Research Interests: variability of sea ice thickness interactions among ocean, seaice and atmosphere climate modelling

Transcription

1 Dr. Richard Moritz Department Chair, Polar Science Center M.S. (Yale University, 1979), M.A. (University of Colorado, 1979), PhD (Yale University, 1988) Research Interests: variability of sea ice thickness interactions among ocean, seaice and atmosphere climate modelling

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3 WEEKLY TO INTERANNUAL VARIATIONS OF SEA-ICE DRAFT DISTRIBUTION AT THE NORTH POLE ENVIRONMENTAL OBSERVATORY Richard Moritz Contributing NPEO PI s: Rebecca Woodgate, Knut Aagaard and Jamie Morison Polar Science Center, Applied Physics Laboratory, University of Washington Applied Physics Lab Polar Science Center ARCTIC FORUM, 13 May, 2004

4 OUTLINE The NPEO Motivation, Background Ice Draft Distribution Moored Sonar Measurements Ice Draft Time Series Estimated Distributions & Statistics Features of the Temporal Variation Comparison with Published Data Preliminary Consideration of Causes Conclusions 5/20/04 Arctic Forum

5 The NPEO Established in 2000 (NSF LTO s) Time Series and Repeat Measurements Key Variables in Central Arctic Emphasis on Tracking Changes UW, OSU, NPS, CRREL, JAMSTEC, PMEL ELEMENTS Spring Surveys (CTD, Ocean Chemistry) Autonomous Drifting Stations (OAI) Bottom-anchored Mooring ( )

6 NPEO MAP:

7 NPEO 2004 Mooring

8 NPEO DRIFTING STATION WEB PAGE psc.apl.washington.edu/northpole/index.html

9 NPEO DRIFTING STATION NOAA/PMEL Web Cam TODAY 13 May, 2004 Camera #2 Camera #1

10 Interdecadal Changes (Rothrock, et al., 1999): Ice Draft near NP Decreased ~ 1.5 m NPEO POSITION 5/20/04 Arctic Forum

11 Regional, Interdecadal Changes (Rothrock,, et al., 1999, Data adjusted seasonally to 15 September using a model) Time Continuity, Trends, & Interannual changes? Causes Radiative Forcing? Advection? Ocean Heating? How influential are the seasonal adjustments? NPEO aims to help address these questions. 5/20/04 Arctic Forum

12 Background Ice Thickness is Fundamental to Arctic Ocean: Ice Mass Budget Heat Budget Freshwater Budget Ice Thickness h(x,t) varies chaotically on small scales: _ ~ 2 m Autocorrelation Length scale ~ 150 m Time scale ~ 5 min Aggregate the data for relevance to climate: Area Statistics Ice Thickness Distribution Atmos/Ocean Forcing & Response Scales ~ km Ice Thickness Distribution temporal variations are not well known on intraseasonal, interseasonal and interannual time scales 5/20/04 Arctic Forum

13 Spatial Variations in Ice Thickness: Multiyear and First Year Ice ICEX-03 Camp MY Thickness 3 10 m FY Thickness 1 2 m Ridges spaced < 100m 5/20/04 Arctic Forum

14 Spatial Variations: Leads and Young Ice Ice Thickness < 15 cm Ice Thickness > 30 cm 5/20/04 Arctic Forum

15 Spatial Variations of the Underside of Sea Ice Blocks in Deformed First Year Ice Some Smooth Multi-Year Ice 5/20/04 Arctic Forum

16 Temporal Variations of Morphology Air View of ice station SHEBA before melting Air View of ice station SHEBA during melting 5/20/04 Arctic Forum

17 Much of this variation can be summarized by the Ice Thickness Distribution Total Area Ao Areas where thickness is on {h, h+dh} Definition (Thorndike, et al., 1976): g(h)dh = [1/Ao] * A(h, h+dh) 5/20/04 Arctic Forum

