Variations in Climate Part II: The Subtropics

    In the last article, I talked about how the intense heating of the sun in the tropics causes not only warm temperatures, but heavy thunderstorms, which is why the tropics are associated with very wet weather.  This week,  I am going to talk about how the circulation in the tropics (i.e. Hadley Cell), causes the weather in the subtropics.
    The region of the the subtropical climates are usually located in between latitudes of 25 and 35 degrees north and south latitude.  Many of the world's largest deserts are located here.  These deserts include the Sahara desert, the Atacama Desert, the American Southwest (technically this is not a true desert and I will explain why later.), and many others.  These deserts usually are the hottest places in the world, mainly because they rarely receive rain or any kind precipitation for that matter.  The strange thing about many of the deserts, is that when they do receive rainfall, which is about once every couple of years, it comes in the form of intense downpours, and usually leads to catastrophic flash flooding.  Not only are the world's driest places located within this region but also some of the world's wettest places.  With this in mind, why is it that some locations within the subtropics are extremely dry and some are extremely wet?  Or more importantly why is it that some of these places are extremely hot, but can also be wet too?  The next several paragraphs should answer these puzzling questions.
     The circulation within the subtropical region is mostly influenced by the Hadley Cell, and the Intertropical Convergence Zone, which means that this region usually sees the most hurricanes of any other region in the world.  Because the Intertropical Convergence Zone shifts with the seasons, many places in the Subtropical region see an extremely wet and dry season, especially in Southern Asia, where the Intertropical Convergence Zone shifts the most.  As we learned in the last article, the Hadley cell begins when the equator is intensely heated by the sun.  Since most areas surrounding the equator, is ocean, the heated air, rises and condenses forming huge, towering thunderstorms, that produce torrential rainfall.  As the rising air nears the tropopause, it spreads laterally toward the poles.  Due to radiational cooling and convergence of the air, the layer of air causes strong areas of sinking air or subsidence, near latitudes of about 25 to 30 degrees north and south latitude.  Since earth's surface is not uniform, these high pressure ridges form not in bands, but in several cells that stretch around the world.  These cells also change position and actually move differently due to the differentiation of earth's surface and the seasons.  The areas that are generally under the influence of these high pressure ridges, are usually where the deserts are.  These strong areas of high pressure cause large areas of subsidence and compression.  When the air mass compresses, it dries the air mass out, causing few clouds and clear conditions.  The compression also causes a warming of the air mass, which partially explains the heat over the deserts.  Clear skies during the day signal intense surface heating from the high angle sun, causing even hotter conditions and it becomes a cycle of becoming hotter and hotter until the high pressure cell retreats or shifts.
     These high pressure cells are usually called semi-permanent, because they can be stationary for very long periods of time, causing intense heat waves over the deserts.  Areas that are near the oceans, such as the east coast of Saudi Arabia and the Southeastern United States, usually have very high dew points, which combined with the incredible heat, make working outside unbearable.  Other areas, such as the west coasts of continents, are usually influenced by cold ocean currents.  These cold currents cool the intensely heated surface, causing dense fog and a stratus deck of clouds.  This limits the amount of daytime heating along the west coasts of continents.  This also causes deserts to form along west coasts that aren't hot at all, but very dry because the stable air mass along with the subtropical highs would cause subsidence, fog, stratus clouds, and no precipitation.  Even the driest desert in the world, the Atacama, has daily fog and drizzle, but very little, if any, rainfall.  