The autumn mixes the water layers formed in the summer
The autumn thoroughly mixes up the water masses of Airisto. In the summer, the temperature of the surface water can rise to over 20 degrees, but deeper down the water temperature remains well under 10 degrees even in the middle of summer. In other words, the water column becomes layered, with the warm surface waters and the colder waters near the bottom not mixing during the summer. The layer separating them, where the temperature changes rapidly with depth, is called the thermocline. The layering is caused by the temperature of the water: the warmer surface water is lighter than the colder water near the bottom.
In the autumn, the surface waters cool down, and as their temperature falls to around 10 degrees Celsius, the vertical layering of the water collapses and the masses of water are effectively mixed from the surface all the way to the bottom. The mixing is further facilitated by strong autumn storms. This occurrence is called the autumn overturn. The annual autumn overturn is important because it also supplies oxygen-rich water to the sea bottom, which may be suffering from a lack of oxygen after the summer.
