Storm Surge Almanac for Southwestern British Columbia:
Fall/Winter 2007-2008
Draft
Scott Tinis
Institute of Ocean Sciences, Sidney, BC
1. Overview
This document provides a summary of the marine environment of southern British Columbia as it
relates to the upcoming 2007/08 winter storm season. Since the risks of exposure to extreme water
levels are highest for the lower mainland, this report will concentrate on water levels at Point Atkinson
and the risk of exceeding the historical highs there. The main factors controlling extreme water levels
are seasonally high tides, intensity of storms (both winds and sea-level pressures) and ambient coastal
water levels due to basin scale phenomena such as the El Niño/Southern Oscillation (ENSO).
2. Tides
The historical high water level at Point Atkinson occurred on December 16, 1982 when the water
gauge measured a total water level of 5.61 m. The factors that led to the extreme water level were a
combination of seasonally high tide, strong winds, low pressure and an ambient sea-level height of
about 0.2 m due to one of the most intense El Niño events on record.
The highest tides of the year, known as the perigean spring tides, are seen twice a year, near the
summer and winter solstices. High tides during this period reach 5.0 m at Point Atkinson; a coincident
storm surge of 0.6 m could cause a total water level equivalent to the 1982 peak, even without the
presence of an El Niño. The predicted spring tides occurring monthly from October 2007 to February
2008 are shown in Figures 1a-1d. There are periods of high tide approaching 5.0 m in each of these
months, but there is a period of persistently high tides at the end of December that should be noted.
This period offers a broad window whereby a storm could cause extreme water levels to occur.
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Figure 1. Predicted spring (highest monthly) tidal heights for Point Atkinson for (a) October, (b)
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November, (c) December 2007 and (d) January 2008. The highest spring (perigean) tides during this
four month period occur in November and December 2007 (circled in red).
3. Sea Surface Height (SSH) Anomaly
Sea surface height data as measured by the JASON radar altimetry satellite are processed by the Jet
Propulsion Laboratory (JPL) at the California Institute of Technology to show the interannual
variability of SSH by removing the mean and seasonal signals, and the trend. The remaining
anomalies can indicate effects by large scale interannual phenomena, such as ENSO. As of October 1,
2007, no positive sea level anomalies that could potentially strengthen storm surge peaks in our area
are seen in the altimetry data.
Figure ?. Jet Propulsion Laboratory image of 10 day averaged SSH anomaly over the Pacific Ocean. The
equatorial pattern indicates La Niña conditions are present and there is little or no SSH anomaly currently off
the coast of British Columbia.
4. El Niño/La Niña Projection
The Climate Prediction Center (CPC) of the National Center for Environmental Prediction (NCEP)
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provides regular updates to the ENSO state of the Pacific Ocean. As of October 11, 2007 a La Niña
condition existed and was strengthening. Moderate La Niña conditions are expected to persist into
2008. The complete commentary is available at:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf
From the CPC commentary:
“Expected La Niña impacts during October – December include a continuation of above-average precipitation
over Indonesia and below-average precipitation over the central equatorial Pacific. For the contiguous United
States, potential impacts include above average precipitation in the Pacific Northwest, and continued below
average precipitation in the Southwest.”
With the projected persistent La Niña and concomitant above-average precipitation it is expected that
during this storm season southern British Columbia will experience more frequent or more intense
storms, but little or no increase in ambient sea-level due to large scale ENSO effects.
5. Summary
From the data available as of October 2007, the winter 2007/08 storm season will be characterized by:
1. tides reaching 5.0 m at Point Atkinson during parts of all months from late November to early
March, but particularly high tides will be seen daily from December 23 to December 27, 2007,
2. above average frequency or intensity of storms based on historical effects of moderated La
Niña conditions in winter,
3. normal (near-zero) ambient SSH anomalies due to ENSO effects.
The critical times to watch for extreme sea-levels will be during periods where high tides at Point
Atkinson approach 5.0 m. There is no expected ambient SSH anomaly to augment storm surges, and
since peak surges due to wind and sea-level pressure are normally 0.6-0.8 m, the historical extreme sea
level at Point Atkinson of 5.61 m is likely to be challenged only during periods of extreme tides.
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