Physical OceanographyAn Introduction

8/14/2019 Physical OceanographyAn Introduction

1/84


2/84

The distinction between these two is often ill-defined, particularly with the recent explosive

growth in numerical modeling, which is used forboth process modeling and ocean simulation, andassimilation of data into numerical models, mainlyfor simulation and predictive capability. Perhaps a

more useful categorization of scientificapproaches is the distinction between those withthe goal of understanding a specific process andthose with the goal of basic description orsimulation of the oceans motions. Observationsand numerical modeling are used for both of thesegoals.


3/84

The goal of descriptive physical oceanographyis to obtain a clear and systematic descriptionof the oceans, sufficiently quantitative to permitus to predict some aspects of their behavior inthe future with some certainty. Understandingthe basic elements of the ocean environmentfocuses dynamical understanding and permitsuseful, quantitative evaluation of ocean models.


4/84


5/84

Why Study Ocean Physics There are many reasons for developing our knowledge of

the oceans. As sources of food, of chemicals and of power,they are as yet only exploited to a minor degree. Theoceans provide a vitally important avenue of transportation. They form a sink into which industrial and

human waste is dumped, but they do not form a bottomlesspit into which material like radioactive waste can bethrown without due thought being given to where it mightbe carried by currents. The large heat capacity of theoceans exerts a significant and in some cases a controllingeffect on the earths climate, while the continuousmovement of the currents and waves along the coast mustbe taken into account when piers, breakwaters and otherstructures are built.


6/84

Where does the water go?

In all of these applications, and in many others, knowledgeof the ocean circulation is needed. One goal of physicaloceanography is to obtain a systematic, quantitativedescription of the character of the ocean waters, theirgeographic distribution and of their movements. The latterinclude the major ocean currents that circulatecontinuously but with fluctuating velocity and position,medium and small-scale circulation features called themesoscale features that correspond to weather in the

atmosphere, the variable coastal currents, the predictablyreversing tidal currents, the rise and fall of the tide, and thewaves generated by winds or earthquakes.


7/84

Character of the Ocean

The character of the ocean waters includes aspects such astemperature and salt content, which together determinedensity and hence vertical movement, and also includesother dissolved substances (oxygen, nutrients, chemicalspecies, etc.) or biological species insofar as they yieldinformation about the currents.


8/84

Descriptive vs Dynamical In the descriptive approach to physical oceanography,observations are made of specific features. These are

reduced to as simple a statement as possible of thecharacter of the features themselves and of their relationsto other features. The dynamical or theoretical approach isto apply the already known laws of physics to the ocean,regarding it as a body acted upon by forces, and toendeavor to solve the resulting mathematical equations toobtain information on the motions to be expected from the

forces acting. Numerical modeling is often an adjunct of theoretical physical oceanography; with the goal of understanding well-defined processes with more complexphysics than can be treated theoretically.


9/84


10/84

Our present knowledge in physical oceanographyrepresents an accumulation of data, most of which

have been gathered during the past 150 years. Thepurpose of this class is to summarize some of theconcepts resulting from studies of these data togive an idea of what we now know about the

distribution of the physical characteristics of theocean waters and of their circulation. We includesome of the achievements of dynamical physicaloceanography as important context for description.A full treatment of dynamical oceanography is

contained in other classes.


11/84

History of Physical Oceanography Physical oceanography has gone through several

historical phases. Presumably sailors have alwaysbeen concerned with ocean currents as they affecttheir ships courses and changes in oceantemperature or surface condition. Many of theearlier navigators, such as Cook and Vancouver,made valuable scientific observations during theirvoyages in the late 1700s, but it is generallyconsidered that Mathew Fontaine Maury (1855)started the systematic large-scale collection of ocean current data, using ships navigation logs ashis source of information.


12/84


13/84


14/84


15/84


16/84


17/84

Todays Descriptive Physical Oceanography

With small data sets, each data point can be consideredcarefully while statistical analysis is not feasible.Similarly, sparse data sets continue to be collected today,for instance for chemical constituents of seawater, andcontinue to be analyzed point by point. However, manymodern observational techniques generate large volumesof information on currents and water properties. Satellitesprovide large amounts of data on surface conditions.Within the water column a variety of floats provide

nearly continuous mapping of currents and temperature.


