L. 13 Global Circulation

The line between where Polar cells and Ferrel cells begin and end are at a fixed point on the Earth's surface.

False

Local scale winds averaged over the long term are how _____________ scale winds are calculated.

Global

The highest daily amounts of incoming energy occur at:

high latitudes in summer.

A. _1_ cell consists of rising air at the _2_ and sinking air in the _3_.

1) Hadley 2) Equator 3) Subtropical High

Most of the world's deserts are found at what latitude(s)?

30 North and South

In terms of the three-cell general circulation model, the driest regions of the earth should be near:

30 degree latitude and the polar regions

Subtropical high

A belt of high pressure with descending dry air. Deserts located at subtropical high-pressure areas, characterized by both hot and dry air.

According to the three-cell general circulation model, at the equator we would not expect to find:

A ridge of high pressure

Westerlies

Air moving toward the poles deflects toward the east (because of Earth's rotation), resulting in a westerly air flow. This air pattern is called, logically, the westerlies. This is the reason why wind in Michigan (and generally regions between Mexico and Canada) frequently flows from the west.

A jet streak is a place where ______________________ is (are) often found.

All the above; clear air turbulence strong vertical wind speed shear very strong winds

Jet Stream

As mentioned previously in this lesson, winds over the midlatitudes in both hemispheres generally blow from west to east. Rapid changes in pressure in a relatively narrow region set up a steep pressure gradient that intensifies the wind speed and creates the jet stream. Jet streams are usually found in the tropopause of the atmosphere (approximately 10 to 15 kilometers above the surface). West --> East in both hemispheres Jetstreams --> Tropopause Influences daily weather pattens. The movement of the jet stream strongly influences daily weather patterns. For example, when the polar jet stream dips farther south (in the Northern Hemisphere), it brings cold air to that region. Additionally, we will see in a future lesson that the movement of the jet stream is an important component of midlatitude cyclonic storms.

Doldrums

As trade winds pass over the warm oceans while converging at the equator, they pick up moisture, which results in increased precipitation in the region. At the Equator we find that winds are fairly calm due to the even pressure gradient and the rising air. These areas with calm winds are known as the doldrums. The rising warm area creates large cumulus clouds and thunderstorms. CALM WINDS AT THE EQUATOR

The 'real world' pressure pattern in the Southern Hemisphere is more homogeneous than the Northern Hemisphere because the Northern Hemisphere is covered mostly by oceans.

False

The One-cell Model describes a large circulation cell, called a Hadley cell, in which air rises over the poles, travels towards the equator aloft, subsides at the equator, and then returns to the poles at lower altitudes.

False

Single-cell model

Assuming that... -The earth's surface is covered uniformly with water. -The sun is always directly above the Equator. -The earth does not rotate. Temp and wind removed from the equation. When we have a situation using the assumptions just described, atmospheric circulation would consist of one very large convection cell in each hemisphere.

In the general circulation of the atmosphere, one would find the region called the doldrums:

At the equator

Global Scale Circulation of winds is also known as:

Atmospheric General Circulation

The Southern Oscillation deals with:

Barometric pressure differences

As the air descends at 30 degrees in the Hadley cell, the air is very __________.

Dry

The name given to the current of warm water that replaces cold surface water along the coast of Peru and Ecuador during December is:

El Nino

The El Niño Southern Oscillation can wreak havoc on world weather systems. This is all because of the upwelling warm or cold water off the coast of tropical Peru and Ecuador. Which involves warm water and which involves cold water?

El Niño involves warm water, while La Niña involves cold water.

When we divide the world up in to latitudinal 'zones,' which of these is not a zone?

Equatorial

In the one-cell model, we are creating a ____________ cell.

Hadley

The large thermally-driven convection cell that is driven by convective 'hot' towers along the equator is the _________________________.

Hadley Cell

Highs and Lows Bullshit

In January, land cools more than oceans and temperatures are therefore higher over the ocean. Low pressure centers are found over oceans and high pressure centers over continents in the Northern Hemisphere, and vice versa in the Southern Hemisphere. You can also see the lows in tropical regions along the Intertropical Convergence Zone (ITCZ), and highs in the subtropical regions. In the July map, you can see that global pressure patterns are reversed in summer, with generally low pressure over land and high over oceans. Additionally, the Pacific High and Bermuda High affecting the United States have strengthened, while there is the development of a thermal low over the desert Southwest. Notice that over the Southern Hemisphere, the pressure pattern is more homogeneous. This is because the Southern Hemisphere is covered mostly by oceans. Also, note the actual observed pressure patterns here are consistent with the Three-cell model, with a line of high pressure over the subtropical region near 30° latitude and lows near 60° latitude. The observed global mean wind patterns shown on these figures are also consistent with the Three-cell model.

