Strongest upper-level winds associated with flow toward the jet maximum

• Strongest low-level winds are a result of mass balance and Indian Monsoon

JAN

JUL



Vertical Wind (mb/s):

• Two orders of magnitude smaller than either zonal or meridional wind (strong motions confined to small scales)

• Dominated by Hadley Cell circulation

• Upward motion indicates the zonal mean

location of the ITCZ convection

• Sinking motion indicates the zonal mean

locations of the clear, dry subtropical highs (most deserts located here)

• Double upward maxima are related to N-S shift of ITCZ over land masses

• Low-level maxima in NH near 30º-40ºN related to Indian Monsoonal flow

JAN

JULJUL



Moisture :

Total Precipitable Water (mm)

• Tropical maxima follows the ITCZ as it moves N-S (land and water)

• Minima associated with subtropical highs, mountain ranges, deserts, and polar regions

• Double ITCZ in January is the South

Pacific Convergence Zone (SPCZ) associated with subtropical jet

• ITCZ tends to cross equator toward summer hemisphere

pt

po

qdpg

PW1

JUL

JAN



Moisture:

Relative Humidity (%)

• Maxima near the surface (ocean source)

• Maxima associated with ascent (ITCZ) in

the Hadley cell

• Minima associated with descent in the Hadley cells (subtropical highs)

JAN

JUL

D

W

D

W

DD



Precipitation (mm/day):

• Tropical maxima follows the ITCZ as it moves N-S (land and water)

• Local maxima associated with ascent over major mountain ranges

• Minima associated with subtropical highs and polar regions

• Local minima associated with descent

beyond major mountain ranges

• Double ITCZ in January is the South Pacific Convergence Zone (SPCZ) associated with subtropical jet

JUL

JAN


The Tropics: Large-Scale Circulations

Hadley Circulation:

• Zonally symmetric over-turning circulation that dominates (~defines) the Tropics

• Ascent near the equator is thermally driven by solar heating maximum and latent heat release (which is partially balanced by adiabatic cooling)

• Ascent is not a a uniform band but rather multiple localized “hot towers” (embedded within the ITCZ) that are more efficient at transporting the heat aloft

• Forced divergence at the stable tropopause leads to poleward flow, which via Coriolis, turns into westerly flow (i.e. jet streams)

• Descent is thermally driven by radiative cooling (which is partially balanced by adiabatic warming)

• The near-surface equatorward (or return) flow is, via Coriolis, turned into easterly flow (i.e. trade winds) and converges more heat and moisture toward ITCZ

• Migrates north and south following the Sun



Hadley Circulation:

• North-south extension is a function of Earth’s rotation rate (i.e. Coriolis force)

Earth: One rotation in 24 hours 6 zonal bands

Jupiter: One rotation in 10 hours 12 zonal bands



Hadley Circulation:

• North-south extension is a function of Earth’s rotation rate (i.e. Coriolis force)

Saturn: One rotation in 11 hours 10 zonal bands

Venus: One rotation in 243 days 2 zonal bands



Hadley Circulation: An Exercise

How long does it take a parcel to complete one full circulation?

How many revolutions of the Earth does the parcel complete in this time?

Assumptions:

• Parcel begins at 0°N, 0°W, at the surface

• Troposphere is 15 km deep• Earth’s circumference is 40,000 km• Cell extends 3,000 km to the north

• Parcel rises rapidly through an ITCZ thunderstorm at 15 m/s• Other motions follow climatological mean winds



Walker Circulation:

• Zonally asymmetric over-turning that strongly influences zonal flow near the equator• Thermally-direct circulations forced by E-W gradients in SST induced by wind-driven ocean currents and the global land configuration (one in each equatorial ocean)• Ascent occurs over regions of warmer SSTs and decent is a result of radiative cooling

• Linked to the El Nino Southern Oscillation (ENSO)• Impacts Atlantic Tropical Cyclones

The dominantPacific component

of the Walker Circulation



Walker Circulation:

• Pacific component shifts east

during El Nino events

• Descent occurs in western Pacific

• Pacific component is still stronger than the Atlantic, and thus tends to increase westerly winds aloft and descent over the Atlantic ocean, which promotes increased vertical shear and less convection, which leads to less Atlantic TCs

El Nino

Normal or La Nina


The Tropics: Climatology and Large-Scale Circulations

Summary:

• Need for radiation (energy) balance

• Land /Ocean forcing (heat and moisture sources)• Horizontal, Vertical, and Seasonal Variations of temperature, winds, moisture, and precipitation

• Hadley Circulation (forcing and seasonality) • Walker Circulation (forcing and impacts)


ReferencesClimate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage

( http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl )

Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-year Reanalysis Project. Bull Amer Met. Soc., 77, 437-471.

http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl

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The Tropics: Climatology and Large-Scale Circulations

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