18/84

All of these observations are meshed withincreasingly complex numerical simulations of

ocean processes. Todays physical oceanographermay no longer have the luxury of knowing eachdata point and may instead use statistical methodsto analyze the large quantities of data now

available. Descriptive physical oceanographyskills have had to expand to the statisticaldescriptions of data along with numericalsimulations of the ocean environment. Basicfamiliarity with ocean circulation and waterproperties remains a necessary foundation .


19/84

The ocean and the atmosphere

It will become apparent during our description that thereare strong interactions between the ocean and theatmosphere. An example is the El Nino - SouthernOscillation (ENSO) phenomenon which although localizedin the tropical Pacific affects climate on time scales of several years over much of the world. To understand suchinteractions it is necessary to understand the coupledocean-atmosphere system. In consequence,oceanographers and meteorologists need to work closely

together in studying both the hydrosphere and atmosphereand their interactions.


20/84

History of Physical Oceanography

The science of oceanography is fairly young. Itsorigins are in a great variety of earlier studiesincluding some of the earliest applications of physics and mathematics to Earth processes.

Some say that Archimedes was one of the earliestphysical oceanographers. The familiarArchimedes principle describes the displacementof water by a body placed in the water.Archimedes also made extensive studies of harbors to fortify them against enemy attack.


21/84

History of Physical Oceanography

Many early mathematicians also used their skills to studythe ocean. Sir Isaac Newton didnt directly work onproblems of the ocean but his principle of universalgravitation was an essential building block inunderstanding the tides. Both LaPlace and LeGendre put alot of work into a formal solution of the tides; LaPlacesequation is a fundamental element in a description of thetides. Other mathematicians worked on a mathematicaldescription of the ocean waves that surrounded their

English homeland. All of these studies are clearly part of what we now know as physical oceanography.


22/84


23/84


24/84


25/84

Charles Darwin and the Beagle

Another source of sea-going physical studies of the oceancame from studies made by naturalists who went alongon British exploring expeditions. One example wasCharles Darwin who went along as the ships naturalist of the HMS Beagle on a voyage to chart the southeast shoreof South America. This journey included many long visitsto the South American continent where Darwin formulatedmany of his ideas about the origin of species. During thecruise he took measurements of physical ocean parameters

such as surface temperature and surface salinity.


26/84

There were so many naturalists traveling on Britishvessels in the early 1800s that the Royal Society in

London decided to design a set of uniformmeasurements. Then Royal Society secretary RobertHooke was commissioned to develop the suite of instruments that would be carried by all Britishgovernment ships. One noteworthy device was asystem to measure the bottom depth of the deep ocean.It consisted of a wooden ball float attached to an ironweight. The pair was to be dropped from the ship todescend to the ocean floor where the weight would be

dropped; the wooden ball would then ascend to thesurface where it would be spotted and collected by theship.


27/84

Organized Expeditions

In 1838 the US Congress had the Navy organize and

execute the United States Exploring Expedition to collectoceanographic information from all over the world. Manyof the backers of this expedition saw it as a potentialeconomic boon but others were more concerned with thescientific promise of the expedition. In 1836, $300,000had been appropriated for this expedition. As originallyconceived the expedition was to be of particular benefit tonatural history, including geology, mineralogy, botany,vegetable chemistry, zoology, ichthyology, ornithologyand ethnology. Some practical studies such asmeteorology and astronomy were also included in the

program.


28/84


29/84

Unlike other later and more significant single-shipexpeditions, 6 naval vessels carried out the United StatesExploring Expedition. Starting in Norfolk, Virginia, theexpedition sailed across the Atlantic to Madeira, recrossedto Rio de Janeiro, then south around Cape Horn and intothe Pacific Ocean. By the time the ships had sailed up thewest coast of South America to Callao, Peru, storms had

put three ships out of commission. What remained of theexpedition crossed the Pacific and while the scientificgentlemen were busy making collections in New Hollandand New Zealand, two ships, the Vinennes and thePorpoise , sailed south into the Antarctic region whereWilkes believed that there was a large land mass behind abarrier of ice.


