Jonathan Sharples

Semi-diurnal and longer period stability cycles in the Liverpool Bay region of freshwater influence

Abstract Detailed observations from a 12.5 h anchor station and a 45 day mooring deployment in the Liverpool Bay region of freshwater influence are presented. The anchor station data, showing vertical and temporal variations of current, density and gradient Richardson number, demonstrate a semi-diurnal stratification cycle driven by tidal straining of a freshwater-induced horizontal density gradient. The current meter mooring observations demonstrate the regular repetition of this cycle of semi-diurnal stratification and also indicate a period of more enduring stratification following a neap tide. This longer period of stability is caused by the decrease in tidal mixing energy allowing the gravitational relaxation of the horizontal density structure, with the increasing mixing towards the next spring tide returning the region to a vertically mixed state. It is suggested that the periods of post-neaps stability in Liverpool Bay will not occur more frequently than monthly, due to an N 2 modulation of the spring-neaps mixing cycle. A one-dimensional model incorporating a level 2 turbulence closure scheme reproduces the main features of the observations and is used to investigate the residual current behaviour. The model indicates that pulses in the residual flow are associated with periods of low turbulence at each slackwater, as the reduced frictional coupling through the water column allows the acceleration of the density-driven flow for a brief period. Similarly the general reduction in turbulent intensities at neaps should result, according to the model, in increased current speeds in directions closer to that of the isopycnals as the residual flow approaches geostrophy. Neither of these predictions of the model can be identified convincingly in the data because of other sources of variability.

Introduction to the Physical and Biological Oceanography of Shelf Seas: Life in the shelf seas

In this exciting and innovative textbook, two leading oceanographers bring together the fundamental physics and biology of the coastal ocean in a quantitative but accessible way for undergraduate and graduate students. Shelf sea processes are comprehensively explained from first principles using an integrated approach to oceanography – helping to build a clear understanding of how shelf sea physics underpins key biological processes in these environmentally sensitive and economically important regions. Using many observational and model examples, worked problems, and software tools, they explain the range of physical controls on primary biological production and shelf sea ecosystems.

Waves, turbulent motions and mixing

In this chapter we shall look at waves and turbulence, two forms of motion which are of particular importance in the shelf seas because of their roles in bringing about the mixing which re-distributes properties such as heat, salt, momentum and substances dissolved or suspended in the water. There is a marked contrast in character between the two: waves are generally highly ordered motions which are amenable to precise mathematical description, while turbulence is chaotic in nature and it can usually only be represented in terms of its statistical properties. Both waves and turbulence are large scientific topics in their own right and are the subject of more than a few specialised textbooks. Here, we shall focus on those aspects of surface waves, internal waves and turbulence theory which are necessary to the understanding of processes in shelf seas, and we shall leave the more specialised aspects for the interested student to pursue from the further reading list. Surface waves We have already developed the theory of long waves in Chapter 3 from the basic equations and shown how such waves can help us to understand tidal motions in shelf seas. The more general theory of surface wave motions, in which there is no restriction on wavelength, is more involved and a full treatment is beyond the scope of this book. Here we shall simply present the assumptions and the principal results of the theory of infinitesimal waves and give a physical description of the motion involved. As well as being useful in themselves, the results of surface wave theory introduce us to many of the concepts relevant to the understanding of the more complicated motions involved in internal waves.