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Watching the Tides: The 100th Anniversary of the Portland, Maine Tidal Station
Inspecting Sea Level Trends
One of the benefits of longer-term water level records is the ability to inspect sea level trends. For annualized data from 1912 through the end of 2011, the Portland tide gauge has shown an increase in mean sea level of approximately 1.9 mm per year (R2 = 0.75), or about 7.5 inches over the past 100 years (Figure 2). This rate has mirrored global ocean sea level changes of 1.7 mm per year over about the last century, according to the IPCC (2007). Most of these global measurements have been derived from tidal station data until the mid-1990s.
Since 1993, satellite altimetry has been used to monitor global ocean sea level changes, as shown by the University of Colorado's Sea Level Research Group. This data shows that global sea level change rates have slightly increased to over 3 mm per year, or about 1 foot per century, based on data from 1993 to 2012 (Figure 3, Nerem and others, 2010), including seasonal variability. During the same time period, however, sea level at the Portland tidal station rose at a rate of over 4 mm per year, or 1.4 feet per century (Figure 4). It is important to note that seasonal variability was left in these annualized sea levels over this shorter time period, resulting in a relatively low R2 value (0.32). Seasonal variability will be discussed below.
Seasonal Sea Level Variability
In general, sea levels along the Maine coastline also undergo seasonal variability, driven mostly by weather patterns. Typically, sea levels are lower in the winter months than the summer months, as shown in Figure 6. This figure shows the range of the mean sea level data values (by month) in blue dots from 1912 to 2011. It also shows each monthly average as a red line.
It is clear that overall average monthly mean sea levels tend to be lower, but slightly more variable, during the winter months (December to March). This is mainly due to a strong northwesterly (offshore) flow during the winter months, which basically "blows" water away from the coast, and lowers sea levels (Sweet and Zervas, 2011). However, the increased winter month variability (larger range of low to high values) is due to the influence of more frequently occurring storm events and storm surges, which can elevate or depress water levels significantly (Wood, 2001a).
The highest average monthly mean sea level measurements during the entire data period (1912-2011) are labeled in Figure 6. The two highest values are from the months of February and March 2010. In February, a series of northeasters piled water up along the coast due to onshore winds and surges in the 1-2-foot or higher range for extended periods of time, elevating the overall water levels (both low and high tides) significantly for that month (Figure 7). During this month, the highest surge was over 4 feet above the predicted water level, occurring during a strong northeaster on February 26, 2010 (Figure 8); this was one of the highest measured surges in the past several decades, but luckily coincided with a falling tide. Higher than normal sea levels (both highs and lows) continued into March 2010 due to another series of storms, resulting in the second highest monthly mean water level for the entire record at the tidal station. The higher tides also had a dramatic geological impact at Popham Beach State Park (Dickson, 2011).
During the summer and early fall months (April to November), sea levels are typically higher, with less variability. This is due to a relatively consistent onshore flow that typically develops during warmer weather, which piles water along the coast. This is typically experienced as the summer "sea breeze" that forms almost each day due to temperature gradients between the land and the ocean. The variability during these months is generally less due to less influence of storm events. At the Portland tidal station, June typically appears to be the month with the highest average monthly mean sea level, as denoted by the highest value along the red "average" line (Figure 6).
Interestingly, during the summer of 2009, mean sea levels along the entire eastern coast of the United States were higher than normal in the months of June and July (which typically correlate with Maine's highest mean sea levels). The NOAA CO-OPS prepared a special report (Sweet and others, 2009) regarding this anomaly. During this time, predicted tides were being exceeded by anywhere from half-a-foot in New England, to near 2-feet along the mid-Atlantic seaboard. NOAA attributed this anomaly to steady and persistent northeast winds, combined with a weakening of the Florida Current transport (a current leading to the Gulf Stream), and timing with astronomically high tides or perigean spring tides (Wood, 2001b). Additional analysis found tidal surges and northeasters are more common during strong El Niņo years, including the winter of 2009-2010 (Sweet and Zervas, 2011).
Highest Monthly Water Levels
Using Tidal Station Data to Analyze Flooding
In Portland Harbor, it is locally known that "flood stage" occurs when water levels meet or exceed 12 feet Mean Lower Low Water (MLLW), as described by Cannon and others (2009). This means that coastal flooding is expected once water levels reach this elevation. Using this elevation as a baseline, a tool developed by the NOAA CO-OPS called the Inundation Analysis Tool (IAT) can be used to look at the frequency of past flooding events, and how potential sea level rise may impact the frequency of those events. This tool can output the number of events that met or exceeded a given elevation, in addition to the duration (in hours) that the given elevation was met or exceeded.
