3Oceanographic Analysis and Fleet Numerical Meteorology and Oceanography Center (FNMOC) Products


420. Wave-height analysis

421. Sea-surface temperature analysis

422. Layer-depth analysis

423. Combined wave height products

424. Fronts and eddies’ analysis considerations

425. FNMOC models and products

THERE are many different types of oceanographic analyses and products produced by the Fleet Numerical Meteorology and Oceanography Center (FNMOC) located in Monterey, California. We will only look at a few. However, the objective of the following lessons is to familiarize you with the need for such analyses, the evaluation of the data, analysis procedures, and some of the unclassified products FNMOC produces.

420. Wave-height analysis

The state of the sea is ever important in naval operations. It may aid, hinder, or negate maneuvers and operations, and is a primary consideration in routing. Ocean transits are not conducted without taking climatic, current, and forecast sea conditions into consideration. For example, wintertime ocean crossings in the North Atlantic Ocean are primarily conducted along more southerly routes. These routes are longer, but climatically they provide the best weather and seas. For current and future operations, knowing the location of favorable and unfavorable seas may be one of your jobs and one of the reasons to analyze sea heights. Beside transits, at-sea refuelings, replenishments, helicopter operations, and antisubmarine operations are all effected by the state of the sea.

There are different types of wave-height analyses. You may analyze wind waves, swell waves, combined waves, significant waves, etc. The only difference in the analyses is the wave type.

Heights, and directions of waves are plotted from shipboard synoptic observations. The plotted information consists of the following elements:

Wave-height plots are shown in figure 3–1. Note that the wave period is not plotted or analyzed.

As with any product analysis, you should review history, especially the most recent analysis. The most recent analysis is of primary importance, because waves are not very conservative. Sea heights are subject to significant change over relatively short periods of time. The most recent analysis gives you an idea of what to look for on the current product. We recommend transferring the highest labeled sea-height contours (high sea areas) onto the current product and beginning your analysis in one of these areas.

Another aid in analyzing sea heights is the current and past sea-level pressure (surface) products. Because most waves are wind generated, the wind directions, speeds, fetch, and duration are elements that will help you with your analysis. These elements are either plotted on the surface product or can be determined from the plotted data.

The sea-height analysis consists of contours drawn at 3-foot intervals, i.e., 3, 6, 9, 12. Much like isobaric analysis, you connect the reports of equal sea heights. Scan the product for reports that provide a continuous closed contour. In the high sea areas, you might begin with a 9-foot contour.

Connecting points of equal sea heights seems easy; however, a proper analysis calls for far more than connecting a series of like sea-heights. For example, a single contour may outline hundreds or thousands of square miles based on the reported heights but be incorrect. The reason the contour could be wrong can often be attributed to the analyst not taking into account wind and wave directions. A proper analysis requires that you check the wind and wave directions, as well as the heights.

Along coasts, the height of wind waves is controlled by the wind velocity (speed and direction), and bottom topography. If the wind is blowing onshore and is quite strong, the innermost sea-height contour might be quite high. With an offshore wind, the innermost contour will be small, usually 0 or 3. When the wind blows parallel to the shore, the sea heights may be large or small, depending on the speed and duration of the wind.

All sea heights near the shores of continents and large islands are influenced by land and sea breezes. Modification of the easterly wind field by these breezes can increase or decrease sea heights. Also, strong gravity winds that form when cold dense continental air flows down from on top of continental highlands produce high waves for short distances from shore.

Another factor affecting wave height is water depth. Shallow water, water that has a depth that is less than one-half the length of the waves passing over it, slows the waves and increases their height.

In the open sea, the height and direction of the sea is, for the most part, directly related to the wind. Where waves move with a current, the wave lengths increase and the heights lessen. The opposite holds true when seas oppose a current. In areas where strong currents oppose one another, waves can steepen even to the point of breaking. The North Wall of the Gulf Stream, where the Gulf Stream meets the Labrador Current, is one such area.

Sea-height contours are drawn as closed loops or continuous lines that begin and end along a coast. The closed contours delineate maximum or minimum sea areas such as those shown in figure 3–2.

Figure 3–2. Wave height analysis.

