SWOT Fast-sampling OrbitInternal Tide Beams

James B. Girton and ZhongXiang Zhao APL, University of Washington

!Matthew Alford Scripps, UCSD

A fast-repeat phase observable: Internal Tide Beams• Internal Tides: internal waves at tidal frequency generated by

linear topographic features. • Beams may propagate across entire ocean basins (1000s of km). • M2 mode-1 semidiurnal tide maps generated by fitting plane

waves (amplitude, phase, direction) to multi-satellite altimetry. • Many passes will be needed to separate narrowband internal tide

from broadband eddy spectrum, but how many? (12 days aliasing period will cover M2 phase.)

• Total available passes: 90 in fast-repeat phase, 55 in 3-year science phase.

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North PacificHawaii, Aleutians, Mendocino

Hawaiian Ridge

Aleutian Islands

Mendocino Escarpment

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Southeast PacificFrench Polynesia, Easter Island

Easter Ridge

Tuamotu Ridge

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North AtlanticSeamounts, Cape Verde, Mid-Atlantic Ridge

Vema Fracture Zone

Cape Verde

Seew

arte

Se

amou

nts

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South AtlanticSpiderwebs

Vitoria–Trinidade Ridge

OptionsSeveral internal tide beams propagate in directions suitable for sampling over a long distance by the SWOT fast-sampling phase track (after some shift).

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The current track does not capture any well-sampled internal tide beams suitable for validation.

Mendocino Escarpment is the nearest well-sampled target (Althaus et al, JPO 2003; Alford, JPO 2010)—reachable with a 1–2° westward shift.

But other interesting features are covered by the orbit…

Girton Progress Report 4

Table 1: RRC track shift (negative indicates westward shift) needed to target prominent internal tide beams.

Topographic Feature IT Beam Direction RRC track shiftEast Polynesian Islands northeastward �2�

Kaua’i Channel (Hawai’i) northeastward �7�

Aleutian Islands southeastward �10�

Mendocino Escarpment southwestward �2�

must establish its own credibility in terms of both instrument performance and in-situ validation of SSHsignals. Since a significant portion of the SSH signal on 10–200 km scales is in the internal tide, making atleast some attempt to (a) target known IT features with AirSWOT and (b) verify these signals with in-situmeasurements would seem to be essential. Our multi-satellite altimetry IT analysis (Fig. 1) can provide agood starting point for IT targeting, and our team can also offer economical options for in-situ measurementsin the form of moorings or profiling floats to characterize the amplitude, propagation direction, and verticalstructure of the internal tide. SIO or APL-UW mooring instrumentation and expertise can be providedfor a selected AirSWOT location, and the APL-UW pool of profiling EM-APEX floats can be used for ashort-term spatial array without the need for a full-sized research vessel for deployment and recovery. Eachplatform has advantages for particular locations and/or processes and should be considered for use by theAirSWOT team.

3.3 Surface signature of broadband internal wave continuum

Finally, we have also been exploring methods for estimating the SSH signature of non-tidal internal wavesfrom the starting point of the plane-wave pressure signal as a function of the wave internal displacementamplitude, frequency, and vertical wavenumbers (as well as the local buoyancy and Coriolis frequencies).Alternatively, the signal can be derived from the velocity amplitude, so that vertical profile spectra of shearand strain can be used to predict SSH spectra in the context of the Garrett and Munk (1975) (GM) internalwave model. Of course, the horizontal wavenumber dependence of the GM spectrum has undergone verylittle observational testing, and SWOT and AirSWOT will both have the opportunity to improve our knowl-edge in this respect. For the immediate future, it is sufficient to point out that both narrowband internal tidesand a broadband GM internal wave spectrum have an SSH signature that can be estimated from subsurfaceprofiling timeseries measurements that span a significant portion of the water column (enough to resolve thefirst baroclinic mode). For this reason, such measurements will be an essential component of both AirSWOTand SWOT validation.

Vitoria–Trinidade Ridge southeastward +1° Vema Fracture Zone northward 0°

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Greenland–Scotland Ridge OverflowsDenmark Strait, Faroe Bank Channel, Iceland–Faroes Ridge

Denmark

Strait

FBC

WTB

IFR

Iceland

Greenland

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Greenland–Scotland Ridge OverflowsDenmark Strait, Faroe Bank Channel, Iceland–Faroes Ridge

Denmark

Strait

FBC

WTB

IFR

Iceland

Greenland

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Greenland–Scotland Ridge OverflowsDenmark Strait, Faroe Bank Channel, Iceland–Faroe Ridge

Denmark

Strait

FBC

WTB

IFR

Iceland

Greenland