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Eight other satellites that have provided oceanographic data are Seasat, TOPEX-Poseidon, QuickScat, CZCS, SeaWiFS, MOS, ERS-1 and ADEOS. The first three, discussed on this page, are radar-based observers. Each provided important information on various aspects of sea state including wind direction and velocity which helped to forecast wave conditions. The most advanced of these was ADEOS, a Japanese-directed satellite that had many sensors; unfortunately, a malfunction put it out of commission after only two months.


Seasat, TOPEX-Poseidon, Jason-1, NSCAT, Quicksat, ERS-1, and SeaWinds

Temperature variations are major factors in the development, strength, and directional behavior of moving atmospheric gases (winds). Their prevailing motions change over time, but tend to follow certain preferred paths in various parts of the world. We determine wind directions indirectly, by relating them to the patterns of waves they produce, especially in the open seas. Data from the scatterometer on Seasat helped to generalize wind patterns over the Pacific Ocean, shown in this image:

Seasat Scatterometer illustration of general wind patterns over the Pacific Ocean.

14-29: Why do we want to know about wind patterns? ANSWER

Seasat was the first U.S. satellite that, as an original intention, had as its primary mission, monitoring oceanic surface phenomena, such as sea state (surface wave parameters, including wavelength, period, and height), surface wind fields, internal waves, currents and eddies, and sea ice characteristics. The microwave systems on Seasat were described on page 8-6; on that same page is the remarkable map of fractures systems in the global ocean floor. Here we present a JPL image of worldwide generalized sea surface height derived from Seasat data:

Sea surface height variations as determined by Seasat.

Radar images from Seasat over land surfaces proved so invaluable that the impression may be that we subordinated the marine applications. Nevertheless, the system performed well as intended and we gleaned much information about the oceans during its 98 days of service in 1978, before an electrical short circuit disabled it.

Operating at a high depression angle (67 to 73°) to optimize wave detection , the L-band radar on Seasat (with its 25-m resolution) provided some exceptional images of ocean waves. We show an enlarged image of the North Sea to illustrate this (the image requires careful inspection, possibly requiring higher monitor resolution:

Seasat radar image of ocean waves in the North Sea.

14-30: How many directions of advancing waves can you detect in the above image? ANSWER

Intersecting waves are strongly expressed in this Terra ASTER (sensor) image of waters in the Bay of Bengal east of India.

Multiple wave patterns in the open ocean of the Bay of Bengal.

Waves interact with adjacent incoming waves and with land runoff as shown in this ERS-1 radar image taken off the Pacific coast near Los Angeles:

ERS-1 SAR image of waves moving onto the coast near Los Angeles.

Many wave sets have wavelengths and amplitudes that are small compared to those in the open oceans, in part because of limited fetch (area over which the waves are generated by winds). These require high resolution imagery to detect. Here is a 4-meter resolution IKONOS blue band image of a lake in Mississippi in which the wave train has a northeast directionality:

Close-spaced waves trending northeast (moving southeast) in a lake in the southern U.S.

Surface wave expressions related to variations in bathymetric depths are evident in the right Seasat image (north to upper left), showing the Nantucket Shoals off Rhode Island. Nantucket Island is at the bottom left.

Seasat image of the Nantuckett Shoals off of Rhode Island.

Internal waves, such as those shown in the Sulu Sea (Pacific Ocean), result when a subsurface layer of water with different density and temperature is moved by currents against the upper layer. No surface crests are formed but the motion produces an interference effect visible as dark bands.

Aqua MODIS image of internal waves off an island in the Sulu Sea.

Internal (subsurface) waves in shallow waters in the Gulf of California appear in the next Seasat image on the left (north is towards the lower right).

Seasat image of the Gulf of California.

An interesting phenomenon known as "wake effect" is evident in this SeaWiFS (see next page) image of the Windward Islands in the eastern Caribbean. Prevailing winds from the east encounter topographic highs (mostly volcanic peaks) and are disrupted and slowed down. This leaves calmer surface winds, and much reduced wave heights, on the leeward side. Sun glint helps to emphasize the contrast.

SeaWiFS image of the Windward Islands, producing tapered zones of calmer waters (wakes) on the western (leeward) side of those islands with notable topographic elevation.

We first described the TOPEX-Poseidon (T/P) mission, run by JPL, in page 8-7. We suggest you access this (outstanding tutorial) prepared by the TOPEX-Poseidon team, which explores the kinds of information that radar altimetry and scatterometry can acquire.

