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Chapter 6: Atmospheric and Oceanic Circulations

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Transcript of Chapter 6: Atmospheric and Oceanic Circulations

Chapter 6: Atmospheric and Oceanic Circulations
Geography 1180
Wind Essentials
Winds are driven by the imbalance between equatorial energy surpluses and polar energy deficits
Earth’s atmospheric circulation transfers both energy and mass on a grand scale, determining Earth’s weather patterns and the flow of ocean current
Atmospheric circulation also spreads air pollutants, whether natural or human-caused, worldwide
Atmospheric Circulation
Atmospheric circulation is categorized at three levels:

1. primary circulation, consisting of general worldwide circulation

2. secondary circulation of migratory high-pres-sure and low-pressure systems

3. tertiary circulation, which includes local winds and temporal weather patterns

• Winds that move principally north or south along meridians of longitude are meridional flows
• Winds moving east or west along parallels of latitude are zonal flows
Air Pressure
Air pressure—the weight of the atmosphere described as force per unit area—is key to understanding wind
The molecules that constitute air create air pressure through their motion, size, and number, and this pressure is exerted on all surfaces in contact with air
both pressure and density decrease with altitude in the atmosphere.
• The low density in the upper atmosphere means the molecules are far apart, making collisions between them less frequent and thereby reducing pressure
When air in the atmosphere is heated, molecular activity increases and temperature rises.
Warmer air is less dense, or lighter, than colder air, and exerts less pressure
The amount of water vapour in the air also affects its density

Moist air is lighter because the molecular weight of water is less than that of the molecules making up dry air
As water vapour in the air increases, density decreases, so humid air exerts less pressure than dry air
• The end result over Earth’s surface is that warm, humid air is associated with low pressure and cold, dry air is associated with high pressure
Air Pressure and Measurement
Air pressure, including its measurement and expression, is key to understanding wind
• The molecules that constitute air create air pressure through their motion, size, and number—the factors that determine the temperature and density of the air
• Pressure is exerted on all surfaces in contact with air
Using similar instruments, scientists set a standard of normal sea-level pressure at 1013.2 mb (millibar, which expresses force per square meter of surface area) or 29.92 in. of mercury (Hg).
In Canada and other countries, normal air pressure is expressed as 101.32 kPa (kilopascal; 1 kPa = 10 mb).

Wind: Description and Measurement
Wind is generally the horizontal motion of air across Earth’s surface
• Turbulence adds wind updrafts and downdrafts and a vertical component to this definition
Differences in air pressure (density) between one location and another produce wind
• Wind’s two principal properties are speed and direction, and instruments measure each
Wind Speed measured =

Wind direction =
wind vane
Winds are named for the direction from which they originate
Beaufort Scale
Driving Forces within the Atmosphere
Four forces determine both speed and direction of winds
1. Earth's Gravity

2. Pressure Gradient Force

3. Coriolis Force

4. Friction Force
Earth's Gravity
The pressure that Earth’s gravitational force exerts on the atmosphere is virtually uniform
Gravity equally compresses the atmosphere worldwide, with the density decreasing as altitude increases

The gravitational force counteracts the outward centrifugal force acting on Earth’s spinning surface and atmosphere

Without gravity, there would be no atmospheric pressure—or atmosphere, for that matter