421. Sea-surface temperature analysis

Sea-surface temperature (SST) observations are plotted and analyzed to delineate relative warm and cold surface water areas. Figures 3–3 and 3–4 show two SST analyses examples. The fundamental information derived from SST analysis has many uses. In search and rescue operations, the temperatures are used to determine survival times. The table below shows sea water survival times based on temperature. In submarine and antisubmarine warfare operations, regions of strong SST gradients are extremely important because of their impact on sonar performance. And regarding meteorology, sea-surface temperatures play an important role in the development and dissipation of sea fogs, thunderstorms, sea and land breezes, low clouds, and tropical storms.

Figure 3–3. SST analysis of the western Mediterranean Sea.

Figure 3–4. SST analysis of the Newfoundland area.

Water Temp (° F)

Exhausted or Unconscious

Expected Survival Time


Less than 15 minutes

less than 15 - 45 minutes

32.5 - 40.0

15 - 3- minutes

30 - 90 minutes

40.0 - 50.0

30 –60 minutes

1 - 3 hours

50.0 - 60.0

1 - 2 hours

1 - 6 hours

60.0 - 70.0

2 - 7 hours

2 - 40 hours

70.0 - 80.0

3 - 12 hours

3 hours - indefinite

> 80.0



Plotted data

Usually, a single day’s collection of SST observations is insufficient for the preparation of a SST regional analysis. Therefore, several days worth of SST observations are plotted on the same product. The exact number of days may vary, but most products consist of 5 days’ worth of data. Such products are called composite products, because they are composed of more than one day’s data. Such an analysis is possible because sea water temperatures are quite conservative and are slow to change.

Erroneous SST values, and incorrect positioning of correct values can change the entire analysis picture as well as that of all the resultant products. Therefore, check all questionable temperatures.

Analysis procedures

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 analyst. The analysis is accomplished in much the same manner as meteorological products. For example, the first step is to transpose history onto the current product. The temperature patterns on the current product 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 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 2°C intervals. However, in areas of weak horizontal temperature gradients it may be necessary to analyze the isotherms to the nearest 1°C or even ½°C.

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

Current structure

Currents transport warm and cold water throughout the world’s 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, let’s 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 24°C, while the inshore shelf and slope water have temperatures of 14°C, and 18° C, respectively. Seaward of the Gulf Stream, the SST of the Sargasso Sea is 24°C. 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 (fig. 3–5.)

Figure 3–5. Gulf Stream current structure of the western North Atlantic Ocean in the winter.

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 a 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 isotherms tend to follow the outline of the bottom contours as you can see in figure 3–6.

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 world’s oceans are affected by freshwater runoff from continents by major river systems. The intrusion of less saline, cold water can create cold tongues in these regions.

Figure 3–6. Similarity of isobaths and isotherms.


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


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 counterclockwise 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 caused by the wind. Cross-current wind causes warm-water or cold-water advection, while wind that blows parallel to ocean currents causes no advective change in the SST.

422. Layer-depth analysis

The idea of the three-layered ocean–mixed layer, main thermocline, and deep layer–was mentioned earlier in this volume. Of these three layers, the mixed layer is the most variable in its properties (primarily depth), and requires considerable attention.

Mixed layer

We determine layer depths from temperature versus depth profiles obtained in bathythermograph observations. The mixed-layer depth is determined by finding the first subsurface depth with a temperature at least 1°C colder than the surface temperature. From this point, continue back up the profile to the previous depth–this is the mixed-layer depth, or MLD.

The temperature-versus-depth profile in figure 3–7 shows an isothermal layer of 24.5°C water in the first 15 meters. Between 15 meters and 55 meters the temperature decreases to 22.1°C. This temperature is maintained to a depth of 75 meters. Between 75 meters and the last reported depth of 145 meters the temperature drops rapidly to 7.6°C.

The first reported depth with a temperature at least 1°C colder than the surface temperature is 5 meters. The preceding temperature is 24.5°C at 15 meters. This is the depth of the mixed layer. Had the isothermal layer at the surface not been in existence and the temperature had simply decreased from the surface to 55 meters, the MLD would have been at the surface. In other words there would have been no mixed layer.

The importance of the mixed layer lies in the fact that it plays a vital role in anti-submarine warfare (ASW) operations. The mixed layer also plays a key role in the food chain. Some of the most productive fishing areas are found where strong mechanical and/or convective mixing occurs.

Plotted data

Like SST data, more than one day’s worth of layer-depth data (fig. 3–8) is plotted on the same product. Plotted layer depths may or may not have a (+) or a (p) plotted behind the report.

