Water and Oceans
|Q: How does salinity affect ocean currents?||A: Salinity has an effect on the density of water. The higher the salinity the larger the density as long as all other density influencing factors (e.g. temperature) stay the same. Density, like in the atmosphere is one of the driving factors of flows. Remembe,r if the density of an object is lower than its surrounding it will rise, if its higher it will sink. It's because the larger a density the larger it's weight per m3. The two main factors to increase density of water in the ocean are low temperature (but be careful, the temperature passes a certain point density decreases again, water is very special in this case) and high salinity.|
For the thermohaline circulation we need water masses to sink in some regions and rise in others. The two main areas of sinking are in the northern Atlantic (East of Greenland) and the southern Atlantic (near Antarctica). Here the water density becomes respectively high because the temperatures are very low and ice formation increases the salinity.
To understand why the salinity is increased, think about a bucket of salt water. In this bucket the amount of water is staying the same but can exist in liquid or solid (ice) form. If you cool down the temperature ice will form. We know that ice is solid fresh water and does not include salt. So the salt that was in the frozen water before it froze will be given up the remaining liquid water. The more ice is formed the less liquid water is left, which has to hold the salt and the salinity increases.
This is what happens in the two before mentioned regions as well. In the Arctic and Antarctic lots of ice is formed, which leads to an increased amount of salt. Cold and salty water means high density which again means that the water will sink. If we wouldn't have these regions of deep water formation the whole thermohaline circulation wouldn't work. Therefore salinity and temperature are two important factors in the formation of ocean currents.
|Q: Because Nansen did his experiment in ice and not liquid water, does this mean that the Coriolis effect will lead to Ekman spirals the same way under both circumstances? One would think wind would push liquid water at a faster rate than ice, or that something else might be different.||A: It might be actually the other way around. Since ice sticks out if the water there is a larger surface the wind can act on. For an ice berg the wind can push on the sides and produce friction on top, which helps moving it. Whereas for water the wind c an only act on a flat surface by friction. Imagine you have a piece of paper. If you put it flat on a surface and start a fan the sheet might move only by a bit, if you put it perpendicular to the surface blocking the wind it would move very fast.|
However this is just explaining the speed at which the water or ice moves. The Ekman spiral is defined by its characteristic deflection. In the case of an ice berg it can stick deep into the ocean surface layer (only about 1/10 of it sticks out of the water). So actually its movement can be influenced by deeper layer. Therefore it can actually happen that a deep reaching ice berg would move into another direction than a thin ice sheet or highest layer of water right at the ocean. The theory is still the same (same reasons why it happens).
But however the Ekman spirals that can be seen on Earth are usually not perfectly following the model. The Ekman spiral is a theoretical model and we know that on Earth (including atmosphere and ocean) things are very complex and therefore most processes don't work exactly how they work in theory.
|I don't understand the concept of Ekman spiral and how it relates to Coriolis Effect.||The Ekman spiral is produced by the deflection due to Coriolis force. This is the same effect that we find acting on winds. (To find more information about what the Coriolis effect is, please refer to the post in the atmospheres section.)|
When wind blows over water, friction moves momentum downward into the water. This means it starts moving the water.
As we know on Earth (actually on any rotating system) the Coriolis force acts on all fluid flows. Now if we have a look at a layer in the ocean, the forces acting on this layer are the friction induced by the movement of the layer above and the Coriolis force. These two forces are facing different directions. The resulting force is than the added vectors (a vector tells you the exact direction and the strength) of these two. But be careful, it's not just the added total forces only the vectors. Because the Coriolis force and the friction are not into the same direction, the resulting force will be directed between these two forces and therefore the flow will be deflected. This is the reason why we see a turning of the flow direction with depth in the Ekman spiral.
Now the deeper we go in the ocean the more the friction will slow down the flow. This is the reason why the flow strength decreases with depth in the Ekman spiral.