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Adapted from An Introduction to Satellite Image Interpretation, Eric D. Conway and the Maryland Space Grant Consortium, ©1997, Johns Hopkins University Press, Baltimore, 255 pp with Interactive CD-ROM.
For more information about this book and how to order copies go to the JHU Press On-line Catalog
The use of environmental satellites has greatly improved our ability to study the characteristics of the oceans. From a satellite, large areas of the oceans, including remote regions, can be observed at the same time. This allows entire oceans to be monitored and studied on a useful time scale and at a lower cost. This activity will discuss several applications of weather satellite imagery to oceanographic studies.
At the end of the activity you should be able to describe why weather satellites are a useful tool for studying oceanography (as opposed to other methods of oceanographic research) and describe the patterns seen in infrared imagery to map the location of different ocean currents. Once you have mastered this skill, you will then be able to create ocean circulation maps showing the location of the various thermal ocean currents in the Atlantic, Pacific, and other oceans.
Spiral ocean eddies in the North Central Atlantic Ocean
[eddies.gif ]
Diagram of general oceanic circulation [ocean.jpg]
False-color Sea Surfac Temperature image of Atlantic Ocean [sst1_atl.gif]
False-color Sea Surfac Temperature image of Atlantic Ocean [sst2_atl.gif]
Multi-Channel Sea Surface Temperature - Grayscale version - off coast of
North Carolina (dark is wamer) [Gulf_st2.gif]
Infrared HRPT image of East Coast, US showing Gulf Stream [gulf_str.gif]
One of the most useful data sets offered by remote sensing
of oceans is sea surface temperature (SST).
IR sensors on environmental satellites can be used to measure the temperature
across large expanses of the ocean surface. This data has many important
applications that permit scientists to use ocean temperatures to observe
ocean circulation and locate major ocean currents. Ocean current analysis
can facilitate ocean transportation, much as jet stream analysis is used
for routing aircraft. Additionally, by using SST, scientists can monitor
changes in ocean temperatures and relate these to weather and climate changes.
SST can also aid in the detection of newly formed sea ice, which might otherwise
go unnoticed. The same data can also be used to monitor the amount of ice
on inland bodies of water such as the Great Lakes and Hudson Bay. Finally,
SST can be used to help locate living resources that are associated with
specific thermal features in the oceans, such as fish that prefer a specific
temperature range.
The oceans are an important part of the Earth's heat exchange
system. Ocean water at the equatorial regions of the Earth absorbs heat
from the sun. These warm ocean currents then flow toward the poles, carrying
heat away from the equator and distributing it to higher latitudes. Cold-water
currents travel 
from the polar regions toward the equator,
where they become heated again. This circulation is mainly wind driven and
often matches the wind patterns across the globe. In the Atlantic and Pacific
Basins in the Northern Hemisphere, currents generally flow clockwise (anticyclonically)
as persistent high pressure near 30° latitude forces ocean circulation.
In the Southern Hemisphere the circulation is mainly counterclockwise. These
circulation patterns are known as gyres. This map illustrates a generalized
circulation pattern for the world. Typically, the currents that flow poleward
are warm-water currents and the currents flowing toward the equator are
cold-water currents. It is important to understand that ocean circulation
is very dynamic, and this map only shows the average position of these currents.
Additionally, this map does not show subsurface and deep ocean circulation,
both of which are very important in ocean dynamics.
Oceanic fronts are boundaries between water masses of different density. Density is a function of temperature and salinity (the amount of dissolved salts in water); therefore, both thermal (temperature) fronts and haline (salinity) fronts exist in the ocean. A thermal front is a zone with a pronounced horizontal temperature gradient, while a haline front exhibits a horizontal salinity gradient. Ocean fronts can extend from the surface to the very deep layers of the ocean, often separating very large volumes of ocean water.
Using IR satellite imagery on a relatively cloud-free day, it is possible to detect ocean thermal fronts in the surface layers of the ocean. This is accomplished by locating in an image a distinct gray shade difference that results from the horizontal temperature gradient across a thermal ocean front. Since these temperature differences are often on a magnitude of 2­5° C, image enhancement techniques are often used to highlight the temperature range of the ocean surface and improve the contrast between small differences in sea surface temperature.
Oceanic fronts can be permanent or transient features. Permanent oceanic fronts include the Gulf Stream front , located off the east coast of North America, and the Kuroshio Current front, located off the east coast of Asia. These frontal boundaries always exhibit a pronounced horizontal temperature gradient and can be up to 1000 meters deep. Transient oceanic fronts usually occur seasonally and are generally weaker, with more diffuse boundaries. Transient fronts may only appear in the ocean for a few weeks during the year; however, they are important components of the ocean system.
The Gulf Stream is a strong warm-water current that generally
flows northward, nearly parallel with the Atlantic coast of the United States.
The Florida Current is the southern portion of the Gulf Stream that extends
from the southeastern tip of Florida to Cape Hatteras, North Carolina. At
Cape Hatteras, 
the Gulf Stream turns eastward
and flows into the northern Atlantic Ocean, where it slowly cools. This
current is studied on a regular basis, and its position is charted regularly
to aid shipping in the oceans around North America. The Gulf Stream is also
an important component of weather over the oceans. As air comes into contact
with this warm-water current, it is heated. Thunderstorms are often seen
forming over the Gulf Stream, making it especially important to transatlantic
shipping
and air travel. This current is also responsible for bringing warmer air
temperatures to the United Kingdom, moderating the climate in several cities,
including London. In this image the Florida Current and the Gulf Stream
are visible off the east coast of the United States. Since it is warmer
than the surrounding waters, the frontal system appears as a region in the
water that is a darker shade of gray.
Gulf Stream flow is characterized by large-amplitude meanders that often break off and form eddies. These patterns are similar to those observed in the atmosphere. Cyclonic eddies are formed by southward meanders of the Gulf Stream that become very large, break away from the Gulf Stream, and develop separate, closed circulations (counterclockwise in the Northern Hemisphere; clockwise in the Southern Hemisphere). Cyclonic eddies are often termed cold-core eddies, since the central area of the eddy contains cooler slope water surrounded by a ring of warmer Gulf Stream water.
Anticyclonic eddies are formed by northward meanders of
the Gulf Stream that break away from the Gulf Stream and develop separate,
closed circulations. Anticyclonic eddies are often called warm-core 
eddies since the central area of the eddy contains warmer water
from the Gulf Stream surrounded by a ring of cooler shelf water. These eddies
often form in conjunction with the bottom topography of the ocean, especially
along the edge of the continental shelf boundary. Once they form, they spin
off by themselves and decay slowly. In this image, several warm-core eddies
can be seen along the northern edge of the Gulf Stream. The most distinct
eddy is located east of Cape Cod. It is easily identified by the ring of
warm water surrounded by a ring of cooler water. To the east of this eddy
is a large-amplitude meander that may become cut off as a warm-core eddy.
