Interaction of convection and horizontal gyre circulation in the North Atlantic Current
Project Leader: Bishakhdatta Gayen
Collaborators: Dr Catherine Vreugdenhil (University Cambridge, UK), Prof Ross W. Griffiths (ANU, Australia), Prof Graham Hughes (Imperial College London)
Primary Contact: Bishakhdatta Gayen (firstname.lastname@example.org)
Keywords: computational fluid dynamics; fluid dynamics; turbulence
Disciplines: Mechanical Engineering
The Atlantic circulation plays an essential role in global climate. It maintains the climate of Europe by increasing northward heat transport. The temperature of Europe could be much colder if the vertical component of the circulation, called the Atlantic Meridional Overturning (AMOC) were to slow down. The AMOC involves motion with a large range of length scales, including 1 km scale vertical convection in some regions at high latitudes, 100 km scale eddies spun off from the Gulf Stream, westward propagating planetary waves and basin-scale horizontal gyres. Convective mixing in the surface mixed layer, ‘deep convection’ to over 1 km depth spreading over high laditude gyre and dense currents sinking along the continental slopes are all key aspects of the circulation. Large-scale surface flows including ocean gyres are thought to be predominantly wind driven and have been studied extensively using models that are hydrostatic and do not resolve convection. Therefore, we have limited understanding of the impact of convection in driving circulation in the North Atlantic basin.
The project will examine in detail the relative roles of convection and wind stress in maintaining the surface gyres at the basin scale. We will simulate the flow under a prescribed surface wind stress and buoyancy (temperature) whilst maintaining geophysically realistic meridional distributions of buoyancy. The magnitudes of both temperature and wind stress will be independently varied in order to understand and predict their impacts on the circulation at different forcing scenario over the North Atlantic ocean in future. We will examine the effects of Ekman transport, vertical Ekman pumping, Rossby wave propagation, and intensification of western boundary currents. Thus, for the first time, the dynamics of a wind-driven ocean will be coupled to those of fully resolved turbulent convective circulation. The response of the flow to temporal variability of the surface forcing will also be examined. We will also study the sensitivity of the circulation to wind and buoyancy forcing, which will give us a better picture of circulation patterns and help us to develop a robust theoretical understanding for this system. The simulations will be extended to Large Eddy Simulation (LES), in which the smallest dissipation scales of turbulence will be modeled using a technique that couples the model to the grid resolution. LES allows for computation of the flow in a domain approximately two orders of magnitude larger than is feasible with DNS, while remaining physically accurate.