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8/13/2019 Oceanography OE ALL

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The temperature of the top (mixed) layer varies seasonally in response to variations in solar

heating. The solid curve shows the winter conditions in the mixed layer and the dotted curve

shows the seasonal thermocline that exists during spring warming. Note that this seasonal

thermocline is confined to the mixed layer. The dashed curve is the seasonal thermocline in

extreme summer conditions. The permanent thermocline is not found at high latitudes where the

surface waters are very cold and there is little variation in water temperature with depth. It is in

these high latitude regions that cold surface waters can sink to depth. This process is responsiblefor the vertical circulation of ocean waters, called thermohaline circulation because density

differences due to temperature and salinity differences are responsible for the circulation. Twoother terms, parallel to the term themocline, are used in oceanography. The halocline is the zone

of rapidly changing salinity between relatively low salinity surface waters and deeper more saline

waters. The pycnocline marks a zone of rapidly changing density.

Upwelling, the movement of deeper water towards the surface and downwelling, movement ofsurface waters to greater depth, are important processes in the ocean. Upwelling and downwelling

can occur locally in coastal waters or on a larger scale involving significant volumes of ocean

water. During upwelling deeper cold water is brought towards the surface and isotherms deflect

upwards. During downwelling warm surface water is moved to greater depths and isotherms are

deflected downwards. There are five basic types of upwelling (Figure 3). Note that downwellingis simply the reverse of these processes. For example, if the wind is blowing onshore, rather than

offshore (Figure 3b), downwelling will occur.


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A. Ekman-transport upwelling.Wind blowing across the surface water sets the water intomotion. Because of the Coriolis effect water in the Northern Hemisphere is deflected to

the right of the wind direction and water in the Southern Hemisphere is deflected to the

left. Some of the momentum of the moving surface waters is transferred to the deeper

waters setting them in motion. As we go down in the water column the momentum

transfer decreases and at some depth the water motion is zero. At all the depths above this

the water is deflected to the right (NH) or left (SH), the slower the water movement thegreater the amount of deflection. This variation in direction of water movement with

depth is referred to as the Ekman spiral (Figure 4). If we sum up the total watermovement throughout the Ekman spiral the net movement is at 90o to the surface wind.

Referring to Figure 3a, you are in the Northern Hemisphere and the wind is blowing to

the north (a southerly wind since winds are named for the direction from which they

come). The net movement of water is to the right away from the shore. The offshore

movement of the surface water is offset by the movement of deeper waters to the surface,i. e., upwelling.

B. Wind-driven upwelling. In this case an offshore wind causes water to move awayfrom the coastline. This water is replaced by deeper water moving to the surface (Figure

3b).

C. Open-ocean Coriolis-effectupwelling. The divergence of

surface currents causes deeper

waters to move to the surface

replacing surface waters that

move away from the zone of

divergence (Figure 3c).

D. Obstruction upwelling. Acurrent moving past a headland

or other obstruction will drawwater away from the obstacle

and upwelling will occur

(Figure 3d).

E. Density-driven upwelling. Colddenser surface waters sinking to

depth force less dense waters to

the surface (Figure 3e).

Figure 4.Diagram illustrating the Ekman spiral. (a) A body of water can be thought of as a set ofslabs, the top one driven by the wind and each one below set in motion by momentum transfer.

With depth each layer moves at a slower speed. (b) Vectors showing water motion as a function

of depth. Average waver movement is at 90 to the surface wind direction. The example is set in

the Northern Hemisphere. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2ndEd. New York: Freeman, p. 73.


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1. Figure 5 is a temperature cross section off Point Conception. Contour the profile down tothe 8C isotherm using a 0.5C contour interval. It is suggested that you begin contouring

at the bottom (8C isotherm) and work upward.

a. What process is indicated by the shape of the contours at shallower depths?

Figure 5.


