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Purpose: This lab will provide:
An understanding of global wind and ocean surface current patterns.
An appreciation and understanding of temperature and salinity and its variability across the oceans.
An understanding of vertical circulation (upwelling and down-welling) as it is controlled by the vertical thermal gradient in oceans and wind.
Objectives: In this exercise you will:
Use graphs and participate in a classroom demonstration to learn how temperature and salinity affect seawater density.
Examine and consider characteristics and differences/similarities in wind and ocean surface current patterns.
Examine sea-surface temperature and salinity differences at various latitudes.
Consider the causes and affects of thermohaline circulation and its role in moderating climates.
INTRODUCTION
INTRODUCTION
Most people are very well aware of currents in the ocean. Anyone who has been to the beach has probably experienced the effects of current as it carries you while relaxing on a float. Currents may be best described as continuous flow in one direction. Surface currents (in ocean waters less than ~1000 m depth) are driven by the dominant wind direction. In contrast, deep currents (in ocean waters greater than 1000 m depth) are not affected by atmospheric winds, and instead are driven by differences in seawater density that occurs due to variation in temperatures and salinity that originates at the ocean’s surface.
Hopefuls sending "messages in a bottle" have relied on currents to deliver letters around the world for over 200 years. Many experiments are still being done using floating objects to study currents around the world, but they are now made of biodegradable and non-toxic materials so they pose minimal threat to wildlife. Read more about the history of messages in a bottle and drift card research.
GLOBAL WIND PATTERNS
GLOBAL WIND PATTERNS
Oceanic surface current patterns are often in the shape of circular paths, called gyres (Fig. 6.1). This circular pattern results from the geometry of ocean basins restricted between landmasses and dominant wind directions that are influenced by the Coriolis Effect. There are three dominant convention cells in both the northern and southern hemispheres: Hadley, Ferrell and Polar cells. Wind circulation is limited to the Hadley cells between 0° and 30° latitude, the Ferrell cells between 30° and 60°, and the Polar cells between 60° and 90° (Fig. 6.2).
Figure 6.1 Map of global surface currents. The gyres are the big circulation regions in the ocean.
Image available at http://oceanmotion.org.
Image available at http://oceanmotion.org.
Figure 6.2 Global wind convection cells. Note the dominant wind direction is deflected to the right (red arrows across the land’s surface) of the direction of circulation in the convection cell (red arrows in convection cell) in the north hemisphere. Conversely, the deflection direction is to the left in the southern hemisphere.
Image available at http://serc.carleton.edu/earthlabs/climate/5.html.
Image available at http://serc.carleton.edu/earthlabs/climate/5.html.
THE CORIOLIS EFFECT
THE CORIOLIS EFFECT
The coriolis effect causes air masses to be deflected from a direct path of travel as the air mass moves across global distances (Fig. 6.3). The deflection occurs due to the difference in the rate of the planet’s rotation at different latitudes. Because the planet is “wider” at the equator, it must rotate at a faster rate compared to the rotation of polar regions to complete one revolution in 24 hours. Given the circumference of the Earth to be ~2500 miles, rotational velocity at the equator is ~1000 miles per hour (~2500 miles / 24 hr), whereas at the poles its essentially ~0 mph.
Consider the example as seen in Fig. 6.3, as air moves from 30°N to 60°N, it not only travels with a north-directed velocity that results from windspeed, but also with an east-directed velocity equal to the earth’s rotation at its point of origin. By the time the air mass reaches ~60°N, the air mass has traveled farther to the east than the Earth has rotated at 60°N latitude, making the path of air movement appear deflected to the right. The apparent deflection of the air mass is caused by the coriolis effect, and by applying the same logic for wind traveling from north to south or in convection cells in the southern hemisphere, one can deduce that the direction of deflection is always to the right of the travel path in the northern hemisphere, and to the left in the southern hemisphere. This explains why, in the northern hemisphere, wind is deflected from northeast to southwest as it travels in both the Hadley and Polar cells, and from southwest to northeast in the Ferrell Cell.
By comparing Figures 6.1 and 6.2, you will see how dominant wind directions influence ocean surface current patterns. For example, the clock-wise rotation of the North Atlantic Ocean forms where the Northeast trade winds meets the Westerlies. Portuguese sailors named this area the Sargasso Sea, after the Sargassum seagrass that collects in the center of the gyre.
Still confused about the Coriolis effect? Watch this great video!
