Ocean Deep Convection Research Articles

Thermals with background rotation and stratification

Scaling analysis and experiments are used to study the evolution of thermals in the presence of background rotation. When the ambient environment is homogeneous, the thermal rises and expands until it reaches a critical height where the Rossby number becomes ∼ 1. The thermal then stops expanding and rises in a column. Both the critical height and column radius scale with (F0f-2)1/4. F0 is the initial thermal buoyancy and f is the Coriolis frequency. The thermal vertical velocity is independent of f. When the background is stratified with buoyancy frequency N, the thermal rises to a neutral buoyancy level which scales with (F0N-2)1/4. For N/f [Lt ] 0.6 column formation occurs before the thermal reaches the neutral level. For N/f [Gt ] 0.6 the thermal reaches the neutral level before rotation is important. In both regimes, geostrophic adjustment eventually causes the formation of a baroclinic vortex consisting of an anticyclonic lens of thermal fluid at the neutral level and cyclonic circulation below. The lens has Nh/fl ∼ 1. The lens thickness 2h and the radius l obey relations of the form (F0N-2)1/4 (N/f)m. However, the exponents m are different in the two regimes. The relevance of these results to deep-ocean convection and hydrothermal venting is discussed.

Sedimentation on the Feni Drift and late Glacial bottom water production in the northern Rockall Trough

Between the Last Glacial Maximum (18 ka) and the present day, a variety of contrasting oceanographic conditions, including changes in surface and bottom water circulation patterns have existed in the North Atlantic. Evidence for the presence of bottom currents in winnowed silt horizons in the Feni Drift suggests that deep water may have been produced in this region during glacial—deglacial intervals. Current activity, and thus perhaps deep water production, was apparently intermittent, and may have been controlled by a process analogous to the formation of Labrador Sea Water. This involves cooling and deep ocean convection during winter months. Physical factors controlling this production would be temperature and salinity of upper waters which would govern the density, and wind stress necessary for vertical overturn coupled with absence of ice cover. Evidence of bottom current activity suggests that deep water production occurred in the Rockall Trough throughout Termination 1A which closely corresponds to Fairbanks (1989) meltwater pulse 1a (mwp 1a). Production of deep water in this region apparently ceased shortly after the decline in influence of the meltwater influx.

Flows caused by mass forcing in a stratified ocean

Flows due to point sources and sinks of mass are considered on both f- and β-planes. The fluid is assumed to be continuously stratified with constant buoyancy frequency, N, and has no boundaries. Solutions to the flow field giving both horizontal and vertical structure are obtained on large timescales compared with an inertial period for the case when the source/sink flow is started from rest. Steady solutions are obtained through the inclusion of simple Rayleigh damping. On an f-plane the flow is radially symmetric, and a sink (source) generates cyclonic (anticyclonic) swirl due to angular momentum conservation. The nature of the flow of fluid toward the sink depends critically on the ratio f/ N. On a β-plane a sink (source) also generates cyclonic (anticyclonic) swirl, but the velocity field is no longer radially symmetric and the circulation about the source/sink drifts westward. Also, the flow of fluid toward the sink intensifies to an eastward flowing jet. The behaviour is described both in terms of vorticity arguments and in terms of the westward propagation of energy by very low frequency Rossby waves. The solution is then used to calculate the surface circulation and vertical density structure in the vicinity of deep ocean convection events where the process is modelled by a simple mass transfer over depth.