18 The Ice Draft Distribution Summarizes the Amount (Area) of Ice in Each Draft Interval (Reminder: Thickness is approx 1.2 * Draft, & Draft is what we measure here) 5/20/04 Arctic Forum

19 Statistical Parameters of the I.D.D MODE Count (<5 cm) OWF MEAN ~ Area STDV SLOPE Ice Draft (m) 5/20/04 Arctic Forum

20 NPEO Ocean-Ice Mooring (Water Depth ~ 4200 m) Year 1: 4/10/02-04/21/02, Lat N Lon E Ice Draft Year2: 4/23/02-04/23/03 Lat N Lon E Ice Velocity Drawing: Jim Johnson 5/20/04 Arctic Forum

21 Estimating Sea Ice Draft with a Moored ULS Sea Level (SLP from IABP) Draft Thickness Ice Velocity (ADCP) Depth Range Draft = Depth Range Thickness = 1.2 * Draft ULS (measures range and depth) SSP, DENS from T,S T,S from PHC Anchor 5/20/04 Arctic Forum

22 APL ULS Mark 2 Specifications Frequency: 300 khz Beam Width: 2 half power Nominal Footprint (55 m depth): 2 meter diameter Pulse Length: 1 millisecond Sampling Intervals: Low-Resolution: 5 minutes, continuous High-Resolution: 10 seconds for 25 minutes twice per day Typical Overall Errors: Approx. +/- 2 cm for level ice in summer Larger in Winter than in Summer (O.W. offsets rarer) Larger for deformed ice (Draft computed from first echo) Uncertainties Echoes from off-axis targets Variations in Sound Speed and Density (T & S) 5/20/04 Arctic Forum

23 25 Minute Sample of High-Res. ULS Measurements 1-D Profile of Ice Bottom 5/20/04 Arctic Forum

24 ADCP Ice Velocity Data Complete Annual Record of Ice Speed Daily Ice Motion & Wind Correlated 88% of Speeds > 2 cm/s Ice Lockup is Rare 5/20/04 Arctic Forum

25 Eulerian Time Series: The ULS Samples a Streak Line on the Ice Bottom Surface Ice Velocity ULS t1 t2 Ice Velocity data can benefit the analysis: * How much ice was sampled (length of streak) * Where is the ice that was sampled 5/20/04 Arctic Forum

26 Eulerian Time Series: The ULS Samples a Streak Line on the Ice Bottom Surface Ice Velocity t1 ULS t2 Sample from t1 to t2 Ice Draft Histogram ULS Ice Draft ~ ADCP Ice Speed Compute Stats Pseudo-Spatial Statistics 5/20/04 Arctic Forum

27 Pseudo-Spatial Statistics (e.g. Melling,, et al., 1995, J. Atm. Oc.. Tech.) Probability ~ Distance ~ Speed*Time PS-Weight ~ Distance Comparison of time and p.s. Statistics ULS DATA 9-22 May Low Speed, Anomalous Draft ADCP DATA 5/20/04 Arctic Forum

28 Complete 5-minute spacing Annual Time Series, Year 1 107,467 Obs 5/20/04 Arctic Forum

29 Complete 10-minute spacing Annual Time Series, Year 2 53,791 Obs 5/20/04 Arctic Forum

30 Annual Ice Draft Histograms for NPEO Year 1 and Year 2 Apr 2001 Apr 2002 Apr 2002 Apr 2003 Count MEAN = 3.0 m MODE = 2.0 m OWF = 2% SLOPE = Count MEAN = 3.5 m MODE = 2.2 m OWF = 1% SLOPE = Ice Draft (m) Ice Draft (m) 5/20/04 Arctic Forum

31 Annual Ice Draft PDF for NPEO Year 1 and Year 2 5/20/04 Arctic Forum

32 RESOLVING INTRASEASONAL VARIATIONS Divide the Time Series into Non-overlapping 2-week segments Use the ADCP Data to Weight Time Intervals ~ Speed Estimate Time Series Stats and Pseudo Spatial Series Stats Plot the Annual Variations of 2-Week sample statistics 5/20/04 Arctic Forum

