Ocean-Atmosphere Coupling

Lecture Figures (powerpoint); Additional figures (powerpoint).

Take away ideas and understandings:

  1. Meridional heat and freshwater transfer: The ocean and atmosphere work together to move heat and freshwater across latitudes, as required to maintain a quasi-stationary climate pattern. The wind-driven and thermohaline ocean circulation accomplish this task for the ocean, by moving warm waters poleward, colder water toward the Equator. On average the ocean meridional heat flux is higher or at least equivalent to that of the atmosphere between the equator and 30° latitude, with the atmosphere becoming dominate at higher latitudes. Ocean currents of differing salinity also move freshwater from place to place to close the global hydrological budget. For example, salty water flows away from the evaporative subtropics to be replaced with lower salinity water from the tropics.
  2. Fluxes across the sea-atmosphere interface: Heat exchange between ocean and atmosphere is a product of a number of processes: solar radiation heats the ocean; net long wave back radiation cools the ocean; heat transfer by conduction and convection between the air and water generally cools the ocean as does evaporation of water from the ocean surface
  3. Any imbalance of the heat or freshwater budgets due to sea-atmosphere fluxes is compensated by transfer of heat and freshwater by ocean currents. Generally heat transport across latitudes is from the tropics to the polar regions, but in the South Atlantic Ocean the oceanic heat transport is directed towards the equator. This is due to the thermohaline circulation - as warm upper kilometer water is carried northward, across the equator, offsetting the southward flow of cooler North Atlantic Deep Water near 3000 m. Much of the heat lost to the atmosphere in the North Atlantic is derived from this cross equatorial heat transfer. The flux of freshwater in the North and South Atlantic is southward, as freshwater excess of the Arctic is brought into off set the net evaporation and influx of salty water from the Indian Ocean

To maintain an approximate steady state climate the ocean and atmosphere must move excess heat from the tropics to the heat deficit polar regions (Fig. 1). Additionally the ocean and atmosphere must move freshwater to balance regions with excess dryness with those of excess rainfall. The movement of freshwater in its vapor, liquid and solid state is referred to as the hydrological cycle.

In low latitudes the ocean moves more heat poleward than does the atmosphere (Fig. 2), but at higher latitudes the atmosphere becomes the big carrier. The wind driven ocean circulation moves heat mainly on the horizontal plane. For example, in the North Atlantic, warm surface water move northward within the Gulf Stream on the western side of the ocean, to be balanced by cold surface water moving southward within the Canary Current on the eastern side of the ocean (see ocean circulation lecture). The thermohaline circulation moves heat mainly in the vertical plane. For example, North Atlantic Deep Water with a temperature of about 2°C flows towards the south in the depth range 2000 to 4000 meters to be balanced by warmer water (greater than 4°C) flowing northward within the upper 1000 meters.

The ocean role in climate would be zero if there were an impervious lid over the ocean, but there is not, across the sea surface pass heat, water, momentum, gases and other materials (Figs. 3, 4, 5, 6). The wind exerts a stress on the sea surface that induces the Ekman transport and wind driven circulation as discussed in the ocean circulation lecture.

Sea-Air Heat Exchange (Fig. 3)

Solar Radiation: Much of the direct and diffuse solar short wave (less than 2 micros, mostly in the visible range) electromagnetic radiation that reaches the sea surface penetrates the ocean (the ocean has a low albedo, except when the sun is close to the horizon), heating the sea water down to about 100 to 200 meters, depending on the water clarity. It is within this thin sunlit surface layer of the ocean that the process of photosynthesis can occur. Solar heating of the ocean on a global average is 168 watts per square meter.

Net Back Radiation: The ocean transmits electromagnetic radiation into the atmosphere in proportion to the fourth power of the sea surface temperature (black-body radiation). This radiation is at much longer wavelengths than that of the solar radiation (greater than 10 micros, in the infrared range), because the ocean surface is far cooler that the sun's surface. The infrared radiation emitted from the ocean is quickly absorbed and re-emitted by water vapor and carbon dioxide and other greenhouse gases residing in the lower atmosphere. Much of the radiation from the atmospheric gases, also in the infrared range, is transmitted back to the ocean, reducing the net long wave radiation heat loss of the ocean. The warmer the ocean the warmer and more humid is the air, increasing its greenhouse abilities. Thus it is very difficult for the ocean to transmit heat by long wave radiation into the atmosphere; the greenhouse gases just kick it back, notably water vapor whose concentration is proportional to the air temperature. Net back radiation cools the ocean, on a global average by 66 watts per square meter.

