Introduction  Following a 10 year gap at the end of the first global ocean color mission, the Japanese National Space Development Agency (NASDA) launched the Advanced Earth Observation Satellite, also called "Midori" in August 1996. On the Midori platform, both the Ocean Color and Temperature Scanner (OCTS) and POLDER (Polarization and Directionality of the Earth's Reflectances) provided phytoplankton chlorophyll a maps of the global ocean from November 1996 through June 1997. Here we describe the 8 month OCTS time series. (Data from the fourth satellite ocean color mission, SeaWiFS, have also been prepared under this project and are currently avaliable through the SMP-LAS server.) These ocean color data sets have been prepared to help understand temporal and spatial variability in the global ocean and how chlorophyll a variability may be related to other fundamental biogeochemical measurements. 

Starting with 8-day global composite OCTS chlorophyll a imagery, we averaged the data in space and time (to reduce the number of cloud-obscured pixels) to build an 8 month time series at 1 degree spatial resolution with nominal 8-day temporal resolution. 

Methods  Global (9-km and 8-day resolution) OCTS chlorophyll concentration images for the period November 1996 through June 1997 were acquired from NASA's archive (http://eosdata.gsfc.nasa.gov/data/dataset/OCTS). NASA designates these data, which are image representations of binned data products, as "Level 3 Standard Mapped Image (SMI) files". "Bins" correspond to grid cells on a global grid, each cell approximately 81 square kilometers in size. All the data for each grid cell is accumulated for 8 days and placed in the same "bin".

The NASA OCTS data archive was created by a collaboration of the NASA SeaWiFS and SIMBIOS (Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies) Projects and NASDA. SIMBIOS-NASDA-OCTS data were processed using the same software and algorithms employed for standard SeaWiFS processing to minimize processing-related differences (atmospheric correction and bio-optical algorithms) between the OCTS and SeaWiFS missions. Preliminary descriptons of the OCTS-specific modifications to the algorithms and the statistical comparisons between OCTS, POLDER and SeaWiFS can be found at http://seawifs.gsfc.nasa.gov/SEAWIFS/RECAL/OCTS_Repro1/. Methods describing NASA's SeaWiFS data processing are discussed in McClain et al., 1998, Robinson et al., 2000, and O'Reilly et al., 2000. 

We averaged the OCTS data within each 8-day array onto a 0.25 x 0.25 degree grid using a maximum likelihood estimator (MLE) as described by Campbell et al. 1995. We then applied a 1 x 1 degree running box median filter to remove small-scale variability and noise. The resultant images were then sub-sampled to reduce the spatial resolution of the arrays to 1 x 1 degree. 

As satellite chlorophyll data can be approximated with a log-normal distribution, we log-transformed the data and then used a 3-point (24-day) running mean (and when necessary, used 1 of the 3 points to represent the mean) to smooth the time series in each pixel. Our final space-time cube (x,y,time) consisted of 31 arrays (maps) of ocean chlorophyll coverage. 

There were no OCTS data acquired for the period 2 through 9 February 1997. This interval was filled in during the time series smoothing step so that the resultant data set is gap free. With the exception of the final image, which has data only in the eastern Pacific and Atlantic Oceans, individual OCTS images provide good global chlorophyll coverage, having an average of 82% valid ocean pixels. However, due to a moving latitudinal band of no data which appears between 31 October and 9 December in the Southern Hemisphere and 1 May and 3 July in the Northern Hemisiphere, only 12,136 (31%) ocean pixel locations have valid data for the 30 image time series.

The OCTS data set is particularly important, because it captures the initiation of the 1997/1998 El Nino, as well as the pre-El Nino conditions.

Campbell, J.W., J.M. Blaisdell and M. Darzi. 1995. Level-3 SeaWiFS data products: spatial and temporal binning algorithms. SeaWiFS Technical Report Series, NASA Technical Memorandum 104566, Vol. 32. Goddard Space Flight Center, Greenbelt, Maryland. 

McClain, C.R., M.L. Cleave, G.C. Feldman, W.W. Gregg, S.B. Hooker and N. Kuring, 1998. Science quality SeaWiFS data for global biosphere research. Sea Technology, 39: 10-16. 

O'Reilly, J.E., S. Maritorena, D.A. Siegel, M.C. O'Brien, D.Toole, F.P. Chavez, P. Strutton, G.F. Cota, S.B. Hooker, C.R. McClain, K.L. Carder, F. Muller-Karger, L. Harding, A. Magnuson, D. Phinney, G.F. Moore, J. Aiken, K.R. Arrigo, R.Letelier, and M. Culver, 2000. Ocean Chlorophyll a Algorithms for SeaWiFS, OC2, and OC4: Version 4. In O'Reilly, J.E., and 24 Coauthors, SeaWiFS Postlaunch Calibration and Validation Analyses, Part 3. NASA Tech. Memo. 2000-206892, Vol. 11, 9-19. 

James A. Yoder
Graduate School of
Oceanography 
University of Rhode Island 
South Ferry Road 
Narragansett, RI 02882
401-874-6864
jyoder@.gso.uri.edu