North Slope Trends in Sea Level, Storm Frequency, Duration and Intensity

North Slope Trends in Sea Level, Storm Frequency, Duration and Intensity Nels J. Sultan, Kenton W. Braun and Dempsey S. Thieman PND Engineers, Inc. Anchorage, Alaska, USA nsultan@pndengineers.com ABSTRACT The tide gage at Prudhoe Bay provides the only continuous long term direct measurement of sea level on the Alaska North Slope. In addition to trends in mean sea level, it provides a record of storm surges and setdowns. Analysis of the sea level time series from 1993 to 2010 reveals no statistically significant trends in relative sea level, storm frequency, intensity and duration. The return period of sea level extremes are estimated. KEY WORDS: sea level; storm intensity; storm frequency; climate change INTRODUCTION This paper presents analysis of sea level measured at the Prudhoe Bay tide gage. The purpose is to identify trends that may exist in relative sea level, and storm frequency, intensity and duration. The information is needed for coastal and offshore engineering design criteria, and for planning to account for potential climate change. Another application is that it provides context for field measurements of more limited time duration, allowing for better interpretation of the data. Sea level is an important design criteria for for ice, because a higher sea level allows ice runs to travel further inland and overtop higher structures. The arctic North Slope offers unique coastal and offshore challenges, including eroding shorelines, logistics and schedule constraints, sensitive arctic environmental issues, and limited ice and met-ocean data. Arctic climate change, including possible acceleration of environmental change is an issue potentially related to reduced ice cover and global climate change. Limited met-ocean data complicates the design of arctic shore protection and offshore structures. For example, the lack of long term wave buoy data prevents direct calculations of the wave height return period, and verification of numerical models. Ice characteristics, ocean currents and other important variables also lack long-term, continuous recorded measurements. No data is a common challenge to coastal and ocean engineers working in the arctic, resulting in reliance on numerical models for determining key design criteria. Airport. The Prudhoe Bay tide gage provides a record of sea surface elevation from 1993 to present (NOAA, 2010a). The tide record is useful for identifying storm surge and set-down events, and long term trends in relative sea level. The Prudhoe Bay tide gage is the only continuous long term direct measurement of sea level on the Alaska North Slope. In some ways it is the only continuous, long term direct measurement of any oceanographic variable on the Alaska North Slope. Relative sea level can be influenced by a number of factors besides astronomic tides, including storms surges and setdown, ground elevation changes (subsidence or uplift), the supply of sediment from rivers and global (eustatic) sea level changes. In arctic regions, glacial rebound is typically an important factor. Seasonal trends in weather and currents, and decadal cycles can also be significant. For example, during the strong El Nino of 1997-98 the sea level was elevated approximately 0.3 meters for a number of months at tide gages in the northeast Pacific. Local sea level can also be influenced by short duration infra-gravity waves, with a wave period of 1 to 2 minutes, although these are typically not included in tide gage records. Rare events such as tsunamis with a longer wave period are sometimes recorded by tide gages depending on the instrument technology. Global sea level rise projections are available from many sources and are the subject of some controversy. Useful guidance on planning and design for potential sea level rise at a specific site is included in a recent document from the US Army Corps of Engineers (2009). The tide range on the Alaska North Slope is small. Mean Higher High Water (MHHW) is at elevation +0.21 meters above Mean Lower Low Water (MLLW) datum. Mean Sea Level is at elevation +0.11 meters, MLLW. Storms on the North Slope result in setup or setdown much larger than the astronomic tides. The largest recorded water level at Prudhoe Bay from 1993-2010 was +1.47 m on August 11, 2000, and the lowest recorded water level was -0.91 m on October 10, 2006. Figure 1 plots the predicted (astronomic) tide level and the measured sea level time series, as an example. A set-down in sea level lasting approximately 2 days is shown (NOAA, 2010a). However, long term wind and sea level data are available near Prudhoe Bay. Wind data starting in 1974 is available from the Deadhorse Paper No. ICETECH10-155-R0 Sultan Page number: 1

