listening from the ocean bottom: a broadband view of ocean ... · 11/13/2017  · 1 listening from...

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Listening from the ocean bottom:

A broadband view of ocean waves

Justin Stopa1, Fabrice Ardhuin1, Charles Peureux1, Jean-Yves-Royer2

1Laboratoire d'Océanographie Physique et Spatiale, Brest, France2Laboratoire Géosciences Océan, Brest, France

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adapted from Walter Munk (1950)

15 km150 m1.5 m Wavelength200 km

1. The surface wave spectrum

What this talk is about is in this grey box

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1. The surface wave spectrum

What this talk is about is in this grey box

And it can all be recorded from the ocean bottom

(Duennebier et al. JGR 2012)

100 10 1 0.1 0.01 Acoustic frequency

? 5 0.5 0.1 0.01 Surface gravity wave frequency

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Let's start with the very low frequencies: IG waves

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IG waves can be really big, and contribute to flooding (see recent storms in Fidji … )

2. IG waves

Here are some local examples:

(Sheremet et al., GRL 2014)

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But we have very little data available– Review by Webb et al. (JGR 1991) : few data, ~ 1 cm height – Aucan & Ardhuin (GRL 2013) : analysis of tsunami warning stations :

2. Open ocean IG wave data

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→ further analysis by Ardhuin et al. (OM 2014) :– New experiments (Waimea 2012)– Use of various datasets

2. model for Infragravity waves

Coastal sources

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→ Frequency dependance? Empirical relation based on very few data ….

2. model for Infragravity waves

Coastal sources

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Global map of free IG wave heights :

Top : Jan & Feb

Bottom: July & August

For year 2008

2. Infragravity waves

Global model result

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Sources of IG waves recorded off Japan are in North America

(Rawat et al., GRL 2014)

But big errors in South Pacific …

2. Building a global IG wave model

Model validation : global scale

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IG waves and Earth's hum

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Model application : estimation of sources of microseisms (the hum)... first validation by Ardhuin et al. (GRL 2015)

Ongoing work (Deen et al. at IPGP)

3. IG waves & hum

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And throughout the year … rms accelerations, at SSB (LHZ)7 to 20 mHz(dB re 1 m/s2)

3. IG waves and hum sources

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Conclusions and perspectives

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Conclusions

1) Very long periods (30 to 300 s) and short periods ( < 2 s) are poorly known.

Both can be observed at the ocean bottom : – Linear response in infragravity waves– Non-linear interactions for short waves (e.g. Farrell & Munk 2008, Duennebier

et al. 2012, Ardhuin et al. JASA 2013 … )

Observation needs : long time series on the ocean floor with broadband instruments

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Verification of same model at global scale : using DART bottom pressure recorders

2. Building a global IG wave model

It works … not well everywhere...

datamodel

U.S. West coast U.S. West coast

Peru

U.S. East coast