C H A P T E R

1

Introduction to Descriptive PhysicalOceanography

Oceanography is the general name given tothe scientific study of the oceans. It is histori-cally divided into physical, biological, chemical,and geological oceanography. This book is con-cerned primarily with the physics of the ocean,approached mainly, but not exclusively, fromobservations, and focusing mainly, but notexclusively, on the larger space and timescalesof the open ocean rather than on the near-coastaland shoreline regions.

Descriptive physical oceanography approachesthe ocean through both observations andcomplex numerical model output used todescribe the fluid motions as quantitatively aspossible.Dynamical physical oceanography seeksto understand the processes that govern the fluidmotions in the ocean mainly through theoreticalstudies and process-based numerical modelexperiments. This book is mainly concernedwith description based in observations (similarto previous editions of this text); however, inthis edition we include some of the concepts ofdynamical physical oceanography as an impor-tant context for the description. A full treatmentof dynamical oceanography is contained in othertexts. Thermodynamics also clearly enters into ourdiscussion of the ocean through the processesthat govern its heat and salt content, and there-fore its density distribution.

Chapter 2 describes the ocean basins and theirtopography.Thenext three chapters introduce thephysical (and some chemical) properties of fresh-water and seawater (Chapter 3), an overview ofthe distribution of water characteristics (Chapter 4),and the sources and sinks of heat and freshwater(Chapter 5). The next three chapters cover datacollection and analysis techniques (Chapter 6and supplemental material listed as Chapter S6on the textbook Web site http://booksite.academicpress.com/DPO/; “S” denotes supple-mentalmaterial.), an introduction to geophysicalfluid dynamics for graduate students who havevarying mathematics backgrounds (Chapter 7),and then basicwaves and tideswith an introduc-tion to coastal oceanography (Chapter 8). Thelast six chapters of the book introduce the circu-lation and water properties of each of the indi-vidual oceans (Chapters 9 through 13) endingwith a summary of the global ocean in Chapter 14.

Accompanying the text is the Web sitementioned in the previous paragraph. It hasfour aspects:

1. Textbook chapters on climate variability andoceanographic instrumentation that do notappear in the print version

2. Expanded material and additional figures formany other chapters

1Descriptive Physical Oceanography

� 2011. Lynne Talley, George Pickard, William Emery and James Swift.

Published by Elsevier Ltd. All rights reserved.

3. A full set of tutorials for descriptiveoceanography with data and sample scriptsprovided based on the Java Ocean Atlas(Osborne & Swift, 2009)

4. All figures from the text, with many more incolor than in the text, for lectures andpresentations.

1.1. OVERVIEW

There are many reasons for developing ourknowledge of the ocean. Near-shore currentsand waves affect navigation and construction ofpiers, breakwaters, and other coastal structures.The large heat capacity of the oceans exertsa significant and in some cases a controlling effecton the earth’s climate. The ocean and atmosphereinteract on short to long timescales; for example,the El NinoeSouthern Oscillation (ENSO)phenomenon that, although driven locally in thetropical Pacific, affects climate on timescales ofseveral years over much of the world. To under-stand these interactions it is necessary to under-stand the coupled ocean-atmosphere system. Tounderstand the coupled system, it is first neces-sary to have a solid base of knowledge aboutboth the ocean and atmosphere separately.

In these and many other applications, knowl-edge of the ocean’s motion and water propertiesis essential. This includes the major oceancurrents that circulate continuously but withfluctuating velocity and position, the variablecoastal currents, the rise and fall of the tides,and the waves generated by winds or earth-quakes. Temperature and salt content determinedensity and hence vertical movement. They alsoaffect horizontal movement as the density affectsthe horizontal pressure distribution. Sea ice hasits own full set of processes and is important fornavigation, ocean circulation, and climate. Otherdissolved substances such as oxygen, nutrients,and other chemical species, and even some ofthe biological aspects such as chlorophyllcontent, are used in the study of ocean physics.

Our present knowledge in physical oceanog-raphy rests on an accumulation of data, most ofwhich were gathered during the past 150 years,with a large increase of in situ data collection(within the actual water) starting in the 1950sand an order of magnitude growth in availabledata as satellites began making ocean measure-ments (starting in the 1970s).

