On this day in history...

...in 1978, the summary paper of the Mid-Ocean Dynamics Experiment was accepted for publication on January 15, after being received the previous July and revised in December 1977. Drafted by W. Simmons on behalf of the MODE Group, a collective of theoretical, experimental, and numerical oceanographers from institutions across the United States and the United Kingdom, the paper appeared in Deep-Sea Research as "The Mid-Ocean Dynamics Experiment." It synthesised the results of a field campaign that had taken place nearly five years earlier, southwest of Bermuda, and that had set out to answer a question physical oceanography could not yet answer with confidence: were the energetic mesoscale eddies that John Swallow had glimpsed by accident in 1959 a real, structured, and ubiquitous feature of the open ocean, or an isolated curiosity?

The discovery that prompted MODE had been almost accidental. Swallow floats tracked from nearby vessels at 41°N, 14°W in 1958, and near Bermuda in 1959 and 1960, had revealed energetic motions roughly 200 kilometres across, drifting westward at a few centimetres per second and occupying the entire water column. The observation also exposed a hard truth: the oceanographic instrumentation of the time was, in the paper's own words, disappointingly inadequate for measuring such motions systematically. More than a decade was needed to develop and perfect the long-term current meter arrays and high-resolution density measurements that a real eddy survey would require. MODE was the result of that decade of instrumental development, finally brought to bear on a single, well-chosen patch of ocean.

The experiment unfolded in stages. MODE-0, a preliminary campaign of moorings and hydrographic work, ran from autumn 1971 through winter 1972. The main event, MODE-1, took place from March through mid-July 1973, in a region generally within a 300-kilometre radius circle centred at 26°N, 69°40'W, an area chosen for its varied topography: a flat abyssal plain in the west rising toward the continental rise, and rougher seafloor relief to the east. Six research vessels and an extraordinary diversity of instruments were deployed simultaneously: arrays of moored current meters and temperature-pressure recorders, repeated hydrographic and CTD stations, neutrally buoyant floats drifting at 1,500 metres and tracked acoustically over long range by the SOFAR system that Swallow's invention had made possible, and acoustic and electromagnetic velocity profilers. A smaller, complementary survey called MINIMODE combined ship-tracked floats, CTD sampling, and a moored current meter array within the same region. It was, by a wide margin, the most spatially and temporally intensive direct study of ocean interior motion ever attempted.

The results, as the 1978 paper documented across more than fifty pages of description, dynamics, and numerical interpretation, were unambiguous: mid-ocean eddies were real, structured, and energetic, not statistical noise or an artefact of sparse sampling. In the region studied, current variability was organised in a band with periods around 100 days and horizontal scales near 70 kilometres, in which the currents were nearly isotropic and the vertical scales extended over the full depth of the water column. The eddy field, the paper concluded, formed an energetic and structured variability superimposed on a much weaker mean gyre-scale circulation, a result that inverted the assumptions many ocean models of the time had quietly relied upon. Direct comparisons of measured vertical current shear against geostrophic estimates confirmed that the eddies were, to within instrumental error, in close geostrophic and quasi-geostrophic balance, giving theoreticians a solid empirical foundation on which to build dynamical models. The experiment also yielded a wealth of secondary results: the first systematic measurement of bottom mixed layers beneath the abyssal plain, evidence that the surface temperature field was shaped advectively by the eddies from below rather than purely forced from above by the wind, and detailed observations of internal tides and high-frequency internal wave motions superimposed on the mesoscale field.

MODE did not, by its own authors' admission, solve the eddy problem. It was, in their words, an eddy experiment at one small place for one short period. But it proved decisively that the phenomenon was general, established for the first time a quantitative description of its statistics and structure, and provided the empirical foundation against which theoretical models of geostrophic turbulence, baroclinic instability, and quasi-geostrophic dynamics could finally be tested with real data. It also set a template, both scientific and organisational, for the international, multi-platform, multi-institution ocean experiments that would follow: the larger Atlantic-and-Pacific POLYMODE programme of the late 1970s, and ultimately the global eddy-resolving observational and modelling systems of today.

The contributions of MODE to physical oceanography can be summarised across several interconnected areas:

• Definitive confirmation of mid-ocean mesoscale eddies: MODE-1 provided the first comprehensive, spatially and temporally resolved evidence that energetic eddies are a structured and ubiquitous feature of the open ocean interior, ending a decade of uncertainty since Swallow's initial float observations.
• Quantitative description of eddy statistics: The experiment established concrete estimates of the dominant space and time scales of mid-ocean variability, roughly 70 kilometres and 100 days in the region studied, providing the first solid statistical basis for later theoretical and modelling work on ocean turbulence.
• Empirical validation of quasi-geostrophic dynamics: Direct comparisons between measured current shear and geostrophic estimates confirmed that mesoscale eddies are close to geostrophic balance, giving theoretical oceanographers an observationally grounded starting point for models of eddy dynamics that remains foundational today.
• Demonstration of large-scale multi-platform ocean experiments: MODE's combination of moored arrays, hydrographic surveys, SOFAR-tracked floats, and profiling instruments deployed simultaneously from six vessels established the organisational and technical template for the large international ocean process experiments that followed, including POLYMODE.
• Reassessment of the relative importance of eddies and mean circulation: By showing that eddy kinetic energy could dominate over the mean gyre-scale flow in the central ocean, MODE forced a fundamental revision of how oceanographers and modellers conceived of the balance between mean and fluctuating components of ocean circulation, a revision whose consequences are still being worked out in eddy-resolving climate models.

The MODE summary paper, accepted on January 15, 1978, closed a chapter that had opened almost twenty years earlier with a chance observation off Bermuda. What Swallow's floats had hinted at in 1959, an ocean far less quiescent in its interior than anyone had assumed, MODE confirmed with the full weight of a coordinated, intensive, multi-year campaign. The mesoscale eddy field that the experiment characterised remains, to this day, one of the central objects of physical oceanography: the feature that every eddy-resolving ocean model must reproduce, and the variability that every satellite altimeter has been built, in part, to observe.

Sources

• The MODE Group (1978). The Mid-Ocean Dynamics Experiment. Deep-Sea Research, 25, 859–910.
• John C. Swallow - Wikipedia
• Woods Hole Oceanographic Institution