Video 16: Atmospheric Circulation

layers of atmosphere

troposphere, the stratosphere, and the ozone layer, and the upper atmosphere Troposphere contains 75% of gases in the atmosphere

what happens when hurricanes move into the middle latitude

they are infleunced by the prevailing westerlies which cause them to move from west to east

Hadley Cells

(0 to 30N and S)

Ferrel Cells

(30 to 60N and S)

Polar Cells

(60 to 90N and S)

4 main currents that comprise of subtropical gyres

-equatorial currents -western boundary currents -northern or southern boundary currents -eastern currents -equatorial countercurrents

what factors complicate the idealized circulation cell model

1. The tilt of Earth's rotation axis, which produces seasons SEASONAL CHANGES (tilt of earth) During winter, therefore, the continents usually develop atmospheric high-pressure cells from the weight of cold air centered over them and, during the summer, they usually develop low-pressure cells Seasonal shifts in atmospheric pressure over Asia cause monsoon winds, which have dramatic effects on indian ocean currents 2. DISTRIBUTIONS OF CONTINENTS AND OCEAN The lower heat capacity of continental rock compared to seawater, which makes the air over continents colder in winter and warmer in summer than the air over adjacent oceans The uneven distribution of land and ocean over Earth's surface, which particularly affects patterns in the Northern Hemisphere In the winter, the continents cool down much more than the oceans do → develop high pressure zones around them In the summer, the continents heat up much more than the oceans and develop low pressure zones over them

Equatorial countercurrents:

A large volume of water is driven westward by the north and south equatorial currents and piles up water on the western side of an ocean basin near the equator, creating higher sea level there. As a result, this bulge of water flows downhill toward the east under the influence of gravity. This current, called the equatorial countercurrent, is a narrow, easterly flow of water that occurs counter to and between the adjoining equatorial currents.

POLAR EASTERLIES

Air moves away from the high pressure at the poles, too, producing the polar easterly wind belts. The Coriolis effect is maximized at high latitudes, so these winds are deflected strongly. The polar easterlies blow from the northeast in the Northern Hemisphere, and from the southeast in the Southern Hemisphere. When the polar easterlies come into contact with the prevailing westerlies near the subpolar low pressure belts (at 60 degrees north and south latitude), the warmer, less dense air of the prevailing westerlies rises above the colder, more dense air of the polar easterlies.

Physical properties of atmosphere

At HIGHER TEMPERATURES, for example, air molecules move more quickly, take up more space, and DENSITY IS DECREASED. Thus, the general relationship between density and temperature is as follows: Warm air, less dense (Rises) heat rises high heat = low pressure A column of warm, less dense air causes low pressure at the surface, which will lead to rising air (movement away from the surface and expansion) Cool air, more dense (sinks) cool air = high pressure When cool dense air sinks at the surface it will create a high pressure on the surface

Land breeze

At night, the land sur- face cools about five times more rapidly than the ocean and cools the air around it. This cool, high-density air sinks, creating a high-pressure region that causes the wind to blow from the land. This is known as a land breeze, and it is most promi- nent in the late evening and early morning hours.

subtropical region climate

Belts of high pressure are centered there, so the dry, descending air produces little precipitation and a high rate of evaporation, resulting in the highest surface salinities in the open ocean

Northern or Southern Boundary Currents:

Between 30 and 60 degrees latitude, the prevailing westerlies blow from the northwest in the Southern Hemisphere and from the southwest in the Northern Hemisphere. These winds direct ocean surface water in an easterly direction across an ocean basin

what causes the coriolis effect?

Difference in the speed of earth's rotation at different latitudes causes the coriolis effect Maximum coriolis effect at poles: no coriolis effect at equator Magnitude of the coriolis effect depends much more on the length of time the object is in motion

hadley cell

Equatorial regions: warm, lots of rain, doldrums b/c air moves up - usually little horizontal wind Tropical regions: warm, less rain, trade winds (ext. N and S to T of Cancer and T of Capricorn, respectively) Subtropical regions - warm, high rate of evaporation, weak winds

why do hurricanes lose energy over land

Hurricanes die off over land because hurricanes take their energy from converging winds over humid air rising from the warm water surface and are therefore reliant on water vapor. Upon changing from water vapor to liquid water (rain), the latent heat of vaporization (remember, 540 calories for every one gram of water) is released into the air, "feeding" the hurricane. So, hurricanes "feed" off the latent heat released into the atmosphere, and therefore the storm, that is associated with the phase changes of atmospheric water vapor when it condenses. Over land, there is not the source of the constant water evaporating and therefore then condensing and releasing this energy to "feed" the storm. A hurricane requires at least 100-200 feet of very warm water to maintain its energy.

coastal upwelling

If the winds are from the north (Figure 7.12a), Ekman transport moves the coastal water to the right of the wind direction, causing the water to flow away from the shoreline. Water rises from below to replace the water moving away from shore in a process called coastal upwelling . Areas where coastal upwelling occurs, such as the West Coast of the United States, are characterized by high concentrations of nutrients, resulting in high productivity and rich marine life.

