Most mariners have heard of the terms ‘horse latitudes’ and ‘the doldrums’ but are less familiar with the meteorological name of ‘the subtropical ridge’.
   Doldrums in earlier English usage meant the opposite of a tantrum; that is a fit of dullness and sloth. Sailors stuck with no wind and not much to do coined the term to describe the region and the state of wind. Horse latitudes came about by vessels caught in these areas with no wind or rain would run short of water. Horses that were cargo were thrown overboard as a water saving measure, hence the name. Similar procedures were used on slavers with the weak and sick ‘human cargo’ being jettisoned.
   These areas all refer to the belts of high pressure around the globe at 30° to 35° in latitude; the sub tropical ridge.
   In a nutshell hot air rises in equatorial regions and sinks into belts of high pressure systems (northern and southern hemisphere) called the subtropical ridge. As the air sinking into these highs has come from high altitudes where it is very cold the air is also very dry. The sinking air generally brings light winds. Land areas that are affected by the subtropical ridge are often dry – Australia, North Africa, southern USA and Mexico.
   Some of the high pressure systems over the ocean are semi-permanent in nature and as a result drive large surface ocean currents or gyres. There are five main oceanic gyres and like high pressure systems rotate anticlockwise in the southern hemisphere and clockwise in the northern hemisphere. Due to the current’s rotation, water ‘piles up’ in the relative calm of the centre of these gyres.
   Most of us, especially city dwellers, know of areas where the autumn leaves and rubbish collect, blown by the winds. There are areas in the oceans where a similar process happens. Rubbish collects in the five gyres, and the more populated the adjacent regions, the greater the amount of rubbish collected.
   Despite the care we all take, there are not too many vessels that have not lost a plastic bag or bottle over the side. Plastic being durable becomes long term flotsam and collects in the gyres. With the large populations on the coasts of the northern Pacific, there has been much research into the North Pacific gyre which may contain up to 100 million tons of flotsam.
   “Unfortunately some species of seabirds mistake plastic flotsam for food,” said Dr Eric Woehler of Birds Tasmania. “The parent birds regurgitate the flotsam as ‘food’ to their young. The young birds typically die onshore at the nest from the ingestion of plastic before they are old enough to fly. The adult birds are much more likely to die at sea, with the flotsam in their stomach being returned to the ocean as their carcass decomposes.”
   An interesting video on this subject by Captain Charles Moore can be found on the TED (Technology, Entertainment, Design) website at http://www.ted.org/index.php/talks/capt_charles_moore_on_the_seas_of_plastic.html
   There are strict rules regarding disposal of waste at sea; check with your local marine authority. Local authorities do prosecute vessels and hefty fines, court and clean up costs fines can be awarded against the owners, master of the vessel and involved crewmembers.

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of QLD and is a Master V. He gives education courses on Marine Meteorology.