Blowing out the Ocean’s Waters

A fellow blogger over at Spoonbeams read my post on king tides and beachcoming  and asked what caused this Sunday’s extreme low tide in the Hudson way up near Germantown, which was baring previously unseen underwater obstacles.

Photo credit: Spoonbeam (Do you have any pictures of this past weekend's low sea levels? If so, pass them on!)

Here’s a plot showing how the sea level near there, at Norrie Point, varied over the past week, demonstrating how low the tides fell.  This was observed all over the entire region’s tidal waterways and beaches.

Time history of water elevation in feet: tide predictions (blue), NYHOPS model predictions (magenta) and observations (red dots) at Norrie Point along the Hudson (near Poughkeepsie). Zero elevation is mean-lower-low-water (MLLW), the typical daily low tide level. Not doing bad, NYHOPS!

It was actually driven by strong winds blowing from west to east — it pushed New Jersey coastal ocean waters offshore and can have an effect similar to the height of the tide, basically a reverse storm surge, a “blowout”.  Here’s some wind data — get my point?

Wind observations at JFK Airport, from the Urban Ocean Observatory (which is the parent site of SSWS).

Winds were blowing 22-30 knots (25-35 mph) from the west (arrows toward the right) on February 25th for about a day straight.  That’ll do it … I looked at 3-month view of data in SSWS for a few winters and you can see that this is about the lowest water level that is ever seen, as she surmised.  It occurs typically 1-2 times per winter.  It’s impressive that Spoonbeams noticed – it’s nice to know people with their eyes on the water who notice these things!

For future reference, instead of tide-tables or other tide prediction websites, people in our area should check http://stevens.edu/SSWS for tide predictions that incorporate this effect, as well as rain-driven water level changes that can be large in the Hudson.

Interestingly Earth’s rotation was also likely at play, causing Long Island coastal waters to also be deflected toward the right if the wind blows over a day or longer, which it did. That’s the Coriolis effect, and would let the west winds blow the water toward the southeast, also offshore.

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2 Responses to Blowing out the Ocean’s Waters

  1. spoonbeams says:

    Looks like I asked the right person! Thank you!

  2. tugster says:

    cool post! wish i’d known in advance about the blowout. i’d have gone upriver to check some submerged wreckage that might have been exposed up in the rondout.
    keep up the good work!

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