EarthsClimate_Web_Chapter.pdf, p. 22-24 Lecture 7: The Oceans (1) EarthsClimate_Web_Chapter.pdf, p. 22-24 This lecture is the first of the three-part series about the role of oceans in the global climate system. The materials correspond to the Ruddiman book, Ch. 2, p. 39-44. Additional reading info is provided in the course webpage.

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 Let’s first begin the general features of the oceans. Ask the students first what they can come up with a list of as many features as possible. Then showing the list. 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 Temperature: heated from above. Salinity: higher in the lower layers as the surface layers are subject to precipitation and runoff. Unlike the atmosphere, which is unstable in the troposphere, ocean is a stable system such that the warm light waters in the surface and cold and heave salty water in the bottom. 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 However, the surface currents in the oceans are primarily driven by the winds. Once the water move, they are subject to the Coriolis effects. Different parts of the oceans have different sea levels. They are not LEVEL. Therefore, there is a pressure gradient force, which moves the water from high levels to low levels. Global currents in each of the major ocean basins show a giant circular pattern – called gyre. These surface currents are important to distribute the heat around the globe. 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 Currents turn poleward where they encounter land barriers (e.g., Gulf Stream) How do the winds drive ocean currents? 4. Eastward flow of currents is enhanced by the Westerlies 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 The effects of the surface currents are significant on redistribution of the heat in the world, thus making climate equable.

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 by swirling through the cooler water to the north and east. An example of the heat transfer in the North Atlantic Ocean associated with the Gulf Stream.

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 Need to explain an important concept of the Ekman Spiral, which explains how the surface winds act on the surface ocean waters progressively from the surface down to about 100 m below the surface. The spiral is produced because of the Coriolis effects, which always deflect the moving water to the right of its intended direction in the northern hemisphere. (to the left in the Southern Hemisphere.)

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). With the Spirial concept, let’s look at again the subtropical gyre. 4. Spinning clockwise in the northern hemisphere – a balancing act of pressure gradient force and Coriolis force.

Upwelling and Downwelling a. Deflection of water away from continent b. Upwelling of deeper water to replace surface water c. Commonly nutrient rich The Ekman Spirial and the Ekman transport (i.e. the net transport of the surface water at a right angle away from the surface wind) can explain uppwelling and downwelling. 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. An example of upwelling and the ocean surface temperature.

Zones of upwelling in global oceans Globally, where are the upwelling zones? Notice the Coastal Peru is rich in nutrients, good for fish industry. El Nono can interrupt this – to be discussed on Friday’s class.