Pressure Gradient Force
Pressure gradient force drives air from areas of higher barometric pressure (more dense air) to areas of lower barometric pressure (less dense air), thereby causing winds
High- and low-pressure areas exist in the atmosphere principally because Earth’s surface is unequally heated
Meteorologists often connect Barometers to barographs and take reading of changing pressure over time
These readings are collated and analyzed
Reading taken at elevation must be corrected
Corrected for comparison to the Sea-level equivalent
This numerical value is then added to the pressure
Horizontal differences in air pressure are seldom extreme
~30 millibars above 1013.25mb or 60 mb below
Canadian North in winter
Land constantly radiates heat (not replaced)
Cold air has tightly packed molecules
Thus high density ( = high pressure)
Gulf of Mexico in winter
Warm air = low barometric pressure
These pressure differences establish a pressure gradient force
An isobar is an isoline (a line along which there is a constant value) plotted on a weather map to connect points of equal pressure
A pattern of isobars on a weather map provides a portrait of the pressure gradient between an area of higher pressure and one of lower pressure
The spacing between isobars indicates the intensity of the pressure difference, or pressure gradient
A steep gradient causes faster air movement from a high-pressure area to a low-pressure area.
Isobars spaced wider apart from one another mark a more gradual pressure gradient, one that creates a slower airflow
Along a horizontal surface, the pressure gradient force alone acts at right angles to the isobars, so wind blows across isobars at right angles
Along a horizontal surface, the pressure gradient force alone acts at right angles to the isobars, so wind blows across isobars at right angles
A field of subsiding, or sinking, air develops in a high-pressure area
Air descends in a high-pressure area and diverges outward at the surface in all directions
In a low-pressure area, as air rises, it pulls air from all directions, converging into the area of lower pressure at the surface
Coriolis Force
On our rotating planet, the Coriolis force deflects anything that flies or flows across Earth’s surface—wind, an airplane, or ocean currents—from a straight path
This force is an effect of Earth’s rotation
Earth’s rotational speed varies with latitude
0 kmph at the poles (surface is at Earth’s axis)
1675 kmph (1041 mph) at the equator
Because Earth rotates eastward, objects appear to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
The Coriolis force is zero along the equator
half the maximum deflection at 30° N and 30° S latitude
reaches maximum deflection at the poles.
The effect of the Coriolis force increases as the speed of the moving object increases; thus, the faster the wind speed, the greater its apparent deflection
The Coriolis force does not normally affect small-scale motions that cover insignificant distance and time
Coriolis Force and Wind
As air rises from the surface through the lowest levels of the atmosphere, it leaves the drag of surface friction behind and increases speed
This increases the Coriolis force, spiraling the winds to the right in the Northern Hemisphere or to the left in the Southern Hemisphere,
generally producing upper-air westerly winds from the subtropics to the poles
In the upper troposphere, the Coriolis force just balances the pressure gradient force
Consequently, the winds between higher-pressure and lower-pressure areas in the upper troposphere flow parallel to the isobars
Such winds are
geostrophic winds
and are characteristic of upper tropospheric circulation
Friction Force
The effect of surface friction extends to a height of about 500 m and varies with surface texture, wind speed, time of day and year, and atmospheric conditions.
In general, rougher surfaces produce more friction
Near the surface, friction disrupts the equilibrium established in geostrophic wind flows between the pressure gradient and Coriolis forces
surface friction decreases wind speed, it reduces the effect of the Coriolis force and causes winds to move across isobars at an angle
Summary of Physical Forces on Winds
Winds are a result of the combination of these physical forces
When the pressure gradient acts alone, winds flow from areas of high pressure to areas of low pressure.
The combined effect of the pressure gradient force and the Coriolis force on air currents in the upper atmosphere, above about 1000 m
they produce winds that do not flow directly from high to low, but that flow around the pressure areas, remaining parallel to the isobars =
geostrophic winds
Near the surface, friction prevents the equilibrium between the pressure radient and Coriolis forces that results in geostrophic wind flows in the upper atmosphere
Because surface friction decreases wind speed, it reduces the effect of the Coriolis force and causes winds to move across isobars at an angle
Atmospheric Patterns of Motion
The warmer, less-dense air along the equator rises, creating low pressure at the surface

The colder, more-dense air at the poles sinks, creating high pressure
If Earth did not rotate, the result would be a simple wind flow from the poles to the equator, a meridional flow caused solely by pressure gradient
On a rotating Earth, the poles-to-equator flow is predominantly zonal (latitudinal), both at the surface and aloft.