30/84

In the austral summer of 1839-40 Wilkes sailed his shipssouth until blocked by the northern edge of the pack ice. He

then sailed west along the ice barrier and was able to getclose enough to see the land. At one point he came within anautical mile of the coast of Termination Land as Wilkesnamed it. This was the most interesting part of the expeditionas far as Wilkes was concerned. His alleged discovery of Antarctica was strongly contested by the British explorer SirJames Clark Ross but it remains as the only well-knownbenefit of this mission


31/84

Matthew Fountaine Maury

During this same period there was an importantdevelopment in the U.S. A Navy lieutenant, MatthewFontaine Maury, was seriously injured in a carriageaccident and was not able to go to sea for many years.Instead he was put in charge of a fairly obscure Navyoffice called the Depot of Charts and Instruments (1842 -

1861). This later became the U.S. Naval Observatory.This depot was responsible for the care of the navigationequipment in use at that time. In addition it received andsent out logs to be filled out by the bridge crew ships.Maury soon realized that the growing number of ship logsin his keeping was an important resource that could be

used to benefit many.


32/84


33/84

It was under Maurys guidance that a Lt. Baker developedone of the first deep-sea sounding devices. Lt. Baker stuck with the age-old concept of measuring the ocean depth by

dropping a line from the surface. The problem had beenthat in 4,000 m of water the line became too heavy toretrieve from the surface. Lt Baker designed a new metalline whose cross section varied from a very narrow gauge

wire at the bottom to a much thicker wire nearer thesurface. In addition, Baker followed one aspect of Hookes design and dropped the weight at the bottom,again making the system much lighter for retrieval. Alater addition was a small corer added to the end of the lineto collect a short (few cms) core of the top-layer of sediment.


34/84

This device led to the first comprehensive mapof bottom topography of the North Atlantic.Unfortunately for Maury, when the civil warbroke out he returned to his native south andspent most of the war developing explosive

devices to destroy enemy ships and to barricadeharbors. An important part of Maurys legacyis a book, which he wrote in 1985 and which isstill in print, the Physical Geography of the

Sea.


35/84

The Challenger Expedition The first global oceanographic cruise was made on the British ship the

HMS Challenger . This three-year (1872-1876) expedition (Fig. 1.2)

was driven primarily by the interest of a pair of biologists (William B.Carpenter and Charles Wyville Thomson) in determining whether ornot there is marine life in the great depths of the open ocean.Thomson was a Scot educated as a botanist at the University of Edinburgh and in the late 1860s he was a professor of natural history

at Belfast, Ireland. He had been working with his friend Carpenter, amedical doctor, to discover if the contention by another Britishnaturalist (Edward Forbes) that there was no life below 600 m (calledthe azoic zone) was true or not. Even in the early phase of theChallenger expedition dredges of bottom material from as much as2,000 m had demonstrated the great variety of life that exists at theocean bottom. In addition to biological samples this expeditioncollected a great number of physical measurements of the sea such assea surface temperature and samples of the min-max temperatures atvarious depths.


36/84


37/84


38/84

Along with Thomson and Carpenter, the Challenger scientific staff consistedof a naturalist John Murray and a young chemist, John Young Buchanan,both from the University of Edinburgh. The youngest scientist on the staff was twenty-five-year-old German naturalist Rudolf von Willemo s-Suhm

who gave up a position at the University of Munich to join the expedition.Henry Nottidge Moseley, another British naturalist who had also studied bothmedicine and science, joined the expedition after returning from aGovernment Expedition to Ceylon. Completing the staff was the expeditionsartist and secretary, James John Wild. Much of the visual documentation that

we have from the Challenger Expedition came from the able pen of JamesWild. The addition of John Murray was fortuitous in that he later saw to thepublication of the scientific results of the expedition. Upon return, it wassoon found that the Challenger expedition had exhausted the funds availablefor the publication of the results. Fortunately Murray, who was really astudent from the University of Edinburgh, recognized the value of thephosphate formations that dominated Christmas Island. Claiming the islandfor England, Murray later set up mining operations on the island. The incomefrom this operation was later used to publish the Challenger reports.