TOPEX-Poseidon uses two radar altimeters to measure distance from the satellite to the sea's surface (to a precision of 4.3 cm). From such data, we derive global maps that show Dynamic Ocean Topography (rises ["hills"] and depressions ["valleys"]), Sea Surface Variability, Wave Heights, and Wind Speeds. A separate microwave radiometer on this mission determines Precipitable Water Vapor, which allows for corrections in the pulse transit time to improve distance accuracy.

14-31: How would T/P determine wind speeds from the observables? ANSWER

The first TOPEX-Poseidon maps we examine show ocean topographic variations. Below is a plot of large-scale variations during September 1992 in ocean elevations relative to the Earth's geoid. The data used to construct the map come from other sources, such as gravitational perturbations of satellite orbits. Note that, as much as 150 m (492 ft) of relief exist on the seas between the Atlantic Ocean (lower, green) and the western Pacific (higher, orange-red).

Colorized TOPEX/Poseidon global Sea Surface Variation map, September 1992.

14-32: Where are the oceans the highest? ANSWER

Seasonal differences do occur because of departures from the norms in height, as water expands or contracts in each hemisphere. The pair of maps below show Sea Surface Height (SSH) variationin the northern Fall (upper) and Spring (lower), in a plot centered on the Pacific Ocean. The differences in elevation lead to redistribution of water through thermally-induced currents.

Colorized TOPEX/Poseidon global Sea Surface Height variation map - Fall and Spring plots.

The next pair of maps are plots of wind speeds and wave heights during June 1995. There is some correlation between these parameters: greater speeds tend to cause higher waves.

TOPEX/Poseidon maps of wind speed (top) and wave heights, June. 1995.

TOPEX-Poseidon also has shed new light on the oceans tides. There has been an ongoing mystery as to balancing the energy provided by the Moon's gravitational attraction, which produces the tides, and the dissipation of that energy. What was known is that much of the energy goes into setting up surficial ocean currents that carry water from higher areas to lower. Ocean heights measurements by T-P have now better fixed the areas of the seas that are higher and lower than mean sea level. This image shows a general pattern:

Tidally-induced highs and lows on the Earth's oceanic surface.

And this image shows more details, representing a data sets for six years of observations. The so-called tidal energy dissipation thus displayed is affected in part by variations in sea surface heights which establish gradients that cause water flow that influences tidal rises or falls (see page I-1b.

Tidal energy dissipation on the world's seas, as determined from Topex-Poseidon observations of sea heights.

From this can be derived this broad outline of the current flow lines outward from the highs:

Directional flow patterns of tidal currents, as established by TOPEX-Poseidon.

The shortfall of about 30% for the accounting of energy balance in tidal energy distribution has from the T-P observations now been explained by work done in changing ocean volume below the surface. The configuration of the sea floor contributes to this.

Topex-Poseidon was decommissioned (ceased data gathering) on January 6, 2008.

A follow-up to TOPEX-Poseidon, named JASON-1, is a component of the EOS program (see Section 16). Operated jointly by NASA JPL/CNES, this spacecrafts sensors include C and Ku band radar altimeters, a microwave radiometer, and a Doppler radar. Again, sea surface heights (SSH) are the main oceanographic phenomenon being measured; for Jason, differences in SSH as small as 4.1 cm can be determined.

The first SSH map made from Jason-1 measurements.

Data from Jason-1 are now being processed routinely by various government and private centers engaged in marine studies. This map of global tropical zones was produced by the Space Center at the Univesity of Texas-Austin.

SSH variations in a map generated by the University of Texas Space Center.

Just as has been done by Topex-Poseidon for the last 10 years, Jason-1 is gathering daily information on the buildup of hot waters that lead to the El Niño that forms every several years in the Pacific. Here is an October, 2002 Jason-1 map that shows the warming trend in the eastern Pacific - as the winter of 2002-03 ensues, unusual weather has indeed become common in the United States.

The warm waters (white) of El Niño in October 2002, in the map constructed from Jason-1 data.

One aspect of oceanographic studies and management that has received considerable attention lately is the state of health of corals - the animals (polyps) that make the foundation of coral reefs which support a wide variety of biota. Various satellites are contributing to an organized monitoring program designed to gather long term data and to "flag" potential local to worldwide conditions that threaten coral populations. Landsat 7 is a mainstay of this effort. Here is a Landsat false color view of part of the Florida Keys (see also page 8-6) which illustrates the monitoring capabilities from satellites.

Part of the Florida Keys in a false color image obtained by Landsat 7.

