Unusually Low Pressure Caused Sea Level to Rise

Have you felt lightheaded during the past day?  

Perhaps you should have, because the atmospheric pressure has been unusually low.

Low enough to cause a significant rise in sea level.

Consider a plot of sea-level pressure for SeaTac Airport since July (below).  The pressure dropped to 990 hPa, the lowest observed during the period, by a LOT.

Our extremely low pressure was associated with a deep low-pressure center immediately to our west (see the analysis at 4 AM Friday).   At this time, the low center had a central pressure of 968 hPa, which is low for this latitude.

The average pressure for Seattle in November is 1017 hPa (the average around the nation is 1013 hPa), so our pressure on Friday was 27 hPa below normal.

Localized low pressure causes sea level to rise by approximately 1 cm for every decline of 1 hPa.  (See schematic below).

Thus, sea level over Seattle was elevated by 27 cm or 10.6 inches by the low pressure.  Almost a foot!

You want to see the low pressure in action on sea level?   

Below is a plot below of predicted (blue) versus observed (red) sea level in Seattle.   The difference between the two has increased this week as the low-pressure center approached, with sea level 2.2 feet above normal.  

So about 40% of the anomalous sea level rise was due to the low pressure.

But the effects of the low center on sea level do not end there.

The low center is associated with powerful westerly (from the west) and southwesterly winds (see forecast surface winds at 1 PM Friday below).   Winds near the coast were strongly from the south (southerly)

Such southerly winds push on the ocean surface, which flows TOWARDS the coast by the Coriolis force, resulting in a HIGHER water level.  See schematic below of this situation.  Want to impress your friends?  This is called Ekman Transport.

But the high water fun caused by the low center does not end there!

The offshore low with strong winds over a large area (long fetch) produces big waves that can bring the water levels up even more on the coast (see forecast wave heights for Friday morning below).

There are other, more minor contributors to short-term high water levels, such as a Kelvin wave associated with the El Nino moving up the coast (several inches).     

The good news is that water levels should drop as the low center weakens over the weekend.