39/84

Scandinavian contributions and the dynamic method

In the last quarter of the nineteenth century a group of Scandinavian scientists began to investigate the theoreticalcomplexities of the sea in motion. In the late 1870s, aSwedish chemist, Gustav Ekman, began studying thephysical conditions of the Skagerack, part of the waterwayconnecting the Baltic and the North Sea. Motivated byfisheries problems, Ekman wanted to explain shoals of herring that had suddenly reappeared in the Skagerack after an absence of 70 years.


40/84


41/84

Pettersson and Ekman both understood that to obtain auseful picture of the circulation a series of expeditions

involving several vessels that could work together at manytimes throughout each year would have to be organized.This was a new approach to the study of the sea. In thename of fisheries research such a series of research cruiseswas begun in the early 1890s. These were some of thefirst cruises that emphasized the physical parameters of theocean. For the vertical profiling of the ocean temperaturea new device was available. Since 1874, the English firmNegretti and Zambra had manufactured a reversing

thermometer that would give accurate temperatures atdepth.


42/84

Fridtjof Nansen During this time, another Scandinavian broke new ground

in the rush to reach the North Pole. As a young man of 16,Fridtjof Nansen from Norway was the first person to walk across Greenland. This exploring spirit led Nansen topropose a Norwegian effort to reach the North Pole. From

his studies of various evidences, Nansen decided that therewas a northwestward circulation of ice in the Arctic.Instead of mounting a large attack on the Arctic, Nansenwanted to build a special ship that could withstand the

pressures of the sea ice when the ship was frozen into theArctic pack ice.


43/84

He believed that if he could sail as Far East as possible insummer he could then freeze his ship into the pack ice and

be carried to the northwest. His plan was to get as close aspossible to the North Pole at which time he and acompanion would use dog sleds to reach the pole and thenreturn to the ship. Named the Fram (forward inNorwegian) this unique ship was too small to carry a largecrew. Instead Nansen gathered a group of nine men whowould be able to adapt to this unique experience. Alwaysa scientist, Nansen planned a large number of measurements to be made during the Fram s time in the

ice pack.


44/84

On March 1895 the Fram reached 84 N about 360 miles from thepole. Nansen believed that this was about as far north as the Fram was likely to get. In the company of Frederik Hjalmar Johansen and alarge number of dogs, Nansen left the relative comfort of the Fram

and set off to drive the dog sleds to the North Pole. They drove slowlynorth over drifting ice until they were within 225 miles of their goal,farther north than any person had been before. For three months theyhad traveled over extremely rough ice, crossing what Nansen referredto as congealed breakers and they had lost their way. From their

farthest north point they turned south eventually reaching Franz Josef Land where they hoped to encounter a fishing boat in the shortsummer season. Surviving by eating their dogs, Nansen and Johansenwere very fortunate to meet a British expedition led by Frederick Jackson. In the summer of 1896 they sailed home to Oslo aboard theWindward . Meanwhile the Fram drifted further west and south andemerged from the ice pack just north of Spitsbergen. She sailed back to Oslo and arrived just a week after Nansen and Johansen.


45/84

One of Nansens primary objectives in the Fram

Expedition was to form a more complete idea of thecirculations of the northern seas. To achieve this the Fram had taken systematic measurements of the temperaturesand salinities of the Arctic water. Using one of

Petterssons insulated water bottles, Nansen had attached areversing thermometer to sample the temperature andsalinity profiles. This arrangement, known as a Nansen

bottle is still in use today.


46/84

The Ekman Spiral . Working in the Geophysical Institute of the University of Bergen, Norway,

Nansen tried to explain the measurements made by the Fram . Thehydrographic measurements suggested a very complex connection betweenthe Norwegian and Arctic Seas. The daily position information from theFram was also of great interest for this study. As a young student, Ekmanworked on this problem with Nansen. Both were interested to note that theFram did not drift in the same direction as the prevailing wind but insteaddiffered from the wind by about 20 - 40 to the right.

