Transcription
Have you ever wondered just how ocean circulation works? What drives it? And what impacts it has on other Earth Systems? Like how changes in ocean circulation patterns might affect climate or ecosystems?
Well, in this video, we are going to be talking about just that: the effects of changing ocean circulation. How it works, why it's changing, what's changing it, what's impacting it, why, and what it is impacting. So, let's get started.
First, I'll talk about surface ocean circulation and how it works and all that. And then we'll move on to deep ocean circulation, because, turns out, they're pretty different.
The circulation of the surface ocean is driven mainly by wind. Global wind patterns drive gyres, or circular rotating currents, in all of the major ocean basins. These gyres are impacted by the Coriolis force, in other words, Earth's rotation. Because Earth is spinning on its axis, the Northern Hemisphere currents rotate clockwise, and the Southern Hemisphere ones rotate counterclockwise.
And this circulation of the surface ocean does a lot of transferring of heat from tropics to polar regions, as does deep ocean circulation, which we'll talk about later. In any case, it's a very similar concept to atmospheric circulation, which we talked about in my recent atmospheric video. Essentially, atmospheric circulation is driven by the temperature gradient from equator to poles due to the angle in which the solar radiation hits the surface. And then these create atmospheric circulation cells, which are affected by the Coriolis force, and therefore drive east-west trade winds, which drive surface ocean currents.
The loss of heat once water reaches the poles cools the surface water until its density exceeds that of the underlying water. Once this happens, this leads to something called downwelling, or the overturn of the surface waters to the deep waters. Essentially, eventually the water travels to the pole, it gets cold, and it sinks to the bottom due to its cold, dense, saline content. And this leads to vertical mixing of the ocean.
So, so far, we've really only talked about the surface. Um, but that leads to the vertical mixing of the surface waters with deeper waters. And this is really important for nutrient cycling. This is because in most of the ocean, vertical mixing is limited across the pycnocline, or the boundary in which the density changes drastically from surface to deep waters. Through most of the ocean, this density difference keeps vertical mixing from happening. It limits vertical mixing, and so the surface and deep waters are very different in density and don't mix in constituents very often.
However, along coastlines where the downwelling or vertical mixing occurs, this allows nutrients to cycle back to the surface ocean from the deep ocean, helping to fuel biological activity in the surface ocean. So, therefore, most of the ocean where there's limited vertical mixing is stratified. It's layered in a way based on density, and also this leads to stratification based on other factors as well: chemistry, temperature, and we'll talk more about that later.
But the extent of vertical mixing that occurs at the poles varies drastically based on season. For example, in winter, freshwater is removed as ice freezes, and the salinity of the water in that region, and thus the density, increases. And that increases the downwelling, increases the mixing. Whereas during the summer, the freshwater is added from the ice to the water, and it lowers the salinity and lowers the density, and therefore it decreases the amount of downwelling, and therefore vertical mixing.
The extent of vertical mixing at poles has also varied over long geologic time scales, depending on, for example, whether there was ice at the poles at all or not. In periods where there was no ice at the poles, for example, in the Cretaceous, there is obviously weak mixing due to the lack of downwelling, due to the lack of differences in density at the poles. And this led to a very strong stratification of the water column, uh, due to this lack of mixing. And this led to ocean anoxia, or very low oxygen levels, to, you know, completely zero oxygen in some regions, um, in the deep water, because vertical mixing allows the delivery of oxygen from surface to deep waters. And when that doesn't happen, the deep waters can become very anoxic, which can be very detrimental to life, obviously.
Now, moving to deep ocean circulation. So, we've talked about surface ocean circulation, driven mainly by wind, and also a little bit about the vertical mixing between the surface and the deep ocean. But deep ocean circulation, instead of driven by wind, is mainly driven by temperature and salinity. Just like what we we talked about with the downwelling, where it comes down to a decrease in temperature and an increase in salinity causes it to become denser and sink to the bottom. The same thing is true anywhere on Earth, not just at the poles.
And so this leads to a whole belt of circulation called thermohaline circulation, also nicknamed, uh, the conveyor belt, the global ocean conveyor belt, um, because this thermohaline circulation belt kind of does connect all of the ocean basins and lead to fully mixed, uh, oceans due to the way it transports surface and deep waters across this ocean belt. So, in contrast to the relatively isolated gyres within each ocean basin, the thermohaline conveyor belt transfers seawater between the major ocean basins, which is really important for a well-mixed ocean.
The modern ocean conveyor belt, or the modern, you know, pattern or shape that this belt takes place in, has developed relatively recently in the Cenozoic. Specifically, the Antarctic Circumpolar Current, which is kind of the isolated current around Antarctica. The event that allowed isolation of waters to circulate around Antarctica, uh, was the isolation of Antarctica as a continent, which happened over time when, you know, all the other continents kind of moved away from it, uh, as it broke up from being Gondwana. And the last few to do this were India, then Australia, and then, of course, uh, South America broke away, um, because it was connected for a long time, and, uh, it broke away, forming the Drake Passage, which now has allowed circulation to go through and, and kind of isolate itself around Antarctica. Which actually was the first major step in beginning our current Cenozoic cooling trend that eventually led us into our current modern ice age.
