Background ScenarioThe Gulf Stream historically has been studied as a pathway from the tropics to the northern latitudes. As part of the global wind driven circulation model, the Gulf Stream is of particular interest because of its role in heat transport to the North Atlantic. As the Gulf Stream meanders, mesoscale eddies, that are 50 - 200 km in diameter, develop and become separated as distinct water masses. These parcels of water with their special characteristics are part of the dynamics of the North Atlantic gyre. While eddies off the Southeast coast of the United States and in the Gulf of Mexico influence local weather and major storm development, other surface current eddies around the world also affect climate conditions. An understanding of this complex air/sea relationship helps oceanographers and climatologists more accurately forecast changes and patterns in the global weather and the development of tropical storms.
Global Circulation of the Ocean's Waters
The ocean's circulation patterns are the result of the complexity of seawater and external forces that act on the waters. The deep-water convection that occurs exists because of differences in seawater density causing parcels to sink or rise in the water column. Surface ocean currents are influenced globally by the prevailing wind bands, the earth's rotation and the deflective processes known as the Coriolis effect, the continental and local topography, gravity, and weather events that control heating and cooling, evaporation and precipitation in the top layers. These massive gyres are confined to the ocean's basins and are detected in the top 1000 meters of the water column. Wind driven gyres correspond to the patterns of wind in each of the hemispheres, thus moving clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.Our knowledge of these surface currents dates to historical times of trade when it was noted that the journey across the Atlantic from the New World could be made in less time if the vessel had an additional velocity added by the Gulf Stream. The adventurers of the 21st century that sail the ocean use sophisticated imagery to forecast and take advantage of the position of these powerful ocean surface currents. The scientific community studies the role of ocean surface circulation in long range climatology, short term cyclonic forecasting, the flow of nutrients as well as pollutants and the biological distribution of sea organisms.
Monitoring the Currents
Traditionally, the study of the ocean's surface dynamics has been through the use of float bottles or drifters that have been deployed by ships. Since science is often a study of observation, a variety of unexpected tools have provided data about global ocean gyres. Glass fishing floats from the Sea of Japan have been transported across the North Pacific by the strong clockwise flow known as the Kuroshio Current. These glass floaters appear along the California and Oregon coastlines for collection. As a result of an accidental spill of sneakers into the Kuroshio stream, scientists were able to chart the position and time of landfall for the sneakers on the Pacific Americas coastline. This data provided a clearer picture of the motion and strength of the Pacific gyre. The Kuroshio Current is a strong western boundary current that parallels the intensity of the North Atlantic western boundary current better known as the Gulf Stream.This boundary acts as a front to separate the cold waters of the Labrador Current from the warm very saline waters of the Sargasso Sea. The North Atlantic circulation is forced by the trade winds in the lower latitudes blowing along the southern front of the Sargasso Sea and by the westerlies blowing along the north side of this calm Sea. The North Atlantic Current defines the most northern part of this circulation. The return currents that complete the North Atlantic gyre are the Canary and the North Equatorial Currents.
Information gathered for YOTO drifters, while still poorly understood, does provide some explanation of the waters that feed the North Atlantic gyre. The Gulf Stream has its beginnings in the Caribbean sea coming together from various small passages of water between the islands of Trinidad and Puerto Rico, and joining some water from the Mona and Windward Passages of the Greater Antilles. These combined waters concentrate into the Caribbean Current that flows out of the Straits of Florida as the Florida current. The Gulf Stream runs northward close to the Southeastern United States coastline until it sharply turns towards the east off the coast of Hatteras, N.C. The Gulf Stream at its widest and deepest, transports 140 million tons of water per second towards the continent of Europe. "There is a river in the ocean" wrote Maury in 1855 and like a river the Gulf Stream meanders and cuts its path through a cold North Atlantic Ocean warming the land, creating fog, and transporting heat energy from the tropics to the British Isles.
Satellite imagery has enhanced and supported the data from drift arrays as well as providing greater understanding of the interaction between air and sea. Warm core and cold core eddies that separate from the main stream can be monitored using altimeters on NOAA satellites. Sea surface temperatures can be precisely measured using AVHHR sensors on satellites; interpolations of this data can provide daily models of the movement of the Gulf Stream and the meso-scale eddies.
