Oceanographic Observations - Remote Sensing Application - facegis.com
Oceanographic Observations

We now transit from viewing atmospheric weather systems from space to observing the oceans, which contain about 360 million cubic miles of saline water (97% of all surface/near surface water; the rest is classified as fresh). Much of the oceanic information, as gathered from space, comes as secondary information from Metsats, although a growing number of satellites, and a few Shuttle missions, are/were specifically dedicated to collecting oceanographic data. The kinds of data acquired by the sensors include the following: sea-surface temperature, oceanic-current patterns, formation of eddies and rings, upwelling, surface-wind action, wave motions, ocean color (in part indicative of phytoplankton concentrations), and sea ice status in the high latitudes. Coastal and shelf waters adjacent to continental margins generally show considerable variation in near-surface temperatures. Some variation is due to inflow and mixing of river waters but ocean currents and upwelling also modify the patterns. Look at these thermal-IR images, made from NOAA AVHRR data, of part of the California coastline from Mendocino, south to Lompoc (top), and the big Island of Hawaii (bottom), in which offshore warmer waters are displayed in lighter tones.

NOAA AVHHR thermal IR image of the California coastline.
NOAA AVHHR thermal IR image of the big Island of Hawaii.

In recent years these temperature maps are now routinely rendered in color, as this map of the coastalwaters off southern California aptly illustrates:

NOAA AVHRR image of water temperature variations off the California coast.

Ocean currents, such as the Gulf Stream off the eastern U.S. coast, and the Pacific current, off the west coast, result from redistribution of warm water that collects in tropical regions and flows towards cooler zones at higher latitudes. A color-coded rendition (below, top) of part of a Day-Thermal HCMM image (page 9-8) shows the well-defined Gulf Stream off the North Carolina-Virginia coast. Paired with this image is one of the East Coast showing surface temperatures calculated from algorithms that process multi-channel data obtained by NOAA-14's AVHRR.

Color-coded HCMM Day-Thermal image of the Gulf Stream off of the North Carolina-Virginia coast.
Colorized NOAA-14 AVHHR image of the East Coast of the U.S.

That second image also shows the Gulf Stream, which, in places, breaks into warm core rings, i.e., some meanders in the current get pinched off (cold core rings also occur). Temperature values for the colors include: orange = 25-28 ° C (77-82 ° F); yellow = 23 ° C (72 ° F); green = 14 ° C (57 ° F); blue = 5 ° C (41 ° F).

A recent image from Terra's MODIS, using bands at 11 and 12 µm, shows how sharp the temperature contrast can be between the main Gulf stream (red) and surrounding waters:

The Gulf Stream, as imaged by ASTER on Terra.

Another MODIS image of the warm Gulf Stream emphasizes its tendency to meander as it moves northward:

MODIS image showing sharp meanders in the offshore Gulf Stream.

Daily, Metsats also routinely procure global observations of temperatures in marine waters (known as Sea Surface Temperature or SST). Here for example is a map of SST made in late September of 1987.

A Sea Surface Temperature (SST) map made by thermal bands on a NOAA AVHRR, covering most of the Earth�s oceans; in this and subsequent similar maps cold waters shown in purple and blue and warm waters in yellow, reds, whites.

We can integrate SST values into calendar intervals and thus compare them month by month or between equivalent periods in years. Below are SSTs for the months of January (top) and July (bottom) in 1993, as determined from NOAA AVHRR data.

Colorized global Sea Surface Temperature map for January, 1993, taken from NOAA AVHHR data.

Colorized global Sea Surface Temperature map for July, 1993, taken from NOAA AVHHR data.

Again, reds and yellows show warm water, and blues and purples depict cold water. At first glance, we don't see much seasonal variation, when we view it worldwide, although close inspection reveals real differences. In general, the oceans tend to maintain their average temperatures with notably less variations than the atmosphere above them.