A brief history of physical oceanographywithillustrations is provided as supplemental mate-rial on the textbook Web site (Chapter 1 supple-ment is listed as Chapter S1 on the Web site).Historically, sailors have always been concernedwith how ocean currents affect their ships’courses as well as changes in ocean temperatureand surface condition. Many of the earlier navi-gators, such as Cook and Vancouver, made valu-able scientific observations during their voyagesin the late 1700s, but it is generally consideredthat Mathew Fontaine Maury (1855) started thesystematic large-scale collection of ocean currentdata using ship’s navigation logs as his source ofinformation. The firstmajor expedition designedexpressly to study all scientific aspects of theoceans was carried out on the British H.M.S.Challenger that circumnavigated the globe from1872 to 1876. The first large-scale expeditionorganized primarily to gather physical oceano-graphic data was carried out on the German FSMeteor, which studied the Atlantic Ocean from1925 to 1927 (Spiess, 1928). A number of photosfrom that expedition are reproduced on theaccompanying Web site. Some of the earliesttheoretical studies of the sea included the surfacetides by Newton, Laplace, and Legendre (e.g.,Wilson, 2002) and waves by Gerstner and Stokes(e.g., Craik, 2005). Around 1896, some of theScandinavian meteorologists started to turntheir attention to the ocean, because dynamicalmeteorology and dynamical oceanographyhave many common characteristics. Currentknowledge of dynamical oceanography owesits progress to the work of Bjerknes, Bjerknes,Solberg, and Bergeron (1933), Ekman (1905,1923), Helland-Hansen (1934), and others.

1. INTRODUCTION TO DESCRIPTIVE PHYSICAL OCEANOGRAPHY2

The post-war 1940s through 1960s began toproduce much of the data and especially theoret-ical understanding for large-scale ocean circula-tion. With the advent of moored and satelliteinstrumentation in the 1960s and 1970s, thesmaller scale, energetically varying part of theocean circulation d the mesoscale d began tobe studied in earnest. Platforms expanded fromresearch and merchant ships to global satelliteand autonomous instrument sampling. Futuredecades should take global description andmodeling to even smaller scales (submesoscale)as satellite observations and numerical modelingresolution continue to evolve, and different typesof autonomous sampling within the watercolumn become routine. Physical oceanographyhas retained an aspect of individual explorationbut large, multi-investigator, multinationalprograms have increasingly provided many ofthe new data sets and understanding. Currentresearch efforts in physical oceanography arefocused on developing an understanding of thevariability of the ocean and its relation to theatmosphere and climate as well as continuing todescribe its steady-state conditions.

1.2. SPACE AND TIMESCALES OFPHYSICAL OCEANOGRAPHIC

PHENOMENA

The ocean is a fluid in constant motion witha very large range of spatial and temporalscales. The complexity of this fluid is nicely rep-resented in the sea surface temperature image ofthe Gulf Stream captured from a satellite shownin Figure 1.1a. The Gulf Stream is the westernboundary current of the permanent, large-scaleclockwise gyre circulation of the subtropicalNorth Atlantic. The Gulf Stream has a width of100 km, and its gyre has a spatial scale of thou-sands of kilometers. The narrow, warm core ofthe Gulf Stream (red in Figure 1.1a) carrieswarm subtropical water northward from theCaribbean, loops through the Gulf of Mexico

around Florida and northward along the eastcoast of North America, leaves the coast atCape Hatteras, and moves out to sea. Itsstrength and temperature contrast decay east-ward. Its large meanders and rings, with spatialscales of approximately 100 km, are consideredmesoscale (eddy) variability evolving on time-scales of weeks. The satellite image also showsthe general decrease in surface temperaturetoward the north and a large amount of small-scale eddy variability. The permanence of theGulf Stream is apparent when currents andtemperatures are averaged in time. Averagingmakes the Gulf Stream appear wider, especiallyafter the separation at Cape Hatteras where thewide envelope of meanders creates a wide,weak average eastward flow.

The Gulf Stream has been known and chartedfor centuries, beginning with the Spanish expe-ditions in the sixteenth century (e.g., Peterson,Stramma, & Kortum 1996). It was first mappedaccurately in 1769 by Benjamin Franklin

FIGURE 1.1 (a) Sea surface temperature from a satelliteadvanced very high resolution radiometer (AVHRR)instrument (Otis Brown, personal communication, 2009).This figure can also be found in the color insert.

SPACE AND TIMESCALES OF PHYSICAL OCEANOGRAPHIC PHENOMENA 3

working together with whaling captain TimothyFolger (Figure 1.1b; from Richardson, 1980a).1

The narrow current along the coast of theUnited States is remarkably accurate. Thewidening envelope of the Franklin/Folgercurrent after separation from Cape Hatteras isan accurate depiction of the envelope ofmeandering apparent in the satellite image.

When time-mean averages of the Gulf Streambased on modern measurements are con-structed, they look remarkably similar to thisFranklin/Folger map.

The space and timescales of many of theimportant physical oceanography processes areshown schematically in Figure 1.2. At the small-est scale is molecular mixing. At small,

FIGURE 1.1 (b) Franklin-Folger map of the Gulf Stream. Source: From Richardson (1980a).

