U6115 Climate and Water

Ocean Stratification and Circulation

Martin Visbeck


Take away ideas and understandings:

  1. The ocean holds 98% of the 1.4 billion cubic kilometers of water on the planet. Exchange of this water with the water vapor within the atmosphere is a key part of the global hydrological cycle. In addition, the high capacity and density of water relative to the atmosphere, and the great amount of energy required to change its phases (solid to liquid to gas) makes the ocean a powerful factor of the Earth's climate system. The ocean attenuates winter/summer and day/night changes in air temperature, and moves significant amounts of heat from low to higher latitudes, it provides moisture to the atmosphere.
  2. The link between ocean and climate depends on exchange of energy (mainly heat and radiation) and materials (water, gases) across the sea surface. The atmosphere 'sees', influences and responds to the sea surface temperature (SST), by way of sea-air heat flux. SST generally cools with increasing latitude, but important deviations from a pure latitudinal dependence occurs. These are generally due to the movement of sea water in both the horizontal and vertical directions. Temperature and density of ocean water are related inversely: warm water means low density, cold water means denser sea water.
  3. Sea water is about a 3.49% salt solution, the rest is freshwater. The more saline, the denser the sea water. As the range of salt concentration in the ocean varies from about 3.2 to 3.8%, oceanographers, who refer to salt content as 'salinity', express salt concentration as parts per thousand; 34.9 ppt is the average salinity. As sea water 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 tend 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. Waters warmer than 10C dominate the sea surface but do not extend much below 500 m in the ocean; the warm waters provide just a veneer of warmth over a cold ocean. The sharp drop off in temperature with depth is called the thermocline. Deeper cold waters derive their properties at the sea surface during winter at high latitude.
  5. The deep Atlantic is relatively saline. This water is derived from the sinking of chilled saline surface water in the northern North Atlantic, and is called North Atlantic Deep Water (NADW). 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 buoyant to sink. Towards the sea floor, temperatures are near 0C marking the presence of Antarctic Bottom Water derived from the dense water along the shores of Antarctica. At the base of the thermocline is the low salinity Antarctic Intermediate water derived from the Antarctic Circumpolar Current.
  6. Ocean circulation is produced by the wind stress acting on the sea surface and by buoyancy (heat and freshwater) fluxes between the ocean and atmosphere. The former induces the wind driven ocean circulation, the latter the thermohaline circulation. The wind driven flow is by far the more energetic and for the most part resides in the upper kilometer. The sluggish thermohaline circulation forces ocean overturning reaching in some regions to the sea floor, resulting in the formation of the major water masses of the global ocean: North Atlantic Deep Water and Antarctic Bottom Water.
  7. The wind stress produces Ekman Layer transport in the surface layer (upper 100 to 300 meters) of the ocean, which redistributes the buoyant surface water producing sea level hills in convergence regions, depressions in divergent regions, of about 1 meter amplitude. These irregularities in sea level induce horizontal pressure gradients, initiate the wind driven geostrophic currents.
  8. The ocean currents are for the most part geostrophic, meaning that the horizontal pressure gradients are balanced by the Coriolis Force.
  9. Ocean overturning, product of buoyancy exchange with the atmosphere, form specific water masses which spread at depth throughout the ocean. The thermohaline circulation engages the full volume of the ocean into the climate system, by allowing all of the ocean water to 'meet' and interact directly the atmosphere, that is to exchange heat and freshwater with the atmosphere.

