Oceanography

Forces acting on seawater surface

            As in the case with most of the processes on earth the ultimate source of energy providing the power for the atmospheric circulation is the sun. unlike liquids, gases are easily compressed, so where the atmospheric pressure is greater, the air is also correspondingly denser. Atmospheric pressure is decreased by half for every 6 km of elevation and it gets thinner at the same rate. For example at an elevation of 6 km, air pressure is only one-half as dense. At 12 km, the pressure and density are both reduced by another factor of half, being quarter of their sea level values.

        Our atmosphere blocks the infrared radiation given off by the earth. But at higher altitudes the air is thinner, so it is easier for radiated heat to pass on out into space. Consequently the atmosphere generally gets cooler with altitude up to a height of about 12 km above sea level where it is roughly – 60⁰ C.

           In equatorial regions, air rises for two reasons, the obvious one is that warmer air is less dense and therefore lighter. The other, more subtle reason is that the water molecules are actually lighter than the nitrogen or oxygen molecules. Since there is a good deal of evaporation from the oceans and rain forests in equatorial regions, the air there contains relatively more water molecules and less of heavier molecules than elsewhere, although only by a fraction of a percent. In polar regions, the high altitude air masses are very cold and therefore dense, so they sink.

           If the earth were not spinning the movement of air masses and the circulation of the atmosphere would be of a simple pattern. Around the equator the air warmed from below, rises. Once aloft, the air flows towards the poles where it is cooled from below and sinks to flow back towards the equator. The surface winds would always blow towards the equator, being prevailing northerlies (coming from the north) in the Northen Hemisphere, and prevailing southerlies in the Southern Hemisphere. It should be noted that winds are named for the direction from which they blow.

              The real earth however, spins and therefore there is a deflection of moving air masses due to Coriolis. The effect of earth’s rotation on objects moving above it is called the Coriolis effect. It was first described by Gaspad Gustavede Coriolis (1792 – 1843), a French engineer. The coriolis force is maximum at the poles and diminishes towards the equator, where it goes though zero and changes direction.

             The atmosphere moves somewhat independent of the Earths’s surface, as there is little or low friction between the earth’s surface and the atmosphere. A parcel of air that appears to be stationary above a point on the equator is actually turning with the Earth and moving eastward at a speed of about 1700 km/hr (1050 miles/hr). if the south wind blows this parcel of air due north, it moves across circles of latitude with progressively smaller circumferences. At these higher latitudes, points on the Earth’s surface move eastward more slowly. At 60 ⁰ c N, the eastward speed of a point on the Earth’s surface due to rotation is only half of the speed (850km/hr) at the equator. Because of this, a parcel of air that was originally moving only northward relative to the earth at the equator, carries with its equatorial eastward speed. Therefore the air parcel is displaced to the ease, relative to the earth’s surface, as it moves from low to high latitude. In the northern hemisphere this deflection is to right of the direction of the air motion. If the same situation occurs in the southern hemisphere with a parcel of air moving southward from the equator, due to a north wind, the deflection is still to the east relative to the earth’s surface. However this deflection is now to the left of the direction of the air motion.

       In the northern hemisphere, the air rising at the equator turns poleward gets deflected towards the right and soon heads eastwards, when it reaches higher altitude and latitude, at about 30 ⁰  ,the air looses much of its heat by radiation to space and the cold heavy air descends. Another factor that contributes to the descent is that the poleward moving air is moving eastward at about 30 ⁰   latitude and therefore piles up and this contributes to the descent. The descending air is warm by comparison, making this region near 30   latitude, warm,clear, and dry. Many of the warm desert areas are near 30 ⁰  north and south of the equator.  

              The descending air divides, some continuing towards the poles, and the rest returns to the equatorial regions. The surface air returning towards the equator is acted on by the coriolis effect that turns it westward producing the easterly trade winds which were used by the sailing ships on their way towards the new world. In the region form the equator to 30 north and south latitudes, the surface winds are easterly and the winds aloft are westerly. These westerly winds aloft move from west to east, commonly at a speed exceeding 160 km/hr (100 milles/hr)

          Between 60 ⁰ N and the north pole, the winds blow from the north and east while between 60   S and the south pole they blow from south and east. In both cases they are called polar easterlies. In the intermediate latitude the third circulation cell is established with the air sinking near 30 ⁰   N along with that from the tropical cell, and rising near 60 ⁰  N   along with that form the north polar cell. The surface winds in this intermediate cell are deflected and produce winds that blow from the south and west between 30  N and  60 ⁰  N and from north and west between 30 ⁰  S and 60 ⁰  S. In both hemispheres these winds are called westerlies. The air returning aloft to  30 ⁰ latitude in this intermediate cell is also a westerly wind because it has enough westerly momentum to overcome the coriolis effect. Thus in the region between 30 ⁰ – 60 ⁰  latitude the winds are westerlies at all elevation.

The place where the westerly winds of the temperate area meets the cold polar air is called the polar front. Its location changes, especially with season, but it generally varies between 40 ⁰-60 ⁰. latitude. At the equator and 60 ⁰. N and S, moist, low-density air rises; these are areas atmospheric pressure zones of clouds and rain. Zone of high density descending air at 30 ⁰. And 90 ⁰. North and south are areas of high atmospheric pressure zones of low precipitation and clear skies. Air flows over the Earth surface from regions of high atmospheric pressure to low atmospheric pressure. The area of rising air at the equator is known as the doldrums, and high pressure areas at 30 ⁰. N and S are known as horse latitudes In these areas sailing ships could find themselves becalmed for days.

 The momentum imparted to the sea by these major wind drive regular patterns of broad, slow, relatively shallow ocean surface currents. Some currents transport more than one hundred times the volume of water carried by all of the Earth’s rivers combined. Currents of such magnitude greatly affect the distribution of marine organisms and the rate of heat transport from tropical to polar regions. As the surface water is moved horizontally by the wind, momentum is transferred downward. The speed of the deeper water diminishes steadily as momentum is lost to overcome the viscosity of water. Eventually at depths generally less than 200 m, the speed of wind driven currents become negligible. Surface currents set up by the winds move at about 2% of the speed of the wind that caused them. For instance, a wind blowing at 10m/sec would cause a surface current of about 20 cm/sec. Wind driven surface water sets the water immediately below it in motion. But due to low friction, this next deeper layer is deflected to the right (Northern Hemisphere) or left (Southern Hemisphere) of the surface layer direction. The same is true for the next layer down and the next.

The result is a spiral in which each deeper layer moves slowly, and with a greater angle of deflection than the layer above. This current spiral is called the Ekman spiral, after the Swedish Physicist V. Walfrid Ekman, who in 1902 demonstrated mathematically that under ideal conditions a systematic decrease in current speed and a change in its direction occurred at increasing depths. (Fig.9).

        Under natural conditions however the Ekman spiral does not operate in its predicted theoretical fashion. Oceans are not in uniform state, and a single wind impact is not so prolonged. The net motion of the entire mass of moving water flows at right angles to the wind direction is called Ekman transport. The Ekman layer marks a depth of 100m; their currents and frictional force defining the Ekman spiral are quite active. Below this depth these forces are virtually non existent.