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The Tropics: Climatology and Large-Scale Circulations

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. Sun. Earth. The Tropics: Climatology.

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The Tropics: Climatology and Large-Scale Circulations

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  1. The Tropics: Climatology and Large-Scale Circulations M. D. Eastin

  2. Outline • Climatology • Radiation • Land / Ocean • Temperature • Winds • Moisture • Clouds and Precipitation • Large-Scale Circulations • Hadley Cell • Walker Circulation M. D. Eastin

  3. Sun Earth The Tropics: Climatology The need for radiation balance: Incoming Shortwave Radiation Outgoing Longwave Radiation OLR M. D. Eastin

  4. The Tropics: Climatology M. D. Eastin

  5. 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) M. D. Eastin

  6. 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 • 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) M. D. Eastin

  7. 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 M. D. Eastin

  8. The Tropics: Climatology • Meridional Cross Sections: • Constructed from zonal means • (averages around latitude circles) longitude zonal mean 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 JUL M. D. Eastin

  9. The Tropics: Climatology 200 mb 200 mb JAN JUL 850 mb 850 mb JAN JUL M. D. Eastin

  10. 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 • 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 W E JAN W E W JUL M. D. Eastin

  11. The Tropics: Climatology • 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 M. D. Eastin

  12. The Tropics: Climatology • 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 JUL JUL M. D. Eastin

  13. The Tropics: Climatology • 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 JAN JUL M. D. Eastin

  14. The Tropics: Climatology • 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) D D W JAN D D JUL W M. D. Eastin

  15. 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 • Local minima associated with descent • beyond major mountain ranges • Double ITCZ in January is the South • Pacific Convergence Zone (SPCZ) • associated with subtropical jet JAN JUL M. D. Eastin

  16. 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 M. D. Eastin

  17. 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 Jupiter: One rotation in 10 hours 12 zonal bands M. D. Eastin

  18. 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 Venus: One rotation in 243 days 2 zonal bands M. D. Eastin

  19. 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 M. D. Eastin

  20. 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 M. D. Eastin

  21. 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 M. D. Eastin

  22. 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) M. D. Eastin

  23. 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. M. D. Eastin

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