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Deepak Cherian

physical oceanographer; project scientist I
National Center for Atmospheric Research
| +1 303-497-1713 | Mesa Lab 400

Hello.

I am a physical oceanographer.

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I am part of the Ocean Section of the Climate and Global Dynamics Lab at the National Center for Atmospheric Research in Boulder, Colorado. Currently, I study problems that involve

  1. small-scale turbulence & mixing,
  2. baroclinic instabilities and mesoscale oceanic eddies,
  3. continental shelf flows
  4. equatorial flows.

using observational records from moored, shipboard and satellite platforms; as well as high resolution realistic and idealized numerical models (see below)

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I like software tools that enable easy, intuitive, convenient and scalable analysis of datasets, both big and small. So I help with building and maintaining xarray; and contribute to the wider Pangeo ecosystem of software tools (such as xgcm). Come join us!

Research

Linking microstructure measurements to the mesoscale

With Emily Shroyer (OSU), Jonathan Nash (OSU)

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We are working to estimate lateral (mesoscale) diffusivity coefficients from two classes of microstructure measurements:

  1. moored measurements from the χpod of Moum & Nash (2009), and
  2. basin-wide GO-SHIP transects from the CTD-χpod (Pickering & Nash, unpublished).

We will interpret the χpod measurements using the triple decomposition framework of Garrett (2001). Above [left] is a preliminary estimate for the P06 section at 24°S in the Pacific Ocean. For comparison, we show an estimate based on Argo finestructure (Cole et. al., 2015) as well as an estimate using passive tracers advected by altimetric velocities (Abernathey & Marshall, 2013). [right] Density bins used for the computation are marked on a T-S plot.

Turbulence in the equatorial Pacific cold tongue

With Dan Whitt (NCAR), Scott Bachman (NCAR), Ryan Holmes (UNSW), Bill Large (NCAR), R-C. Lien (UW APL)

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We are studying turbulence in the equatorial cold tongue using a series of high resolution nested simulations of the eastern equatorial Pacific. This work is funded by NOAA’s Climate Variability and Predictability (CVP) program.

One cool result so far is the presence of deep cycle turbulence off the equator in the cold cusp of Tropical Instability Waves (TIWs). This video demonstrates this using an animation of 4-hourly SST (top) and average turbulence heat flux in the top 150m (bottom; parameterized using KPP). Initially the mixed layer is included and there is a pronounced daily cycle throughout the domain. About 20 seconds in, the mixed layer is removed and a daily cycle of mixing persists in the TIW cold cusp both on and off the equator. TIWs have been observed to modulate the equatorial deep cycle at 140W (Moum et al, 2009; Inoue et al 2012; Inoue et al 2019) but no observations of an off-equatorial deep cycle exist to date.

Turbulence and lack of turbulence in the Bay of Bengal

With Emily Shroyer (OSU), Jim Moum (OSU) and the OSU Ocean Mixing Group | Read the paper! | More

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Multiple year-long moored turbulent mixing measurements collected using fast temperature sensors (χ-pods) as part of the ASIRI/EBoB/RAMA programmes paint a picture of mixing across the Bay of Bengal that spans multiple time scales: interannual to diurnal and shorter.

Interesting signals include interannual & intraseasonal variability, a daily cycle in turbulence, depressed turbulence in barrier layers, elevated mixing associated with the tropical cyclones and a quiet ocean below 50m. To me, the coolest result is the absence of turbulence in April at 100m i.e. inferred diffusivity values are near-molecular!

Shelf flows forced by mesoscale eddies

Advisor: Ken Brink (WHOI) | Read the paper! | More

One chapter of my thesis focused on the shelf flows forced by mesoscale eddies translating at the shelfbreak. The flow field is summarized below. What I found most interesting was the difference in vertical structure of the cross-shelfbreak flow. The shelf-water outflow is approximately vertically uniform whereas the eddy- and slope-water inflow is strongly sheared. Our paper explains why this happens.

shelf-flow-summary.png

Cross-shelfbreak exchange by mesoscale eddies

Advisor: Ken Brink (WHOI) | Read the paper! | More

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My dissertation looked at the interaction of deep-ocean mesoscale eddies with continental shelf-slope topography.

