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.


  1. IPCC4 estimates are from 0.5-2 feet in the next 100 years, and since then those estimates have been revised upwards in the literature to 2-6 feet in the next 100 years. That makes the statement, "yesterday's tide would be an everyday tide by midcentury", to be only hyperbole in that its more likely that yesterday's tide would be a yearly king tide occurrence by midcentury.

  2. Cliff, following up on comment/observation from a few days ago: current forecast for Snoqualmie Pass (For Thursday) is high of 26 and low of 18, yet the observed temperature is now 34. What's going on?

  3. Cliff this was a really interesting post and provided some context for an internal discussion at our yacht club re marina renovation.

    question came up at the bus stop this morning: are the weather patterns we are see ( as adults) different than what we recall as children-- are we really having wetter, longer storms than we used to?

    brad on Vashon

  4. Cliff,
    I wish your Earth Orbit illustration had a larger minor axis. Right now it looks like parhelion is not Earth's closest approach to the sun.
    Otherwise, great post!

  5. Cliff, appreciate the nice summary explaining the event was really based on coincidence of independent events which has a low overall probability of occuring. To add to your remarks, Global sea level changes may be non-changing or falling in our region but the other component that affects local sea level change is vertical land movement. In Seattle, and Central Puget Sound land is subsiding. Therefore there is an effective local sea level rise which shouldn't be ignored. This can be seen in the change in mean sea level each time NOAA updates their tidal epoch. The most recent change from epoch 1960-1978 to 1983-2001 was 0.18 feet (2.16 inches),+Puget+Sound,+WA

  6. Lunar perigee (more significant than solar perihelion?) coincided with new moon, but that was six days earlier! Re the wind direction, tidal flooding also occurred at Fay Bainbridge park on the east side of Bainbridge Island, which would seem contradictory.

  7. Clif Mass is my forecaster, it shall not snow.

    He predicteth the sunny green pastures, He leadeth me to avoid the troubled waters, He restoreth my faith in the science of Meteorolgy.

    Yea, tho I ride through valley of the shadow of stupid freaking cell-phone drivers, I shall fear no snowfall, for thou art doing thy job. Thy barometer and thy thermometer, they comfort me.

  8. You write, 'Interestingly, the eastern Pacific is a region where sea level has not been rising recently. '

    Is sea level not constant over the planet? Are there regions on the planet where sea levels are different from other regions, even if adjusted for winds, currents, tides, etc? Why would the sea level rise (rate of change) in one area be greater than another area?

    Lots of questions, I know!

    Thanks for the great information and scientific approach to a fascinating subject, Cliff!


  9. Cliff, first want to mention that I’ve much enjoyed reading your blog. Have a few comments here though usage of the phrase “King Tides” which has long been a sort of pet peeve of mine. To explain why, need to first expand a bit on the background information about tides you provided.

    Larger astronomical tidal ranges occur (1) around the times of the full moon and new moon (recent record water level event associated with the latter) when the earth moon and sun are approximately in a line, and (2) when the moon is near perigee (closest point of approach to the earth in its 28-day orbit). So it’s when these 2 combine, when the occurrence of a full or new moon coincides with the moon being at perigee in its 28-day orbit around the earth, that the highest high tides will occur. This tends to happen 3 or 4 times per year.

    But then as Cliff mentions, there’s also (3) the variation in distance of the earth from the sun, with tidal ranges enhanced when the earth is closest (perihelion, closest approach, occurs around Jan 2 each year).

    So it’s when all 3 of these most come together that the highest astronomical high tides of the year will occur: and these are the so-called “King Tides.”

    My issue with all this is that the magnitudes of both (2) and (3) are pretty small. If we look for example at the 2012 NOAA Astronomical Tide Chart for Seattle, the highest value for December is 13.0 ft (6:50 am Sat Dec 15 and 7:34 am Dec 16) – indeed the highest high tide of the year – but the highest value for November, 12.6 ft on Nov 16, was only 5 inches lower. And the highest back in Feb, 12.7 ft, was just 4 inches lower.

    So calling them “King Tides” and corresponding media usage of adjectival phrases like “extreme high tides” – like in this New York Times article

    strikes me as kind of hyperbolic (or at least we should admit that it’s a pretty small Kingdom!).


  10. ptwise...

    one other factor regarding tides I here mentioned deals with tetonic plates - that the area in and around Olympia and Tacoma is "sinking" faster while other areas are "rising" - thus increasing or decreasing the chances of record tides

    is this significant?

  11. It should be noted that in general in the psrts of the northern hemisphere I've been able to observe (decades worth) in winter the biggest tides are at the new moon, not the full moon. Reference: tide tables.

  12. So, IPCC AR5 is leaked, study states no increase in atmospheric water vapor, no global lower tropo temp increase in over 16 years, IPCC climate models overstate ( some in a major way ) future temp increase.

    And you ignore what solar cycle 24 and 25 may well be suggesting, and now state that Snoqualmie pass will have no snow in 2050 ? Not, 2030 now ?

    I hope you live long enough to watch the planet cool.

  13. This was indeed devastating. Most areas were badly flooded multiple times this past year. Don't let the water get you down, hire a Seattle Water Cleanup orofessional.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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