Case Study 2: U.S. West Coast, El Nino & Seasonal Sea Level Changes

Evidence for non-tidal sea level change over longer-periods of time

West Coast Case Study

Now that we are familiar with daily tide gauge data (you will work with more data later on) and storm-surge impacts, let's explore some longer-term (but still short-term in a geologic context) water level data from the west coast. Washington, Oregon, and California routinely experience coastal flooding from storms, bizarre extreme waves, and other phenomena that last for days even weeks and occasionally for a few months.

Case Study:

Researchers in Washington State have a website that explores the impacts of coastal processes. One website: http://web.archive.org/web/20170506101704/http://www.ecy.wa.gov/programs/sea/coast/storms/elnino.html (opens a new window) identifies some of the impacts of the El Niño of 1997/1998. One tide gauge station at Grays Harbor, WA reported a 14.52 meter high wave that caused massive erosion on the shore. It was a freakishly large wave produced by a storm system that itself was superimposed upon elevated water levels caused by other factors that persisted for a period of months. This website shows an un-sourced dataset that indicates that the high water levels of 1997-1998 coincided with an El Niño, and were preceded by high water levels some 15 years earlier during 1982-1983, which also were coincident with a strong El Niño event. As shown, the time intervals immediately following high sea levels were unusually low, i.e., during the La Niña events. These ideas are interesting and worth exploring just a little bit. Scientists at the USGS have also explored the same 1997-1998 event because of the hundreds of millions of dollars in damage it produced in the San Francisco Bay region alone. The report is available at: http://pubs.usgs.gov/fs/1999/fs175-99/ and explores the event in a bit more detail. Please feel free to read the report and the study the imagery as you have time. We will focus on one component of the report, and that is the record of sea level at Fort Point in San Francisco, which is shown below in Figure 4.11.

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Figure 4.11: Excerpt from a USGS publication showing the sea level data from Fort Point, California from 1900 to 2000. The graphic shows data from daily tides, annual sea level cycles, and longer‐term trends of sea level change. Major high‐water events are labeled with the dates of particularly strong El Niño events.
Credit: <span style="font-size: 12px;">USGS: Screen clipping taken: 1/14/2014 12:52 PM</span>

In the record shown, USGS scientists have plotted sea level change from 1900 through ~2000 (the longest record from the west coast). They have used a statistical averaging technique to condense the daily tidal signals in order to look at the longer-term signal. In this data, they have pointed out five different episodes of major high water intervals for San Francisco. These high water events each coincided with an El Niño. For more info on El Niño, check out NASA's SEA LEVEL VIEWER (Figure 4.12):

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Figure 4.12: NASA's Sea Level Viewer.
Credit: <span style="font-size: 12px;">NASA: Screen clipping taken 1/15/2014 11:45 AM</span>

The USGS report explores the influence of daily tides, annual sea level cycles and the occurrence of El Niño/La Niña events, and also explores the apparent longer-term trend of rising sea level evident in the century-long data set for this particular area.

It is clear from this dataset that we still have not explored all of the reasons why sea level can and does change in time and space so we have more work to do. It is also clear that climate factors (and not just individual weather/storm events) together with various oceanographic phenomena might play roles in local sea level change. We will need to explore these ideas before we can look at even longer-term patterns of sea level change.

In the discussions above, we have highlighted a couple of factors that contribute to sea level, and sea level change that have yet to be defined and discussed. So, we will do that now. When looking at the multiplicity of variables that contribute to sea level change, it is important to think about the root driving mechanisms behind each variable that influences sea level on different time and spatial scales.