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Geophysical fluid dynamics Introduction

Highlights of work in geophysical fluid dynamics this year included laboratory and theoretical fluid dynamics studies modelling lava flows, where cooling, solidification and yield-strength are important factors. Experiments have focused on the flow of yield-strength fluid that is also cooling and solidifying as it flows down a sloping channel. A variety of inertial, viscous, plastic and cooling-controlled flow regimes have been found.

Cartoon of how noble gases may be partially retained in the mantle despite losses due to melting under mid-ocean ridges. Some of the melt is trapped, in the form of hybrid pyroxenite, and recycled internally without losing its gases. The melt that reaches the surface forms oceanic crust that degasses.

The geophysical fluid dynamics laboratory has also seen a renewed effort to understand the three-dimensional flow in mantle subduction zones, including the influences of an over-riding plate, back-arc spreading, the effects of a thick keel on the over-riding plate, and the behaviour of a hot mantle plume ascending under the over-riding plate. This work relied on an extended visit by Prof Kincaid and a student from the University of Rhode Island. The interaction of ascending mantle plumes and subduction zones is being examined with a view to explaining the distribution and ages of the Columbia River Basalts and volcanism of the Yellowstone hotspot. The work has shown that previously unsuspected patterns of volcanism can be produced and many aspects of the volcanism, including the age distributions, around Yellowstone and the Lava High Plains of the northwest USA can be explained by interaction of a plume and subduction zone.

Lessons learnt from several years of numerical modelling of the combined chemical and thermal evolution of the mantle are now bearing fruit in two directions.  The models are being extended to Venus' mantle to test whether the 'basalt barrier' mechanism, reported last year, can explain the outburst of volcanism that completely resurfaced Venus about 500 Myr ago.  Initial results are promising.  

The insights from the numerical modelling have also fed into a new hypothesis to reconcile mantle chemistry with mantle dynamics.  The idea is that only a fraction of the melt generated under a mid-ocean ridge actually reaches the surface.  The remaining melt is trapped in the mantle, and carries the so-called incompatible elements.  This hypothesis removes the need for a postulated deep, hidden reservoir containing 'missing' incompatible elements.  It can also explain the presence of enigmatic 'unradiogenic' helium and other noble gases that emanate from some hotspot volcanos.

Studies of the fundamental dynamics of the ocean's meridional overturning circulation continued in the geophysical fluid dynamics laboratory. A new approach to the energetics of the circulation developed this year has elucidated the way in which energy supplied to irreversible turbulent mixing from the winds and tides must be in balance with the available potential energy supplied by the surface buoyancy fluxes. The two are closely tied, the buoyancy fluxes providing the driving force while the turbulent mixing maintains the stratification, and hence the strength of the forcing and the consequent rate of overturning. Numerical solutions have revealed the presence of significant internal gravity wave activity generated by the convection. It will require further work to determine whether there is likely to be substantial wave generation under oceanic conditions, and whether the wave energy can contribute to the vertical mixing. The steady-state dynamics were also examined in experiments with the case of a large ocean basin connected to a marginal sea by flow over a topographic sill. The exchange flow can influence the circulation and stratification in the ocean.

A video clip of flow in the laboratory convection model, in which the base is heated near both ends and cooled over the central half. The right hand section of the base is heated by an applied heat input 10% smaller than that applied to the left hand section of the base. Hence the plume at the left hand end is stronger than that at the right end and fills the top of the box. In the oceans this is analogous to the strongest sinking region producing the bottom waters.

In other experiments the response to small changes in the surface boundary conditions, such those as implied by global warming, has been investigated. The conditions leading to a potential shutdown of the deep sinking leg of the overturning circulation in simplified cases have been outlined. A first set of experiments with periodic oscillatory surface forcing is also providing insights into whether the global circulation is influenced by fluctuations having periods from the annual seasonal cycle (which is known to force variations in the deep sinking of cold waters) to millennia (which is the time scale for complete equilibration of the stratification to new surface conditions).

In another laboratory study, the dynamics of wakes behind islands and headlands were shown to be sensitive to eddy disturbances or turbulence carried from upstream of the topographic feature. The incident disturbances cause a faster dissipation of wake instabilities with distance downstream, hence a smaller recirculation region. This study is currently being extended to a practical application involving the dispersion of wastewater released into a major estuary in which there is a separation of the flow in the main channel and a relatively slow flushing of a shallow region to one side. Preliminary experiments in a water flume are exploring the roles of wastewater outflow location and tides on the flushing time.