Viewing a single comment thread. View all comments

jellyfixh t1_is1t12y wrote

Reply to comment by Chlorophilia in Does the salinity of ocean water increase as depth increases? by rhinotomus

Thank you for the correction, I had never heard of the mechanical input to the MOC. So it seems density drives the initial formation but mechanical energy is needed to have the water circulate back to surface?

297

Chlorophilia t1_is1ugnv wrote

Yes exactly. As you completely correctly wrote, the parts of the ocean with the right conditions to create very dense water masses (e.g. the marginal seas of the North Atlantic and Southern Ocean) are the parts of the ocean where deep water formation occurs. You can't have an overturning circulation without deep water formation. But this isn't an energy source, because no energy input is required for dense waters to sink below lighter waters. The problem is that, in order to have vigorous overturning, these dense waters have to somehow be returned to the surface (and at a significant rate). There's no process in the deep-ocean that adds (non-negligible) freshwater or heat, so the deep waters can't rise buoyantly. As a result, the only way water can return to the surface is by being dragged up by wind-induced upwelling, or (probably to a lesser extent) mechanically mixed up, probably mainly due to tides. What exactly are the key processes that determine the strength of the AMOC is an active research question, and the freshening of the North Atlantic is absolutely capable of reducing the AMOC strength (because if you're generating deep water at a lower rate, you're also going to upwell less deep water), but the point is that deep water formation isn't a driver (or energy source) of the overturning.

216

Bear_Wills t1_is1xafc wrote

>There's no process in the deep-ocean that adds (non-negligible) freshwater or heat, so the deep waters can't rise buoyantly.

Freshwater makes sense, but do geothermal vents in the deep-ocean not add non-negligible heat? (Apologies if that is a silly question, just found the conversation very interesting as someone with little knowledge in this area, but that part stood out to me)

83

Chlorophilia t1_is1yzjg wrote

This is a good point! Geothermal vents are too localised to be significant, but there is a geothermal heat flux everywhere on the ocean floor (due to heat escaping from the Earth's interior). However, this heat flux is on the order of 0.1W/m^2. By contrast, the heat flux at the ocean surface from the sun is of order 100W/m^2. So for the heat budget of the ocean as a whole, the geothermal heat flux is negligible. Locally, at the ocean floor, it has been argued that the geothermal heat flux could be non-negligible. However, this is not routinely incorporated into ocean or climate models (I will admit that I didn't even know this had been properly looked into before your comment made me look it up!) and, whilst it's possible that it could have some second-order effect, it's orders of magnitude too small to drive the kind of large-scale overturning we see in the modern Atlantic.

150

phantasmagorical_owl t1_is28geh wrote

I can't speak for other ocean or climate models, bit geothermal heating is included in NASA's ECCO ocean models and state estimates. Its magnitude is small relative to ocean surface heat fluxes but geothermal heating does help maintain a more realistic deep ocean state by reducing the drift in deep temperatures.

38

Chlorophilia t1_is28uwa wrote

> I can't speak for other ocean or climate models, bit geothermal heating is included in NASA's ECCO ocean models and state estimates.

That's good to know! I was not aware of this.

22

Aeellron t1_is24aot wrote

Man the facts about the sun just never cease to amaze.

Orders of magnitude more heat introduced than geothermal vents.

As soon as you think about it it makes sense.

14

Shaetane t1_is2zdnv wrote

We really aint nothing without our resident well-distanced, well-temperatured, star

14

salsashark99 t1_is39c04 wrote

What is the range of salinity of ocean water?

2

Chlorophilia t1_is3aa0f wrote

Most of the ocean is between 32-36g/L (so quite a tight range!). You can get more extreme values in some marginal seas, and of course places like lagoons and estuaries.

6

Swiss_cake_raul t1_is3xvlq wrote

I actually knew this fact already but reading it written as g/L instead of ppt just made me realize it's the same ratio of salt:water as my sourdough recipe!

2

phantasmagorical_owl t1_is299zb wrote

Wouldn't one expect local Ekman pumping (wind induced upwelling) and internal tidal mixing to be indifferent to the rate of remote deep water formation? The properties of the upwelled deep waters certainly vary based on what their surface properties when they descended.

