Lesson 5: Pressure, Winds and Circulation

Conditions during El Nino years and non-ENSO years

wildfires, floods, unseasonable weather, droughts, hurricanes, disease outbreaks.

Differences in warm and cold air

-As air is warmed at the surface, molecules spread out which make it less dense. Low pressure means that there are fewer air molecules in a given unit of space. -Warm air, less dense, rises. -Cold air, denser, means air sinks.

Conditions during typical years

-In the tropical pacific region trade winds dominate and blow FROM THE EAST TO THE WEST -In the tropics, these winds flow from subtropical highs over S. America to a low pressure system over Indonesia and Australia -As the air flows in the lower atmosphere over the tropical Pacific, it drags warm surface water with it. -The surface of the ocean then becomes raised in the WESTERN tropical pacific and are typically warmer in the western tropical pacific than in the east. -Energy and heat stored in western tropical pacific -land breezes blow offshore from the South American coast and a deep upwelling ocean current (Peru current) flows north toward the equator and then turns toward the west -Upwelling brings cold nutrient filled water from the eastern tropical pacific and replaces the warm surface water that is being dragged westward by the trade winds -This massive conveyor belt, 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. -In terms of precipitation, the cool upwelling water off the South American coast tends to cool the air above it, making the air too dense to rise and produce precipitation. -In the western Pacific, the warm surface waters warm the air above them causing the lower atmosphere there to become destabilized, which increases the likelihood of 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.

Low Pressure Systems

-Low pressure systems are referred to as cyclones. -The vertical flow of air consists of warmer, ascending air that converges at the center of a cyclone. -Because air rises in the center of a low, the central part of the system has the lowest pressure. -The horizontal flow around the center of a low in the Northern Hemisphere is COUNTERCLOCKWISE (looking at it from above). -Low pressure centers are generally associated with cloudy or stormy weather. STORMS WARM ASCENDING AIR COUNTER CLOCKWISE IN N. HEMISPHERE LOW PRESSURE IN MIDDLE OF CYCLONE DOMINANT AT THE EQUATOR

Winter Monsoon

-Strong high pressure anticyclone cools the norther part of Asian Landmass AT THE SAME TIME, the INTERTROPICAL CONVERGENCE ZONE is located over the Indian ocean creating an area of low pressure. AS A RESULT, this steep pressure gradient, cold, dry surface winds blow from the interior of the Asian landmass toward the coast.

Thermocline

A layer of water separating the warm, mixed surface waters of the ocean and the cool, deep water. -Shallower during non-ESNO years in the eastern tropical Pacific than in the west where it is pushed deeper by the mounded, warm surface waters.

Monsoonal Winds; winter and summer monsoons; why moonsonal winds occur

An example of the seasonal migration of air pressures. -This shift occurs at a subcontinental scale as prevailing winds change direction over the course of the year in response to the migration of the Intertropical Convergence Zone and the unequal heating of land and water. -The monsoon occurs in many SUBTROPICAL PLACES, but it is most pronounced in SOUTHEAST ASIA.

Polar Front

Around 60° north and south latitude, warm moist air from the westerlies meets cold, dry air from the poles. The boundary between these two contrasting air masses is the polar front.

El nina

Component in a natural climate cycle called the El nino southern oscillation cycle that takes place in the pacific ocean. -A pool of very cool water shifts from the EASTERN tropical Pacific to the WEST -Abnormally high pressure occurs over the eastern tropical pacific, which intensifies trade winds -Cool deep upwelling waters off central america dominate the tropical pacific and migrate more west than normal years -DURING LA NINA YEARS PRECIPITATION IS ABUNDANT IN THE WESTERN TROPICAL PACIFIC, BUT SCARCE IN OTHER PARTS OF THE WORLD. -'Cool phase'

Upwelling and downwelling currents

Downwelling: As water is circulated into the higher latitudes, downwelling occurs due to a decreasing temperature and high salinity. The DENSE WATER SINKS DOWNWARD, DEEP INTO THE OCEAN. Upwelling: As water moves into the tropics, it warms and produced upwelling whereby LESS DENSE WATER RISES FROM THE DEPTHS OF THE OCEAN.

Thermohaline Circulation

The global oceanic circulatory system driven by differences in water density. -Drives vertical deep currents -Water at the surface is warmer and less salty than deeper oceans

Seasonal migrations of pressure systems (locations throughout the year)

The high and low pressures stay the same with the sun, however the earth's tilt creates different pressures for places all over the globe. -The Intertropical Convergence Zone locates itself where there is the most radiation.

