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Most meteorological satellites designed to monitor the atmosphere do not provide "good looks" at the ocean, largely because they are purposely lower resolution systems that need to monitor atmospheric properties at gross scales. Their thermal bands do broadly indicate temperature differences in oceanic water that give some information on current flow. The NOAA satellites, for example, produce valuable thermal data that can be plotted at regional to global scales. But, other aspects of the marine surface waters including sediment patterns, sea state, wind flow, wave patterns, planktonic life, etc. are of vital interest to oceanographers. A number of satellites dedicated to gathering oceanic data have notably increased our understanding of these great bodies of water; some of this information bears directly on weather and climate control. This page describes some general types of observational data from such satellites and shows the value of continual monitoring by focusing on the El Nino and La Nino thermal events in the Pacific.


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 of this variation is due to inflow and mixing of river waters but ocean currents and upwelling also modify the patterns. Look at the two 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.

14-26: Comment on the thermal patterns in both of the above images. ANSWER

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 above 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.

14-27: Birdwatchers today are taking a new kind of bird trip: the pelagic (off-shore ocean) boat trip to see many varieties of sea birds. Off the Virginia and North Carolina coasts, these trips often venture as much as 60 to 80 miles seaward from the coast. Can you surmise as to why? ANSWER

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 the date (Feb 26-27, 1999) in which the writer actually downloaded a completed plot of SST during the afternoon the the 27th.

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

Satellite observations were instrumental in identifying and understanding the "El Niño" weather phenomenon. The term (Spanish for "the little child") refers to a weather pattern in the southern hemisphere around Christmastime. An El Niño event is characterized as an anomaly in normal sea surface temperatures. It 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 (as much as 8° C [56° F]) leading to increased southern-hemisphere summer-storm activity. In North America, 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.

The overall El Niño condition was particularly active in the early 1980s, 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.

This onset of marine warming, in fact, 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.

The next image pair shows the observed SST between Australia and South America during January of 1983 and a prediction of the expected temperature distribution based on twelve months of earlier observations. The close correspondence between these two El Niño data sets implies that, even then, the model explaining this phenomenon had become fine-tuned.

 SST map of the South Pacific between Australia and South America showing temperature differences on January 1983.

Below are four plots of SST from October into November of 1994, showing the progressive shift of warmer waters to the east.

Colorized Sea Surface Temperature map (showing four separate plots) of the area between Australia and South America, from October to November 1994.

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. We see that El Niño have a profound effect on global weather systems. We support this idea with 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, setting up the conditions associated with a La Niña, as shown in the Topex/Poseidon illustration below. The purple band denotes cooled (contracted) 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 a JPL site.

14-28: What can we do now to stop or control the Niño(a)'s? ANSWER

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Primary Author: Nicholas M. Short, Sr. email: [email protected]