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High-frequency and high-wavenumber variability in the
California Current: Evaluating model requirements for SWOT assimilation
Sarah Gille1, Matthew Mazloff1, Jinbo Wang2,
Teresa Chereskin1, Bruce Cornuelle1, Dimitris Menemenlis2, Marcello Passaro3,
Cesar Rocha4
1Scripps Institution of Oceanography, 2Jet Propulsion Laboratory
3Technischen Universität Munchen 4Woods Hole Oceanographic Institution
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California Current: Test bed for SWOT Goal: Develop regional version of MITgcm to assimilate SWOT’s high-wavenumber measurements Build on existing regional ECCO machinery and network of observations • SWOT (swath boundaries) • Nadir altimetry (Jason) • Moorings • HF radar • Buoys (NDBC) • Glider lines
Regional MITgcm built to match MITgcm (llc4320) global model
• ~2 km resolution • Tidal forcing on
boundaries and surface
• 90 vertical levels allows internal waves to propagate
Tide in 2016 for Los Angeles replicates major features
of tide gauge observations
MBARI M2 mooring (June-Sept) Dynamic height
Time (days from 1 June)
Frequency spectra
Regional models
Global model Observations
Can a regional model generate enough internal wave energy?
Regional tests
• Mooring has high-frequency energy
• Global model (llc4320 MITgcm) replicates mooring energy
• Regional MITgcm and ROMS missing high-frequency energy
Hypotheses: • Mooring data noisy; global model too energetic • Interannual variability in observations • Open boundaries don’t let in enough energy
Regional models
Global model Observations
CCE1 mooring (June-Sept)
Time (days from 1 June)
• 2016 and 2017 differ, • … but spectra similar
• Global model replicates mooring,
• … but regional model lacks high-frequency variability
Can a regional model generate enough internal wave energy?
Regional tests
• Mooring has high-frequency energy in 2016 and 2017
• Global model (llc4320 MITgcm) replicates mooring energy
• Regional MITgcm missing energy at high frequency
Hypotheses: • Mooring data noisy; global model too energetic • Interannual variability in observations • Open boundaries don’t let in enough energy
Sea surface height wavenumber spectra 500 100 20 10 5
Wavelength (km)
10-3
10-2
10-1
Along-track wavenumber (cpkm)
10-7
10-6
10-5
10-4
10-3
10-2
10-1
po
wer
sp
ectr
al d
en
sit
y (
m2/c
pkm
)
-2
-4
global model (line 90)
model daily-averaged
regional model (line 90)
• Global model: spectra from hourly output vs daily averages
• Regional model: less energetic than global model at high wavenumbers---more like daily averages Adapted from Chereskin et al, submitted, JGR-Oceans, 2018
Sea surface height wavenumber spectra 500 100 20 10 2 1 0.2
Wavelength (km)
10-2
100
Along-track wavenumber (cpkm)
10-4
10-3
10-2
10-1
100
po
wer
sp
ectr
al d
en
sit
y (
m2/c
pkm
)
-2-4
global model (line 90)
model daily-averaged
regional model (line 90)
• Global model: spectra from hourly output vs daily averages
• Regional model: less energetic than global model at high wavenumbers---more like daily averages Adapted from Chereskin et al, submitted, JGR-Oceans, 2018
Sea surface height wavenumber spectra 500 100 20 10 2 1 0.2
Wavelength (km)
10-2
100
Along-track wavenumber (cpkm)
10-4
10-3
10-2
10-1
100
po
wer
sp
ectr
al d
en
sit
y (
m2/c
pkm
)
-2-4
95%
Sentinel 3
AltiKa
Jason 1-2 (track 195)
global model (line 90)
model daily-averaged
regional model (line 90)
• Global model: spectra from hourly output vs daily averages
• Regional model: less energetic than global model at high wavenumbers---more like daily averages
• Altimeter spectra more energetic than models from 100-50 km and flatten out (implying “noise”) for scales smaller than ~50 km.
Adapted from Chereskin et al, submitted, JGR-Oceans, 2018
Vertical velocity
Global model: llc4320 Regional model: MITgcm with open boundaries
Larger vertical velocity variance in global model
W variance at 500 m; black contour = 3000 m bathymetry
Global model (llc4320 MITgcm)
Regional model (MITgcm)
W variance at 500 m; black contour = 3000 m bathymetry
Global model (llc4320 MITgcm)
Regional model (MITgcm)
Mendocino escarpment generates tidal beam
Mendocino escarpment tidal beam less pronounced
Larger vertical velocity variance in global model
Global model (llc4320 MITgcm)
Regional model (MITgcm)
Mendocino escarpment generates tidal beam
Mendocino escarpment tidal beam less pronounced
Zoom in on Santa Barbara Channel
Larger vertical velocity variance in global model
Vertical velocity comparable in SoCal Bight, in lee of islands
Global model (llc4320 MITgcm)
Regional model (MITgcm)
Wave energy flux (u’p’)
Regional
Global 90
22
-38
-59
-55
-51
Megawatts (MW)
74
-165
Positive: energy into the domain. Negative: energy out of the domain.
North West South
Summary and Conclusions • Small-scale and high-frequency
processes occur in the California Current region in observations and global model, but not in regional model
• Energy originates outside of regional domain (e.g. Hawaii and western Pacific).
• Future work: Regional models that represent internal waves will need a new strategy to input energy at open boundaries.
Global
Regional
Vertical velocities
Global model: llc4320 Regional model: MITgcm with open boundaries
Characterizing the California Current
Acoustic Doppler Current Profiler data: 1993-2004, 39 cruises
Helmholtz decomposition to separate rotational and divergent components
Chereskin et al, submitted, JGR-Oceans, 2018