Physical and ChemicalFeatures of the Gulf Stream
The Gulf Stream is a rich source for studying fluid dynamics. The force of the water on the surface as it is deflected around continental features and as it passes through narrow straits models examples of physical concepts. The western intensification of the Gulf Stream as the gyre circulates in the North Atlantic is key to understanding the role of the Coriolis force, and the relationship of the surface wind bands to the surface water. The Gulf Stream off the coast of North Carolina tend to become much stronger, much faster, and much narrower due to the combination of increased Coriolis effect, the global winds in that latitude, and frictional effects of the landmasses. The average speed of the Gulf Stream is about .5 meters/second (much slower than the wind force above the water. The warm Gulf Stream current flows much faster when this large volume of water is squeezed into a narrow channel held in place by the cold waters of the North Atlantic. The volume of water transported by the Gulf Stream is about 55 to 106 cubic meters per second, or about 500 times the flow of the Amazon River.As the gyre rotates clockwise in the Northern Hemisphere, the water is pushed more toward the center. Deflection of the water through the column spirally increases as depth increases following the Ekman Transport model. There exists a calm area of sea in the middle of the North Atlantic commonly referred to as the Sargasso Sea where a myriad of unique ocean plants and animals have uniquely adapted to the quiet environment. As more and more water channels into this central core, the surface bulges more than 1 meter above the equilibrium sea level, which in turn exerts an additional pressure on the waters below. This process continues until the force of gravity that causes water to flow downward and away from the mound is equal to the force deflecting the water into the core. The balanced effect is referred to as geostrophic flow; the Sargasso Sea in the North Atlantic is in geostrophic balance.
The physical nature of the water in the North Atlantic is of interest to climatologists who study the change in the position and intensity of the Gulf Stream. The heat energy that is transferred via this surface current as well as the untapped kinetic energy of the moving water is of interest to alternative energy research. As the current brings warm water from the tropics to the northern latitudes and warms the landmass of Europe, it also is responsible for supplying some of the deep water that is part of the global thermohaline circulation. It is this very salty and now cold water that sinks off the coast of Newfoundland or in the Norwegian Sea becoming a part of the North Atlantic deep water. The subtle changes in the water chemistry due to changes in temperature, surface evaporation, overturning, and many other external variables pose a challenge to oceanographers to more fully understand the water planet.
Eddies or Ocean Storms
One of the most interesting features of the most western boundary currents is the tendency of these streams to have variation in the rate of flow. Much like a fast spray of water into a slow moving swimming pool, the pattern of waves develops as the fast spray catches some of the water of the pool. The fast moving spray wobbles forming pockets of energy that may eventually become separated from the main stream. These are known as eddies. Eddies can rotate clockwise or counterclockwise depending on the nature of their origin. Cold water eddies can be seen to the south of the Gulf Stream as portions of the Labrador current have been pinched off due to the meandering of the current. Altimeter readings enable oceanographers to tract the motion of the eddies and to hypothesize about their longevity and their influence on weather patterns. Cold core eddies are characterized by cyclonic circulation (counterclockwise) and are of negative value to the sea surface. Warm core eddies spin with anticyclonic circulation (clockwise) pushing warm surface water lower in the water column. Warm water expands more than colder water, so the surface within a warm eddy is elevated with respect to the surrounding water. Absolute sea height is not actually measured. What is measured is the difference from the mean sea height as calculated from several local and global models.Satellites and Tropical Storm Development
The ocean currents and the eddies may be studied using satellite altimeter maps that indicate sea surface height. Anomalies of sea surface height, which are the difference between the mean sea surface values and satellite altimeter readings, are related to the amount of heat in the upper layer of the ocean. The amount of heat energy may also be seen on a SST satellite diagram. Tropical storm development, cyclogenesis, is related to the amount of available heat in the surface water to fuel the atmosphere. A good example of a large sea surface height anomaly was seen during the development of Hurricane Floyd in the Atlantic Ocean in September 1999. In the Gulf of Mexico, the role of sea surface height anomalies has been correlated to the rapid intensification of Hurricane Bret and Hurricane Opal.
