Introduction

Sea level fluctuations due to winds have received considerable attention recently because of the flooding that often ensues. However, some organisms in coastal ecosystems have evolved to depend upon intermittent flooding to bring prey, nutrients, sediment etc. Others are negatively affected.  Sea levels have been rising and this will change the frequency and duration to flooding. Some places will be affected more than others. Most recent work has focused on the return frequency of catastrophic events. Here we exploit the available data to assess what the more subtle effects are likely to be and whether we can detect the influence of changing wind patterns. 

Discussion

We have analyzed sea level observations obtained at the eastern and western end of LIS. The tidal effects were extracted using NOAA tidal predictions and the statistics of the anomalies summarized. The theory of Garvine (1985) and Wong (1990) predicts that the amplitude of anomalies driven by winds from the northeast will be large at the western end of LIS compared to those in the east and the distribution of variance in the de-tided records by month of the year is consistent with that prediction. We show that the frequency distribution of the DJF anomalies is significantly impacted by sea level rise, particularly at New London and in the summer at Willets-Kings point. De-trending the anomalies largely removes this effect. At Willets-Kings Point station, the DJF percentile bands narrow from 1960 to 2015. This is consistent with a reduction in the magnitude of the along-Sound wind stress driving the local setup in the Sound.

(a) shows the distribution of the de-trended December to February (DJF) water level anomalies at New London in five year time-bins. The blue lines bound the water level range containing 99% of the observations. The green, red, cyan and magenta lines contain the 98%, 95%, 80% and 50% of the observations, respectively. (b) shows the same statistics for the June-August interval. Note the change of scale.

a) shows the distribution of the de-trended December to February (DJF) water level anomalies at Willets-Kings Point computed in the same manner as in the figure above. (b) shows the same statistics for the June-August interval

Since the effects of wind on sea level in the winter are so large in the western Sound, the impact of sea level rise is not very significant. In the summer and in the eastern Sound, the impact is large and clearly observable in the anomaly data distributions. The examples of the impacts on the change in the frequency of flooding that the mean sea level change has caused are quite significant. The significance scales with the ratio of the magnitude of the change in mean level to the standard deviation in the wind driven anomalies. When the distribution is narrow, a small change in the mean level can lead to a large change the frequency of flooding. We show the empirical frequency distribution is narrow in the summer and in the eastern Sound. Coastal ecosystems that are sensitive, either positively or negatively, to changes in flooding frequency will be most impacted in the eastern Sound.  
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References

Garvine, R.W. (1985). A simple model of estuarine subtidal fluctuations forced by local and remote wind stress. J. Geophys. Res., 90(C6), 11,945–11,948.

Foreman, M. (1980). The harmonic analysis of tidal model time series. Adv. Water Resources. 12. 109-120.

O'Donnell, J. and J.E.D. O'Donnell (2012). Coastal Vulnerability in Long Island Sound: The Spatial Structure of Extreme Sea Level Statistics. Oceans, 2012, pp.1-4, 14-19 Oct. 2012.

Wong, K. -C., (1990). Sea level variability in Long Island Sound. Estuaries, 13, 362-372.

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