Figure 6.3 The Coriolis effect causes air masses that move across global distances to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This occurs because equatorial regions are rotating about the earth’s axis faster than polar regions.
OCEAN SURFACE CURRENTS
OCEAN SURFACE CURRENTS
Circulating surface waters will form fast, narrow and deep currents on the west side of a gyre and slow, wide and shallow currents on the east side of a gyre. For example, six times as much water flows through the Kuroshio Current at an average rate of 10 kilometers per hour on the west side of the Pacific, in contrast to much slower flow at 2 kilometers per hour in the California Current, which is also a quarter as wide. This phenomenon is known as westward intensification and is also attributed to the Coriolis effect. The difference in the earth’s rotation rate at different latitudes becomes increasingly larger at higher latitudes so that the coriolis effect is zero at the equator and greatest at the poles. Consider for example, the North Atlantic Ocean with clockwise flow from the Gulf Stream to the Canary Current. Water flowing in the Gulf Stream experiences an increasing Coriolis effect as it flows northward and this maximizes the right-ward deflection and acts to speed up the current. Conversely, water flowing in the Canary current is experiencing a decreasing Coriolis effect, minimizing deflection to the right and creating a counter-current to slow the overall southward flow of water.
At this point, open the Lab 6 Answersheet on a separate window so you can answer the questions as they come up.
Q1. Using the maps of oceanic surface currents (Fig 6.1) and global wind patterns (Fig 6.2) to help you.
a) What is the dominant surface current direction just North of the equator? ____________________________
b) Which wind controls the current north and south of the equator? (See Figure 6.2) _____________________________
Q2. The West Wind Drift is one of the largest currents in the ocean and is used often by mariners sailing from west to east across the globe. Which dominant winds cause this current to form? (Hint: Use the two images below to help you) ___________________________
THERMOHALINE CIRCULATION
THERMOHALINE CIRCULATION
Deep ocean currents are driven by density differences that occur due to differences in temperature and salinity of surface waters at different latitudes. Let's look at these two factors and how they interact in the ocean.
TEMPERATURE
TEMPERATURE
Sea surface temperatures vary as one might expect from warmest waters at the equator to colder waters at the poles (Fig. 6.4). Note how the isotherms, or lines connecting points of equal temperature, reflect this latitudinal change in temperature across the ocean’s surface. Figure 6.5 shows how ocean temperatures change with depth. Average surface temperatures often extend down to ~300 meters depth during cool, winter months due to high wave action and/or currents in what is called the mixed layer. During warm, calm weather periods with little wave action, the mixed layer may become more shallow and limited to the upper ~100 meters and a seasonal thermocline, or layer in which temperature changes rapidly with depth, will develop.
Regardless of this seasonal variation in surface temperatures and mixing, warm surface waters at the equator and mid-latitudes are underlain by a permanent thermocline, within which temperatures decrease so rapidly with depth, that the layer acts as a density barrier separating exchange between warm surface waters and cold, dense bottom waters. Vertical circulation between surface and deep waters is limited to polar regions where surface waters are so cold that there is very little temperature gradient, or permanent thermocline, to separate the two water layers.
Figure 6.4 Map of average sea surface temperatures.
Image available at http://aquarius.nasa.gov/images/global_sst_map.jpg.
Image available at http://aquarius.nasa.gov/images/global_sst_map.jpg.
Examine the image of average sea surface temperatures (Fig. 6.4) and answer the following questions.
Examine the image of average sea surface temperatures (Fig. 6.4) and answer the following questions.
Q3. Where are temperatures the warmest? ___________________________
Q4. What is the maximum average surface temperature? ______________________
Q5. a) Where are temperatures the coolest? __________________
b) Record the minimum average surface temperature. ________________
Q6. What is the overall temperature range of the oceans? _________________
Q7. Consider the temperatures of the equatorial Pacific…
a) Are temperatures warmer in the east, west, or are they the same? ____________________
b) Why do you think this occurs? (Hint: think wind currents)
Q8. Based on the surface current map (Fig. 6.1) and what you know about ocean temperatures and westward intensification, describe the following currents as either relatively warm or cold, and fast or slow.
The permanent thermocline is always present, doesn't change with the seasons, and acts as a separation barrier between surface and deeper waters.
A seasonal thermocline may develop in the upper 300 meters of the ocean during period of little wave action and minimal mixing of solar-heated surface waters.
Figure 6.5 A thermocline is an ocean layer in which temperature changes rapidly with depth.
