PHS111-10 Oceanography Notes

Zachary Burell, Instructor

March 9, 2015

Oceanography - The Study of the ocean.Oceanography covers a wide range of topics, including marine life and ecosys-

tems, ocean circulation, plate tectonics and the geology of the sea floor, and thechemical and physical properties of the ocean.

Just as there are many specialties within the medical field, there are manydisciplines within oceanography.

Biological oceanographers and marine biologists study plants and ani-mals in the marine environment. They are interested in the numbers of marineorganisms and how these organisms develop, relate to one another, adapt totheir environment, and interact with it. To accomplish their work, they mayuse field observations, computer models, or laboratory and field experiments.

Chemical oceanographers and marine chemists study the compositionof seawater, its processes and cycles, and the chemical interaction of seawaterwith the atmosphere and sea floor. Their work may include analysis of seawatercomponents, the effects of pollutants, and the impacts of chemical processeson marine organisms. They may also use chemistry to understand how oceancurrents move seawater around the globe and how the ocean affects climate orto identify potentially beneficial ocean resources such as natural products thatcan be used as medicines.

Geological oceanographers and marine geologists explore the ocean floorand the processes that form its mountains, canyons, and valleys. Throughsampling, they look at millions of years of history of sea-floor spreading, platetectonics, and oceanic circulation and climates. They also examine volcanicprocesses, mantle circulation, hydrothermal circulation, magma genesis, andcrustal formation. The results of their work help us understand the processesthat created the ocean basins and the interactions between the ocean and thesea floor.

Physical oceanographers study the physical conditions and physical pro-cesses within the ocean such as waves, currents, eddies, gyres and tides; thetransport of sand on and off beaches; coastal erosion; and the interactions of theatmosphere and the ocean. They examine deep currents, the ocean-atmosphererelationship that influences weather and climate, the transmission of light andsound through water, and the oceans interactions with its boundaries at thesea floor and the coast.

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All of these fields are intertwined, and thus all oceanographers must have akeen understanding of biology, chemistry, geology, and physics to unravel themysteries of the world ocean and to understand processes within it.

CurrentsOceanic currents are driven by tides, winds, and differences in water density.Currents are essential for maintaining the existing balance of life on Earth, butthey can be deadly as well.

When used in association with water, the term "current" describes themotion of the water. Some currents you may be familiar with are the motionof rainwater as it flows down the street, or the motion of the water in a creek,stream, or river flowing from higher elevation to lower elevation. This motionis caused by gravity. The speed and direction (velocity) of currents can bemeasured and recorded.

Oceanic currents are driven by several factors. One is the rise and fall ofthe tides, which is driven by the gravitational attraction of the sun and moonon Earths oceans. Tides create a current in the oceans, near the shore, andin bays and estuaries along the coast. These are called "tidal currents." Tidalcurrents are the only type of currents that change in a very regular pattern andcan be predicted for future dates.

A second factor that drives ocean currents is wind. Winds drive currentsthat are at or near the oceans surface. These currents are generally measuredin meters per second or in knots (1 knot = 1.15 miles per hour or 1.85 kilometersper hour). Winds drive currents near coastal areas on a localized scale, and inthe open ocean on a global scale.

A third factor that drives currents is thermohaline circulation - a processdriven by density differences in water due to temperature (thermo) and salinity(haline) in different parts of the ocean. Currents driven by thermohaline circu-lation occur at both deep and shallow ocean levels and move much slower thantidal or surface currents.

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Tidal Currents

Tidal currents occur in conjunction with the rise and fall of the tide. Thevertical motion of the tides near the shore causes the water to move horizon-tally, creating currents. When a tidal current moves toward the land and awayfrom the sea, it floods. When it moves toward the sea away from the land, itebbs. These tidal currents that ebb and flood in opposite directions are calledrectilinear or reversing currents.

