El Niño is characterized by coupled oceanic and atmospheric conditions which lead to a slackening or reversal of the equatorial trade winds, which typically blow from east to west. This allows a warm pool of equatorial ocean water to migrate eastward towards the coast of South America. The changes in the ocean and atmosphere as a result of El Niño produce more southerly storm tracks, which bring increased precipitation and coastal storm events (storm surge and waves) to the California coast. The warm water associated with El Niño also exacerbates storm surge conditions through thermal expansion of ocean waters, which can further elevate high tides by 0.5 to 1.0 feet above normal.
The figure below shows the average sea surface temperature (SST) anomalies for July 1, 2015 relative to typical conditions. As can be seen, the entire northeast Pacific basin is warmer than usual, with SSTs running up to 3 deg C above normal in the eastern equatorial Pacific.
So what does this all mean for the California coast? Well, if the historical record tells us anything, it's that any old run of the mill El Niño is not a guarantee of wet weather, extreme tides, and large waves. Wet and stormy winters are most likely to occur during strong El Niño events. The most severe El Niños in recent memory - the 1982-83 and 1997-98 events were strong El Niños that persisted for the duration of the winter. Strong El Niño conditions persisted from September through March in 1982-83 (with a peak index of 2.2) and from July through February in 1997-98 (with a peak index of 2.4).
Both winters brought intense rainfall, extreme tides, and large storm waves to the coast. In fact, many tide stations along the California coast recorded their highest ever tide levels during these two events (read more about California coastal flood processes here). We can use our historical record of past El Niño events combined with predicted astronomical tides and El Niño forecast to project the likelihood of extreme tide flooding along the California coast for the coming winter. To accomplish this, I analyzed the storm surge patterns during past El Niño events and developed statistical relationships to project the occurrence of extreme tide events for the coming months.
First, let's take a look at the occurrence of extreme tide events during a typical winter - one characterized by neutral El Niño conditions. A commonly used benchmark for high winter tides is the King Tide which represents the highest annual winter tides each year (read more about King Tides here). The figure below shows the highest predicted tides during February 2016. I typically consider a King Tide to be around 7 ft, so these high tides are pretty close. We know that when we get tides around 7 ft, we start to see some nuisance flooding impacts along areas such as San Francisco's Embarcadero and other areas such as Mill Valley. So we'll use the 7 ft threshold to define a significant tide event in San Francisco Bay.
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| Predicted tides at San Francisco showing King Tides around February 7-9, 2016. |
I paired the predicted tides for November through April with the historical storm surge climatology for neutral El Niño conditions to produce a baseline for the expected number of King Tide days for the coming winter, in the absence of El Niño conditions. And you know what? This year is not a great year for King Tides. Based on my analysis, I project we will have approximately four King Tide days this year (i.e., tides that exceed the 7 ft benchmark). So, depending on the timing of Pacific storm events relative to the high tides we may see more or fewer than four - but that's my best estimate. Four King Tides is the median, or expected, value based on the statistical analysis - the 95% confidence limits on that estimate are between 0 and 12 King Tide days. The figure below shows the projected tide conditions for February 2016 for neutral El Niño conditions. The most likely timing for King Tides appears to be around February 7-9.
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| Projected tide conditions for February 2016 under neutral El Nino conditions. |
Given the projected El Niño conditions for the coming fall and winter, however, we can expect to see much more frequent storm surge events and generally elevated water levels as a result of the prevailing El Niño conditions. This means that if we have a moderate or strong El Niño in the coming months, we will see a much greater frequency of King Tides this year. How many more? Well, it depends on the strength and duration of the El Niño conditions and the timing of storm events with high tides.
If we have a moderate El Niño event through the winter, we can expect to see approximately nine King Tide events. The figure below shows the projected tides for February under moderate El Nino conditions. During moderate El Niños, higher than normal tides can occur; however, not with the same severity as during strong El Niños. Looking at the projected tide levels, you can see that the tides are generally shifted upwards, and a greater number of high tides have the potential to exceed the King Tide benchmark.
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If we have a strong El Niño event that lasts through the winter, we can expect to see approximately 30 King Tide events! That's on par with the 1982-83 and 1997-98 winters, which saw approximately 31 and 32 King Tide events, respectively. The figure below shows the projected tides for February under a strong El Nino event. What's remarkable about this projection is the long string of nearly equal high tides between February 12-21 - all of which have the potential to exceed the King Tide benchmark. That will be a critical time period for communities to be prepared for flood impacts.
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I've summarized the results by month for neutral, moderate, and strong El Niño conditions in the table below. What's interesting about the strong El Niño events is that while 30 King Tides sounds like a lot, the 95% confidence limits on the projection puts the upper limit at 48 King Tide events. That is extremely unlikely to occur and is more a product of the statistical analysis methods used in the simulations - truly a worst-case scenario. Anything greater than 30 will be extraordinary but 48 events would definitely be one for the record books!
Projected Number of King Tide Events for Neutral, Moderate, and Strong El Niño Winter (2015-2016)
Neutral
|
Moderate
|
Strong
|
1982-83
Winter |
1997-98
Winter |
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Low
|
Mid
|
High
|
Low
|
Mid
|
High
|
Low
|
Mid
|
High
|
|||
November
|
0
|
1
|
2
|
0
|
1
|
2
|
2
|
5
|
7
|
4
|
7
|
December
|
0
|
1
|
3
|
0
|
2
|
4
|
1
|
4
|
7
|
6
|
8
|
January
|
0
|
1
|
3
|
0
|
2
|
5
|
1
|
5
|
8
|
9
|
11
|
February
|
0
|
1
|
3
|
0
|
2
|
5
|
4
|
8
|
13
|
6
|
6
|
March
|
0
|
0
|
1
|
0
|
2
|
5
|
3
|
8
|
13
|
6
|
0
|
Total
|
4
|
9
|
30
|
31
|
32
|
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So stay tuned for more as we continue to watch the developing conditions in the Pacific. The timing of this year's El Niño on the heels of our historic drought highlights the extreme nature of the California climate. A strong El Niño this year would be both a blessing and a curse, as the same storms that would fill our reservoirs may wreak havoc on our coastline.
