SEA


Air-Sea Exchange

For centuries, the surface temperature distribution of the oceans has been known to provide information on the locations of strong ocean currents such as the Gulf Stream, Somali, and Kuroshio currents. Only recently, however, has the profound role that ocean temperature plays in regulating weather and climate been understood. The oceans and atmosphere form a coupled system separated by an interface through which heat, moisture, and momentum are exchanged. The state of the ocean-atmosphere system is driven by radiation received from the Sun, which causes uneven heating of the Earth's surface, in particular excess heating at the equator relative to the poles. The surface temperature gradients cause fluid motions in the atmosphere and oceans that redistribute heat towards the Poles, and which depend on dynamical processes that have long-term mean, seasonal, diurnal, and transient components.


Sea Surface Temperatures

Sea surface temperature is an important factor in the physical processes underlying the surface energy balance, the sensible and latent heat exchanges at the air-sea interface, and the circulation of the atmosphere and oceans. The heat fluxes depend critically on sea surface temperature, as does the upward longwave radiation emitted from the ocean. Ocean circulation is influenced by the sea surface temperature distribution through its effect on atmospheric circulation and surface winds, which, in turn, drive the ocean currents, and also through its effect on sea water density, which contributes a buoyancy component to the ocean circulation.

These global sea surface temperature images from 1984 show normal temperatures over a 6-month cycle. The colors range from blue, which represents 3° C, to dark orange, which represents 30° C.

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The image at right shows sea surface temperature conditions just after an El Niño in 1983. By clicking on the June 20 - July 4, 1984 marker, you can see where conditions had returned to normal one year later. Deviations in this normal pattern, such as El Niño, are key indicators of changes in the environment. The anomalies may be transient, or cyclical, leading to relatively short-term but potentially damaging changes in global weather patterns. They may also indicate long-term warming (or cooling) trends with serious implications for global environmental change.

Anomalies, such as El Niño, may indicate serious global climate changes, leading to precipitation-level changes causing flooding and droughts, and higher sea levels.

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Evaporation from the Ocean

Water is the essential element for life, and evaporation is the principal mechanism to transport water from the surface of the Earth to the atmosphere. Over 70% of the Earth's surface is covered by ocean, which is the largest reservoir of water on Earth. With their high specific heat and large thermal inertia, the oceans are also the largest reservoir of heat and the flywheel of the global heat engine. Since water has high latent heat, evaporation is also an efficient way to transfer the energy. Besides releasing latent heat to the atmosphere, the water transported from the ocean to the atmosphere forms clouds which absorb and reflect radiation. Water vapor is also an important greenhouse gas which absorbs more longwave radiation emitted by the Earth than the shortwave radiation from the Sun.

Redistribution of clouds and water vapor changes the Earth's radiation balance and affects climate. The differential heating of the atmosphere by the ocean fuels atmospheric circulation which, in turn, drives ocean currents. Both winds and currents transport and redistribute heat and greenhouse gases.

Water vapor and latent heat are transported from the ocean surface to the atmosphere mainly by turbulence. The images above show the distribution of evaporation (Figure 1); precipitation (Figure 2); and the divergence of moisture transport (Figure 3) in October 1987. All values are multiplied by the latent heat of vaporization and expressed in heat flux units.

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Surface Wind Fields Over the Oceans

In general, there is a high degree of correlation between wind speed and wave height. As a result, wind speed is determined from the strength of the return pulse of the radar altimeter. A calm sea is a good reflector and returns a strong pulse, whereas a rough sea tends to scatter the signal and return a weak pulse.

High wind speeds are represented in red. The highest winds generally occur in southern oceans, where winds over 15 meters per second can be found. The strongest waves are also generally found in this region. The weakest windsindicated by dark blueare found in the tropical and subtropical areas of the Pacific, Atlantic and Indian Oceans.

Wind Movie



Wind Speed and Wave Height

Simultaneous observations of wind speed and wave height will help to improve forecasts of ocean waves, which have great value to shipping interests. In addition, a key scientific issue of NASA's Earth Science Enterprise is the exchange of energy between the atmosphere and the ocean, and strong winds favor increased energy exchange.

The top image shows the global distribution of wind speed at the ocean's surface. Radar pulses are transmitted from the satellite to the ocean surface below and wind speed is determined by the strength of the pulses reflected by the ocean's surface and returned to the satellite. A calm sea serves as a good mirror-like reflector and returns a strong radar pulse in one direction. On the other hand, a rough sea tends to scatter the radar signals and returns a weak pulse. In this image, the strongest winds are found in the Southern Ocean and are indicated by red. The highest waves also are located in this region (in the bottom figure). In general, there is a high degree of correlation between wind speed and wave height. The weakest winds (represented by dark blue) are found in the western tropical Pacific Ocean, the tropical Atlantic Ocean, and the tropical Indian Ocean.

The bottom image indicates the wave heights in the oceans during the same period. Wave height is determined by the shape of the radar pulse returned by the ocean's surface. In this image, the highest waves occur in the Southern Ocean, where waves up to 6 meters high (represented in red) are found. The lowest waves (indicated by dark blue) are found primarily in the tropical and subtropical oceans, where the winds tend to be lighter.