The Tropics: Climatology and
Large-Scale Circulations
Tropical
M. D. Eastin
Outline
Climatology
• Radiation
• Land / Ocean
• Temperature
• Winds
• Moisture
• Clouds and Precipitation
Large-Scale Circulations
• Hadley Cell
• Walker Circulation
Tropical
M. D. Eastin
The Tropics: Climatology
The need for radiation balance:
Incoming Shortwave
Radiation
Sun
Earth
Outgoing Longwave
Radiation OLR
Tropical
M. D. Eastin
The Tropics: Climatology
Tropical
M. D. Eastin
The Tropics: Climatology
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)
Tropical
M. D. Eastin
The Tropics: Climatology
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
Sea-Surface Temperatures (C)
• 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)
Tropical
M. D. Eastin
The Tropics: Climatology
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
JAN
JUL
Tropical
M. D. Eastin
The Tropics: Climatology
Meridional Cross Sections:
• Constructed from zonal means
(averages around latitude circles)
x  
zonal
mean
1
2
2
 xd 
longitude
0
variable in question
(e.g. temperature)
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
Tropical
JUL
M. D. Eastin
The Tropics: Climatology
200 mb
200 mb
JAN
850 mb
850 mb
JAN
Tropical
JUL
JUL
M. D. Eastin
The Tropics: Climatology
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
W
W
E
JAN
• 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?
W
E
W
JUL
Tropical
M. D. Eastin
The Tropics: Climatology
Meridional (north-south) Wind (m/s):
JAN
• 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
JUL
Tropical
M. D. Eastin
The Tropics: Climatology
Vertical Wind (mb/s):
JAN
• 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)
JUL
JUL
• 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
Tropical
M. D. Eastin
The Tropics: Climatology
Moisture :
Total Precipitable Water (mm)
PW 
1
g
pt
 qdp
po
• Tropical maxima follows the ITCZ
as it moves N-S (land and water)
JAN
• 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
Tropical
JUL
M. D. Eastin
The Tropics: Climatology
Moisture:
Relative Humidity (%)
D
D
• 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)
W
JAN
D
D
W
Tropical
JUL
M. D. Eastin
The Tropics: Climatology
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
JAN
• 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
Tropical
M. D. Eastin
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
Tropical
M. D. Eastin
The Tropics: Large-Scale Circulations
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
Tropical
Jupiter: One rotation in 10 hours
12 zonal bands
M. D. Eastin
The Tropics: Large-Scale Circulations
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
Tropical
Venus: One rotation in 243 days
2 zonal bands
M. D. Eastin
The Tropics: Large-Scale Circulations
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
Tropical
M. D. Eastin
The Tropics: Large-Scale Circulations
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 dominant
Pacific component
of the
Walker Circulation
Tropical
M. D. Eastin
The Tropics: Large-Scale Circulations
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
Normal or La Nina
El Nino
Tropical
M. D. Eastin
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)
Tropical
M. D. Eastin
References
Climate 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.
Tropical
M. D. Eastin