GENERAL CIRCULATION THE SOUTHERN HEMISPHERE

by Mark A. Thornton - October 2005

A Record Shattered

Clipper ship from www.eraoftheclipperships.com

In early 1848, Baltimore’s waterfront was abuzz with news of the successful return of the W.H.D.C. Wright, a sleek clipper ship, from Rio de Janeiro, Brazil. Captain Jackson, of the Wright, had managed to complete the 8,000 mile round trip in 75 days instead of the usual 110. By reducing the trip’s duration and associated operating expenses, Jackson had dramatically increased his profit margin on the journey. Jackson’s courage not only rewarded him with a record-setting trip, it ushered in the adoption of an entirely new understanding of the patterns of wind and currents on the oceans.

General Circulation
If you chose to closely observe the wind at a specific location over the course of several days, you would likely experience considerable variations in direction and speed. Over a much longer time period and a larger geographic area, however, the wind settles into a general pattern of speed and direction, with predictable changes associated with seasonal transitions in the weather. It is this long-term, generally predictable pattern in the wind that meteorologists refer to as general circulation.

 

Solar heating is largely confined to the region

 between 30°N and 30° S latitude.

COMET Multimedia Database

The driving force of general circulation is the Sun’s unequal heating of the Earth’s surface. The diagram to the left demonstrates that over the course of a year, the Sun’s heating power is at its maximum between latitudes 30° south and 30° north, the region generally defined as the Tropics. Due to the tilt of the Earth on its axis, even the tropics are not equally heated. The months of December through February are considered meteorological summer in the Southern Hemisphere, and during this period the Sun’s rays are more directly focused on the southern reaches of the Tropics.

 

However, during the months of June, July and August, the Earth has progressed in its orbit and the Sun shines more directly over the northern portion of the Tropics. This unequal heating results in significant temperature differences between the tropics and the poles. A variety of mechanisms are constantly at work transferring warm air toward the poles and cool air toward the equator in an effort to diminish these differences.

 

Atmospheric Principles

Weather map for October 10, 2005 showing areas of(L)ow and (H)igh Pressure Australian Bureau of Meteorology

Before we can more closely examine the concept of general circulation, we need to review a few basic atmospheric principles. The air above the earth at any give location, also referred to as an air column, has weight. Meteorologists determine the weight of an air column using an instrument called a barometer, and use the term barometric pressure for the resulting atmospheric variable. Because the heights of air columns in cities at higher elevations tend to be shorter than those of cities at sea level, adjustments are made to the pressure readings at higher altitude locations to equate them to sea level pressure. This adjustment allows for a direct comparison of barometric pressure, regardless of the altitude at which the observation takes place.

Everyone is familiar with weather maps, such as the one to the left, dotted with “L”s  for low pressure and “H”s for high pressure. Simply put, regions identified with an “L” are those where the air column weighs less than the air columns surrounding it. Air columns associated with low pressure are usually characterized by rising air parcels which are likely to result in clouds and precipitation. Conversely, areas of high pressure are characterized by the clear skies and fair weather associated with falling air parcels.

Low Pressure/High Pressure

Australian Bureau of Meteorology

It is important to understand that areas of low and high pressure aren’t merely surface features; they have a corresponding offsetting component much higher in the atmosphere. For example, a surface low is unable to persist unless the higher levels of the atmosphere above it shed more air from the column than the amount that is entering it at lower levels. The animations to the right demonstrate the overall structure and air flow in an air column of both low and high pressure in the Southern Hemisphere.

Wind pattern associated with Southern Hemisphere (H)igh and (L)ow Pressure

Wind at the Earth’s surface flows from areas of high to low pressure. The speed of the wind is determined by the difference in barometric pressure between the air columns and the linear distance over which the transition takes place. However, the path taken is not a direct line drawn from the center of high pressure to the center of low pressure. Instead, friction and the Coriolis force combine to modify the flow of surface wind, as show in the graphic to the right. The result is that surface air in the Southern Hemisphere flows in an outward, counter-clockwise fashion from areas of high pressure and in an inward, clockwise manner to an area of low pressure. The surface wind in the Northern Hemisphere flows in the opposite direction.