There are some deserts that are really not true deserts because even though they are in the subtropical region, the subtropical high is only part of the reason for these deserts.  These deserts are called rainshadow deserts, and are some of the driest in the world.  They form very short distances away from extremely wet areas.  One such place is the American Southwest, where storm systems may occasionally bring heavy rainfall, mainly during the winter and sometimes summer, but most of the time the storm systems that do make it over the mountains, usually produce little precipitation.  This desert in the Southwest forms because as moist air is lifted over a mountain by a strong low pressure system, it causes heavy and intense rainfall for days on the windward side of the mountain.  As it starts to descend the mountain, it is now dry because it used very large portion of its moisture on the windward side of the mountain.  The air descends and warms by compression at the dry adiabatic rate of 10 degrees celsius for every 1000 meters.  Since the air on the windward side was saturated it cools at a lower rate than when it warms on the leeward side of a mountain.  This causes extremely hot temperatures in the leeward valley (such as Death Valley), along with dry conditions.
     The high pressure cells that cause the heat waves and fog, shift from winter into summer.  In the winter, the subtropical highs tend to be along the west coasts of continents, causing the most fog and the least precipitation to be in the winter.  In the summer, the subtropical highs are usually located closer to the east coasts of continents but still sitting close to the centers of the oceans.  When the subtropical highs sit near the east coasts of continents, they cause warm, humid to be pumped onto the continents, usually the precursor to intense heat waves in the middle-latitudes.  Also when the ridges shift north and eastward, the ITCZ also makes a shift northward, which usually means the beginning of hurricane season.  When the high is oriented such that the ITCZ is just to the south of it, then expect several hurricanes to form and traipse eastward.
    Many people wonder why, if there are so many deserts located in these regions, why are there extremely wet areas around the world that are in the subtropical regions.  One reason that there are extremely wet area is because of the ITCZ.  In areas around the world where the ITCZ extends farther northward than in other areas, they have a wet and dry season.  During the winter in their dry season, these locations usually see very hot temperatures, all the way into late spring.  Then as soon as monsoon season hits in late May, these areas see extremely heavy rainfall, sometimes amounting to 200 inches for their average annual precipitation.  When the wet monsoon hits, the average temperatures drop where they would normally rise to even hotter levels.  This can seen in a climate graph of Calcutta, India.  Another reason that some areas in the Subtropics are extremely wet is because when the subtropical highs move toward the east coasts of continents, the high produces an onshore flow of very humid, air which heats intensely from the sun when it moves on land.  This causes the air mass to become extremely unstable, and because the air mass is very moist, it will rise, condense, and produce towering thunderstorms similar to that of the ITCZ.  This effect can be seen in a graph of Orlando, Florida precipitation which usually receives over 55 inches of rain every year.  You can also see from their climate graph, that they have wintertime minimum and summertime maximum precipitation.  Farther north, in Georgia and Alabama, locations around there are wet all year, mainly because they are hit by numerous low pressure systems during the wintertime, while receiving heavy summertime thunderstorms.
       In my next article, we will be focusing on the mid-latitude circulations and climates.  Middle latitude climates are much more complicated mainly because our weather is controlled by the jet stream.  As we all know the jet stream is variable and can change rapidly, causing rapidly changing surface conditions.  Even as our understanding of the atmosphere has increased, we still do not fully understand the processes that control the climates of the world.