Using the measurements made by the Fram along with simple tank models of the Fram , Ekman developed his theory of the wind-drivencirculation of the ocean. Published in 1905 as part of the Fram report, Ekmanpostulated the response of the ocean to a steady wind in a uniform direction.Making some simple assumptions about the turbulent viscosity of the ocean,Ekman could show how the ocean current response to a steady wind musthave a surface current 45 to the right of the wind in the NorthernHemisphere. Below that there is a clockwise (NH.) spiral of currents (called

the Ekman spiral) down to a depth where the current vanishes.


47/84


48/84

Johan Sandstr m and Bjorn Helland-Hansen . The Norwegian Board of Sea Fisheries had invited Helland-Hansen and

Nansen, Johan Hjort, to participate in the first cruise of their new research

vessel. They were responsible for the collection of hydrographic measurements.A new problem cropped up. In their process of measuring salinity it wasnecessary to have a reference sea water to make the measurement precise,since slightly different methods and procedures were being used. At this time aDanish physicist, Martin Knudsen, was working on a set of hydrographicaltables that would clearly define the relationship between temperature, salinityand density. At the 1899 meeting of the International Council for theExploration of the Sea (ICES), Knudsen had proposed that such tables bepublished in order to facilitate the standardization of hydrographic work. Forthis same reason Knudsen suggested that a Standard or Normal water be createdand distributed to oceanographic laboratories throughout the world as a standardagainst which all salinity measurements could be compared. Knudsen thenproceeded to set up the Hydrographical Laboratory for ICES in Copenhagenand the standard seawater later became known as Copenhagen Water. He alsopublished standard tables called Knudsen Tables which displayed therelationships between chlorinity, salinity, densities and temperature.


49/84


50/84

S i d hi id P f Alf d M h h


51/84

Spiess presented his idea to Prof. Alfred Merz, then thehead of the Oceanographic Institute in Berlin. Merzhad been educated as a physical geographer but he had

always worked on the physics of the ocean. He washappy to accept the role of scientific leader of thefuture ocean expedition. This interest included theparticipation of his son-in-law and former studentGeorg W st, mentioned above with respect to his useof the dynamic method.

Merz and W st had collected together all the German andBritish hydrographic observations and had come up with anew vision of the horizontal and vertical circulation in theAtlantic with different water masses in thick layers (Fig.1.3). Our present view of the Atlantics overturningcirculation is not very different from their concept.


52/84


53/84

The verification and improved resolution of this proposedcirculation became the focus for the expedition. Since the

Meteor was not a very large ship it was decided that thecrew would have to help out in many measurementprograms. As a consequence, many crewmembers weresent to school at the Oceanography Institute in Berlin. In

addition it was decided to execute a test or shakedowncruise to determine if all the equipment was workingproperly. This cruise went from Wilhelmshaven on the

North Sea to the Azores and back.


54/84

This pre-cruise turned out to be a very wise move,resulting in a number of very basic changes. The

smokestack was lengthened in an effort to get theheat of the engines higher off the deck. In thetropics the lack of good ventilation on the shipbecame a serious problem and a lot of work had to

be done on the deck. The unique systemdeveloped for the Meteor to anchor in the deepocean had to be corrected. In addition, the forwardmast was set up to carry more sail to save coal onsome of the longer sections (Fig 1.4).


55/84


56/84


57/84

There were also some interesting personnel changes thatwere arranged after the pre-expedition. Most important

was the fact that a chemist who was to be in charge of thesalinity titrations was found to be colorblind. (Thetitration has a color change at the end point.) It was thennecessary to find someone who could do the salinitytitrations. The solution was that W st, although notoriginally slated to participate in the expedition, was takenalong to titrate the salinity samples. This later becamevery important since the expedition leader, Dr. Merz,passed away in Montevideo after the first of the Meteors

east-west sections had been completed.


