Just where IS the Southern Ocean?

My recent interview on ABC Conversations elicited a response from one listener about just how I was defining the geographical reach of the Southern Ocean. I responded with a brief explanation of how I had approached this contentious question, and I thought it would be worth reproducing it here in case others might be wondering.

My research indicates that the northern geographical limit of the Southern Ocean became a matter of international disagreement in 1928, and that it remains contested today with various member countries of the International Hydrographic Organisation adopting their own version in accordance with their national interests.

1928 Limits of Oceans and SeasThe first edition of Limits of Oceans and Seas published by the International Hydrographic Bureau in 1928 (above) showed the Southern Ocean touching the southernmost coastlines of Australia, NZ, South America and South Africa. This ‘boundary’ reflected mariners’ understanding of the geographical extent of the Southern Ocean at that time. Some IHO member states subsequently disputed the boundary and, by the time the second edition was issued in 1937, the Southern Ocean boundary had been redrawn southward as shown below.

1937 Limits of Oceans and SeasThe Southern Ocean was omitted entirely in the third edition in 1953. The IHO explained that, according to opinions received since the 1937 edition, ‘…there exists no real justification for applying the term Ocean to this body of water, the northern limits of which are difficult to lay down owing to their seasonal change. The limits of the Atlantic, Pacific and Indian Oceans have therefore been extended South to the Antarctic Continent. Hydrographic Offices who issue separate publications dealing with this area are therefore left to decide their own northern limits. (Great Britain uses the Latitude of 55° South)’.

The IHO proposed a draft boundary in 2002 but it was never ratified by member states, and the 1953 edition effectively remains in force today. As a result, the geographical area of the Southern Ocean is represented differently on world and nautical maps and charts according to the country of issue. I briefly summarised this history in endnote 2 of the Prelude (p. 206) in Wild Sea as follows:

As the introductory map shows, the Southern Ocean’s geographical limits have long been a matter of disagreement amongst maritime nations. The International Hydrographic Bureau (now Organisation), formed by maritime nations in 1921 to create a uniform system of nautical maps and charts, issued the first edition of Limits of Oceans and Seas in 1928 showing the Southern Ocean extending to the southern coastlines of South America, Australia, New Zealand and South America. In the second edition published in 1937, the northern boundary had been moved southward and, by the third edition in 1953, the Southern Ocean was erased altogether and nations advised to determine their own boundary. The United States, for example, adopted the line of latitude at 60° South and Great Britain latitude 55° South, while Australia defined the Southern Ocean as all the waters south of its coastline from latitude 40° South to the Antarctic continent. The Southern Ocean was declared the world’s fifth ocean in 2000 in recognition of its physically and biologically distinctive waters south of the Antarctic Convergence, and the IHO issued a draft boundary in 2002, but its northern limits remain contested.

However, my decision to present the Southern Ocean’s northern limits as flowing as far north as the southern coastlines of Australia, New Zealand, South America and South Africa was based on a number of considerations. During my research, I concluded that the ocean’s geographical extent, as defined by the IHO and interpreted variously by its member nations, is only one approach to representing the Southern Ocean, and that there are also other ‘constructions’ of the Southern Ocean based on:

  • its biological characteristics as defined in the Convention on the Conservation of Antarctic Marine Living Resources (latitude 50° S);
  • its geopolitical boundary as defined by the Antarctic Treaty (latitude 60° S);
  • historical perceptions (such as early mariners who described the Southern Ocean as the region south from the Roaring Forties;
  • the region where the temperature of water and air suddenly drop along what is now described by oceanographers as the Subtropical Front (roughly latitude 40° S);
  • cultural perceptions, including the Indigenous peoples of the southern coastal regions whose ancestral ‘country’, I discovered, extends into the Southern Ocean; and
  • personal perceptions of southern Australians such as myself who have grown up with the (inherited) knowledge that the ocean that flows along their coastline is the Southern Ocean! Nowadays this is reflected, rightly or wrongly, in tourism maps and literature!

So as you see, I decided that there was no simple way of delineating the Southern Ocean because of the fact that its northern ‘boundary’ is fluid, its waters intermingling with those of the Indian, Atlantic and Pacific Oceans.


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