World Building - Oceanography, anyone ?

[rust]

Although this is a rather specific question, perhaps there is some-
one on this forum who could help me with it ?

As mentioned elsewhere, my setting is a pure water world (axial tilt
4.6°, no land at all, warm climate without permanent polar ice caps,
average sea water salinity 3.4 %) with a human colony on it.

The colony's council (of PCs) recently came up with the idea to build
a first free-floating settlement that should drift on the main ocean cur-
rent of the northern hemisphere, but unfortunately I have no idea what
that current should look like, and my research did not really find any
convincing answers to my questions.

From what I was able to find, it seems the current should move clock-
wise at a speed of perhaps 5 km/h between the equator and the nor-
thern polar region, while there should be an almost current-free region
immediately north of the equator and a circumpolar current around the
northern polar region.

This looks good and simple enough, but I do not really trust it. If this
model were true, there obviously would have to be several (three ?)
such clockwise current in the northern hemisphere, and I do not know
how the heat exchange between the equatorial region and the polar
region could work in such a model (deep sea currents ?).

So, if any of you have some knowledge of planetology or oceanography,
please (please !) come to my rescue, because otherwise I will just have
to make this up - and then be at the mercy of my players, who will of
course discover any mistake I made ...

Thank you very much !

 

[EDG]

As mentioned elsewhere, my setting is a pure water world (axial tilt 4.6°, no land at all, warm climate without permanent polar ice caps, average sea water salinity 3.4 %) with a human colony on it.

OK, so by "pure water world" you really mean it's got 100% water coverage, not that the water itself is "pure water" (since you're talking about salinity here.

The colony's council (of PCs) recently came up with the idea to build a first free-floating settlement that should drift on the main ocean current of the northern hemisphere, but unfortunately I have no idea what
that current should look like, and my research did not really find any convincing answers to my questions.

First, I'm a bit rusty on this so don't take this as 100% accurate! As far as I can recall, the currents depend on a lot of things. On a grand scale, water will circulate in an anti-clockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere (the different directions are due to coriolis effects). I think this is also to do with warm water flowing to colder areas and cold water flowing to warmer areas too.

I think if you had an ideal very deep (hundreds of km) ocean then you'd have a number of large circulating cells in each hemisphere. However, unless you've got a panthalassic world you'll have two things changing that - topography on the ocean floor and the salinity of the water. On Earth you get cold water sinking at the poles and warm water rising at the equator, and those circulate around to make a "Great Ocean Conveyor belt" ( http://science.nasa.gov/headlines/y2004/05mar_arctic.htm ). And of course on Earth we have land masses in the way too. All of those factors combine to give you whatever currents you end up with. ( http://en.wikipedia.org/wiki/Ocean_current )

From what I was able to find, it seems the current should move clockwise at a speed of perhaps 5 km/h between the equator and the nor-
thern polar region, while there should be an almost current-free region
immediately north of the equator and a circumpolar current around the
northern polar region.

Sure.

Sounds probable. Though I have no idea if you accounted for atmosphere here too - winds will drive some of the flow too, and you can get a massive fetch on a land-less ocean. Basically, you can have huge hurricane zones because there's no land to stop them from building up - they'd make Katrina look like a light breeze.

This looks good and simple enough, but I do not really trust it. If this model were true, there obviously would have to be several (three ?)
such clockwise current in the northern hemisphere, and I do not know
how the heat exchange between the equatorial region and the polar
region could work in such a model (deep sea currents ?).

If you're that interested in it, get some university level Ocean Science textbooks (Open University did a really good series IIRC). But this is getting into the realm of full-on ocean modelling (where you have to worry about ocean-atmosphere interaction too).

Alternatively, get the Blue Planet RPG. It's got a massive amount of info on a specific ocean planet, and it was written by people who knew what they were talking about (at least get the Player and Moderator guides, and Natural Selection).

In fact, I'd strongly recommend getting Blue Planet and working out from there if you need more info.

 

[rust]

Thank you very much for your fast and comprehensive answer !

I will take a look at the Open University material and try to get Blue Planet
(which is on my "to buy" list anyways).

Until then I will continue to make it very difficult for the players to get the
floating settlement ready (they want adventures, don't they ?), and if they
handle all the challenges well, I can still have the settlement drift happily
in a circling current until I have understood and worked out more of the
entire current scheme.

Thank you again !

 

[EDG]

I am pretty sure that the "hypercanes" (the really huge hurricanes) are going to do their best to demolish any floating settlements. As soon as the settlement crossed into the danger zone ( a wide belt around the equator IIRC) then it's going to be in trouble.

 

[rust]

As soon as the settlement crossed into the danger zone ( a wide belt around the equator IIRC) then it's going to be in trouble.

Ah, very good (for me, this is) !

This will give the players' characters something to work on (I think they
will come up with drives for their floating settlement ...) and - more im-
portantly - will give me an opportunity for a dramatic adventure, like a
high-risk rescue mission (once someone has sabotaged the drives, and
the settlement keeps drifting into that "hypercane" ...).