Sea Level (m, MLLW) 0.3 0.25 0.2 0.15 0.1 5 0 5 0.1 0.15 0.2 Measured Sea Level vs. Predicted Astronomic Tide predicted measured 1/5/07 1/7/07 1/9/07 1/11/07 1/13/07 1/15/07 1/17/07 1/19/07 Figure 1. Sea Level Time Series at Prudhoe Bay Storm Setdown Because the continental shelf offshore from Prudhoe Bay is wide and flat, the storm surge or setdown at the shoreline is relatively large. Positive storm surges are caused by westerly storms. Westerly winds and waves create currents moving toward the east that are then deflected to the right by the Coriolis effect, onto the shallow continental shelf, pushing up the water at the shoreline. The opposite occurs with easterly storms. Easterly storms cause the currents to move toward the west, that are then deflected by the Coriolis effect to the right and offshore, moving the water away from shore and causing a drawdown in the water level. The prevailing winds on the North Slope are from the east, generally parallel to the shore, and easterly storms with a setdown are more common. However, westerly storms can be stronger. Because of the associated positive storm surge, westerly storms can result in larger wave heights and wave loads on shore protection structures in shallow water, where wave heights are depth limited. Analysis of the sea level record at Prudhoe Bay provides information on the storm frequency, direction, intensity and duration. Trends in the sea level parameters also provide insights into climate change. TECHNICAL DATA The Prudhoe Bay tide gage has been operated continuously by the National Oceanographic and Atmospheric Administration (NOAA) since 1993. The station was initially established in 1990. However, there are significant gaps in the data between 1990 and 1993 and some of the data was not verified (Kent, 2010). To simplify the analysis only hourly data between June 27, 1993 and April 30, 2010 are analyzed for this paper. The length of record is 16.8 years, and includes gaps totaling 28 days (%). The tide gage uses an acoustic sensor during the ice-free summer months, and a pressure sensor when the sea is covered in ice. The sensors are checked each year including checking the elevation relative to local bench marks. the solid fill causeway creates the possibility of local bias in sea level, because local wind and/or wave set-up may be higher on one side of the causeway than the other for easterly vs. westerly storm directions. However, the tide gage is located approximately 200 meters south of a gap in the causeway designed to allow water circulation. Therefore, no significant difference in sea level should exist on either side of the causeway near the tide gage for easterly vs. westerly storms. SEA LEVEL EXTREMES Table 1 lists ten highest and lowest sea level elevations recorded at the Prudhoe Bay tide gage. The daily maximum sea levels in Table 1 include all effects, such as astronomic tide, rather than filtering out the astronomic tide to derive the sea level anomaly primarily caused by storm events. The daily maximum and minimum sea levels are analyzed to identify extreme sea level events. Events are identified based on whether the sea level exceeds a threshold. The threshold is above 0.91 m for surge and below 0.46 m for setdown, which results in 21 surge events and 26 set-down events. Table 1. Prudhoe Bay Sea Level Extremes 1993 to 2010 [1] Surge Setdown No. Date (m, MLLW) No. Date (m, MLLW) 1 8/11/2000 1.47 1 10/10/2006-0.91 2 7/30/2003 1.36 2 12/4/2007-0.84 3 10/9/2002 1.26 3 11/25/2007-0.73 4 7/31/2008 1.24 4 12/31/1993-0.66 5 8/15/2002 1.21 5 1/16/1994-0.66 6 8/1/2008 1.20 6 10/7/1994-0.63 7 8/17/2002 1.18 7 11/30/2007-0.62 8 8/5/2003 1.13 8 12/25/1993-8 9 7/30/1993 8 9 10/25/1998-6 10 7/31/2004 8 10 2/7/2003-5 [1] Extremes sea levels include storm surge, astronomic tide and seasonal cycle. Sea level is in meters above MLLW. The data are fitted to a statistical distribution to estimate the return period of extreme sea levels. Figure 2 and 3 show the storm surge and setdown sea levels, with a Weibull distribution line of best fit. Table 2 lists the calculated return period and water level. The 25 year return period sea level is +1.5 m for a storm surge and -0.9 m for a storm setdown. Considering the length of record of 16.8 years, predictions of return period water levels exceeding a 25 year return period may not be reliable. 90% statistical confidence limits are shown in the figures. The tide gage records the sea level every 6 minutes with the data uploaded via satellite to the NOAA database. The data is verified monthly with quality control checks and made available for internet download on the CO-OPS web site (NOAA, 2010a). Additional information is included in a report by UNESCO on the status of arctic tide gages (Plag, 2000). The tide gage is located on the east side of the West Dock causeway, a 3,500 meter north-south causeway connected to shore. The location on Paper No. ICETECH10-155-R0 Sultan Page number: 2

