Ekman, V. Walfrid

V. Walfrid Ekman, in full Vagn Walfrid Ekman, (born May 3, 1874, Stockholm, Sweden—died March 9, 1954, Gostad, near Stockaryd), Swedish physical oceanographer best known for his studies of the dynamics of ocean currents. The common oceanographic terms Ekman layer, denoting certain oceanic or atmospheric layers occurring at various interfaces; Ekman spiral, used in connection with vertical oceanic velocity; and Ekman transport, denoting wind-driven currents, derive from his research.

Ekman was the youngest son of Fredrik Laurentz Ekman, a Swedish physical oceanographer. After finishing secondary school in Stockholm, Ekman studied at the University of Uppsala, where he majored in physics. But lectures on hydrodynamics in 1897 by Vilhelm Bjerknes, one of the founders of meteorology and oceanography, definitely decided the direction of Ekman’s work.

While still a student at Uppsala, Ekman made important contributions to oceanography. When it was observed, during the Norwegian North Polar Expedition, that drift ice did not follow the wind direction but deviated by 20° to 40°, Bjerknes chose Ekman to make a theoretical study of the problem. In his report, published in 1902, Ekman took into account the balance of the frictions between the wind and sea surface, within layers of water, and the deflecting force due to the Earth’s rotation (Coriolis force).

After taking his degree at Uppsala in 1902, he joined the staff of the International Laboratory for Oceanographic Research in Oslo, where he remained until 1909. During those years he proved to be a skilled inventor and experimentalist. The Ekman current meter, an instrument with a simple and reliable mechanism, has been used, with subsequent improvements, to the present, while the Ekman reversing water bottle is used in freshwater lakes and sometimes in the ocean to obtain water samples at different depths with a simultaneous measurement of water temperatures. He displayed his theoretical and experimental talents in his study of so-called dead water, which causes slow-moving boats to become stuck because of a thin layer of nearly fresh water spreading over the sea from melting ice. This phenomenon, frequently occurring in fjords, seriously impeded the Norwegian explorer Fridtjof Nansen in Arctic waters. Ekman demonstrated by experiments in a wave tank that the resistance to the motion of the vessels is increased by the waves that are formed at the interface between layers of water of different densities.

He also derived an empirical formula for the mean compressibility (compression ratio divided by pressure) of seawater as a function of pressure and temperature. This formula is still in use today to determine density of deep seawater which is compressed by hydrostatic pressure.

From 1910 to 1939, Ekman was professor of mechanics and mathematical physics at the University of Lund in Sweden, where he pursued his main interest, the dynamics of ocean currents. He published theories on wind-driven ocean currents, including the effects of coasts and bottom topography, and on the dynamics of the Gulf Stream. He also tried, with partial success, to solve the complex problem of ocean turbulence.

In 1925 Ekman participated in a cruise of a German research ship to the Canary Islands. On finding that data on currents obtained for several days at several marine stations between the Bay of Biscay and the Canary Islands were not sufficient to obtain an average figure, he and a colleague, during the years 1922–29, improved the technique for measuring currents for a prolonged period by collecting data from an anchored ship. After several preparatory cruises for that purpose off the Norwegian coast aboard a Norwegian research vessel, they made a cruise to the trade-wind region off northwestern Africa in the summer of 1930 to determine the average current at various ocean depths at stations occupied for two weeks or longer. Preliminary reports were published soon after the cruise, but Ekman wrote the final report in 1953 at the age of 79. The long delay in publication, partly owing to the loss of important data during the German occupation of Norway, also indicates the unparalleled care he took with his work.

Although his name and achievements were well known among oceanographers, he rarely attended international meetings—but his genuine kindliness prevented him from becoming a recluse. Most of his teachers and friends, such as Nansen and Bjerknes, were Norwegian; he spent many vacations in Bergen. He sang a beautiful bass, spent much time at the piano, and occasionally composed music. In the fall of 1953, he began a study of turbid currents, which he continued until a few days before his death.