Thursday, December 8, 2011

High pressure, low water

I would never have considered it.  Or if I had, I would have assumed that the effect was small enough to be of academic interest only.

But Cliff Mass, our primo weather blogger here in Seattle, says otherwise - and proves it.  Cliff points out that our recent record high atmospheric pressure was accompanied by significantly lower tides.  Stealing a couple of public-domain images from his blog (you should read the whole story there), we see:

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.
Two feet is not of academic interest - it is significant in the real world.  It could easily make the difference between passing over a sandbar in Killisut Bay and being grounded on it.  Something else to keep in mind...

I suppose that the opposite effect - higher sea level with lower pressure - explains at least a part of the storm surge associated with hurricanes.

I recommend Cliff's blog to you.  It is usually Seattle-oriented, with our unique weather tweaks, and it is informative.

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