Ocean Stratification

The ocean composes 70.8% of the earth's surface. Sea water fills the basins separating the continents (Fig. 1) with an average depth of 3795 meters. The transition from the continental to the deep ocean or continental margins extend from the sea shore to around 2500 meters depth, it covers 40.7% of the ocean (29% of Earth surface). The deep basins composes of extensive flat plains of 4000 to 5000 meters depth, and the mid-ocean ridge, marking the axis of sea floor spreading where the crust tectonic plates of the earth form. The deep ocean covers about 59.3% of the ocean's surface (42% of Earths surface). The deepest ocean is found in the trenches where the plates are subducted, the Mariana Trench is 11035 meters deep (compared to the 8848 meter height of Mount Everest). If the solid earth were made into a flat plain, the sea water 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 50-60S. 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 or 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 (Fig. 2), 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 state to the ocean is a topic of importance in our warming planet, in terms of rising sea level (which amounts to about 0.3 cm/year, though mostly in the last 100 years from melting of alpine glaciers). 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 reservoirs (ocean, land atmosphere) by evaporation, precipitation and river runoff is called the hydrological cycle.

The properties of water are unique, and essentially to maintaining the Earth as a habitable planet (Fig. 3). One of the most important properties 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. 4). This property gives the ocean great heat 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 (Fig. 5) 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. 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 100F! The change of temperature of the surface ocean and surface air versus latitude (Fig. 6) 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 pattern of sea surface temperature, SST, (Fig. 7) is the result of many factors. Sea air exchange of heat is the most important factor (discussed in Ocean-Atmosphere coupling lecture). Another factor is ocean circulation, including both the horizontal and vertical movements of sea water. Ocean currents, the flow of sea water along the horizontal plane, transports warm water of the tropics to higher latitudes (e.g. see the SST off the east coast of the US, where the Gulf Stream advects warm water towards the north) and cold water away from the polar regions towards lower latitudes (e.g. see the eastern margins of the North Atlantic near 30N where cooler SST is advected southward). Circulation of waters of differing temperature provide for the oceans contribution to the meridional heat flux requirement for maintaining a quasi steady state climate. The transfer of warm SST into the northern North Atlantic is a particularly important part of the climate system. The ocean heat represented by that warm surface water is slowly released to the atmosphere, warming the air that passes over northern Europe, making for a much more moderate climate that one might expect for a high latitude environment. It is the cooling of this water (cooler water is more dense than warmer water) that leads to sinking of surface water into the depths of the ocean marking the formation of North Atlantic Deep Water (discussed below). In the polar regions SST approaches 2C, this is possible because the salt within sea water lowers the freezing point by approximately 2C.

As the ocean becomes cooler with increasing depth, upwelling of water from a few hundred meters depth to the sea surface acts to cool the ocean surface. Wind induced upwelling is responsible for the cooler ocean surface in the eastern subtropical regions of each ocean. The seasonal shift of the solar radiation input causes a corresponding shift of SST (Fig. 8), which represents the extent of seasonal heat storage by the ocean.

Sea water is about a 3.49% solution of salt (Fig. 9), or about 96.5% freshwater. The more saline, the denser the sea water. 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, making 34.9 ppt the average salinity. Salinity changes the properties of water from that of pure water. 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. 10) tend to become salty, while the areas of excess rainfall become fresher (Fig. 11). The tropical belt, or Intra-tropical Convergence Zone is such an area. Ocean circulation acts to move lower salinity sea water 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 stands out. 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. The effects of ocean circulation are seen in the transfer of saline surface water into the northern North Atlantic. Cooling of this water leads to the North Atlantic Deep Water formation.

In the polar regions sea water freezes (Fig. 12). The resulting ice contains only part of the sea water salt, about 0.5% (5 ppt), hence ice formation like evaporation, concentrates salt in the remaining body of sea water. This causes very dense water (cold and salty), which in some regions in the Southern Ocean leads to deep reaching convection, called Antarctic Bottom water. The very low salinity of the Arctic prohibits the development of deep reaching convection. Southern ocean ice exhibits lots of seasonal variability, and is generally only 0.5 meters thick. There is evidence of greater amount of vertical overturning in the southern Ocean, as deep water upwells to be replaced by sinking cold polar water. In sharp contrast is the Arctic sea where the sea ice is usually about 2 to 3 meters thick with a lesser amount of seasonality, and a water column which is very stratified. There is some evidence that global warming is reducing the area of year round sea ice in Arctic, but not (yet?) within the Southern Ocean.

