The Branches of The South

Immagine correlata
Credit: Te Ara

When you think about the ocean, much than the land, you cannot expect a linear response due to human action. The ocean is not a place that we can space, or shape or limit. For me the ocean is the higher form of the nature where everything retours to its place, probably due to the constant presence of the water. Today we will talk about one of the oceans that have a huge power in the global circulation, in the climate regulation and also in the biological distribution all around the globe: the Southern ocean

The absence of land barriers all around the Antarctic play a key role on the dynamics of currents in the Southern ocean, and this take also to the presence of a circular current called Antartic Circumpolar Current (ACC). The ACC consists of a number of circumpolar fronts, which correspond to water mass boundaries. The two main fronts are the Sub-antarctic and Polar Fronts. 

In the south of the Polar Front, the Antarctic Divergence is located. It is the place where high salinity NADW (North Atlantic Deep Water; you can see what means in the first articles about the global climate change) reaches the surface coming from depths of more than 2,000 m. The portion of this water mass reaching the upper 200 m warms the surface waters and melts the sea ice. In the Polar Front part of this water mass, now warmed and diluted by melt water, creating a new water mass known as Antarctic Intermediate Water (AAIW). It can be traced in all of the three world oceans, characterized by its low salinity. Close to its formation region it has a salinity of about 33.8 psu and a temperature of 2.2°C. The AAIW is located below the thermocline, but in depths shallower than about 1,500 m.

Freezing of sea ice on the Antarctic continental shelf leads to bottom water formation. The salt rejected by newly formed ice increases the salinity and so the density on the shelves. If there is wind forced ice export from the shelves the process of ice production can continue to inject salt into the upper layer of the ocean. When the salinity reaches a threshold value the water is dense enough to sink down over the shelf edges into the deep basins. On its way down the slope of the shelf intense mixing processes with water derived from the Circumpolar Current (CDW circumpolar deep water) form the properties of Antarctic Bottom Water (AABW). Therefore it would not be correct to account AABW production only to convection; subsurface mixing surely has an important role. This kind of processes are confined to some specific regions. Main area for AABW production is the Weddell Sea and in its initial stages AABW has a salinity of 34.66 psu and a temperature of about – O.8°C. The Antarctic Bottom Water is the coldest, and the densest, deep water in the open ocean. The AABW flows into the South Atlantic and eastward through the Indian and Pacific sectors of the Southern Ocean. Below 4000m depth, all Atlantic Ocean basins are mainly occupied by the Antarctic Bottom Water.

The two water masses of which we have just spoken therefore reach a different depth all the oceans and for this reason their indispensable role is evident.

From the casual distribution of land through the geological eras and the movements of the tectonic plates has been favored the creation of a separate continent from the rest of the world by the so-called Drake strait. The most remote part of the world, which shows no hospitality for man, is actually able to regulate global ocean circulation and more. These places should be protected so not only because they are the home of charismatic creatures like penguins and seals, but these places must resist, because we need them, they allow us to live. I think that the first of the two reasons is enough because if we protect nature we will also protect the humankind, but obviously it is not for everyone.

And that is why we are compromised.

Maria Bruno

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