In the early days of oceanography, from the middle of the 19th century, scientists had a simple way of investigating ocean currents: masters of German merchant vessels were given an empty bottle and a sheet of paper, which they completed by filling in the date before throwing the tightly sealed message-bearing bottle overboard at a specified geographic longitude. Finders of these message-bearing bottles were asked to fill in the date and place where they found it and return the form to "Deutsche Seewarte"
(German Marine Observatory) in Hamburg, predecessor of the Federal Maritime and Hydrographic Agency of Germany. Experts estimate that the probability of a bottle message being found somewhere undamaged is about ten percent. Nevertheless, scientists hoped to be able to trace back the drift path of the bottles and, in this way, gather information about ocean currents.
In Germany, Georg Ritter von Neumayer, a scholar who was to become the first director of Deutsche Seewarte in Hamburg, is considered to be the inventor of this "bottle-message based research". There also are historical accounts according to which experiments involving more or less watertight, unsinkable bottles have been made since antiquity.
- The Greek philosopher and researcher Theophrastos (371 - 287 BC), a pupil of Aristotle, assumed that conditions in the Mediterranean Sea were influenced by the Atlantic Ocean. To prove this, he carried out experiments using sealed jugs which were thrown into the Aegean Sea near Athens.
- When Christopher Columbus got into a storm on his return voyage and feared that the ship would sink he is said to have thrown a small cedarwood barrel overboard containing notes on the discovery of America.
- Under the reign of Queen Elizabeth I of England, a law was adopted in 1590 which was to prevent unauthorised persons from opening officially sealed bottles containing, for example, messages from British war ships. Only the "official uncorker of bottles" was authorised to open such bottles. This law remained in force for as long as 200 years.
- First oceanographic research involving the use of sealed bottles was conducted as early as 1786 in the Bay of Biscay and, at the same time, on the other side of the Atlantic by Benjamin Franklin, who also invented the lightning conductor.
The BSH's collection, comprising about 660 bottle messages that have been found and returned, probably is the biggest collection of this type worldwide. The messages, once contained in bottles floating in the oceans, have been compiled chronologically from 1864 to 1933 in four thick black folders, with the exception of the documents from 1901 to 1927 which were probably lost in the aftermath of the First World War.
The oldest document of this impressive collection dates back to 14 July 1864, when von Neumayer himself, sailing on the "Norfolk", threw it overboard near Cape Horn in a tightly sealed rum bottle. Three years later, exactly on 9 June 1867, at noon, the bottle was found by the worker Micheal O'Donohue on the Australian coast near Portland, as documented on the blue cardboard square. The bottle had travelled 8,532 nautical miles (15,800 kilometres) at an average speed of about eight nautical miles a day.
Oldest Message in a Bottle from 1864
Multilingual Message in a Bottle
Messages in bottles have long been replaced by other methods because reliable ocean circulation data could not be obtained in this way. They permitted rough estimates of the distance covered and of the current speed. This was about all the data that could be derived. Today, several methods are available:
These modern successors to the message in a bottle are autonomous free-drifting instruments sending measurement data automatically via satellite to data centres. The data are available to scientists and operational forecasting services within 24 hours. About 3,300 ocean floats equipped with latest state-of-the-art oceanographic measuring instruments and transmission technology are drifting in the oceans as part of the global ocean observing programme "Argo"
. After deployment, the float sinks to a pre-specified depth of about 2,000 metres and stays at this depth for two weeks, drifting with the ocean currents. The float then rises to the sea surface, measuring temperatures and salinity in the water column on its way up. It transmits these data and its position data to orbiting satellites, then it sinks to its former depth, and another 14-day cycle begins. Ocean circulation data at the floating depth and at the surface are computed on the basis of the position data.
The Federal Republic of Germany contributes 50 drifters per year to the programme, which in Germany is supervised by the BSH.
Surface floats, which also drift across the oceans, measure not only water temperature and salinity but additionally meteorological parameters such as atmospheric pressure and air temperatures. The global surface float programme is co-ordinated by the World Meteorological Organization (WMO) and the National Oceanic
and Atmospheric Administration (NOAA). The drift paths of currently more than 1,100
surface floats are monitored via satellite and allow
a very precise determination of local surface currents.
Stationary measurements of local current and wave conditions are made using instruments moored to the seabed. Such measurements are very expensive and hard on the materials used. They offer the advantage of long measuring series and a statistics-based computation of mean currents. The instruments can remain moored to the seabed for up to two years, after which the moorings are normally removed, the data are read and the instruments are overhauled. Methods for real-time transmission of data via glass fibre
cable have been used lately, for example in the tsunami warning system in the Indian Ocean. The worldwide network of deepsea moorings is co-ordinated internationally within the framework of the "OceanSITES" programme. In the North and Baltic Sea region, the BSH is operating current meter moorings at the MARNET positions.
Current measurements on board research vessels
The majority of research vessels are equipped with acoustic current meters. Current speeds in the water column are determined using the doppler effect of the emitted sound signal. Currents to a depth of 2000 metres can be measured along the shipping routes in continuous operation. The measurement data are stored in the databases of national data centres and are available on request.
Yellow rubber ducks travel across the oceans
Oceanography also has its entertaining aspects - in this case 30,000 rubber bath tub toys which went overboard in a container more than 15 years ago when a container ship got into a tropical storm in the Pacific Ocean. Since then, the tub toys have been turning up on Atlantic and Pacific beaches. Following retirement, the U.S. oceanographer Dr. Curtis Ebbesmeyer has dedicated his time to investigating
their drift paths.
Two thirds of the tub toys drifted south, stranding on the coasts of Australia, Indonesia, and South America. The other third embarked on a long voyage which ended 15 years later on the coasts of England. To get there, the ducks had to cross the Bering Strait separating Alaska from Siberia. Having battled their way across the icy Arctic Sea for several long years, they headed south along the east coast of Greenland and finally ended up in the North Atlantic Current, a branch of the Gulf Stream, which carried
them to their final destinations on the coasts of northwest Europe. In this way, yellow rubber ducks have provided valuable information about ocean current conditions.