The Atmosphere and Ocean

Manhattan College SCI 210: Oceanography Chapter 5: The Atmosphere and Ocean Learning Objectives In this chapter, we examine radiational heating and cooling of the Earth-atmosphere system: 1. the interaction of incoming solar radiation with the atmosphere, ocean, and continents, 2. the flow of infrared radiation to space, and 3. the greenhouse effect. We also discuss heat transport by atmospheric and oceanic circulations 1. Understand atmospheric circulation as it relates to ocean circulation. INTRODUCTION: At middle latitudes, prevailing winds blow from west to east. Sea surface temperatures (SST) change relatively little through the course of a year. This stable SST regime dampens the summer-to-winter temperature contrast of air flowing over the ocean to downwind Western Europe. • In Western Europe, the air temperature contrast between summer and winter is less than it is over most of North America. Driving Question: What role does the ocean play in the long-term average state of the atmosphere? WEATHER AND CLIMATE We can think of weather as the state of the atmosphere at some place and time – described in terms of such variables as temperature, precipitation, cloud cover, and wind speed. Climate is popularly defined as weather at a particular place averaged over a specific interval of time. – By international convention, average values of weather elements such as temperature or precipitation are computed over a 30-year period beginning with the first year of a decade. Heating and Cooling Earth’s Surface 1. As Earth orbits the sun, its atmosphere and surface are absorbing energy radiated by the sun (mostly between 0.25 and 2.5 micrometers). 2. Absorption of solar radiation heats the Earth-atmosphere system. 3. At the same time the entire planet is emitting infrared radiation to space (mostly between 4 and 24 micrometers), which has a cooling effect on the Earth-atmosphere system. 4. Over the long term, radiational cooling of the planet essentially balances radiational heating of the planet so that Earth remains in radiative equilibrium with surrounding space. SOLAR RADIATION 1. Once every 24 hrs, Earth completes one rotation on its axis. 2. At any instant, half the planet is illuminated by solar radiation while the other half is in darkness. 3. The tilt of Earth’s spin axis (23 degrees 27 minutes) is responsible for the seasons. 4. the Northern Hemisphere tilts away from the sun in fall and winter, and toward the sun in spring and summer a. Annual periodic changes in the planet’s orientation to the sun result in changes in a.i. solar altitude (the angle of the sun above the horizon) and . length of daylight (elapsed time between sunrise and sunset) b. Solar altitude varies from 0º (at sunrise or sunset) to as much as 90º (sun directly overhead) c. At middle and high latitudes, the altitude of the noon sun is higher, daylight is longer, and solar radiation is more intense in summer than in winter. d. The intensity of solar radiation striking Earth’s surface per unit area varies with the solar altitude. e. With increasing solar altitude, more solar energy strikes a unit area of Earth’s surface in a unit of time f. Greater solar altitudes in the tropics translate into more intense radiation and higher temperatures at Earth’s surface. Consider this analogy: (A) A flashlight beam shines on a horizontal surface most intensely when the flashlight shines from directly overhead (analogous to a solar altitude of 90 degrees). (B) At an angle decreasing from 90 degrees, the flashlight beam spreads over an increasing area of the horizontal surface so that the light is less concentrated (less radiational energy received per unit area). Proximity to large bodies of water also affects the timing of the average warmest and coldest time of the year. 1. Outside of the tropics, the annual temperature cycles lags the annual solar radiation cycle. 2. In the interior United States, the air temperature cycle lags the solar radiation cycle by an average of 27 days. 3. But in coastal localities having a strong maritime influence (e.g., coastal California, Florida), the average lag time is up to 36 days. SOLAR RADIATION BUDGET Solar radiation intercepted by Earth travels through the atmosphere and interacts with its component gases and aerosols. These interactions consist of a) scattering, b) reflection, and c) absorption. 1. With scattering, a particle disperses radiation in all directions: up, down, and sideways. 2. Reflection is a special case of scattering in which a large surface area redirects radiation in a backward direction. • The fraction of incident radiation reflected by a surface is known as the albedo of that surface, i.e., albedo = [(reflected radiation)/(incident radiation)] × 100%. – Within the atmosphere, the tops of clouds are the most important reflectors of incoming visible sunlight. 3. Absorption is a process whereby some of the radiation that strikes an object is converted to heat energy. • Oxygen, ozone, water vapor, and various aerosols (including cloud particles) absorb solar radiation. a. The strong absorption of ultraviolet (UV) radiation by oxygen and ozone (O3) in the stratosphere shields organisms from exposure to potentially lethal intensities of UV. b. These absorption processes create the so-called stratospheric ozone shield. Solar radiation not scattered or reflected to space or absorbed by atmospheric gases or aerosols reaches Earth’s surface where it is either reflected or absorbed. b.1. High-albedo surfaces reflect a considerable amount of incident solar radiation whereas low-albedo surfaces reflect much less incident solar radiation. – The albedo of the ocean surface varies with solar altitude. a. Under clear skies, the albedo of a flat, tranquil water surface decreases with increasing solar altitude. b. With cloud-covered skies, only diffuse solar radiation strikes the water surface; the albedo varies little with solar altitude and is uniformly less than 10%. b.2. Considering that the ocean covers about 71% of the surface of the planet, the ocean is the principal sink (absorber) for solar radiation striking Earth’s surface. b.3. Under clear skies, the albedo of a flat and undisturbed water. b.4. Surface changes with solar altitude. b.5. A wave-covered water surface has a slightly higher albedo at high solar altitudes and a slightly lower albedo at low solar altitudes. Earth’s planetary albedo is about 30%. a.1. The Earth-atmosphere system reflects or scatters back to space on average about 31% of the solar radiation intercepted by the planet. a.2. The atmosphere (i.e., gases, aerosols, clouds) absorbs only about 20% of the total solar radiation intercepted. a.3. The remaining 49% of solar radiation is absorbed by Earth’s surface—mostly the ocean. SOLAR RADIATION AND THE OCEAN The ocean’s absorption of the visible portion of solar radiation is selective by wavelength. • Water absorbs the longer wavelengths (i.e., reds and yellows) of visible light more efficiently than the shorter wavelengths (i.e., greens and blues) so that green and blue penetrate to greater depths. – This explains the blue/green color of the open ocean. The sunlit surface layer of the ocean, down to the depth where light is just sufficient for photosynthesis, is termed the photic zone 1. In clear ocean waters, this depth is usually from 100 to 200 m (330 to 650 ft) but is much shallower in highly productive or turbid waters. 2. As light becomes dimmer with increasing depth, its color also changes. a. This color change affects plant production because each plant pigment is most efficient with a specific color of light. b. The combination of pigments in any type of phytoplankton determines its optimal depth distribution. INFRARED RADIATION AND THE GREENHOUSE EFFECT Emission of heat to space in the form of infrared radiation balances solar radiational heating of the Earth-atmosphere system. This is global radiative equilibrium While the clear atmosphere is relatively transparent to solar radiation, certain gases in the atmosphere known as greenhouse gases impede the escape of infrared radiation to space thereby elevating the temperature of the lower atmosphere. This important climate control is the so-called greenhouse effect.