DECADAL CHANGES IN VERY LARGE WAVE OCCURRENCE ON THE US PACIFIC COAST

Richard J. Seymour1, Fellow, ASCE

The occurrence of severe wave events (Hm0 >6m for 24 hours) is examined from 1984-2007 along the Pacific Coast. This coast is divided here into four regions and the storminess of each is considered, as well as the occurrence of storms large enough to occur in two or more regions simultaneously. The resulting statistics are compared between two 12 year periods, 1984-1995 and 1996-2007. The latter period has approximately triple the number of large storms as the former in the three northernmost regions. The potential influences of ocean water temperature and the Southern Oscillation (El Nino/La Nina conditions) are examined and found to have no significant correlation with the remarkable differences between the two time periods. One storm was found to result in an Hm0 greater than the study threshold over all four regions, spanning the entire West Coast on a single day.

INTRODUCTION

A number of papers have addressed temporal changes in the wave climate within regions of the US Pacific Coast and the possible relationships of climatological conditions with those changes (Seymour et al, 1984, Seymour, 1996, Seymour, 1998, Allan and Komar, 2000 and 2001, Seymour, 2002, Bromirski et al, 2003 and 2005, Graham, 2005, Adams et al, 2008.) Other papers have addressed the impacts of specific extreme wave events or seasons containing such events (Earle et al, 1984, Komar, 1986, Komar, 1998, Storlazzi et al, 2000.) Seymour (1996) established a list, based upon hindcasts and observations of extreme wave events (Hirio exceeding 6m) along the Pacific Coast for the period 1900 to 1995, without relating those events to specific regions. During the past two decades the density of continuous wave measurement stations has increased sufficiently to allow reasonably uniform consideration of the entire coastline between the Canadian and the Mexican borders. This paper extends the identification of storm events exceeding 6m over a 24 year interval from 1984 through the winter of 2007. The Pacific Coast is divided into four regions for this study and particular attention is given to extreme wave events that occur across multiple regions.

Because the wave climate has been shown in a number of the referenced papers to vary substantially at the decadal scale, attempts have been made to relate these changes to quasi-decadal climate change factors. Among the climatological indices considered are SOI - Southern Oscillation Index - a global scale climate shift associated with the El Nino/La Nina (ENSO) events (http://www.jisao.washington.edu/data/walker/), PDO - Pacific Decadal Oscillation - an index of Pacific Ocean sea surface temperature change (http://www.iisao.washington.edu/pdo/PDO.Iatest) and MEI - Multivariant ENSO Index - which combines sea-level pressure, zonal and meridional

1 Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, La Jolla, CA, 92093-0214, USA

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components of the surface wind, sea surface temperature, surface air temperature, and total cloudiness fraction of the sky with SOI to characterize the ENSO. (http://www.cdc.noaa.gov/peopIe/klaus.wolter/MEI/mei.htmI).

The correlation of ENSO indices with changes in wave climate are investigated in Seymour (1996,1998), Allan and Komar (2000,2006), Graham (2005), and those of the PDO index in Adams et al (2008). Correlations between large scale atmospheric pressure anomalies and NTR (non-tidal residue, a proxy for wave storminess derived from tidal records) are studied in Bromirski et al (2003). The MEI, the SOI and the PDO are evaluated in this paper as predictors of the occurrence of extreme wave events.

PACIFIC COAST WAVE DATA

Period of the Study

1984 was the earliest year in which adequate wave measurement data were available for the whole coast. This allowed the data set to be divided into two 12 year periods - 1984 to 1995 and 1996 to 2007. In addition, because all of the extreme events occurred during the traditional winter - November through April - the years referred to in the paper describe the winter season beginning in the Autumn of that year and extending into the Spring of the following year.

Data Sources

Wave measurement buoys are maintained by two agencies along the US Pacific Coast - the National Data Buoy Office (NDBC) of the National Oceanic and Atmospheric Administration (NOAA) and the Coastal Data Information Program (CDIP) operated by the Scripps Institution of Oceanography (SIO) at the University of California San Diego (UCSD). The buoy locations used in this study are shown in Figure 1. Because buoy availability is not constant, the largest number of buoys representing deep water and unsheltered conditions was selected - a total of 27 instruments. Approximately 2.4 million records of hourly data including Hm0, peak period, Hmax and, in some cases, mean direction were downloaded from their respective data repositories (http://www.ndbc.noaa.gov/. and http://cdip.ucsd.edu/). These were averaged over each day to yield approximately 100,000 records, constituting the data set for this study. The coastline was divided into four regions as shown in Figure 2 and the data from buoys within that region were segregated by region.

