|
Lines in the Sea This article has been commissioned by the IOCCG and has appeared in the backscatter magazine, published by the Alliance of Marine Remote Sensing (AMRS) Since the earliest times, sailors and scientists and have noted the occurrence of dramatic contrasts in the colour of the ocean over relatively short horizontal distances. These have been studied in coastal waters, or where the coastal waters meet those of the open sea but until recently have not been well documented in the deep ocean. In 1992, Dr Jim Yoder, Professor of at the Graduate School of Oceanography, was working in the equatorial Pacific Ocean, known to be a very dynamic environment and one of the most productive areas in the open sea. The project was part of the long-term Joint Global Ocean Flux Study (JGOFS) studying the role of the upper ocean in the global carbon cycle. Prof. Yoder made measurements from a P-3 aircraft using a laser-induced chlorophyll fluorometer, concurrent with ship-based measurements of phytoplankton abundance and related oceanographic properties across the equatorial upwelling system. As it happened, this section of the ocean was also being monitored by astronauts aboard the Space Shuttle, Atlantis who supplied the photograph in Figure 1.
A band of unusually dense aggregations of the buoyant diatom Rhizosolenia sp., some 10 -- 20 nautical miles wide, was observed at the convergence of the North Equatorial Countercurrent and the cooler, denser water of the South Equatorial Current. The nearest land would be Christmas Island, roughly 2000 km to the west. Chlorophyll-{\it a} concentrations within the band ranged from 5 to 29 mg m-3, compared with nearby waters where the concentration was only 0.3 mg m-3. An oceanographic front, rendered visible by breaking waves (whitecaps) and by the extremely high contrast in ocean colour, made a distinct line in the sea that could be traced from space and from the aircraft for hundreds of kilometers (Figure 1). The oceanographic interpretation of this front is that it is a tropical instability wave (propagating to the west at about 50 km a day) at the boundary between the two water masses. The waves are a persistent feature in AVHRR imagery taken during the summer and autumn in this area. |
The distinction between the deep green patches of water containing
diatoms and the adjacent blue waters on the other side of the
convergence were also clearly visible from the ship (Figure 2). It was
an easy matter to sample the green water and identify the organisms
responsible for the colour.
The first results of this work were published as a cover story in Nature (Yoder et al., 1994). The most probable explanation for the biological phenomenon associated with the front is that primary production was higher on the colder side of the front, and that the buoyant Rhizosolenia was maintained at its surface outcropping despite the strong horizontal convergence across it (Acoustic Doppler current profiler measurements indicated cross-frontal velocities at about 40 cm s-1; downwelling velocities at the front were estimated to be about 1 cm s-1). Prof. Yoder's work is a beautiful example of what can be done with remotely-sensed imagery of the ocean. It is a forerunner of the exciting research that can be accomplished with the new generation of ocean-colour sensors. It also points out that interpretation of imagery collected from satellites will be optimised only if we continue to enjoy the support of ocean-class research vessels. Reference List Yoder, J.A., S.G. Ackelson, R.T. Barber, P. Flaments, and W.M. Balch, ``A line in the sea'', Nature, 371 (6499), 689-692, 1994. This article appeared in the February 1997 issue of the backscatter magazine, published by the Alliance of Marine Remote Sensing |