Three-cell model

In contrast to the Single-cell Model, the earth is rotating in the Three-cell Model. The rotation of the earth on its axis breaks the single cell into three cells. Now, three cells (Hadley cell, Ferrel cell, and Polar cell) instead of the one are redistributing energy in the atmosphere. Hadley Cell (Thermal Direct) In the Hadley cell, air rises near the Equator and then descends near 30° latitude, where a belt of high pressure is found (a subtropical high, or anticyclone). The air in this region warms as it descends, which produces clear skies and warm temperatures. The descending air of the subtropical high is dry, because moisture is lost as precipitation over the equator before reaching these locations. Most major deserts on the earth's surface are located in these subtropical high-pressure areas, characterized by both hot and dry air. DESERTS The specific characteristics of each area in the Three-Cell Model also result in precipitation patterns in each area. The graphic below is a cross-section of earth in the Three-Cell Model (notice the latitudes at the top of the graphic), along with associated precipitation and weather patterns. Key: 1. precipitation in all seasons (A climates) 2. summer precipitation, winter dryness (some C climates) 3. dry in all seasons (B climates) 4. winter rain, summer dryness (some C climates) 5. precipitation in all seasons, but more in the summer (some C and D climates) 6. sparse precipitation in all seasons; cold (E climates)

Polar Front

In meteorology, the polar front is the boundary between the polar cell and the Ferrel cell around the 60° latitude in each hemisphere. At this boundary a sharp gradient in temperature occurs between these two air masses, each at very different temperatures.

Ferrel Cell

In the Ferrel cell, which is between the Hadley and the Polar cells (sometimes called a midlatitude cell as it is between approximately 30° and 60° latitude), air is carried towards the poles near the surface of the earth, and towards the equator aloft in the atmosphere.

Polar Cell

In the Polar cell, cold air moving from the poles rises when it reaches the polar front near 60° latitude. The polar front is the area where warm, moist air from the midlatitudes meets cold, dry air from the poles. The rising air aloft returns to the north where it sinks back to the poles. Behind the polar front, the Coriolis force deflects cold air from the poles, so air flows from the northeast. These winds are referred to as polar easterlies.