33 6 19 June, /20/04 Arctic Forum

34 6-19 June, 2001 ICE DRAFT (m) 5/20/04 Arctic Forum

35 4 17 July, 2001 Ice Draft (m) 5/20/04 Arctic Forum

36 26 Sept 9 Oct, /20/04 Arctic Forum

37 26 March 8 April, /20/04 Arctic Forum

38 Interseasonal Variations Rothrock NP 1990 s 5/20/04 Arctic Forum

39 Regional, Interdecadal Changes (Rothrock,, et al., 1999, Data adjusted seasonally to 15 September using a model) 2.45 m 5/20/04 Arctic Forum

40 Interseasonal Variations Rothrock NP 1990 s 5/20/04 Arctic Forum

41 Open Water Fraction and Slope SHEBA 1998 Air Photos (Perovich( Perovich,, et al) 5/20/04 Arctic Forum

42 NPEO ULS 12 5/20/04 Arctic Forum

43 NPEO ULS 12 5/20/04 Arctic Forum

44 NPEO ULS 12 5/20/04 Arctic Forum

45 NPEO /20/04 Arctic Forum

46 Framework for Understanding Time Changes H = Mean ice draft dh/dt = G + A + D G = Growth (Melt when negative) A = Advection (Veloc( across H contours) D = Divergence 5/20/04 Arctic Forum

47 Thermodynamic Effects: Variations of Thickness and Temperature at a Lagrangian Point Snow 2.6 m Ice Top 1.5 m Ice Temperature Ice Bottom 20 May 15 Sep Oct 1997 (M.Y. Ice at SHEBA, Perovich,, et al., 1999) Oct /20/04 Arctic Forum

48 Thermodynamics: No Melt Ponds, July, 2002 No melt ponds on web cam in year 2 5/20/04 Arctic Forum

49 Thermodynamics: Melt ponds in July, 2003 No melt ponds on web cam in year 2 5/20/04 Arctic Forum

50 NPEO Mooring Location and 2001 Ice Displacements NPEO 5/20/04 Arctic Forum

51 2002 Ice Displacements 5/20/04 Arctic Forum

52 SEASONAL SLP ANOMALIES, 2001 H L L L 5/20/04 Arctic Forum

53 SEASONAL SLP ANOMALIES, 2002 L H L 5/20/04 Arctic Forum L

54 MULTI-YEAR ICE CONCENTRATION Overall Low values, Max s s along Greenland-Archipelago 5/20/04 Arctic Forum

55 MULTI-YEAR ICE CONCENTRATION Overall high values, Max s s away coasts 5/20/04 Arctic Forum

56 MULTI-YEAR ICE CONCENTRATION Lower values, Max spread out toward Eurasian Basin 5/20/04 Arctic Forum

57 MULTI-YEAR ICE CONCENTRATION 5/20/04 Arctic Forum

58 MULTI-YEAR ICE CONCENTRATION 5/20/04 Arctic Forum

59 MULTI-YEAR ICE CONCENTRATION Lower values, Max spread out toward Eurasian Basin 5/20/04 Arctic Forum

60 Conclusions Year 1 The ADCP ice velocities are consistent with the IABP Geostrophic Winds and our knowledge of daily ice motion. The ADCP ice velocities are used to estimate pseudo-spatial statistics of Ice Draft. Overall, differences between two-week sample time and pseudo-spatial statistics are small. The two-week samples resolve interseasonal variations of the mean, standard deviation, modes, open water fraction and ridged ice slope. The sample mean draft in Year 1 varied from 3.7 meters in early April to 2.3 meters in late August, with an overall annual mean of 3.0 meters. The sample mean draft estimated for 15 September, 2001 is 2.3 meters, a value near the middle of the Rothrock et al. estimates for NP in the 1990 s. Changes in the modal draft lag behind the sample mean, with extrema of 2.3 meters in late June, 2001 and 1.6 meters in early January, The January minimum is more than 3 months later than one expects from thermodynamic forcing only. The open water fraction reached a maximum of 13 percent in July, and is not inconsistent with air photo estimates from SHEBA during /20/04 Arctic Forum

