Welcome to the third video in lecture two. In this video we'll look at some examples of drifters
and floats as well as ROVs, AUVs and gliders. So drifters and floats are generally a type of
autonomous measurement platform. The surface drifter is the one I described earlier here,
which drifts at the surface but usually captures currents in the surface mix layer. And then we
have profiling float, which is shown here, often called the ARGO, which is basically the ARGO
float. But there are other types of drifters that won't get too much into here, like a deep drifter
which floats at a particular depth that we can set and follows the currents at this depth. But in
order to capture its position, we need either a ship to accompany it, or maybe several ships,
or to have some sort of moored receiver or something to send a signal to the float to
track it as it moves throughout the ocean.
So on the left plot here, we have an example of a drifter, a surface drifter. A surface drifter
tracks off the coast of Namibia. So again, this is this example of a filament, which we can see
here by the lighter yellow color, bringing cold water offshore. Okay, so I'll remove that for now
so it doesn't block our view. So this is these two eddies which drive this filament offshore.
And we placed these blue dots, so where we place some drifters. And we can see the black lines
track the movement of these drifters as they move offshore. So they can show, so they're very useful
in pretty small scales as you can see, but also the larger scale pattern. So as I sort of tried
to highlight here, they cover quite a range of scales from sort of the sub meso one kilometer
scale up to hundreds if not thousands of kilometers depending on how long they last.
Essentially until we can't get their, can't receive their signal anymore. The smaller scale
basically depends on how often we receive their precision, how what we set their frequency of
their transmission to. The floats, these Argo floats, which also provide profiles, work in a
pretty similar manner. So when they're always being driven by currents, whatever depth they're
at, but they can also provide vertical information in various parts of the ocean. And essentially
the idea is that you drop them off somewhere and you just let them go and they continue to drift
and record as long as the instruments continue to function. So this is a plot from 2017. I have
an updated plot later, but this is a plot that shows all the floats throughout the world, the
location of all these Argo floats throughout the world. So you can see there are really,
there are thousands, so there are almost 4,000 floats covering almost every part of the
world's oceans with the exception of the Arctic. And so you can get on one hand, you can get in
the vertical quite small scales. I think it's probably on the order of meters or tens of meters.
But in the horizontal you can get basin scale or even global scale
information by combining the data from all of these different drifters. And so that's what the
plot in the bottom is showing. It's showing temperature at a thousand meters measured by
Argo floats and you can get a global perspective at a particular date or probably over a better
set over a particular period of what, of the variability of ocean temperature.
And on top of that you have a time series. You could look at how this evolves over time as these
measurements, as these instruments continue to record. So remotely obligated or automonas
underwater vehicles are sort of the next step beyond a Argo float. So probably the most,
the most similar is a, is a glider, which you see here on the left. And the big difference,
of course, is that it has wings here and a tail fin. And so by changing its buoyancy,
like the Argo float, it start, it can, it can rise and fall within the water column.
But because of the wings on its side, it descends at an angle and therefore also covers
not only a vertical distance, like the Argo float, but also a horizontal.
And so you can tell it to, you can, you can say, give it a starting point
and tell it to meet you, so to say, at a new location. So you can start it,
you drop it off here and you say, I want you to get to these coordinates. And so using its,
its tail fin, which it can, which can then help it turn, it can use the existing currents in the
water column to gradually make its way to the end point. This doesn't, of course, always worked,
depends on the strength of the currents. But eventually, but, but in any case,
you tell it to, every time it reaches the surface, or every time you tell it to reach the surface,
which may not be for every profile, it sends its signal back to your satellite. So you can track
its position. And if you can see that it's really not making its destination, then you know where,
at least where you can go and pick it up, or tell it to go to some other new destination update its,
its, its guidance. And these are just some kind of some numbers of typical speeds. So it can cover
about 10 kilometers in eight hours, or that's probably very highly variable, and moves about
20 to 40, 20 to 40 centimeters per second. And these platforms started out with, you know,
it's very simple, well, simple, the basic measurements of conductivity, temperature,
and pressure, but gradually more and more instruments are being added to them depending
on the particular research question that was trying to be answered from microstructure
profiles to look at turbulence, like these, like these Calvin Hamilton stabilities that we looked at,
or biogeochemical tracers to look at, for example, chlorophyll concentration, or pH levels.
And one of the biggest advantages of gliders, or these type of
autonomous vehicles, is that you can send them out in any kind of weather,
and they continue to measure no matter what the conditions. So what I didn't mention with ships
is one of the biggest drawbacks is they're limited just to some extent by the weather.
So when there's a big storm, you can't continue to measure, maybe you have to even find a safe
harbor. But with these gliders, you can send them out in any conditions, and they'll continue to
capture measurements, which can be extremely useful. And they're under, they can be underway for
months, providing really high resolution data in the vertical and horizontal, but also in time,
and over a long time period. The top left here is just an example of a remotely operated vehicle,
which kind of has a similar principle that you just can mount instruments to it. It can then
go through the water column and capture a variety of scales, depending on the instruments that are
mounted to it in the distance that you ask it to cover. But they can't, they are generally,
in this case, this would be an example of one that's connected to the ship.
Well, actually, I'm not sure that's true. But anyways, they're often connected to the ship,
or they're limited, much more limited in the time they can be out there based on because of
a power limitation. So they need to, in order to power the engine that guides them,
they can't, they need more energy and can't carry enough batteries to last as long as a
glider, which needs very little energy, since it's just changing its buoyancy generally.
So this is an example of glider data collected in the North Sea over several weeks.
And on the top, we have a plot of temperature with these white lines marking the picnocline.
And here is also in the middle is the thermocline. The gray is the bottom of the North Sea.
And you can see the really strong stratification at the start of this campaign. And what the
glider captured is a very strong storm, storm Bertha from 2014, and how this storm completely
mixed, mixed the four water columns went from a really strongly stratified system. So on the
bottom plot, we can see two different ways of calculating the thickness of the thermocline,
and then the red line shows the temperature difference. So we had a temperature difference
of almost of more than five degrees. So quite a strong thermocline, and quite thick, so five
meters to 10 meters. And then by the end of the storm, the water column, the full water column,
which is almost 40 meters deep, was completely mixed. And this is something that you couldn't
capture with a ship. You couldn't be out there in the conditions of the storm. And so you wouldn't
be able to see how this storm evolves. And in this case, they can look at in quite high detail
because the image instruments that were mounted on the glider, what happens in this period during
the storm, what are the mechanisms that drive this mixing that led to the mixing of the full
water column. And this concludes the third video in lecture two.