El nino

The largest transient oceanic/atmospheric disturbance that takes place in the tropics. -Affects weather across the globe -Easterly Trade winds weaken and sometimes reverse -Warm surface water moves east toward south america -results in lowered ocean surface in the west and an elevated warm ocean surface in the east -High pressure over northern australia -Deepened low-pressure trough forms over the eastern tropical pacific -Warm surface waters build up in the eastern tropical pacific -A pool of warm water also shifts from the western side of the tropic pacific to the east -Ultimately, the deep upwelling ocean current that normally dominates off the South American coastline is overtaken by the warm surface waters, which pushes the thermocline deeper than usual in the east as 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 in the east during an El Niño year. This has a significant impact on the coastal environment of Peru and Ecuador (as well as the climate of eastern Australia and beyond). -'Warm phase' -DURING A TYPICAL EL NINO EVENT, PRECIPITATION IS ABUNDANT OVER THE CENTRAL PACIFIC AND DROUGHT CONDITIONS DEVELOP IN THE WESTERN TROPICAL PACIFIC

Trade Winds

Wind blowing steadily toward the equator from the northeast in the northern hemisphere or southeast in the southern hemisphere. -Pick up a great deal of moisture from warm oceans

Global Circulation Model (dominant winds and pressure systems)

Climate model. (wind and pressures) Primary Pressure Areas -Polar highs -Subpolar lows -STHs -Equator low Primary Winds -Polar easterlies -westerlies -trade winds -westerlies -polar easterlies

Convection

Due to unequal heating. The vertical movement of air (also liquid) due to differences in temperature. (cool air sinks, warm air rises) -Occurs when air is heated

Cyclones

LOWPRESSURE

Ocean Movements

Tidal: -Caused by rise and fall of tides -Gravitational attraction to the moon and sun Surface Wind Currents: -Oscillatory ( back and fourth) motions of the surface water -Surface currents related to wind and pressure systems -N. Hemisphere = circulatory patterns around the subtropical highs Deep Currents: -Movement of streams of water through the ocean -Vertical and horizontal currents are set in motion by surface contrasts in temperature and salinity and by the force of the wind.

Hadley Cell

At low latitudes (from about 0° to 30° north and south latitude), the flow to and from the Intertropical Convergence Zone and subtropical high forms a three-dimensional atmospheric circulation cell

High Pressure Systems (what they are, characteristics)

-High pressure systems are referred to as anticyclones. -The vertical flow of air consists of cooler, descending air at the center of a high that diverges (spreads apart) at the surface. -Because air sinks in the center of a high, the highest pressure is in the central part of the system. -The horizontal air flow around the center of a high is CLOCKWISE in the Northern Hemisphere. -High pressure centers are generally associated with fair weather. FAIR WEATHER COLD DESCENDING AIR CLOCKWISE IN N. HEMISPHERE AIR PRESSURE HIGH IN MIDDLE OF ANTICYCLONE

Cold air drainage (Katabatibc winds)

During COLDER MONTHS, cold air accumulates over higher altitude regions, then flows out as katabatibc winds (STRONG, COLD WINDS) into nearby lower altitude regions under the influence of gravity. Ex: Santa Ana winds in Cali

Variables affecting large-scale winds

UNEQUAL HEATING: The equator gets more heat and insolation than poles resulting in unequal heating across the earth's surface which means that air density, and therefore pressure, differs from place to place creating global wind patterns. The CORIOLIS FORCE: Created by Earth's rotation which causes air to be deflected as Earth moves below. (N. Hemisphere wind is deflected to the right) FRICTIONAL FORCE: Below 1,500 meters wind is slowed by this. Created by the drag that results from features, like mountains, forests, and buildings, that serve to slow wind down. The higher up, the less friction.

Equatorial Low-Pressure Through (Intertropical Convergence Zone) ; Climate Characteristics

Air at the equator is low pressure and warm. When it rises it cools and can't hold moisture so it rains.

Intertropical Convergence Zone

Band of low pressure, characterized by calm winds, cloud cover and high rainfall. LOW PRESSURE DOMINATES THE EQUATOR. Low pressure with a center or low pressure center rises or ascends.

Wind

Because air rises in the center of a low, a void is created that must be filled. This void ultimately becomes filled by air that flows outward from the center of the high and into the low. We feel this exchange of air as wind. HIGH AND LOW PRESSURE SYSTEMS ARE GENERALLY ADJACENT

Frictional Forces

Below 1,500 meters wind is slowed by this. Created by the drag that results from features, like mountains, forests, and buildings, that serve to slow wind down. The higher up, the less friction.