Sea Level (m, MLLW) 2.5 1.5 2 4 6 8 10 20 40 60 80100 Return Period (years) Figure 2. Sea Level Surge Return Period Sea Level (m, MLLW) -0.2-0.4-0.6-0.8 - -1.2-1.4-1.6 Return Period (years) Data Weibull Distribution (k=1.40) 90% Confidence Interval, upper 90% Confidence Interval, lower 2 4 6 8 10 20 40 60 80100 Data Weibull Distribution (k=0) 90% Confidence Interval, upper 90% Confidence Interval, lower Figure 3. Sea Level Setdown Return Period Table 2. Extreme Sea Level Return Period Prudhoe Bay [1] Surge Setdown Years (m, MLLW) Years (m, MLLW) 2 1.11 2-9 5 1.25 5-0.69 10 1.34 10-0.78 25 1.45 25-0.89 50 1.54 50-0.97 100 1.61 100-5 [1] Extreme sea level includes storm surge, astronomic tide and seasonal cycle and is based on limited data (16.8 years). Extreme sea levels may not be reliable beyond a 25 year return period. Surge elevations and extreme sea levels are an important design parameter, although direct measurements of sea level during storm surges are not common. Previous investigators have used the elevations of lines of driftwood on the North Slope to infer the height of storm surges. Reimnitz and Maurer (1979) report a storm surge elevation at Prudhoe Bay of to 2.5 m, based on measurements of driftwood elevations on the mainland and barrier islands. They concluded that 1970 storm surge elevation was not equaled in the previous 90 to 100 years. Driftwood lines measured during the 1970 event indicated storm surge elevations along the North Slope shoreline varying from less than 1.5 m to greater than 2.5 m. MONTHLY MEAN SEA LEVEL Trends in the sea level (sea level rise) are analyzed by first calculating the mean sea level for each month between June 1993 and April 2010 from the hourly time series. Figure 4 plots the mean sea level for each month of the year, showing the seasonal cycle. Included in Figure 4 are error bars at one standard deviation. The monthly mean sea level is highest during the summer (0.32 m, MLLW in August) and lowest in winter (-2 m, MLLW in March). The seasonal variation is subtracted from the monthly time series and the results are shown in Figure 5. Linear regression applied to the mean sea level from 1993 to 2010 shows a small trend of +0.23 m per 100 years, but with a 95% statistical confidence interval of +/- 0.26 m per 100 years. The correlation co-efficient is less than 2%. The results are interpreted to mean essentially no statistically significant trend in relative sea level at Prudhoe Bay, based on measurements of the monthly mean with the seasonal cycle removed. The methodology is described in more detail by NOAA (2009) in a report analyzing sea level variations in the United States. NOAA researchers also have concluded that there is essentially no significant trend in local sea level at the Prudhoe Bay tide gage, based on analysis of a longer length of record from the tide gage at Prudhoe Bay. A trend of -0.26 ± 2.47 mm/yr. was found for the time period 1990-2010 (Kent, 2010). Sea level trends have been analyzed at a number of other tide gages in the arctic, although the only tide gage data in the Beaufort Sea spanning more than one year appears to be at Prudhoe Bay and at Tuktoyaktuk. The status of the arctic tide gages worldwide and recommendations for improvements are summarized in Plag (2000). At Tuktoyaktuk on the Canadian Beaufort Sea a statistically significant sea level trend of +0.35 m/100 years ± 0.11 is reported by Manson and Solomon (2007). The Tuktoyaktuk sea level data analyzed is from 1962 to 1997 with gaps. The Tuktoyaktuk site was re-established in 2003 as a permanent tide gage station operated by the Canadian government. METERS, MLLW 0.4 0.3 0.2 0.1 0.1 0.2 Seasonal Cycle Mean Sea Level Prudhoe Bay Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 4. Seasonal Cycle Mean Sea Level - Prudhoe Bay Paper No. ICETECH10-155-R0 Sultan Page number: 3

