The sun and the atmosphere drive directly or indirectly almost all dynamical processes in the ocean. The dominant external sources and sinks of energy are sunlight, evaporation, infrared emissions from the sea surface, and sensible heating of the sea by warm or cold winds. Winds drive the ocean s surface circulation down to depths of around a kilometer. Wind and tidal mixing drive the deeper currents in the ocean.
The oceans, in turn, is the dominant source of heat that drives the atmospheric circulation. The uneven distribution of heat loss and gain by the ocean leads to winds in the atmosphere. Sunlight warms the tropical oceans, which evaporate, transferring heat in the form of moisture to the atmosphere. The heat is released when the vapor condenses as rain. Winds and ocean currents carry heat poleward, where it is lost to space.
Because the atmosphere drives the ocean, and the ocean drives the atmosphere, we must consider the ocean and the atmosphere as a coupled dynamic system. In this chapter we will look at the exchange of momentum between the atmosphere and the ocean. In Chapter 5, we will explore heat exchanges. In Chapter 14, we will explore how the ocean and the atmosphere interact in the Pacific to produce El Niño.
4.1 The Earth in Space
The Earth's orbit about the sun is nearly circular at a mean distance of 1.5 × 108 km. The eccentricity of the orbit is small, 0.0168. Thus Earth is 103.4% further from the Sun at aphelion than at perihelion, the time of closest approach to the sun. Perihelion occurs every year in January, and the exact time changes by about 20 minutes per year. In 1995, it occurred on 3 January. Earth's axis of rotation is inclined 23.45° to the plane of Earth's orbit around the sun (Figure 4.1). The orientation is such that the sun is directly overhead at the Equator on the vernal and autumnal equinoxes, which occur on or about 21 March and 21 September each year.
The latitudes of 23.45° North and South are the Tropics of Cancer and Capricorn respectively. The tropics lie equatorward of these latitudes. As a result of the eccentricity of Earth's orbit, maximum solar insolation averaged over the surface of the Earth occurs in early January each year. As a result of the inclination of Earth's axis of rotation, the maximum insolation at any location outside the tropics occurs around 21 June in the northern hemisphere, and around 21 December in the southern hemisphere.
If the insolation were rapidly and efficiently redistributed over Earth, maximum temperature would occur in January. Conversely, if heat were poorly redistributed, maximum temperature in the northern hemisphere would occur in summer. So it is clear that heat is not rapidly redistributed by winds and currents.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, email@example.com
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Updated on September 3, 2008