El Nino and La Nina

In tropical latitudes. El Niño and La Niña are opposing parts of the natural climatic cycle called the El Niño-Southern Oscillation (ENSO) cycle, which occurs in the Pacific Ocean. During El Niño years, a pool of very warm surface water moves from the western side of the tropical Pacific (off the coast of Australia and the Indonesian Archipelago) to the east (just off the coast of Ecuador and Peru). During La Niña years, a pool of very cool water shifts from the eastern tropical Pacific to the west. El Niño and La Niña represent nearly opposing sides of the ENSO climatic cycle. Therefore, sometimes people refer to El Niño as the warm phase of the ENSO cycle and La Niña as the cool phase. Southern Oscillation refers to the changes in barometric pressure on opposing sides of the tropical Pacific that accompany El Niño and La Niña events. The effects (altered rainfall and wind) of these shifts in oceanic and atmospheric circulations are felt not only in the tropical Pacific, but around the world. According to the International Research Institute for Climate Prediction, the ENSO cycle is only second to the seasons in driving weather patterns at the global scale! _________________________________________ To understand how El Niño affects atmospheric and oceanic circulation, we must first understand what conditions are like during a typical, non-ENSO year. Trade winds dominate most of the time in the tropical Pacific region. As you learned earlier in this lesson, they blow FROM the east TO the west. In the tropics, these winds flow from STHs (subtropical highs) over western South America to a low-pressure system over northern Australia. As this air flows westward in the lower atmosphere over the tropics, it drags warm surface water with it. The water warms even more as it travels across the Pacific. Because the winds are carrying warm surface water into the region, the surface of the ocean in the western tropical Pacific becomes raised (as much as 60 centimeters or 2 feet). During non-ENSO years, sea surface temperatures are generally 6 to 8° Celsius (about 10 to 15° Fahrenheit) warmer in the western tropical Pacific than in the east. As the ocean's surface is rising with warm water in the western tropics, land breezes are blowing offshore from the South American coast, and a deep upwelling ocean current (called the Humboldt or Peru current) flows north towards the equator and then turns toward the west. This upwelling current carries in cold, nutrient-rich water from deep in the eastern tropical Pacific, essentially replacing the surface water being dragged westward by the trade winds. This massive conveyor belt of ocean water, transporting warm water to the west and bringing up cool water from deep in the east, continuously recycles water in the tropical Pacific during non-ENSO years. Regarding precipitation, the cool upwelling water off the South American coast tends to cool the air above it. This makes the air too dense to rise and produce precipitation. In the western Pacific, however, the warm surface waters warm the air above them, causing the lower atmosphere there to become destabilized--increasing the chance for precipitation. As a result, an eastern tropical Pacific location may receive 90 centimeters (35 inches) of rain in a normal year, whereas a western location may receive 250 centimeters (98 inches) of rain in a normal year. EL NINO During El Niño years, the trade winds weaken and sometimes reverse. This weakening or reversal of the trade winds prevents warm surface water from building up in the western Pacific. Instead, this warm surface water migrates east towards the South American coast (around Ecuador and Peru). This eastward journey takes a few months, and it results in a lowered ocean surface in the west and an elevated and warm ocean surface in the east. High pressure develops over northern Australia and a low-pressure trough forms over the eastern tropical Pacific near Peru. Ultimately, the warm surface waters overtake the normally dominant deep upwelling ocean current off the South American coastline. This pushes the thermocline (the transition between surface and deep water) deeper than usual in the east, and surface water temperatures here can rise up to 8° Celsius (46° Fahrenheit). Alternatively, the thermocline becomes shallower in the west as the surface water cools. Cool, nutrient-rich water is no longer able to upwell along the coast during an El Niño year. This change has a significant impact on the coastal environment of Peru and Ecuador. For example, because the cool, nutrient-rich waters are no longer near the surface, fish are no longer near the surface. This obviously would hurt the fishing industry in these areas during an El Niño year, as your textbook describes. In summary, during El Niño years, precipitation is abundant over the central Pacific and drought conditions develop in the western tropical Pacific. LA NINA La Niña conditions are nearly the opposite of El Niño conditions. La Niña occurs when abnormally high pressure occurs over the eastern tropical Pacific, intensifying trade winds. As a result, the cool deep upwelling waters off the coast of South America will dominate the tropical Pacific and migrate further west than in non-ENSO years. During a La Niña event, the sea surface temperature in the eastern tropical Pacific averages between 1 to 4° Celsius (2 to 7° Fahrenheit) lower than normal. If you recall, this cool water upwelled from great depths is rich in nutrients. During La Niña years, precipitation is abundant in the western tropical Pacific, but scarce in other parts of the world. IMPACTS As has been mentioned previously, the changing patterns in ocean circulation in the tropics during El Niño/La Niña years also affect weather patterns around the world. Let us take a look at how it affects weather in the United States. The impacts of El Niño/La Niña on weather in temperate latitudes are most recognizable during winter. During the wintertime of an El Niño year, weather will be generally more wet and cooler over California and the southern United States. El Niño winters are mild over western Canada and parts of the northern United States (such as Michigan). A La Niña winter is characterized by colder weather in the western United States, higher than average precipitation in the Pacific Northwest and Upper Midwest, and warmer/drier weather in the southern United States.

What does ITCZ stand for?

Inter-tropical convergence Zone

Why is a one-model of circulation considered okay even if it assumes such strange circumstances?

It is a model, which is a simplification of the real world

In a Hadley cell, there is a large __________________ at the equator.

Low Pressure

The northern-most and southern-most cells are known as ____________ cells.

Polar Cells

The wind belt observed on the poleward side of the polar front is called the:

Polar Easterlies

El Nino and La Nina deal with:

Sea surface temps

La Nina years see a _______________ in the trade winds, causing more precipitation in the western Pacific.

Strengthening

The semi-permanent pressure systems associated with the polar front are called:

Subpolar lows

Thermal direction cell

The Hadley cell is a thermal direct cell where warm air rises and cold air sinks. By thermal direct, we mean that the cell exists because of surface temperatures and the cell's thermal characteristics override the effects of any other weather phenomena.

