Wednesday, December 19, 2012

Record Water Levels in Seattle: Why Did It Occur and Is Global Warming Important?

On Monday morning, during a modest windstorm, the Seattle tidal station (NOAA gauge 9447130) measured the highest water level since measurements there began in 1901.  The results was beach flooding and damage to docks, beachfront homes, and other coastal facilities (see pictures below).  Now we didn't beat the old record by much.   Water reached 14.51 feet and the mean low water mark exceeded the previous record, 14.49 feet, set in January 1983, by .02 feet.   Well, a record is a record, even if you win by a whisker!  
West Seattle During the Storm (courtesy W. Seattle Blog)
Alki Beach (courtesy W. Seattle Blog)
Submerged fishing dock in Mukilteo (courtesy Bob Donegan, Ivars, Inc)
 The media, such as the Seattle Times, had some big headlines on this new record, and some of them suggested that either global warming had a hand in this, or the high tide was a warning of things to come:  "Yesterday's tide would be an everyday tide by midcentury."

So what is the truth about this sea-level record?  Why did the record occur on Monday?  Some of the answers might surprise, perhaps. 

The high water on Monday had a number of contributing factors.

First, we started with astronomically very high tides, sometimes called "King Tides",  that occur during midwinter.   Tides are mainly caused by the gravitational attraction of the moon, but the sun also contributes.  The highest tides tend to occur when the sun and moon are lined up, which happens during a full/new moon, and when the moon is closest to the earth (perigee).   During our midwinter, the sun is closest to the earth (discussed in an earlier blog), and thus its gravitational attraction is stronger.  The result, the highest "King" tides.  Nothing to do with a certainly local TV station.
Centered on January 2, the earth is closest to the Sun, enhancing the tides.
 The following figure shows you the predicted tides, what actually happened, and the difference on Monday (12/17), Tuesday, and part of Wednesday (time in GMT).  Our big observed tide occurred around 1600 GMT (8 AM) Monday.  The predictions were too low.  Why?  The big reason is that we had a strong low pressure area over us (the storm) and low pressure caused sea level to rise.

Low atmospheric pressure results in a higher sea level. This is known as the "inverse barometer effect", with water level rising about 1 cm for every 1 mb (1 hPa) drop in pressure.  Here is the pressure at Seattle during the period.  The pressure had dropped to 

around 980 mb, with the lowest pressure around 0900 GMT (roughly 1 AM),  hours before the astronomical high tide.  If average pressure is about 1013 mb, the pressure dropped to about 33 mb below normal, which would bring the water level up by about 33 cm or about 1.1 feet.  This is one reason why the difference between actual and predicted was largest in the early morning hours.  

Another factor that brings up sea level is the storm surge effect, the influence of strong winds pushing water up on the beach.  At Sea Tac Airport, the winds switched from southerly (roughly parallel to the shore) to SW (more of an onshore component) around 2-3 AM, and the winds increased rapidly during the early morning hours (see graphics).
The implication is that we were lucky:  the storm hit too early to maximize the water level.  It if had hit 4-6 hrs later, the tides might have been a half a foot or so higher.  Then we would have really had a record!

We have King tides every year, but how often do we get 980 hPa lows like Monday's storms?  Well, lets check the UW records back to 1996.  Here is a plot of sea level pressure from Sea Tac for the period.   Looks like we get that low approximately every other year.  
So to be in record territory we need to get a King Tide, which is limited to midwinter, and get a major storm phasing in with relatively close timing.   Plus you need a significant onshore component to the wind to push water up on the beach. You can see why it is hard to break records.  Talking to some of my colleagues, we really had the feeling that this storm had unusually strong onshore flow (more of westerly component to the wind) along the eastern shore of Puget Sound.  This map illustrates:
It all came together for this storm.

What about sea level rise?  How did it contribute?

 Here is the sea level rise over the past century at Seattle.  An upward trend of about 8.1 inches over the last century.  But if you look closely and ignore the trend line, you will see there really little trend during the past 30 years.  Much of the trend in sea level rise began before humans could have had a large impact on sea level by increasing greenhouse gases (human influence becomes significant around 1970).  So both natural and anthropogenic (human-induced) warming has contributed to sea level rise in Seattle.  Sea level rise is making such records easier to break, but it is important to keep in mind that local sea level has not changed since the time of the previous record (1983). 
Interestingly, the eastern Pacific is a region where sea level has not been rising recently.  To illustrate this, check out a figure from the IPCC (2007) report (IPCC is collection of international scientists working on the global warming issue). This graphic shows change in sea level from 1985-2010 from satellite data.  In our area, general sea level has remained the same or fallen!  
 In the long term, the earth will warm profoundly from increases in greenhouse gases such as CO2 and that will surely cause substantial increases in sea level everywhere.  But we must be careful in pointing the finger at human-caused global warming for extreme weather events today, such as the high tides in Seattle during our Monday storm.

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