Waves and fetch in the Marginal Ice Zone

Jim Thomson 1013 NE 40th St, Seattle WA 98105

phone: (206) 616-0858 fax: (206) 616-5467 email: jthomson@apl.uw.edu

Award Number: N00014-12-1-0113 http://www.apl.washington.edu/MIZ

FY14 report (year 3 of 5)

LONG-TERM GOALS

The long-term goal is to improve prediction of the arctic Marginal Ice Zone (MIZ) by improving basic understanding of the interaction between waves, sea ice, and open water (i.e., fetch).

OBJECTIVES

The primary objective is to improve wave source/sink parameterizations by directly measuring the growth and dissipation of waves in the MIZ. The secondary objective is to develop a surface wave climatology of the arctic ocean and the relation to the seasonal MIZ.

APPROACH

The technical approach is to use Surface Wave Instrument Floats with Tracking (SWIFT) buoys to measure waves, winds, and turbulence at the air-sea-ice interface (Thomson, 2012). The SWIFT deployments were successfully completed during the summer of 2014. These seasonal SWIFT measurements are placed in context using mutliyear subsurface Acoustic Wave And Current (AWAC) moorings and regional WAVEWATCH3 model results.

WORK COMPLETED

A total of six SWIFT buoys were deployed in the Beaufort and Chuckchi Seas during the summer of 2014: three from the R/V Ukpik in July, one from the R/V Araon in August, and two from the USCGC Healy in Auguest. Of these six, two from the Ukpik were allowed drift freely across the region in an endurance mode for two months. The other deployments were shorter and focused on detailed measuremens of processes, including an opportunistic dataset collected from the R/V Ukpik with waves incident on brash ice.

The endurance SWIFTs drifted westward and spent most the later part of August measuring the waves incident to the large array deployed by other DRI members. Around September 1, one of the SWIFTs (#10) drifted into the ice (as indicated by telemetry images from the onboard camera) and immediately measured a sharp reduction in the waves (Figure 1). This SWIFT also reported reduced turbulence, lower salinity, and lower temperatures (both air and water) once in the ice. The winds, however were similar, suggesting that much of the wind stress in the ice goes directly to the ice (not the water or the waves).

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Figure 1. SWIFT wave height data from the endurance deployments during the 2014 MIZ experiment. SWIFT #10 shows strong reduction of waves upon entering the ice around Sept 1. The

turbulence is similary reduced (not shown).

In addition, the first year-long time series from the AWAC on a Beaufort Gyre mooring ‘A’ was analyzed to determine the wave dependence on fetch. The instrument was redeployed on mooring ‘A’ (N 75, W 150), and another AWAC was added to mooring ‘D’ (N 74, W 140). These instruments will be recovered, offloaded, and redeployed yearly through 2015.

Data from the 2014 effort are posted and linked to the program website (apl.uw.edu/miz). Data processing is underway.

RESULTS

Data from the process measurements collected during the R/V Ukpik cruise show a clear link between waves, wind, turbulence, and ice. By placing a SWIFT offshore of the ice and then reseting it progressively farther into the ice, we measured a reduction in the high frequency tail of the wave spectrum via ice damping (Figure 2a). This high frequency tailed is termed the equilibrium range, and in open water it is directly related to the wind stress and the surface turbulence (Thomson et al, 2013).

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In the ice, this portion of the spectrum is reduced, even though the measured winds are the same. Likewise, the turbulence is reduced (Figure 2b). This implies that the effective stress on the waves and water is reduced in the ice, likely because a portion of the wind stress goes directly to the ice. This is confirmed by visual observation of very smooth water surface within the ice floes (i.e., damping short waves). This is consistent with existing models of frequency-dependent wave attentuation in ice, however it is the first known direct observation of the associated change in turbulence (and implied stress partitioning).

Figure 2. Wave energy frequency spectra and profiles of the turbulent dissipation rate measured by a SWIFT buoy at various ice concentrations (indicated by color).

In a complimentary effort, data analyzed from the AWAC mooring measurements, combined with ice maps from the National Ice Center (NIC), show a strong dependence of wave height on the open water distance (Figure 3). This is large scale effect of sea ice (as opposed to the local effects shown in Figure 2).

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Geophysical Research Letters 10.1002/2014GL059983

Figure 3. Scaling of waves in the Arctic Ocean, using nondimensionalwave energy versus nondimensional fetch. There are 1880 points (eachcorresponding to an hourly observation from the in situ mooring at75◦N, 150◦W). Symbols as in Figure 2. The dashed line is a regressionwith a logarithmic slope of 1.6. The Pierson–Moskowitz limit for purewind seas is shown at = 3.64 × 10−3.

scaling, confirms that the fetch correla-tion is not spurious. Furthermore, there isno systematic dependence or correlationwith wind direction, which is distributednearly uniformly over the record inthe range 0–15 m s−1. Other relation-ships, however, are apparent in the data.Dimensionless peak period has a strongpositive correlation with dimensionlessfetch (R2 = 0.7) and a dependence 1∕2.Dimensionless energy has a strong cor-relation (R2 = 0.5) with wave age and adependence (c∕U)4.