METERS, MLLW 1.5 Monthly Mean Sea Level Prudhoe Bay +0.23 Meters/100 years, (+/ 0.26) analysis of storm variability along the California coast based on data from the San Francisco tide gage. One advantage of using sea level data is that the record is naturally smoothed, without the temporal and spatial variability of other parameters, such as windspeed. For the Prudhoe Bay sea level data, a threshold of +0.61 m is used to define a positive surge (associated with westerly storms), and -0.30 m is the threshold for a negative setdown (associated with easterly storms). 1.5 1993.0 1995.0 1997.0 1999.0 200 2003.0 2005.0 2007.0 2009.0 201 Figure 5. Monthly Mean Sea Level Trend 1993-2010 - Prudhoe Bay (Seasonal cycle removed) SST. Anomoly Nino Region Degree C 3.0 3.0 Sea Surface Temp. Anomoly (El Nino/La Nina) 1993.0 1995.0 1997.0 1999.0 200 2003.0 2005.0 2007.0 2009.0 201 Figure 6. Oceanic Nino Index - Sea Surface Temperature Anomaly (NOAA 2010b) It is interesting to note the sea level anomaly during parts of 2003-04, and 2007-08 evident in Figure 5. The monthly mean sea level was elevated approximately 0.25 m for 2003-04, and lowered approximately 0.3 m for 2007-08. The persistent sea level anomaly seems to be more than random fluctuations in the data. The anomaly does not appear to be correlated with the Oceanic Nina Index (Figure 6). Sea level can fluctuate with ocean current and temperature changes associated with El Nino/La Nina events. The strong 1997-98 El Nino event, when sea level was raised approximately 0.3 m in the northeast Pacific Ocean, does not appear to have influenced sea level at Prudhoe Bay. However, storm frequency in the southern Beaufort Sea is reported to have increased during El Nino years based on analysis of wind data from 1973-95 (Hudak and Young, 2002). The cause of the persistent sea level anomalies lasting on the order of 1 year (Figure 5) have not been determined. Further investigation such as correlations with wind data, ice cover, ocean currents or oceanographic data may provide insights. STORM VARIABILITY Trends in storminess have been investigated by first subtracting the predicted (astronomic) tide from the measured tide. The remaining non-tide residual time series is then analyzed to identify anomalies in the sea level. Sea level data is commonly used to investigate storm variability and intensity. See for example Bromirski, et al (2003) for Each surge or set-down event is analyzed to identify the water level peak-over-threshold (in meters), and the duration (in days). The peak water level is a proxy for the storm intensity. A third parameter called the storm magnitude is calculated by multiplying the elevation of the sea level anomaly by the duration. This gives a measure of the total energy of the event with units of meters days. For example, an event with the sea level averaging meters above the astronomic tide, lasting 3 days, would have a storm magnitude of 1.5 m days. A similar storm power statistic has been used by others in the analysis of wind data (Atkinson, 2005). The storm variability is shown in plots in the Appendix. Figure A1 shows the peak water level for each event from 1993 to 2010. Figure A2 plots the duration that each water level anomaly exceeded the threshold. Figure A3 plots the storm magnitude. The thresholds were chosen arbitrarily to provide a reasonable number of events. The record was not further smoothed to combine events that are close together into a single event. Each up and down-crossing past the threshold is used to define an event, resulting in 68 surge events (westerly) and 491 set-down events (easterly). A more sophisticated statistical analysis with variable thresholds and combining consecutive storms within a defined time spacing may provide additional useful information. However, the advantage to using a single threshold criterion is that it avoids more arbitrary decisions as to what constitutes a single storm event. The larger number of events also, arguably, presents a finer resolution of the data, which can be combined by eye into a larger single events in a qualitative evaluation of the time series. Linear regression analysis of the storm variability data (Figures A1 to A3) shows no significant statistical trends in storm intensity, duration, frequency and magnitude. The frequency of events is analyzed based on the number of events per year. The distribution of events between easterly (setdown) and westerly (surge) also does not appear to have changed. No. Events 50 45 40 35 30 25 20 15 10 5 0 setdown easterly surge westerly Water Level Anomaly Events 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 Figure 7. Sea Level Anomaly Frequency (sea level >0.61m for surge, sea level <0.30m for setdown) Paper No. ICETECH10-155-R0 Sultan Page number: 4