1 Franklin noted on his frequent trips between the United States and Europe that some trips were considerably quicker than

others. He decided that this was due to a strong ocean current flowing from the west to the east. He observed marked

changes in surface conditions and reasoned that this ocean current might be marked by a change in sea surface

temperature. Franklin began making measurements of the ocean surface temperature during his travels. Using a simple

mercury-in-glass thermometer, he was able to determine the position of the current.

1. INTRODUCTION TO DESCRIPTIVE PHYSICAL OCEANOGRAPHY4

macroscopic scales of centimeters, microstruc-ture (vertical layering at the centimeter level)and capillary waves occur. At the slightly largerscale of meters surface waves are found, whichhave rapid timescales and somewhat long-livedvertical layers (fine structure). At scales of tensof meters are the internal waves with timescalesof up to a day. Tides have the same timescalesas internal waves, but much larger spatial scalesof hundreds to thousands of kilometers. Surfacewaves, internal waves, and tides are describedin Chapter 8.

Mesoscale eddies and strong ocean currentssuch as the Gulf Stream are found at spatialscales of tens of kilometers to several hundredkilometers and timescales of weeks to years(Figure 1.1a,b). The large-scale ocean circulationhas a spatial scale the size of ocean basins up tothe global ocean and a timescale ranging fromseasonal to permanent, which is the timescaleof plate tectonics that rearranges the oceanboundaries (Chapter 2). The timescales forwind-driven and thermohaline circulation inFigure 1.2 are actually the same for circulationof the flow through those systems (ten yearsaround the gyre, hundreds of years through thefull ocean); these are time-mean features of the

ocean and havemuch longer timescales. Climatevariability affects the ocean, represented inFigure 1.2 by the El Nino, which has an interan-nual timescale (several years; Chapter 10);decadal and longer timescales of variability ofthe ocean circulation and properties are alsoimportant and described in each of the oceanbasin and global circulation chapters.

We see in Figure 1.2 that short spatial scalesgenerally have short timescales, and long spatialscales generally have long timescales. There aresome exceptions to this, most notably in thetides and tsunamis as well as in some fine-struc-ture phenomena with longer timescales thanmight be expected from their short spatialscales.

In Chapter 7, where ocean dynamics are dis-cussed, some formal non-dimensional parametersincorporating the approximate space and time-scales for these different types of phenomenaare introduced (see also Pedlosky, 1987). Anon-dimensional parameter is the ratio ofdimensional parameters with identical dimen-sional scales, such as time, length, mass, etc.,which are intrinsic properties of the flowphenomenon being described or modeled. Ofspecial importance is whether the timescale of

Bubbles

Deep-watersurface waves Tsunamis

Tides

Internalwaves

Mesoscaleeddies

Thermo-haline circulationWind-

driven circulation

El Niño

Capillary waves

Milankovitch

Length Scale 500–5,000 km

1,000s years100 years

10 years

1–7 years

wks – months

12–24 hrs

1 min – 20 hrs0.1 sec – 1 min

0.1 sec

0.01 sec

Tim

esca

le

mm cm 1–100m 0.1–100km 100–1,000km 5,000 km 10,000 km

FIGURE 1.2 Time and spacescales of physical oceanographicphenomena from bubbles andcapillary waves to changes inocean circulation associated withEarth’s orbit variations.

SPACE AND TIMESCALES OF PHYSICAL OCEANOGRAPHIC PHENOMENA 5

an oceanmotion is greater than or less than abouta day, which is the timescale for the earth’s rota-tion. Earth’s rotation has an enormous effect onhow the ocean water moves in response toa force; if the force and motion are sustainedfor days or longer, then the motion is stronglyinfluenced by the rotation. Therefore, an espe-cially useful parameter is the ratio of the time-scale of Earth’s rotation to the timescale of themotion. This ratio is called the Rossby number.For the very small, fast motions in Figure 1.2,this ratio is large and rotation is not important.For the slow, large-scale part of the spectrum,the Rossby number is small and Earth’s rotationis fundamental. A second very important non-dimensional parameter is the ratio of the verticallength scale (height) to the horizontal length

scale; this is called the aspect ratio. For large-scaleflows, this ratio is very small since the verticalscale can be no larger than the ocean depth. Forsurface and internal gravity waves, the aspectratio is order 1. We will also see that dissipationis very weak in the sense that the timescale fordissipation to act is long compared with boththe timescale of Earth’s rotation and the time-scale for the circulation to move water fromone place to another; the relevant non-dimen-sional parameters are the Ekman number andReynolds number, respectively. Understandinghow the small Rossby number, small aspectratio, and nearly frictionless fluid ocean behaveshas depended on observations of the circulationandwater propertiesmade over the past century.These are the principal subjects of this text.

1. INTRODUCTION TO DESCRIPTIVE PHYSICAL OCEANOGRAPHY6