Variation in Climate Part One: The Tropics

Usually, when we go on a vacation to a place we have never been before, we find out what the climate is like first before we start packing. Finding out the climate before we go on vacation helps us to know what kind of clothes to pack, whether to bring umbrellas or not, whether to bring tents or tarps, and so much more.    But what exactly IS a climate?  Climate is the average weather of any given location, including how much precipitation it usually has during a given period of time, and what kind of extremes to expect at this location.

As most of you know, the sun controls the weather, therefore it also controls the climate of a location. For example: the farther north you travel (or if you live south of the equator, the farther south you travel) the less the sun angle is. When the sun angle is more acute to the ground (a lesser sun angle), it is usually colder than a place that has a higher sun angle. With this in mind, the higher the latitude, the colder it gets. But why then, is a city such as Eureka, California, that is at the same latitude as New York City, warmer then New York during the winter and colder in the summer, or why is a southern city such as Tampa Bay, Florida warmer then some cities at the equator, which has a higher sun angle than Tampa Bay? In the next few paragraphs, I will attempt to answer these questions as best as I can, seeing as that some of the climate questions are still puzzling meteorologists and climatologists alike.

The first climate we will start with is the Tropics. Usually climatologists think of the Tropics as the climate that controls the rest of the climates around the world. One of the main proponents in a tropical climate is the intensity of the sunlight hitting the earth. The intense sunlight causes intense heating of the surface and warm water temperatures. The intense heat from the surface rises, cools and condenses producing towering thunderstorms that produce heavy rainfall in a band that stretches around the world, called the Intertropical Convergence Zone (ITCZ). Thus, since maximum surface heating occurs where the most intense sunlight radiates the earth, the ITCZ moves with the sun and the seasons. When the heated air rises, expands, and condenses, it causes upper level divergence of the air, and as I stated in my last article, upper level divergence encourages surface convergence which further aids the development of thunderstorms. This is an example of a thermal low, one that is produced by heat and not by the jet stream. These are usually much weaker than storm systems in the middle latitudes. Usually the ITCZ, is located over or just south of the equator during January and is usually located around 23-25 degrees north during July, but extends much farther north over Asia during the summer. The reason for this is because land is heated more intensely than water and since Asia is the biggest continent it absorbs much more sunlight than in the western hemisphere at the same latitude and causes the air mass over this area to be very hot, humid and unstable. Because of this, there is a very large difference in the position of the ITCZ over Asia from the summer to the winter season, and is the main reason for the wet and dry season over southeast Asia, or the monsoonal seasons. Where the Intertropical Convergence Zone is located, is where the most rainfall will occur. (usually over the rainforests of South America, the Caribbean, Africa, and Southeast Asia) Also where most of the rainfall occurs, the summertime temperatures are usually colder then at a location that is located in the subtropical zones or even at a location in the middle latitudes. Not only does the ITCZ affect the weather in the tropics, but it also affects the weather of the subtropics and middle latitudes. When the ITCZ extends in between latitudes of 15 and 25 degrees north, the Coriolis force is now enough to turn the normally southeast winds on the south side of the boundary, when it is at the equator, to southwesterly winds. When the northeast winds on the north side of the boundary converge with the southwesterly winds on the south side of the boundary, they produce cyclonic rotation and vorticity, and when a tropical wave interacts with the area of vorticity, the tropical wave becomes organized and could develop into a hurricane if wind shear and dry air are not involved. The organized tropical wave then travels along the ITCZ until a subtropical high begins to steer the system northward into the United States.

This is a model that simplifies the basic climate variations in the world.

When sailors from the Old World first sailed in this region of the world (to get to the tropics they used the trade winds, the northeast and southeast winds that converge along the ITCZ) they called the area around the ITCZ the doldrums, because of the light winds and violent thunderstorms that were not reliable for wind power. This is because within the tropics, there is very little difference in sea level pressure. Without much wind, sea breezes and land breezes are much more noticeable in the tropics than in the rest of the world.
The circulation of the tropics is  involved with a circulation cell in the atmosphere, which is why I mentioned before that it is the driving force of most of the other climates of the world. This circulation cell is called a Hadley Cell, named after Sir George Hadley, who was using his theory to explain world circulation patterns in 1735, such as the trade winds.  His idea is today used as an idealistic model in tropical atmospheric circulation.
This is the model that is used today: the hot air mass over the tropics rises and condenses producing heavy thunderstorms over the ITCZ.  When this air hits the tropopause it spreads laterally. One: because the stratosphere (the layer above the troposphere) is stable, and because there is so much upward motion in this humid air mass, it spreads laterally toward the poles; and two: the pressure gradient force between the poles and the tropics causes air to move towards the poles. This air then begins to cool due to radiation and it begins to sink at about 30 degrees latitude on both sides of the equator, causing large high pressure cells to dominate the climate of the subtropics, which I will continue to explain in next week’s article.
Next week, I will talk about how the air flow in the tropics influences the climate of the subtropics, which is where the world’s largest deserts are located such as the Sahara Desert and the Atacama Desert in Chile—the driest desert in the world. Back to the weather around here, temperatures will definitely start to warm to a hotter drier pattern over the next several days, as the monsoonal trough develops over the American Southwest deserts. When this trough develops, a large subtropical high will move over the central plains, causing a major and intense heat wave to develop over the Midwest and East. According to long range forecasts, this heat wave could last to Independence Day or longer, before a cold front moves through temporarily ending the heat, only to return again after a few days.  The recent heat and the heat that is forecasted to come has lead me to change my own summer forecast to a very warm one. Other than that, have a great week!