Physical and Optical Structures in the Upper Ocean of the East (Japan) Sea
Burton H. Jones, University of Southern California
Los Angeles, CA 90089-0371
Phone: (213) 740-5765
Fax: (213) 740-8143
The overall objective of this project is to understand the processes that control bio-optical response to physical forcing in the upper ocean of the East/Japan Sea. Specifically, desire to understand:
- The bio-optical response to seasonal forcing of the upper ocean
- The role of the subpolar front on the bio-optical properties of the central basin
- Contrasting seasonal and coastal/central basin bio-optical variability
Two cruises, the first in May 1999 followed by a second in January 2000, sampled the upper ocean and atmospheric boundary layer variability in the Japan/East Sea (Drs. C. Dorman, SIO; R. Beardsley, and J. Edson, WHOI). The spring cruise focused on frontal dynamics, characterizing bio-optical variability associated with the spring phytoplankton bloom and documenting the location, range, and properties of water masses formed at the subpolar front during the preceding winter.
The wintertime cruise documented the upper ocean response to a series of cold air outbreaks with particular attention to processes associated with water mass formation and subduction at the subpolar front. Both cruises employed a towed, undulating profiler (SeaSoar) to make highly-resolved observations of the upper ocean.
Inherent optical properties were obtained using a Sea Soar equipped with a Wetlabs AC-9 9-wavelength absorption/attenuation meter and a HOBI Labs Hydroscat-6 6-wavelength optical backscatter meter. Chlorophyll fluorescence and beam transmission (660 nm) were also measured on the Sea Soar. Repeated intensive grid surveys provided approximately synoptic, three-dimensional coverage in the region of the sub-polar front. Longer sections documented oceanic and atmospheric boundary layer variability away from the front. The backscattering data is available from the spring 1999 cruise, and the AC9 data is available for the January 2000 cruise.
The primary source of bio-optical signals at the subpolar front was from phytoplankton biomass. Elevated signals were observed at the front during both winter and spring periods. During both seasons the phytoplankton biomass, and therefore the bio-optical signatures, were strongest on the northern side of the front. In both seasonal cases, there was evidence of subduction of biomass along the base of the subpolar front, extending from nearsurface on the north side of the front, to subsurface beneath the warmer southern side of the front. During the spring observations, evidence of subducted phytoplankton was observed to nearly 200 meters. The shape of the backscatter spectrum provided indirect evidence of the particle size distribution in the region. The shape of the spectrum was bimodal with a smaller absolute exponent in the upper layer and a larger absolute exponent in the upper layer. The spectral shape of backscatter for the tongue of subducted particles was similar to the spectral shape of the backscatter in the upper layer, consistent with the source of these particles coming from the cool, low salinity region north of the front.
In January 2000, four repeat surveys sampled subpolar front evolution through three cold air outbreaks, including a particularly strong event that occurred 24-26 January, between the first and second surveys. Mixed layers deepened and cooled with the passage of each successive storm system, consistent with a largely one-dimensional response in which intense surface cooling and convective overturning play important roles. Between cold-air outbreaks, advective effects associated with both the front and a nearby eddy likely govern mixed layer evolution.
As in the spring 1999 observations, chlorophyll concentration, and therefore inherent optical properties, were elevated on the northern side of the front. This was despite the strong wind forcing from the cold air outbreaks. Subduction of surface water from north of the front was also evident in the inherent optical properties. Both CDOM absorption and chlorophyll absorption showed subducted features south of the surface front. The structure was much more patchy than in spring. Subduction of inherent optical properties was evident down to only 100-120 meters beneath the surface, much shallower the spring observation.