Pacific high

The North Pacific High is a semi-permanent, subtropical anticyclone located in the northeastern portion of the Pacific Ocean, located northeast of Hawaii and west of California.

polar jetstream

The momentum the air has as it travels around the earth is conserved, which means that as air over the equator starts moving poleward, it keeps its eastward motion constant. The Earth below the air, however, moves more slowly as that air travels toward the poles. As a result, the air moves faster and faster from west to east the farther it moves from the equator. In addition, as the Three-cell model showed earlier, the regions around 30° North/South and 50-60° North/South are areas where temperature changes are the greatest. As the difference in temperature between the two locations increases, the strength of the wind increases. Therefore, the regions around 30° North/South and 50-60° North/South are also regions where wind in the upper atmosphere is the strongest. These two areas are referred to as the subtropical jet stream (near 30° latitude) and the polar jet stream (approximately 60° latitude).

subtropical jetstream

The momentum the air has as it travels around the earth is conserved, which means that as air over the equator starts moving poleward, it keeps its eastward motion constant. The Earth below the air, however, moves more slowly as that air travels toward the poles. As a result, the air moves faster and faster from west to east the farther it moves from the equator. In addition, as the Three-cell model showed earlier, the regions around 30° North/South and 50-60° North/South are areas where temperature changes are the greatest. As the difference in temperature between the two locations increases, the strength of the wind increases. Therefore, the regions around 30° North/South and 50-60° North/South are also regions where wind in the upper atmosphere is the strongest. These two areas are referred to as the subtropical jet stream (near 30° latitude) and the polar jet stream (approximately 60° latitude).

Polar easterlies

The polar easterlies (also Polar Hadley cells) are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the North and South Poles towards low-pressure areas within the Westerlies at high latitudes.

A Hadley cell is __________________ cell, meaning that surface temperature is the most important thing.

Thermal Direct

Trade Winds

Trade winds are named for their ability to quickly move trading ships across the ocean. They are steady and blow at about 11 to 13 miles per hour. As trade winds pass over the warm oceans while converging at the equator, they pick up moisture, which results in increased precipitation in the region. At the Equator we find that winds are fairly calm due to the even pressure gradient and the rising air. These areas with calm winds are known as the doldrums. The rising warm area creates large cumulus clouds and thunderstorms.

Energy moves from an area of surplus to an area of energy deficit.

True

In an El Nino year, nutrient rich waters off of Peru go away because the water is much warmer.

True

La Nina is the cool phase in terms of sea surface temperatures

True

Most deserts worldwide are found under the subtropical highs.

True

During an El Niño year, fishermen along the western coast of South America may catch fewer fish because:

Warm surface waters prevent the cold and nutrient-rich waters at greater depths from reaching the surface.

El Nino in Michigan typically brings a _________________ climate.

Warmer, drier

Chicago, Illinois (latitude 42° north) is located in the _____________________.

Westerlies

Michigan's dominant wind pattern is a formed by the _________________.

Westerlies

The majority of the United States lies within which wind belt?

Westerlies

Hadley cell

When we have a situation using the assumptions just described, atmospheric circulation would consist of one very large convection cell in each hemisphere. This creates a large circulation cell, called a Hadley cell, in which air rises over the Equator, travels towards the poles aloft, subsides over the poles, and then returns to the Equator at lower altitudes. The Hadley cell is a thermal direct cell where warm air rises and cold air sinks. By thermal direct, we mean that the cell exists because of surface temperatures and the cell's thermal characteristics override the effects of any other weather phenomena.

Atmospheric General Circulation

Winds at a global scale. ~~~Just as climate is the composite of weather patterns over a long period of time, so is general circulation an average of local wind patterns. ** The underlying cause for global circulation is the uneven heating of Earth by the Sun. Tropical latitudes receive more solar energy than polar regions. The atmosphere transports heat from the tropics to the poles, and cold air from the poles to tropics, creating global circulation! The process of transporting warm and cold air within the atmosphere is extremely complex, therefore, we can use models to give a general description of this process. In the next section of this lesson, we will cover two models: 1) the single-cell model of atmospheric circulation, and 2) the three-cell model.

Intertropical Convergence Zone

Winds in the subtropical high diverge at the surface as they descend. As they diverge on the southern side of the system (in the Northern Hemisphere), they flow back to the intertropical convergence zone (ITCZ), forming the northeasterly trade winds. That is, the air flows from a zone of high pressure (the subtropical high), to a zone of low pressure (the intertropical convergence zone), and winds result. In the Southern Hemisphere, this flow is reversed, and air flow diverges on the northern side of the system, forming the southeasterly trade winds.

bermuda high

a semipermanent area of high pressure located over Bermuda in summer and fall that steers many storm systems westward across the Atlantic.

During El Niño years, precipitation is:

abundant over the central Pacific and scarce in the western tropical Pacific.

There is an energy _______ at the poles and an energy ________ at the equator.

deficit; surplus

Atmospheric global circulation transports:

heat from the tropics to the poles and cold air from the poles to tropics.

On a weather map of the Northern Hemisphere, the trade winds would be observed:

south of the subtropical highs.