BSPH U6115 Climate and Water

Ocean-Atmosphere Coupling, El Nino Southern Oscillation (ENSO)

Martin Visbeck


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.

2.     Fluxes across the sea surface interface: Heat exchange between ocean and atmosphere across the sea surface is a product of a number of processes: solar radiation heats the ocean; long wave back radiation cools the ocean; heat transfer by conduction and convection between the air and water, generally cools the ocean; and evaporation of water from the ocean surface cools the ocean. Any imbalance of these exchange terms over the course of a year is made up by heat transfer by ocean currents. Evaporation from the ocean and precipitation on the ocean surface couple the ocean and atmosphere hydrological cycle.

3.     Mean climatology of the tropical Pacific ocean-atmosphere system.

4.     Changes in equatorial Pacific ocean and atmosphere circulation associated with El Niño and La Niña events.

5.     Two dynamic feedback processes which act to intensify El Niño and La Niña events.

6.     Why this system oscillates and the time-scale of this oscillation.

7.     Effects of El Niño/La Niña on regional and global climate.


 


I. Ocean Atmosphere Coupling

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

The ocean transmits electromagnetic radiation into the atmosphere in proportion to the fourth power of the sea surface temperature (°K). This radiation is at much longer wave lengths (greater than 10 micros, in the infrared range) than that of the sun, because the ocean surface is far cooler that the sun's surface. The net long wave radiation from the ocean surface is surprisingly uniform over the global. Why? 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 infra red 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, it just gets kicked back by the greenhouse gases, notably water vapor whose maximum concentration is proportional to the air temperature. Net back radiation cools the ocean, on a global average by 66 watts per square meter.

When air is contact with the ocean is at a different temperature than that the sea surface, heat transfer by conduction takes place. The ocean is on global average 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 into the lower atmosphere. If the ocean were colder than the atmosphere (which of course happens, just not quite as common as a warmer ocean) the air in contact with the ocean cools, becoming denser and hence more stable, more stratified. As such it does a poor job of carrying the ocean 'cool' into the lower atmosphere. This occurs over the subtropical upwelling regions of the ocean (Cape Verde climate) 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.

The largest heat loss for the ocean is evaporation, (which links heat exchange with hydrological cycle). 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 are needed to evaporate one gram 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.

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 (Fig. 6) 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. 7) 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 Intra Tropical Convergence Zone (ITCZ) where the trades winds of northern and southern hemisphere meet, forcing updrafts of air.


II. General Background about ENSO

 

III. Impacts of 1982/83 El Nino Episode (Fig 4)

 

IV. Mean Tropical Pacific Ocean-Atmosphere Climatology

 

V. General History of ENSO Research; Sir Gilbert Walker (Fig 9)

 

VI. End Members of ENSO Circulation

 

VII. Mean Sea Level Pressure (MSLP) Index of ENSO

 

VIII. General History of ENSO Research; Jacob Bjerknes (Fig 17)

 

IX. General Description of ENSO Processes: Why is There an El Nino State?

 

X. Why Does ENSO State Tend to Oscillate?


Reading:

Resources


lecture text by Mark Cane, Peter deMenocal, Arnold Gordon and Jim Simpson.