A (+) denotes the reported value is or may be greater than that indicated. A (p) is plotted if a positive temperature gradient exists from the surface to the MLD. If both a positive gradient and a possibly deeper layer depth exist, the (p) and (+) are both plotted.

Analysis procedure

The mixed layer is analyzed by drawing isopleths of equal layer depth. Like isobars, these lines should be smooth flowing and never cross or touch. Unlike isobars, there will be no sharp turns or kinks. Isopleths are drawn at 60-meter intervals in winter and 15-meter intervals in summer. In winter, the mixed layer extends to much greater depths than in summer; therefore, the 60-foot interval is used. In summer, the mixed layer is much shallower and the 15-meter interval is required.

As you might expect, there are even less bathythermograph observations to work with than there are SST observations. Therefore, history is extremely important.

Another aid in regions with few or no layer-depth observations is the latest SST analysis. The SST analysis is used to determine the SST advection pattern, which in turn is used to evaluate reported layer depths and to justify increasing or decreasing layer depths in a region.

In regions where cold SST advection takes place, the mixed layer generally increases; it gets deeper. The opposite applies in regions of warm advection; the layer depth generally decreases.

Variation in the depth of the sonic layer may occur as a result of several different factors, one of which is diurnal heating and cooling. Usually, the near-surface diurnal temperature variation averages about 0.5° C; however, under extreme conditions the SST may vary as much as 3°C. As the sea surface heats during the day, a conditional thermocline (NOT to be confused with the main thermocline) often develops within 10 meters of the surface. The maximum temperature gradient associated with this conditional thermocline occurs during the late afternoon, and the normal layer depth is all but destroyed. Such an occurrence is called the "afternoon effect." At night when the sea surface cools, the conditional thermocline disappears and the sonic layer again descends to the depth of the main thermocline. That daytime as well as nighttime sea-surface temperatures are plotted on composite products helps to overcome such errors in layer-depth (LD) analysis.

423. Combined wave height products

These products depict the instantaneous state of the seas as a combined effect of wind and swell waves. The combined sea height products use the following formula to compute a single height value from wind and swell waves:


Hc = Combined sea height.

Hw = Height of wind wave.

Hs = Height of swell wave.

The combined height value is plotted along with the heading of the swell wave (an arrow) and is transmitted from the weather centers over HF broadcasts. Isoheights for ocean products are normally drawn in three-foot increments, and labeled. Heavy seas are emphasized by enhancing the 12-foot contour line. Maximum heights and minimum heights are marked and labeled within the appropriate contour/region.

424. Fronts and eddies’ analysis considerations

The synoptic analysis of fronts and eddies presents a difficult problem due to poor alphanumeric data distribution/quality and weather systems that impair satellite interpretation of ocean thermal contrasts. Computer graphics, available from Naval Oceanographic Office (NAVOCEANO) by computer link, maintain a good history of documented fronts and eddies and provide a starting point from which to build the next composite analysis. Greater technical expertise in satellite interpretation is required to accomplish an adequate analysis than in the past.


Most alphanumeric data are concentrated along major shipping lanes. With large data void areas across most of the world’s oceans, satellite imagery becomes a major source of information. Computer graphics require human input, and may lag behind current satellite data. Here, the satellite interpretation would override the graphic product. NOTE: A satellite image may be the only input you have for updating fronts.


SST reports have an uncertainty of at least 1°C even in data dense areas, due to the following reasons:


Since sea water is super ultraconservative regarding temperature changes over time, history should be the back bone of the analysis. Once a good analysis has been established, the changes should be small from day to day.

Analysis techniques

Know the climatology of the region. Also, know the following characteristics of the region:

Do not radically change major features from day to day. Start the analysis in regions that have dense coverage.

Analysis types

There are two types of analysis to consider when analyzing for ocean eddies and fronts.

Horizontal analysis

Maintain an awareness of data scarcity especially near frontal boundaries. The data available may not allow for proper analysis of the gradients present. Consider the norms for the water masses present. Ensure that gradients are representative. Allow openings in isotherms for the flow of currents. If data sparse areas exist, request, if possible, additional bathythermograph drops from ships and aircraft. Lastly, stack the analysis and maintain continuity and slope (cold water wedges under warm water).

