This book presents the views of leading scientists on the knowledge of the global ocean circulation following the completion of the observational phase of the World Ocean Circulation Experiment. WOCE's in situ physical and chemical measurements together with satellite altimetry have produced a data set which provides for development of ocean and coupled ocean-atmosphere circulation models used for understanding ocean and climate variability and projecting climate change. This book guides the reader through the analysis, interpretation, modelling and synthesis of this data.
Basic questions of physical processes in large-scale dynamics of the oceans are discussed in this book. These large-scale circulations influence the climate of the earth and are of great importance for the future development of our climate system.
The past decade has seen tremendous progress in the application of ocean remote sensing to the study of the global ocean circulation. This chapter provides a summary of the resultant advances in our understanding of the key processes of the ocean that affect climate variability. Many of the advances result from the combined usage of remote sensing from multiple types of measurement and in situ observations. Remotely sensed ocean variables include sea surface height, wind, temperature, salinity and color, as well as the variable mass of the ocean and ice from spaceborne measurement of the earth’s gravity field. These observations have often been analyzed with various in situ observations, including moored buoys, hydrographic profiles, surface drifters, and Argo floats. The general circulation of the ocean as manifested by the ocean surface dynamic topography from satellite altimetry, and the geoid from satellite gravity measurements, can now be determined at scales approaching 100km. The information from surface drifters and Argo floats has added more details through the upper ocean depths. The large-scale changes of the ocean on decadal scales reveal complex geographic patterns in relation to the changes in the atmospheric forcing. The causes for the slow rise of the global mean sea level are diagnosed in terms of the steric and mass change of the ocean. The bottom pressure inferred from ocean mass change measured from space provides direct observation of the barotropic variability of the ocean. The detailed information of ocean surface wind measured from scatterometry and temperature from infrared and microwave radiometry reveals a positive correlation between the two, leading to new understanding of air–sea interactions at scales below 1000km. Data combined from multiple satellite altimeters through optimally designed processing have revolutionized the study of the global ocean mesoscale processes, revealing new information on the spectral transfer of energy and on global eddy propagation characteristics, which vary in relation to the mean circulation, bottom topography, and the nonlinearity of eddy dynamics. The gridded fields of remote sensing data have made satellite observations routinely accessible to general users for scientific and operational applications. The outlook for future development in ocean remote sensing is also discussed.
The Southern Ocean circulation connects the ocean basins as well as the upper and deep layers of the ocean. As a result, the region has a profound influence on the global ocean circulation and climate. The Antarctic Circumpolar Current and the overturning circulation are dynamically linked through interactions between the mean flow, eddies, topography, air–sea forcing, and mixing and stirring by small- and mesoscale processes. A new dynamical paradigm is emerging that emphasizes the fully three-dimensional nature of the circulation, including the localization of meridional and vertical exchange of momentum, vorticity, and tracers by interactions between the flow and topography. Changes observed in the Southern Ocean in recent decades have implications for global climate and provide insight into the response of the Southern Ocean circulation to changes in forcing.
Conceptual models are a vital tool for understanding the processes that maintain the global ocean circulation, both in nature and in complex numerical ocean models. In this chapter we provide a broad overview of our conceptual understanding of the wind-driven circulation, the thermohaline circulation, and their transient behavior. While our conceptual understanding of the time-mean wind-driven circulation is now fairly mature, basic questions remain regarding the thermohaline circulation, for example, surrounding its overall strength and stability. Similarly, basic questions remain regarding the transient adjustment and internal variability of the ocean circulation.
The book represents all the knowledge we currently have on ocean circulation. It presents an up-to-date summary of the state of the science relating to the role of the oceans in the physical climate system. The book is structured to guide the reader through the wide range of world ocean circulation experiment (WOCE) science in a consistent way. Cross-references between contributors have been added, and the book has a comprehensive index and unified reference list. The book is simple to read, at the undergraduate level. It was written by the best scientists in the world who have collaborated to carry out years of experiments to better understand ocean circulation. Presents in situ and remote observations with worldwide coverage Provides theoretical understanding of processes within the ocean and at its boundaries to other Earth System components Allows for simulating ocean and climate processes in the past, present and future using a hierarchy of physical-biogeochemical models
This chapter focuses on numerical models used to understand and predict large-scale circulation, such as the circulation comprising basin and global scales. It is organized according to two themes. The first addresses physical and numerical topics forming a foundation for ocean models. We focus here on the science of ocean models, in which we ask questions about fundamental processes and develop the mathematical equations for ocean thermo-hydrodynamics. We also touch upon various methods used to represent the continuum ocean fluid with a discrete computer model, raising such topics as the finite volume formulation of the ocean equations; the choice for vertical coordinate; the complementary issues related to horizontal gridding; and the pervasive questions of subgrid scale parameterizations. The second theme of this chapter concerns the applications of ocean models, in particular how to design an experiment and how to analyze results. This material forms the basis for ocean modelling, with the aim being to mechanistically describe, interpret, understand, and predict emergent features of the simulated, and ultimately the observed, ocean.