General Circulation in the Tropics and SubtropicsIntense solar heating in the region near the Equator is responsible for creating a snake-like band of rising air circling the planet. This area can be easily identified on satellite images, such as the one below, by the nearly constant appearance of towering cumulus clouds and intense thunderstorms. It is easy to discern these cloud bands that pass just north of Australia and across the African Continent on this image from Southern Hemisphere summer. 

A satellite image showing the ring of cumulus clouds and thunderstorms associated with the ITCZ. Space Science and Engineering Center at the University of Wisconsin-Madison

There is a limit, of course, to how high these rising parcels can travel before they reach the barrier known as the tropopause. Since they are unable to continue upward, the air parcels have no option but to head toward the North and South poles. 

The prevailing winds resulting from Hadley, Ferrel and Polar cells.

As these air parcels head toward the poles, they tend to cool and converge in a band around the Earth, located at approximately 30° latitude in both the Northern and Southern Hemisphere. This convergence and cooling contributes weight to the air mass, resulting in an area of high pressure at the surface, referred to as the subtropical highs. Some of the air from these subtropical highs flows back along the surface, and while doing it is shifted by the Coriolis force, resulting in steady northeasterly winds in the Northern Hemisphere and southeasterly winds in the Southern Hemisphere.

This loop of air circulating from the tropics toward the subtropical highs and back again is known as a Hadley Cell, after George Hadley who originally proposed the concept in the 1700s. Although Hadley’s hypothesis of general circulation wasn’t entirely correct, it was remarkable at the time.  Atmospheric scientists now understand that three such cells exist in each hemisphere, transporting warmer air to higher latitudes and colder air back toward the tropics. The graphic at right provides a picture of the various dynamics at work, as well as the prevailing winds that result from the circulations of the cells.  

The region near the Equator where these winds converge is called the intertropical convergence zone (ITCZ). Due to the unequal distribution of land masses between the Northern and Southern hemisphere, the ITCZ does not precisely overlay the Equator. Instead, it weaves back and forth, generally following the area of greatest solar heating. 

Source: USA Today

The ITCZ tends to shift in a northerly direction during Northern hemisphere summer and southerly during Northern hemisphere winter. The area is characterized by light surface winds and considerable precipitation. The ITCZ during Southern Hemisphere summer is identified on this annotated satellite image.

General Circulation in History
Our understanding of the mechanics of general circulation is a fairly recent scientific achievement when compared to our awareness of its manifestations. The winds flowing from the subtropical highs toward the Equator from the northeast in the Northern Hemisphere, and from the southeast in the Southern Hemisphere, have been used by mariners for thousands of years. During the age of exploration and the development of the spice trade, these winds came to be known as the trade winds. The trade winds were a fairly reliable conveyor belt that transported ships to nearly every corner of the globe and back to Europe, laden with riches and unbelievable tales of exotic lands.

 

The location of the horse latitudes and doldrums along with flow of the trade winds. Base map from The University of Texas - Austin

In the context of human history, a ship’s ability to sail effectively within 35° or 40° of the direction of the wind is a recent occurrence. Prior to the development of efficient sail and hull designs, ships generally traveled parallel to or away from the wind by relying upon large square sails to simply catch the wind and push the vessel along, more or less as the wind would force a barn door open. Early explorers, restricted in the direction they could travel, were well aware of the trade winds and navigated carefully to take full advantage of the propulsion they had to offer. In the days before engines, the lack of favorable winds could, and frequently did, lead to disaster.

The areas associated with the subtropical highs (30° north and south latitude) came to be known as the horse latitudes by early voyagers. Sailing lore suggests that the term was adopted because ships caught in this area of light winds and dry air jettisoned horses and livestock overboard in order to save water – a precious commodity on journeys of unknown duration. Early sailors were also familiar with the ITCZ, referring to the region as the doldrums. For lack of steady winds, crews frequently suffered under the scorching sun for days or weeks while their ships were becalmed. Sailors who managed to pass through the doldrums without delay considered themselves very fortunate.

