The Coriolis Effect and Its Contribution to the Formation of Geostrophic Winds

The Coriolis Effect and its Contribution to the

Formation of Geostrophic Winds

  In 1835, French engineer-mathematician Gustave-Gaspard Coriolis described the Coriolis effect as an inertial force.   The Coriolis effect shows that Newtonian laws of motion of bodies should be used in a rotating frame of reference.  The effect of the Coriolis force is an apparent deviation of the direction of an object within a rotary axis move.  An object on the ground along a longitudinal line, or north-south path, will appear to veer to the left in the Southern Hemisphere and to the right in the Northern Hemisphere.  There are two reasons for this:  first, a function of latitude is the diverging velocity of a point on the earth, and second, the Earth’s constant eastward rotation.  The object does not really diverge from its path, but because of the rotation of the earth on its axis it does seem that way.  The Coriolis deflection is therefore related to the latitude, the motion of the earth and the motion of the object.  The Coriolis effect reduces at lower latitudes, and doesn’t have an impact on the equator.  The strength and velocity of wind depends on the intensity of the pressure gradient that produces the wind.  The rate of change in the atmospheric pressure between two points is known as the pressure gradient, shown on weather maps by the spacing of isobars.  When the Coriolis effect is equally counterbalanced by the pressure gradient force, geostrophic wind will flow parallel to the isobars.  Because of the gradient and geostrophic wind relationships the wind field responds to semi-annual and regular annual changes.  The cycle in surface mean geostrophic flow has a large year-to-year variation and is greatest in the subtropics and tropics.  The annual wave is associated with minimum easterly flow and maximum westerly flow in the subtropics in winter and maximum westerly flow in summer.  Geostrophic winds can control entire climate patterns and affects weather globally and nationally.

Geostrophic winds contribute to the fact that the eastern part of South Africa is moister than the western part.  The reason for this is because air moves to the equator from an initial position with higher latitude to lower latitude.  Divergence occurs between the two latitudes because the air accelerates.  The air moving to the poles over the east coast of South Africa will, however, be convergent.  This effect may not be strong, but according to Tyson & Preston (2012), it does help to explain that the vertical motion is enhanced on the western side of the South Indian anti-cyclone over eastern South Africa and inhibited on the eastern side of the South Atlantic anticyclone over the west coast.    

Different regions on Earth have different prevailing winds.  Winds that blow across a particular region are called prevailing winds.  The prevailing winds blow toward the poles at thirty to sixty degrees latitude.  Winds are named after the direction the come from, so these winds are called westerlies.  They are responsible for much of the weather across Canada and the United States.  Winds that form when the air over the poles cools and sinks to the surface are called polar easterlies.  

Sailors and meteorologists aren’t the only ones who have to contend with the Coriolis effect.  Aircraft covers large distances in short periods of time, which is why pilots must also take its influence into account when charting the paths for their flights.  If they don’t, they might end up in a completely different location than where they planned to go.

Wind stress makes the upper-most layer of the ocean move.  The top layer of water will cause the layer beneath to move as well.  In the southern hemisphere, the water will move to the left because of the Coriolis effect.  This causes an Ekman spiral of water movement in the current.  The current weakens with depth, where it will start moving in the opposite direction and more slowly to the wind at the surface.  Water will then either be piled up against the coastline or removed from it depending on the direction of the wind to the coast.  On the west coast of southern Africa water is moved away from the coast, because of winds of a southerly element.  The sea surface is deprived of water near the coast so, in compensation, an upwelling of cool water occurs.  On the south-east coast of South Africa, south westerly winds will pile up against the coastline, sea levels will rise which will result in the surface water sinking in compensation.  When the surface of the ocean is lowered or raised, slopes form, and geostrophic winds blow parallel to the slope.  These slopes results in having lighter, warmer water to the left of the slope and denser, cooler water to the right.  The opposite would occur in the northern hemisphere.  The slope of the sea surface contributes to the barotropic component of the pressure gradient that drives the geostrophic current (Tyson & Preston, 2012).  

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