Once we look below the sea surface we see a richness of the temperature and salinity range (Fig. 13, Fig. 14), and further differences between the three oceans. Waters warmer than 10C 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 40N to 40S is called the thermocline. The waters below the thermocline exhibit much reduced temperature decrease with depth. 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. 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 0C marking the presence of Antarctic Bottom Water derived from the shores of Antarctica. Below the thermocline is a low salinity layer derived from the Antarctic Circumpolar Current. This water mass, made relatively fresh by excess precipitation of the circum-Antarctic belt is called the Antarctic Intermediate water. A low salinity intermediate of more limited extent forms in the North Pacific.

Oceanographers often use a temperature/salinity (T/S) diagram to determine the origin of the sea water properties (Fig. 15). A parcel of sea water 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 sea water 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.

The viewer (Fig. 16) may be used to explore the distribution of temperature and salinity at the sea surface and within the ocean interior.

 

Ocean Circulation

The ocean would have a significant role in governing climate, even if it did not circulate. The ocean mixed (surface) layer storage of solar excess heating in summer and the release of that heat to the atmosphere in winter, would mitigate the seasonal extremes of the atmosphere temperature even without an ocean circulation. However, the ocean waters do circulate, as currents carry low latitude warm water to higher latitudes where the heat can be released to the atmosphere, working with the atmosphere to balance the earth's heat budget. The horizontal circulation shapes the sea surface temperature (SST) pattern moving heat from the tropics to the polar regions (Fig. 1). The same is true for freshwater, as the ocean circulation moves saline water formed in the excess evaporative regions to the excess precipitation regions, subtropical to tropical respectively, as part of the global hydrological cycle.

The ocean circulation includes flow in the horizontal, such as flow along the sea surface, and flow in the vertical that overturns the ocean, engaging the cold deep interior in the climate system, by coupling it to the atmosphere. The ocean circulation, both horizontal and vertical is induced by the wind acting on the sea surface and by buoyancy fluxes between the ocean and atmosphere. The buoyancy forces are capable of inducing overturning that reach from the sea surface to the sea floor. Buoyancy fluxes are those fluxes between air and water that alter the density of the sea water. Cooling of the ocean make the ocean denser, removing buoyancy; evaporation makes the ocean saltier and hence denser, again removing buoyancy. Heating and excess precipitation has the opposite effect, they add buoyancy to the ocean. These topics are covered in the next lecture.

Oceanographers usually divide the ocean circulation into two parts, the wind driven and the thermohaline or buoyancy driven circulation. (Ocean tidal currents are not dealt with in this lecture, some oceanographer think their effects on ocean mixing is important to the climate system.) We will first deal with the wind driven circulation, which by far is the more energetic, though confined mostly to the upper kilometer of the ocean and generally moves water in the horizontal plane. A important product of the wind driven circulation is the Gulf Stream off the east coast of the US (Fig. 2). The Gulf Stream, as other western boundary currents, advects (transports) very warm tropical water to high latitudes and is thus a major player in meridional heat flux. Much of this heat reaches into the northern North Atlantic, to heat the atmosphere over northern Europe. It is the resultant cooling of Gulf Stream and its northward extension that make the surface water dense enough to sinking into the deep North Atlantic as part of the thermohaline circulation associated with globally important North Atlantic Deep Water.

As mentioned above most of the ocean circulation is driven by the wind (Fig. 3). The surface winds are part of the global atmospheric. The trades winds blow towards the equator. The trade winds of the two hemispheres meet at the intratropical convergence zone (ITCZ), where updrafts of air induce high precipitation. The ITCZ falls over the warmest band of SST. Note that on annual average it occurs in the northern hemisphere, near 5N. Poleward of the trades are the westerlies, and still further poleward are the polar easterlies. The westerlies are associated with the strongest wind, but often very variable, as the wind field there is shaped by storms.

