Atmospheric Circulation

The atmosphere and ocean circulation work together to absorb heat and redistribute it from one part of the globe to another. Otherwise, the tropics would get hotter and hotter, and the polar regions would get colder and colder due to the unequal distribution of insolation at the Earth's surface.

Air pressure

Air pressure is the force that a column of overlying air presses on an area of the Earth's surface. At sea level, the average pressure is 1 kg/cm2 (or 14.7 lb/in2). Winds are created when air moves from areas of high air pressure to areas of low air pressure. Heating air and the addition of water vapor create lighter, rising air (that is, LOW pressure zones), while cooling and low water vapor content create dense, sinking air (that is, HIGH pressure zones).

Global wind belts

The atmospheric circulation diagram below shows the generalized wind patterns on Earth. It does not account for the influence of continents. The red lines and arrows show how the lower atmosphere circulates vertically and at high altitudes. A persistent low pressure zone is created near the equator called the Intertropical convergence zone (ITCZ), where massive amounts of evaporation cause air to rise. Condensation, precipitation and cooling of the air creates a belt of descending air called the subtropical high pressure zone (around 30 degrees N and S latitude). A zone of low pressure, the Polar front, forms where the cold polar air converges with warmer midlatitude air forcing the latter to lift or rise.

The black lines and arrows show an idealized map of the surface wind patterns. The surface winds form three belts north and south of the ITCZ. These belts are 1) the Trade Winds, 2) the Westerlies, and 3) the Polar Easterlies. Winds are named for the direction from which they blow.

Coriolis effect

The generalized global wind pattern map above shows that surface winds follow a curved path. This deflection from a straight line is called the Coriolis effect. The Coriolis effect deflects any particle in motion and arises because Earth's rotational speed varies with latitude. At the equator the Earth rotates the full circumference of the earth in a 24 hour period (i.e. 40,000 km/24 hr ~ 1670 km/hr). At 60° N and S latitude, where the circumference is one half the distance of the equator, the earth still rotates once in 24 hr (i.e.20,000 km/24 ~ 835 km/hr). The rotational speed is zero at the poles. Because of these speed differences at different latitudes, as air or water moves from one latitude to another it retains its initial eastward speed. This causes the wind or water to curve or "be deflected" to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Aerosols as a tracer

Surface winds do not maintain three distinct belts as illustrated above. Continental land masses affect the flow of air by slowing its movement by friction and by seasonal variation in heating. The atmosphere is heated from below by heat radiated from Earth's surface. Continents experience high fluctuations in surface temperatures between excessive heating in the summer time to extreme heat loss in winter time. These fluctuations affect the flow of air and causes the surface winds to rotate around high and low pressure centers within the wind belts.

Satellite images of particles (aerosols) in the atmosphere are one way to illustrate the path of global atmospheric circulation. The animation below shows daily global images of the Total Ozone Mapping Spectrometer (TOMS) aerosol index indicating the location of UV-absorbing tropospheric aerosols from July through September, 1988. Light brown indicates the smallest amount of dust/smoke in the atmosphere, with dark brown indicating the largest amount.

A number of things can be seen in July, including desert dust being blown from the Sahara across the Atlantic to the Caribbean and smoke from biomass burning in Africa south of the equator. Dust can also be seen over Saudi Arabia and in the Takla Makan desert in western China. Beginning in August, smoke from biomass burning can be seen in South America. Notice the transport of this smoke into and across the southern Atlantic Ocean, and the transport of smoke from Africa both west into the Atlantic and southeast into the Indian Ocean. Finally, smoke from the large number of forest fires in the western United States (particularly Yellowstone) is seen in late August, early September. Watch as this smoke travels across the northern hemisphere.

Volcanic eruptions also eject gases and particles that can be traced in the atmosphere. An 18 km-high volcanic plume from one of a series of explosive eruptions of Mount Pinatubo beginning on 12 June 1991, viewed from Clark Air Base (about 20 km east of the volcano). Three days later, the most powerful eruption produced a plume that rose nearly 40 km, penetrating well into the stratosphere. (Photograph by David H. Harlow, USGS.)

NASA's Volcanology Team uses measurements from TOMS instruments onboard the Nimbus-7 satellite to detect sulfur dioxide gas (SO2) and ash fallout produced by volcanoes.

These images show the SO2 cloud detected by Nimbus-7/TOMS from the June 16, 1991 eruption of Mt. Pinatubo, the largest eruption of the last half-century, to June 30, 1991. The images show the SO2 index (SOI); the value of the SOI is proportional the amount of SO2 present up to a value of 200. At that point the SOI saturates, and the images shown below reflect this saturation. Actual SO2 amounts after the initial eruption were above 1000 DU (Dobson units). Color scale of the images shows purple and blue at the low end of the scale at 30-70 SOI, and orange and red at the high end of the scale at 140-200 SOI.

The Surface winds

The NASA Scatterometer (NSCAT) instrument was launched onboard the Japanese Advanced Earth Observing Satellite (ADEOS I) in August 1996. The NSCAT mission was to collect data on the global climate, weather changes, air-sea interaction, and the water cycle.