58/84

This left the ship without a science leader. Although W stwas the most knowledgeable, he was considered too junior

to take over as expedition leader. Instead Captain Spiessofficially took over both as scientific leader and navalcaptain. In practice, however, it was W st who guided theexecution of the many measurements in physicaloceanography. He was committed to testing the schemethat he and Merz had developed for the circulation of theAtlantic. He was also a careful and painstaking collectorof new measurements, making sure that no short-cutswere taken in collecting or processing the measurements.

O A il 16 1925 h l f Wilh l h h


59/84

On April 16, 1925 the Meteor left Wilhelmshaven on herway to Buenos Aires, Argentina, which was to be thestarting point of the expedition. Outfitted with every new

instrument possible the Meteor was the first ocean researchcruise to concentrate primarily on the physical aspects of the ocean. She carried not one but two new echo-soundingsystems, which were to accurately measure, the depth of the ocean beneath the ship. With no computer or evenanalog storage machines it was necessary for someone tolisten continually to the pings of the unit. Crewmenwere enlisted in this operation and two sailors had to be inthe room 24-hours a day listening to pings and writing

down the travel times.


60/84


61/84

In addition the Meteor had a new system that would thatwould enable it to anchor in the deep ocean. It isinteresting that Walfrid Ekman went along on the pre-expedition trip to the Azores. In response to the fact thatthe Meteor was to be able to moor itself in the deep ocean,Ekman developed a current meter that could be usedmultiple times when suspended from the mainhydrographic wire (Fig 1.5). Ekman did not go along onthe main cruise but his current meter was used repeatedlyduring the deep-sea anchor stations.


62/84


63/84


64/84


65/84


66/84

A WHOI H Bi l d h fi di i i f hi


67/84

At WHOI Henry Bigelow was made the first director in spite of hisgenuine distaste for administrative duties. Originally WHOI was onlyto be operated in the summer leaving Bigelow the rest of the year forhis scientific and hobbies (fishing). Bigelow was so convinced of theimportance of having a fine, seaworthy vessel capable of making longvoyages in the stormy North Atlantic that he dodged the efforts of many to donate old pleasure yachts or tired fishing vessels. Instead heagreed to spend $175,000 on the largest steel-hulled ketch in theworld. A sailing ship with a powerful auxiliary engine was chosen

over a steamship because of problems with carrying sufficient coal forlong distance cruising. The contract was awarded to a Danishshipbuilding company and included two laboratories, two winches andquarters for 6 scientists and 17 crew. After delivery in the summer of 1931 Bigelow hired his former student, Columbus ODonnel Iselin, as

master of the research vessel named Atlantis . Iselin later became thedirector of WHOI and left a legacy of some very importantdevelopments in the study of the water masses of the ocean.

A SIO H ld S d hi d di i


68/84

At SIO, Harald Sverdrup was hired as a new director in1936, bringing from the Bergen school an emphasis onphysical oceanography. Within a year of his arrival, SIO

purchased a movie stars pleasure yacht and converted herinto the research vessel E.W. Scripps . Sverdrup had earlierbeen involved with an international effort to sail asubmarine under the North Polar ice cap. During a test it

was discovered that the submarine, named the Nautilus, had lost a diving rudder and would not be able to cruisebeneath the ice. It was not until 1957 that anothersubmarine named Nautilus was to cruise beneath the Northpolar ice cap and even to surface in one of the larger leadsin the ice pack.


69/84

O SIO H ld S d d hi d W l


70/84

Out west, at SIO, Harald Sverdrup and his student WalterMunk were studying the dynamics of wind-drivencurrents. At WHOI, Henry Stommel was also involved in

these studies. Basic models of the wind driven circulationemerged from these studies starting with Sverdrups modelwhich explained the basic balance between the majorcurrents and the pressure gradients, followed by

Stommels model and its explanation of the westwardintensification that closed the major ocean gyres at thewestern end. Munks model, with a slightly differentexplanation for the westward intensification, put it alltogether, giving a realistic circulation in response to asimplification of the meridional wind profile. Thesemodels were the basis for future more complex and

eventually numerical models of the ocean circulation.