Doldrums

Calm winds due to even pressure gradient and the rising air at the equator.

Coriolis Force

Created by Earth's rotation which causes air to be deflected as Earth moves below. (N. Hemisphere wind is deflected to the right)

Atmospheric Pressure

Differences in atmospheric pressure create wind. -Atmosphere transfers energy

Westerlies

Dominate in the midlatitudes -North side of the subtropical high, air diverges and flows to the northeast in the Northern hemisphere and southeast in the Southern Hemisphere.

Anticyclones

HIGHPRESSURE

Subtropical high pressure system; climate characteristics

High altitude air on the northern side of the system flows northward, while air on the southern side flows to the south. As the air rises and cools it becomes more dense and sinks. -Around 25° to 30° north and south latitude -Creates a zone of SINKING AIR on either side of the equator -Results in high pressure, hot, dry air -AIR FLOWS FROM SUBTROPIC HIGHS TO INTERTROPICAL CONVERGENCE ZONE OF LOW PRESSURE

Local Wind systems; land-sea breezes

Land Sea Breezes: A result of the differential heating and cooling of land and water. -Located along SHORES of MAJOR BODIES of water, such as large lakes and oceans. -During the day, air above LAND IS HOTTER than air above SEA. This results in HIGH PRESSURE OVER THE SEA and LOW PRESSURE ABOVE LAND. This creates SEABREEZE: the air moves from sea toward land, or from high to low pressure. -At night, the air over land cools more rapidly than the air over the sea, causing HIGH PRESSURE ABOVE LAND AND LOW PRESSURE ABOVE SEA, resulting in air that moves from land to sea = LAND BREEZE

Ocean currents and movements

Ocean currents are a second way that heat is transferred around the globe. -Mirrors atmospheric circulation -Driven largely by winds -In the NORTHERN HEMISPHERE, OCEAN CURRENTS ARE GENERALLY CLOCKWISE DIRECTION -In the SOUTHERN HEMISPHERE, OCEAN CURRENTS ARE GENERALLY COUNTERCLOCKWISE

AREAS OF HIGH PRESSURE MOVE TOWARD AREAS OF LOW PRESSURE

POOL EXAMPLE

Relation between pressure and altitude

Pressure=weight of air -Air pressure measured in millibars Air pressure at any altitude is equal to the weight of a column of air extending above that altitude to the top of the atmosphere. -Air pressure decreases with increasing altitude because there is less air above at higher latitudes and the weight of the air is also less. More dense closer to the surface of the Earth.

Southern Oscillation

Refers to the changes in barometric pressure on opposing sides of the tropical Pacific that accompany El Nino and El Nina events. -These shifts in oceanic circulation are linked to shifts in atmospheric circulation in the tropical Pacific but their effects (rainfall) can be felt around the world.

Summer Monsoon

Reverse of winter -The INTERTROPICAL CONVERGENCE ZONE shifts northward over Asia bringing low pressure and a subtropical high is now over the Indian ocean. -The warm winds from this high pressure anticyclone pick up moisture as they pass over the Indian Ocean. The air is pushed upward by the warmth of India and then pushed up further by the Himalayas. -AS THE AIR RISES, IT COOLS AND PRECIPITATION OCCURS - THEIR SUMMER SEASON IS RAINY AF

Polar High

The Polar High is a mass of descending air that rotates clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, directing cold, dry air toward the polar front. The resulting winds are called polar easterlies. WEAK HIGH PRESSURE SYSTEM

Geostrophic Winds

The combination of pressure gradient force and coriolis force. How they're formed: -Initially, air will start to move in the direction of the pressure gradient, that is, from high to low pressure. -At some point, the Coriolis force will take over and force wind to the right in the Northern Hemisphere. This is more pronounced at higher latitudes. -Ultimately, the pressure gradient force and the Coriolis force balance each other out, and WIND WILL BLOW PARALLEL TO THE ISOBARS. These parallel winds, which occur in the OCCUR IN THE UPPER ATMOSPHERE, are geostrophic winds.

Pressure Gradient Force

The difference in pressure between two such areas (high and low pressure) that results in air flow. The > Greater the pressure gradient force (the diff between highs and lows), the stronger the force and thus the more intense resulting winds.

Isobars

a line on a map connecting points having the same atmospheric pressure at a given time or on average over a given period. On map : enclosed in a circle