Data indicates that in 2011, flood stage was exceeded 11 times, for a duration of about only 8 hours. This represented about 1.6% of all high tides that occurred in 2011, meaning only about 2% of the tides that occurred in 2011 exceeded the flood stage. However, if sea level rose 1 foot (0.3 m), the frequency of flooding would increase to 98 times, for a duration of 141 hours total, and account for roughly 14% of all high tides. In a 2 foot SLR scenario, these numbers increased to 281 times flooding would occur, for a duration of 570 hours, and almost 40% of high tides. It is important to note that these estimates are based simply on a repeat of the 2011 tide and storm surge history.
This kind of analysis is vital to understanding how sea level rise may impact the frequency of static flooding. Much of this data was presented to the City's Transportation, Energy, and Sustainability Committee by MGS scientists in February 2012 as part of a Vulnerability Assessment completed by MGS for the City, highlights of which are available on the City of Portland website.
Tidal stations are invaluable in helping to understand short term and seasonal variation of water levels, impacts of storms, and longer term sea level trends. Continued operation of the Portland, Maine tidal station will help ensure sound scientific planning in response to flood hazards now and into the future. Happy 100th Birthday Portland, Maine Tidal Station 8418150!
Cannon, J.W., Bogden, P.S., Morse, R.R., Ogilvie, I.S., and Shyka, T.A., 2009, The development of a coastal flood nomogram for southwest coastal Maine and the seacoast of New Hampshire: Eastern Region Technical Attachment No. 2009-01, January, 2009.
Dickson, S.M., 2007, Portland tide gauge and waterfront: Maine Geological Survey March 2007 Site of the Month.
Dickson, S.M., 2011, Setting the stage for a course change at Popham Beach State Park: February 2011 Site of the Month.
Intergovernmental Panel on Climate Change, 2007, IPCC Fourth Assessment Report: Climate Change 2007.
Nerem, R. S., Chambers, D., Choe, C., and Mitchum, G.T., 2010, Estimating mean sea level change from the TOPEX and Jason altimeter missions: Marine Geodesy, v. 33, no. 1 supp. 1 (2010), p. 435.
Sweet, W., Zervas, C., and Gill, S., 2009, Elevated East Coast sea levels anomaly: July - June 2009: NOAA Technical Report, NOS CO-OPS 051.
Sweet, W.V. and Zervas, C., 2011, Cool-season sea level anomalies and storm surges along the U.S. East Coast: Climatology and comparison with the 2009/10 El Nino: Monthly Weather Review, v. 139, p. 2290-2299. DOI 10.1175/MWR-D-10-05043.1.
Wood, F.J., 2001a, Tidal Dynamics, Volume II: Extreme tidal peaks and coastal flooding: The Coastal Education & Research Foundation [CERF], West Palm Beach, Florida, 387 p.
Wood, F.J., 2001b, Tidal Dynamics, Volume I: Theory and analysis of tidal forces: The Coastal Education & Research Foundation [CERF], West Palm Beach, Florida, 326 p.
Camill, P., Hearn, M., Bahm, K., and Johnson, E., 2012, Using a boundary organization approach to develop a sea level rise and storm surge impact analysis framework for coastal communities in Maine: Journal of Environmental Studies and Sciences, DOI 10.1007/s13412-011-0056-6.
Colgan, C.S., 2008, The effects of climate change on economic activity in Maine: Coastal York County Case Study: Maine Policy Review, v. 17, no. 2, p. 66-79.
Kirshen, P., Merrill, S., Slovinsky, P., and Richardson, N., 2011, Simplified method for scenario-based assessment adaptation planning in the coastal zone: Climate Change, DOI 10.1007/s10584-011-0379-z.
Pugh, D.T., 1987, Tides, surges and mean sea-level: A handbook for engineers and scientists: John Wiley and Sons, New York, 472 p.
Shepard, C.C., Agostini, V.N., Gilmer, B., Allen, T., Stone, J., Brooks, W., and Beck, M.W., 2012, Assessing future risk: quantifying the effects of sea level rise on storm surge risk for the southern shores of Long Island, New York: Natural Hazards, v. 60, p. 727-745.
Strauss, B.H., Ziemlinski, R., Weiss, J.L., and Overpect, J.T., 2012, Tidally adjusted estimates of topographic vulnerability to sea level rise and flooding for the contiguous United States: Environmental Research Letters, v. 7, 12 p., DOI 10.1088/1748-9326/7/1/014033.
Tebaldi, C., Strauss, B.H., and Zervas, C.E., 2012, Modelling sea level rise impacts on storm surges along U.S. coasts: Environmental Research Letters, v. 7, 11 p., DOI 10.1088/1748-9326/7/1/014032.
Website by Peter A. Slovinsky.
Originally published on the web as the May 2012 Site of the Month.
Last updated on April 30, 2012
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