Conduction: When air is contact with the ocean is at a different temperature than that the sea surface; heat transfer by conduction takes place. On average the ocean is about 1 or 2 degrees warmer than the atmosphere so on average ocean heat is transferred from ocean to atmosphere by conduction. The heated air is more buoyant than the air above it, so it convects the ocean heat upward into the atmosphere. If the ocean were colder than the atmosphere (which of course happens) the air in contact with the ocean cools, becoming denser and hence more stable, more stratified. As such the conduction process does a poor job of carrying the atmosphere heat into the cool ocean. This occurs over the subtropical upwelling regions of the ocean. The transfer of heat between ocean and atmosphere by conduction is more efficient when the ocean is warmer than the air it is in contact with. On global average the oceanic heat loss by conduction is only 24 watts per square meter.

Latent Heat: The largest heat loss for the ocean is due to evaporation, which links heat exchange with hydrological cycle (Fig. 4). On global average the heat loss by evaporation is 78 watts per square meter. Why so large? Its because of the large heat of vaporization (or latent heat) of water, a product of the polar bonding of the H2O molecule, as discussed in the Ocean Stratification lecture. Approximately 570 calories (2.45 x 106 joules) are needed to evaporate one gram (kilogram) of water! A gram of water is roughly one cubic centimeter, amounts to a loss of one centimeter of water per a square centimeter of ocean surface area. The water vapor leaving the ocean is transferred by the atmosphere eventually condensing into water droplets forming clouds, releasing its latent heat of vaporization in the atmosphere, usually quite remote from the site of the evaporation, thus representing a significant form of heat transfer, later heat transfer.

The annual heat flux between ocean and atmosphere (Fig. 5) is formed by the sum of all of the heat transfer process: solar and terrestrial radiation; heat conduction and evaporation. While the ocean gains heat in low latitudes and losses heat in high latitudes, the largest heat loss is drawn from the warm Gulf Stream waters off the east coast of the US during the winter, when cold dry continental air spreads over the ocean. An equivalent pattern is found near Japan, where the Kuroshio Current is influenced by the winter winds off Asia. It is in these regions that the atmosphere takes over as the major meridional heat transfer agent.

The annual freshwater flux between ocean and atmosphere (Fig. 4) reflects the water vapor content (relative humidity) of the atmosphere, resulting from the general circulation of the atmosphere. The dry regions of the subtropics where the air subsides along the poleward edges of the Hadley Cell; the rainy Intertropical Convergence Zone (ITCZ) where the trades winds of northern and southern hemisphere meet, forcing updrafts of air.

In a steady state condition ocean currents must carry heat from the ocean areas with excess heating to regions with a deficit of heat, similarly with freshwater fluxes (Figs. 7 and 8). The meridional flux of mass at various depths within the ocean, or of heat or freshwater flux varies not just with latitude, but also between the ocean basins (Figs. 9, 10, 11). Both the thermohaline and wind driven circulation contribute to these meridional fluxes. It is difficult to calculate the heat and freshwater exchange, as measurements of ocean and air temperature, of atmospheric humidity and winds strength, all of which govern the sea-air coupling, are sparse and often inaccurate. This leads to large uncertainties in the meridional ocean heat flux. But even with uncertainty in mind, the South Atlantic stands out as anomalous.

Generally heat transport across latitudes is from the tropics to the polar regions, but in the South Atlantic Ocean the oceanic heat transport is directed towards the equator! This is due to the thermohaline circulation- as warm upper kilometer water is carried northward, across the equator, offsetting the southward flow of cooler North Atlantic Deep Water near 3000 m. Much of the heat lost to the atmosphere in the North Atlantic is derived from this cross equatorial heat transfer. The flux of freshwater in the North and South Atlantic is southward, as freshwater excess of the Arctic is brought into off set the net evaporation and influx of salty water from the Indian Ocean.

As mentioned in previous lectures sea-air fluxes in polar and sub-polar regions drives the thermohaline circulation. In the northern North Atlantic winter cooling produces very deep-reaching mixed layers (often called convective "chimneys") that inject cold water into North Atlantic Deep Water (Figs. 12, 13), ventilating the deep ocean with water recently in contact with the atmosphere. In the Southern Ocean (Fig. 14) subsurface deep water is relatively warm (1° to 2°C); the surface water is cold, near the freezing point of seawater, -1.9°C, covered with sea ice in winter. If the warm subsurface water were to reach the sea surface it would prohibit the formation of winter sea ice. This generally does not happen, but in the mid-1970s it did, at least in one region near the Greenwich Meridian (Figs. 14, 15), causing a "polynya". During this event winter mixed layers reached down to 3000 m, vigorously ventilating the deep ocean.


Text by Arnold Gordon, 2004.