Unit 9: Circulation Patterns of the Atmosphere Simple models of the atmospheric circulation Actual circulation of the atmosphere Global, Regional Upper atmosphere, jet streams Action in the atmosphere—cloud formation over Easter Island in the South Pacific Ocean. (Authors’ photo)

OBJECTIVES  Develop a simple model of the global atmospheric circulation Discuss the pressure systems and wind belts of that model circulation, and the complications that arise when the model is compared to the actual atmospheric circulation Introduce the basic workings of the upper atmosphere’s circulation  Understand the importance of the upper level jet stream and its seasonal variability

Thermal Cell Circulation based on uneven heating, but no rotation Hypothetical atmospheric circulation on a featureless, nonrotating Earth. Polar high pressure and equatorial low pressure would result in northerly surface winds in the Northern Hemisphere and southerly surface winds in the Southern Hemisphere. The rotation of the Earth, the annual variation in the latitude of the vertical noon Sun, the distribution of oceans and continents, and land/water heating contrasts at the surface prevent this simple general circulation from developing.  

Simplified Model of Atmospheric Circulation including uneven heating, rotation, and conservation of angular momentum A conceptual model of the global atmospheric circulation pattern showing the major surface pressure belts, the prevailing surface wind systems that develop from them, the upper-level jet streams, and the Coriolis deflection of surface winds.

General Atmospheric Circulation Figure 4.13b

Features of the Idealized 3-Cell Model Equitorial Low or Inter-Tropical Convergence Zone (ITCZ) warm air rises near the equator where trades meet; cloudy, rainy ITCZ shifts with movement of sun, north in our summer, south in our winter. Shift is greater over continents. Subtropical Highs occur where rising tropical air at the equator moves poleward, cooling and sinking creating warm, dry conditions. Trade Winds. Air flowing from subtropical highs towards ITCZ deflects by Coriolis forces into NE trades and SE trades. Trade winds are the most consistent winds in the atmosphere. Westerlies. Air flowing poleward from subtropical highs are deflected by Coriolis force into westerlies at mid-latitudes. Polar Highs occur at the two polar regions as cold air sinks and spreads equatorward. Coriolis deflection turns these high latitude winds into polar easterlies. Polar front. Where cold polar easterlies meet warmer westerlies, low pressure and rising air forms the polar front, a zone of traveling cyclones.

Global mean sea-level pressure (mb) patterns in January (A) and July (B).

Features of the Global Seasonal Pressure Patterns Equitorial Low or Inter-Tropical Convergence Zone (ITCZ) warm air rises near the equator where trades meet; cloudy, rainy ITCZ shifts with movement of sun, north in our summer, south in our winter. Shift is greater over continents. Subtropical Highs occur where rising tropical air at the equator moves poleward, cooling and sinking creating warm, dry conditions. These highs also shift with the sun, being larger, stronger over ocean basins in summer. Westerlies. Air flowing poleward from subtropical highs are deflected into westerlies at mid-latitudes. Polar Highs occur at the two polar regions as cold air sinks and spreads equatorward. Coriolis deflection turns these high latitude winds into polar easterlies. Polar highs become more dominant in winter over high latitude continents, such as Siberian High. Subpolar lows occur over high latitude oceans in winter, such as the Icelandic and Aleutian Lows. These are mean positions of frequent cyclones that move along the polar front into N. America and Eurasia. In the S. Hemisphere subpolar lows form a year-round belt of stormy weather.

Fraction of surface area covered by land as a function of latitude.

The Hadley Cell Circulation A cross-section of the Hadley Cell circulation. Surface convergence of the trade winds feeds convective lifting in updraft zones that are embedded along the Equatorial Low (ITCZ). Westerly acceleration of upper-level winds leads to the formation of the Subtropical Jet Stream (STJ), which initiates convergence of air and general subsidence at about 30 latitude. The resulting Subtropical High (STH) is a broad and continuous belt of high pressure that encircles the Earth.

The Inter-Tropical Convergence Zone Mean positions of the Inter-Tropical Convergence Zone (ITCZ) in January and July. Note the larger migration of the ITCZ over continents and oceans surrounded by continents.

Secondary Surface Circulation: Monsoon Winds Monsoon is a seasonal reversal of winds caused by differential heating of land and ocean. In winter, the land cools with high pressure and offshore winds. In summer, the land heats rapidly creating low pressure and onshore winds. This is also the rainy season for much of the subcontinent. General airflow patterns over the Indian subcontinent during the winter (top) and summer monsoon season (bottom). The regional circulation system involves strong seasonal changes in the heating patterns over the Indian subcontinent and Tibetan plateau, seasonal reversal of offshore and onshore winds over India, shifts in the position of the Subtropical and Equatorial Jet Streams, and distinct winter and summer precipitation patterns.

Monsoon Winds The Himalayas form a giant barrier to summer monsoon winds pushing them westward into the Ganges River Valley.

Circulation of the Upper Atmosphere In the upper troposphere, Coriolis force turns poleward flowing air into westerly winds. The strongest westerlies is called the jet stream. Note the winter jet stream is stronger and further south than in summer. Zonal mean wind speeds (m s-1) by latitude and altitude averaged across all longitudes for multiple decades of time for the Northern Hemisphere in summer and winter. Easterly zonal winds are shaded, westerly zonal winds are unshaded.

Upper Air Rossby Waves Upper air westerlies form waves with flow moving equatorward producing troughs, while poleward flow forms ridges. How many waves can you see? Mean 500-mb height (m) in the Northern Hemisphere for January.  

Wind Portrait of the Pacific Ocean Figure 4.6