Open Ocean Storm Waves in the Arctic

Transcription

1 International Workshop on Wave Hindcasting and Forecasting/Coastal Hazards Symposium Liverpool, UK, September 2017 Open Ocean Storm Waves in the Arctic Takuji Waseda,Adrean Webb (UTokyo, Japan) Kazutoshi Sato,Jun Inoue (NIPR, Japan) Alison Kohout (NIWA, NZ) Bill Penrose, Scott Penrose (P.A.S. Consultants, AU) Arctic Challenge for Sustainability (ArCS) Program of the Japanese Ministry of Education, Culture, Sports, and Technology

2 Reduced Sea Ice Extent = Increased Area with Waves mean wave height March September Walsh 2014 Wind waves are generated in the emerging ice-free waters as large as 1000 km x 1000 km Babanin et al. 2014

3 Objective To identify the long-term trend of the extreme wave height in the Arctic Ocean for the safe navigation of ships in the Northern Sea Route Methods Drifting wave buoy measurements were conducted in 2016 Sep. to Nov. Validate numerical wave model in the ice-free and open waters The long-term trend was studied using the 38 year-long ERAinterim reanalysis ( ) Conclusion Clear trend of the expected maximum significant wave height from the Laptev to Beaufort Seas was found. The cause of the increase is likely the increase of the expected maximum wind speed in the icefree waters. Waseda, Webb, Sato, Inoue, Kohout, Penrose and Penrose, 2017 under review

4 Maximum wave height in the ice-free waters estimating the trend of extreme values in changing wave climate c.d.f. of the significant wave height P H s Probability of maximum Hs in N number of samples 1 P H s = 1 N N # of grids in ice free waters Maximum Hs in the ice-free waters (areal maximum) H s = P N Assume Weibull distribution x c P H s = 1 e k H s = c ln N 1 k Outstanding Question: Which represents the trends in the Arctic Ocean, c or N?

5 Trend of Hs distribution(beaufort & Chukchi sea) Trend of the Weibull scale parameter c Decadal trend of the pdf of significant wave height gradually shifting to the higher Hs p.d.f. f H s = k c x c k 1 e x c k c.d.f. F H s x c = 1 e k Thomson et al. 2016

6 Fetch Law Increased ice-free water distance longer fetch higher waves H s = c ln N 1 k Thomson & Rogers 2014 GRL

7 2016 summer wave observation JAMSTEC MR16-06 R. V. Mirai Cruise 2016 August 22 October 5

8 Wave Observation Sep.10 Nov R/V Mirai (Oceanography, 45d) Arctic sea ice extent (Arctic Data archive System) 11/2 stop 9/10 deployment

9 Waves In Ice (WII) buoy Specifications Height without Keel - 778mm Diameter mm Weight without Keel - 20Kg Weight with Long Keel -35Kg Battery capacity 7.2V - 60 Amp Hour Battery life without charging - 3 weeks at full load 3 External solar panels 3 temperature probes, air, sea and internal GPS Receiver Iridium SBD modem 9602 Intel Edison processor (includes WiFi) 9 degrees of freedom IMU MPU9250 (3-axis gyroscope, accelerometer and compass) P.A.S. Consultants Australia 15 kg Power spectral density Spectral moments Wave direction etc.

10 Buoy Trajectories High-pass filter Mean of all the observed wave spectrum sensor noise wind-sea Buoy Deployment NO Date (YYYYMMDD) Longitude (deg W) Latitude (deg N) 1 10-Sep :45:18 UTC Sep :45:32 UTC Buoy Lost conact NO Date (YYYYMMDD) Longitude (deg W) Latitude (deg N) 1 02-Nov :45:41 UTC Nov :45:19 UTC Default setting of the cut-off period was set to 33 s, which is too long for Arctic waves. Post Processing: An adaptive filter was applied.

