ATMOSPHERIC AND OCEANIC CIRCULATION

Earth’s atmospheric and oceanic circulations represent a vast heat engine powered by the Sun. Volcanic eruptions such as those of Tambora in 1815 and Mount Pinatubo in 1991 demonstrate the power of global winds to disperse aerosols and pollution

AIR PRESSURE AND ITS MEASUREMENTS

The weight of the atmosphere (created by the motion, size, and number of air molecules) is air pressure. The pressure exerts an average force of approximately 1 kg/cm2 (14.7 lb/in.2).  Air pressure is measured with a mercury barometer developed by Torricelli or an aneroid barometer (a closed cell, partially evacuated of air, that detects changes in pressure).

WINDS

Wind is the horizontal movement of air across Earth’s surface. Its speed is measured with an anemometer (a device with cups that are pushed by the wind -mph) and Wind direction is detected with a Wind vane (a flat blade that is directed by the wind, 33’ above ground).

The four driving forces within the atmosphere

a) Gravity,  b) Pressure gradient force, c) Coriolis force, &  d) Friction force. Earth’s gravitational force on the atmosphere operates uniformly worldwide.

Winds are directed and driven by the pressure gradient force (moves air from areas of high pressure to areas of low pressure), The Coriolis force is a deflection in the path of winds or ocean currents caused by the rotation of Earth; Coriolis force deflects objects to the RIGHT in the Northern Hemisphere and to the LEFT in the Southern Hemisphere), Friction force is caused by Earth’s varied surfaces that exert a drag on wind movements in opposition to the pressure gradient. Air pressure patterns are portrayed on maps using the isobar—an isoline that connects points of equal pressure.

Geostrophic winds are wind circulations that occur in the upper troposphere. The combined effects of the Coriolis and the Pressure gradient forces cause the winds. The winds blow around pressure areas parallel to the isobars. In High pressure areas, winds descend and diverge, spiraling outward to form an anticyclone (clockwise in the Northern Hemisphere). In Low pressure zones, winds converge and ascend, spiraling upward to form a cyclone (counterclockwise in the Northern Hemisphere). The pattern of high and low pressures on Earth in generalized belts in each hemisphere produces the distribution of specific wind systems.

The primary Air Pressure Regions are:

1)              the equatorial low-pressure trough

2)              the weak polar high-pressure cells (at both the North and South Poles),

3)              the subtropical high-pressure cells and

4)              subpolar low-pressure cells.

Along the equator, winds converge into the equatorial low creating the intertropical convergence zone (ITCZ). Air rises along the equator and descends in the subtropics, in each hemisphere. The winds returning to the ITCZ from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere produce the TRADE WINDS. Winds flowing out of the Subtropics to higher latitudes produce the WESTERLIES in either hemisphere.  The weak and variable polar EASTERLIES diverge from the polar high-pressure cells, particularly the Antarctic high.

The subtropical high-pressure cells on Earth, generally between 20° and 35° in either hemisphere, are variously named the Bermuda high, Azores high, and Pacific high. Aleutian low and Icelandic low are migrating low pressure cells. In summer the cells are located in high latitude zones of the North Pacific and Atlantic oceans respectively bringing precipitation to Ireland, and Pacific Northwest of USA. In January the subpolar lows migrate to lower latitudes causing cyclonic storms of the West Coast of USA and Europe.

UPPER AIR CIRCULATION.

Within the geostrophic winds of the upper Westerlies are fast flowing undulating wave motions called Rossby waves. The prominent movements in upper level, westerly winds are streams of high speed winds called the jet streams. Depending on their latitudinal position in either hemisphere they are termed the polar jet stream or the subtropical jet stream.

LOCAL WINDS

Different heating characteristics of land during the day and water surfaces during the night create land and sea breezes.

Mountain air cools rapidly at NIGHT creating a heavy air that subsides downslope into the valleys as Mountain breeze (katabatic winds)

Valley air gains heat rapidly during the DAY forcing its way up the slopes during the afternoon as Valley breeze (anabatic winds)

MONSOON WINDS

Intense seasonal heating of the interior of continents causes wind systems to shift bringing seasonal rainfall onto adjacent lands. These winds involve an annual cycle of returning precipitation with the summer Sun and are named after the Arabic word for season, mausim, or monsoon. The monsoons of Southern and Eastern Asia are driven by the location and size of the Asian landmass and its proximity to the Indian Ocean. Other areas that experience Monsoon winds are: northern Australia, West Africa, and southern Arizona.

OCEAN CURRENTS

Ocean currents are primarily caused by the frictional drag of wind and occur worldwide at varying intensities, temperatures, and speeds.

Other factors that shape the flow of ocean currents

a)               Coriolis force

b)              Density differences associated with salinity and temperature

c)               Configuration of continents and the ocean floor

d)              The tides

Ocean Currents that flow from the North & South poles towards the equator distribute cool temperatures (They produce Cool Ocean Currents). Ocean Currents that flow from the Equator towards the poles distribute warm temperatures (and produce Warm Ocean Currents). Trade winds converge along the ITCZ and push quantities of water in a process known as the western intensification. 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 nutrient-rich and rises from great depths to replace the vacating water. In other portions of the sea where there is an accumulation of water the excess water gravitates downward in a downwelling current. These currents generate important mixing actions that flow along the ocean floor and travel the full extent of the ocean basins, carrying heat energy and salinity.