Rectilinear tidal currents, which typically are found in coastal rivers andestuaries, experience a slack water period of no velocity as they move fromthe ebbing to flooding stage, and vice versa. After a brief slack period, whichcan range from seconds to several minutes and generally coincides with high orlow tide, the current switches direction and increases in velocity.

Distances between earth, sun and moon The relationship between the massesof the Earth, moon, and sun and their distances to each other play critical rolesin affecting tides and the currents they produce. Click the image for a largerview.

Tidal currents are the only type of current affected by the interactions ofthe Earth, sun, and moon. The moons force is much greater than that of thesun because it is 389 times closer to the Earth than the sun is. Tidal currents,just like tides, are affected by the different phases of the moon. When themoon is at full or new phases, tidal current velocities are strong and are calledspring currents. When the moon is at first or third quarter phases, tidal currentvelocities are weak and are called neap currents.

Also similar to tides, tidal currents are affected by the relative positions of

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the moon and Earth. When the moon and Earth are positioned nearest to eachother (perigee), the currents are stronger than average and are called perigeancurrents. When the moon and Earth are at their farthest distance from eachother (apogee), the currents are weaker and are called apogean currents.

The shape of bays and estuaries also can magnify the intensity of tides andthe currents they produce. Funnel-shaped bays in particular can dramaticallyalter tidal current magnitude. The Bay of Fundy in Nova Scotia is a classicexample of this effect, and has the highest tides in the world - over 15 meters.

Coastal CurrentsCoastal currents are intricately tied to winds, waves, and land formations.Winds that blow along the shorelinelongshore windsaffect waves and, there-fore, currents.

Waves

Before one can understand any type of surface current, one must understandhow wind and waves operate. Wave height is affected by wind speed, windduration (or how long the wind blows), and fetch, which is the distance overwater that the wind blows in a single direction. If wind speed is slow, only smallwaves result, regardless of wind duration or fetch. If the wind speed is great butit only blows for a few minutes, no large waves will result even if the wind speedis strong and fetch is unlimited. Also, if strong winds blow for a long period oftime but over a short fetch, no large waves form. Large waves occur only whenall three factors combine.

As wind-driven waves approach the shore, friction between the sea floor andthe water causes the water to form increasingly steep angles. Waves that becometoo steep and unstable are termed breakers or breaking waves.

The highest surface part of a wave is called the crest, and the lowest partis the trough. The vertical distance between the crest and the trough is thewave height. The horizontal distance between two adjacent crests or troughs isknown as the wavelength.

Wave height is affected by wind speed, wind duration, or how long the windblows, and fetch, which is the distance over water that the wind blows in a singledirection. If wind speed is slow, only small waves result. If the wind speed isgreat but it only blows for a few minutes, no large waves will occur. Also, ifstrong winds blow for a long period of time but over a short fetch, no largewaves form. Large waves occur only when all three factors combine .

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Longshore Currents

The speed at which waves approach the shore depends on sea floor andshoreline features and the depth of the water. As a wave moves toward thebeach, different segments of the wave encounter the beach before others, whichslows these segments down. As a result, the wave tends to bend and conform tothe general shape of the coastline. Also, waves do not typically reach the beachperfectly parallel to the shoreline. Rather, they arrive at a slight angle, calledthe angle of wave approach.

When a wave reaches a beach or coastline, it releases a burst of energy thatgenerates a current, which runs parallel to the shoreline. This type of currentis called a longshore current.

Longshore currents are affected by the velocity and angle of a wave. Whena wave breaks at a more acute (steep) angle on a beach, encounters a steeperbeach slope, or is very high, longshore currents increase in velocity. Conversely,a wider breaking angle, gentler beach slope, and lower wave height slows alongshore currents velocity. In either case, the water in a longshore currentflows up onto the beach, and back into the ocean, as it moves in a sheetformation.

As this sheet of water moves on and off the beach, it can capture andtransport beach sediment back out to sea. This process, known as longshoredrift, can cause significant beach erosion.