The dominant processes leading to lateral transport by the general ocean circulation are reviewed. The general circulation is distinguished from a theoretical steady flow by the effects of mesoscale eddies. The general circulation flow may be averaged over the scale of the eddies, but averaging does not eliminate correlations among eddy variables. The present state of understanding of the transport by these eddy correlations, and how they are parameterized in models, is discussed in some detail. Satellite, drifter, and model estimates of eddy statistics are compared. Particular emphasis is placed on the direction, heterogeneity, and anisotropy of eddy-induced diffusion, advection, and transport.
This book offers a comprehensive overview of the models and methods employed in the rapidly advancing field of numerical ocean circulation modeling. For those new to the field, concise reviews of the equations of oceanic motion, sub-grid-scale parameterization, and numerical approximation techniques are presented and four specific numerical models, chosen to span the range of current practice, are described in detail. For more advanced users, a suite of model test problems is developed to illustrate the differences among models, and to serve as a first stage in the quantitative evaluation of future algorithms. The extensive list of references makes this book a valuable text for both graduate students and postdoctoral researchers in the marine sciences and in related fields such as meteorology, and climate and coupled biogeochemical modeling.
Interocean and interbasin exchanges occur at choke points of relatively limited extent and so provide natural geographic constraints for observing the variability of the global circulation. However, the complex bathymetry of interconnected straits and sills at these choke points and the many unique dynamical processes associated with interocean and interbasin exchanges provide challenges for observations and models alike. While overall the exchanges tend to reduce property gradients between and within ocean basins and marginal seas, they may also introduce contrasting thermohaline fluxes that can potentially influence the strength and stability of the meridional overturning circulation. In this chapter, the present knowledge of interocean exchange through the high-latitude Drake Passage and Agulhas system in the Southern Ocean and the low-latitude Indonesian seas is discussed. Examples of interbasin exchange from marginal seas illustrate their importance as source regions for the forcing of the circulation, or as regions where water masses are formed that modify and mark the variability in the global climate system. Finally, deep passage overflows that permit the exchange of deep and bottom waters between neighboring ocean basins, their characteristics and dynamics are reviewed.
Strong, persistent currents along the western boundaries of the world’s major ocean basins are called “western boundary currents” (WBCs). This chapter describes the structure and dynamics of WBCs, their roles in basin-scale circulation, regional variability, and their influence on atmosphere and climate. WBCs are largely a manifestation of wind-driven circulation; they compensate the meridional Sverdrup transport induced by the winds over the ocean interior. Some WBCs also play a role in the global thermohaline circulation, through inter-gyre and inter-basin water exchanges. After separation from the boundary, most WBCs have zonal extensions, which exhibit high eddy kinetic energy due to flow instabilities, and large surface fluxes of heat and carbon dioxide. The WBCs described here in detail are the Gulf Stream, Brazil and Malvinas Currents in the Atlantic, the Somali and Agulhas Currents in the Indian, and the Kuroshio and East Australian Current in the Pacific Ocean.
The modelling of ocean circulation is important not only for its own sake, but also in terms of the prediction of weather patterns and the effects of climate change. This 2007 book introduces the basic computational techniques necessary for all models of the ocean and atmosphere, and the conditions they must satisfy. It describes the workings of ocean models, the problems that must be solved in their construction, and how to evaluate computational results. Major emphasis is placed on examining ocean models critically, and determining what they do well and what they do poorly. Numerical analysis is introduced as needed, and exercises are included to illustrate major points. Developed from notes for a course taught in physical oceanography at the College of Oceanic and Atmospheric Sciences at Oregon State University, this book is ideal for graduate students of oceanography, geophysics, climatology and atmospheric science, and researchers in oceanography and atmospheric science.
The tropical oceans play important roles in the global climate system through ocean transports of heat and freshwater as well as ocean–atmosphere interactions. The developments in observational networks during recent decades have helped us to quantify the strength and variability of most of the ocean general circulations responsible for the transports. Those are discussed in detail in individual sections covering each tropical basin separately with a special emphasis on recent research results. Shallow overturning cells observed in all three tropical basins as well as the deep Atlantic meridional overturning circulation are such examples that are linked to ocean and climate variations on multiple timescales. In addition, tropical ocean–atmosphere interactions associated with oceanic planetary waves cause large-scale climate variations such as El Niño/Southern Oscillation (ENSO), Indian Ocean Dipole, Atlantic Niño, and ENSO Modoki. Recent advances in numerical modeling augmented by in situ and satellite observations are helping the research community to understand ocean process and to predict associated climate variations on seasonal to longer timescales.
