Chapter 18: The Oceans and Their Margins J. Bruce H. Shyu May 24, 2010
Introduction: The World’s Oceans • Seawater covers 70.8 percent of Earth’s surface, in three huge interconnected basins: • The Pacific Ocean (太平洋). • The Atlantic Ocean (大西洋). • The Indian Ocean (印度洋).
The Oceans’ Characteristics • The greatest ocean depth yet measured (11,035 m) lies in the Mariana Trench (馬里亞納海溝). • The average depth of the oceans, is about 3.8 km. • The present volume of seawater is about 1.35 billion cubic kilometers. • More than half this volume resides in the Pacific Ocean.
Ocean Salinity (1) • Salinity (鹽度) is the measure of the sea’s saltiness, expressed in parts per mil (‰ = parts per thousand). • The salinity of seawater normally ranges between 33 and 37‰. • The principal elements that contribute to this salinity are sodium and chlorine.
Ocean Salinity (2) • More than 99.9 percent of the ocean’s salinity reflects the presence of only eight ions: • Chloride. • Sodium. • Sulfate. • Magnesium. • Calcium. • Potassium. • Bicarbonate. • Bromine.
海水中的元素組成 鹽度 : 35gm/1000gm 9 種主要元素: 氯 (Cl) 鈉 (Na) 硫酸鹽 (SO4-2) 鎂 (Mg) 鈣 (Ca) 鉀 (K) 次碳酸鹽 (HCO3-) 溴 (Br) 鍶 (Sr)
Ocean Salinity (3) • Cations are released by chemical weathering processes on land. • Each year streams carry 2.5 billion tons of dissolved substances to the sea. • The principal anions found in seawater are believed to have come from the mantle. Chemical analyses of gases released during volcanic eruptions show that the most important volatiles are water vapor (steam), carbon dioxide (CO2), and the chloride (Cl-) and sulfate (SO42-) anions.
Salinity of the oceans Figure 18.3A
Temperature and Heat Capacity of the Ocean (1) • Global summer sea-surface temperature is displayed with isotherms (等溫度線) that lie approximately parallel to the equator. • The warmest waters during August (>28°C) occur in a discontinuous belt between about 30° N and 10° S latitude. • In winter, the belt of warm water moves south until it is largely below the equator.
Temperature of the oceans in August Figure 18.3B
Temperature and Heat Capacity of the Ocean (2) • Waters become progressively cooler both north and south of this belt. • Since the water has a high heat capacity (熱容量), both the total range and the seasonal changes in ocean temperatures are much less than what we find on land. • Coastal inhabitants benefit from the mild climate resulting from this natural ocean thermostat.
Vertical Stratification (1) • Temperature and other physical properties of seawater vary with depth. • When fresh river water meets salty ocean water at a coast, the fresh water, being less dense, flows over the denser saltwater, resulting in stratified water bodies. • The oceans also are vertically stratified as a result of variation in the density of seawater.
Vertical Stratification (2) • Seawater become denser as: • Its temperature decreases. • Its salinity increases. • Gravity pulls dense water downward until it reaches a level where the surrounding water has the same density. • These density-driven movements lead both to stratification of the oceans and to circulation in the deep ocean.
海洋環流 表面洋流：風吹流 由行星風系所驅動 運行速度快 中層流與底層流：溫鹽環流 由海水的密度差所驅動 運行速度緩慢 兩者共同形成全球的海水輸送帶
Ocean Circulation • Surface ocean currents (表面洋流) are broad, slow drifts of surface water set in motion by the prevailing surface winds. • A current of water is rarely more than 50 to 100 m deep. • The direction taken by ocean currents is also influenced by the Coriolis effect (科氏力效應).
Current Systems • Each major current is part of a large subcircular current system called a gyre (環流). • The Earth has five major ocean gyres. • Two are in the Pacific Ocean. • Two are in the Atlantic Ocean. • One is in the Indian Ocean.
觀測：用衛星追蹤浮球 冬 季 速度比率 50 cm/s 緯 度 夏 季 速度比率 50 cm/s 緯 度 細線條是1988~2006年間衛星追蹤約850個浮球隨流漂移的軌跡（又稱海流麵條圖），箭矢是從軌跡計算出的海流平均速度。（台灣海洋大學胡健驊教授）
Major Water Masses • Ocean waters also circulate on a large scale within the deep ocean, driven by differences in water density. • The water of the oceans is organized into major water masses, each having a characteristic range of: • Temperature. • Salinity.
