Ocean Stratification

Lecture slides (powerpoint)

Take away ideas:

  1. Water & Climate: The ocean extends over 70.8% of the earth's surface. The ocean holds 98% of the 1.4 billion cubic kilometers of water on the planet, divided within three major basins. The high capacity and density of water relative to the atmosphere, and the great amount of energy required for change of phase of water (solid - liquid - vapor) makes the ocean a powerful and stabilizing force of the Earth's climate system. One obvious consequence of the ocean's influence is the "marine effect", which acts to attenuate winter/summer and day/night extremes of air temperature. Another is that the ocean circulation transfers significant amount of heat from low to higher latitudes, helping the climate system to attain an approximate steady state condition. The ocean is a key part of the global hydrological cycle, providing moisture for the atmosphere; ocean circulation of freshwater balances the net evaporative and precipitation belts.
  2. Sea Surface Temperature: The temperature of the sea surface is high (27-30°C) near the equator, often the maximum value occurs a few degrees of latitude north of the equator and low (at the sea water freezing point of ®1.9°C) within the polar oceans. However, there are also changes of sea surface temperature with longitude. Warmer water projects poleward along the western boundaries of the ocean. The eastern tropical regions of each ocean are cooler than the western tropical margin. These are due to the movement of seawater in the horizontal (ocean currents) and vertical (upwelling/sinking) directions. Temperature and density of ocean water are related inversely: warm water means low density, cold water means denser seawater. The salt content of the water also affects Ocean density.
  3. Sea Surface Salinity: Sea water is about a 3.49% salt solution, the rest is freshwater. The more saline, the denser the seawater. As the range of salt concentration in the ocean varies from about 3.2 to 3.8%, oceanographers refer to salt content as 'salinity', express salt concentration as parts per thousand; 34.9 ppt is the average salinity. As seawater evaporates the salt remains behind, only the freshwater is transferred from the ocean to the atmosphere. A region of excess evaporation, such as the subtropics tends to become salty, while the areas of excess rainfall become fresher. Sea ice formation also removes freshwater from the ocean, leaving behind a more saline solution. Along the shores of Antarctica this process produces dense water. Salinity reflects the workings of the hydrological cycle: the movement of freshwater through the earth/ocean/atmosphere system.
  4. Below the Sea Surface: Waters warmer than 10°C dominate the sea surface but do not extend much below 500 m within the ocean; tropical and subtropical surface water provide is just a veneer of warmth over a cold ocean; typical deep ocean temperature vary from ®1° to 3°C. The sharp drop off in temperature with depth is called the thermocline. The warm surface water is generally saltier than the cooler deep or polar waters. The halocline marks the drop of salinity with depth that accompanies the thermocline. The surface water warmth overrides the saltiness in governing density, so that the warm surface water regions coincide with buoyant (less dense) water. In polar regions buoyancy of the surface layer is mainly a consequence of the freshness of the surface water. Deep cold waters derive their properties at the sea surface during winter at high latitude.
  5. Deep Water Masses: The deep Atlantic is relatively salty (34.9). This water is derived from the sinking of chilled saline surface water in the northern North Atlantic. The cooling makes the surface water dense, forcing it to sink, or convect into the deep ocean, and spread southward at depth. It is called North Atlantic Deep Water (NADW). In contrast the deep Pacific is lower in salinity (34.7), as it experiences no deep convection of cooled salty surface water, its surface layer is too fresh and buoyant to sink. Pacific deep water is derived from the lower salinity water column of the southern ocean. Towards the sea floor, temperatures are near 0°C marking the presence of Antarctic Bottom Water (AABW) derived from the very cold (-1.9°C; 34.65), dense water along the shores of Antarctica. At the base of the thermocline is the low salinity Antarctic Intermediate Water (AAIW) derived from sinking of cool (3° to 4°C), low salinity waters (34.4) from 50° - 60°S marking the Antarctic Circumpolar Current and ocean polar front zone.

The ocean extends over 70.8% of the earth's surface. Seawater fills the basins separating the continents (Fig. 1) with an average depth of 3795 meters. The continental margins, extending from the seashore to around 2500 meters depth, cover 40.7% of the ocean (29% of Earth surface). The deep ocean covers about 59.3% of the ocean's surface (42% of Earth's surface). The extensive flat plains of the deep basins range 4000 to 5000 meters in depth. The mid-ocean ridge, marking the spreading axis of the crust tectonic plates is marked by rugged topography; the ridge crest is about 2500 m deep. The deepest ocean is found in the trenches where the plates are subducted, the Mariana Trench is 11,035 meters deep (compared to the 8848 meter height of Mount Everest). If the solid earth were made into a flat plain, the seawater would cover the entire earth to a depth of 2440 meters. If all of the water vapor in the atmosphere were converted to liquid it would cover the smoothed earth surface by about an 1-inch.

