I live on the coast in NE U.S. and we just went through a Nor’easter. The tides were greatly impacted with very high (high and low) tides. This was very noticeable behind the barrier islands. At one point, low tide was higher than a normal high tide. The explanation often given is that the wind holds the high tides back and prevents the tide from retreating out the inlet that it had just come in AND that the wind pushes even more water in on the next incoming tide. The cycle repeats and the high tide line keeps moving higher. I’ve heard it called “stacking”.
So, to what depth does a wind impact the flow of water? If an inlet is 25’ deep, can the wind slow the flow of water all the way to the bottom? I assume the higher the windspeed, the greater the depth but what depth? Is there a formula for this? The sheer volume and mass of water would seem impossible to slow significantly with 40-50 mph winds but my eyes weren’t lying to me. Is it just “surface” water being pushed in? How deep would this “surface” water be?
Also, during the peak high tides, the wind was blowing perpendicular to the inlets (locally, anyway) and had been for hours. It would seem that the water should be flowing across the mouth of the inlet and along the beach and not into the inlet and back bays. In fact, at the surf line it was easy to see the water moving down the beach. It wasn’t making a 90 degree turn at the mouth of of the inlet, right? How could it?
You ask a lot of questions some beyond my science.
I grew up on Long Island. Both kinds of storms spin counter-clockwise due to the Coriolis effect, yet Nor’easters have that distinct comma shape which changes their winds.
The Moon (now in late 3/4) has a big impact on tidal height when full or new. Yet if you’re seeking big surfing waves, you need sustained wind over a long distance. You can get that without either kind of storm, yet are more likely at some point if a hurricane has struck somewhere nearby because of how compact it is,
Sorry, can’t explain your last paragraph. I doubt there’s a “formula” for it all, just old sailor knowledge.
Winds can absolutely affect water levels beyond what the tides would do.
It’s called a storm surge. Most notable during hurricanes where they can be a serious problem (other strong storms can do it too as you just noted). As for the math of it all I have no idea. They have almighty computers help them do those kinds of calculations but that is far beyond me.
Storm surge is the abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide. The surge is caused primarily by a storm’s winds pushing water onshore. The amplitude of the storm surge at any given location depends on the orientation of the coast line with the storm track; the intensity, size, and speed of the storm; and the local bathymetry.
Storm tide is the total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide. Astronomical tides are caused by the gravitational pull of the sun and the moon and have their greatest effects on seawater level during new and full moons—when the sun, the moon, and the Earth are in alignment. As a result, the highest storm tides are often observed during storms that coincide with a new or full moon.
Your chart makes anomalies seem not so significant. But as anomalies are won’t to do, they are unpredictable. If a hurricane of 3 2+ gets into NY harbor we’ve already seen the subways flooded. “It wasn’t supposed to be this high of a surge” is weak when the streets are also underwater.
ETA: As storms generally flow west to east, they often hit the UK with sub-tropical yet significant force. People on the West coasts of Ireland and UK have not seen these kinds of storms that wash right over sea-walls that have been good for a hundred years.
Isn’t one component of storm surge the low atmospheric pressure usually associated with storms? Not really suction, but water being pushed up by surrounding high pressure systems?
The wind pushes the surface water that in turn pushes the water beneath that and in turn the water beneath that. It, of course, diminishes with depth but even at 1000 feet, 5% of the wind current at the surface is still detectable. A chart is untenable because the sea floor also has an effect and is highly variable in depth.
I also thought this might be the case but after my OP, I checked. It seems pressure plays a role but it minor compared to the wind. I’m a minor weather geek and the forces of nature can be amazing. A less-than-hurricane force wind overcoming the tide is one of them. At least, I think so. And the idea that wind impacting the current 1000’ down is another. I never knew that.
Fascinating! A couple of months ago I learned (right here in FQ) that oceans don’t “level out” so the effects of rising sea levels varies by location. I think of that as similar to “standing waves” on a global scale.
I understand that differential air pressures cause wind, but does the movement of air over the surface of the water also cause a reduction in air pressure like the “lift” on an airplane wing, or is that effect only present with both a top and bottom surface with differential wind speeds? Complete ignorance of the physics of air movement here, please bear with me.
sunacres is correct that low barometric pressure results in increased sea level, even in the absence of wind. This effect is very significant, adding half a metre or more to astronomical tides. Similarly in high pressure conditions the water level can be half a metre below the charted depths at low tide, which can lead to embarrassing and costly incidents.
And it should be noted that the most powerful hurricanes do not necessarily create the highest storm surges. The key factors are time, breadth of field, and shoreline geography.
A Cat 1/2 with a broad wind field moving at medium speed over a long reach of water will often generate more surge (based on shoreline) than a quickly moving smallish Cat 4/5 since there is less time for the stacking/push to occur.