Rip Currents

When waves travel from deep to shallow water, they break near the shorelineand generate currents. A rip current forms when a narrow, fast-moving sectionof water travels in an offshore direction. Rip current speeds as high as 8 feetper second have been measuredfaster than an Olympic swimmer can sprint!This makes rip currents especially dangerous to beachgoers as these currentscan sweep even the strongest swimmer out to sea.

As longshore currents move on and off the beach, rip currents may formaround low spots or breaks in sandbars, and also near structures such as jettiesand piers. A rip current, sometimes incorrectly called a rip tide, is a localizedcurrent that flows away from the shoreline toward the ocean, perpendicular orat an acute angle to the shoreline. It usually breaks up not far from shore andis generally not more than 25 meters (80 feet) wide.

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Rip currents typically reach speeds of 1 to 2 feet per second. However, somerip currents have been measured at 8 feet per secondfaster than any Olympicswimmer ever recorded . If wave activity is slight, several low rip currents canform, in various sizes and velocities. But in heavier wave action, fewer, moreconcentrated rip currents can form.

Because rip currents move perpendicular to shore and can be very strong,beach swimmers need to be careful. A person caught in a rip can be swept awayfrom shore very quickly. The best way to escape a rip current is by swimmingparallel to the shore instead of towards it, since most rip currents are less than80 feet wide. A swimmer can also let the current carry him or her out to seauntil the force weakens, because rip currents stay close to shore and usuallydissipate just beyond the line of breaking waves. Occasionally, however, a ripcurrent can push someone hundreds of yards offshore. The most important thingto remember if you are ever caught in a rip current is not to panic. Continue tobreathe, try to keep your head above water, and dont exhaust yourself fightingagainst the force of the current.

Upwelling

Winds blowing across the ocean surface often push water away from an area.When this occurs, water rises up from beneath the surface to replace the di-verging surface water. This process is known as upwelling.

Upwelling occurs in the open ocean and along coastlines. The reverse pro-cess, called downwelling, also occurs when wind causes surface water to buildup along a coastline. The surface water eventually sinks toward the bottom.

Subsurface water that rises to the surface as a result of upwelling is typicallycolder, rich in nutrients, and biologically productive. Therefore, good fishinggrounds typically are found where upwelling is common. For example, the richfishing grounds along the west coasts of Africa and South America are supportedby year-round coastal upwelling.

Seasonal upwelling and downwelling also occur along the West Coast of theUnited States. In winter, winds blow from the south to the north, resultingin downwelling. During the summer, winds blow from the north to the south,

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and water moves offshore, resulting in upwelling along the coast. This summerupwelling produces cold coastal waters in the San Francisco area, contributingto the frequent summer fogs.

Surface Ocean CurrentsThe Coriolis Effect

Coastal currents are affected by local winds. Surface ocean currents, which occuron the open ocean, are driven by a complex global wind system. To understandthe effects of winds on ocean currents, one first needs to understand the Coriolisforce and the Ekman spiral.

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If the Earth did not rotate and remained stationary, the atmosphere wouldcirculate between the poles (high pressure areas) and the equator (a low pres-sure area) in a simple back-and-forth pattern. But because the Earth rotates,circulating air is deflected. Instead of circulating in a straight pattern, the airdeflects toward the right in the Northern Hemisphere and toward the left in theSouthern Hemisphere, resulting in curved paths. This deflection is called theCoriolis effect. It is named after the French mathematician Gaspard Gustavede Coriolis (1792-1843), who studied the transfer of energy in rotating systemslike waterwheels.

Trade WindsIn the Northern Hemisphere, warm air around the equator rises and flows northtoward the pole. As the air moves away from the equator, the Coriolis effect de-flects it toward the right. It cools and descends near 30 degrees North latitude.The descending air blows from the northeast to the southwest, back toward theequator (Ross, 1995). A similar wind pattern occurs in the Southern Hemi-sphere; these winds blow from the southeast toward the northwest and descendnear 30 degrees South latitude.

