The last glacial cycle was characterized by substantial millennial-scale climate fluctuations1,2,3,4,5, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces6,7, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution8 or higher1. Records based on oxygen isotopes, however, contain uncertainties in the range of ±30 m, or ±1 °C in deep sea temperature9,10. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth—and hence global sea level—over the past 470,000 years (470 kyr). Our reconstruction is accurate to within ±12 m, and gives a centennial-scale resolution from 70 to 25 kyr before present. We find that sea-level changes of up to 35 m, at rates of up to 2 cm yr-1, occurred, coincident with abrupt changes in climate.

Sea-level fluctuations during the last glacial cycle

The subject of ocean turbulence is in a state of discovery and development with many intellectual challenges. This book describes the principal dynamic processes that control the distribution of turbulence, its dissipation of kinetic energy and its effects on the dispersion of properties such as heat, salinity, and dissolved or suspended matter in the deep ocean, the shallow coastal and the continental shelf seas. It focuses on the measurement of turbulence, and the consequences of turbulent motion in the oceanic boundary layers at the sea surface and near the seabed. Processes are illustrated by examples of laboratory experiments and field observations. The Turbulent Ocean provides an excellent resource for senior undergraduate and graduate courses, as well as an introduction and general overview for researchers. It will be of interest to all those involved in the study of fluid motion, in particular geophysical fluid mechanics, meteorology and the dynamics of lakes.

https://assets.cambridge.org/97805218/35435/frontmatter/9780521835435_frontmatter.pdf

The Turbulent Ocean

The emergence of coherent vortex structures is a characteristic feature of quasi-geostrophic or two-dimensional turbulence and because of their relevance to large-scale geophysical flows, the dynamics of these structures has been studied increasingly over the past decade. In the oceans and in the atmospheres vortices or eddies are abundant. For this reason, the study of isolated vortices in rotating fluids-including their dynamics, instability properties, mutual interaction behavior, and effects due to bottom topog­ raphy-is of fundamental interest for refined models of geostrophic turbulence and of the general oceanic and atmospheric circulations. Pro­ cesses of heat transfer, dispersion of biochemical components, and the transport of other physical properties are closely connected with coherent structures. The dynamics of two-dimensional vortices in homogeneous fluids is also a key problem in free shear flows. With or without background rotation, the main question concerns the stability of these vortices to three­ dimensional disturbances. In addition, two-dimensional vortices also play an important role in tokamak-confined plasmas as well as in astrophysical situations such as accretion discs of neutron stars. During the past decade, considerable insight has been gained in the

Vortices in Rotating Fluids

This review is intended as a comprehensive but concise summary of present capabilities in coastal pollutant, ecosystem and water quality modelling. It reflects the recent rapid developments in multidisciplinary modelling in shelf seas. The behaviour of conservative pollutants that act as passive tracers is contrasted with those that have more complex behaviours, including oil spills. The importance of sediment modelling is emphasised, since contaminants commonly exist in both a dissolved and a particulate state, or adhere to sediments. Recently developed ecological models can have great complexity, reflecting the complexity of the real ecosystem. These models are now being linked to physical models of coastal waters and run with the same resolution. This has become possible only recently because of increases in computer power, particularly the availability of parallel systems at reasonable cost. The main advances in physical modelling are likely to come through greater understanding of turbulence and other sub-gridscale processes as well as increased resolution. In the coastal seas there is often a lack of oceanographic data, which is even greater for the many biological and chemical variables than it is for physical variables. This is probably the single most important factor limiting the progress of operational water quality models.  2002 Elsevier Science Ltd. All rights reserved.

Modelling pollution dispersion, the ecosystem and water quality in coastal waters: a review

The structural features of turbulence at the free surface of a channel flow have been experimentally investigated. The experiments were conducted in a horizontal channel of large aspect ratio in the (depth based) Reynolds number range of 2800–8800. The results indicate that the persistent structures on the free surface can be classified as upwellings, downdrafts, and spiral eddies. Upwellings are shown to be related to the bursts originating in the sheared region at the channel bottom and the eddies are seen to be generated at the edges of the upwellings. The eddies often merge if rotating in the same direction, and form “pairs” if rotating in opposite directions—though there are occasional mergers of such counter-rotating ones. The spiral eddies decay slowly and are sometimes annihilated by fresh upwellings. The population densities and the persistence times of the various structures were measured for different flow conditions. The resulting data show that the physical parameters characterizing the struc...

An experimental investigation of the characteristics of free-surface turbulence in channel flow

We report results of experiments with a field of two-layer geostrophic turbulence that is driven internally by baroclinic instability. Turbulence is first produced at a sharp, unstable density front and propagates horizontally until all of the stratified fluid contains quasi two-dimensional eddy motions. Continuing instability blocks the inverse energy cascade that is otherwise characteristic of two-dimensional turbulence, and causes the dominant length scale to be approximately constant. Ekman friction, however, causes the turbulence intensity to decay exponentially in time. Measurements of the dispersion of neutrally buoyant particles indicate an energy spectrum of the form E ( k ) ∼ k −2.46±0.02 , where k is the horizontal wavenumber, at scales less than that of energy injection. The experiments also have implications for horizontal mixing by mesoscale turbulence at density fronts. The spreading of the upper-layer fluid proceeds like t 1 2 over time scales much less than that for the exponential decay of fluid velocities, but on larger time scales mixing is reduced, and can be halted, by Ekman dissipation.

R.W. Griffiths

Papers

The structure of mesoscale turbulence and horizontal spreading at ocean fronts