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I am a physical oceanographer in the Atmospheric and Oceanic Sciences Program at Princeton University.
My research interests are in the dynamics of the ocean and their implications for climate. In my work I use a
combination of theory, process-oriented numerical simulations, and observations. A few current research projects are described below:
Deep stratification and overturning circulation of the ocean
The meridional overturning circulation 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 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. (Upper panel) Theoretical solution for stratification, shown by temperature isolines in (oC) (solid lines), and the residual overturning circulation in (Sv) (blue/red) in the deep ocean; (lower panel) solution obtained with a coarse-resolution GCM configured in an idealized domain.
Animations: sensitivity to Southern Ocean wind and NH temperature
Dissipation of geostrophic eddies and mixing
The wind power input into geostrophic flows in the ocean is estimated to be of O(1) TW and is dominated by the work done in the Southern Ocean. This wind power input drives the Antarctic Circumpolar Current (ACC) system and is eventually converted, through baroclinic instability, into a vigorous geostrophic eddy field. The partial absence of meridional boundaries in the Southern Ocean suggests that geostrophic eddies dissipate through interaction with the bottom boundary either through generation of turbulence in the bottom boundary layer or through generation of internal waves at rough topography. While the former process is better understood and likely dominates the total energy dissipation, the latter one has stronger impact on the overturning circulation as it radiates energy away from the bottom boundary into the stratified interior. I explore the dynamics and energetics of geostrophic eddies using high-resolution numerical simulations explicitly resolving meso-scale, submeso-scale, and internal wave dynamics.
Figure. Evolution of temperature (from 0oC to 1.2oC) in a baroclinically unstable zonal flow over rough topography. Horizontal axes of the plot
are in (km) and the vertical axis is in (m). The simulation is carried
out using the MITgcm in a zonally periodic domain with 100 m
horizontal and 10 m vertical resolution. [large]
Dissipation of internal tides and mixing
Turbulent mixing plays an important role for the circulation and
stratification of the ocean. Observations indicate that the turbulent
mixing is enhanced above rough topography. Enhanced mixing is associated
with internal wave breaking and, in many regions of the ocean, has been
linked to the breaking and dissipation of internal tides. To understand
the physical processes leading to wave breaking and mixing, I study the
generation and dissipation of internal tides using a high-resolution
numerical model configured in an idealized domain with a realistic
topography and flow characteristics.
Figure. Evolution of an isopycnal surface (green) and temperature anomaly (blue/red from -0.02oC to 0.02oC) forced by the M2
barotropic tidal flow of 2.5 cm/s over abyssal hill topography
characteristic of the Brazil Basin region. Horizontal axes of the plot
are in (km) and the vertical axis is in (m). The simulation is carried
out using the MITgcm in a horizontally periodic domain with 50 m
horizontal and 5 m vertical resolution. [large]
Global energy conversion from geostrophic eddies into internal lee waves
A global estimate of the energy conversion rate from geostrophic flows into internal waves at rough topography is computed by applying a linear theory of lee wave generation to topographic spectra estimated from single beam soundings, bottom stratification estimated from climatology, and bottom geostrophic flows obtained from a global ocean model.
Figure. Energy flux into internal waves in log10(mWm-2) estimated by applying a linear theory of lee wave generation to topographic spectra
estimated from single beam soundings from NGDC, to bottom stratification estimated
from WOCE hydrographic atlas, and to bottom geostrophic flows obtained from the GFDL
global isopycnal ocean model.
Publications:
Nikurashin, M., and G. Vallis, 2012: A theory of the interhemispheric meridional overturning circulation and associated stratification, Journal of Physical Oceanography, submitted. [PDF]
Nikurashin, M., and R. Ferrari, 2011: Global energy conversion rate from geostrophic flows into internal lee waves in the deep ocean, Geophysical Research Letters, 38, doi:10.1029/2011GL046576. [PDF]
Nikurashin, M., and G. Vallis, 2011: A theory of deep stratification and overturning circulation in the ocean, Journal of Physical Oceanography, 41(3), 485-502. [PDF]
Nikurashin, M., and S. Legg, 2011: A mechanism for local dissipation of internal tides generated at rough topography, Journal of Physical Oceanography, 41(2), 378-395. [PDF]
Ferrari, R., and M. Nikurashin, 2010: Suppression of eddy diffusivity across jets in the Southern Ocean, Journal of Physical Oceanography, 40, 1501-1519. [PDF]
Nikurashin, M., and R. Ferrari, 2010: 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. [PDF]
Nikurashin, M., and R. Ferrari, 2010: Radiation and dissipation of
internal waves generated by geostrophic motions impinging on
small-scale topography: Theory. Journal of Physical Oceanography, 40, 1055-1074. [PDF]
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