The reliability of the trade winds were an important aspect in many famous journeys. Although much maligned by history, Captain William Bligh’s (1754-1817) adventure following 1789 mutiny on board the HMS Bounty stands out as a remarkable success against overwhelming odds. After the successful mutiny, Fletcher Christian set Bligh and eighteen crew members adrift in a 23’ open boat, assuming that all would be lost at sea--along with news of the mutiny.

  

 

Bligh's route following the 1789 Bounty mutiny

However, Bligh was an accomplished navigator, having refined his skills at the side of Captain James Cook, the era’s leading navigator.

Although Tahiti was a mere 500 miles away, Bligh knew better than attempt to buck the trade winds, even for such a relatively short distance. Instead, he took advantage of the reliable southeasterly trades and sailed west towards the East Indies. Forty-eight days later, Bligh and all but one of his crew arrived safely at Coupang, Timor. With only a sextant, logline (a device for measuring speed) and his knowledge of and steadfast reliance upon the trades, Bligh had flawlessly navigated little more than a rowboat nearly 3,600 miles.

A Better Understanding of General Circulation
Captain Jackson of the W.H.D.C. Wright was a veteran of the coffee trade and more than familiar with the trade winds and the standard sailing routes associated with them. Tradition dictated that a vessel bound for Rio de Janeiro must first sail east to the coast of Africa in order to safely pass Brazil’s Cape de Sao Roque. This route extended the time spent transiting the doldrums (ITCZ), and required that the Atlantic Ocean be crossed twice. However, Jackson was a daring man who was willing to experiment with a set of radical new sailing instructions offered by a US Navy Lieutenant named Matthew Fontaine Maury (1806-1873).  

Matthew Fontaine Maury

www.eraoftheclipperships.com

Maury joined the Navy in 1825 and was on board the USS Vincennes when she became the first US naval vessel to circumnavigate the world in 1829. Maury was already a keen observer of weather phenomena, having correctly deduced the mechanics of what we now refer to as a “sea-breeze front” in 1827. The Vincennes’ cruise provided Maury ample opportunities to make extensive observations of wind direction and currents on several of the world’s Southern oceans.

Maury’s voyaging days were all but over after an 1839 stagecoach accident severely and permanently injured his right leg. With little else to occupy his mind, Maury began a systematic review of wind direction and other meteorological observations locked away in thousands of dusty and long-forgotten ship’s logs warehoused by the Navy.

 

In acknowledgment of Maury’s scientific inclinations and writing abilities, he was appointed superintendent of the Navy’s Depot of Charts and Instruments (later to become the US Naval Observatory) in 1842. In this official capacity, Maury continued the work that would result in the publication of both Wind and Current Charts and Sailing Directions (1847), and The Physical Geography of the Sea (1855). The knowledge of wind, ocean currents and weather contained in these publications resulted in faster, less dangerous and more comfortable journeys. No one of this era had a better understanding of the ocean’s wind patterns than Matthew Maury.

Maury’s sailing instructions, subsequently used by Captain Jackson during his 1848 trip to Rio, documented that a ship taking the more direct route to Brazil would encounter favorable winds and, more importantly, a much faster transit of the ITCZ.

"the calm belts of the sea, like mountains on the land, stand mightily in the way of the voyager.  Like mountains on the land, they have their passes and their gaps." (Maury)

The accuracy of the instructions, and the monetary rewards earned from adopting them, motivated even the crustiest of old salts to abandon the traditional routes. Within just a few years, a review of ship’s logs revealed that virtually everyone was relying upon Maury’s guidance.

Maury’s pilot charts were a remarkably clever invention that provided the reader with the distribution of winds by direction for each month of the year for a roughly 300 square mile region of the ocean. The Storm and Rain Chart covered a similar geographic area, and provided data on the frequency of rain, fog and other inclement weather. For the first time in history, a ship’s captain had the ability to foresee the likely sailing conditions they would experience while following their charted course.