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2. Temperature-salinity diagrams and the identification of water masses

A water mass is a large volume of water that can be identified as having a common origin or

source area. Water masses are formed through interactions with the atmosphere or by the mixing

of two or more bodies of water. Because mixing between water masses and their surroundings is

slow, water masses tend to retain their original temperature and salinity. These distinctive

temperatures and salinities can be used to identify the water masses. This identification isimportant because it gives us information about their place of origin, deep circulation, and the

rates at which waters of different densities mix. Table 2 lists the characteristics of the water

masses found in the North Atlantic Ocean.


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2. Oceanographic data from a typical station in the North Atlantic located at about 20Nlatitude are listed in Table 3. Plot these data on Figure 6 and draw a line connecting all

the points in the order of depth. Using the diagnostic parameters in Table 2, name the

major water masses represented in the diagram.

Figure 6.Blank T-S diagram for question 2. Plot the data from Table 3 on this diagram and draw

a line connecting all of the points in order of increasing depth. From Pipkin et al., 1987.

Laboratory Exercises in Oceanography, 2nd Ed.New York: Freeman, p. 95.


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3. Tides and Currents

Tide-Producing Forces

Tides are produced by the gravitational attraction between the Earths ocean water and the moon

and sun. Even though the sun is a much larger and more massive astronomical object than the

moon, it is much farther away from the Earth, so it turns out that the moons pull on the ocean isabout twice as strong as that of the sun. If we consider only the moons effect on the ocean, the

resultant simplified tides would consist of two large bulges (Figure 7). The bulge on the side of

the Earth facing the moon is the result of the moons gravity pulling on the water. The bulge on

the side facing away from the moon is due to inertia(centrifugal forces) resulting from the

two-body rotation of the Earth-moon system (like two figure skaters holding hands and spinning

around together). Of course the Earth also rotates on its axis with a 24-hour period, so it appears

that the bulges shown in the figure rotate around the planet. If these were the only forces affecting

the tides, each oceanic location on Earth would experience two high tides each day as the bulges

pass by, and two low tides each day in between the bulges. Unfortunately, its more complicated

than that.

Figure 7.The formation of tidal bulges at points toward and away from the moon.

We also have to consider the smaller gravitational pull of the sun in conjunction with the moons

effects described above. As Figure 8 shows, if the Earth, moon, and sun are all in a line, the lunar

and solar tides will be additive, resulting in higher high tides and lower low tides. Tides duringsuch conditions are called spring tides, and coincide with a full moon or new moon. [Note: The

term springtide has nothing to do with the season spring] If the Earth, moon, and sun form a

right angle, the gravitational forces from the sun and moon act at crosspurposes,and this results

in lower high tides and higher low tides (i.e., less extreme water levels). Tides during such a

configuration are known as neap tides, which coincide with quarter moon phases. Since the

moon revolvesaround the Earth on an approximately 28-day cycle, we observe spring tidesabout

every two weeks, and likewise for neap tides. This means that we often see two spring tide


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episodes and two neap tide episodes each month (not exactly, because our calendar months dont

exactly coincide with the 28-day lunar month).

Figure 8. Relative positions of the Earth, moon, and sun during spring and neap tides.

Classification of Tides

The combined effects of the lunar tide and solar tide, combined with the complicating effects of

the shapes of ocean basins and land masses on Earth results in daily tidal variations considerablymore complicated than the simple two-bulge system depicted in Figure 8. Tides can be classified

into three general types based on their daily variations. See Figure 9 below.

Semidiurnal tides: Have two high and two low tides each tidal day (also called lunar day, which

is 24 hours, 50 minutes), with little or no difference between consecutive high and low tides. Thistype of tide is characteristic of the East Coast of the United States.

Diurnal tides: Have only one high and one low tide per day. The U.S. Gulf of Mexico coastexhibits this type of tide.

Mixed tides: Have both diurnal and semidiurnal characteristics. The two high tides are unequal in

height, and the two low tides are also unequal. These differences are called diurnal inequalities.

