A Brief History of Seawater Temperature Observations in the 20th Century
World War II (1939–1945)
It was the ‘afternoon effect’ that drew the navy’s attention to the seawater temperature change. The acoustic energy near the sea surface is trapped as the result of solar heating on days with low winds. During the wartime, the navy sonars experienced a loss of range in the afternoons and evenings of calm sunny days. Recording water temperature, therefore, became a military priority in a bid to accurately interpreting sonar readings so that navies can locate submarines. The research by Carl-Gustaf Rossby and his student Athelstan Spilhaus was enlisted for the job. The idea of what we now called the mechanical bathythermograph (MBT) was originated in 1935 when Rossby experimented drawing a continuous pressure/temperature graph by a stylus scratching a plate of smoked brass foil. The first version of a reliable and reusable device was made in 1938. The MBT resembles a torpedo in order to minimize the water resistance. Inside the instrument is a Bourdon tube carrying a capillary tube with xylene, which senses the temperature change and unwinds the Bourdon tube as temperature rises. The uncoiling of the tube renders the attached stylus into horizontal movements which scratch a plate of smoked glass. Meanwhile, a spring and a piston passe the increasing pressure to the stylus to pull it downwards. By the year of 1946, MBTs are capable of reaching to 900 feet (~275m) which is more than twice of the initial achievable depths of 400 feet (~122m). However, most MBTs remained working in the upper 450 feet due to significant depth calibration issues. 73% of the more than 1.5 million MBT records from 1939 to 1967 are deployed by the U.S. Navy, and the rest are from other countries, notably among which are Japan dan the Soviet Union. MBTs are still in use after 1967, and around 800,000 deployments are recorded between 1968 and 1990.
The evolution of bathythermograph has not met its end. The U.S. Navy replaced the MBT with the expendable bathythermograph (XBT) in 1966.
The Advent of the Thermistor
Micheal Faraday FRS has noted the relationship between the conductivity of certain elements and the changes in temperature as early as 1833. However, technology at that time was not sophisticated enough to produce thermal resistors which are coined as thermistors until 1946. It was not until almost a decade later, Bruce Valton Hamon (who lived 97 years) and N. L. Brown designed and tested a prototype conductivity-temperature-depth (CTD) device which incorporated a thermistor. This device commenced commercial production in 1964 and continued to be one of the predominant measuring equipment in the present time. The accuracy of the thermistor is remarkable. With proper calibration, it is accurate to 0.001ºC.
The aforementioned XBT also uses thermistors, however since it is discarded after use, the thermistor included are cheaper and hence of 0.01ºC of accuracy. Compared with MBT, XBT has significant advantages, such as cheaper, more precise, easier to deploy, calibrate and retrieve data, and can be used on a ship moving faster than 15 knots and up to 30 knots.
I had a chance to fire some XBT on a research voyage. It was quite memorable, although it only took a second and a tip-over the ‘gun’.
There were XCTD used during the inception of XBT and CTD, but not as prevalent as XBT and CTD.
Whatever FLOATS your boat
FLIP (floating instrument platform) is a ship that can actually flip into the ocean and stand upright to take measurements of the ocean. It was implemented in 1962 by Scripps Institution of Oceanography and the Office of Naval Research jointly.
Here is a video of the dramatic transformation from a 108-meter long ship into a manmade island sticking out of the ocean. (Jump to 02:00, if you are impatient like me.)
https://www.youtube.com/watch?v=azZIcoPI_CU
TAO (Tropical Atmosphere Ocean) array was established with the intention to monitor and understand the El Niño phenomenon. Initiated in 1979, it consists of an army of moored buoys across the equatorial Pacific.
PIRATA (Pilot Research Moored Array in the Tropical Atlantic) array was started in the mid-1990s.
RAMA (Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction) was set up in the early 2000s, and not completed yet.