When visualized using passive tracer fields (red tracks eddy water and blue, shelf-slope water), the interaction clearly results in the formation of smaller-scale secondary vortices. We term these ’stacked’ vortices to reflect their (unexpected) vertical structure wherein shelf-slope water is stacked over eddy water. Observational evidence for these features remains elusive.

Here’s a video showing the evolution of a passive tracer. The southern boundary is the coast, the eddy is started in the northeast in deep water (flat bottom) and the β > 0. The lower panel shows a time series of volume flux of shelf water: defined to be water parcels that start on the shelf at t=0. The shelf is ≈ 40 km wide and the continental slope is 50 km wide.

Inertial-gravity waves in the equatorial Pacific

With Tom Farrar (WHOI) & Ted Durland (OSU) | Code

farrar-durland-spectrum-deepak.png

Satellite observations give humanity an unprecedented detailed look at the surface ocean. The vertical structure of variability associated with surface signals is relatively less known, and the relevance of theoretical structures derived using strict assumptions is debated; viz., the so-called baroclinic vertical modes.

Motivated by the [zonal wavenumber]-frequency spectra of dynamic height calculated by Farrar & Durland (2012) — see image on right — my goal is to infer the vertical structure of 7-day period inertial-gravity waves in the equatorial Pacific (filter band marked by horizontal lines). I am using long term subsurface temperature measurements and inferred dynamic height from the TAO/TRITON project.

Publications

  • Rypina I.I., Pratt L.J.; Entner S.; Anderson A.; Cherian D.A. accepted. (2020) The Influence of an Eddy in the Success Rates and Distributions of Passively Advected or Actively Swimming Biological Organisms Crossing the Continental Slope. Journal of Physical Oceanography
  • Cherian, D.A., Shroyer, E.L., Wijesekera, H.W., Moum, J.N. (2020) The seasonal cycle of upper-ocean mixing at 8°N in the Bay of Bengal. Journal of Physical Oceanography. 50, 323-342 DOI PDF.
  • Cherian, D.A., Brink, K.H. (2018) Shelf flows forced by deep-ocean anticyclonic eddies at the shelfbreak. Journal of Physical Oceanography. 48, 1117–1138. DOI PDF.
  • Cherian, D.A., Brink, K.H. (2016) Offshore Transport of Shelf Water by Deep-Ocean Eddies. Journal of Physical Oceanography. 46 3599–3621. DOI PDF
  • Haine T.W.N., Cherian D.A. (2013) Analogies of Ocean/Atmosphere Rotating Fluid Dynamics with Gyroscopes: Teaching Opportunities. Bull. Amer. Meteor. Soc.. 94:684. DOI PDF Supplement
  • Brink K.H., Cherian D.A. (2013) Instability of an idealized tidal mixing front: Symmetric instabilities and frictional effects. Journal of Marine Research. 71(6):26. DOI PDF
  • Thesis: Cherian D.A. (2016) When an eddy encounters shelf-slope topography. PDF

Posts

Teaching

While at MIT, I took the semester-long Teaching Certificate Program. I learned that it is generally more effective to have students work through a derivation primarily on their own in class with hints. Following that advice, I created worksheets that guide students through a derivation, guiding them toward important implications and reasoning for various steps. Here are the ones I have so far.

  1. Rossby adjustment - for OSU’s Geophysical Waves class
  2. Non-hydrostatic internal waves - for OSU’s Geophysical Waves class
  3. Sverdrup balance - for MIT’s 12.808 - Observational Physical Oceanography

These are targeted at beginning graduate students. Any comments you might have on these are welcome. Please send me an email.

Latex source is also available on request.

Contact

Email:

Office Address: 1850 Table Mesa Drive, Mesa Lab 400, Boulder, CO

Phone: +1 303-497-1713

Acknowledgments

My work has been funded by the US National Science Foundation (NSF), National Oceanic and Atmospheric Administration (NOAA), National Aeronautics and Space Administration (NASA) and the Office of Naval Research (ONR).

Styling gratefully borrowed from Ethan Schoonover, Nicolas P. Rougier and Matthew Butterick.

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