Presentation on theme: "Chapter 1 Introduction Fig.1-1 The spiral eddies in the central Mediterranean Sea were photographed from the Space Shuttle Challenger. The waters of the."— Presentation transcript:
Chapter 1 Introduction Fig.1-1 The spiral eddies in the central Mediterranean Sea were photographed from the Space Shuttle Challenger. The waters of the ocean are continually moving – in powerful currents like the Gulf Stream, in large gyres, and in smaller swirls and eddies ranging in size down to a centimeter across or less.)
What drives all this motion? energy from the Sun, and the rotation of the Earth. Sun: winds Energy is transferred from winds to the upper layers of the ocean through frictional coupling between the ocean and the atmosphere at the sea-surface. variations in the temperature and salinity of seawater (in term density) Changes in temperatures are caused by fluxes of heat across the air-sea boundary. Changes in salinity are brought about by additional or removal of freshwater, mainly through evaporation and precipitation, but also, in polar regions, by the freezing and melting of ice. If surface water becomes denser than the underlying water, the situation is unstable and the denser surface water sinks. Vertical, density-driven circulation is known as thermohaline circulation.
How does the rotation of the Earth contribute to ocean circulation patterns? (1) Consider a missile fired northwards on the Equator (Fig. 1.2(a)). The missile is moving eastwards at the same velocity as the Earth’s surface as well as moving northwards at its firing velocity. Initially, because it has the same eastward velocity as the surface of the Earth, the missile appears to travel in a straight line. The eastward velocity is greatest at the Equator and decreases towards the poles. As a result, in relation to the Earth, the missile is moving not only northwards but also eastwards, at a progressively greater rate (Fig. 1.2(b)).
How does the rotation of the Earth contribute to ocean circulation patterns? (2) Fig.1-2(a) A missile launched from equator has not only its northward firing velocity but also the same eastward velocity. (b) The path taken by the missile in relation to the surface. This apparent deflection of objects that are moving over the surface of the Earth without being frictionally bound to it be the missiles, parcels of water or parcels of air is explained in terms of an apparent force known as the Coriolis force.
Q1.1 What can you say about the Coriolis force acting on a body moving the curved surface of a hypothetical cylindrical Earth rotating about its axis? Fig.1.3 A hypothetical cylindrical Earth On the real Earth, the eastward velocity of the surface is greatest at the Equator and zero at the poles. For the hypothetical cylindrical Earth, the eastward velocity of the curved surface would be the same whatever the distance from the poles, and so a body moving above the surface would not appear to be deflected, and there would be no Coriolis force.
The rotation of the (spherical) Earth about its axis causes deflection of currents, winds and projectiles. 1.The magnitude of the Coriolis force increases from zero at the Equator to a maximum at the poles. 2.The Coriolis force acts at right angles to the direction of motion, so as to cause deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Consider, for example, a current flowing with a speed of 0.5 m/s at about 45 o of latitude. Water in the current will travel approximately 1800 m in an hour, and during that hour the Coriolis force will have deflected it about 300 m from its original path. The Coriolis force has the visible effect of deflection ocean currents. Study Currents and winds within which the Coriolis force is balanced by horizontal forces resulting from pressure gradients. Such flows are described as geostrophic.
1.1 The radiation balance of the Earth-ocean-atmosphere system Fig.1.4(a) The radiation balance at the top of the atmosphere plotted against latitude for (i) the northern summer and (ii) the southern summer. The red solid line is the intensity of incoming solar radiation and the red dashed line is the intensity of radiation lost to space. (b) The average temperature of surface waters at different latitudes. At a given latitude, there will be surface waters whose annual mean temperatures are higher or lower than shown by the curve; this range is represented by the thickness of the pink envelope.
The intensity of incoming solar radiation is greatest for mid- latitudes in the hemisphere experiencing summer, while for high latitudes in the winter hemisphere, the oblique angle of the Sun’s rays, combined with the long periods of winter darkness, results in amounts of radiation received being low. The Earth not only receives short-wave radiation from the Sun, it also re-emits radiation of a longer wavelength. Little of this long- wave radiation is radiated directly into space; most of it is absorbed by the atmosphere, particularly by the carbon dioxide, water vapor and cloud droplets. Thus, the atmosphere is heated from beneath by the Earth and itself re-emits long-wave radiation into space. The intensity of the radiation emitted into space does not vary greatly with latitude, although as can be seen from the dashed curves in Fig.1.4(a), it is highest for the subtropics and lowest for high latitudes in the hemisphere experiencing winter.
