Direct Mixing Measurements using χpods in IWISE

Profiling Dissipation Measurements using χpods on Moored Profilers (Moum/Nash)

Shipboard LADCP/χpod profiling of Internal Wave Structure (Nash/Moum)

 Jonathan D. Nash & James N. Moum

 College of Earth, Ocean and Atmospheric Sciences

Oregon State University

with help from: Byungho Lim (OSU), Andy Pickering and Matthew Alford (APL-UW)and thanks to Ming-Huei Chang, Maarten Buijsman, Luc Rainville, Alexander Perlin, Ray Kreth, Mike Neeley-Brown, John Mickett, Eric Boget, Amy Waterhouse, Zoe Parsons, Jen MacKinnon, Harper Simmons

ObjectivesGeneral• quantify turbulence dissipation where large amplitude

internal waves are generated

Particular• capture the energetics of the largest scales that directly

extract energy from the barotropic tides • while simultaneously measuring mixing associated with the

turbulence that occurs at millimeter and millisecond scales. • through direct observation, to assess the means by which

waves form and break, elucidate the structure/evolution of the wave breaking, and quantify the dissipation that induces irreversible mixing

Methods• χpods on moorings

Stablemoor

Methods• χpods on moorings

• χpod-like devices on moored profilers

Methods• χpods on moorings

• χpod-like devices on moored

• χpod on shipboard CTD for full ocean depth turbulence profiling

χpod -LADCP

Methods• χpods on moorings

• χpod-like devices on moored profilers

• χpod on shipboard CTD for full ocean depth turbulence profiling

• fabricated and deployed 5-component array of moorings to capture the 2D evolution of the larger-scale dynamics

 

MP χpods

{

data example

Kraichnan theoretical spectrum

εχ=N2 χ/(2 Γ Tz

2)

Tested – Puget Sound 2009Refined for 2010 (MPN)Deployed 2011 on N1, N2 all data returned

Profiling Dissipation Measurements using χpods on Moored Profilers

Products → LT Thorpe (overturn) scalesχT temperature variance dissipation rateKT turbulence diffusivity (Osborn-Cox)ε TKE dissipation rate (indirect)Km turbulence viscosity

fast thermistors on APL MP

1st continuous deep-ocean profiling experimentmesoscale current (Kuroshio?) dominates 2nd half• elevated turbulence at base of current • is this friction on a western boundary current?

χpod

speed sensor at low f

ε=2x10-7 m2s-3

dissipation sensor at high f

mooring N1 – χpod at 2000 m

1 day time series

inexpensive, lightweight, low power, standalone velocity sensor

characterization of sensor includes tests in • wind tunnel • tidal channel• P / T chambers

New mean speed / dissipation sensor for use on χpods and in general on moorings

compensated pitot

tube

leading to a new GustT combination probe

not acoustic, hence requires no

scatterersquiet

2nd continuous deep turbulence profiling time series

N2 (1830m water depth)

2011

2 units constructed and deployed – both worked – only 1 MP profiled

Chipod-LADCP-CTD

Above: TKE dissipation rate from LADCP/chipods (green) and Thorpe analyses (blue) at one of the most energetic stations sampled during IWISE.

directturbulence

(green)

inferredturbulence

(blue)

OSU Ocean Mixing χ-pod/LADCPdirect measurements of abyssal turbulence from standard shipboard CTD. permits rapid deep profiling direct turbulence differs from that inferred from overturnslow noise-floor (but N2-dependent)

fast-T3-axis accel3-axis gyrocompassUSB-data

Nash & Moum

MPchipods

1) broadly-distributed dissipation on the east ridge

2) big breaking lee waves on the west ridge

eastwest

contrasting structures from detailed measurements at 2 ridges

mid-column dissipation not dominated by a single breaking wave…

A1 – mid-column dissipation at the generation site 1440 m water depth

Byungho Lim

A1 – mid-column dissipation at the generation siteobservation / model comparison (MITgcm /

Buijsman)

similar tidal fields, but water-column instabilities are not captured by MITgcm and model dissipation is mostly near the bottom.

