Last Thursday, Torge & I invited his “atmosphere & ocean dynamics class” to a virtual excursion into my kitchen — to do some cool experiments. As you know, I have the DIYnamics rotating table setup at home, so this is what it looked like:
A reader of my blog, Rocío*, sent me this beautiful image from Arnao beach (Castrillón- Asturias-Spain), and I asked if I could use it in a #friendlywaves post. He agreed, so here we go!
First, let’s check out the original image in all its beauty, before I start scribbling on it. What features of the waves stand out to you that you find especially interesting?
For me, what I think is especially awesome here, is how the behaviour of the waves lets you draw conclusions about the sea floor underneath. Look at all the wave crests coming in nice and parallel. Far offshore, it’s difficult to even see wave crests (marked orange, for example), only when they come closer to the shore and the sea gets shallower, they start to build up, get a distinct shape. Yet in some places they become a lot steeper and start breaking a lot further offshore (red marks) than in others — why?
Because in those spots the sea is shallower, thus the interaction with the seafloor is a lot stronger. If you look at the yellow mark, for example: Offshore of it the wave crests are still very shallow and not pointy, and then all of the sudden they break. Here the water is deep until there is a very fast change and then it’s suddenly very shallow (and probably rocky, hence all the turbulence).
And then, if you come closer towards the shore, there is an area that has only a very gradual incline, where the shape of the waves hardly changes any more (blue marks).
And then there is a small inlet to a large puddle that acts as “slit” (albeit a fairly wide one) and lets waves radiate as half circles from where they enter through the slit.
I love how in such a beautiful image of such a beautiful landscape, there is so much physics that we can discover if we only choose to look! :-)
*I asked how I could credit the picture to Rocío, but he doesn’t have Twitter or a website and wrote “I only want you to explain it for people i love your blog and your information you are doing a great job”. Aaaaw, thank you!!! :-) And thanks for sending me this beautiful picture!
This might not be the kind of water that people usually like to look at during their lunch break — I admit, it does not look particularly inviting. But look at how much the mud can tell us about what’s going on in the water!
I love how the overexposed seagull is stirring up a muddy wake as it paddles through the shallow water. Would you have thought that it was THAT shallow and that a small bird paddling along would create such a mess?
I love all the different kinds of structures in these pictures. The mud that has fallen dry and that shows all these canyon-like shapes where the water has run off. The mixing of waters with different mud concentrations, thus different colors. And then the muddy trajectory of the seagull.
Focussing on the mixing of water masses: See the lighter, sediment-loaded plume flowing from somewhere to the right over to the lower left of the picture below? And do you notice how different the shear-induced mixing looks from the paddling-induced one?
Here is the same spot again, only seconds later. See how dynamic the plume is, entraining clearer water as it flows along, eddying at its sides?
Now that the weather is nice and sunny again, here is what it looked like only last Saturday. It wasn’t even really stormy, but windy enough so that the ships leaving the locks at Kiel Holtenau were working a lot harder than usual. Especially difficult when you are almost empty and then there is a lot of wind! See that wake?
Right behind the ship you see above, there was a second ship leaving the locks. See how milky the water looks where the first ship went from all the air bubbles that were pushed under water by the ship’s propeller? You can even see some of that water spreading underneath that floating barrier in the foreground!
And see the difference between the waves on the upwind side of the ship and the downwind side?
Here is the picture that my friend sent me that she took from inside of the café that we were sitting in before I HAD to go outside and take pictures. If I am being sent pictures of my back every week by my friends, are they trying to tell me something? :-D
I just love this picture: The two boats in the front are going at the same speed (the trainer is driving right next to the person in the row boat over a long distance), yet look at how different the two ships’ wakes look!
The motor boat has this huge, breaking, turbulent wake. Even though it rides so high up in the water, it displaces a lot of water and creates a wake with a large amplitude (how large the amplitude is is visible in the picture below, where some poor people were sitting in row boats when a motor boat sped past. But also here: Look at how cool these feathery waves that constitute the wake look together!).
But then, going back to the original picture (which I am showing again below) — look in contrast at the row boat’s wake. You see the paired eddies where the oars were in the water, and you see a tiny little trail where the body of the ship went. But that’s all. Yet both boats are going at exactly the same speed! Pretty cool, isn’t it? (Also pretty scary how much energy the motor boat is spending on moving water and moving a larger hull and a heavy engine rather than just propulsion when the payload of both boats is more or less the same — one person)
Speaking of wake watching, the other day I wrote about long distance wave watching in the sunset, i.e. what kind of things one can deduce on surface roughness (and its causes) from different reflections of the setting sun on the water. And then I was asked why ships’ wakes were still visible for such a long time after the ships had already sailed. So here is my attempt at an explanation:
Check out the pictures above and below. In both you see the turbulent wake of the RV Kristine Bonnevie on our recent student cruise. You clearly see where the ship’s hull has passed through the water, moved forward by the ship’s propeller, which is very clearly introducing a lot of turbulence. And you very clearly see where the ship has not been: a more or less undisturbed wave field full of small wind waves, that looks substantially different from the turbulent wake.