Q1.2 (a) According to Fig.1.4(a), for what latitude band is there always a net surplus of radiation? At what latitude there is always a deficit? (b) (i) Approximately how much warmer than surface waters poleward of 70 o are surface ocean water within 10 o of the Equator? (ii) ̒ Sea-surface temperatures are highest in the vicinity of the Equator because of the long day-length there. ̓ True or false? (a) Surplus - all year within about 10 13 o of the Equator. Deficit - poleward of about 75 o latitude. (b) (i) Surface waters within 10 o of the Equator average about 27 o C, while those poleward of 70 o average about 1 o C. (ii) False. In the vicinity of the Equator, day and night are of comparable length, being equal at equinoxes. The high equatorial sea-surface temperatures are the result of relatively large amounts of radiation being received all year i.e. a high average insolation. The largest amounts of incoming solar radiation are received by mid-latitudes in summer.
In fig.1.4(a), the two areas labeled ‘net surplus’ are together about equal to those labeled ‘net loss’. This suggests that the radiation budget for the Earth-ocean-atmosphere system as a whole is in balance. The positive radiation balance at low latitudes, and the negative one at high latitudes, results in a net transfer of heat energy from low to high latitudes, by means of wind systems in the atmosphere and current systems in the ocean. There has been much debate about the relative importance of the atmosphere and ocean in the poleward transport of heat. Fig.1.5 shows estimates of poleward heat transport, based on satellite observations of the upper atmosphere, and measurements within the atmosphere, published in 1985.
Fig.1.5 Estimates of the contributions to poleward heat transport (red curve) by the ocean (solid blue curve) and the atmosphere (dashed blue curve). Positive values are northward heat transport, negative values are southward heat transport. The total heat transport was derived from satellite, heat transport by the atmosphere was estimated using measurements of atmospheric heat fluxes, and the heat transported by the ocean was calculated as the difference between the two.
Q1.3 (a) How does the total poleward transport of heat in the ocean compare with that in the atmosphere, according to Fig.1.5? (b) Over which parts of the globe does the ocean contribute significantly more to poleward heat transport, and over which parts does the atmosphere contribute significantly more? (a) the area under the solid blue curve in Fig. 1.5 is very slightly greater than that under the dashed blue curve, indicating that the ocean transports polewards fractionally more heat than does the atmosphere. (b) The ocean contributes more to poleward heat transport in the tropics (between about 20 o S and 30 o N) and the atmosphere contributes more in mid-latitudes ( 30 o 55 o S and 40 o 60 o N).
Wind systems redistribute heat partly by the advection of warm air masses into cooler region, and partly by the transfer of latent heat which is taken up when water is converted into heat vapor, and released when the water vapor condenses in a cooler environment. The tropical storms known as cyclones or hurricanes are dramatic manifestations of the transfer of energy from ocean to atmosphere in the form of latent heat. The generation of cyclones, and their role in carrying heat away from the tropical oceans, will be described in Chapter 2.
1.2 Summary 1.Circulation in both the oceans and the atmosphere is driven by energy from the Sun and modified by the Earth’s rotation. 2.The radiation balance of the Earth-ocean-atmosphere system is positive at low latitudes and negative at high latitudes. Heat is redistributed from low to high latitudes by means of wind systems in the atmosphere and currents systems in the ocean. There are two principal components of the ocean circulation: wind-driven surface currents and the density-driven (thermo-haline) deep circulation. 3.Air and water masses moving over the surface of the Earth are only weakly bound to it by friction and so are subject to the Coriolis force. The Coriolis force acts at right angles to the direction of motion, so as to deflect winds and currents to the right in the Northern Hemisphere.
Q1.4(a) A missile is fired southwards from the Equator. Explain what will happen to the direction of its path in relation to the Earth. (b) In which direction will a current be deflected by the Coriolis force if it is initially flowing (i) eastwards at 45 o N, (ii) westwards on the Equator? (a) The missile has the eastward velocity of the Earth’s surface at the Equator. As the missile travels southwards, the eastward velocity of the Earth beneath it becomes less and less, so that in relation to the Earth the missile is moving not only southwards but eastwards. Thus, a missile fired southwards from the Equator is deflect towards the east. (b)(i) In the Northern Hemisphere, the Coriolis force deflects currents to the right, and so would tend to deflect southwards any current flowing initially eastwards. (ii) On the Equator, the Coliolis force is zero and there is no deflection.
Q1.5 In Fig.1.4(a), the horizontal axis is scaled according the surface area of the Earth in different latitude bands. Why do you think the horizontal axis of Fig.1.4(b), showing the annual mean temperature of surface ocean waters, is more compressed at northern than southern high latitudes? Like that in Fig.1.4(a), the horizontal axis of (b) is scaled according to surface area, but in this case it is not the area of the Earth’s surface that is relevant but the area of ocean surface. The proportion of the Earth’s surface that is ocean is much smaller in the Northern Hemisphere than in the Southern Hemisphere, which between 30 o and 65 o of latitude is largely ocean. The horizontal axis is therefore even more compressed at northern high latitudes than southern high latitudes.