CONCLUSIONS AND RECOMMENDATIONS There are no significant trends in the observed sea level, storm intensity and duration based on analysis of the tide gage data at Prudhoe Bay. The length of record analyzed is limited to 16.8 years and therefore definitive conclusions cannot be made about regional climate change. The study of trends in storm frequency and intensity would be improved with analysis of the long term record of wind speed and direction at the Deadhorse airport at Prudhoe Bay. Additional work is recommended to correlate the met-ocean trends in the sea level time series with trends derived from wind data. As arctic energy exploration and development continues it will involve new challenges in the coastal, nearshore and offshore environments. Innovative solutions and improved data collection are required. The lack of continuous, measured oceanographic data in the region is a major shortcoming, resulting in an over-reliance on numerical model output to determine key oceanographic design variables, such as sea level (storm surge) and wave heights. The authors suggest that a high priority should be collecting long term, continuous measurements of sea level. Measurement of sea level from a regional network of tide gages would be relatively simple and low cost, and would provide a wealth of useful information. In addition to trends in sea level, the data would allow calculation of the return period of storm events and extreme water levels, verification/calibration of oceanographic numerical models, and the study of climate change in general. Also important is determining the variability of storm surge elevations along the North Slope shoreline. Sea level during storm surges can vary substantially at different areas as indicated by studies of driftwood elevation on the North Slope (Reimnitz and Maurer, 1979). A Joint Industry Project to collect continuous, long term oceanographic data is recommended. Properties that should be measured include sea level, waves, currents, and ice, including ice run events. ACKNOWLEDGEMENTS The authors thank ExxonMobil for whom much of the work was done, Ajay Sampath for assistance with the data analysis, and Jena Kent with the National Oceanographic and Atmospheric Administration for valuable discussions. REFERENCES Atkinson, D.E., Observed storminess patterns and trends in the circum- Arctic coastal regime, Geo-Marine Letters, 25(2-3), June 4, 2005. Bromirski, R.E., Flick, R.E. and Cayan, D.R., Decadal Storminess Variability Along the California Coast: 1858-2000, Journal of Climate, 16(6): p. 982-993, 2003. Hudak, D.R. and Young, J.M.C., Storm Climatology of the Southern Beaufort Sea, Atmosphere-Oceans, 40(2), p. 145-158, 2002. Kent, J., National Oceanographic and Atmospheric Administration, personal communication, 2010. Manson, G.K., and Solomon, S.M., Past and Future Forcing of Beaufort Sea Coastal Change, Atmosphere-Oceans, 45 (2) p. 107 122, 2007. NOAA, Sea Level Variations of the United States 1854-2006, National Oceaongraphic and Atmospheric Administration, National Ocean Service, Center for Operational Oceanographic Products and Services, Technical Report NOS CO-OPS 053, 2009. NOAA, Sea Levels Online,National Oceaongraphic and Atmospheric Administration, 2010a. web addresss: <http://tidesandcurrents.noaa.gov/sltrends/sltrends.html>. NOAA, Oceanic Nino Index Cold and Warm Episodes by Season, 2010b. web address: <http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoy ears.shtml>. Plag, H.P., Arctic tide gauges: a status report, Intergovernmental Oceanographic Commission (of UNESCO), 30 pp., 2000. Reimnitz, E. and Maurer, D.K., Effects of Storm Surges on the Beaufort Sea Coast, Northern Alaska, Arctic, 32(4), p. 329-344, Dec. 1979. US Army Corps of Engineers, Water Resource Policies and Authorities Incorporating Sea-Level Change Considerations in Civil Works Programs, Circular No. EC-1165-2-211, July 1, 2009. Paper No. ICETECH10-155-R0 Sultan Page number: 5

APPENDIX: Prudhoe Bay Tide Gage Data Trends in Storm Frequency, Duration and Intensity. (Storm Surge/Setdwon, meters 1.5 1.5 storm surge westerly storm setdown easterly Water Level Above/Below Astronomic Tide (>0.61 m Surge, < 0.30 m Setdown) 5/11/93 5/11/94 5/11/95 5/10/96 5/11/97 5/11/98 5/11/99 5/10/00 5/11/01 5/11/02 5/11/03 5/10/04 5/11/05 5/11/06 5/11/07 5/10/08 5/11/09 5/11/10 Figure A1. Peak Water Level Surge or Setdown - Prudhoe Bay Tide Gage 1993-2010 Storm Duration (Days) 4.0 4.0 6.0 8.0 1 1 14.0 storm surge westerly storm setdown easterly Water Level Anomaly Storm Duration (Days) 5/11/93 5/11/94 5/11/95 5/10/96 5/11/97 5/11/98 5/11/99 5/10/00 5/11/01 5/11/02 5/11/03 5/10/04 5/11/05 5/11/06 5/11/07 5/10/08 5/11/09 5/11/10 Figure A2. Water Level Anomaly (Storm) Duration - Prudhoe Bay Tide Gage 1993-2010 Storm Energy (Meter Days) 4.0 3.0 3.0 4.0 5.0 6.0 storm surge westerly storm setdown easterly Water Level Anomaly Storm Magnitude (Meter Days) 5/11/93 5/11/94 5/11/95 5/10/96 5/11/97 5/11/98 5/11/99 5/10/00 5/11/01 5/11/02 5/11/03 5/10/04 5/11/05 5/11/06 5/11/07 5/10/08 5/11/09 5/11/10 Figure A3. Water Level Anomaly (Storm) Magnitude - Prudhoe Bay Tide Gage 1993-2010 Paper No. ICETECH10-155-R0 Sultan Page number: 6