Lidar mounted on aircraft have proved capable of determining the shape, and to some extent, depth of coral reef complexes off island shores. Here is a relief image of Johnson's Reef off St. Johns Island in the American Virgin Islands:

EAARL image of mapped subsurface reef off St. Johns Island.

Turning again to the Japanese ADEOS (Midori), as expected it carries a Vis-NIR imager (called AVNIR). Here is an image of some of the island reefs in the Great Barrier Reef of northeast Australia.

Image taken by the AVNIR on ADEOS showing a quasi-true color view of some of the reef islands in the Great Barrier Reef of northern Australia.

ADEOS also produced some interesting ocean color images, none more so than this view of the sea around Japan itself, here with the natural colors modified to the full range of blues through reds to emphasize currents and other water circulation patterns:

Ocean color patterns in the ocean around Japan, rendered in colors normally associated with temperature variations.

The Jet Propulsion Laboratory (JPL) developed, in cooperation with Japan's NASDA, a follow-on instrument to the Seasat Scatterometer, called NSCAT (NASA Scatterometer). This flew as part of the multiple payloads on ADEOS, launched from Japan on August 16, 1996, and operated successfully for 9 months. The view below shows the sophisticated nature of this satellite:

Artistís rendition of the ADEOS satellite with its complement of instruments.

NSCAT was a microwave radar scatterometer that transmitted pulses at 13.995 GHz and measured their reflections (backscatter) from ocean ripples and other surfaces. Six antennas provided eight beams that extended over two wide bands. The returned signal is subjected to Doppler processing. In general, rougher seas correlate with greater wind speeds; in turn, rough seas increase backscatter. Operating at 50 km spatial resolution, the system distinguished and recorded 268,000 wind vectors, derived from wave-induced backscatter data.

The principal use for NSCAT was in determining wind-direction, from which is derived useful information relative to ocean waves and global climate patterns. The instrument could also operate over land (see page 3-5) and produce sea-ice images (page 14-14). Here is a global, wind-vector map, color coded for grouped intervals of different velocities.

Color-coded NSCAT global wind vector map.

Further processing allows imposition of pseudo-streamlines indicating wind direction, as shown here:

Another example of a NSCAT color-coded map of wind velocities, over which are plotted wind streamlines (see top figure on this page).

On July 19, 1999 NASA JPL launched QuickScat, a satellite whose prime sensor (SeaWinds) is a radar with 25 meter resolution. Its primary mission is to provide near-realtime measurements of surface roughness that translate into wind velocities.

Wind velocity map of the Pacific Ocean hemisphere made by Quickscat; the yellow area south of Japan is a typhoon (Olga).

A detailed map of wind speed intervals associated with Typhoon Nesat in the Pacific, on June 6, 2005, shows the details obtainable with the SewWinds instrument

Typhoon Nesat, June 6, 2005, imaged by Quicksat.

Below is a map of Hurricane Alley in low latitudes of the Atlantic, on which color shading indicative of wind speeds and vectors showing prevailing wind directions are plotted.

And here is a QuickScat SeaWinds map that includes an actual hurricane, Floyd, a destructive one that hit the southern U.S. in September, 1999; the immediate area of the hurricane appears as an expanded inset:

SeaWinds map of the hemisphere containing the Atlantic Ocean, showing the winds associated with Hurricane Floyd off the Florida Coast; data for September 13, 1999.

That localized area is shown in more detail in this SeaWinds map of wind flow in the Gulf of Mexico and the Atlantic during Hurricane Floyd on September 16, 1999:

SeaWinds map of major wind patterns during Hurricane Floyd.

NASA sponsored another SeaWinds radar scatterometer as an instrument package onboard ADEOS-2 (renamed Midori-2) launched on December 14, 2002 by the Japanese NASDA program. Primarily an ocean surface monitor, this sensor can identify sea ice and can, like its predecessors above, determine wind velocities. Here is the first returned data set acquired on January 28-29, 2003:

SeaWinds January 2003 global data showing polar ice as gray and ocean wind speeds as low (blue) to high (red).

The Japanese have also launched several Marine Observation Satellites (MOS) that include their own scanning radiometer (MESSR), a Visible-Thermal instrument (VTIR), and a microwave unit (MSR). These sensors are operated over both land and sea. This MESSR image of the south-central coast of Japan, showing bays below the city of Nagoya, is typical.

MOS-1 image of the Nagoya area, Japan.

To close this topic, you may wish to "attend" a JPL lecture on radar scatterometry. Access this at von Karman lectures, choosing the topic "The Winds and Beyond: The Radar Scatterometer", January 14, 2002.

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Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net