6
Bruce A. Warren
These subsurface, ocean-scale maps were never synoptic maps (the data were
not collected at one time), and rarely maps of mean distributions (except in the
North Atlantic not many observations were repeated nearby and then averaged), but
composite maps (all reliable observations made whenever in an ocean plotted together
and contoured). The fact that coherent composite maps with smoothly contourable
features could be prepared at all argued for great stability in the property fields.
While oceanographers were aware of some mesoscale variability (current meanders,
or wiggles in isopleths on sections, if nothing else), the revelation brought about by
direct, sustained current measurements that the motion field was generally dominated
by its fluctuations (even with tides and internal waves filtered out), and that the ocean
was full of eddies, came as a great surprise in the 1960s.
Coverage with hydrographic stations was concentrated in the Atlantic before
midcentury because the Indian Ocean was far from laboratories equipped for prolonged voyages, and the Pacific was too broad for pre-1960s research vessels to make
transoceanic sections like those in the Atlantic. But neither remoteness nor climatic
severity discouraged the men of the Discovery Investigations from their long exploration of the Southern Ocean in the 1930s (G. Deacon, 1937; 1984, pp. 63–73).
Two technical developments around the time of the Second World War promoted rapid mapping of the path of the Gulf Stream, by tracking its sharp, subsurface, cross-stream temperature gradient. With the bathythermograph, temperature
soundings could be taken to depths of 150–250 m from a ship underway, and with
the Loran-A shore-based radio-navigation system, the ship’s position could be determined at any time within some 1000 km of the east coasts of the United States and
Canada, to an accuracy of about 1 km. Following the Stream east of Cape Hatteras
disclosed that it was not at all the broad diffuse flow indicated there by the ship-drift
maps, but a narrow, swift current taking sinuous, changing paths, whose meanders occasionally closed and broke off as great current rings (e.g., Fuglister and Worthington,
1951). In addition to changing drastically the characterization of the instantaneous
Gulf Stream, these novel observations influenced later conceptions of the Kuroshio
(Kawai, 1972), and, especially, of the Agulhas Retroflection and formation of Agulhas
Rings (Lutjeharms et al., 1992).
3. GEOSTROPHY AND DYNAMIC CALCULATIONS
Geostrophy entered oceanography surreptitiously, cloaked in Bjerknes’s circulation
theorem. Even The Oceans (Sverdrup et al., 1942) does not include the term, let
alone “thermal wind.” Mohn (1885) had given the formula connecting (geostrophic)
velocity to the slopes of isobaric surfaces, but when Sandstr¨ om and Helland-Hansen
(1903) came to work out procedures for computing the (geostrophic) velocities and
volume transports related to the baroclinic density field (“dynamic calculations”),
they started with the circulation theorem. Although the word “geostrophic” would
Bruce A. Warren
These subsurface, ocean-scale maps were never synoptic maps (the data were
not collected at one time), and rarely maps of mean distributions (except in the
North Atlantic not many observations were repeated nearby and then averaged), but
composite maps (all reliable observations made whenever in an ocean plotted together
and contoured). The fact that coherent composite maps with smoothly contourable
features could be prepared at all argued for great stability in the property fields.
While oceanographers were aware of some mesoscale variability (current meanders,
or wiggles in isopleths on sections, if nothing else), the revelation brought about by
direct, sustained current measurements that the motion field was generally dominated
by its fluctuations (even with tides and internal waves filtered out), and that the ocean
was full of eddies, came as a great surprise in the 1960s.
Coverage with hydrographic stations was concentrated in the Atlantic before
midcentury because the Indian Ocean was far from laboratories equipped for prolonged voyages, and the Pacific was too broad for pre-1960s research vessels to make
transoceanic sections like those in the Atlantic. But neither remoteness nor climatic
severity discouraged the men of the Discovery Investigations from their long exploration of the Southern Ocean in the 1930s (G. Deacon, 1937; 1984, pp. 63–73).
Two technical developments around the time of the Second World War promoted rapid mapping of the path of the Gulf Stream, by tracking its sharp, subsurface, cross-stream temperature gradient. With the bathythermograph, temperature
soundings could be taken to depths of 150–250 m from a ship underway, and with
the Loran-A shore-based radio-navigation system, the ship’s position could be determined at any time within some 1000 km of the east coasts of the United States and
Canada, to an accuracy of about 1 km. Following the Stream east of Cape Hatteras
disclosed that it was not at all the broad diffuse flow indicated there by the ship-drift
maps, but a narrow, swift current taking sinuous, changing paths, whose meanders occasionally closed and broke off as great current rings (e.g., Fuglister and Worthington,
1951). In addition to changing drastically the characterization of the instantaneous
Gulf Stream, these novel observations influenced later conceptions of the Kuroshio
(Kawai, 1972), and, especially, of the Agulhas Retroflection and formation of Agulhas
Rings (Lutjeharms et al., 1992).
3. GEOSTROPHY AND DYNAMIC CALCULATIONS
Geostrophy entered oceanography surreptitiously, cloaked in Bjerknes’s circulation
theorem. Even The Oceans (Sverdrup et al., 1942) does not include the term, let
alone “thermal wind.” Mohn (1885) had given the formula connecting (geostrophic)
velocity to the slopes of isobaric surfaces, but when Sandstr¨ om and Helland-Hansen
(1903) came to work out procedures for computing the (geostrophic) velocities and
volume transports related to the baroclinic density field (“dynamic calculations”),
they started with the circulation theorem. Although the word “geostrophic” would