Byungho Lim

T-Chains on the West Ridge

Buijsman et al

T1T2

T3T4

N2

30-50 m sensor spacing to detect overturns

2-sec sampling to capture inertial subrange

Vertical synopticity (test sampling schemes of other platforms)

3 months data

700 m

500 moverturns

waves

west

east

eastwardwestward

Jonathan Nash

Mooring T3 during spring + diurnal inequality

T1T2

T3T4N2

T-Chains on the West Ridge spring tides / diurnal inequality

Mooring T3 during neap/semidiurnal period

T1T2

T3T4N2

neap tides / semidiurnal period

T-Chains on the West Ridge

T1T2

T3T4N2

Dissipation tied to lee waves

Strong spring/neap changes

Isopycnals displaced down in the mean?

Lee-wave shifts closer to ridge crest during neaps?

Springs(diurnal)

Neaps(semidiurnal)

Time – mean structure / east ridge

T1T2T3T4

N2

T1T2

T3T4N2

dissipation scales with u3bt

(nonlinear!)

consistent with Klymak et al (2010)’s “recipe” for ε over a supercritical ridge

… u3 because flux into trapped lee waves ~( ubt x u2

bt ) …

ε~ u3bt

Dissipation evolution and scaling

Summary Results

• 1st continuous turbulence profiling away from ship-based upper ocean measurements

• χpod-CTD measurements have led to beginning of contribution to Global Repeat Hydrogaphy Program

• NEW VELOCITY SENSOR - speed + turbulenceleading way to new possibilities

• observational confirmation of Klymak etal (2010) ε scaling

• breaking waves: vertically-integrated ε O(1 W/m2)comparable to flux divergence 5 kW/5 kmsuggests significant local dissipative losses

• vertically-distributed turbulence may be difficult to model but significant to water mass mixing through vertical flux divergence

Summary Results

continued contributions to NRL field scienceMORT Mixing Over Rough TopographyBWE Breaking Wave Effects in High Winds

technological:• loan of OSU-developed instrumentation• technical-level analysis

scientific:• participation in science-level analysis• contribution to publications

LT – large-eddy length scale statistic simply computed from 1D profiles - but an imperfect statistic in an evolving 3D field

Lo – large-eddy length scale defining buoyancy limit on turbulenceLo = √(ε/N3)

if LT = Lo, then ε = LT2

N3

is LT = Lo ?

Moored profiler χpod estimates of turbulence dissipation rate, ε

Moored profiler χpod estimates of turbulence dissipation rate, ε

LT – large-eddy length scale statistic simply computed from 1D profiles - but an imperfect statistic in an evolving 3D field

Lo – large-eddy length scale defining buoyancy limit on turbulenceLo = √(ε/N3)

if LT = Lo, then ε = LT2

N3

is LT = Lo ?

same data – different definition of N2

How do we know χpods work?

5 χpods on TAO mooring yields 5 time series of χ, ε

7 χpods on EQUIX mooring yields 7 time series of χ, ε

24h continuous profiling of χ, ε 6-10 profiles/h

16-day experiment at 0, 140WOct/Nov 2008

Equatorial Internal Wave Experiment 2008

Perlin & Moum, 2012 JAOTech

pod

How do we know χpods work?

Perlin & Moum, 2012 JAOTech

χ ε

profilerχpodsχpods

pod

Comparison of ε computed from χpod and from pitot tube

A1 – mid-column dissipation at the generation site

Observation / Model comparison at T3

Andy Pickering

Observation / model comparison at T3

model does a pretty good job with the vertical distribution and daily-averages

details are a little different

Andy Pickering

T1T2T3T4

N2

MITgcm / Buijsmann et al 2013

Dissipation evolution / compare to MITgcm

T1T2

T3T4N2

mesoscale current (Kuroshio?) dominates 2nd half elevated turbulence at base of current is this friction on a western boundary current?

First continuous deep turbulence profiling time series

MP-N 2010

diurnal composite / spring

semidiurnal composite / neap

T-Chains on the West Ridge

spring diurnals vs. neap semidiurnals

ConclusionsBuijsman et al

log10 εobserved

Integrated Dissipation from big breaking waves contributes O(1 W/m2) vertically-integrated ε this suggests ΔFε= 5 kW/m in 5km… ε is significant to FE!

ε~ u3bt

Distributed Mixing (detached from bottom) is difficult to model

ε~ u3bt

Can models accurately capture mid-column ε and

its vertical distribution?

Can we assign errorbounds on model ε?

Summary Results