The tides of the Pacific Coast of the United States, including Puget Sound, are of the mixed type.


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Figure 9.Typical tide curves for the three common types of tides.

Tide Heights and the Tidal Datum

Tide heights are measured (and predicted in tide tables) with respect to a local zero or base level,

known as a tidal datum. While it may seem that it would have been logical to choose a standard

worldwide reference datum like mean sea level to use everywhere, that has not been the historical

standard used, and were stuck with something else. In some areas the datum chosen is mean lowwater (MLW), which is the average of all low tide levels at a particular tide station. The typical

datum used in the United States, however, is mean lower low water (MLLW), which is the

average of the lowest tide each day at a given station (remember there are often diurnal

inequalities). Once this datum is established for any location, it becomes the zero point for all

tide measurements and predictions at that location.

Figure 10.Depths for various tidal datum planes.


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PLOTTING A TIDE GRAPH

Complete the tide curve graph for Days 2, 3, and 4 by plotting the tide heights given in the table:

Based on your tide graph, answer the following questions:


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1. During what day(s) is the tide diurnal?

2. During what day(s) is the tide semidiurnal?

3. During what day(s) is the tide mixed?

4. What is the largest range, and on what day does it occur?

5. What is the smallest range, and on what day does it occur?

6. What is the largest diurnal inequality, and what day does it occur? [Hint: Be sure to see your

lab background information for the definition of diurnal inequality. There will be one number

for high tides and one number for low tides.]


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4. Currents

Origin of surface currents

Surface currents arise due to the interaction of the prevailing winds and the ocean surface. Hence

the surface wind pattern (Figure 11) plays a key role in establishing the surface currents. Once the

water is set in motion it is acted upon by the Coriolis force. This force causes moving water in the

Northern Hemisphere to deflect to the right and moving water in the Southern Hemisphere todeflect to the left. These deflections set up the large scale gyres that characterize the surface

circulation pattern (Figure 12). Because the earth rotates from east to west, the centers of the

oceanic gyres are offset to the west. Thus currents on the western side of ocean basins are

narrower then currents on the eastern side. What this means is that in order for the volume of

water moving through the gyres to remain constant currents must flow faster on the western side

of the ocean basin. This is referred to as the westward intensification of oceanic currents. Also

note that the surface currents move warm water from lower latitudes to higher latitudes, and

conversely move colder water from higher latitudes to lower latitudes.

Figure 11.Major wind belts of the earth and zones of high and low pressure. Note that the

meteorological equator is 5 10 Degree north of the geographical equator.


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Figure 12.Major surface currents of the worlds oceans.

2. With reference to Figure 12, describe the following currents as either warm or cold, and fast or

slow.

Bathymetric Charts


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5. BATHYMETRIC CHARTS

Nautical charts are maps of a region of the ocean used primarily for navigation and piloting.

These charts display the bathymetry or depths of the sea floor below sea level. Historically, the

sea floor depths were obtained by lowering a weighted cable to the sea floor. Today, sea floor

depths are obtained with a ship-mounted sonic depth recorder which bounces sound waves off

the sea floor (Figure 13-1a). A sound generator on the ship emits sound waves that strike the seafloor and are reflected upward to a listening device called a hydrophone. The method is faster,

more accurate and allows continuous depth determination as a ship travels. Each measurement of

depth to the sea floor is called a sounding.

Figure 13.Acoustic Depth-Sounding.A ships hull-mounted acoustic pinger emits a sound

pulse that travels to the seafloor, reflects, and then travels back to the ships listening devise

called a hydrophone. The roundtrip travel time of the sound pulse is recorded and the depth is

computed (see formula). The depth recorder operates continuously making a dense set of depth

measurements along the ships track.

Shipboard computers record the round-trip travel time of the sound waves and calculate depth by

multiplying the known speed of sound in water (Sw = 1460 meters/second) by half of the traveltime:

D =Sw x travel time ,

where the depth of the water D in meters is the product of the sound speed in water and one half

of the total travel time to the bottom and back.


