William J. Emery

Instruments and Methods

Publisher Summary This chapter focuses on methods and instrumentation for measuring the large-scale circulation and water properties of the ocean. Traditional deep oceanographic profiles are made from research ships to study the very largest spatial and temporal scales of the ocean circulation and property distributions. These remain the only way to measure the deep ocean with high accuracy, and the only way to make most chemical measurements. A deep oceanographic station can take up to three hours and a cross-section across an ocean can take up to two months, posing limitations to interpretation. The individual, widely separated profiles cannot be used to study tides, internal waves, or eddies, for instance, but these and other smaller scale motions affect the individual station measurements. There are, however, useful ways to process and analyze the data so that they can be used to study the large space and timescales of interest. Satellite altimeters measure the oceans surface height, passing over each point on the oceans surface every week or two. The chapter discusses some of the sampling issues for physical oceanography. Platforms for observations are described, and instruments for in situ observations (within the water column) are reviewed. The chapter also provides an overview of satellite remote sensing, and oceanographic archives.

Physical Properties of Seawater

This chapter focuses on the physical properties of seawater. Many of the unique characteristics of the ocean can be ascribed to the nature of water itself. Consisting of two positively charged hydrogen ions and a single negatively charged oxygen ion, water is arranged as a polar molecule having positive and negative sides. This molecular polarity leads to waters high dielectric constant (ability to withstand or balance an electric field). Water is able to dissolve many substances because the polar water molecules align to shield each ion, resisting the recombination of the ions. As seawater is heated, molecular activity increases and thermal expansion occurs, reducing the density. Another important physical characteristic of seawater is its temperature. In most of the ocean, temperature is the primary determinant of density. The chapter discusses molecular properties of water, and thermal properties of seawater. Effects of temperature and salinity on density, and concepts related to sound and light in the sea are also explained in the chapter.

Introduction to Descriptive Physical Oceanography

This chapter provides an introduction to descriptive physical oceanography. Oceanography is the general name given to the scientific study of the oceans. It is historically divided into physical, biological, chemical, and geological oceanography. Descriptive physical oceanography approaches the ocean through both observations and complex numerical model output used to describe the fluid motions as quantitatively as possible. This book is concerned primarily with the physics of the ocean. Chapter 2 describes the ocean basins and their topography. The next three chapters introduce the physical (and some chemical) properties of freshwater and seawater ( Chapter 3 ), an overview of the distribution of water characteristics ( Chapter 4 ), and the sources and sinks of heat and freshwater ( Chapter 5 ). The next three chapters cover data collection and analysis techniques ( Chapter 6 ), an introduction to geophysical fluid dynamics ( Chapter 7 ), and then basic waves and tides with an introduction to coastal oceanography ( Chapter 8 ). The last six chapters of the book introduce the circulation and water properties of each of the individual oceans ( Chapters 9–13 Chapter 9 Chapter 10 Chapter 11 Chapter 12 Chapter 13 ) ending with a summary of the global ocean in Chapter 14 .

Typical Distributions of Water Characteristics

This chapter describes the typical distributions of water properties such as temperature, salinity, oxygen, and nutrients. Most water characteristics have large and typical variations in the vertical direction, which encompasses an average of 5 km in the deep ocean, whereas variations of similar magnitude in the horizontal occur over vastly greater distances. For instance, near the equator, the temperature of the water may drop from 25°C at the surface to 5°C at a depth of 1 km, but it may be necessary to go 5000 km north or south from the equator to reach a latitude where the surface temperature has fallen to 5°C. Much of the geographic variation in properties in the oceans and atmosphere occurs in the north–south (meridional) direction. Properties are often much more uniform in the eastwest (zonal) direction. A principal exception to the latter is the important zonal variation near boundaries, especially on the west sides of ocean basins. The chapter discusses the temperature distribution of the oceans. It also gives details about salinity distribution, surface salinity, and upper layer salinity. Concepts related to density distribution and density at the sea surface and in the upper layer are also explained in the chapter.

Global Circulation and Water Properties

This chapter summarizes the circulation and water properties at a global scale, synthesizing the regional elements from the individual ocean basins, and presents some evolving views of the global overturning circulation. The surface circulation systems have been observed for centuries in all of their complexity, and are the best mapped part of the circulation because of ease of access. These circulations impact navigation, pollutant dispersal, the upper oceans productive euphotic zone, and continental shelves and coastal zones. As the interface with the atmosphere, the surface layer and circulation are directly involved in ocean-atmosphere feedbacks that affect both the mean states of the ocean and atmosphere and also seasonal to climate scale variability. Just a few hundred meters below the sea surface, some parts of the circulation change dramatically as the wind-driven gyres contract and weaken. At intermediate and abyssal depths, the circulation is dominated by the deep penetration of the most vigorous surface currents, and by circulation associated with large-scale buoyancy forcing and weak diapycnal processes that can change the density of the water internally. The chapter discusses upper ocean circulation systems as well as intermediate and deep circulation systems. It explains basics of mass transports in layers into closed regions. A detailed discussion on eddy energy and lateral eddy diffusivity distributions is also presented in the chapter.

