Lecture 12 Notes  October 22, 2004

Nutrient Cycle

Phytoplankton produce energy for life from photosynthesis, which means they intake carbon dioxide and produce oxygen. They live in the upper ocean where they can absorbe light. They are colorful and actually alter the color of the ocean. They thrive where the ocean is nutrient rich. Deep water is relatively nutrient rich because phytoplankton deplete nutrients near the surface, and phytoplankton fall to the ocean depth when they die. On their path to the ocean floor they deposit nutrients in the water below the surface.

Phytoplankton are food for fish and serve as a marker for upwelling. Check out the Seawifs Figure 5-13 in your text or here. The ocean color is shown measured from satellite, but the color is enhanced a great deal to exagerate the detail. Lighter colors (like orange and yellow) are high in phytoplankton, indicating upwelling from coastal upwelling and mixing. The term mixing refers to upward and downward motions that occur intermittently in space and time in a region when the density profile is favorable for vertical motions (i.e., nearly unstable). Mixing in the high latitudes causes these regions to be nutrient (and hence phytoplankton) rich.

Isotopes and Carbon-14 and ocean age at depth

Isotopes are defined in Chapter 5. I described how they are used to infer information about climate. For example, water isotopes evaporate at different rates depending on the temperature of the liquid layer.  The underlying reason is that heavier water isotopes require more energy. Condensation also occurs at different rates for different water isotopes. Consequently the ratio of isotopes in ice and snow tell us the temperature history of their lifetime since evaporation.

Photosynthesis is another example of a process that depends on isotopes: Isotopes of carbon dioxide are consumed at different rates depending on a number of things like plant type, temperature, etc. Live biota acquire carbon-14 from breathing in carbon dioxide. Once they die, the source of carbon-14 is gone, and the supply of carbon-14 slowly decays because carbon-14 is radioactive. It has a halflife of 5730 years, so it is useful for looking at century and milennial timescales.

Carbon-14 that is dissolved in the water is used as a marker for the time since ocean water has been near the ocean surface. This is also called the "age" of the water. Carbon-14 is acquired at the ocean surface from contact with the atmosphere. The carbon-14 is then carried with the ocean  water, but it is decaying all the time, albeit slowly. Consequently the concentration of carbon-14 gives an indication of how long it has been since the water has been at the surface. See Figure 5-11 in the text or here.

El Nino

Normal conditions in the tropical Pacific result from steady easterly trade winds on the equator that drag the warm sea surface temperature towards the western Pacific. Offshore winds (easterly) off South America  create coastal upwelling off shore there (in contrast coastal upwelling away from the equator is typically due to Ekman transport), so the sea surface temperature is lower in the eastern Pacific. This temperature gradient across the Pacific causes a circulating cell known as the Walker circulations (see Figure at left and below) that is driven by rising over the warm pool in the west and sinking in the east.


El nino conditions arise when the easterly trade winds are weaker than usual or perhaps even in the opposite direction from normal. This occurs for complex reasons that we will not delve into in this course. The warm pool then relaxes eastward, back towards the central Pacific. The eastern Pacific sea surface temperature increases too. The Walker circulation is shifted a great deal to the east and another west-east cell develops in the western Pacific as shown.

Figure 15-13 shows a positive feedback loop for the tropical Pacific, where if the sea surface temperature in the west increases, so does the strength of Walker circulation, which reinforces the easterlies at the surface. The strengthened easterlies then further build high sea surface temperature in the west.

The atmospheric circulation in the tropical Pacific gives rise to a pressure gradient from east to west that is known as the Southern Oscillation. The term ENSO is a combination of El Nino + Southern Oscillation.
The Southern Oscillation is defined as the pressure in the west minus the pressure in the east, so it is more negative than usual during El Nino conditions. Usually El Nino is measured in terms of the sea surface temperature of the eastern Pacific, so it is more positive than usual during an El Nino.  Thus the two best known measure of ENSO have opposite signs during a big El Nino (or La Nina, roughly the opposite of El Nino). Data are taken in a giant array of buoys in the tropical Pacific that is one of the crowning achievements of climate research. These data provide enough information to allow a great deal of research about current conditions, in part to make forecasts of the future. Presently the sea surface temperature in the eastern Pacific is a little higher than normal - a 0.5 on one El Nino scale, where 1.0 is a pretty decent event. Forecasts indicate we might get up to 0.7 in the next couple of months.

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Contact the instructor at: atms211@atmos.washington.edu

Last Updated: 10/22/2004