The discovery of gold in California in 1848 prompted Maury to develop sailing instructions that included the most favorable path for the dreaded passage around Cape Horn.  Ships were known to spend weeks tacking back and forth below the Horn, hoping for a slight shift in the wind that would allow passage into the Pacific Ocean. By 1855, the average trip from New York to California was 136 days, down from 187. The respect for Maury and the usefulness of his charts became so universal that by the mid-1850s insurance underwriters were requiring them on all insured merchant vessels.

Maury's work continued until the outbreak of the Civil War. Like many Virginians, he resigned his commission and offered his services to the Confederacy when his home state seceded. He provided an abundance useful information to Raphael Semmes, the captain of the remarkably successful raider CSS Alabama. Surrender at Appomattox effectively ended the Civil War, but Maury found himself one of a handful of Confederate officers who were initially denied amnesty. Fortunately, his duties had taken him to Europe where he remained until the clamor for his punishment died down in 1868. He then returned to the United States and spent his final years as a professor of meteorology at the Virginia Military Institute.

Matthew Maury provided mariners of his era with an improved understanding and the necessary tools to take better advantage of the Earth’s general circulation. It is unfortunate that his contribution has largely been lost to history.

A Modern Understanding of General Circulation
Our knowledge of the mechanics of general circulation has improved dramatically since the 1850s and modern sailors have been quick to take advantage of these scientific advances. In particular, the imagery and remote sensing capabilities of weather satellites provide a wealth of data on the structure and seasonal changes associated with general circulation in the tropics and subtropics.

Many 21st century sailors still yearn to sail the high seas or perhaps complete a circumnavigation of the globe, with the most daring still eager to challenge Cape Horn. Those who are less adventurous can live vicariously through the participants in the Volvo Ocean Race, which begins on November 12, 2005, and pits seven high-tech sailboats in what amounts to a drag race across some of the planet’s most treacherous waters.

The race is divided into a number of legs, and is scheduled to finish in Gothenburg, Sweden in June of 2006. The competitors in such an event typically have on-shore support teams to assist with logistics and to provide real-time weather forecasts. Let’s examine some of the climatological data that the support teams might take into consideration as they attempt to plan the fastest course around the Southern Hemisphere. The climatological charts that will be discussed present the long-term mean for various meteorological variables. The actual conditions experienced on the race course may vary considerably from the long-term mean.

NCEP/NCAR Reanalysis

1000mb Vector Wind (m/s) For November

Climatology 1968 - 1996

(click here for full chart)

When the starting gun fires at Portsmouth, England on November 12th, the North Atlantic will be settling into a winter weather pattern, while meteorological summer approaches in the Southern Hemisphere. The first order of business for the competitors find the northeasterly trade winds originating from the Northern Hemisphere subtropical high, and head for the northeast coast of Brazil. A review of the mean surface wind chart to the right suggests the participants will encounter wind of approximately 9 meters per second (17.5 knots) as they head southwest toward the Equator. Such a fresh breeze suggests that spinnakers are likely to be a steady companion on this portion of the course.

As progress is made on the first leg, the satisfaction of approaching the warmer weather of the Southern Hemisphere will quickly give way to concerns over transiting the Intertropical Convergence Zone (ITCZ), known by sailors as the doldrums.  Based upon the convergence of the northeast and southeast trade winds, the chart to the right shows that historically the ITCZ occupies the area just above the Equator from approximately 5° to 10° north latitude during November.

Meteorologists have a variety of methods to identify the location of the ITCZ.  Let’s examine a few of them while the Volvo Race crews douse the spinnakers, hoist the jibs and set a course through this tricky area.

 

NCEP/NCAR Reanalysis 1000mb Temperature (C) For November

Climatology 1968 - 1996 (click here for full chart)

 

NCEP/NCAR Reanalysis Sea Level Pressure (mb) For November

Climatology 1968 - 1996 (click here for full chart)

The ITCZ is closely aligned with the ribbon of maximum surface heating that circles the Earth and is known as the thermal equator. The plot of maximum air temperatures for November (Chart 2) shows the region just north of the Equator having values of approximately 26°C, while air temperatures in the adjacent area is 22°C. As the air above the ITCZ warms, its density tends to decrease, promoting the development of lower sea level pressure. As the chart of sea level pressure (Chart 3) shows, the region of the ITCZ has a pressure of 1010mb which is moderately lower than the region immediately to the north and south.