Extreme Event Identification

The problems introduced by two buoy types and by varying data availability among stations were alleviated by selecting the maximum daily average Hm0

from each region to represent that region. Whenever this significant height exceeded 6m, that day marked the beginning of an extreme event. That storm continued until the average Hm0 for the day was reduced below the 6m value. In a few instances, the Hm0 was below the threshold by a very small amount for

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Figure 1. Wave measurement buoys utilized in this study

one day and then was followed by a day or more of above threshold values. In these cases the storm interval was judged to bridge the slightly-below-threshold day. If adjacent regions were above the threshold on the same day, this would also be considered a multiple region storm. There were many instances of storms propagating down the coast such that a region would achieve threshold a day or so later than its upcoast neighbor. This occurrence was considered to be two separate regional storms. The maximum measured wave height in the region, or multi-region area, during the period of the storm was identified with that storm as a separate statistic.

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EXTREME WAVE OCCURRENCE

Regional Storms

A summary of the regional extreme wave events is shown in Table 1 and the annual occurrence is shown graphically in Figure 2. It is immediately obvious that the second (1956-2007) epoch was substantially stormier that the previous 12 years. The Washington and Oregon regions are shown to have substantially more extreme waves than the California regions, but that Northern California far exceeds the storminess of Southern California. Figure 2 also emphasizes the extreme storminess of the winter season beginning in 1997.

1985 1990 1995 2000 Winter Season Beginning in Year

2005

Figure 2. The number of extreme wave events in annual winter seasons in each region. The vertical line separates the 12 year periods.

Multi-Regional Storms

As previously defined, multi-region storms are those adjacent regions that exceed threshold simultaneously for at least one day. The occurrence of these large storms is shown by epoch in Table 2 and the annual variation is shown in Figure 3. From Table 2 it can readily be seen that the later epoch not only produced many more over-threshold events, but also that these storms were substantially larger in geographic extent - almost 5 times as many multi-regional storms - including one that occurred in all four regions simultaneously.

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Table 1. Winter

Region

Washington Oregon No. California So. California

Total

Large Events by Region

1984-1995

11 12 4 2

29

1996-2007

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94

Table 2. Winter Season Multi-Regional Large Wave Events

Number of Regions Involved Epoch 1 2 3 4 Total

1984-95 15 6 1 0 22

1996-07 19 20 13 1 53

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The December 16, 2002 Super Storm

The atmospheric pressure low that forced this storm is shown in Figure 4. The entire dark central area is at or below 980 mb, which is typically the lowest pressure designated by NOAA on weather maps. The long dimension of this core is about 3000 km which is more than adequate to generate fully developed seas in the fetch on the southern edge that would be directed at the West Coast. The maximum height observed in this storm was 11.1m and occurred in the Oregon region. The Super Storm occurred over an area larger than any observed since the inception of these observations, although it did not achieve the highest waves. The largest recorded wave height in this study was 14m and occurred during the storm of March 3, 1999, again in Oregon.

CORRELATIONS AMONG CLIMATE INDICES AND EXTREME WAVES

Climate Index Variation

The MEI (Multivariate ENSO Index) is scaled such that a value greater than 0.5 represents a moderate El Nino and greater than 1.0 a strong El Nino. Similarly, negative values represent moderate or strong La Ninas. Papers cited in the introduction suggest that MEI is positively correlated with Southern California wave climate (that is, strong El Ninos produce stormier winters) and, to a lesser degree, negatively correlated with the wave climate in Oregon. Adams et al (2008) suggests stormier winters in Southern California when the PDO (Pacific Decadal Oscillation) is greater (warmer) than its long term average. The variations in the winter season mean values of these two indices and the SOI (Southern Oscillation Index), the traditional indicator of El Nino/La Nina conditions, over the period of this study are shown in Figure 5.

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Figure 4. The December 16, 2002 condition of the atmospheric pressure low that forced the storm of that date, resulting in significant wave heights exceeding 6m from Canada to Mexico.

1990 1995 2000 Year Winter Season Begins

2005

Figure 5. Winter-season-averaged values of climate indices compared to their long term averages. Dashed lines are mean values over a 12 year period.