Thank you once more !

 

[EDG]

You might have a bit of trouble finding BP books in stores now (get the v2 ones if you can find them) - but FFG are still selling them on their online store at http://store.fantasyflightgames.com/blueplanet.aspx . If you pick up the Essential Collection, the Players Guide, the Moderators Guide, and Ancient Echoes (not listed there, it's the cetacean sourcebook) then you'll have the whole v2 line.

Redbrick Limited have also recently licensed the game (they have Fading Suns and Earthdawn too) and are going to be re-releasing BP on PDF soon apparently.

 

[Myrm]

Redbrick Limited have also recently licensed the game (they have Fading Suns and Earthdawn too) and are going to be re-releasing BP on PDF soon apparently.

Hmm, there are several players I know who are going to be very happy about that....

 

[AKAramis]

Redbrick Limited have also recently licensed the game (they have Fading Suns and Earthdawn too) and are going to be re-releasing BP on PDF soon apparently.

Hmm, there are several players I know who are going to be very happy about that....

http://rpg.drivethrustuff.com/index.php?cPath=4464 is where they will show up whence they arrive.

 

[rust]

Thank you, I will keep an eye on the Drivethru website !

 

[Thunderbolt]

I've been watching the BBC series, "The Living Planet" and the episode I just finished attributes the presence of polar ice at the poles to the effect of distributed land masses.

These disrupt the flow of warmer water from equatorial areas that might otherwise keep the poles ice free.

Your water world may well have no ice at the poles at any time in the year as a result of circulation.

 

[daryen]

To extend the discussion, I remember reading an article about the possibility of a total water world (I forget the actual term). The idea of such a world is that there is no ocean floor. It is just water all the way down until the pressure turns it into whatever unimaginable pressure would turn water into. (There may or may not be an actual metal core. If there is one, it is just too far down to make a sea floor.)

Is such a world actually possible? And would a significant human-compatible ecosystem be possible without an actual ocean floor?

 

[Silvereye]

I'm kind of sleepy, so I'll just fire some of what I remember about oceanography down. Its your game, so use what sounds coolest.

With Earth, the biggest influences on the water are the gravitational pulls of the Sun, Moon (and then Jupiter - I think). The tidal forces, the tides are more extreme toward the poles, than they are at the equator. The next biggest drivers are winds, and atmospheric pressure differences. A decent wind (over a long distance) can add a couple of metres to a tidal range.

Now the tides really need a land mass to crash against to dissipate their energy, as your world has no land masses for this to happen, those tidal forces will continue to build into some pretty violent seas. Your best bet to see what effect they could be like is to have a look at articles on the Southern Ocean (around Antarctica). I'd guess that the best way to simulate world this would be to look at how atmospheric winds (jet streams) work (its a cellular system I think), and apply it to to the water as well.

Trying to remember, I think oceanic circulations are based on factors like temperature and salinity which both mess about with water density (less salinity is less dense - there is a really cool underwater 'lake' of more saline water somewhere on a seabed on earth) and the winds stirring up the surface. Another factor is the seabed topography - lots of valleys and sea mounts and things which stir up the circulation like mountain ranges and valleys stir up the atmosphere.

Now, you've not said anything about the biologicals, but most of the seas surface on Earth is even more devoid of life than most deserts. The nutrient rich waters are located deep underwater, and where they come to the surface (such as sea mounts and coastal shelves) you get the hot-spots of life.

If it's not too late in your world building, I'd definitely add long underwater mountain ridges (you can also have the hydrothermal vents and stuff here), and the occasional exposed volcano on the surface, your weather will still be pretty brutal, likely to scour away and break any surface rocks over a relatively short time. It would also give you a decent current circulation, and biological potential.

 

[EDG]

To extend the discussion, I remember reading an article about the possibility of a total water world (I forget the actual term). The idea of such a world is that there is no ocean floor. It is just water all the way down until the pressure turns it into whatever unimaginable pressure would turn water into. (There may or may not be an actual metal core. If there is one, it is just too far down to make a sea floor.)

Is such a world actually possible? And would a significant human-compatible ecosystem be possible without an actual ocean floor?

That'd be the Panthalassic worldtype (roughly) - take a rocky planet a bit more massive than Earth (1.5- 2), add an earth mass or two of water to the top. The water compresses into high pressure ice phases with depth (you'd probably have dozens of km of ice at the bottom of the ocean) and then below that is a rocky crust, partially molten mantle, and inner/outer core.

They are possible, though nobody's detected one yet (some of the low mass worlds recently detected around other stars might be though). As for life, it's anyone's guess.

 

[rust]

Your water world may well have no ice at the poles at any time in the year as a result of circulation.

Thank you very much, this is a most welcome information, because I pre-
fer my setting without any kind of polar ice caps - it makes sea traffic
across the polar regions that much easier.