In both hemispheres
Winds are westerly (eastward-moving) in the middle and high latitudes
Winds are easterly (westward-moving) in the low latitudes toward the equator
Earth’s global circulation system transfers thermal energy, air, and water masses from equatorial energy surpluses to polar energy deficits, using:
Primary High-Pressure and Low-Pressure Areas
The primary high- and low-pressure areas of Earth’s general circulation appear on maps as cells or uneven belts of similar pressure that stretch across the face of the planet, interrupted by landmasses
Between these areas flow the primary winds
Secondary highs and lows, from a few hundred to a few thousand kilometers in diameter and hundreds to thousands of meters high, form within these primary pressure areas
In each hemisphere, two of the pressure areas are stimulated by
thermal (temperature)
equatorial low-pressure trough (marked by the ITCZ line on the maps)
weak polar high-pressure cells at the North and South poles
In each hemisphere, two of the pressure areas are are formed by dynamic (mechanical) factors
the subtropical high-pressure cells (H)
subpolar low-pressure cells (L)
Equatorial Low-Pressure Trough—ITCZ: Warm and Rainy
Constant high Sun altitude and consistent daylength (12 hours a day, year-round) make large amounts of energy available in this region throughout the
The warming creates lighter, less-dense, ascending air, with surface winds converging along the entire extent of the low-pressure trough.
This converging air is extremely moist and full of latent heat energy

As it rises, the air expands and cools, producing condensation; consequently, rainfall is heavy throughout this zone.
Vertical cloud columns frequently reach the tropopause, in thunderous strength and intensity.

Tropical Rainfall Monitoring Mission (TRMM)
Trade Winds
The winds converging on the equatorial low-pressure trough are known as the trade winds, or trades.
Northeast trade winds blow in the Northern Hemisphere and southeast trade winds in the Southern Hemisphere.

The trade winds pick up large quantities of moisture as they return through the
Hadley circulation cell
for another cycle of uplift and condensation
Hadley Cell Cycle -
Air converges via trade winds at/neer equator = ITCZ
Rises and moves north/south-ward into subtropics
Descends (subtropical highs) to surface and returns to ITCZ - cycles again
Subtropical Highs: Hot and Dry
Equitorial to 20° N/S
20° to 35° N/S
A broad high­pressure zone of hot, dry air brings clear, frequently cloudless skies over the Sahara and the Arabian Deserts and portions of the Indian Ocean
subtropical anticyclones form as air above the subtropics is mechanically pushed downward and heats by compression on its descent to the surface
Warmer air has a greater capacity to absorb water vapour
this descending warm air is relatively dry
the anticyclonic movement means the easten side of the high-pressure cells are drier and more stable (less convection) - and circulate cooler ocean water and influence the west coasts of continents = deserts
The westerlies, which are the dominant winds flowing from the subtropics toward higher latitudes
the westerlies are less consistent than the trade winds, with variability resulting from midlatitude migratory pressure systems and topographic barriers that can change wind direction
Subpolar Lows: Cool and Moist
~35° to ~60° N/S
~60° - 90° N/S
In January, two low-pressure cyclonic cells exist over the oceans around 60° N latitude, near their namesake islands: the North Pacific Aleutian Low and the North Atlantic Icelandic Low
Both cells are dominant in winter and weaken or disappear in summer due to strengthening high­pressure systems
North Pacific Aleutian Low
North Atlantic Icelandic Low
Polar Highs: Frigid and Dry
Polar high­pressure cells are weak.
The polar atmospheric mass is small,
receives little energy from the Sun to put it into motion.
Variable winds, cold and dry, move away from the polar region in an anticyclonic direction