The empirical scaling (Figure 3) obscuresmany of the details of the wave actionbalance and wave-ice interactions.In contrast to pure wind sea studies[Dobson et al., 1989; Stiassnie, 2012],the scaling includes swell (e.g., domi-nant wave periods of up to 10 s duringthe September storm), and the valuesgreatly exceed the Pierson–Moskowitzlimit. The swell is not driven by the local

wind and thus is an external source of wave energy in the scaling. To isolate points with swell, the pointsin Figure 3 are separated by nondimensional wave age, c

U, which is the ratio of the dominant wave celer-

ity to the wind speed. Pure wind seas have a wave age less than one, indicating that the wind is drivingthe waves, and these points cluster largely below the Pierson–Moskowitz limit. Swells have a wave agegreater than one, indicating that the waves are outrunning the wind, and these points extend well abovethe Pierson–Moskowitz limit. The seas have the expected dependence of 1, whereas the swells have adependence closer to 1.7. The overall fit is 1.6.

The scaling is successful across both categories, because these waves represent a continuum of sea andswell. The synoptic wind fields that drive the wind sea are large enough in scale to be relevant to the wholebasin as the wind sea evolves into swell. This is similar to the results of Young [2006], in which both sea andswell follow a fetch law in Hurricanes (his Figure 11) because under extreme forcing the swell remains partof the wind and wave system. Here the distance of open water is relevant to the nonlinear interactions thatcontrol the development of swells (because the nonlinearity is weak and requires large propagation dis-tances) [Hasselmann, 1962]. In the limit of pure swell, the local wind is not important, but the scaling of waveheight and basin size is still valid because space is required to build the swell. For swells, the length scale can be interpreted as the basin scale (although the actual fetch is still used), because a mooring in the centerof the basin will have a similar value (regardless of wind direction) that represents the characteristic distanceto an ice boundary.

4. Conclusion

The Arctic Ocean is unique in annual variation of length scale, with the maxima increasing dramaticallyin recent years. Although the dynamics are more general than the conventional fetch dependence ofocean waves, the result is simple: open water distances are the controlling variable for wave heights in theBeaufort Sea (and likely the rest of the Arctic Ocean). Future scenarios for reduced seasonal sea ice cover inthe Arctic suggest that larger waves are to be expected and that swells will be more common. Swells carrymore energy and have longer attenuation scales within ice [Squire et al., 1995; Squire, 2007] and thus willbe more effective at breaking up the remaining ice. It is possible that the increased wave activity will be thefeedback mechanism which drives the Arctic system toward an ice-free summer. This would be a remark-able departure from historical conditions in the Arctic, with potentially wide-ranging implications for theair-water-ice system and the humans attempting to operate there.

THOMSON AND ROGERS ©2014. The Authors. 4

Wav

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)2 / U

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102 ice sea swell

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10 1

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P M limit

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Fetch (scaled), g x / U2

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Figure 3. Wave energy, scaled by wind, versus open water fetch, also scaled by wind. Open water

fetch is the strongest indicator of wave energy in the western Arctic Ocean. IMPACT/APPLICATIONS Improved wave and MIZ predictions in the Arctic Ocean will enable safe naval operations in the region. RELATED PROJECTS A completed contract with Scitor Corp. supported analysis of declassified satellite images for wave information in the Beaufort region. Resources are data are shared with the “Sea State and Boundary Layer Physics of the Emerging Arctic Ocean” DRI. More information is at http://www.apl.washington.edu/project/project.php?id=arctic_sea_state PUBLICATIONS Thomson, J. and E. Rogers, Swell and sea in the emerging Arctic Ocean, Geophys. Res. Lett., 41 (2014). [published, refereed]. Thomson, J., E. D’Asaro, M. Cronin, E. Rogers, R. Harcourt, and A. Shcherbina, Waves and the equilibrium range at Ocean Weather Station P, J. Geophys. Res., 118 (2013). [published, refereed]. Thomson, J. “Observations of wave breaking dissipation with SWIFT drifters,” J. Atmos. and Ocean. Tech., 2012. [published, refereed]. Craig, L. et al, “Science Plan for the Mariginal Ice Zone program,” APL-UW Tech. Report 1201, 2012. [published].

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