OC103 Lesson #13: Chemical and Physical Structure of the Oceans Almost all of the ocean is very dark and cold (just a few degrees above freezing). I know that sounds like western Oregon in December, but this is much worse. Only the upper few hundred meters of the ocean ever sees any sunlight or ever warms up much above freezing. Not surprisingly, that upper few hundred meters is where most of the biological activity takes place. This lesson covers some of the chemical and physical properties of the ocean, how they vary with depth and from place to place, and how that affects biological activity. Color • Why is the Ocean Blue (or Greenish-Blue)? o Colors other than blue (and sometimes green too) are absorbed by water molecules. Only blue (and some green) are reflected back to your eye, reds and yellows are absorbed by the water. • Light Absorption in the oceans o 2/3 of the sunlight striking the ocean surface is absorbed by the first several meters of seawater. That absorbed light energy converts to heat, which significantly warms the upper several meters of the surface ocean. o Red and yellow light are absorbed first, green and blue light penetrate farther into the ocean before they are absorbed. The figure at right shows how deep the different colors of light penetrate into the ocean. o 99% of light is absorbed in the top 100 m or so, so at about 100 m the light is very dim, and gets even dimmer down deeper. These shallower depths, where 99% of the light is, are called the photic zone because photosynthetic plants (of which there are many in the oceans) can survive there. This 100 m depth for the photic zone is an average that varies depending upon the amount of suspended sediments and organisms in the water. In areas of unusually clear water, the photic zone may extend as deep as 150–200 m; and in areas of unusually cloudy water, it may only extend to 20–50 m deep. o Below about 100 m is the aphotic zone, where photosynthesis is not possible because there is not enough light.Temperature Remember from the previous lesson how the high heat capacity of water makes it hard to warm up and cool down the ocean? Although the ocean does not experience the 10–20°C daily swings in temperature that can occur on land, over the course of days, and with the seasons, the surface temperature of the ocean can change. Warming by sunlight and cooling by the atmosphere are the major causes of this. We can observe ocean temperatures in two ways: • Scientists can make maps of global sea surface temperatures using sensors on satellites. A map of sea surface temperatures in August (see below; red areas are warmer, blue areas are colder) shows the strong solar warming near the Equator due to the intense sunlight there. Surface currents on the west side of the Atlantic and Pacific spread this warm water away from the equator, as shown by the arrows in the figure.Temperatures in the Ocean • Scientists can also measure temperature at any given spot in the ocean by lowering instruments into the ocean on cables and generating a vertical temperature profile for a location. Comparing vertical profiles taken at the same place at different times of year shows how the sun significantly warms the upper part of the ocean during the summer. The figure at right is a set of typical profiles showing how the upper 50–60 m of the ocean is cold in the winter, but the longer days of summer sunshine lead to gradual warming of the upper ocean. By mid to late summer the uppermost part of the ocean in this example is nearly 10°C warmer than the water just 30–40 m deeper. The transition layer where there is a sharp change in temperature with depth (at about 20-60 m in this example, but can be as deep as 1000 m in places) is called the thermocline. Have you ever been swimming in a lake and dove down deep enough to pass into much colder water? Many lakes in temperate regions also have thermoclines caused by seasonal solar heating of the surface layer. • Solar heating in temperate and tropical areas of the ocean is typically the biggest factor causing this warm upper layer. Notice how on the left side of the figure below (showing a comparison of vertical temperature profiles) that the deep ocean is just as cold in temperate and tropical areas as it is near the poles, but the surface ocean is warmer as you approach the equator. The right side of the figure shows this graphically along a line from the South to the North Pacific. Note that almost all of the water in the oceans is very cold (<4°C), but there is a thin surface layer of warmer water (except near the poles) separated from the cold deep water by a thermocline.Salinity The salinity of the surface ocean varies, generally ranging from 34‰ to 37‰ (parts per thousand). The Atlantic Ocean is generally saltier than the Pacific Ocean (see map at right), even though more rivers empty into Atlantic. The reason the Atlantic is saltier is because there is a competition between evaporation and precipitation in the oceans, and in the Atlantic precipitation loses. The climate around the Atlantic is dryer, so there is more evaporation from the ocean surface than there is precipitation. The figure at right shows how sea surface salinity is related to the balance between evaporation (which removes fresh water from the sea surface) and precipitation (which adds fresh water). Notice how the maximum sea surface salinities are in areas where the evaporation exceeds precipitation. This is mainly in subtropical areas 20–30° from the equator. These areas are the ones that show up as high-salinities (36–37‰) on the map above in both the Atlantic and Pacific. Areas right along the equator also have high evaporation, but they also have very high precipitation, and all that rain offsets the evaporation. Just as with temperature, salinity in the oceans varies with depth, and most of the variation is relatively close to the ocean surface. The figure at right shows example salinity profiles from three different places in the oceans. Notice that the salinity of the water in the upper few hundred meters varies from <34‰ to ~38‰, even though all three profiles have very similar salinities below that (~34–35‰). Using similar terminology as for the thermocline, the sharp gradient in salinity at ~200–500 m in these example profiles is called the halocline (halo is the Greek word for salt).Density The density of