December 07, 2011

High Pressure Produces Low Sea Level

During roughly the past week, sea level pressure has been unusually high--including the record-breaking high pressure observed on December 1 (1043.4 hPa).   Here is the pressure of the last four weeks.  The first three weeks had relatively normal sea level pressures (1000-1020 mb), but the last week or so, pressures have generally ranged from roughly 1030-1035 mb.   Very unusual to stay that high for so long.

We have also been observing another anomaly:  the height of the water levels in the region have been unusually LOW, particularly along the Pacific coast.  NOAA produces water-level predictions = (the tide tables we know so well) and these predictions are generally quite good, since we understand very well what produces tides and their periodicities.  But recently the tide predictions have been greatly in error, forecasting tides that are much too high by one or two feet!  Here is the predicted and observed water level at Neah Bay, provided to me by UW's Dr. Nate Mantua.

Turns out these two anomalies (high pressure and low water levels) are directly connected, with high pressure pushing water levels down.

The general term for this mechanism is the inverse barometer, and is often used to explain unusually high water levels when low pressure is over a water-covered area (see graphic)--and you may

remember that I wrote about this a few years ago when record low pressure spread over the west coast (click here for my past blog).  In general,  the water level should sink around 1 cm for every mb pressure increases. We have experienced about a 20 mb high-pressure anomaly, and thus one would expect water levels around 20 cm (8 inches) below normal.

However, there is something else going on here.  The center of the high pressure has generally been offshore and as a result northerly winds have been persistent along the Northwest coast (see graphic from WRF model forecast):

Northerly winds put a force on the water to the south, but because of the Coriolis effect, there is an offshore component to the surface currents.  Thus, surface currents are pulling water AWAY from the coast, further reducing the height of the water surface. 

And it is better than that if you like low water levels.  With a complete absence of storms there are no strong winds pushing water up on the coast and very weak wave action.  Take a look at the wave heights along the Washington and Oregon coasts at two NWS coastal buoys for the past five days (46041 off of WA, 46050 off of Oregon).  Waves progressively decreased to 4-5 ft.  Good time for fishing?

We will be stuck in this pattern (high pressure, low clouds west of the Cascade crest, air pollution) for several more days, so you better get used to it.  Want sun, head into the mountains or eastern WA or OR.

PS:  If you want me to answer a question on-air on KPLU on Friday at 9 AM, you can leave it at this link.


  1. Cliff,

    My recent flight from Atlanta was 1 hour shorter than they had predicted. Is this due, in part, to the high pressures we're feeling here in the PNW?

  2. This has been worse than rain. Grey, dark, and bad air. How I wish for a good strong blow to mix things up and create some breaks in the clouds! Gasp.

  3. Man I hate high pressure....any idea when we can get out of this and return to a good weather pattern(rain/mtn snow)?

  4. Cliff,
    what are the chances of clear sky for the lunar total eclipse on Saturday morning? Alternate viewing sites?

  5. Fishing any good?

    Looks like boats out of San Juan are catching fish. But nowhere else.

  6. the water level should rise around 1 cm for every mb pressure increases

    Isn't this backwards? Intuitively (and per what we've been seeing), the water level should go down for high pressure, no?

  7. are of course correct...I need to proof these blogs before I send them out! Sorry...cliff

  8. Cliff,

    The anomaly in the sea levels reminds me of a phenomenon in the Great Lakes called a seiche. This happens when sustained high winds or extreme barometric events causes water to get pushed to one end of a lake, then is sloshes back to the other end after conditions change. They can cause a rapid change in lake levels of many feet, and often without warning. Wikipedia has several good examples (I remember the 1995 event):

    Lake seiches can occur very quickly: on July 13, 1995, a big seiche on Lake Superior caused the water level to fall and then rise again by three feet (one meter) within fifteen minutes, leaving some boats hanging from the docks on their mooring lines when the water retreated. The same storm system that caused the 1995 seiche on Lake Superior produced a similar effect in Lake Huron, in which the water level at Port Huron changed by six feet (1.8 m) over two hours. On Lake Michigan, eight fishermen were swept away and drowned when a 10-foot seiche hit the Chicago waterfront on June 26, 1954.

    So, my questions: is the "inverse barometer effect" causing a seiche? Can a seiche occur on the coast of an open ocean? Can a seiche have a duration of several weeks, as noted in your tide graphs of Neah Bay and Astoria?

    I love your blog, keep up the great work!


  9. Good time to walk the tide pools at low tide.

  10. It's been really nice for beach walks on Sinclair Island.

  11. The Tacoma Aroma hit me smack in the face when I opened the kitchen door this morning. (I could actually smell it about a foot away from the door, with the door closed)

    Normally, you can't smell it, but I wish the paper mill wouldn't operate on bad air quality days.

  12. Jen, air quality is good, all the Puget Sound Clean Air Agency sensors were showing good air quality this Thursday afternoon. Maybe the north breeze is why you smell the mill--or maybe they had to curtail operations when ventilation was poor and started up again?

    In fact, PSCAA lifted the burn ban for Pierce and Snohomish counties--at least for now. There will be a relatively short period of poor ventilation, probably from late tonight til Saturday morning. That might be too short a time to put a burn ban back up, but we will see.

    With a northerly breeze and a fairly deep mixed layer up into the clouds today, ventilation was good. It will probably be poor Friday and Friday night as it warms aloft and a stronger inversion sets up. In fact, the clouds may clear and then be replaced by a lower cloud deck, or more likely fog. The lower the inversion, the shallower the mixed layer, and the greater chance to trap pollutants.

    It will be interesting to see if as the gradients turn from northerly to easterly, some gap winds can push through the Cascades. That makes the amount of clearing and where the fog sets up a trickier forecast.

  13. Did anyone else feel the slight constant vibration in the earth over the last 6-7 days? It threw out my furnace blower just enough so that it was very loud and shook the floor when on. Outside I could feel the traffic on my street as each vehicle went by the ground felt like a waterbed. At work, up the high rise, the floor vibrated ever so slightly and the air blower was really loud shaking the floor when on. Now- today-nothing-the house is back to rock solid no vibration. I am wondering if this was some sort of seismic event or possibly if high pressure also affects the water table causing any of this since the timing coincides.


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