The other major event that helped this along, uh, occurred around 3 million years ago, and this was the connection between North and South America. This connection not only allowed for migration of animals across this boundary and bio-diversification in their respective continents, but also caused or exacerbated the cooling. Because before the connection, the Atlantic waters mixed with Pacific waters, decreasing their differences in salinity. And thus, now with the connection there, the Atlantic water has relatively high salinity, causing downwelling before it reaches the Arctic and allowing the Arctic to cool.
But downwelling is only half of the story regarding the ocean's vertical mixing. Upwelling can also occur, and this can be driven by a few different processes. But one major one is surface winds along the coasts. And this upwelling allows cold, nutrient-rich deep waters to be brought to the surface, and therefore fuel primary productivity or algal blooms at the surface because of the nutrient influx.
And during periods in which there's been global warming, the temperature gradients from equator to poles that drive the mixing, this vertical mixing becomes weaker. And this leads to stronger ocean stratification, like I mentioned earlier in the Cretaceous. And we've seen a similar trend in recent years, where down and upwelling have slowed due to the current warming trend. And like I mentioned earlier, when there's a major stratification or stagnation of the water column and the deep water becomes really anoxic, this can be devastating for life. And we've already seen some major devastation due to this ocean stratification in recent years among marine life, especially during El Niño years.
So, first things first, El Niño versus La Niña. La Niña years are essentially when there's stronger trade winds that cause an increase in the strength of upwelling, and thus cooler sea surface temperatures in the central and eastern equatorial Pacific. Whereas El Niño years are characterized by weaker trade winds, which obviously weaken upwelling and temperature gradients across the Pacific, which leads to less mixing, and therefore more stratified ocean columns with warmer surface waters and very oxygen-depleted or anoxic bottom waters or deep waters. And for this reason, El Niño years typically cause more devastation to marine life than La Niña years, especially to coral reef ecosystems.
This is because corals have a symbiotic relationship with algae that kind of live in their polyps and help provide them food. And these algae require very specific temperature ranges. And so as water warms rapidly, the algae basically get the heck out of there. And that's really not great for the coral because in it, it becomes bleached. And this is why we see all these corals being bleached. They're not necessarily dead when they're bleached, but when they're bleached, it's a very, very small chance that they will ever live again, and therefore they will die, um, because to kind of be recuperated from that, the algae would have to come back. And unless the water went back to the temperature range they like, uh, they're not coming back.
But upwelling also leads to the release of CO2 from the ocean to the atmosphere. So, weaker upwelling also leads to higher marine CO2 levels, and thus more calcium carbonate dissolution. Yeah, we don't necessarily want the CO2 to be going into the atmosphere from upwelling because the atmosphere's already got increasing amounts of CO2. But it's also not great if it stays in the ocean because the ocean is also dealing with increasing CO2, which is causing ocean acidification. Essentially, CO2 reacts with water to form carbonic acid, and this combination leads to the dissolution of calcium carbonate minerals.
Why is that important? Well, many corals, mollusks, sponges, and other marine life build their skeletons using calcium carbonate. And so if the ocean continues down this pathway of increased CO2 and increased acidity, it will become harder and harder for these organisms to produce their skeletons, and in some regions, they might even start to dissolve.
Summary: Increasing CO2 levels in the ocean and calcium carbonate dissolution is bad for reef ecosystems. Moreover, carbon dioxide solubility increases in cold deep waters. So, carbon dioxide concentrations in the ocean are higher in colder, deeper waters, meaning that the surface corals that are getting bleached can't necessarily rely on deep corals for the survival of that group of organism because the deep corals are being dissolved. So, both shallow and deep corals are in serious danger.
Also, the weakened mixing, vertical mixing of the ocean, and intensified stratification of the ocean water column will lead to ocean anoxia, like I talked about that happened in the Cretaceous and other periods in Earth's history. Um, this oxygen depletion will also cause devastation among marine life because, just like terrestrial animals, marine animals need oxygen to live.
And in addition to stratification causing ocean anoxia, there's also another process that is leading to ocean anoxia or exacerbating the already anoxic or hypoxic low oxygen regions of the ocean, and that is an increase in nutrient influx to the ocean, which is basically an increase in like nitrogen and phosphorus compounds from over-fertilization being delivered to the ocean through river, uh, inflow, which is delivering all these nutrients to these primary productivity or algal blooms, which are causing algal blooms to basically overproduce organic carbon to the point that it becomes buried so quickly and in so much abundance that the oxygen is used up to decompose it in the water column beneath the bloom. And with the stratification of the water column, the oxygen from the surface won't get mixed back into it, and therefore the deep water column will just continue to become more and more oxygen-depleted as it tries to keep up with the overproduction of organic carbon falling to the seafloor from these algal blooms. This is called eutrophication, and it's not great.
But if you want to hear about an idea that people have had about increasing algal bloom activity and organic carbon burial without the ocean anoxia repercussions, there are some ideas that people have about that, and I talk about that in my ocean fertilization video. So I will link that to the top right if you're interested.
So anyway, I hope you guys enjoyed learning about ocean circulation in today's video and the impacts it has on other systems, as well as vice versa. If you did enjoy this video, I highly recommend that you check out next week and the week after's video. I'm I'm planning to make them both kind of extensions of this one. Next week's being all about ocean composition and chemistry changes and how that impacts other systems and vice versa. And then the following week is all about current ocean changes that are happening and their impacts on mainly life and climate and other things. So if either of those are out by now, then I'll link one up here. And if not, uh, come back next week and check it out, and I will see you guys there. Bye.