The mean surface positions of the major oceanic frontal systems of the world are depicted in figure 3-1-1. You will note that most of the fronts are located along the boundaries of surface oceanic currents. These fronts are termed "permanent", because they are observed during all seasons in the same general geo-graphic location. These fronts do not move significantly from their mean position. Oceanic fronts that are comparatively short lived and show considerable variation in location are termed "transient". Such fronts may exist from a few days to several months. They are primarily the result of seasonal water changes, regional upwelling, open-sea convergence and diver-gences, pronounced surface heating or cooling, or river runoff.

Oceanic fronts separate water masses of different densities, and since the density of seawater is a function of temperature and salinity, there are thermal (temperature) fronts and haline (salinity) fronts. Oceanic fronts are found in the upper layers of the oceans in areas of pronounced horizontal temperature and/or salinity gradients. One such area is off the east coast of the United States where the Gulf Stream interacts with the much cooler coastal waters and the cold Labrador current.

The Gulf Stream front is located in the region of the sharply defined thermal gradient that exists between the Gulf Stream water and the coastal waters. A typical vertical cross section of temperature across the Gulf Stream during spring is shown in figure 3-1-2. Note the sharp vertical temperature gradient on the coastal side of the Gulf Stream. The persistence of this gradient has given rise to the term north wall to describe this portion of the Gulf Stream. The north wall, as revealed by the temperature contrasts in satellite imagery, delineates the surface synoptic location of the Gulf Stream frontal system. In addition to the temperature and/or salinity differences across oceanic fronts, there may be differences in water color, wave height, and current velocity.

Oceanic fronts and currents can be monitored in infrared satellite imagery. Temperature differences across the frontal zones produce distinct gray shade patterns which reveal the frontal systems, including meanders and eddies. Regions of upwelling, river runoff, and current boundaries can also be distinguished in imagery through gray shade differences. Figure 3-1-3, an infrared picture taken in March 1988, shows the shades of gray associated with the thermal contrasts along the eastern coast of the United States.

Distinguishing oceanic features in visual imagery is far more difficult than it is in infrared imagery. It is possibIe to distinguish a front that lies in an area of sunglint if the difference in sea states across the front is strong enough to produce light and dark shading in the frontal zone. It is important to recognize that the oceanic frontal features seen in satellite imagery are those of the oceans upper layers only. Just as meteorological fronts extend upward into the atmosphere, oceanic fronts extend downward into the ocean. For example, fronts associated with major currents often extend to considerable depth (Gulf Stream 3,300 ft; Kuroshio 2,300 ft), while fronts formed by surface heating and cooling or river runoff are quite shallow (165 ft, or less). Below the surface across these fronts, there may be differences in light transmission, dissolved chemicals, biological population, and sound velocity propagation. The latter two are very important in sonar applications.

Knowledge of oceanic fronts is extremely important in submarine and anti-submarine operations, as they are the basis of many tactical decisions.

Learning Objective: Identify the two types of oceanic eddies and the type of satellite imagery used to locate and differentiate between them.

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