OBITUARY: John Swallow

John Crossley Swallow, oceanographer: born New Mill, Yorkshire 11 October 1923; physical oceanographer, Institute of Oceanographic Sciences 1954-83; FRS 1968; Rossby Fellow, Woods Hole Oceanographic Institute 1973-74; married 1958 Mary Morgan (on e stepdaughter deceased); died Gunnislake, Cornwall 3 December 1994.

John Swallow was an oceanographer of outstanding international stature, and a man of great integrity and gentleness. His invention of a novel technique for measuring ocean currents, employing the neutrally buoyant float - now known as the Swallow float -radically altered the way in which we view the world's oceans and their dynamics.

Swallow started his first degree at St John's College, Cambridge, in 1941 and from 1943 to 1946 did military service at the Admiralty Signals Establishment in Haslemere and in Ceylon. He returned to Cambridge to complete his degree and begin a PhD in theDepartment of Geodesy and Geophysics on seismic investigation of the sea floor. His initial research interests led him in 1950 to join the round-the-world voyage of the survey vessel HMS Challenger, on which he shared the geophysics work. Steve Richie, later Hydrographer of the Navy, was Captain.

Swallow was recruited in 1954 by the then Director of the newly formed National Institute of Oceanography (NIO), G.E.R. (later Sir George) Deacon, to work on direct methods of measuring currents at all depths in the ocean. The project planned to track acoustically a slowly sinking sound source (an analogue of an atmospheric radiosonde, the balloons sent up by meteorological stations), but Swallow did not think much of it and the concept was abandoned. Instead he had the elegant idea of acoustically tracking neutrally buoyant floats that would act as markers of water movement. (Water is compressible and so an instrument designed to be less compressible than seawater could be ballasted to sink to a depth at which its density equalled the density of the water around it - and there it would stay, neutrally buoyant and moving with the water.) It could then be tracked acoustically from an attendant ship.

The prototype Swallow floats were constructed of aluminium scaffold tube, the dimensions of which were "adjusted" in baths of caustic soda in order to achieve the correct compressibility. The floats had then to be assembled with their batteries, primitive electronics and "navy reject" nickel scroll transducers and the complete assembly of several tens of kilograms weighed in water to within one gram in order for them to attain the planned pressure level.

The initial Swallow float trials in 1955 in the north-east Atlantic gave the first direct measurements of deep currents in the open ocean and were followed by measurements of the subsurface Mediterranean outflow at Gibraltar and of the flow of Arctic water past the Faroes. Henry Stommel at Woods Hole Oceanographic Institution, in Massachusetts, had predicted that there would be a southward-going undercurrent beneath the Gulf Stream and in 1957 a joint US/UK experiment using Swallow floats observed this flow. This work started a lifelong scientific co-operation and friendship with Henry Stommel that lasted until his death in 1992.

The success of the Gulf Stream experiment led to an attempt in 1959-60 to measure the slower interior recirculation of the oceans using Swallow floats. The predictions of very sluggish flows (in the order of a few millimetres per second) were this time found wanting. Currents were an order of magnitude greater and were discovered to vary on timescales of tens of days. These results from the Aries experiment, named after the 100ft ketch on which the work was carried out south of Bermuda, gave the first glimpses of an ocean now known to be populated with energetic eddies - the analogues of atmospheric weather systems.

This was arguably the most significant discovery about the nature of the ocean made in the 20th century. The phenomenon was not fully explored until the mid-1970s, when US scientists started to use navy hydrophones to track Swallow floats with much lowerfrequency acoustic signals over ranges of thousands of kilometres rather than the few tens of kilometres possible from ship-based tracking.

In the meantime Swallow thrust himself wholeheartedly into the International Indian Ocean Expedition in the early 1960s aboard Britain's new Royal Research Ship Discovery, the successor of Scott's famous ship. This started a continuing fascination with the oceanography of the Indian Ocean.

John Swallow married Mary Morgan, then Librarian at NIO, in 1958 and their home in Surrey became a mecca for visiting oceanographers from all over the world. John's culinary skills, developed of necessity when cooks on the Challenger and Aries were less than acceptable, were put to good use.

In 1978 John Swallow suffered a severe heart attack but made a good recovery and, following his retirement from the Institute of Oceanographic Sciences (the successor of NIO) in 1983, the Swallows moved to Cornwall. His recovery was sufficient to allow him to go back to sea on Discovery, and on French and German expeditions to the Indian Ocean. His research continued and his study overlooking Dartmoor was the hub of an international network of Indian Ocean researchers.

Swallow received much international recognition and numerous prestigious international awards, including Fellowship of the Royal Society in 1968. The most poignant came last February when he and his wife went back to Woods Hole shortly after her daughterLucy's untimely death, for John to receive the first award of the Stommel Gold Medal - fitting tribute to the two men's lifelong friendship and scientific collaboration.

John Swallow leaves a scientific legacy that is incalculable. Almost 2,000 Swallow-type floats are now helping to reveal the role that the global ocean circulation plays in the earth's climate as part of the World Ocean Circulation Experiment. These floats are the direct descendants of Swallow's initial experiments. But most of all, to those who worked with him, he will be remembered for his incisive mind, his hard work, his soft Yorkshire voice and his generosity of spirit.

John Gould