These prevailing winds, known as the trade winds, meet at the IntertropicalConvergence Zone (also called the doldrums) between 5 degrees North and 5

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degrees South latitude, where the winds are calm. The remaining air (air thatdoes not descend at 30 degrees North or South latitude) continues toward thepoles and is known as the westerly winds, or westerlies. The trade winds areso named because ships have historically taken advantage of them to aid theirjournies between Europe and the Americas

Boundary Currents

Global winds drag on the waters surface, causing it to move and build up inthe direction that the wind is blowing. And just as the Coriolis effect deflectswinds to the right in the Northern Hemisphere and to the left in the SouthernHemisphere, it also results in the deflection of major surface ocean currents tothe right in the Northern Hemisphere (in a clockwise spiral) and to the left inthe Southern Hemisphere (in a counter-clockwise spiral). These major spiralsof ocean-circling currents are called gyres and occur north and south of theequator. They do not occur at the equator, where the Coriolis effect is notpresent (Ross, 1995).

There are five major ocean-wide gyresthe North Atlantic, South At-lantic, North Pacific, South Pacific, and Indian Ocean gyres. Each isflanked by a strong and narrow western boundary current, and a weak andbroad eastern boundary current

The Gulf Stream is a powerful western boundary current in the NorthAtlantic Ocean that strongly influences the climate of the East Coast of theUnited States and many Western European countries. The Gulf Stream, pairedwith the eastern boundary Canary Current, flanks the North Atlantic gyre. TheGulf Stream, also called the North Atlantic Drift, originates in the Gulf of Mex-ico, exits through the Strait of Florida, and follows the eastern coastline of theUnited States and Newfoundland. It travels at speeds of 25 to 75 miles per dayat about one to three knots (1.15-3.45 miles per hour or 1.85-5.55 kilometersper hour). It influences the climate of the east coast of Florida, keeping tem-peratures warmer in the winter and cooler than the other southeastern states inthe summer. Since it also extends toward Europe, it warms western Europeancountries as well.

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The Ekman Spiral

The Ekman spiral occurs as a consequence of the Coriolis effect.The Ekman spiral, named after Swedish scientist VagnWalfrid Ekman (1874-

1954) who first theorized it in 1902, is a consequence of the Coriolis effect. Whensurface water molecules move by the force of the wind, they, in turn, drag deeperlayers of water molecules below them. Each layer of water molecules is moved byfriction from the shallower layer, and each deeper layer moves more slowly thanthe layer above it, until the movement ceases at a depth of about 100 meters(330 feet). Like the surface water, however, the deeper water is deflected bythe Coriolis effectto the right in the Northern Hemisphere and to the left inthe Southern Hemisphere. As a result, each successively deeper layer of watermoves more slowly to the right or left, creating a spiral effect. Because thedeeper layers of water move more slowly than the shallower layers, they tend totwist around and flow opposite to the surface current.

The Global Conveyor BeltThermohaline Circulation

Winds drive ocean currents in the upper 100 meters of the oceans surface.However, ocean currents also flow thousands of meters below the surface. Thesedeep-ocean currents are driven by differences in the waters density, which iscontrolled by temperature (thermo) and salinity (haline). This process is knownas thermohaline circulation.

In the Earths polar regions ocean water gets very cold, forming sea ice. As aconsequence the surrounding seawater gets saltier, because when sea ice forms,the salt is left behind. As the seawater gets saltier, its density increases, andit starts to sink. Surface water is pulled in to replace the sinking water, whichin turn eventually becomes cold and salty enough to sink. This initiates thedeep-ocean currents driving the global conveyer belt.

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Global Conveyor Belt

Thermohaline circulation drives a global-scale system of currents called theglobal conveyor belt. The conveyor belt begins on the surface of the oceannear the pole in the North Atlantic. Here, the water is chilled by arctic temper-atures. It also gets saltier because when sea ice forms, the salt does not freezeand is left behind in the surrounding water. The cold water is now more dense,due to the added salts, and sinks toward the ocean bottom. Surface water movesin to replace the sinking water, thus creating a current.