71/84

Modern Day Physical Oceanography


72/84

Modern Day Physical Oceanography

Today many institutions around the world carry outresearch in physical oceanography, including thedescendants of the European and Japanese laboratories thatpioneered oceanographic research in the 1800s and firstpart of the 20 th century. Oceanographic research hasgrown to include many government institutions. Becauseof the importance of large-scale oceanography to climate,most climate-modeling laboratories support oceanographic

modeling along with atmospheric modeling.

Oceanographic Meetings


73/84

The biggest meetings for physical oceanographers are

sponsored by the American Geophysical Union (AGU) andEuropean Geophysical Union. Canada and Japan alsohave vigorous annual meetings. International meetingssuch as the meetings of the International Association for

the Physical Study of the Ocean (IAPSO) as part of themeeting each four years of the International Union of Geology and Geophysicists attract a large number of physical oceanographers to an international forum.

There has been a dramatic shift in emphasis of research in physical


74/84

There has been a dramatic shift in emphasis of research in physicaloceanography near the end of the 20th century. A global survey of ocean circulation (World Ocean Circulation Experiment or WOCE),whose main purpose was to assist, through careful observations, the

development of numerical ocean circulation models used for climatemodeling, and an intensive ocean-atmosphere study of processesgoverning El Nino in the tropical Pacific (Tropical Ocean GlobalAtmosphere or TOGA) have been completed. Many of the programsthat are continuing past these focus on the relationship between ocean

physics and the climate. At the same time the practical importance of ocean physics in the coastal ocean is emerging. Even the U.S. Navy isnow more interested in developing an understanding of the physics of the coastal oceans than in knowing something about the deep ocean.The need for military operations in the ocean has shifted to the coastslargely in support of other land operations. At the same time oiloperations are primarily restricted to the shallow water of the coastalregions where tension with the local environment requires even greaterstudy of the coastal ocean.

Shifts in Modern Sampling Methods


75/84

The most dramatic shifts in physical oceanographic

methods at the turn of the 21st

century are to extensiveremote sensing, in the form of both satellite and moreautomated in situ observations, and to ever-growingreliance on complex computer models. Satellitesmeasuring sea surface height, surface temperature, andmost of the components of forcing for the oceans are nowin place. Broad observational networks measuring tidesand sea level and upper ocean temperatures in the mid-to-late 20 th century have been greatly expanded.

These networks now include continuous current and temperature


76/84

pmonitoring in regions where the ocean's conditions strongly affectclimate, such as the tropical Pacific and Atlantic, and growingmonitoring of coastal regions. Global arrays of drifters measuring

surface currents and temperature, and subsurface floats measuringdeeper currents and ocean properties between the surface and about2000 m depth are now expanding. Meanwhile the enormous growth inavailable computational power and numbers of scientists engaged inocean modeling is expanding our modeling capability and ability to

simulate ocean conditions and study particular ocean processes. Withincreasing amounts of globally-distributed data available in near real-time, numerical ocean modelers are now beginning to combine dataand models to improve ocean analysis and possibly prediction of ocean circulation changes, in a development similar to that fornumerical weather prediction in the twentieth century. Full climatemodeling includes ocean modeling, and many oceanographers arebeginning to focus on the ocean component of climate modeling.These trends are likely to continue for some time.


77/84

These networks now include continuous current and temperature monitoring in regions


78/84

where the ocean's conditions strongly affect climate, such as the tropical Pacific andAtlantic, and growing monitoring of coastal regions. Global arrays of drifters measuringsurface currents and temperature, and subsurface floats measuring deeper currents andocean properties between the surface and about 2000 m depth are now expanding.

Meanwhile the enormous growth in available computational power and numbers of scientists engaged in ocean modeling is expanding our modeling capability and ability tosimulate ocean conditions and study particular ocean processes. With increasingamounts of globally-distributed data available in near real-time, numerical oceanmodelers are now beginning to combine data and models to improve ocean analysis andpossibly prediction of ocean circulation changes, in a development similar to that fornumerical weather prediction in the twentieth century. Full climate modeling includesocean modeling, and many oceanographers are beginning to focus on the oceancomponent of climate modeling. These trends are likely to continue for some time.