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Correlation of Climate Indices with Storm Occurrence

No significant correlations were found between the winter-season-averaged climate indices, MEI, PDO, and SOI and the number of large wave events during that winter. Although the annual values had no predictive value for storminess, it is interesting to consider the mean values over the two 12 year periods. During 1984-95, the mean MEI value was about at the moderate El Nino level, while in the 96-07 period it was essentially neutral. The SOI, as expected, performs as the negative of the MEI. The ocean temperature index, PDO, is above its long term average during 1984-95 and at its long term average for the 1986-2007 period. This tends to contradict the suggestion in Adams et al (2008) that warmer water produces bigger waves. Seymour et al (1984) shows six storms in Southern California exceeding 6m significant height in the winter of 1982 (although the duration requirement was only 9 hours rather than the 24 hour requirement in the present paper.) These observations, coupled with high correlations between El Ninos and hindcasts for the period since 1900, led to the assumption that El Ninos would provide an effective predictor of winters with many large wave events (at least in Southern California.) The average value of MEI during the 1982 winter was 2.62 and the PDO was 0.7.In comparison, the maximum average winter value of MEI in the 1984-2007 period occurred during the winter of 1997 when it reached 2.41 with a PDO value of 1.3. Thus, both winters were marked by strong El Ninos and warmer than average water. In strong contrast with the 1982 winter, there were no extreme wave events in Southern California in the 1987 winter, although there was one in Northern California and Oregon, and three in Washington. The only extreme events in Southern California during this study occurred in the winters of 1988, 1995, 2002 and 2007. The MEIs were -1.28, -0.51, 0.98 and -1.23 and the PDOs were -0.56, 0.45, 1.56 and -0.9, respectively. So, only the single event of the winter of 2002 fitted the earlier model of correlation between strong El Ninos and warm water with big waves in Southern California.

The maximum observed wave height during the storms in this study is one method for rating their relative severity. Figures 6 and 7 show the relationships among maximum height, MEI and PDO for the two time periods.

In the 84-95 period, water warmth appears to have been a more important parameter in determining both the number and the magnitude of the storms. The larger number of storms in the strong El Nino regime compared to the La Nina events is consistent with the preponderance of El Nino conditions during this period, as shown in Figure 5. In the 1996-2007 period, there is no significant influence of either water temperature or climate phase on the numbers of storms.

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Conc lus ions

There was a distinct and significant difference in the large wave event regime between the two twelve year long periods studied here. The number of events

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exceeding threshold tripled between the first and the second period and the number of geographically large, multiple region storms greatly increased.

In the first 12 year period, warmer water appeared to produce more storms. More storms occurred during El Nino than La Nina events, but not greater than would have been predicted by chance, given the relative times these conditions existed. In the second 12 year period there was no significant difference between the numbers of storms produced during El Nino or La Nina events, nor was there any significant difference attributable to water temperature. The December 2002 storm appears to have established a modern record for geographic extent on this coast. There was obviously a radical change in the weather patterns in the North Pacific between these two periods. The resulting change in the occurrence of very large wave events along the West Coast of the United States cannot be explained by the periodic variation in mean water temperature or in the Eastern Pacific phenomena that drive the El Nino/La Nina conditions. The record is far too short to infer that this is a climate change trend.

Acknowledgements

This research was funded by the United States Army Corps of Engineers and the California Department of Boating and Waterways.

References

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Graham, N.E. 2005. Coastal impacts of North Pacific winter wave climate variability: the Southern California Bight and the Gulf of the Farallones. Calif. Energy Comm. CEC-500-20005-018, 34pp. Komar, P.D. 1986. The 1982-83 El Nino and erosion on the coast of Oregon. Shore and Beach. 66 (3), 33-41. Komar, P.D. 1998. The 1997-98 El Nino and erosion on the Oregon Coast. Shore and Beach, 66(3), 33-41. Seymour, R.J. 1996. Wave climate variability in Southern California. J. of Waterway, Port, Coastal, and Ocean Engineering, 122(4), 182-186. Seymour, R.J. 1998. Effects of El Nino on the West Coast Wave Climate. Shore & Beach, 66(3), 3-6. Seymour, R.J. 2002. The Influence of Global Climate Change on Extreme Wave Occurrence on the West Coast of the United States, Proc, 28th Int. Conf. on Coastal Engineering, 1,52-60. Seymour, R.J., R.R. Strange III, D.R. Cayan, and R.A. Nathan. 1984. Influence of El Ninos on California's wave climate. Proc. 19th Int. Conf on Coastal Engineering, 1, 577-592. Storlazzi, CD. and G.B. Griggs. 2000. The influence of El Nino-Southern Oscillation (ENSO) events on the evolution of central California's shoreline. Geol. Soc. ofAmer. Bull. 112( 2), 236-249.