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Bathymetric charts are constructed from arrays of depth soundings by drawing a set of contour

lines (or isobaths); each of which connects points of equal depth. The example in Fig. 14shows

an idealized region in which the sea floor slopes smoothly away from the coast and howthat is

represented on a nautical chart. The 20 contour line connects all 20 ft sea floor depthsrelative

to the mean sea level (i.e., the 0 ft datum or long-term time-average of sea level). Likewise the

40 contour connects depths of 40 ft below sea level and so on. The numbered contour lines are

the index contours; the unnumbered contour lines are the supplemental contours. The differencebetween two adjacent contours is called the contour interval, which is 10 ft for the Fig. 14

example.

Figure 14a. The bathymetry or depth distribution of the ocean in the upper panel is depicted by

the set of depth contour lines (in units of feet) on chart below. Note that the closer together the

contours, the steeper the slope of the sea floor.


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Figure 14b. The bathymetry or depth distribution of the ocean

CHART SCALE AND HORIZONTAL DISTANCE

Charts represent the Earth's surface, but at a reduced size. To interpret the chart properly, it is

important to know the chart scale, that is the fixed relationship between a distance on the chart

and the corresponding distance on the Earth. For example, when one centimeter (cm) on the chart

equals 125,000 cm (which equals 1250 meters (m) or 1.25 kilometers (km)) on the Earth. The

chart scale can be given as a fraction 1/125,000 or the ratio 1:125,000. Effectively the size of theEarth's surface has been reduced or scaled down by 125,000 times so that it can fit on the chart.

All useful charts contain a bar scale (Fig. 15) which is used to interpret chart distances in terms

of real Earth distances. The total length of the bar scale in Fig. 15 represents a total Earth distance

of four km which is subdivided into both 1 km and 0.25 km segments.

Figure 15. Graphic bar scale. Always note the 0 position.

DETERMINING SLOPE OR GRADIENT

The slope of the sea floor (or gradient) may be numerically expressed as a ratio, percentage, or

angle. Slope is the ratio of the relief (or change in depth of a sea floor feature) to the horizontal

distance over which the slope is measured, according to

Slope = relief/horizontal distance of slope ,


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where Slope units can be feet/mile, meter/kilometer, fathom/mile, or fathom/kilometer. The slope

can be converted to a percentage by converting the slope ratio to a quantity without units and

multiplying it by 100% according to

% Slope = (relief/horizontal distance of change in the same units) x 100%

For example, a slope of 100 ft/mi would have a percent slope of

(100 ft/mi) x (1 mi/5280 ft) x 100% = 1.9%.

Slopes and percent slopes are given in angles (with units of degrees ) in Figure 16. Note that a

horizontal line has 0% slope and an angle of 0; and that a 100% slope has angle of 45. A

vertical line has an angle of 90 and a percent slope of infinity.

Figure 16. The different slopes are given as percent slope, angle in degrees, and feet per mile in

the picture above.

Vertical exaggeration of profile plots

For a graphical profile to illustrate the true shape of the sea floor, a ratio of 1:1 for vertical and

horizontal distances must be the same or have a ratio of 1:1. This means that one unit on the

vertical scale is the same distance as one unit on the horizontal scale.

However, the slopes of ocean features (i.e., relief) are generally so small that it is difficult to see

sea floor features. Typical Atlantic Ocean basin features are only a few kilometers high, while the

basin itself extends laterally for thousands of kilometers. If the profile were displayed with a 1: 1

ratio on a regular sheet of paper, it would appear as a flat line.

In order to illustrate the details of the sea floor relief, the depth scale of an ocean profile is

vertically exaggerated (stretched) relative to the horizontal scale. Vertical exaggeration causes


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distortion in the shapes of the bathymetric features that are being illustrated; with the amount of

distortion increasing with the amount of exaggeration. To convince yourself of this, draw a circle

on a wide rubber band. As the rubber band is stretched, the circle will be distorted into an oval

and eventually an ellipse. As vertical exaggeration increases on a profile, hills appear to be

higher, valleys deeper and the slopes between them become much steeper. Slopes, that in reality

are gentle, will look steep; steep slopes will appear to be precipitous.