Vertical analysis (track lines and vertical cross sections)

The change between points is interpolated and may not represent high gradient areas (points along a line are great distances apart). Ensure you apply a cross section from the proper perspective. Your orientation changes if end points on a line are selected in a different order (e.g., E to W or W to E).

425. FNMOC models and products

There are many computer-generated surface, upper-air, and oceanography products available from FNMOC. In this lesson we will only look at some of the systems, models and products that FNMOC produces.

Navy Oceanographic Data Distribution System (NODDS)

NODDS provides the users the accessibility to build their own products from the FNMOC data base. It used a PC and phone modem to access the data. Numerous products can be provided to the user in any global region.


The purpose of NODDS is to upgrade and improve the support for Naval Oceanography Command (NAVOCEANCOM) activities by providing a method of obtaining tailored environmental products for areas of responsibility.

Regional centers

Currently there are five regional centers that collect and compile gridded field data from the FNMOC and transmit over high-frequency communications to the fleet. Each center provides tailored product transmission according to fleet requirements.

Naval Eastern Oceanography Center (NAVEASTOCEANCEN)

This center is located in Norfolk, Virginia and is responsible for the support of the Atlantic Ocean from 60° N to 60° S.

Naval Oceanography Command Center (NAVOCEANCOMCEN)

There are two locations responsible for this large area. The first center is located in Rota, Spain and is responsible for the Mediterranean Sea area. The second center in Guam is responsible for the West Pacific and Indian Ocean from 60° N to 60° S.

Naval Western Oceanography Center (NAVWESTOCEANCEN)

Located in Pearl Harbor, Hawaii this center is responsible for the Eastern Pacific from 60° N to 60° S.

Naval Polar Oceanography Center (NAVPOLAROCEANCEN)

This center located in Suitland, Maryland is responsible for supporting the Arctic and Antarctic regions.


NODDS uses a desktop microcomputer to define geographical areas of interest and allow selection of required environmental data. An automatic link is established to access FNMOC’s worldwide environmental data bases using commercial communications’ software and value-added network data lines. Subsets of information are extracted and compacted for transmission back to the microcomputer. Automation keeps the communications connect time to a minimum and a shell program provides the mainframe computer security. Information is received from FNMOC in grid format and the microcomputer calculates isoline or wind barb information before displaying. Once processed, the data can be displayed on the defined geographical background.

The flexibility of the microcomputer allows:

Naval Operational Global Atmospheric Prediction System (NOGAPS)

NOGAPS is the principal model system in FNMOC operational data runs for analyzing upper-air data.

Naval Operational Regional Atmospheric Prediction System (NORAPS)

As of this writing, NORAPS is used in operational data runs to provide 36-hour fine-mesh forecasts for four geographical regions: the Mediterranean Sea, western Pacific Ocean, Indian Ocean, and the northwestern Atlantic Ocean. Data from two other regions are also run through the NORAPS program: the Eastern Pacific and the North Pole. However, these two regions are available only on request.

Tactical Environmental Support System (TESS–3)

TESS–3 provides the user with a satellite-link to a regional center to download gridded field data for product assembly.

Optimum Path Aircraft Routing System (OPARS) computer flight plan

The purpose of OPARS is to provide flight planning service to the naval aviation community. OPARS is a set of computer programs that select the optimum routes for aircraft in support of flight operations.

Forward looking infrared forecasts (FLIROUT)

FLIROUT computes the range at which FLIR sensors onboard aircraft will see target types.

Sea water transparency products

At this time sea water transparency products are available for the North Atlantic only. Water transparency affects visual detection of submarines. The factors affecting underwater visibility include:

Bioluminescence products

Submarine bow waves, periscope wakes and torpedo wakes may become apparent when bioluminescent organisms become disturbed. Submarines have been observed to depths of 270 feet when outlined by bioluminescence. The length of the observed wake determines the speed of the submerged craft.

Self-Test Questions

After you complete these questions, you may check your answers at the end of the unit.

