Physical Interactions with the Atmosphere
In broad terms, the ocean interacts with the atmosphere in two main ways.
The first way is physically, through the exchange of heat, water, and
momentum. Covering more than 70 percent of the Earth's surface and
containing about 97 percent of its surface water, the ocean stores vast
amounts of energy in the form of heat. Moreover, the ocean has a
relatively large temperature inertia, or resistance to change. Earlier
scientists perceived the ocean as an unchanging "desert" due to its slow
circulation (relative to the circulation of the atmosphere) and its low
biological productivity. Yet, today we know the biological and physical
functioning of the ocean system can change quickly over both small and
large areas (i.e., during an El Niño). Because it often drives the timing
and patterns of climate change, the ocean was recently labeled by some
scientists as the "global heat engine."
Global climate modelers recently estimated that over the course of the 20th century, the ocean has reduced by about half the expected surface warming due to rising greenhouse gas levels. Scientists have observed an overall cooling trend in the east despite the strong and frequent El Niño events after 1975. Modelers conclude that as a consequence of the exchanges of heat and momentum (or interaction of air and water currents) between ocean and atmosphere, the mean temperature of the Pacific Ocean increases less than it would if it only exchanged heat. Thus, given its efficiency at redistributing heat poleward, the ocean is effectively delaying and regulating global warming.
But scientists don't know if the ocean's role as "climate moderator" will persist over the long term. There is evidence that large and abrupt changes in the ocean's circulation have had major impacts on global climate in the past. Some scientists question whether the existing ocean circulation pattern is stable enough to withstand the stresses of rising temperatures, heightened greenhouse gases and fresh water runoff due to melting glaciers. As the rate of fresh water runoff increases, it lessens the salinity and density of surface water, thereby raising its freezing point. More surface ice would inhibit the escape of heat and could result in a major reorganization of the ocean's circulation system, which would affect global climate.
Chemical Interactions with the Atmosphere
The second way that ocean and atmosphere couple is chemical, as the ocean
is both a source and sink of greenhouse gases. Much of the heat that
escapes the ocean is in the form of evaporated water, the most abundant
greenhouse gas on Earth. Yet, water vapor also contributes to the
formation of clouds, which shade the surface and have a net cooling
effect. In the long run, scientists don't know which process (cloud
shading or water vapor heat retention) will exert the larger influence on
global temperatures.
One of the key questions in climate change research is how the physical and biological processes of the ocean will respond to chemical and physical changes in the atmospheric. For example, will there be increased storms that increase mixing in the upper ocean? Will phytoplankton become more or less productive in such an environment? Windblown dust from soils and desert sands are rich in iron that, when they settle into the ocean, serve as "fertilizer" for the phytoplankton. Will this dust input increase as a result of climate change? Will this increase the efficiency of the biological pump and thereby increase the removal of CO2 from the atmosphere by the ocean?
There is some evidence that larger amounts of iron may be deposited in the ocean in the future as global warming causes increases in windblown continental dust. Theoretically, if iron-containing aerosols ("aerosols" are solid or liquid particles suspended in the atmosphere) fall in large regions of the ocean that are rich in other nutrients, then this would act like "fertilizer" to promote burgeoning phytoplankton populations, thus slowing the rate of CO2 increase in the atmosphere and partially offsetting the anticipated warming.
While we know that short-term events, such as El Niño, dramatically affect phytoplankton concentrations in the Pacific, scientists aren't sure how, over the long-term, changes in ocean circulation and atmospheric forcing will affect the ocean's productivity. During an El Niño event, the whole marine food chain is disrupted. Larger fish and mammals must either starve or move to where phytoplankton are more abundant. Linking these intense events with changes in climate variability is one of the most pressing issues in climate research. Yet, long-term trends are difficult to detect when natural variability is so high. A comprehensive program of observations and modeling is necessary to improve our ability to make predictions on the future course of the Earth system.
Space-based Oceanography
Reliable sea surface temperature measurements from space-based sensors
have been a goal of oceanographers since the late 1960s. For the first
time, Terra will provide oceanographers with the radiometric resolution
and precision, the scientific calibration, the spatial coverage, and the
ability to remove atmospheric effects (such as clouds and aerosols) that
will enable measurements of sea surface temperature accurate to within
0.5°C. These data will enable a better understanding of ocean-atmosphere
coupling-particularly during El Niño events.
Terra will make comprehensive measurements of phytoplankton biomass as well as dissolved organic matter in the upper ocean. These measurements of the "standing stock" of the base of the marine food web are critical to understanding the interactions between ocean circulation and productivity. Terra provides for the first time the ability to measure "fluorescence," or energy emitted by phytoplankton during photosynthesis. Fluorescence measurements will greatly improve the estimates of the rates of marine productivity. The combination of both standing stock (derived from precise measurements of the ocean's color) and growth rate will significantly improve our understanding of the ocean and climate.
An Earth-orbiting NASA scatterometer (a microwave radar called QuickSCAT) will measure wind velocity (both speed and direction) over the ocean. These data will complement Terra by providing essential information on processes that drive ocean circulation. Winds affect the degree of ocean mixing as well as providing the momentum to drive ocean currents. QuikSCAT will provide high-resolution maps of wind velocity to study specific processes such as coastal upwelling, as well as global-scale maps of winds to examine large-scale processes such as El Niño.
Terra, and QuikSCAT (together with other Earth observing satellites such as SeaWiFS, TRMM, and Landsat-7-all of which are addressed in other NASA Fact Sheets) provide oceanographers with unprecedented data for scientific studies of the air-sea couplings and their effects on the marine biosphere. With these data, scientists can refine their models of the ocean's physical, biological, and chemical influences on global climatic and environmental change.
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