"NSCAT uses eight antenna beams to scan two wide bands of ocean, one on each side of the instrument's orbital path. NSCAT transmits short pulses of microwave energy to probe ocean surfaces and then measures the reflected or backscattered power. Variations in the magnitude of this backscattered power are caused by changes in small (centimeter-sized), wind-driven waves. Using a method called Doppler processing (a change in the observed frequency of the radio waves due to relative motion of source and observer), the measured backscattered power is separated into cells at specific locations on Earth's surface; these are then transmitted to the ground for processing. During ground processing, wind direction and speed can be determined from these variations. Within two weeks of receiving the raw data, the ground system will process wind measurements." NSCAT Home page

Below you will find the last real-time images gathered by NSCAT, before it ceased operation due to failure of the ADEOS I satellite on June 29, 1997.

    Animation of surface wind vectors in the North Pacific for a 7 day period

The background colors represent wind speed, blues are low speed (0-8 Meters/second, 0-29 kph, 0-18 mph), magentas moderate (8-20 m/s, 29-72 kph, 18-45 mph ) and yellows high (greater than 20 m/s, 72 kph, 45 mph), the moving arrows show the wind directions.

The image below represents the surface wind over the Pacific Ocean, with North and South America at the right. It was based on more than 150,000 satellite measurements made during a single day in 1978 by a "scatterometer" instrument on the Seasat satellite. The arrows show wind direction and the colors represent wind speed. Blue indicates wind speeds of 1-4 meters/second; gray, 4-6 meter/second; red, 6-16 meters/second; and yellow, 16-20 meters/second.

In areas of low pressure, winds swirl in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. Note the wind speed and direction near storms in the South pacific and near Alaska.

Do Exercise A

Oceanic Circulation

Driving forces of ocean circulation

Atmospheric winds generate horizontal surface currents in the ocean by frictional drag. The uppermost100 m of ocean water is set into motion by the wind. In contrast, deep ocean circulation is driven mainly by density contrasts in the water produced by temperature and salinity variations.

Global ocean circulation

Water is 1000 times more dense than air and moves at much slower speeds, therefore the Coriolis effect acts on the moving water over a longer period of time. When wind blows over water, the surface water does not move directly in front of the wind but moves about 45 degrees toward the right of the wind's motion in the Northern Hemisphere. This deflection is due to the Coriolis effect. Further deflection of surface ocean currents is due to the distribution of continental land masses.

In general the Trade winds produce an equatorial current (parallel to the equator), and the Westerlies produce the North Pacific and North Atlantic currents in the Northern Hemisphere. Circular motion of water in the ocean basins, called a gyre, is set into motion as water flows northward or southward to connect the latitudinal flow. The major surface oceanic currents are illustrated in the diagram below.

Drifters, moored arrays, and satellite data

Drifter buoys released into the ocean, the buoys float with the currents and take measurements of the water with built-in instruments. They are tracked by satellites in orbits far above Earth and transmit data several times a day. Ships and airplanes can drop these buoys into the sea. When released by ships, they have a 98% survival rate; from the air, survival drops to 78%. About half of the drifters lose their ability to communicate with the satellite, for one reason or another, after 440 days. Other buoys last longer and transmit their information for several years. The floater at the top of the buoy sits at the surface of the water and holds an antenna for sending data to a satellite above. Instruments well below the surface cause the ocean currents to take the buoy along instead of the surface wind. The buoy also holds electronic instruments for measuring sea surface temperatures (SST), submergence, irradiance (for sunlight) and barometric pressure. At the top is another device for measuring temperature and conductivity (used to calculate salinity).

World map of global buoys in the ocean in July 1997.

The TOPEX/POSEIDON (T/P) mission (a joint effort between NASA and the French space agency) goal is to understand the dynamics of ocean circulation and the role this circulation plays in climate change. The satellite travels in an orbit that allows coverage of 95% of the ice-free oceans every 10 days. The satellite measures sea levels, current variations, and provides information about tides, waves, and wind. The images are "false color" images. That is, different measured values have been assigned different colors to make them easier to see.The image at the top of this module shows variations in sea level which is due to shifts in atmospheric pressure.

Sea-surface temperatures (SSTs) show the locations of ocean surface currents by delineating the flow of warm and cold waters. The image below shows sea-surface temperatures (SSTs) for the entire world ocean from Sept. 13-16, 1997. Temperatures are in degrees Celsius. Red shows warm temperatures, purple denotes cold temperatures.

NOAA provides near-real time satellite-derived sea surface temperature (SST) images. Data are collected four times daily from the Advanced Very High Resolution Radiometer (AVHRR) instrument aboard NOAA's Polar Orbiting satellites. These data are then processed using a set of NOAA-developed multichannel, atmospherically corrected algorithms, mapped to a series of predefined 1km and 4km regions. These satellites have been critical to long-range forecasting of weather conditions and potential global climate change.

Do Exercise B


What is El Niño?

"El Niño is a disruption of the ocean-atmosphere system in the tropical Pacific. El Niño was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific ocean, occurring near the beginning of the year. El Niño means The Little One in Spanish. This name was used for the tendency of the phenomenum to arrive around Christmas.

