ATMOSPHERIC CIRCULATION

 

 The surface of the Earth has 4 primary environments: atmosphere, hydrosphere (oceans), lithosphere, and the biosphere. All are interconnected but the atmosphere and the hydrosphere are the two most freely interacting primary environments.

The storage and distribution of captured solar energy (heat) which controls weather, climate, is intimately associated with the oceans and the atmosphere.

Weather is the day to day conditions or state of the atmosphere at a particular location or area of the Earth. Climate is the long-term conditions of the atmosphere in a particular area of the Earth averaged over 10 or more years. Elements of weather and climate include heat and temperature changes, pressure and density changes, and water content (humidity) changes.

 

Composition of the AtmosphereThe atmosphere is a mixture of predominantly gases, plus very small quantities of solids and liquids (particulates). These components are traditionally considered as variable and constant components. Variable and constant components together is what we commonly call air.

The constant components are:NITROGEN - 78%OXYGEN - 21%ARGON - 0.9%

The variable components are:CARBON DIOXIDE, WATER VAPOR, OZONE, METHANE, NITROGEN OXIDES, and PARTICULATES (including dust) - 0.1%.

Water vapor, whose main source is evaporation from the ocean surface, is the most variable among the variable components, and can be present up to 4% in lower part of the atmosphere (clouds). It is the dominant factor in heat transfer at the Earth surface, and hence the most important component of weather and climate.

Structure of the AtmosphereThe atmosphere is the most external layer of the Earth, composed dominantly of gases, and held in place by the pull of gravity.

The atmosphere, in turn, consists of 4 layers that are recognized on the basis of temperature trends:

Troposphere - the layer that is closest to the ocean/land surface within which temperature decreases with height. The troposphere is located between 0 - 10 km height above sea level. Stratosphere - the layer immediately above the troposphere (10 - 45 km above sea level) within which temperature increases with height.· Mesophere - the layer that immediately surrounds the stratosphere within which temperature decreases with height.· Thermosphere - the layer that surrounds all the other layers and forms the boundary between Earth and space. There is no sharp, well-defined boundary because the components of the atmosphere decreases gradually. This boundary is conveniently pegged at 100 km. Temperature in this layer increases with height.

All 4 layers exert an average pressure of 1 atmosphere (or 1,000 millibars) on the land or water surfaces. This pressure is exerted by all components of the atmosphere.

These components are not uniformly distributed. Indeed, 60% of air is located in the troposphere, where all weather activities are confined. 99% of air is within the troposphere and the stratosphere. Hence, most of the air in the atmosphere is concentrated near the water and land surfaces where life is present. Moreover, the stratosphere contains nearly all the ozone in the atmosphere. Ozone protects living organisms from harmful high-energy radiation such as ultraviolet rays.

Atmospheric CirculationAtmospheric circulation is the result of solar energy supply and distribution at the Earth's surface. 99.9% of the energy that heats the land and water surfaces, and puts the air and oceans into motion in the form of winds and currents, is solar energy. Without this heat supply, there would be no air movement, no temperature change, no seasons, no rain or snow. Indeed, there would be no weather!

Of the total incoming solar energy received daily:· 30 % is lost by reflection and scattering,· 50% heats up the land /water surface directly, and· 20% heat up clouds directly.

So one can easily sense that the air is scarcely heated directly by the sun's rays. Because visible light consists mostly of short wave radiation, it passes right through the gas components, except where there are clouds as noted above. This radiation heats up the land/water surface during the day. At night, the heated land and water surfaces begin to radiate their own energy, but as longer wavelengths than solar radiation, known as terrestrial radiation (TR). This weaker radiation is easily absorbed by water vapor and carbon dioxide in the atmosphere. Since the bulk of the air components are in the troposphere, the atmosphere is heated up from the ground up. So cloudy nights tend to be warmer as more TR is absorbed and hence, trapped within the lower atmosphere. This trapped heat is further reradiated (or recycled), although relatively small amounts may be lost in the process. This is known as the greenhouse effect.

 

The 50% daily solar heat received on the water/land surface is not uniform because of the curved surface of the Earth. More heat is received at the equator, which decreases gradually to the least amount of heat received at the poles. This pattern is only retained during the fall and spring seasons. However, there is a yearly surplus of heat between the equator and latitudes 30 degrees north (N) and south (S) at the land/ocean surface.

These locations of high/low heat reception shifts with the seasons: In June, the Earth tilts downward and the maximum heat is received at latitude 23.5 degrees north (Tropic of Cancer). This is called northern summer or southern winter. The reverse occurs in December for summer in the southern hemisphere.

In general, the concentration of solar heat around the equator causes the air above it to be constantly warm. Warm air typically rises, hence the air

pressure at the equator is low. When the air has risen sufficiently high in the troposphere, it cools down and part of it turns northward and the other southward as the travel towards the two poles. This area of constantly rising air is the ITCZ (intertropical convergence zone) because the two poleward moving air eventually return at the land/ocean surface, and converge around the equator. The ITCZ does not always coincide with the geographic equator because it shifts seasonally north or south of the geographic equator. It is therefore considered to be the meteorological or thermal equator.

As the moving air approaches latitude 30 degrees N and S, the poleward traveling air masses are now sufficiently cool (having lost nearly all its moisture by raining) and sufficiently dense to sink down to the land/water surface. This sinking cool, dry air causes high pressure at latitudes 30 deg. N and S. Instead of this sinking air accumulating at one spot, most turn south or north, heading back toward the equator where they had began their journeys. Hence, two air circulating cells are formed that way between the equator and latitudes 30 deg. N and S. These are known as the Hadley cells.

