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Chlorophilia t1_is2agyq wrote

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

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.

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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.

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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.

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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?

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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").

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