Sea surface temperature distribution on a worldwide basis can be obtained from NOAA satellite data on a near real time basis. Here is a plot for February 26-27, 1999:

Colorized global Sea Surface Temperature map for Feb 26-27, 1999.

Orbview-2 has a thermal sensor. Here is a view of Central America with the warmer Gulf of Mexico to the upper right and the more variably warm to cool eastern Pacific Ocean to the left.

Orbview-2 thermal image.

El Niño

Ocean surface temperatures have been measured since the late 19th century. It is now well known that the values of these temperatures, and their changes from year to year, are key factors in the global and regional climate and weather patterns. Note the variations from the average of temperatures in part of the Pacific Ocean (this large body of water is the main influence on global meteorological conditions although the Atlantic has a strong effect on Europe and Africa).

Temperature variations in the tropical Pacific Ocean.

These "ups and downs" appear to follow specific trends. Changes can be recognized to define notable variation on a 6 to 18 months timeframe. This next plot reveals a time frame for temperature values in the Pacific Ocean that seems to be on the order of 20 to 30 years (indicated by stretches of dominantly red [hotter] or blue [cooler]):

Pacific Ocean temperature variations in the 20th Century.

The short period variations refer to a condition in the Pacific Ocean called "El Niño" (Spanish for "the little child" namel, Jesus Christ - the term was applied by South American fishermen to refer to warm waters around Christmastime). The long period pattern is called the Pacific Decadal Oscillation (PDO). These two illustrations indicate a general or averaged temperature anomaly (either hotter [red] or cooler [blue]) in the Pacific Ocean for "El Niño" (also called ENSO for "El Niño" Southern Oscillation) and for the PDO.

El Nino temperature anomalies.
The PDO anomaly.

Here is a similar diagram for what is called La Niña, which describes the condition of now colder water off South America, with the previous hot water having migrated westward towards Australia.

The La Nina condition.

Generally, El Niño and La Niña's alternate during several years (but short periods of so-called "normalcy" also are interspersed). We summarize the temperature anomalies of the two phenomena with this well-known illustration:

El Nino and La Nino in the Pacific.

Between times of maximum anomalous temperatures, the temperature distribution in the Pacific shows considerable locational variability, as depicted by these measurement made by Envisat's thermal sensor:

Changes in sea surface temperatures from 1997 to 2000.

An El Niño event results from changes in atmospheric pressures in the eastern Pacific Ocean that cause the normally westward flowing trade winds to reverse direction, which, in turn, diminishes or reverses an upwelling of cold water off the South American coast and displaces the Peruvian current. The surface waters there become warmer (by as much as 8° C [15° F]) leading to increased southern-hemisphere summer-storm activity. This next diagram offers more insight into this behavior:

More information on the ENSO mechanisms.

Above the North American continent, an El Niño can greatly perturb normal weather patterns, causing abnormal rainfall in some parts of the country and droughts elsewhere. Ferocious storms are more frequent and hurricanes may increase or decrease from normal numbers, depending on the effects in the Atlantic and Pacific Oceans. An El Niño usually precedes a La Niña, essentially a reversal of conditions off the western South-American coast, in which colder water than usual comes to the surface.

El Niño's effect on continental weather, such as over North America, is essentially that of its repositioning of the Polar and Subtropical Jet Streams. As a reminder of their general locations around Earth, we return to this illustration from the meteorological Tutorial at the beginning of this Section:

The Jet Streams.

This easterly flow of air both steers weather systems and serves as a "barrier" to air masses. For the Polar Jet Stream, the air to its north is usually cooler, or even cold, relative to the air to the south. The Subtropical Jet Stream helps to position moist air to its south and drier air to its north. The two streams twist (meander) and shift (towards the north or south) in both short time spans (weeks) and longer (seasonal). Thus, one's local climate at some time during a year will be determined in part by the position of the relevant Jet Stream overhead. The effect of an El Niño is to warm or cool the air that then flows laterally to perturb the usual position of the Jet Streams so as to make any given region warmer or colder and wetter or drier than normal.