The growth and evolution of ocean modeling is paced, to a


79/84

g g p ,certain degree, by the growth in computing power overtime. The computational cost of a model is determined byits resolution, that is the range of scales represented; thesize of the domain (basin or global, upper ocean or fulldepth); and the comprehensiveness and complexity of theprocesses, both resolved and parameterized, that are to berepresented. An ocean model is typically first formulated

in terms of the differential equations of fluid mechanics,often applying approximations that eliminate processesthat are of no interest to the study at hand. For example, inthe study of large scale ocean dynamics, sound wavepropagation through the ocean is not of great importance,

so seawater is approximated as an incompressible fluid,thereby filtering sounds waves out of the equations.

These networks now include continuous current and temperature monitoring in regions


80/84

where the ocean's conditions strongly affect climate, such as the tropical Pacific andAtlantic, and growing monitoring of coastal regions. Global arrays of drifters measuringsurface currents and temperature, and subsurface floats measuring deeper currents andocean properties between the surface and about 2000 m depth are now expanding.

Meanwhile the enormous growth in available computational power and numbers of scientists engaged in ocean modeling is expanding our modeling capability and ability tosimulate ocean conditions and study particular ocean processes. With increasingamounts of globally-distributed data available in near real-time, numerical oceanmodelers are now beginning to combine data and models to improve ocean analysis andpossibly prediction of ocean circulation changes, in a development similar to that fornumerical weather prediction in the twentieth century. Full climate modeling includesocean modeling, and many oceanographers are beginning to focus on the oceancomponent of climate modeling. These trends are likely to continue for some time.


81/84

The earliest three-dimensional ocean general circulation


82/84

gmodels, originally developed in the 1960's by Kirk Bryanand colleagues at the NOAA Geophysical Fluid DynamicsLaboratory were based on finite-difference methods usingdepth as the vertical coordinate. Models descended fromthis formulation still comprise the most widely used classof ocean general circulation models, particularly in theclimate system modeling community. The first globalocean simulations carried out with this type of model werelimited by the then available computational resources toresolutions of several hundred kilometers, insufficient torepresent the hydrodynamic instability processesresponsible for generating mesoscal eddies.

In the 1970's as observational technology emerged that showed thef f


83/84

predominance of mesoscale eddies in the ocean, a new class of modelswith simplifications to the physics, e.g. using the quasi-geostrophicrather than the primitive equations, and limited domain sizes but withresolution of a few tens of kilometers was developed, most notably byWilliam Holland, Jim McWilliams and colleagues at NCAR. Modelsof this class contributed greatly to the development of ourunderstanding of the interaction of mesoscale eddies and the large-scale ocean circulation, and to the development of parameterizationsof eddy mixing processes for use in coarser resolution models.Initially developed as a generalization to the quasi-geostrophic eddy-resolving models, isopycnal coordinate models such as that developedby Rainer Bleck and co-workers at the University of Miami havebecome increasingly popular for ocean simulation through the 1980'sand 1990's. Similarly, sigma- or terrain-following coordinate modelsinitially developed primarily in the coastal ocean modeling

community, e.g. by George Mellor and co-workers at PrincetonUniversity, have seen increasing use in basin- to global-scale oceanstudies through the 1980's and 1990's.

In the 21st century we are witnessing both a tighter


84/84

integration of modeling with observational oceanography,for example through the use of data assimilationtechniques, and significant merging and cross-fertilizationof the various approaches to ocean modeling describedabove. Computer power has reached a level where theocean components of fully coupled climate system modelshave sufficient resolution to permit mesoscale eddies,

blurring the distinction between ocean models used forclimate applications and those used to study mesoscaleprocesses. Several new models are emerging with hybridvertical coordinates, bring the best features of depth,isopycnal and terrain-following coordinates into a single

model framework .

Date post:	30-May-2018
Category:	Documents
Upload:	peterttkok
View:	216 times
Download:	0 times

Physical OceanographyAn Introduction

Documents