The ocean basins are divided into three main Oceans; the Pacific Ocean is the largest and deepest (52% of the ocean area, mean depth of 4028 meters); Indian (20% area, mean depth of 3897 m) and the Atlantic Ocean, the shallowest because of the rather narrow deep basins (25% area, mean depth of 3332 m). The arctic is considered part of the Atlantic Ocean; the southern parts of the three Oceans are referred to as the Southern Ocean. The northern hemisphere has less ocean than the southern hemisphere, only about 61% ocean versus 81% for the southern hemisphere. This may account for the more extreme seasonal swing of air temperatures (a more continental climate) experienced over the northern hemisphere.

The oceans are connected by a deep circum-Antarctic channel near 50-60°S. Within this channel is the Antarctic Circumpolar Current (ACC) which carries water from west to east at a rate of 130 million cubic meters per second (this unit of ocean water transport is called a Sverdrup, Sv, the ACC transports 130 Sv, about 100 times the outflow of all of the earth's rivers). The oceans differ slightly in their properties, e.g. the Pacific is the freshest ocean, the Atlantic the saltiest. The differences are not large however thanks to the ACC, which acts, to blend the ocean waters.

98% of the 1.4 billion cubic kilometers of water on the planet resides in the Ocean (Figs. 2, 3); the second largest reservoir is the glacial ice caps of Antarctic and Greenland, amounting to around 1.6%. The transfer of water from the glacial ice to the ocean is a topic of great concern in our warming climate, as it leads to a rising sea level (about 0.3 cm/year in the last 100 years from melting of alpine glaciers and thermal expansion brought about by warming of sea water. If all of the water in the atmosphere were removed it would cover the earth surface only by about 1 inch. Movement of water between the primary water reservoirs (ocean, land, atmosphere, cryosphere) by evaporation, precipitation and river runoff is called the hydrological cycle.

The properties of water (Figs 4, 5, 6) are unique, and essential to maintaining the Earth as a habitable planet. One of the most important attributes of water in terms of the Earth's climate system is the great amount of heat that is required to warm water or change it from a solid to a liquid or to a vapor (Fig. 6,7). This property gives the ocean great Ýthermal inertia”, that is it takes a lot of energy to change the temperature or phase-state of water, heat that would otherwise reside in the atmosphere. It is this property that confines the atmospheric temperature to a narrow range relative to a dry earth. The ocean's great heat capacity takes up excess heat of summer, releases it to the atmosphere in the winter (a similar effect occurs between day and night). This has a modifying effect on the seasonal swings of the air temperature (Fig. 8). The change of temperature of the surface ocean and surface air versus latitude (Fig. 8) reveals not just the extent of the meridional change of temperature from the heat excess of the tropics to the heat deficit polar regions, but also the more 'volatile' nature of the air temperature. The closer to the ocean the greater the attenuation of seasonality. The center of the northern continents experiences the greatest difference between winter and summer air temperature. Siberia experiences a seasonal swing of about 100°F! A marine climate experiences a characteristic seasonal swing of <10°C.

The pattern of sea surface temperature, SST, (Fig. 9) is the result of many factors. The exchange of heat and water between the sea surface and the atmosphere is the most important factor (discussed in Ocean-Atmosphere Coupling lecture). Another factor is ocean circulation, including both the horizontal and vertical movements of seawater. Ocean currents, the flow of seawater along the horizontal plane, transports warm water of the tropics to higher latitudes. The SST off the east coast of the US (Fig. 10) clearly reveals the presences of the northward flowing Gulf Stream that advects warm water towards the north. Ocean currents directed towards lower latitude project cooler SST along the eastern boundaries of the mid-latitudes: the eastern margins of the North Atlantic near 30°N where cooler SST is advected southward. Circulation of waters of differing temperature provide for the ocean's contribution to the meridional heat flux requirement for maintaining a quasi-steady state climate (Fig. 11; discussed in the Ocean-Atmosphere Coupling lecture). The transfer of warm SST into the northern North Atlantic is a particularly important part of the climate system (discussed below). In the polar regions SST approaches -2°C, this is possible because the salt within seawater lowers the freezing point of water from 0°C for freshwater by approximately ®1.9°C for seawater. The seasonal shift of the solar radiation input causes a corresponding shift of SST, which represents the extent of seasonal heat storage by the ocean.

As the ocean becomes cooler with increasing depth, upwelling of water from about hundred meters depth to the sea surface acts to cool the ocean surface. Wind induced upwelling, called Ekman upwelling, is responsible for the cooler ocean surface along the coasts of the eastern subtropical regions of each ocean (Fig. 9). Cooler SST along the Equator (best seen in the Pacific Ocean on Fig. 9) is also a result of Ekman upwelling. Ekman upwelling produces regions of sea surface divergences. Winds may also induce sinking or convergences of surface water. Such divergence/convergences lead to general ocean circulation (discussed in the Ocean Circulation lecture).

Seawater is about a 3.5% solution of salt (Fig. 12), about 96.5% freshwater. As the range of salt concentration varies from about 3.2 to 3.8%, oceanographers, who refer to salt content as 'salinity', express salt concentration as parts per thousand (ppt), 3.5% is equivalent to 35.0 ppt. (By international convention salinity is expressed without units: a solution of 3.5% has a salinity of 35.0.) Salinity changes the properties of water from that of pure water (Figs. 12, 13), for example as salinity increases: the water freezing point is lowered; density is increased. While density is inversely proportional to temperature (colder-denser), the relationship of salinity to dense is direct: increased salt content makes the water denser. In addition, seawater has a slightly lower heat capacity and coefficient of vaporization than does freshwater, but the change is not enough to alter the unique role of water in the climate system.