大西洋的溫鹽環流 NADW 北大西洋深層水 AAIW 亞南極中層水 AABW 南極底層水 Figure 18.5
The Global Ocean Conveyor System (1) • Dense, cold, and/or salty surface waters that flow toward adjacent warmer, less-salty waters will sink until they reach the level of water masses of equal density. • The resulting stratification of water masses is thus based on relative density.
The Global Ocean Conveyor System (2) • The sinking dense water in the North Atlantic propels a global thermohaline circulation system, so called because it involves both the temperature (thermo) and salinity (haline) characteristics of the ocean waters.
The Global Ocean Conveyor System (3) • The Atlantic thermohaline circulation acts like a great conveyor belt, transporting low-density surface water northward and denser deep-ocean water southward. • Heat lost to the atmosphere by thiswarm surface water, together with heat from the warm Gulf Stream, maintains a relatively mild climate in northwestern Europe.
全球海水輸送帶 Figure 18.6B
Ocean Tides (1) • Tides (潮汐): • Twice-daily rise and fall of ocean waters. • Caused by the gravitational attraction between the Moon (and, to lesser degree, the sun) and the Earth. • The Moon exerts a gravitational pull on the solid Earth.
Tide-Raising Force (1) • A water particle in the ocean on the side facing the Moon is attracted more strongly by the Moon’s gravitation than it would be if it were at Earth’s center, which lies at a greater distance. • This creates a bulge on the ocean surface due to the excess inertial force (called the tide-raising force).
Tide-Raising Force (2) • At most places on the ocean margins, two high tides and two low tides are observed each day as a coast encounters both tidal bulges. • Twice during each lunar month, Earth is directly aligned with the Sun and the Moon, whose gravitational effects are thereby reinforced, producing higher high tides and lower low tides.
Tide-Raising Force (3) • In the open sea tides are small (less than 1 m). • Along most coasts the tidal range commonly is less than 2 m. • In bays, straits, estuaries, and other narrow places along coasts, tidal fluctuations are amplified and may reach 16 m or more. • Associated tidal currents (潮汐水流) are often rapid and may approach 25 km/h. • The incoming tide locally can create a wall of water a meter or more high (called a tidal bore).
Ocean Waves (1) • Ocean waves receive their energy from winds that blow across the water surface. • The water particles move in a loop-like, or oscillating manner. • Because waveform is created by this loop-like motion of water parcels, the diameters of the loops at the water surface exactly equal wave height (波高).
Ocean Waves (2) • Downward from the surface, a progressive loss of energy occurs, resulting in a decrease in loop diameter. • “L” is used to represent wavelength (波長), the distance between successive wave crests or troughs. • At a depth equal to half the wavelength (L/2), the diameters of the loops have become so small that motion of the water is negligible.
Wave Base • The depth L/2 is therefore referred to as the wave base (浪基面或波底). • Landward of depth L/2, as the water depth decreases, the orbits of the water parcels become flatter until the movement of water at the seafloor in the shallow water zone is limited to a back-and-forth motion.
由於摩擦力隨深度增加，水分子運動的能量隨水深而不斷減弱，到水深為波長的一半時即消失。由於摩擦力隨深度增加，水分子運動的能量隨水深而不斷減弱，到水深為波長的一半時即消失。 波底 ＝ ½波長水深 wave base = depth of ½ wave length
Breaking Waves • When the wave reaches depth L/2, its base encounters frictional resistance exerted by the seafloor. • This causes the wave height to increase and the wave length to decrease. • Eventually, the front becomes too steep to support the advancing wave and the wave collapses, or breaks.
波浪的形狀從深海進入淺海所發生的變化 波長和波速不斷減小，波高不斷增加。最後波浪愈變愈陡，圓周狀轉動的速度 終於超過波浪本身前進的速度，波浪於是崩潰，形成破浪(breaking wave 或breaker)。
Surf • Such “broken water” is called surf. • The area between the line of breaking waves and the shore is known as the surf zone (破浪帶). • Water piled against the shore returns seaward partly in localized narrow channels as rip currents (離岸流). • The geologic work of waves is mainly accomplished by the direct action of surf.
Wave Refraction (1) • A wave approaching a coast generally does not encounter the bottom simultaneously all along its length. • As any segment of the wave touches the seafloor: • That part slows down. • The wave length begins to decrease. • The wave height increases. • This process is called wave refraction (波浪折屈).