General pattern of warm/cool and wet/dry conditions over North America owing in part to the El Nino and La Nina.

This ability to follow and interpret the Jet Stream shifts resulting from El Niño and La Niña is a key input to prediction models that forecast expected weather conditions for the coming season(s). Here is the forecast for the Winter period 2000-2001; in fact, the actual weather history was similar to this model.

Forecast for the Winter of 2000-2001.

Satellite observations have been instrumental in identifying and understanding the "El Niño" weather phenomenon. Let us now show several examples of this use during one particular time span. These utilize data from TOPEX/Poseidon (next page) which uses radar to characterize sea surface elevations that rise when water is warmer and fall when cooler. The overall El Niño condition was particularly active in the early 1980s - publicity about which made the general public aware of the phenomenon. It was active again in the late 1990's. Experts in early 1997 predicted a very strong El Niño condition for the latter half of that year into 1998 - and this event indeed occurred. Its disaster phase may have started in 1997, with Hurricane Pauline on October 9, the strongest to hit the west Mexican coast in decades (devastating Acapulco). Events into January 1998 bear out the forecast, with heavy rainfall in the Southern and Western U.S., ice storms in New England and Canada, and abnormally balmy weather in some places.

Onset of marine warming began to appear by the time this TOPEX/Poseidon image was taken on September 20, 1997. It shows a broad, elongate band of very hot water stretching westward from Peru across the Pacific.

TOPEX/Poseidon map of derived oceanic temperatures (based on conversion of sea surface heights to thermal expansion) showing El Nino near its maximum on September 20, 1997.

Although regional in extent, this water-temperature perturbation can decidedly influence weather over much of the Earth, as the conditions in the western Pacific also become disturbed. The combination of warm Pacific Ocean temperatures and shifts in the atmospheric jet streams means that certain areas of the world with typically dry conditions instead get heavy rain and potential flooding, while other regions can be drought-stricken. An El Niño occurs every two to seven years, reversing normal weather patterns throughout areas of Canada, the United States, Mexico, South America and as far away as Africa. The previous major El Niño occurred in late 1991 through mid-1992, with a smaller and less destructive one recorded in 1994-95. The weather system can last as long as eighteen months, depending on how rapidly the ocean temperatures cool and return to normal. El Niño has a profound effect on global weather systems. This idea is supported by the recent summary of 1997 worldwide temperatures: that year was the warmest twelve months in terms of average temperature maxima since records for the entire Earth started in 1860. Some of this may also reflect a significant contribution from global warming.

As the summer of 1998 moved into fall, with several major hurricanes including Mitch which killed more than 10,000 people in Honduras and neighboring Central American countries, a transition began, in which the earlier El Niño gradually changed into a La Niña. By mid-October the central band of cold water had largely replaced the equatorial belt of warm water off South America while shifting the warmer water towards Australia, setting up the conditions associated with a La Niña, as shown in the Topex/Poseidon illustration below. The purple band denotes cooled (denser) water some 18 cm (7 in) below normal heights.

Colorized TOPEX/Poseidon image of La Nina, October 1998.

After two years of wild La Niña weather in the U.S. and elsewhere - droughts in the Southwest and Southeast in 2000 and severe conditions in the Midwest and record year round heat over much of the country - at last in June of 2000 this phenomenon appears to be waning, and probably disappearing. Note that the large purple patch in the above image has now dissipated into several smaller, discontinuous blue patches in the June 2000 scene below.

TOPEX/Poseidon SST map indicating that La Nina was disappearing by June, 2000

At present there is an informative Web site sponsored by NOAA that offers an overview of El Niño and daily to monthly reports on their stage of development and related activities. You can access this site through the link NOAA-El Nino. You can access data on El Niño, as monitored by TOPEX-Poseidon (see next page), through this JPL site

Source: http://rst.gsfc.nasa.gov