In the document linked below (in the section with the thermocline circulation demo), there will be a graph you have to plot. Use Figure 6.5 as a guide to label the water layers.
SALINITY
SALINITY
The formal definition of salinity was published by the International Council for the Exploration of the Sea in 1902 as “the total amount of dissolved solids in seawater in parts per thousand (ppt) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized.” Luckily, we will be using we will be using a simpler definition: “the total amount of solid material (in grams) dissolved in one kilogram of seawater.” Because salinity compares grams (1x) to kilograms (1000x), the salinity of water is expressed in parts per thousand (ppt) rather than parts per hundred. Eg. If a bucket of water has a salinity of 2 ppt, that means 2 cups of salt per thousand cups of water, or 2 tablespoons of salt per 1000 tablespoons of water, it's a ratio.
Percent or Per-mil?
Percent or Per-mil?
Sometimes you will see salinity as pph (expressed as % since it's over a hundred), or ppt (written as ‰, and referred to as “per mil” rather than “percent”). This convention is used because the salinity of seawater most commonly varies from 3.3% to about 3.7% dissolved solids, which is not a very wide range when using pph. Slight variations of salinity actually have a big impact on many natural processes and organisms, which is why ppt (33‰ to 37‰ range) or even ppm are often used, expands the range to larger numbers without changing the value
Eg: 4.5% [percent] salinity is the same as 45‰ [per mil] salinity.
Ocean “salt” is composed of ions derived from chemical weathering of rocks on land and from volcanic emissions (Fig. 6.6). Although these ions are delivered to the oceans via rivers and streams, freshwater input actually dilutes the salty ocean, as does rainfall/precipitation which contributes to less saline waters. Salts instead become more concentrated and ocean waters become more saline due to evaporation. Salinity varies from low at the equator, highest at mid-latitudes, and lowest at the poles (Fig. 6.7). Low salinity occurs at the equator even in the presence of high evaporation rates due to the large amounts of rainfall that also occurs there.
Salinity is lowest in cold polar regions due to little evaporation.
Regardless of the range of salinity in the ocean, the ratio of the major dissolved constituents remains unchanged. Because chloride is the most common dissolved ion in sea water and is easily measured, salinity can be determined by measuring the concentration of chlorine in the water and applying the following calculation:
Salinity (‰) = 1.80655 x chlorinity (‰)
Figure 6.7 Average sea-surface salinities. Map source: https://mynasadata.larc.nasa.gov/sites/default/files/inline-images/Salinity.JPGRead more about global ocean salinity and the NASA Aquarius project o monitor salinity across the Earth's oceans here https://salinity.oceansciences.org/overview.htm
Q9. Calculate the concentration of seawater with a salinity of 3.39 % (parts per hundred) in parts per thousand.
Q10. Examine the map of average sea surface salinity values (Fig. 6.7). Note the highest salinity values are not at the equator although this is the warmest place on the planet with high evaporation rates. Why would this be? (Hint: You may have to re-read that section!)
Q11. From the reading, list three processes that cause salinity to vary across the ocean.
Q12. Consider the salinity of the Atlantic and the Pacific Oceans….which ocean, Atlantic or Pacific, is more saline?
Q13. How might you explain the difference between the Pacific and Atlantic Oceans? (Hint: think of the direction of major wind belts and where evaporated waters are carried from the oceans and where precipitation falls).
Q14. What is the salinity of seawater with a chlorinity of 19.65 per mil? Recall: Salinity (per mil) = 1.80655 x chlorinity (per mil).
LAB DEMONSTRATION
LAB DEMONSTRATION
Determining density based on temperature and salinity
Determining density based on temperature and salinity
As stated above, the density, or mass per unit volume, of seawater is a function of temperature and salinity. Figure 6.8 illustrates how the density of seawater varies with changing temperature and salinity. The following demonstration will help you understand the relationship between salt, temperature and density in the ocean. In this lab, the units we will use for density are grams per cubic centimeters (g/cm3).
The density of freshwater = 1.0 g/cm3
Average density of seawater = ~1.025 g/cm3.
If two of the variables are known the third can be determined from the graph. For example, if salinity is 19ppt, density at 30 degrees celsius would be... (follow the line up from 19ppt, until you hit the 30 degrees line, then look at what that value is on the y axis)... so 1.006 gm/cm3.