 

[rust]

If it's not too late in your world building, I'd definitely add long underwater mountain ridges (you can also have the hydrothermal vents and stuff here), and the occasional exposed volcano on the surface, your weather will still be pretty brutal, likely to scour away and break any surface rocks over a relatively short time. It would also give you a decent current circulation, and biological potential.

Yep, I have included quite a number of ridges, trenches and seamounts in
the topography of my water world, for exactly the reasons you mentioned.
However, none of the mountains and volcanoes extends above the surfa-
ce, and the permanent settlement of the human colony is floating above
and built upon a seamount that is about 200 m below the surface - by far
the shallowest region of the planet.

Since I was too lazy to calculate the tables for the tides (and knew that
my players would have asked for them ...), my water world has no moon.
So there are only the solar tides left to consider, and from what I under-
stand of the subject they will not make the already "interesting" weather
of the planet much worse.

As for the biology, there is not much left of it, because a "nearby" (ca. 50
lightyears away) supernova event some centuries ago killed most of the
native biosphere - only some photosynthesis-independent life forms at
hydrothermal vents in the deep sea managed to survive, among them a
perhaps semi-intelligent kind of "octopoid".

My colonists therefore have to build up a new viable biosphere by intro-
ducing life forms from other planets, and the expeditions to search for
suitable creatures, to catch them and bring them to the water world (and
to handle the sometimes unforeseen consequences) are one of the ad-
venture types of the setting (besides alien deep sea ruins, trade and pi-
rates, diplomacy and regional wars, conflicts between factions among the
colonists, and all that usual "stuff").

Thank you very much for the informations and ideas !

 

[E.T.Smith]

...none of the mountains and volcanoes extends above the surfa-
ce, and the permanent settlement of the human colony is floating above
and built upon a seamount that is about 200 m below the surface - by far
the shallowest region of the planet.

Since I was too lazy to calculate the tables for the tides (and knew that
my players would have asked for them ...), my water world has no moon.
So there are only the solar tides left to consider, and from what I under-
stand of the subject they will not make the already "interesting" weather
of the planet much worse.

Still, even though the daily effect is minor, the accumulation of uncounted millenia of solar tidal action on a world-ocean without landmasses to slow it down would likely become staggering. It is possible that the variance would be enough for a seamount 200m below the surface to become an island with a respectable mass above surface level during low tide. Which would make a great evolutionary hotspot,

 

[rust]

It is possible that the variance would be enough for a seamount 200m below the surface to become an island with a respectable mass above surface level during low tide. Which would make a great evolutionary hotspot,

While this is doubtless a very interesting idea, a tide of that kind would
destroy my entire setting: The floating city above the seamount would
crash onto the domed habitats on the seamount ... :shock:

I think I would prefer some handwaving solution, like the seafloor moun-
tain ridges and perhaps ocean and wind currents or planetary climate
changes (with lots of sea ice in the past) absorbing most of the solar tide
effects, to keep my colonists alive a little longer.

However, there is no reason why something of the kind you described
could not have happened in the more or less distant past, and therefore
could be a potential future desaster scenario for the colonists.

It would be somewhat unfair to spring it on them right now, because they
have just discovered that their sun is an irregular long-term variable star
(ah, perhaps there could be some connection with the sea ice that absor-
bed the tidal effects ...?), and that they might have to prepare for out-
bursts of radiation from their sun.

On the other hand, why should I make life easy for them ? :twisted:

Thank you for the idea, there is a scenario on the horizon ...

 

[Ishmael]

I think I would prefer some handwaving solution, like the seafloor mountain ridges and perhaps ocean and wind currents or planetary climate
changes (with lots of sea ice in the past) absorbing most of the solar tide
effects, to keep my colonists alive a little longer.

lots of sea ice in the past?
It melted and the sea levels rose?
maybe there's ruins of an older civilization on some seamount that hasn't been swept away by the tides yet...

 

[EDG]

lots of sea ice in the past?
It melted and the sea levels rose?

Hint: melting sea ice doesn't affect sea levels, it's ice melting on land and the runoff going into the sea that does that .

All the north polar cap (floating on the ocean) on Earth can melt and we wouldn't notice any significant rise in sea level. But if Greenland's and Antarctica's icecaps melt, the sea levels would rise quite a bit.

 

[rust]

lots of sea ice in the past?
It melted and the sea levels rose?

No, as EDG already explained, that would not really change the situation
(although it would indeed be an interesting background for another precur-
sor-civilization on my planet).

I was looking for something that could absorb the accumulating effect of
the solar tides instead of continents. So, if I do not have permanent con-
tinents to "break" the tides, perhaps temporary ones (of sea ice) could
prevent that the tidal effects build up over the millenia ?

If such ice continents, held in place by undersea mountain ranges, could
do the job (with not too much obvious handwaving), an ice age now
and then should be enough to prevent accumulated "monster tides".