The ocean has the largest heat capacity in the climate system and as a result the ocean plays a critical role in the climate. Changes in ocean heat content dominate the Earth’s energy storage; and the ocean’s thermal expansion has been a major contributor to sea-level rise in the twentieth century and likely to be the largest contributor in the twenty-first century. The agreement between changes in ocean heat storage over recent decades and changes in the Earth’s radiative balance, within uncertainties, provides strong support for current understanding of anthropogenic climate change. As a result of improvements in observations and modeling of sea level and components contributing to sea-level change, there is now an improved explanation for twentieth century sea-level rise. Models project a continuing sea-level rise during the twenty-first century and beyond. However, a number of uncertainties remain in our understanding of the global mean and regional distribution of sea-level rise resulting from changes in ocean circulation and changes in the Earth’s gravitational field. Ocean-ice-sheet interactions are important for quantitatively estimating future ice-sheet contributions to sea-level rise.
Observations at and below the surface of the oceans are essential for understanding the ocean system and the role played by the ocean in earth’s climate, for documenting changes and for initializing, validating, and improving ocean models. It is only since the late twentieth century that, thanks to advances in microelectronics, battery technology, and satellite communication, in situ observations (together with satellite observations) have reached a volume and spatial distribution that allow us to track a wide range of global and regional phenomena. This review traces the development of in situ ocean observations primarily from a physical standpoint and describes the internationally coordinated observing networks that now supply these observations. It considers the enormous changes that have occurred in the volume and distribution of these observations and the implication of these changes for defining the evolving state of the global ocean. Finally, there is discussion of the prospects for further improving sustained ocean observations and for the delivery of integrated information from interrelated observing networks.
A broad perspective of the ocean as a key component of the Earth System and of its role in the past, present, and future climate change is provided. The ocean is a huge reservoir of heat, mass, carbon, and many other quantities, and their estimated exchange fluxes suggest characteristic timescales of adjustment ranging from decades to many thousands of years. Surface patterns and meridional fluxes of these quantities highlight the important role of the wind-driven circulation and the deep ocean flow systems through all ocean basins. Ocean-dominated phenomena of natural variability, in particular associated with the tropical oceans, are explained. The relevance of the ocean circulation for abrupt climate change, as recorded from a variety of paleoclimate records, is discussed. This includes the bipolar seesaw concept which explains many features of interhemispheric response during the sequence of rapid warmings in the past ice age. Finally, the ocean’s role during the anthropocene, the time epoch which is dominated by the human-caused increase in greenhouse gases to levels unprecedented in the past 800,000 years, is explored. Both the warming and the increase in atmospheric transport of water polewards create conditions for the ocean that may induce large and irreversible changes in the Atlantic meridional overturning circulation.
In this chapter, we review the physical processes that create the mean and variable circulation features along the eastern margins of the ocean basins. Rather than describing the individual systems, we describe the processes and their variability between the systems, dividing the discussion into the low-, mid- and high-latitude regions. We start with the low latitudes, since their signals often move poleward along the coastal wave guides into the midlatitudes, which are the well-known eastern boundary upwelling systems. Our treatment of the higher latitudes is limited to examples from the better-studied NE Pacific Basin (The Alaska Current).
The ocean–atmosphere exchanges of heat, water, and momentum are key elements of the global climate system. The processes controlling these exchanges, methods by which they may be estimated, and their impacts on the ocean are reviewed, with an emphasis on developments in the past decade. The main characteristics of the long-term mean exchange fields are presented with reference to atmospheric reanalysis, remote sensing, and ship observation-based datasets. Flux measurement and evaluation techniques are discussed in the context of the key observational reference datasets, particularly the critically important growing network of surface flux buoys. A short review of the many flux datasets that are now available is presented with a focus on the ocean heat budget closure problem that remains a leading unresolved issue. Variability in the exchanges is also considered with a focus on (a) changes associated with large-scale modes of atmospheric variability, (b) the effects of anthropogenic climate change, and (c) transfers under extreme conditions. The ocean impacts of changes in the surface fluxes are explored with a focus on modification of near-surface properties, water mass transformation, and the ocean’s overturning circulation. Finally, prospects for future improvements to flux datasets through advances in the synthesis of data from different sources and enhanced observational constraints are discussed.
This chapter summarizes the scientific basis for and the current status of seasonal-to-interannual prediction with particular emphasis on the role of the tropical oceans. The first part of the chapter focuses on oceanic sources of predictability in the tropical Pacific, Atlantic, and Indian Oceans. Seasonal-to-interannual predictability issues in the Northern Hemisphere extratropics are also discussed. Mechanisms that limit predictability, particularly for ENSO, are highlighted. The second part of the chapter describes the forecast quality and procedures in practice today. Finally, the concluding remarks identify some outstanding challenges.