Figure 6.8 Temperature-Salinity-Density graph
*** You will watch a demonstration in the lab, then answer the questions that follow about Thermohaline Circulation ***
*** You will watch a demonstration in the lab, then answer the questions that follow about Thermohaline Circulation ***
Q15. a) Complete the Thermohaline Circulation sheet (as seen to the right) with questions based on the demonstration. You can print the pdf and complete it by hand, edit it digitally using a PDF annotation app, or use the .jpg file (page 1 and page 2) to annotate it using a drawing app. Either way, name it "LastName_Lab6Demo" and scan/ upload it in Question 15 on the Lab 6 Answersheet.
b) YOUR TURN! You will next carry out a mini experiment to continue exploring the effects of temperature on water density.
Beaker 1: Hot tap water (with red food coloring). Temperature: _____________ °C
Beaker 2: Cold tap water (with blue food coloring). Temperature: _____________ °C
i. Which beaker has the water with the greatest density? ______________________________________
Conduct the experiment in your group:
Make sure the divider in the middle of the water tank is pushed firmly to the bottom of the tank.
Pour 200 mL of hot water (red) into one side of the tank at the same time as someone else pours 200 mL of cold water (blue) into the other side.
Make a hypothesis about what will happen when you pull the divider out. When the water stops moving, carefully pull the divider out.
ii. What happens to the water? _____________________________________
Carefully put the divider back into the tank and stir the water only in one side with your finger.
Make a hypothesis about what will happen when you pull the divider out and then carefully pull the divider out.
iv. What happened to the water in this case? ________________________________________________________________________________
THERMOHALINE CIRCULATION
THERMOHALINE CIRCULATION
In the North Atlantic Ocean, seawater temperatures are very cold and when temperatures drop below the freezing point (salt water with 35‰ salinity freezes at -1.91°C), sea ice begins to form. As sea ice forms, the salt is actually expelled from the ice and the salinity of the surrounding seawater increases. The cold sea surface temperatures combined with increased salinity causes the surface waters to become more dense than the water below and the denser surface waters begin to sink. To replace this sinking water, surface water is pulled in from elsewhere and it begins to cool, freeze, and sink. This cycle is termed the thermohaline circulation (thermo = temperature and haline = salt) (Fig 6.9).
Figure 6.9 Water masses of the Atlantic Ocean. This image shows the stable, density driven stratification of the Atlantic Ocean, which occurs as polar surface waters sink to set up deep-water currents such as the North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) currents.
Image source
Image source
Through the global thermohaline circulation, the ocean is able to redistribute heat from the equator to the poles and regulate global climate. In the North Atlantic, for example, warm surface waters are transported away from the tropics by the Gulf Stream towards the Arctic where they are cooled and sink back towards the equator as cold, deep waters and then up-welled back to the surface at lower latitudes (Figure 6.10).
Figure 6.10 Thermohaline circulation. The ocean plays a major role in the distribution of the planet’s heat through deep-sea circulation. This simplified illustration shows this “conveyor belt” circulation, which is ultimately driven by down-welling of cold water in the North Atlantic.
Image and text from http://www.ncdc.noaa.gov/paleo/ctl/thc.html.
Image and text from http://www.ncdc.noaa.gov/paleo/ctl/thc.html.
OTHER VERTICAL CURRENTS
OTHER VERTICAL CURRENTS
Thermohaline circulation is not the only driver that produces vertical currents in the ocean. Wind-driven surface currents can also cause water to sink or rise. When wind blows surface currents against the coastline, an area of surface convergence forms and ocean waters sink, or down-wells. This sinking of ocean surface waters delivers oxygen used by deep-sea creatures living in deeper ocean waters. Conversely, when wind blows surface currents away from the coastline, an area of surface divergence forms and deeper ocean waters rise, or up-wells. The cooler bottom waters may be oxygen-poor, but are loaded with nutrients that fertilize the upper ocean waters and support thriving marine ecosystems.
Q16. Why vertical currents important in oceanography?
Q17. . Why do you suppose water at the equator is the lightest and water at the poles is the densest?
Q18. Global warming is causing the Greenland Ice sheet to melt, which will add a large amount of freshwater to the North Atlantic Ocean, how do you think this would affect the ocean thermohaline circulation? Explain.
Ever heard of Rip Currents? Know how to get out of them if you're ever caught in one, but most importantly, pay attention to signs posted at the beach!
YOU MADE IT to the END!!
YOU MADE IT to the END!!
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