Two other air circulation cells are formed both north and south of the equator. One pair is present between 30 degrees and 60 degrees N and S, and another between 60 deg. and 90 deg. N and S. These cells are known as the Ferrel and Polar cells, respectively. This how the surplus tropical heat, noted above, is distributed over the surface of the Earth.

Prevailing WindsWinds are air movement controlled by pressure differences. The rule-of thumb for wind formation is that "winds blow from high to low pressure." So once there are pressure differences in the atmosphere, there are air flows resulting in winds.

The prevailing winds are the permanent surface winds that blow as a result of the 6 air circulation cells described above.

These winds are expected to blow directly from any permanent high pressure latitude to a low pressure one. However, the direction in which prevailing winds blow is slanted because of the effect of the earth's eastward rotation. This effect is known as the Coriolis effect.

In the case of the Coriolis effect, any object that moves at the Earth's surface is deflected to the right of their travel path in the NH (northern hemisphere), and to the left of their travel path in the SH (southern hemisphere). In general, the Coriolis effect is stronger toward the poles and very weak at the equator.

· Between the equator and 30 deg. N/S, the prevailing wind is the trade winds which tend to blow in a westerly direction.· Between 30deg. N/S and 60deg. N/S, are the westerlies which tend to blow eastward.· Between 60deg. N/S and the poles, are the polar easterlies.

In each hemisphere, prevailing winds are distinguished based on their direction of origin (the direction they come from). Hence, in the NH, the trade winds blow from the northeast and it is called the northeast trade winds, but southeast trade winds in the SH. This pair of winds converge along the ITCZ. In the NH, the westerlies are the south westerlies because they blow from the southwest.

Seasonal Effects and Monsoon WindsThe wind belt patterns described above represent the ideal prevailing wind patterns. But in actuality, there are significant differences between responses of land and ocean to annual solar heating.

The land heats up and cools down faster than the oceans because of their differences in specific heat capacities. These differences are more apparent in

the middle latitudes than around the equator or the poles. In the late spring and summer, the land surfaces tend to be hotter than the oceans which has a high specific heat capacity. Hence, warm air over the heated land surfaces rise upwards causing low pressure zones over the land surface, whereas neighboring ocean surfaces are much cooler and pressure is higher. Begining in the late fall through winter, the reverse process occurs as the land cools faster than the oceans and high pressure develops over the land associated with low pressure over the oceans.

In the NH, because of the extensive areas of continents, the low pressure zones spread and combine across latitude 30 deg. N, breaking this high pressure belt into smaller, discrete high pressure cells located over the North Atlantic and Pacific Oceans. This alters the ideal prevailing wind patterns and displaces the south westerlies and the northeast trades with a clockwise circulation around the discrete high pressure cells. In addition, the ITCZ shifts north of the equator by 5 -10 degrees of latitude dragging the 30 deg. high pressure belt with it adjacent to land masses with low pressure systems. As a result, warm, moist winds blow from the oceans onto the land masses accompanied by heavy, and steady rainfall known as wet monsoon season. Hundreds of millions of people in Western Africa, India, and other Southern Asian countries depend on monsoon rains for drinking and farming.

Based on this heating differences between land and oceans, local, daily onshore and offshore winds develop in coastal areas known as sea breeze and land breezerespectively.

 

El Nino EffectsThe southeast trade winds in the Pacific Ocean typically blows from the high pressure system located just west of Peru/Chile coastline to the low pressure system northeast of Australia. This normal airflow causes warm water to flow toward the western Pacific. Due to still unclear reasons, this normal system breaks down every 4 - 7 years. The Trade wind weakens or reverses direction as the air pressure system breaks down. The accumulated warm surface water begins to flow from west to east. This unusual warm current arrives near the South American coast around Christmas, hence it was called Corriente del Nino, now shortened to El Nino.

EL Nino, which can last from 1- 3 years, has very variable effects:

· disruption of rich, upwelling fisheries off the South American west coast,· number and frequency of coastal storms increase,· flooding in some areas,· droughts in other areas,· mild winters in areas with distinct seasons, and · a decrease in the number of Atlantic Ocean storms (hurricanes) forming.

Normal conditions sometimes return with problems of its own including strong currents, powerful upwelling, chillier temperatures, and stormy conditions. This event is known as La Nina.

Both El Nino and La Nina represents the classic case of how changes in the atmosphere triggers changes in the oceans, leading to additional changes in the atmosphere.

Air Masses, Fronts, and StormsEach of the 6 atmospheric circulation cells noted above contains parcels of air with distinct shape, temperature, pressure and moisture characteristics. These are known as air masses. These air masses are stable and move slowly within or between circulating cells.

Cold air masses are also known as high-pressure cells. Warm air masses are known as low-pressure cells. When these 2 contrasting air masses meet, they form a boundary called a front, which can generate turbulent conditions called storms.

Warm air mass storms are known as tropical cyclones because they come from tropical latitudes. These great, rotating low pressure air masses are known ashurricanes in the Atlantic Ocean, typhoons in the western Pacific Ocean, tropical cyclones in the northern Indian Ocean, and willi-willis in the southern Indian Ocean. These storms produce winds blowing, sometimes up

to at 100 mph (or 160 kmph) or more, and travel east to west (pushed along by the NE Trades, with counterclockwise rotation in the N.H. Extropical cyclones are storms that form between the polar and Ferrel atmospheric cells as cold fronts and/or occluded fronts that rotate counterclockwise in the N.H. These storms travel from west to east in North America, pushed along the by westerlies. They are at least 1,000 km in diameter and can last 2-5 days. The tropical cyclones, such as hurricanes, can move massive quantities of heat from the oceans into the atmosphere that is transported across scores of latitudes and longitudes. Some hurricanes can generate power equivalent to 3 million megawatts-hours per day, exceeding that of a large nuclear bomb.