11 High-pass filter Spectra of the two observed extreme events 2016/9/19 07: /10/22 09:45 Hm0=4.87 Hm0=4.02 The cutoff frequency (fc) is determined from the minimum of the smoothed spectrum (dashed-dotted line). The total energy (m0) is determined from the original spectrum (solid line) applying an ideal high-pass filter at cutoff frequency fc (vertical solid line).

12 Observation vs. ERA-interim Wave height Buoy m 4.0 m Wave height Buoy m 4.7 m See: P67 Ocean wave forecasting system for the Northern Sea Route by Webb

13 Wind field : depression associated w. extreme events Hm0 Significant wave height Wind speed westerly Sea level pressure

14 Post Processing: Comparison with ERA-interim 15 s cut-off filter Era Adaptive cut-off filter Buoy 1: Hs Buoy 2: Hs Hm0 Buoy 1: Hs Buoy 2: Hs c.c c.c c.c c.c Buoy Hm0 Buoy 1: T0-1 Buoy 2: T0-1 Buoy 1: T0-1 Buoy 2: T0-1 c.c c.c c.c c.c Significant improvement of the moment period with adaptive high-pass filter. A similar c.c of Hs (0.91) was obtained w. the SWIFT buoy measurement in 2014 July-Sep

15 Long-term trend of the waves in the Laptev to Beaufort Seas ECMWF: ERA-interim , 0.75 degrees resolution (30kmX110km), 6 hourly

16 September H s Maximum wave height in the Laptev to Beaufort Seas H M s = i=1 H s t i open area (%) Percentage of ice-free and open waters (open area (%)) Laptev East Siberian Beaufort open area (%) # of grids in open waters # of grids in the analysis region Analysis region: E W, N N Chukchi

17 Estimating the expected value of the maximum wave height in the ice-free waters For a given data set, the observed maximum significant wave height is itself a stochastic variable. The p.d.f. of the maximum wave height can be expressed as (c.f. Longuett-Higgins 1952) f H s = N P H s N 1 p H s dh s The expectation of H s can readily be derived: E H N s = 1 P H s 0 dh s The E H s will change with P H s and N. The E H s is estimated in this study as following: E H s M = 1 M H s t i i=1 Extreme value in space, expectation as a time mean M: number of samples in a month

18 Trends of the ERA-interim maximum wave & wind in Laptev to Beaufort Sea There is a clear increasing trend for both wave and wind, rate is largest in October H s 1 to 2 cm/year increasing trend 0.76 m in 38 years (October) U 10 2 to 6 cm/s/year increasing trend 2.3 m/s in 38 years (October) open area (%)

19 Partial correlation analysis : long-term and inter-annual variations August (low) H s U 10 open area H s September (low) H s U 10 open area H s October (low) H s U 10 open area H s August (high) H s U 10 open area H s September (high) H s U 10 open area H s October (high) H s U 10 open area H s H s = H s + H s lll pppp hiii pppp U 10 = U 10 + U 10 lll pppp hiii pppp open area = open area + open area lll pppp hiii pppp : 7 year moving point average H s = c ln N 1 k U 10 = c ln N 1 k High p.c.c. between Hs and U10 implies that the trend can be explained by N, i.e. increase of the number of samples High p.c.c. between Hs and open.area implies that the trend can be explained by c, i.e. the fetch effect High p.c.c. between U10 and open.area needs physical explanation

20 Long-term trends of storms in the Arctic Koyama et al J. Climate No coherent trend in storm frequency Sato & Inoue 2017 Clim Dyn Weak SLP trend compared with the strong trend of ice concentration H s Maximum wind speed including ice-free water, ice-covered water and over land U 10

21 Conclusion WII buoy measurement was conducted in 2016 Sep. to Nov. in the Chukchi & Beaufort Seas ERA-interim wave field was validated by the observation and was used to study the long-term trend of waves in the ice-free waters in the Arctic Ocean The wind in the Arctic region is not strengthening but the wind in the ice-free water is strengthening. It is conjectured that the expected maximum wave height increased in the past 38 years, because, the chances of encountering a strong wind in the ice-free water is increasing.

22 Trend in the mean & scale parameter (Weibull)

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