Deepak Cherian

With Frank Bryan (NCAR), Emily Shroyer (OSU), Jonathan Nash (OSU)

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 basin-wide GO-SHIP transects from the CTD-χpod (Pickering & Nash, unpublished).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.

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

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.

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

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!

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.

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

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.

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

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.