Coastal Downwelling

If the winds are from the south, Figure 7.12b shows that Ekman transport still moves the coastal water to the right of the wind direction but, in this case, the water flows toward the shoreline. The water stacks up along the shoreline and has nowhere to go but down, in a process called coastal down- welling . Areas where coastal downwelling occurs have low productivity and a lack of marine life. If the winds reverse, areas that are typically associated with coastal downwelling can experience upwelling.

Trade winds

NE Trade Winds - In the Northern Hemisphere, however, the northeast trade winds curve to the right due to the Coriolis effect and blow from northeast to southwest SE Trade Winds- the southeast trade winds curve to the left due to the Coriolis effect and blow from southeast to northwest.

Conditions needed to create hurricane

Ocean water with a temperature greater than 25°C (77°F), which provides an abundance of water vapor to the atmosphere through evaporation. Warm, moist air, which supplies vast amounts of latent heat as the water vapor in the air condenses and fuels the storm. The Coriolis effect, which causes the hurricane to spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Tropical Region climate

Regions extend north or south of the equatorial region up to the Tropic of Cancer and the Tropic of Capricorn, respectively. They are characterized by strong trade winds, which blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. These winds push the equatorial currents and create moderately rough seas. Relatively little precipitation falls at higher latitudes within tropical regions, but precipitation increases toward the equator. Once tropical cyclones form, they gain energy here as large quantities of heat are transferred from the ocean to the atmosphere.

origin of hurricane

Remarkably, what powers tropical storms is the release of vast amounts of latent heat of condensation that is carried within water vapor and is released as water condenses to form clouds in a hurricane. A tropical cyclone begins as a low-pressure cell that breaks away from the equatorial low-pressure belt and grows as it picks up heat energy in the following manner. Surface winds feed moisture (in the form of water vapor) into the storm. When water evaporates, it stores tremendous amounts of heat in the form of latent heat of evaporation. When water vapor condenses into a liquid (in this case, clouds and rain), it releases this stored heat—latent heat of condensation—into the surrounding atmosphere, which causes the atmosphere to warm and the air to rise. This rising air causes surface pressure to decrease, drawing additional warm moist surface air into the storm. This air, as it rises and cools, condenses into clouds and releases even more latent heat, further powering the storm and continuously repeating itself as a feedback loop, each time intensifying the storm.

sea breeze

Rising air creates a low-pressure region over the land, pulling the cooler air over the ocean toward land, creating what is known as a sea breeze.

Polar highs:

Similarly, descending air at the poles creates high-pressure regions called the polar highs. These areas have descending air → descending air is quite dry and warm so these areas typically experience dry, clear, fair conditions Not necessarily warm, just dry and clear skies

Prevailing Westerlies

Some of the air that descends in the subtropical regions moves along Earth's surface to higher latitudes as the prevailing westerly wind belts. Because of the Coriolis effect, the prevailing westerlies blow from southwest to northeast in the Northern Hemisphere (move to the right) and from northwest to southeast (move to the left) in the Southern Hemisphere.

polar cell

Subpolar regions- cool, winter sea ice, lots of snow, lots of precipitation Polar regions - cold, sea ice, polar high pressure, no precipitation or snow

polar region climate

Surface temperatures remain at or near freezing in the polar regions, which are covered with ice throughout most of the year. The polar high pressure dominates the area, which includes the Arctic Ocean and the ocean adjacent to Antarctica. There is no sunlight during the winter and constant daylight during the summer.

polar front.

The boundary between the prevailing westerlies and the polar easterlies at 60 degrees north or south latitude This is a battleground for different air masses, so cloudy conditions and lots of precipitation are common here.

horse latitudes.

The boundary between the trade winds and the prevailing westerlies (centered at 30 degrees north or south latitude) Sinking air in these regions causes high atmospheric pressure (associated with the subtropical high pressure) and results in clear, dry, and fair conditions. Because the air is sinking, the horse latitudes are known for surface winds that are light and variable.

