Lecture 7: The Oceans (1) EarthsClimate_Web_Chapter.pdf, p. 22-24
General features of oceans Area: covers ~70% of Earth’s surface Volume: ~97% of all the water on Earth Depth: ~3.5 kilometers Albedo: 5-10%, lowest on Earth’s surface Heat capacity: high; thermal inertia: high Temperature: less variable than in the atmosphere Freezing point: –1.9°C, not 0°C Salinity: water and dissolved salts; most common salt: table salt (NaCl). Density: 1034-1035 kg/m3 (greater than pure water 1000kg/m3) Average salinity = 35 parts per thousand (ppt) or 3.5% by weight Density depends on temperature and salinity: Cold water high density Formation of sea ice high density Evaporation high salinity high density Precipitation and river discharge low salinity low density Two main forms of circulation Surface currents: wind-driven, horizontal, surface waters, fast Deep-ocean circulation: thermohaline, vertical, deep waters, slow Surface is not level due to currents, waves, atmosphere pressure, and variation in gravity.
Vertical Profiles of Temperature and Salinity A. Unlike the atmosphere, which is heated from below, oceans are heated from above, primarily by Sun, largely at the Equator. B. Two overall layers 1. Thin, warm, less dense surface layer well mixed by turbulence generated by wind 2. Thick, cold, denser deep layer that is calm and marked by slow currents 3. Thermocline is the boundary between the layers
Surface currents Wind-induced drag force Coriolis force Pressure gradient force A result of three primary forces Hemispherical Gyres Warm currents (moving out of the tropics): e.g., Gulf Stream Cold currents (moving away from the poles): e.g., California Current
Effects of surface winds on the oceans 1. Currents converge toward Equator following Trade Winds and ITCZ 2. Westward flow along Equator (i.e., North and South Equatorial Currents) 3. Equatorial Currentsturn polewardwhere they encounter land barriers (e.g., Gulf Stream) 4. Eastward flow of currents is enhanced by theWesterlies 5. Currents turn toward the Equator where they encounter land barriers, completing the gyres
Effects of surface currents on heat transfer The equator-to-pole energy transport by the ocean is important in reducing the pole-to-equator temperature differences. Currents moving out of the tropics carry heat poleward Currents moving away from the poles carry cold water equatorward
Heat Transfer in the North Atlantic Ocean Image by NOAA’s AVHRR Satellite in June of 1984. The warm Gulf Stream current (27°C, 80°F) redistribute heat byswirling through the cooler water to the north and east.
Ekman Spiral a. The Coriolis effects cause surface current to move 20–45° from the wind direction (45° in theory) b. Deflection continues with depth, forming a spiral c. To depth of 100 m d. Net transport of water is 90° from the wind direction
More on subtropical gyres 1. Trade winds blowing SW push shallow waters toward the NW. 2. Mid-latitude westerlies blowing NE push surface water to the SE. 3. A lens of warm water piles in the center (2 meters higher than the surrounding ocean). 4. Spinning clockwise in the northern hemisphere – a balancing act of pressure gradient force and Coriolis force.
Upwelling and Downwelling b. Upwelling of deeper water to replace surface water a. Deflection of water away from continent c. Commonly nutrient rich a. Deflection of water towards continent b. Downwelling of surface water to push deep water
Cold Water Upwelling Maps of west coast sea surface temperature indicate regions of significantly cooler water that has up welled from below.