NCEP/NCAR Reanalysis Precipitation Rate (mm) For November. Climatology 1968 - 1996 (click here for full chart)

This area of low pressure, characterized by warm and moist rising columns of air, comprises the rising branch of the Hadley cell and is a breeding ground for thunderstorms. The precipitation rate (millimeters per day) presented on the chart to the left (Chart 4), is an additional variable that can be used to identify the location of the ITCZ, which at this point is just north of the Equator. The maximum values, particularly around the western end of Caribbean, suggest it is likely that the Volvo Race participants will have a wet ride through the doldrums.

After successfully negotiating the ITCZ, the Volvo competitors must contend with the southeasterly trade winds on their way to Cape Town, South Africa. The southeasterly trades emanate from the subtropical high located just to the west of Africa. As shown on the chart on the lower right (Chart 6), for the next several hundred miles the wind is likely to be blowing directly from their intended course--known as “on the nose” in sailing circles--at a speed of 8 to 9 meters per second (15 to 18 knots). Each boat will have to modify their course to strike a fragile compromise between boat speed and progress towards Cape Town. A tactical error here could quickly erase any gains achieved by a speedy transit of the ITCZ.

 

NCEP/NCAR Reanalysis Sea Level Pressure (mb) For November Climatology 1968 - 1996

(click here for full chart)

 

NCEP/NCAR Reanalysis Vector Surface Wind (m/s) For November Climatology 1968 - 1996

(click here for full chart)

ortunately, there are a number of methods available to place the location of the subtropical high and determine the extent of its influence.

The most obvious locator of the subtropical high is sea level pressure. The chart (Chart 5) of mean sea level pressure indicates the subtropical high is located just to the west of Africa's tip. In addition to illustrating the southeasterly trade winds, the counter-clockwise circulation of surface winds around the high is clearly evident on the chart of surface wind (Chart 6). If the wind is consistent with long-term climatology, the Volvo participants are likely to take a more southerly track before turning towards Cape Town.  

The subtropical high marks the region of the descending branch of the Southern Hemisphere Hadley cell. The atmospheric dynamics at work on the high's western side promotes an increase in the overall height of the air column. The increase in the column's height contributes to upward motion which results in increased instability.  

 

NCEP/NCAR Reanalysis Sea Level Pressure (mb) For November Climatology 1968 - 1996 (click here for full chart)

 

NCEP/NCAR Reanalysis Precipitation Rate (mm) For November Climatology 1968 - 1996 (click here for full chart)

Since instability in an air column is related to precipitation, the western area of a subtropical high generally experiences a higher rate of rainfall. The chart on the upper right (Chart 7) shows the areas of higher precipitation rate in red, which nearly outlines the western boundary of the high. In contrast, the eastern side has a much lower precipitation rate which is consistent with the more stable air columns located in this area.

As the races depart Cape Town on leg two on January 2, 2006, they will head east and spend the next two months battling the vicious and unforgiving Southern Ocean. The only stops on the way to Rio de Janeiro will be brief respites in Melbourne, Australia and Wellington, New Zealand. The screaming winds and monstrous seas have earned this expanse of the world the nicknames Roaring Forties, Furious Fifties and Screaming Sixties. As the names suggest, this is no place for the faint-hearted or those prone to seasickness.

The chart below (Chart 8) shows that mean surface wind speed for January and February in the area of 40° to 60° south latitude is generally brisk from a westerly direction with vast regions of 10 meters per second (20 knots). The storms in this region are legendary and frequently possess winds far in excess of the long term mean show here. Largely unimpeded by land masses, the wind driven waves build to towering heights. The competitors and their boats will be pushed to the breaking point.  

