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Section 9.1 Global wind patterns

Subsection 9.1.1 Circulation scales

  • microscale (<1 km in size): PGF, CEN, friction (cumulus clouds, parcels)

  • mesoscale (1-1000 km): PGF, CEN, friction, Cor (thunderstorms, fronts)

  • synoptic scale (>1000 km): geostrophic balance (mid-latitude cyclones)

  • planetary scale (10,000 km): geostrophic balance (atmospheric circulation cells; prevail over longer timescales)

Subsection 9.1.2 Circulation models

  • model: simplified description of a complex system

  • used to explain and predict observations using cause-and-effect relationships

Any circulation model for the Earth should be able to explain:

  • steady and calm winds observed by mariners

  • Regions that lack winds

  • Global patterns of cloudiness

  • Midlatitude cloud patterns

  • Global patterns of precipitation

  • Jet streams

Global Circulation on Non-Rotating Earth

  • single cell circulation (Hadley model)

  • contrast in temperatures between the poles and the equator creates a large convection cell in both hemispheres.

Global Circulation on Rotating Earth

  • three-cell circulation: Hadley, Ferrel, and Polar cells

  • Coriolis effect deflects flow, poleward flows cool and sink

  • Not all cells are meridional (N-S motion), there are also zonal cells (E-W motion)

    • For example, Walker circulation is the zonal component of a Hadley cell

    • this behavior is observed in El Nino Southern Oscillaiton

Subsection 9.1.3 Observed circulation patterns

NOAA Jetstream: https://www.weather.gov/jetstream/circ

Global Circulation Model: Observation

  • tropical trade winds converging in Intertropical convergence zone (ICTZ) (region of the doldrums)

  • midlatitude westerlies encounter the polar easterlies along the polar front

Idealized zonal pressure belts:

  • The equatorial low is an intertropical convergence zone (ITCZ).

  • Subtropical highs (STH) are high-pressure zones in the belts about 20°–35° latitude on either side of the equator.

  • Polar highs near the Earth’s poles are where the polar easterlies originate.

Monsoons

  • seasonal reversal of winds; generally driven by pressure differences.

  • The Asian monsoon, which affects India and its surrounding areas, China, Korea, and Japan.

  • The North American monsoon occurs in the southwestern U.S. and northwestern Mexico: this monsoon is driven by the extreme temperatures, which generate a low-pressure center over Arizona and results in a circulation pattern that brings moist air from the Gulf of California and from the Gulf of Mexico, to a lesser degree.

Cloud and Precipitation patterns also show clear trends related to planetary circulation