61 Conclusions (continued) The annual variation of the modal draft during year 1 is not consistent with a purely thermodynamic forcing. The annual variations of mean, mode and owf in year 2 differ qualitatively and quantitatively from those of year 1. The annual mean draft in year 2 is 3.5 Meters, which is 0.5 meters higher than in year 1. The mean draft for 15 September 2002 is 0.90 meters greater than that of 2001, and lies just above the upper limit of submarine-derived values for the 1990 s s in this region. In year 1 the modal draft decreases monotonically by approx. 0.8 m from June to January. In year 2 the mode fluctuatures,, then increases over this interval. Preliminary, qualitative analysis suggests that both thermodynamics and ice advection contributed significantly to the differences between the time series of the two years 5/20/04 Arctic Forum

62 Acknowledgements Jim Johnson, Mooring Technician, PSC-APL/UW Kay Runciman,, Data Reduction and Analysis, PSC-APL/UW NPEO Logistics Team and Divers CF Alert Funded by the U.S. National Science Foundation ARCSS Program 5/20/04 Arctic Forum

63 THE END 5/20/04 Arctic Forum

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65 ABSTRACT Session JSP04, Arctic Environmental Change Annual variation of sea-ice draft distribution at the North Pole Environmental Observatory R. E. Moritz, R. Woodgate,, K. Aagaard and J. Morison, Polar Science Center, Applied Physics Laboratory, University of Washington. A time series of sea-ice draft D(t) was estimated from measurements made by an Upward Looking Sonar (ULS) from 10 April, 2001 to 21 April, The mooring was located at the North Pole Environmental Observatory (NPEO), Latitude N, Longitude E. Sample statistics and probability density distributions (PDFs)) of D(t) have been estimated by grouping the data in nonoverlapping,, two-week intervals with equal weight given to each observation. The sample mean draft varied from 3.7 meters in early April to 2.3 meters in late August, with an overall annual mean of 3.0 meters. The sample mean draft estimated for 15 September, 2001 is 2.3 meters, a value near the middle of the seasonally adjusted estimates derived from submarine sonar profiles near the North Pole in the 1990's (Rothrock( Rothrock,, et al., GRL, 26, 1999, ). The modal draft of the sample PDF lags behind the sample mean, with extrema of 2.3 meters in late June, 2001 and 1.6 meters in early January, The open water fraction (OWF) is defined by integrating the sample PDFs over ice drafts less than.05 meters. OWF exceeded 1 percent from mid-june to early October, and for a brief period in late December. The maximum OWF of 13 percent occurred in mid-july. The evolution of the ice draft PDF on time scales of a few days to one year is discussed with reference to physical processes and to the problem of estimating trends in the climatology of D. 5/20/04 Arctic Forum

66 April, 2001 May, 2001 June, /20/04 Arctic Forum

67 5/20/04 Arctic Forum

68 Objectives Estimate Ice Draft from Measurements Estimate Ice Thickness Distribution Present Annual Variations in Ice Thickness Distribution Compare with Published Estimates Explain differences between the two years Derive Implications for Lg Scale Ice Physics & Climate Variability Overall Goals Quantify and Understand Temporal Variability of I.T.D. Interseasonal Interannual Climate 5/20/04 Arctic Forum

69 Statistics of Complete Annual Time Series Basic Sample Statistics Exponential Tail Deformed Ice 5/20/04 Arctic Forum

70 Variables Shown on Histograms N Number of Observations MEAN - Sample Mean MODE - Sample Mode STDV - Sample Standard Dev. OWF - Sample Open Water Fraction Slope Exponent for 4-10m ice Subscript t Simple time series statistics p Pseudo-Spatial statistics 5/20/04 Arctic Forum

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