3

Chlorophilia t1_is2agyq wrote

The processes of deep water formation, and return pathways to the surface, are closely coupled because of conservation of volume. Unless you have an enormous expansion of abyssal water masses, Ekman suction + tidal mixing cannot be independent of deep water formation rates. The point is that, regardless of the buoyancy forcing taking place in locations of deep water formation, they're fundamentally limited by the amount of water that you're (mechanically) returning to the surface. You can create as much dense water in the North Atlantic as you want but, if there's no return pathway to the surface, you're not going to generate deep water at a significant rate.

5

phantasmagorical_owl t1_is2l9ad wrote

Ekman suction and tidal mixing are independent of deep water formation rates. Deep water formation by surface buoyancy forcing at high latitudes is not the only way that abyssal waters can be renewed. A thought experiment: an isothermal isohaline ocean is subjected to ekman pumping from surface wind stress in one region. Upwelled water would subsequently redistribute away. Conservation of volume dictates that the upwelled waters are replaced, and of course they are by surrounding waters at depth, possibly many depths. An overturning circulation will become established, with waters away from the upwelling site "sinking" to replace the upwelled waters, although the sinking is more akin to falling, as there is a decrease in the volume of waters below. Possibly the sinking would occur uniformly over the entire non-upwelling basin, or it might be confined to an area around to upwelling, but the resulting overturning circulation would look different than our current MOC.

The rate of surface water transformation depends only on local buoyancy forcing and initial seawater properties, it doesn't know about remote upwelling rates. However, if upwelling ceases, the abyss would eventually fill up with dense transformed water and the rising isopycnal of that "deep water" would eventually limit the depth to which the newly formed dense water sinks. So, in that sense, the rate of deep water formation does eventually depend on there being upwelling or tidal mixing elsewhere.

2

Chlorophilia t1_is2mosd wrote

> Ekman suction and tidal mixing are independent of deep water formation rates.

I think you're misunderstanding what I'm saying, because I'm not disagreeing with you - I'm not saying that Ekman suction and tidal mixing are a function of deep water formation rates. I'm saying that deep water formation rates are (to first-order) a function of Ekman suction and diapycnal mixing. As you say, at equilibrium, the rate of deep water formation is limited by the available return pathways. If upwelling ceases, it is not possible to maintain deep water formation.

2

ReynAetherwindt t1_is32r7a wrote

I don't mean to be obtuse but "deep water formation" sounds like the result of a flood, like, "That there's some deep water, and it weren't there before."

What the heck does it actually mean?

3

Chlorophilia t1_is3561q wrote

It's a good question. The uppermost layer of the ocean is called the 'mixed layer'. As the name suggests, it's a well-mixed layer where the properties are set (over short timescales) by the atmospheric conditions above and, because of weaker stratification at higher latitudes, it tends to be shallow at low latitudes and deep at high latitudes, particularly in the winter. When we talk about a water mass being formed, this usually refers to water leaving the mixed layer, and thereby no longer having its properties directly forced by the atmosphere. This can either occur through a time-mean vertical velocity, or horizontal currents (if the mixed layer profile is sloped). Deep water formation specifically refers to the formation of a water mass that is deep (where "deep" usually means "below the thermocline").

3

TheProfessorO t1_is2x0b0 wrote

Eddy flow over the bottom produces larger vertical velocities than the mean wind driven upwelling

3

Chlorophilia t1_is2ym6p wrote

Eddy velocities are by definition zero in the Eulerian time-mean, so that in itself isn't going to result in a time-mean vertical transport. Eddies in the Southern Ocean actually counteract Ekman suction but I'm not sure on what basis you're arguing that eddies are responsible for most upwelling in the Southern Ocean? Can you provide a study supporting this?

1

TheProfessorO t1_is38c7h wrote

I was not talking about any ocean in particular. The eddy vertical velocity is proportional to the eddy horizontal velocity dotted with the topographic gradient. So the average eddy vertical velocity can be nonzero when the mean eddy horizontal velocity is zero. The importance of this term for mesoscale ocean dynamics was shown by Tom Rossby and a student in the late 80s.

1