420. Wave-height analysis

1. There are different types of wave-height analyses, however, what is the only difference between the types?

2. What information is plotted for a wave-height analysis?

3. Identify some of the different aids used for analyzing sea heights.

4. What contour intervals are used when analyzing sea heights?

5. What controls the height of wind waves along coasts?

6. When analyzing a sea-height product, what would be the innermost contour for an offshore wind?

7. Where do sea-height contours begin and end when not drawn as a closed loop?

421. Sea-surface temperature analysis

1. Why are sea-surface temperature (SST) observations plotted and analyzed?

2. What roles do SSTs play in meteorology?

3. What is the first step in analyzing SSTs?

4. What is the normal isotherm interval for SSTs?

5. What are some of the things to consider before starting your SST analysis?

6. What are some of the local characteristics of SST analysis?

7. Why are eddies so difficult to depict from plotted SST reports?

422. Layer-depth analysis

1. Which layer of the three-layered ocean is the most variable? Explain why.

2. How is the mixed-layer depth determined?

3. What isopleth intervals are used when analyzing the mixed-layer depth?

4. What happen to the mixed layer during cold SST advection? Warm SST advection?

423. Combined wave height products

1. What does the combined wave height product depict?

2. What are the isoheight intervals used on combined wave height products?

424. Fronts and eddies’ analysis considerations

1. Why is satellite data a major source of information when analyzing for the locations of fronts and eddies?

2. Why do SST reports have an uncertainty of at least 1° C even in data dense areas?

3. What is the backbone of any SST analysis?

4. Describe the two types of ocean eddy and front analyses?

425. FNMOC models and products

1. What does the flexibility of the NODDS microcomputer allow for?

2. Match the information on each different FNMOC systems, models and products in column A with its correct name in column B. Items in column B may be used only once.

Column A

___1. Shows submarine bow waves.

___2. Available for the North Atlantic Ocean only.

___3. Provides flight planning service to naval aviation.

___4. Used in operational data runs for four geographical area.

___5. The principal model system for analyzing upper-air data.

___6. Provides the user with a satellite link to a regional center.

___7. Computes the range that aircraft IR sensors will see a target.

___8. Allows the user the accessibility to build their own products.

Column B

a. TESS–3.






g. Bioluminescence products.

h. Sea water transparency products.

3. What factors affect underwater visibility?

Answers to Self-Test Questions


1. The wave type.

2. A circle or dot designating the observation point, a figure representing the wave period, a figure representing the wave height and an arrow representing the direction toward which the waves are moving.

3. Current and past sea-level pressure products.

4. Three-foot contour intervals.

5. The wind velocity (speed and direction) and bottom topography.

6. Ordinarily 0 or 3.

7. Along a coastline.


1. To depict relatively warm and cold surface water areas.

2. The development and dissipation of sea fogs, thunderstorms, sea and land breezes, low clouds, and tropical storms.

3. Place the history onto the current product.

4. 2° C intervals.

5. The structure of the current, bottom topography, local characteristics, and prevailing winds.

6. Areas of freshwater runoff, areas of upwelling and eddies.

7. They are somewhat small, 60 to 100 miles wide.


1. The mixed layer because of its depth.

2. First find the first subsurface depth with a temperature at least 1°C colder than the surface temperature. From there, continue back up the profile to the previous depth.

3. They are drawn at 60-meter intervals in the winter and at 15-meter intervals in the summer.

4. The mixed layer generally increases; it gets deeper. The layer depth generally decreases in regions of warm advection.


1. The instantaneous state of the sea as a combined effect of wind and swell waves.

2. Normally in 3-foot intervals with the 12-foot contour line emphasized during heavy seas.


1. Because most of the data is concentrated along the major shipping lanes leaving large data void areas across the world’s oceans.

2. Because of the uncertainty of the ships’ exact position; the injection temperatures vary from ship to ship and reflect subsurface temperatures, not surface temperatures; bathythermograph temperatures are often sparse and may have encoding errors; SST reports may not be available or not a part of the standard operating procedure for analysis at some commands.

3. History.

4. Horizontal analysis and vertical analysis (track lines and vertical cross sections).


1. Multi-colored displays, zooming of user-selected areas, printing, and data archiving.

2. (1) g.

(2) h.

(3) b.

(4) e.

(5) d.

(6) a.

(7) f.

(8) c.

3. The transparency and color of the water, the amount of surface reflection and refraction, the contrast between the target and the background, the amount of illumination and the optical image of the submerged object.

Unit Review Exercises

Note to Student: Consider all choices carefully, select the best answer to each question, and circle the corresponding letter. When you have completed all unit review exercises, transfer your answers to ECI Form 34, Field Scoring Answer Sheet.

Do not return your answer sheet to ECI.