NCEP/NCAR Reanalysis-Vector Surface Winds (m/s) For January/February-Climatology 1968 - 1996 (click here for full chart)

The chart below (Chart 9) shows that the high southern latitudes are characterized by an extreme sea level pressure gradient. The pressure drops approximately 27mb from the latitude 35° S (the location of the subtropical high) to 65° S. The change in sea level pressure from 50° S to 60° S is particularly steep. It is this pressure gradient that promotes the fierce wind so common in this area. 

NCEP/NCAR Reanalysis-Sea Level Pressure (mb) For January/February-Climatology 1968 - 1996-(click here for full chart)

During January and February, the sailors will share the Southern Ocean with the Subtropical Jet Stream (STJ). The mean wind speed at 200mb (Chart 10) is an excellent way to identify the location of this feature, which at the time of this chart is the band of higher wind speed values from 30° to 55° south latitude. As the chart shows, wind speeds are in the neighborhood of 37 meters per second (72 knots).  

NCEP/NCAR Reanalysis-200mb Vector Winds For January/February-Climatology 1968 - 1996

The STJ is a semi-permanent feature in which the wind speed fluctuates tremendously by season, with the strongest wind occurring during the winter months. Although partially obscured by the chart's label, note the wind speed in the Northern Hemisphere STJ at approximately 35°N latitude and 140°E longitude. At the height of northern hemisphere winter, the wind in this area is approximately 70 meters per second (136 knots). The existence of the STJ is attributed to the atmospheric dynamics associated with air parcels circulating in the upper branch of the Hadley cells from the Equator toward the subtropical highs. Since it is the height of meteorological summer in the summer hemisphere, the Volvo participants will be less affected by the STJ than if they were present during the winter.

The weary crews will have only two weeks in Rio de Janeiro to recuperate from the Southern Ocean before beginning the last leg in the Southern Hemisphere on April 2, 2006. Nearly four months will have elapsed since the uphill climb against the southeasterly trades and the climate in the region will have changed while they were away. 

 

NCEP/NCAR Reanalysis Vector Surface Wind (m/s) For April. Climatology 1968 - 1996 (click here for full chart)

 

NCEP/NCAR Reanalysis Vector Surface Wind (m/s) For November Climatology 1968 - 1996 (click here for full chart)

By April, the ITCZ has shifted south (Chart 11) near the Equator in response to the heating associated with the Southern Hemisphere summer. Between November and April, the surface wind has diminished from approximately 9 meters per second (17.5 knots) to 7.5 meters per second (14.6 knots). The reliable southeasterly trades persist, however, and should be more than adequate to carry the participants back toward the northern hemisphere. 

Not long after departing Rio de Janeiro, the crews will be required to once again chart a course through the ITCZ. There is one additional variable, outgoing long-wave radiation (OLR), that can be used to help place the location of the ITCZ. OLR is a measure of the amount of infrared radiation that a region emits back to space. Warm surfaces emit more radiation than do cold objects, so a weather satellite with an infrared sensing instrument is able to discern which regions are warm and which are cold. Regions with low OLR values therefore either represent areas where the surface is cold, such as the Antarctica, or areas where cold cloud tops exist. Warm areas, such as northern Africa are characterized by relatively high OLR values. The ITCZ is generally characterized by a band of towering cumulus clouds and precipitation, resulting in somewhat lower OLR values. The ITCZ is identified on the above chart (Chart 13) of mean OLR values for April. 

NCEP/NCAR Reanalysis-Outgoing Long-Wave Radiation For April-Climatology 1968 - 1996 (full image)

We'll trust that the Volvo Ocean Race competitors will receive sound weather advice and will successfully return to the northern hemisphere just as Spring arrives. As did the mariners during the Great Age of Sail, they may choose to challenge the extreme conditions of the Southern Hemisphere but at least they do so with far more meteorological data than was contained in Maury's Sailing Directions. A lot has changed in 150 years!

Bibliography

  •  Alexander, Caroline. The Bounty: The True Story of the Mutiny on the Bounty. London: Viking, 2003.
  • Hearn, Chester G. Tracks in the Sea: Matthew Fontaine Maury and the Mapping of the Oceans. New York: International Marine, 2002.
  • Sobel, Dava and William H. Andrewes. Longitude. New York: Walker and Company, 1995.