Data Analysis Concepts and Observational Methods

This chapter provides an overview of the data analysis concepts and observational methods for the study of oceans. In data analysis, observations or determinations of the value of a variable such as pressure, time, temperature, conductivity, oxygen content, and so forth are collected using oceanographic instruments at particular times and locations that are chosen through a sampling strategy. From these imperfect observations, containing both instrumental and sampling error, researchers estimate the true field and its statistical properties as a function of time and/or space. Modern instruments that measure nearly continuous vertical profiles—underway sampling systems such as expendable bathythermographs (XBTs) and acoustic Doppler current profilers (ADCPs), moored current meters, autonomous drifting and guided systems, and satellites—can generate large volumes of digital data. These large data sets can be treated statistically to identify data errors, to map fields, to generate statistical information such as means and trends, and to detect embedded time and space patterns and correlations among different observed parameters. The chapter discusses in detail about oceanographic sampling and observational error. Basic statistical concepts such as mean, variance, standard deviation, standard error, and probability density function are explained in the chapter. The chapter also discusses concepts related to variation in space and variation in time.

Gravity Waves, Tides, and Coastal Oceanography: Supplementary Materials

This chapter provides an overview of gravity waves, tides, and coastal oceanography. River runoff affects coastal regions. It reduces the salinity of the surface layer and even of the deeper water if there is sufficient vertical mixing. It often carries a large amount of suspended sediment, as seen for the Mississippi River outflow and the outflows from the Himalayas into the Bay of Bengal, including the Ganges River. Generally, river runoff has a pronounced seasonal variation, resulting in much larger seasonal fluctuations of salinity in coastal waters than in the open ocean. In a coastal region where precipitation occurs chiefly as rain, the seasonal salinity variation will closely follow the local precipitation pattern. In regions where rivers are fed by meltwater from snowfields or glaciers, the river runoff increases in the summer to many times the winter rate and causes a corresponding decrease of salinity that lags the snowfall by several months. Topography of coral Reefs and water properties in coral reefs are explained in the chapter. Types of estuaries, estuarine circulation, and flushing time of estuaries are also discussed in the chapter.

Dynamical Processes for Descriptive Ocean Circulation

This chapter provides an overview of dynamical processes for descriptive ocean circulation. Motion of water in the ocean is driven by the sun, the moon, or tectonic processes. The suns energy is transferred to the ocean through buoyancy fluxes (heat fluxes and water vapor fluxes) and through the winds. Tides create internal waves that break, creating turbulence and mixing. Earthquakes and turbidity currents create random, irregular waves including tsunamis. Geothermal processes heat the water very gradually with little effect on circulation. Ocean circulation is often divided conceptually into wind-driven and thermohaline (or buoyancy-dominated) components. Wind causes waves, inertial currents, and Langmuir cells. At longer timescales, which involve the Coriolis Effect, wind drives the near-surface frictional layer and, indirectly, the large-scale gyres and currents that are usually referred to as the wind-driven circulation. The chapter explains concepts related to momentum balance, acceleration and advection as they apply to oceans. It also gives details about temperature, salinity, density evolution, and molecular and eddy diffusivity. A detailed discussion on surface mixed layer, bottom mixed layers, and internal mixing layers is also presented in the chapter.

Ocean Dimensions, Shapes, and Bottom Materials

This chapter introduces some nomenclature and features of the basins that have a close connection with the oceans circulation and dynamical processes that are of importance to the physical oceanographer. The shape, depth, and geographic location of an ocean affect the general characteristics of its circulation. Smaller scale features, such as locations of deep sills and fracture zones, seamounts, and bottom roughness, affect often important details of the circulation and of mixing processes that are essential to forcing and water properties. The Atlantic has a very marked “S” shape while the Pacific has a more oval shape. The Atlantic and Indian Oceans are roughly half the eastwest width of the Pacific Ocean, which impacts the way that each oceans circulation adjusts to changes in forcing. The Indian Ocean has no high northern latitudes, and therefore no possibility of cold, dense water formation. The chapter explains concepts of dimensions plate tectonics and deep-sea topography. It explains seafloor features and spatial scales. The chapter also discusses concepts related to shore, coast, beach, continental shelf, slope and rise, deep oceans, and methods for mapping bottom topography.

Arctic Ocean and Nordic Seas

This chapter focuses on Arctic Ocean and Nordic Seas. The Arctic Ocean is a Mediterranean sea surrounded by the North American, European, and Asian continents. It is connected to the Atlantic Ocean on both sides of Greenland and to the Pacific Ocean through the shallow Bering Strait. The Nordic Seas is the region south of Svalbard and north of Iceland. This region is central for transformation and production of some of the densest waters in the global ocean, creating the densest part of the North Atlantic Deep Water, and is a high latitude connection of the fresher North Pacific waters to the saltier North Atlantic waters. The Arctics sea ice cover is a vital component of global climate because of its high albedo (high solar reflectivity. The Arctics sea ice cover is sensitive to climate change. Because of important climate changes and initiation of difficult hydrographic time series in this ice-covered region beginning in the 1990s, there is a large and growing body of information about circulation, water masses, and ice cover in the Arctic. The chapter discusses Nordic Seas circulation and Nordic Seas water masses. Concepts of ice drift and wind forcing, upper layer circulation, and intermediate and deep circulation are also presented in the chapter.