They descend and diverge clockwise in the Northern Hemisphere (counterclockwise in the Southern Hemisphere) and form weak, variable winds of the polar easterlies
Upper Atmospheric Circulation
Circulation in the middle and upper troposphere is an important component of the atmosphere’s general circulation
For surface­-pressure maps, we plot air pressure using the fixed elevation of sea level as a reference datum—a constant height surface
For upper­-atmosphere pressure maps, we use a fixed pressure value of 500 mb as a reference datum and plot its elevation above sea level throughout the map to produce a constant isobaric surface
Similar to surface­pressure maps, closer spacing of the height contours indicates faster winds; wider spacing indicates slower winds
altitude variations in the isobaric surface are ridges for high pressure (with height contours on the map bending poleward) and troughs for low pressure (with height contours on the map bending equatorward)

Upper Atmospheric Circulation
the pattern of ridges and troughs in the upper­air wind flow is important in sustaining surface cyclonic (low­pressure) and anticyclonic (high­pressure) circulation.
Rossby Waves
Within the westerly flow of geostrophic winds are great waving undulations = Rossby Waves
• Rossby waves occur along the polar front, where colder air meets warmer air, and bring tongues of cold air southward, with warmer tropical air moving northward
The development of Rossby waves begins with undulations that then increase in amplitude to form waves
These wave ­and ­eddy formations and upper­-air divergences support cyclonic storm systems at the surface
• Rossby waves develop along the flow axis of a jet stream
Jet Streams
The most prominent movement in the upper-­level westerly geostrophic wind flows are the jet streams
irregular, concentrated bands of wind occurring at several different locations that influence surface weather systems
• The pattern of high­pressure ridges and low­pressure troughs in the meandering jet streams causes variation in jet­stream speeds
• The jet streams normally are 160–480 km wide by 900–2150 m thick, with core speeds that can exceed 300 km·h
Jet streams weaken during the summer and strengthen during its winter as the streams shift closer to the equator
The Asian Monsoon Pattern
The unequal heating bet ween the Asian landmass and the Indian Ocean drives the monsoon of southern and eastern Asia
This process is heavily influenced by the shifting migration of the ITCZ during the year, which brings moisture-laden air northward during the Northern Hemisphere summer
Local Winds
Local winds, which occur on a smaller scale than the global and regional patterns
Three main types:
Land and Sea Breezes
Mountain and Valley Breezes
Gravity Drainage Winds

Land and Sea Breeze
The different heating characteristics of land and water surfaces create these breezes.
Land gains heat energy and warms faster than the water offshore during the day
warm air is less dense, it rises, creating a lower­pressure area that triggers an onshore flow of cooler marine air to replace the rising warm air
the flow is usually strongest in the afternoon, forming a sea breeze.
At night, land cools, by radiating heat energy, faster than offshore waters do
the cooler air over the land subsides (sinks) and flows offshore toward the lower­pressure area over the warmer water, where the air is lifted.
Mountain/Valley Breezes
Mountain and valley breezes are local winds
when mountain air cools rapidly at night and when valley air gains heat energy rapidly during the day
Valley slopes are heated sooner during the day than valley floors.
As the slopes heat up and warm the air above, this warm, less­dense air rises and creates an area of low pressure

At night, heat is lost from the slopes, and the cooler air then subsides downslope in a mountain breeze
Gravity Drainage Winds
a.k.a Katabatic winds, Mistral winds

Cold dense air from higher elevations flows downhill at speed often causing frost damage to crops on the slope or in the valley below
a.k.a Chinook winds

dry, warm downslope winds occurring on the leeward side of mountain ranges such as the Rockies in Alberta and Montana or the Cascades in Washington
These winds are known for their ability to melt snow rapidly by sublimatio
Ocean Currents
The atmospheric and oceanic systems are intimately connected in that the driving force for ocean currents is the frictional drag of the winds
Also important in shaping ocean currents is:
the interplay of the Coriolis force
density differences caused by temperature and salinity
configuration of the continents and ocean floor
astronomical forces that cause tides
Surface Currents
Ocean currents flow over long distances, the Coriolis force deflects them.
their pattern of deflection is not as tightly circular as that of the atmosphere

The oceanic circulation systems are known as
and generally appear to be offset toward the western side of each ocean basin
Equatorial Currents
Trade winds drive the ocean surface waters westward in a concentrated channel along the equator
equatorial currents remain near the equator because of the weakness of the Coriolis force, which diminishes to zero at that latitude
as these surface currents approach the western margins of the oceans, the water actually piles up against the eastern shores of the continents.
This phenomenon is the western intensification.
The piled­up ocean water then goes where it can, spilling northward and southward in strong currents, flowing in tight channels along the eastern shorelines.