subtropical highs

The descending air at about 30 degrees north and south latitude creates high- pressure zones called the subtropical highs

idealized circulation model

The greater heating of the atmosphere over the equator causes the air to expand, to decrease in density, and to rise. As air rises it cools by expansion because pressure is lower; water vapor then condenses and rains over the equator. The resulting dry air mass travels north or south of the equator. Around 30 degrees north and south latitude, the air cools off enough to become denser than the surrounding air, so it begins to descend, completing the loop (Figure 6.12). These circulation cells are called Hadley cells. In addition to Hadley cells, each hemisphere has a Ferrel cell between 30 and 60 degrees latitude and a polar cell between 60 and 90 degrees latitude. All the winds are going from high to low pressure, and being deflected to the right from the perspective object (northern hemisphere)

how are subtropical gyres put in motion

The gyre circulation is wind-driven, the surface water will move in the same direction as the wind that blows on top of it. The trade wind blows toward the equator (deflected to the right in the Northern hemisphere and to the left in Southern Hemisphere in its own perspective due to Coriolis effect). The mid-latitude westerlies wind blows toward the poles (also deflected to the right in the Northern hemisphere and to the left in Southern Hemisphere in its own perspective due to Coriolis effect). These deflections give the Northern hemisphere and the Southern hemisphere each a gyre circulation bounded by continents (flowing clockwise in the Northern hemisphere and counterclockwise in the Southern hemisphere due to the Coriolis effect of the wind that drives it)

subtropical gyre definition

The large, circular-moving loops of water that are driven by the major wind belts of the world are called gyres

equatorial current

The trade winds, which blow from the southeast in the Southern Hemisphere and from the northeast in the Northern Hemisphere, set in motion the water masses between the tropics. The resulting currents are called equatorial currents, which travel westward along the equator and form the equatorial boundary current of subtropical gyres (Figure 7.5). They are called north equatorial currents or south equatorial currents, depending on their position relative to the equator.

equatorial low + subpolar low

Weather conditions: cloudy condition with lots of precipitation, because rising air cools and cannot hold its water vapor A column of warm, low-density air rises away from the surface and creates low pressure.

Eastern Boundary Currents

When currents flow back across the ocean basin, the Coriolis effect and continental barriers turn them toward the equator, creating eastern boundary currents of subtropical gyres along the eastern boundary of the ocean basins. They come from high-latitude regions where water temperatures are cool, so they carry cool water to lower latitudes.

western boundary currents

When equatorial currents reach the western portion of an ocean basin, they must turn because they cannot cross land. The Coriolis effect deflects these currents away from the equator as western boundary currents, which comprise the western boundaries of subtropical gyres. They come from equatorial regions, where water temperatures are warm, so they carry warm water to higher latitudes. Note that Figure 7.5 shows warm currents as red arrows.

Temperate Regions climate

are characterized by strong westerly winds (the prevailing westerlies) that blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere Severe storms are common, especially during winter, and precipitation is heavy. In fact, the North Atlantic is noted for fierce storms, which have claimed many ships and numerous lives over the centuries.

In the Northern Hemisphere, the winds associated with a high pressure system move __________

clockwise and outward from the center (spiraling out) moving outward because pressure moves from high to low

In the Southern Hemisphere, the winds associated with a low pressure system move __________.

clockwise and toward the center (spiraling in toward the low pressure zone)

In the Northern Hemisphere, the winds associated with a low pressure system move __________

counter clock wise, and spiraling inwards (spiraling in toward the low pressure zone) because pressure moves from high to low

In the Southern Hemisphere, the winds associated with a high pressure system move __________.

counterclockwise and outwards (spiraling out)

Coriolis effect:

due to earth's rotation to east and the spherical shape of earth, Causes moving objects on earth to follow curved paths In the Northern Hemisphere, an object will follow a path to the right of its intended direction; In the Southern Hemisphere, an object will follow a path to the left of its intended direction.

Which direction do hurricanes move in

east to west

subpolar region climate

experiences extensive precipitation due to the subpolar low. Sea ice covers the subpolar ocean in winter, but it melts away, for the most part, in summer. Icebergs are common, and the surface temperature seldom

in the winter, the continents cool down

high pressure zones

Tropical cyclones/hurricanes/typhoons

huge rotating masses of low pressure characterized by strong winds and torrential rain.

in the summer, the continents heat up

low pressure zones

equatorial region climate

region spans the equator, which gets an abundance of solar radiation. As a result, the major air movement is upward because heated air rises. Surface winds, therefore, are weak and variable, which is why this region is called the doldrums. Surface waters are warm and the air is saturated with water vapor. Daily rain showers are common, which keeps surface salinity relatively low. The equatorial regions just north or south of the equator are also the breeding grounds for tropical cyclones.

Farrel Cell

temperate regions-strong westerlies Temperatures are temperature, not as warm as equator, but not as cold as the poles

Oceanic Heat Flow

you might expect that over time the equatorial zone grows progressively warmer and the polar regions grow progressively cooler. The polar regions are always considerably colder than the equatorial zone, but the temperature difference remains the same because excess heat is transferred from the equatorial zone to the poles. Low latitude regions receive more solar radiation than high-latitude regions, but oceanic and atmospheric circulation transfer heat around the globe How is this accomplished? Circulation in both the oceans and the atmosphere transfers the heat.