As sea water evaporates the salt remains behind, only the freshwater is transferred from the ocean to the atmosphere, hence a region of excess evaporation, such as the subtropics (Fig. 14) tend to become salty (Fig. 15), while the areas of excess rainfall become fresher. The tropical belt, or Intra-tropical Convergence Zone is such an area. Ocean circulation acts to move lower salinity seawater into evaporative regions, and more saline water into humid regions, this is part of the hydrological cycle.

The relative freshness of the Pacific Ocean surface water contrasted sharply with the surface salinity of the Atlantic Ocean (Fig. 15). Excess evaporation of the Atlantic and excess precipitation of the Pacific are balanced to some measure by an atmospheric flux of water vapor over Central America, amounting to 0.35 Sv. The Arctic Sea is very fresh, due to the enormous amount of river water that drains into it from the northern continents.

In the polar regions seawater freezes (Fig. 16). Southern ocean ice exhibits lots of seasonal variability, and is generally only 0.5 meters thick. Arctic sea ice is thicker, ranging from 2 to 3 meters. The sea ice contains only part of the seawater salt, about 0.5% (5 ppt), hence ice formation like evaporation, concentrates salt in the remaining body of seawater. This causes very dense water (cold), which in some regions in the Southern Ocean leads to deep reaching convection forming a water mass that spreads along the sea floor well into the northern hemisphere. This water mass is called Antarctic Bottom Water (see below). The very low salinity of the Arctic prohibits the development of deep reaching convection in the Arctic Sea.

Once we look below the sea surface we see a richness of the temperature and salinity range (Figs. 20), and further differences between the oceans (Fig. 19). Waters warmer than 10°C which dominate the sea surface do not extend much below 500 m in the ocean; the warm waters provide just a veneer of buoyant warmth over a basically very cold dense ocean. The sharp drop off in temperature with depth, characteristic of the ocean between 40°N to 40°S is called the thermocline. In the salinity field the surface tropical and subtropical ocean is salty, with the deeper waters somewhat fresher. The rapid decrease of salinity with depth, accompanying the thermocline, is called the halocline. The deep Atlantic is relatively saline. This water is derived from the sinking of cooling of saline surface water in the northern North Atlantic (Fig. 20). This is the North Atlantic Deep Water. In contrast the deep Pacific is relatively fresh, as it experiences no deep convection of cooled salty surface water, its surface layer is too fresh and thus buoyant to sink into the deep ocean, at reasonably cool subpolar SST. The deep Atlantic is also warmer than the deep Pacific, as the saline North Atlantic Deep water is sufficiently dense water to inject relatively warm water into the deep layer. Towards the sea floor temperatures reach below 0°C marking the presence of Antarctic Bottom Water (AABW) derived from the shores of Antarctica (Fig. 21). Below the thermocline is a low salinity layer derived from the surface water of the Antarctic Circumpolar Current and the associated ocean polar frontal zone. This water mass, made relatively fresh by excess precipitation of the circum-Antarctic belt is called the Antarctic Intermediate Water (AAIW). The sequence of AAIW, NADW, AABW is clearly seen in the meridional circulation of the Atlantic Ocean (Fig. 22,23) and around Antarctica (Fig. 23).

Additional Information:

Oceanographers often use a temperature/salinity (T/S) diagram to determine the origin of the seawater properties (Fig. 24). A parcel of seawater achieves its temperature and salinity at the sea surface in response to sea-air heat and freshwater exchange. Its surface derived T/S values change within the ocean interior only by mixing with other water parcels. Hence seawater spreading from the surface into the ocean volume can be to trace by T/S properties. That 75% of the ocean volume falls with a narrow range of temperature and salinity indicates that only a small part of the sea surface contributes to the characteristics of the deep ocean. As noted above these are the North Atlantic and the Southern Ocean.

Wind induced upwelling and sinking has an effect on the chlorophyll within the ocean surface layer, a marker of phytoplankton (Fig. 25). Upwelling of cool sub-surface water provides nutrients promoting the growth of phytoplankton, beginning the food chain. The light blue, green and red areas for the ocean (Fig. 25) denote regions of high chlorophyll. Low chlorophyll areas are shown in dark blue. Compare the chlorophyll map with the SST map (Fig. 11).

Besides seasonal variability of SST, there is also interannual variability. One of the most striking and important to the global climate system is the east-west shift of SST in the tropical Pacific Ocean associated with El Niño (Fig 26), a topic of a later lecture.

The temperature of the ocean has been slowly warming in the 20th century due to the rapid increase in greenhouse gases (Fig. 27). There has also been some freshening of the surface layer due to glacial ice melt.

The ocean viewer may be used to explore the distribution of temperature and salinity at the sea surface and within the ocean interior.

Prepared by Arnold L. Gordon, 2004.