The meridional overturning circulation (MOC) of the ocean is of direct importance to the climate system. It transports heat meridionally and is closely tied to the exchange of CO2 with the atmosphere. In past climates, for example at the Last Glacial Maximum, the deep circulation was quite different and may have been responsible for the low CO2 levels in the atmosphere. The stratification and circulation of the upper ocean and thermocline have been extensively studied both theoretically and numerically. In contrast, the stratification and associated meridional overturning circulation of the mid-depth and abyssal ocean are much less understood. In my work I study the dynamics of the deep stratification and overturning circulation using a combination of theory, coarse-resolution and eddy-resolving idealized numerical simulations.
Figure. (Right panel) Theoretical solution for stratification, shown by temperature isolines in (oC) (black contours), and the residual overturning circulation in (Sv) (blue/red) in the deep ocean; (left panel) solution obtained with a coarse GCM configured in an idealized domain.
Nikurashin, M. and Ferrari, R. Overturning circulation driven by breaking internal waves in the deep ocean, Geophysical Research Letters, 40, 1-5, DOI: 10.1002/grl.50542 (2013) [pdf]
Nikurashin, M. and Vallis, G. A theory of the interhemispheric meridional overturning circulation and associated stratification, Journal of Physical Oceanography, 42, 1652-1667 (2012) [pdf]
Nikurashin, M. and Vallis, G. A theory of deep stratification and overturning circulation in the ocean, Journal of Physical Oceanography, 41, 485-502 (2011) [pdf]
The wind power input into the ocean is dominated by the work done in the Southern Ocean, where westerly winds are aligned with the Antarctic Circumpolar Current. This energy is subsequently converted, through baroclinic instability, into a vigorous geostrophic eddy field. Geostrophic eddies dominate kinetic energy of the ocean and can ultimately power turbulence and mixing in the ocean interior. However, the cascade of their energy further down to the dissipation scales and its distribution in the ocean remain largely unknown. In my work I explore the dynamics and energetics of geostrophic eddies using realistic high-resolution numerical simulations explicitly resolving meso-, submeso-, and internal wave scale motions.
Temperature distribution in (oC) (color) and surface flows (black vectors) from a high-resolution numerical simulation of wind-driven geostrophic eddy field with rough small-scale topography configured with parameters typical for the Drake Passage region of the Southern Ocean.
Nikurashin, M., Vallis, G., and Adcroft, A. Routes to energy dissipation for geostrophic flows in the Southern Ocean, Nature Geoscience, 6, 48-51 (2013) [pdf]
Nikurashin, M. and Ferrari, R. Global energy conversion rate from geostrophic flows into internal lee waves in the deep ocean, Geophysical Research Letters, 38, L08610 (2011) [pdf]
Turbulence in the ocean interior greatly enhances mixing of heat, carbon and other tracers and hence plays an important role for the ocean circulation and climate. Observations indicate that turbulent mixing is enhanced in abyssal ocean above rough topography. Enhanced mixing is associated with internal wave breaking and, in many regions of the ocean, has been linked to breaking of internal tides and lee waves generated at rough topography. To understand the physical processes leading to wave breaking and mixing, I study generation, radiation and breaking of topographic internal waves using a combination of available observations and high-resolution numerical simulations.
Nikurashin, M. and Legg, S. A mechanism for local dissipation of internal tides generated at rough topography, Journal of Physical Oceanography, 41, 378-395 (2011) [pdf]
Nikurashin, M. and Ferrari, R. Radiation and dissipation of internal waves generated by geostrophic motions impinging on small-scale topography: Application to the Southern Ocean. Journal of Physical Oceanography, 40, 2025-2042 (2010) [pdf]
Nikurashin, M. and Ferrari, R. Radiation and dissipation of internal waves generated by geostrophic motions impinging on small-scale topography: Theory. Journal of Physical Oceanography, 40, 1055-1074 (2010) [pdf]