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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.