In normal, non-El Niño conditions (top panel of schematic diagram), the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador. The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry. The observations at 110 W (left diagram of 110 W conditions) show that the cool water (below about 17 degrees C, the black band in these plots) is within 50m of the surface.

During El Niño (bottom panel of the schematic diagram), the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. The observations at 110W show, for example, that during 1982-1983, the 17-degree isotherm dropped to about 150m depth. This reduced the efficiency of upwelling to cool the suface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region. The weakening of easterly tradewinds during El Niño is evident in this figure as well. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific." NOAA El Niño Theme Page

The great width of the Pacific Ocean is the main reason we see El Nino Southern Oscillation (ENSO) events in that ocean as compared to the Atlantic and Indian Oceans. Most current theories of ENSO involve planetary scale equatorial waves. The time it takes these waves to cross the Pacific is one of the factors that sets the time scale and amplitude of ENSO climate anomalies. The narrower width of the Atlantic and Indian Oceans means the waves can cross those basins in less time, so that ocean adjusts more quickly to wind variations. Conversely, wind variations in the Pacific Ocean excites waves that take a long time to cross the basin, so that the Pacific adjusts to wind variations more slowly. This slower adjustment time allows the ocean-atmosphere system to drift further from equilibrium than in the narrower Atlantic or Indian Ocean, with the result that interannual climate anomalies (e.g. unusually warm or cold SSTs) are larger in the Pacific.

Monitoring the Equatorial Pacific

As part of the international consortium to monitor the equatorial Pacific Ocean, an array of instruments were moored. The Tropical Atmosphere Ocean (TAO) array measures surface winds, air temperature, relative humidity, sea surface temperature, and subsurface temperatures to a depth of 500 m. The photograph below shows the recovery and servicing of one of the moorings.



Moorings provide real-time oceanographic and meteorological data for the prediction and monitoring of El Niño.

The data presented above show a five day average of the data transmitted by the TAO moored instruments. The upper images show sea surface temperatures in degrees C and wind vectors between 10 degrees N and S latitude in the equatorial Pacific Ocean. The length of the arrow is a measure of the velocity in m/s. The lower image illustrates departures of these values from the norm, i.e. anomalies.

Another way of viewing the development of the 1997 El Niño is the calculated radiation anomalies image above (in Watts per square meters).

How does 1997 compare to other years?

Every El Niño is somewhat different in magnitude and in duration. Plots of Sea Surface Temperature Anomalies (SSTA) from the ENSO Monitor, show El Niño's back to 1982, including the 1982-1983 El Niño, which, until 1997, was the largest El Niño of this century.

How frequently does an El Niño occur?

El Niño's usually occur irregularly, approximately every two to seven years. The El Niño years 1976-1977, 1982-1983, 1986-1987, and 1991-1994 are distinguished by large SST anomalies. The first half of the 1990's is unusual in that the past four years have all been unusually warm in the equatorial Pacific.

El Niño/Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon to cause global climate variability on interannual time scales. Here an attempt to monitor ENSO by basing the Multivariate ENSO Index (MEI) on the six main observed variables over the tropical Pacific is made. Six variables are: sea-level pressure (P), zonal (U) and meridional (V) components of the surface wind, sea surface temperature (S), surface air temperature (A), and total cloudiness fraction of the sky (C) are computed separately for each of twelve bi-monthly seasons. All seasonal values are standardized with respect to each season and to the 1950-93 reference period. The MEI has been developed mainly for research purposes. Positive values of the MEI represent the warm ENSO phase, i.e. El Niño.

What are the global effects of an El Niño?

"During warm (El Niño) episodes abnormal patterns of temperature and precipitation develop in many regions of the globe. These patterns result from changes in the distribution of tropical rainfall and the effects these changes have on the position and intensity of jet streams and the behavior of storms outside of the tropics in both the Northern and Southern Hemispheres. Some areas are already experiencing El Niño-related impacts. Wetter than normal conditions exist over the equatorial central and eastern Pacific. Drier than normal conditions have been experienced in Indonesia, most of Central America, southern Mexico, and in equatorial South America east of the Andes mountains. Drier than normal conditions have also been observed in sections of India. A stronger than normal South Pacific jetstream has contributed to wetter than normal conditions over southern and central portions of South America. There are some indications that the weather pattern over the United States has been influenced by the developing El Niño. The somewhat wetter and cooler than normal conditions over the northern Rockies and sections of the Great Plains, as well as the drier than normal conditions in the mid-Atlantic states are features that have been observed during past El Niño episodes." NOAA El Niño Theme Page

In general, warm ENSO episodes are characterized by an increased number of tropical storms and hurricanes in the eastern Pacific and a decrease in the Gulf of Mexico and the Caribbean Sea. It is believed that El Niño conditions suppress the development of tropical storms and hurricanes in the Atlantic.

Do Exercise C

Other Resources:

U.S. Department of Commerce, NOAA El Nino Theme Page [good 3D animations]

Current El Nino-Related Climate Predictions and Forecasts


World Ocean Circulation Experiment

NASA Scatterometer, sea winds [good animations of sea winds]

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