This deep water moves south, between the continents, past the equator, anddown to the ends of Africa and South America. The current travels aroundthe edge of Antarctica, where the water cools and sinks again, as it does in theNorth Atlantic. Thus, the conveyor belt gets "recharged." As it moves aroundAntarctica, two sections split off the conveyor and turn northward. One sectionmoves into the Indian Ocean, the other into the Pacific Ocean.

These two sections that split off warm up and become less dense as theytravel northward toward the equator, so that they rise to the surface (upwelling).They then loop back southward and westward to the South Atlantic, eventuallyreturning to the North Atlantic, where the cycle begins again.

The conveyor belt moves at much slower speeds (a few centimeters per sec-ond) than wind-driven or tidal currents (tens to hundreds of centimeters persecond). It is estimated that any given cubic meter of water takes about 1,000years to complete the journey along the global conveyor belt. In addition, theconveyor moves an immense volume of watermore than 100 times the flow ofthe Amazon River (Ross, 1995).

The conveyor belt is also a vital component of the global ocean nutrientand carbon dioxide cycles. Warm surface waters are depleted of nutrients andcarbon dioxide, but they are enriched again as they travel through the conveyorbelt as deep or bottom layers. The base of the worlds food chain depends onthe cool, nutrient-rich waters that support the growth of algae and seaweed.

Effects of Climate ChangeThe global conveyor belt is a strong, but easily disrupted process. Research

suggests that the conveyor belt may be affected by climate change. If globalwarming results in increased rainfall in the North Atlantic, and the melting ofglaciers and sea ice, the influx of warm freshwater onto the sea surface couldblock the formation of sea ice, disrupting the sinking of cold, salty water. Thissequence of events could slow or even stop the conveyor belt, which could resultin potentially drastic temperature changes in Europe.

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How Currents affect our livesCurrent measurements are important to shipping, commercial fishing, recre-ational boating, and safety. By using predicted, real-time and short-term fore-casted currents, people can safely dock and undock ships, maneuver them inconfined waterways and safely navigate through coastal waters. With this infor-mation, merchandise and people can arrive on schedule. Lack of this knowledgecan lead to collisions and delayed arrivals.

Search-and-rescue personnel can use real-time and predicted current patternsto determine where the water may carry a missing person or floating object(s).Geographic Information Systems (GIS) programs are used to assist in search-and-rescue efforts as well. These programs use the last-known position of thelost person or item(s), predicted and real-time current and weather data, anddrift patterns to estimate the location of the person or item(s).

Hazardous material (HAZMAT) cleanup operations also use real-time andpredicted current information. Hazardous materials such as oil and fuel fromtankers, typically remain on or near the waters surface, and travel with surfacecurrents and winds. Models created from high-frequency radar, satellite, andwind data help predict where the hazardous material will go.

NOAAs Center for Operational Oceanographic Products and Services (CO-OPS) is working with a prototype of a quick-response buoy that can be deployedwhen a HAZMAT spill occurs. The buoy collects real-time current speed anddirection, wind speed direction and gusts, barometric pressure, and water andair temperature. The buoy can be deployed for up to 30 days.

In addition, scientists study nutrient, sediment, and the concentration ofchemicals which travel in the water column, to understand how currents trans-port these materials locally and globally.

Finally, currents affect swimmers and fishers. Localized currents can beobserved in the form of rip currents at the beach, or a piece of floating woodmeandering in different patterns in a tidal bay or river. Swimming at the beachnear rip currents can be very dangerous. Before going to the beach, learn howto recognize rip currents and strong shore currents, and pay attention to thewarning signs. Recreational and commercial fishers pay close attention to thetiming and strength of currents to maximize their chances of catching fish.

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