Upwelling and Downwelling Flows
Where surface water is swept away from a coast, either by surface divergence (induced by the Coriolis force) or by offshore winds, an upwelling current occurs
This cool water generally is rich in nutrients and rises from great depths to replace the vacating water
In other regions with an accumulation of water such as at the western end of an equatorial current, or in the Labrador Sea, or along the margins of Antarctica—the excess water gravitates downward in a downwelling current

These are the deep currents that flow vertically and along the ocean floor and travel the full extent of the ocean basins, redistributing heat energy and salinity over the globe
Thermohaline Circulation— The Deep Currents
Differences in temperatures and salinity (the amount of salts dissolved in water) produce density differences important to the flow of deep currents
• Travelling at slower speeds than wind­driven surface currents, the thermohaline circulation hauls larger volumes of water
• Ocean surface waters undergo “freshening” in the polar regions because water releases salt when frozen (the salt is essentially squeezed out of the ice structure), and is then salt-free when it melts
Natural Oscillations in Global Circulation
Several system fluctuations that occur in multiyear or shorter periods are important in the global circulation picture
Multiyear oscillations affect temperatures and air pressure patterns and thus affect global winds and climates
El Niño–Southern oscillation
The El Niño–Southern Oscillation (ENSO) in the Pacific Ocean forces the greatest interannual variability of temperature and precipitation on a global scale.
El Niño—ENSO’s Warm Phase
Occasionally, for unexplained reasons, pressure patterns and surface ocean temperatures shift from their usual locations in the Pacific.
Higher pressure than normal develops over the western Pacific, and lower pressure develops over the eastern Pacific.
Trade winds normally moving from east to west weaken and can be reduced or even replaced by an eastward (west­to­east) flow.
The shifting of atmospheric pressure and wind patterns across the Pacific is the Southern Oscillation.

La Niña—ENSO’s Cool Phase
When surface waters in the central and eastern Pacific cool to below normal by 0.4 C° or more, the condition is dubbed La Niña, Spanish for “the girl.”
This condition is weaker and less consistent than El Niño; otherwise, there is no correlation in strength or weakness between the two phases

More Oscillations
Pacific Decadal oscillation
The Pacific Decadal Oscillation (PDO) is a pattern of sea­surface temperatures, air pressure, and winds that shifts between the northern and tropical western Pacific (off the coast of Asia) and the eastern tropical Pacific (along the U.S. West Coast
lasting 20 to 30 years, is longer­lived than the 2­ to 12­year variation in the ENSO
• The PDO negative phase, or
cool phase
, occurs when higher ­than­normal temperatures dominate in the northern and tropical regions of the western Pacific and lower temperatures occur in the eastern tropical region
The PDO positive phase, or
warm phase
, in the PDO ran from 1977 to the 1990s, when lower ­than ­normal temperatures were found in the northern and western Pacific and higher­ than­ normal temperatures dominated the eastern tropical region
North Atlantic and Arctic oscillations
North Atlantic Oscillation (NAO), as pressure differences between the Icelandic Low and the Azores High in the Atlantic alternate from a weak to a strong pressure gradient
The NAO is in its positive phase when a strong pressure gradient is formed by a lower ­than­ normal Icelandic low­pressure system and a higher ­than­ normal Azores high­pressure cell
NAO flips unpredictably between positive and negative phases, sometimes changing from week to week
AO positive, or warm, phase (positive NAO), the pressure gradient is affected by lower pressure than normal over the North Pole region and relatively higher pressures at lower latitudes
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