The analysis of sea-surface temperatures is quite subjective; however, if each analyst adheres to the same general rules, the hand-drawn product should be approximately the same for each individual analyst. The analysis is accomplished in much the same manner as meteorological charts. For example, the first step is to transpose past history onto the current chart. With SST charts, we use a yellow pencil to transpose one or two of the more prominent isotherms. The temperature patterns on the current chart will not differ a great deal from the history because the oceans are very conservative and the temperature patterns change very gradually. This tendency toward gradual change

Table 1-4-1.-Seawater Survival Times

must always be kept in mind, and any data that reflects a major change in a temperature pattern should be closely examined.

With the history transposed, the next step is to draw the isotherms. Sea-surface isotherms are normally drawn at 2C intervals. However, in areas of weak horizontal temperature gradients it may be necessary to analyze the isotherms to the nearest 1C or even 1/2C.

Some of the things you should consider before starting your analysis are current structure, bottom topography, local characteristics, and prevailing winds.

CURRENT STRUCTURE. Currents trans-port warm and cold water throughout the worlds oceans. Knowing the current or currents that exist in an area will help you in evaluating the SST data and in drawing the isotherm patterns. For example, lets look at the Gulf Stream system of the western North Atlantic Ocean in the winter. Off the Virginia coast, typical Gulf water is warmer than 24C, while the inshore shelf and slope water have temperatures of 14C, and 18C, respectively. Seaward of the Gulf Stream, the SST of the Sargasso Sea is 24C. Also, north of Cape Hatteras the warm Gulf Stream meets the much colder Labrador current producing a tightly packed, wave-like isotherm pattern. The strong horizontal temperature gradient makes for a well-defined, sharp boundary on the cold-water side of the Gulf Stream, while the wave-like pattern is created by alternating extensions, or TONGUES, of cold and warm water. See figure 1-4-3.

On the warm-water side of the Gulf Stream there is little temperature contrast between the Gulf Stream and the water of the Sargasso Sea. Because of the large horizontal temperature gradient, this boundary is much more difficult to distinguish through SST analysis. Knowledge of such current information is invaluable in conducting your SST analysis.

BOTTOM TOPOGRAPHY. The ocean floor becomes a factor in SST analysis in shallow waters. Isotherms and isobaths (lines of equal water depth) show marked similarity. The

Figure 1-4-3.-Gulf Stream current structure of the western North Atlantic Ocean in the winter.

isotherms tend to follow the outline of the bot-tom contours. See figure 1-4-4.

LOCAL CHARACTERISTICS. Some local characteristics of SST analysis are areas of freshwater runoff, areas of upwelling, and eddies. 

Freshwater Runoff. Some coastal regions of the worlds oceans are affected by freshwater runoff from continents via major river systems. The intrusion of less saline, cold water can create cold tongues in these regions.

Upwelling. Another area of distinct temperature patterns occurs in regions of upwell-ing. The sea-surface temperatures in these regions are colder than the water surrounding such regions. Also, depending on the strength of the upwelling, the sea-surface temperatures can be colder than what otherwise might be expected.

Eddies. Eddies, independent circulations or rings of cold or warm water, are another feature of SST analysis. They form along major current boundaries and are most prevalent in the western portions of the oceans. For example, warm eddies form on the north side of the Gulf Stream and drift into the colder waters of the Labrador current. The warm eddies maintain a clockwise rotation. Cold eddies form on the south side of the Gulf Stream. They maintain a counter-clockwise circulation. Eddies are difficult to delineate from plotted SST reports, because of their relatively small size (60 to 100 miles wide).

PREVAILING WINDS. The changes that take place in SST patterns can primarily be attributed to the advection of cold or warm water brought about by the wind. Cross-current wind causes warm or cold water advection, while wind that blows parallel to ocean currents causes no advective change in the SST.

Learning Objective: Evaluate layer-depth data and recognize its uses and analysis procedures.

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