SEA

Ocean Topography and Circulation



Ocean Topography

The ocean's circulation plays an important role in the Earth's climate system. Ocean currents move one-third to one-half of the heat from the tropics toward the poles, leading to a moderation of the climate at high latitudes. In addition, dissolved gases, nutrients and other biochemical constituents are stored and transported by the sea's circulation.

Ocean topography is a measure of sea level relative to the Earth's geoid, the surface (coinciding with mean sea level) on which the gravity field is uniform. Oceanographers use ocean topography maps to calculate the speed and direction of ocean currents in much the same way that meteorologists use maps of atmospheric pressure to calculate the speed and direction of winds.

The above image was produced over a 10-day period in October 1992 from preliminary data from the joint U.S.-France TOPEX/Poseidon radar altimeter, a satellite instrument that uses radar to make precise measurements of ocean surface heights.

In this image, the maximum sea level (shown in red and white colors) is located in the western Pacific Ocean and the minimum sea level (shown in blue and purple) is around Antarctica. In the Northern Hemisphere, ocean currents flow clockwise around areas of high sea level, and counterclockwise around areas of low sea level. This phenomenon is reversed in the Southern Hemisphere. Although this image is derived from only 10 days of data, it reveals most of the ocean circulation systems that have been identified by shipboard observations collected over the past 100 years, clearly demonstrating the utility of acquiring Earth system data from orbiting instruments.

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Global Ocean Circulation

Ocean circulation is the large scale movement of waters in the ocean basins. Winds drive surface circulation, and the cooling and sinking of waters in the polar regions drive deep circulation.

Surface circulation carries the warm upper waters poleward from the tropics. Heat is disbursed along the way from the waters to the atmosphere. At the poles, the water is further cooled during winter, and sinks to the deep ocean. This is especially true in the North Atlantic and along Antarctica. Deep ocean water gradually returns to the surface nearly everywhere in the ocean. Once at the surface it is carried back to the tropics, and the cycle begins again. The more efficient the cycle, the more heat is transferred, and the warmer the climate.

Due to the rotation of the earth, currents are deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This effect is known as the "Coriolis force." The deflection leads to highs and lows of sea level directly proportional to the speed of the surface currents. The changes in sea level due to currents are the ocean topography that is observed by TOPEX/Poseidon.

Observations of ocean topography and a knowledge of the Coriolis force permit scientists to map ocean currents using data from the satellite. Every ten days TOPEX/Poseidon produces maps of the currents everywhere in the ocean.

Variations in the ocean's circulation can lead to variations in heat transport and to variations in weather patterns. One important variation in the circulation is the change in the equatorial circulation known as El Niño which occurs with an irregular period of two to five years. The most recent El Niños have been observed with unprecedented accuracy by TOPEX/Poseidon.

Global Ocean Circulation Movie




Gulf Stream Currents

This thermal infrared image of the northwest Atlantic Ocean was taken from a NOAA satellite using the Advanced Very High Resolution Radiometer (AVHRR) instrument, which is sensitive to changes in the temperature of the ocean's surface. Reds are warmest, blues and violets are coolest. The large swirls are eddies associated with the Gulf Stream.

In addition to heat transfer, the temperature of the ocean determines how much carbon dioxide can be absorbed from the atmosphere. Knowing how much is absorbed is important because carbon dioxide is one of the major greenhouse gases that may be responsible for global warming.

Research on processes manifested in sea surface temperature patterns will be performed using data from a number of EOS instruments, including the Moderate-Resolution Imaging Spectroradiometer (MODIS), the Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU), the microwave Humidity Sounder from Brazil (HSB), and the Advanced Microwave Scanning Radiometer (AMSR).

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El Nino/Southern Oscillation

The name "El Niño" refers to a warm ocean current that flows along the coast of Peru every year around Christmas time. In some years the warming persists into the spring and summer. This extended warming not only disrupts the lives of people in South America, but also influences weather patterns in the rest of the world. The name "El Niño" has now become attached to this anomalous event. When this occurs, some parts of the globe suffer from drought, while others experience too much rain. It is important that we better understand the mechanics of El Niño in order to predict the occurrence and effects of this event.

Before an El Niño, strong westerly trade winds in the equatorial Pacific push warm water toward Indonesia. This produces a large pool of 84 degree Fahrenheit water in the Western Pacific. During an El Niño, the trade winds weaken and a wave of warm water moves eastward along the equator toward South America. This warms the central and eastern equatorial Pacific.

As El Niño develops, heavy rains normally found near Indonesia move into the central Pacific, warming the atmosphere there and disrupting the atmospheric circulation throughout the Pacific. Effects have been felt as far north as Canada and as far south as central Chile. Scientists are only beginning to appreciate the far-reaching effects of this phenomenon.

TOPEX/Poseidon is helping scientists better understand the mechanics of El Niño and assisting them with the development of models that will predict future events. This may become even more compelling since in the last four years, El Niño has occurred with more frequency and has lasted longer with greater impact.

The Southern Oscillation is a global-scale seesaw in atmospheric pressure between Indonesia/North Australia and the southeast Pacific. El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the Southern Oscillation pattern.

El Niño Movie