Re: How is the movement of water affected by changes in temperature and salinity?

Dear Marion,

I'm going to discuss mostly the situation in the ocean. Keep in mind that the principles apply to all bodies of water including oceans, rivers, lakes and ponds and is extremely important in estuaries. It is the difference in density between different 'parcels' of water that affects circulation. If the water body is well mixed, there will be little density-driven circulation. This is the case for streams where differences in density are usually obliterated by rapid mixing.

I'll start with the simpler question first. Temperature and salinity both affect water density. An increase in salinity or decrease in temperature will increase the density of water and vice versa. That is, decrease salinity or increase temperature and density decreases. So, warm low salinity water is lightest and cold salty water is heaviest. Some interesting phenomena can occur when you have warm salty water and cold low salt water. More on that later.

The effect of density on circulation is a much larger topic than can be described in detail here. I recommend that you look for an introductory oceanography book to get a more complete description.

The short answer to your question is that currents in water bodies are immensely affected by density. There are two main components to ocean circulation -- wind-driven and density-driven. Near the surface, wind is an important force in circulation and is responsible for many of the swift currents we are familiar with such as the Gulf Stream. However, the effect of wind-driven circulation is limited to a thin layer of water at the surface.

Deep ocean circulation is wholly the result of density-driven circulation. Production of water that circulates to the bottom of the ocean (what oceanographers call 'deep water formation") is the result of density differences at the poles. Here dense water is formed because it gets cold at high latitudes by losing heat to the atmosphere AND gets saltier because as some of the water freezes into ice it leaves behind saltier water. SO here is where that cold salty water is formed and it is the densest ocean water. It sinks down and travels along the bottom of the ocean. It becomes the source of water that is upwelled elsewhere resulting in highly productive areas of the ocean that form famous fishing grounds. It also affects surface circulation because all the water that sinks down is replaced by water pulled from elsewhere at the surface. This is why the Arctic Ocean doesn't run dry AND why the Gulf Stream can continue moving water north without the Arctic Ocean mounding up.

So, density differences can cause water to move. They can also prevent water from moving. How? Well, in the large central areas of the ocean there is very warm water at the surface and very cold, salty water below. This results in a stable layering of water with the densest water on the bottom and the lightest on top. So, there will be very little movement of water from the surface layer down or the deep layer up. Some mixing will inevitably occur, but it is small and requires a great deal of energy to overcome the stable layering. Wind helps but the effect of wind rarely goes below 100 meters. So, generally in the ocean, we find colder saltier water below warmer, fresher water.

Density driven circulation occurs in lakes and ponds, too. However, except for some salt lakes, most of the density differences arise from differences in temperature caused by heating by the sun. This can result in layering in lakes, too. For example, many people swimming in a lake will say it is spring-fed because it is cold down deep and warm at the surface. Actually this is thermal layering. Warm, and therefore light, water is at the top while cold, dense water stays below. The effects of this layering are very important to algae in the water because it cuts them off from nutrients that are regenerated on the bottom. That is until the fall. Then the sun's heating decreases and often wind increases so that the difference in density between the bottom and top get smaller and smaller. When the difference is small enough, the lake will mix to the bottom. This is called ‘turnover' and often results in an algal bloom when that rush of nutrients is unlocked from the deep water.

Finally, an example of a strange density-driven phenomenon. The Mediterranean sea produces very warm and very salty water because it is partly cut off from the Atlantic and because it is so warm. The heat and dryness cause a lot of evaporation leaving behind the warm salty water. This Mediterranean water flows out into the Atlantic at the bottom while colder Atlantic water flows in at the surface. The result is a strange ‘tongue' of warm water at depth, below colder water in the Atlantic. It can only be there because its density is increased by the very high salt content.