Sulfur plumes off Namibia

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Sulfur plumes off Namibia

Photograph

By: United States National Aeronautics and Space Agency (NASA)

Date: April 24, 2002

Source: "Sulfur plumes off Namibia." NASA, November 27, 1981.

About the Organization: The image of sulfur plumes rising up from the bottom of the ocean floor to produce swirls in the waters off the coast of Namibia in southern Africa was created by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite launched by the United States' space agency NASA.

INTRODUCTION

Ocean color is the term used to describe the quantity of light of different wavelengths that reflects off of the surface of the ocean. Various components in the ocean absorb and scatter light differently at different wavelengths. For example, phytoplankton, the microscopic plants that live in the ocean, strongly absorb blue and red light. This means that when there is a lot of phytoplankton in the water, a great deal of green light is scattered out of the ocean. The ocean color is therefore greener and the amount of light measured by a light sensor that detects green wavelengths is greater. In contrast, many types of sands and soils absorb blue light very strongly, making the ocean color more red. Other compounds such as sulfur produce dramatic color swirls and currents. Such sulfur plumes or swirls result from the breakdown of plant matter by anaerobic bacteria (bacteria that do not require oxygen).

Beginning in the 1950s, oceanographers developed instruments to measure ocean color from ships. These instruments, called spectroradiometers, detect the magnitude of light in specific parts of the visible light spectrum. In addition, scientists on ships could sample the seawater and measure the absorption and scattering properties of the various components of seawater. These measurements allowed them to develop mathematical models to predict the concentrations of phytoplankton, terriginous runoff, and other materials in the ocean from measurements of ocean color.

In the 1970s, oceanographers began collaborating with NASA scientists to develop satellites that could measure ocean color from space. The first of these satellites was the Coastal Zone Color Scanner (CZCS). It was launched in 1978 as a test of concept and was only planned to remain functional for one year. Against expectations, ocean color data was collected from CZCS for eight years, until 1986.

Given the successes of CZCS, space agencies from around the world developed satellite-based ocean color sensors. As of March 2005, ten ocean color satellites are functional. MODIS (Moderate Resolution Imaging Spectroradiometer) and SeaWiFS (Sea-viewing Wide Field of View Sensor) are both operated by the United States. Other countries that have successfully launched ocean color satellites include China (COCTS), Japan (OCTS and OCI), Argentina (MMRS), the European Union (MERIS), India (OCM), Korea (OSMI), and France (PARASOL). At least six more ocean color sensors are scheduled for launch by 2009.

PRIMARY SOURCE

SULFUR PLUMES OFF NAMIBIA

See primary source image.

SIGNIFICANCE

The most impressive feature of the space imaging is the high level of spatial complexity. The currents, which are strongly affected by topography of the seafloor, have highly dynamic behaviors. The eddies, rings, and vortices that result from the motion of the currents appear as circular, swirling, and ring-like features in images.

Such imaging has major advantages over shipboard measurements in terms of understanding the complexity of the ocean. Whereas a ship can only measure one point in time and space, satellites can scan large swaths of the ocean instantaneously. The point-by-point measurements can easily mask dynamic changes and complex spatial structure. For example, scientists on a ship could make many samples in a region with a large amount of phytoplankton or sulfur and never know that water with low concentrations of phytoplankton or sulfur occur only a few miles away.

The effects of temporal features have also been studied using information collected from ocean measurement satellites. For example, monsoons have an enormous impact on the growth of phytoplankton in the Arabian Sea. These heavy rains, which impact the region between July and December, result in extremely high concentrations of phytoplankton, as can be seen from satellites. This contrasts with the non-monsoon part of the year, when ocean color images show that the Arabian Sea has very low concentrations of phytoplankton. Ocean color images have also been important in identifying the effects of longer-period global features, like El Niño/La Niña.

Satellite measurements of the ocean have increased oceanographers' understanding of the ocean in other ways as well. Changes in the temperature of the water are often correlated with phytoplankton growth or other submarine growth. This is quite common in upwellings, places where deep, cool water rises to the surface. Ocean color sensors have been used throughout the oceans to identify locations of upwellings and to verify the effects of such upwellings. Ocean color measurements, are for example, used to identify harmful algal blooms, also called red tides, which can result in large fish kills. It can be used to monitor the impacts of pollution and oil spills, the magnitude of dust storms, and even the foraging patterns of sea turtles.