76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023;
Washington, DC
Session L27: Geophysical Fluid Dynamics: Oceanographic II
8:00 AM–10:36 AM,
Monday, November 20, 2023
Room: 151B
Chair: Alexis Kaminski, UC Berkeley
Abstract: L27.00006 : Heat and mass transfer in the North Atlantic Ocean: insights from turbulence and convection resolving simulations
9:05 AM–9:18 AM
Abstract
Presenter:
Bahman Ghasemi
(University of Melbourne)
Authors:
Bahman Ghasemi
(University of Melbourne)
Bishakhdatta Gayen
(University of Melbourne)
Catherine Vreugdenhil
(University of Melbourne)
Taimoor Sohail
(University of New South Wales)
The large-scale circulation attributes to the horizontal geostrophic circulation which is often localized near the ocean surface in the form of gyre and zonal flows and the overturning circulation associated with vertical motion. Mechanical forcing (e.g. wind, tide etc.) has long been known as the primary driver of the meridional overturning or gyre circulations. However, the modern laboratory experiments and high fidelity simulations have revealed the important role of buoyancy forcing in deriving the ocean circulation and maintaining the stratification in the subtropical regions. However, critical questions still remain on the contributions of each parameter to driving the meridional overturning circulation (MOC), gyre circulation, and the processes governing heat and mass transfer in the ocean. In the present work, we investigate the role of buoyancy and wind in driving ocean circulation in the North Atlantic Ocean (NAO) using direct numerical simulations of an idealized ocean at a wide range of surface wind stress and buoyancy forcing. In our model, buoyancy forcing (given by Rayleigh number, Ra ~O( 10^{11} -10^{14}) ) is large enough to sustain turbulent convection and a rich baroclinic eddy field. By varying surface forcing, we can quantify its effect on the heat and mass transfer (strength of the vertical and horizontal stream functions) in the model. The simulations show that the buoyancy forcing can solely drive both horizontal gyre and vertical overturning circulations. Moreover, the evaluation of scaling indicates that the meridional overturning transport scales with thermal wind balance along with the vertical advection-diffusion balance in the thermal boundary layer without considering the surface wind forcing. It shows the negligible effect of the surface wind on the overturning transport in the range of the wind stress investigated in the present work which is comparable to the atmospheric wind forcing relative to the buoyancy forcing. The results show that the horizontal gyre circulation strongly depends on the surface wind stress as it has a component which is linearly proportional to the wind. However, it has another term which associates with the meridional overturning transport which has long been neglected in previous works and the relevant theories.