sea state in cci+ the many faces of ocean waves

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Sea state in CCI+the many faces of ocean waves

Fabrice Ardhuin (LOPS, Brest, France)

with contributions from J. Stopa, J.-R. Bidlot, P. Queffelou …

Happy birthday Joseph!

2wwz.ifremer.fr/iowaga

Outline of this talk

wwz.ifremer.fr/iowaga

0. What is « sea state » ?

1. Who cares about « wave climate ? »

2. From « wave climate » to « wave climate change » :

the COWCLIP initiative

3. Issues and the Sea State CCI proposal / project

4. Connecting with other ECVs

– Sea ice, sea level …


The sea state is the « statistics of wind-generated ocean waves »

Sea-state is usually defined by the elevation spectrum + whitecap propertiesBelow : examples 1 day apart, Katsiveli platform, Black Sea

0. What is sea state?

Let's call it “ocean waves”...

Only one number ?→ significant wave height Hs or SWH or H

m0

(world record is 20.1 m, Hanafin et al. BAMS 2011)


These ocean waves exclude tsunamis : they are generated by winds...but this wind-sea can travel across ocean basins and hit far from storms...

0. What is sea state?

Let's call it “ocean waves” ...

A real issue for many Pacific & Carribean islands...


Any ocean or coastal activity : - design of ships, platforms … → design life time ~ 50 years or more- coastal defenses : parts of Dutch coast have to withstand 1/10000 years storm

(I hope nuclear power plants have same safety requirements ) - marine energy : profitability of investments depend on wind & wave climate- geosciences : waves modify fluxes at air-sea, sea-ice and land-sea interface- remote sensing : corrections for sea level, salinity …

How well do we know today's climate ? Mean wave heights (Hs)… OK in most places.

Extreme values of Hs ?Interannual variability ? : big impact of El Ninos…

Other variables ? – (wave periods T, directions …) → wave « runup » scales with T (g Hs)0.5

– Littoral drift (erosion) is super-sensitive to directions (few degrees change sign)

1. Who cares about « wave climate ? »


SAR-derived surface orbital velocities (Stopa, Ardhuin, Chapron & Collard using Envisat wave mode accepted for JGR)

Previous works : wave heights from wave mode (Lehner et al. ), crossing seas (Collard et al. ), Wind-sea and swell fraction (Chen et al. 2000) …

1. Who cares about wave climate?


Previous work on trends for extremes …

2. Wave climate change & IPCC The COWCLIP project

Mori et al. (2010), from models

Young et al. (2011)...(1985-2008 trend is not secular)


Example of trends in the Arctic :(from altimetry) Less ice means more waves(and more coastal erosion …)

(Stopa et al., The Cryosphere, 2016)

2. Wave climate change The COWCLIP project


Sea state is waves … but cannot ignore winds and currents …

Good ! Altimeters and SARs can give some wind & current information too

3. Issues and the Sea State CCI project


ESA and CNES funded Globwave to reduce inconsistencies


Not planned for climate use : GCOS 154 « There is presently no coordinated sustained effort to deliver global, high-quality sea-state information observations for climate »

Before corrections......worse for extreme values


Standard altimeter products are noisy (not optimized for sea states)→ round robin with several retrackers → important for coasts & currents

New instruments use a different technique (unfocused SAR) : consistency ? Opportunities : less noise. Validation ? (optical imagery from S2 ...)


b. data quality


SAR wave spectra are biased (long periods, improper MTFs, cut-off … )

→ New algorithms→ application to new areas(including sea ice)


a. data quality


We have a requirement for Hs:

There is no way this can be achieved with satellite data alone !

And a typical 1-day coverage :… you can miss big storms ...


b. space-time resolution (sampling)


Sampling is critical for extremes

Wave data can be used in several ways : - identifying and correcting wave model biases (e.g. Ardhuin et al. 2010)- assimilating into reanalyses (e.g. ERA5)

Gridded daily maps are at best useless, at worst misleading.

→ future product based on Sea State CCI will combine model & data


b. space-time resolution (sampling)


~ 25 years of altimetry

~ 35 years for a few buoys

→ link to be done with ship observations & microseisms


c. Short records


The wave climate in the Arctic is changing very fast :

Hs distribution in the Beaufort Sea (ONR Arctic Sea State DRI Team, 2017)

4. Connecting with other ECVssea state and sea ice

Positive feedback on ice retreat...

(ONR Sea State DRI, FP7 SWARP … )


High water at the shoreline is related to wave forcing...→ combined slow MSL rise with wave-driven events and tides

5. Connecting with other ECVssea state and water level

Max water level

Wave power index


In summary

wwz.ifremer.fr/iowaga

1. There is a wave climate community now part of IPCC

2. Climate-quality Wave heights can be provided by altimeters (as proven by Globwave)

3. More work needed on other variables (wind speed, wave periods…) and extremes (especially tropical storms)

4. Connection with other ECVs : at least sea ice.

but also sea level (littoral aspects), SST …

sea state in cci+ the many faces of ocean waves

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