Now why does the turbulent wake look so clearly different from the rest of the water for such a long time, even when the ship is gone? Shouldn’t the wake be invaded by surface waves at their wave speed?
Yes, that should happen, if the wake wasn’t turbulent. As the wake is turbulent, however, there are eddies moving the water around for quite some time after the ship has passed. If the water is being moved faster than the phase speed of the waves, they can’t propagate in there, the “flow” is too fast, the waves are washed away. See where the hydraulic jump is happening in the picture below, and waves seem to be squeezed together outside of the turbulent wake?
Just because it’s fun, here my hydraulic jump animation: the wave (person) is traveling exactly as fast as the current (escalator), it therefore doesn’t move. More Froude number animations + explanations here if you wanna look at what happens if the wave is moving faster or slower than the current…
Anyway. Back to pretty pictures. Below, you see a wake that is a little older: The surface is still a lot smoother than over the rest of the fjord, because the waves still haven’t propagated into the turbulent region. And when they do, longer waves propagate in first, because their phase velocity is faster than that of short waves. But the long waves’ effect on surface roughness is smaller than that of short waves, so the wake still appears smooth for even longer.
Only when all the turbulence has died down and the water is stagnant again (Or moving with the surrounding water) will the wave field be able to grow back to look the same as everywhere else. And therefore, even if you look at water from a distance, you can see where ships have been, even when they’ve long gone themselves.
One of the instruments that was used on our recent student cruise was the so-called MSS (“MicroStructure Sonde”, sometimes also called VMP, “Vertical Microstructure Profiler”) — an instrument that is used to measure how much mixing is going on in the ocean. Those measurements can help us figure out e.g. renewal rates of bottom water in fjords, which are interesting because of the very low oxygen concentrations found there, and their impact on biogeochemistry. And of course it’s also interesting from a purely physical oceanographic curiosity :-)
In the picture below, you see the MSS being deployed: It’s a slim instrument, maybe 1.5m in length, that is attached to an orange cable that runs on a small winch.
And a big THANK YOU to cruise leader Elin (observing from the upper deck) for bringing me along on this cruise! :-)
At the end of the instrument that sticks over the railing in the picture above, you can make out little pins, protected by a metal cage. Those are the sensors for both temperature and velocity shear, both measuring at very high frequency, many many times per second. They are also very sensitve, so in the picture below you see the wooden crate that is used for storing the instrument in between stations.
Once the instrument is deployed into the water, it is not just lowered down in the way a CTD is, but it has to be free-falling through the water. In order to achive that, the person running the winch has to constantly watch the cable going into the water to make sure there is some slack on the cable.
Algot is running the MSS winch
A second way to make sure the instrument is free-falling is to constantly monitor the incoming data on a PC onboard the ship.
Elina is working the MSS “inside job” (Picture by Sonja Wahl)
While the data is being monitored, also the depth the instrument is at is being monitored, or rather its pressure. Since the instrument is free falling, it is not a simple feat to make sure it gets fairly close (approximately 10m) to the bottom, but does not hit the bottom and destroy the sondes. One way we’ve done that on the student cruise is by stopping the outgoing cable when the instrument was at 75% of the water depth and let it fall, and then once the instrument is within 20ish meters of the bottom to start hauling the cable back in (“panic” in the list below ;-))
Snapshot of the piece of paper used to keep track of what’s going on at the current station
Looking at the picture of Algot below, you know that the instrument must be on its way up. Why? Because there is clearly no slack on the cable!
Algot is bringing the MSS back to the surface
In the picture below, do you see the green fringe on the instrument, as well as the rope slung around the metal protection cage thingy for the sondes? Those are there to make sure that no eddies (and especially no trains of eddies) develop while the instrument is falling, because if the instrument was vibrating or moving in some way other than just falling freely, that would influence the data we measure.
The instrument is then brought back on board, and we are ready for the next station!
Algot and Arnt Petter bring the MSS back on board
And which spots did we measure turbulence in? In many, but especially on either side of the fjord’s sill, because that’s where we expect mixing due to tides going in and out (which we also saw in the fjord circulation tank experiment!).