For example, when the vertical scale has been stretched four times relative to the horizontal scale,

we have a vertical exaggeration (VE) of 4. This vertical exaggeration or stretching can be easilydemonstrated with a rubber band. On an unstretched rubber band, measure and mark quarter inch

segments for the distance of an inch. We will assume that an inch represents 100 ft so that each

quarter inch equals 25 ft-as illustrated in Fig. 17. Now stretch the rubber band until the original

inch is 4 inches long. If you measure along the stretched rubber band, then you will discover that

1 inch now represents only 25 ft. Vertical distances have been stretched by a factor of four.

Figure 17. Vertical exaggeration on a rubber band.

Thus for most profiles, there are two scales - one for horizontal distances and an exaggerated one

for the vertical distances. In the above example, the horizontal scale is 1 in = 100 ft and the

vertical scale is 1 in = 25 ft. The vertical exaggeration (VE) associated with a general profile is

found by dividing the horizontal scale by the vertical scale according to:

VE = Horizontal Scale/Vertical Scale

For this example,

VE = (1 in = 100 ft) / (l in = 25 ft) = 4

CONSTRUCTION OF A BATHYMETRIC PROFILE

To construct a bathymetric profile follow the following steps (Fig. 18).


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1. Choose the line, called a trace (or transect), on the chart along which you want to construct the

profile. Determine the total vertical excursion along the transect by subtracting the value of the

shallowest contour from the deepest contour that crosses the trace.

2. Determine the desired vertical exaggeration (VE) and then scale the profile sheet

appropriately. The profile sheet is scaled by drawing a series of equally spaced parallel

lines the length of the transect. The distance between the lines is determined by the verticalexaggeration.

For example, if the horizontal scale is 1 in = 6000 ft and the vertical exaggeration is x 12,

then the parallel lines could be spaced 1/5 inch apart with each line representing a change indepth

of 100 ft according to:

VE = Horizontal Scale/Vertical Scale12 = (1 in = 6000 ft)/Vertical Scale

Vertical Scale = (1 in = 6000 ft)/l2

Vertical Scale = (1 in = 500 ft)

3. Label the lowest line on the profile sheet with the value that is at least one contour intervaldeeper than the deepest contour the trace intersects. For the example in Fig. 18, the deepest

contour crossed by the transect is 300 ft, so label the lowest line 500 ft is

appropriate.

4. Place the profile sheet so that the profile scale lines are parallel to the trace (Fig. 18).

Holding the sheet firmly in place, wherever a contour line intersects the transect, sketch a

faint line perpendicularly upward from the trace to the scaling line on the profile sheet that

corresponds to that depth.

5. After all contours along the transect have been marked on the profile sheet, connect the ends of

the perpendicular lines with a smoothly curving line. This line is the bathymetric profile along the

transect.


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Figure 18.


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CONTOURING OCEAN BATHYMETRY

I. Convert the sounding chart of a portion of the southern Pacific Ocean (Fig. 19) into a

contoured bathymetric chart. Draw contours for 100 m, 200 m, 400 m, 600 m, etc.

Figure 19.Sounding chart of a portion of the southern Pacific Ocean


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II. Concerning Your Pacific Ocean Chart

1. What is the depth of the sea floor at point A? ____________B? ________________

2. What is the relief between points A and B? ____________________

3. Convert the graphic scale into a chart scale as a fraction 1: ___________; ratio 1 in:

____________________

4. What is the depth at point Z? ____________________

5.Using the appropriate profile sheet provided (Fig. 20), draw the profile

between FF'.

6.Determine the vertical exaggeration for each

Figure 20. Southern Pacific Transect FF.

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