How, does the wind induce an ocean circulation? The wind exerts a force or stress on the ocean surface. This stress is proportional to the square of the wind speed. This produces ocean waves and ocean currents. (As with tides, we do not deal with ocean waves in this course.) The wind makes the surface layer of the ocean move, though not in the way that intuition might dictate - its not in the direction of the wind stress, but rather at an angle to it. This is because of the Coriolis Force. Eventually a balance is achieved between the wind stress and the Coriolis Force (Fig. 4). The surface Ekman layer (named after the person who developed the theory around the turn of the century, 1908 to be precise) extends to about 100 to 300 meters depth, is a boundary layer feature in which the direct stress of the wind are felt. The transport within the Ekman layer is 90 towards the right of the wind in the northern hemisphere, to the left in the southern hemisphere. The Ekman transport is proportional to the wind stress, which is proportional to the square of the wind speed. For those more interested in numbers: wind stress = drag coefficient * (wind speed)2. With the drag coefficient equal to around 2 * 10-3. Typically a surface current is around 2 or 3% of the wind speed. One clear effect of the Ekman transport can be seen in the eastern side of the subtropical ocean (Fig. 5), where cold subsurface water is pumped up to the sea surface from a depth of perhaps 200 meters as the sea surface water is forced offshore by the Ekman transport. These regions are rich in nutrients and support important fisheries. The cold SST of these regions also induce a specific climate, called Cape Verde climate, one of a very stable atmosphere, cool, fog, few storms.

It is this movement that produces the wind driven circulation of the ocean. How? Well the wind field changes in its strength and direction from place to place. This causes Ekman transport to either pile up water (convergence) in some places or remove it (divergence) in other regions. As surface water is less dense than deeper water (the ocean is stable, less dense water at top, denser water at the bottom) this has the effect of heaping buoyant water in the convergence regions and removing it from the divergence regions. The hills and valleys of the sea surface produced by the convergence and divergent causes a sea level relief (difference from lowest to highest sea level, neglecting tides and waves) of around 2 meters. To understand the reason hills are produced at the convergences, depressions in the divergences, one needs to invoke the hydrostatic relationship: the hills of buoyant water are balanced by a 'root' of buoyant water, much as a ship or iceberg floats. Removal of buoyant water in the Ekman divergences induce a depression in sea level and upward bowing of the deeper dense water (Fig. 6). The vertical movement of water at the base of the Ekman Layer act to distort the density fields within the deeper water, effectively passing horizontal pressure gradients into the ocean interior.

The pattern of the induced currents are governed not just by the wind stress and its Ekman transport but also by the earth's rotation which causes the Coriolis Force. The Coriolis Force acts at right angles (90) to the ocean current (or wind) direction, to the right in the north hemisphere to the left in the southern hemisphere. The Coriolis Force strength is proportional to the strength of the ocean current. In short: the wind produces convergences and divergences of surface water, which causes hills and depressions in sea level, which produce a horizontal gradient of pressure, or a pressure head reaching down to perhaps 1000 meters. Convergent hills have an outward directed pressure gradient, depression an inward directed pressure gradient (Fig. 7). As the pressure gradients make the water move from high pressure to low pressure, the Coriolis Force starts its action, and eventually a balance is achieved in these two forces, the horizontal pressure gradient equals the magnitude of the Coriolis Force, but is directed in the opposite direction. This balance is called the geostrophic balance, and a current in such a balance is called a geostrophic current. Ocean currents are very close to being in geostrophic balance (Fig. 8, Fig. 9).

The thermohaline circulation is discussed further in the next lecture, but the effects of that circulation is clearly revealed in the ocean stratification (Fig. 10, Fig. 11, Fig. 12, Fig. 13; Figures 10-13 are from the Vertical Section Atlas compiled by Lynne D. Talley at SIO/UCSD.). The various layers of cold water, of layers with high or low salinity all emanate from the sea surface where buoyancy fluxes produce water capable of sinking. The wind and thermohaline forces are really difficult to separate, as they are well linked by the factors that govern sea-air heat, freshwater and momentum fluxes, next lecture.


Text by Arnold Gordon, 2000.