Global Wind Circulations

Learning Goals 9a:

1. Identify the global wind circulations, including the Hadley cell, mid-latitude belt of extratropical cyclones, and Polar cell.
2. Describe how the trade winds, westerlies, and easterlies are influenced by the Coriolis effect

Wind patterns exist at many different scales of motion, from the largest patterns over the whole globe, to winds associated with lows and fronts, to small turbulent eddies.  All these scales are often superimposed on top of each other.  In this section, we will focus on only the larger-scale (global) wind patterns at the earth's surface, as affect sailing large distances across oceans.

In meteorology, we often focus on three latitude bands in the Northern Hemisphere, and similar bands in the Southern Hemisphere.  The figure below gives the terminology used for these latitude bands, and for winds that blow mostly in the east-west direction (zonal winds), and winds that meander quite a bit to the north and south (meridional winds). 

Figure.  Terminology for weather in different latitude bands on earth. 
(Source: Stull, 2017: Practical Meteorology: An Algebra-based Survey of Atmospheric Science. CC-by-nc-sa.) 

Hadley cell

Sunlight heats the air in tropics more than the poles, causing a temperature gradient (change of temperature across a distance) between the equator and the poles.  The warmer air near the equator rises and creates a region of heavy rain and thunderstorms called the Intertropical Convergence Zone (ITCZ), or the Doldrums. Horizontal winds are are often light to calm, making it difficult to sail at the ITCZ.  This warm air rises to the top of the troposphere, and spreads out toward the Northern and Southern Hemispheres.  But these winds get only as far as about 30° latitude before they are turned toward the east (but still at the top of the troposphere). 

This turning is due to the Coriolis effect , which is related to the rotation of the earth. In the Northern Hemisphere, the Coriolis force pushes to the right of the wind direction.  In the Southern Hemisphere, the Coriolis force pushes to the left.

As air accumulates at 30° latitude, the pressure aloft increases, and forces some of the air downward toward the ocean surface.  Air accumulates here, and creates a subtropical high pressure zone .  This is another region of light to calm surface winds where it is difficult to sail.  In the old days when sailing ships were becalmed in this region, the ships often ran out of food and fresh water, causing any animals such as horses on board to die.  These latitude bands became known as the horse latitudes.  The descending air is very dry, so these latitude bands are where deserts are favoured in both the Northern and Southern Hemisphere. 

As surface air blows back toward the equator, it is turned to the right in the N. Hemisphere, and to the left in the S. Hemisphere, due to the Coriolis effect.  The result are the trade winds (winds with a component from the east) over the ocean surface.  In the old days of sailing ships, trans-oceanic commerce and trade could be conducted due to these winds -- hence their name  trade winds. 

The figure below shows the Hadley cells.

Figure. Idealized global circulation for Northern Hemisphere winter. The bulls-eye symbol indicates where a jet stream is coming out of the page toward the reader, and the "X" with a circle around it represents imagined tail feathers of jet-stream wind flowing into the page.  (Source: Stull, 2017: Practical Meteorology: An Algebra-based Survey of Atmospheric Science. CC-by-nc-sa.) 

Mid-latitude Cyclones

At mid-latitudes, such as Canada, there is not a strong vertical circulation cell.  Instead, the winds create large, horizontal eddies or waves (see figure above).  This is the latitude band of swirling low- and high-pressure systems that we see on weather maps (see figure below). The Coriolis effect causes the winds to rotate counterclockwise around lows in the N. Hemisphere, and clockwise around highs.  (Opposite in the S. Hemisphere.)

High-pressure regions (H), called anticyclones, are associated with fair weather, clear skies, but light winds (not good for sailing, but good for picnics and Ultiimate frisbee).

Low-pressure regions (L), called extratropical cyclones, are associated with fronts, bad weather, and strong winds (not good for pleasure sailing). 

Often the best sailing at mid latitudes is in between the highs and lows, where the winds are moderate and weather is still OK.

 

At mid-latitudes is a general west-to-east air flow known as the Westerlies.  The mid-latitude Westerlies exist in both the Northern and Southern Hemispheres, but tend to be stronger in the Southern Hemisphere since they flow mostly over water (less frictional drag). Mid-latitude cyclones (Lows) and anticyclones (Highs) are imbedded in a general westerly flow; hence, these weather systems usually move from west to east. The westerlies in both hemispheres are stronger in winter than in summer. 

Figure.  Surface winds around the globe, as is useful for trans-oceanic sailing. L and H indicate low and high pressure.  (Source: Stull, 2017: Practical Meteorology: An Algebra-based Survey of Atmospheric Science. CC-by-nc-sa.)

Polar Cell

The Polar cells occur between about 60° latitude and the poles. The air movements here are much weaker compared to those in the Hadley cells. The cool polar air meets the warmer mid-latitude air near the 60^th parallel, where the warmer air rises over the colder air. This zone is known as the polar front. Here, winds blow from the east and north, shifting slightly east due to the Coriolis effect and are called the polar easterlies.

Summary

Near the earth's surface at mid-latitudes (such as the latitude of southern Canada) the prevailing winds are from the west .  These winds are called westerlies.  Embedded within this general flow are extratropical cyclones (lows) that are also blown from west to east by the prevailing winds.  But near these lows and their associated fronts, the local winds can deviate from westerly - - the winds blow counterclockwise (in the N. Hemisphere) around the low-pressure center.  Also embedded in the mid-latitude flow are moving anticyclones (highs) that are also blown from west to east.  But near these highs, the winds are very light, and rotation is clockwise around the high center in the N. Hemisphere.  Winds rotation in opposite directions around highs and lows in the S. Hemisphere.

Near the earth's surface at tropical (low) latitudes, the prevailing wind is from the east.  These are called easterlies, and are also known as the trade winds.  Embedded within this general flow can be tropical cyclones (hurricanes, typhoons), which are also initially blown from east to west by the prevailing easterlies. 

The moving/traveling mid-latitude lows and highs are distinct from the semi-stationary bands of high pressure at 30 degree latitude, and semi-stationary bands of low pressure at roughly 60 degrees latitude.

Keywords: Coriolis effect, Hadley cell, Polar cell, Intertropical Convergence Zone (ITCZ), doldrums, subtropical high pressure zone,  horse latitudes, trade winds, westerlies, polar easterlies, polar front, lows, cyclones, highs, anticyclones, zonal winds, meridional winds.

Additional Resources: (non-required material)

Animation of Global Winds: https://earth.nullschool.net/

Videos: (non-required material)

Global Atmospheric Circulation: https://www.youtube.com/watch?v=Ye45DGkqUkE