06/04/02 11:53

Project title: Global Synthesis of POC Using Satellite Data calibrated with Transmissometer and POCData from JGOFS/WOCE

W.D. Gardner, MJ Richardson, A. V. Mishonov

Texas A&M University, Department of Oceanography

A. Data Processing and Analysis

During the second year of the project data collated during the first year of the project were partiallyprocessed and analyzed. The main effort was targeted to analyzing the data from Atlantic and PacificOceans. All beam attenuation data for the Atlantic Ocean were used to calculate the Particular Organic Carbon(POC) concentration and sections of POC were created and posted on the project’s web-site.Results of the our work were presented at the AGU Ocean Science meeting in February 2002 and twopapers were submitted to Deep-Sea Research.

A.1. Global POC assessment.Our extensive collection of transmissometer data has been organized into a coherent database to addressnumerous questions about the global distribution of POC. By interfacing transmissometers with CTDs wehave collected beam attenuation data over the last 15 years during 51 cruises (WOCE, JGOFS, SAVE etc.).The coverage includes basin-wide transects in the North and South Atlantic, North and South Pacific,Indian, and Southern Oceans. Transmissometer profiles were quality controlled, edited, and converted to acommon format. Data were loaded into a database and sections of beam c were constructed for all lines inthe Atlantic, Pacific, Indian, and Southern Oceans. The relationship between beam cp (attenuation due toparticles) and POC was evaluated based on subsets of simultaneously acquired data in the Atlantic, Pacific,Indian and Southern Oceans. These data show reasonably good correlation between the two variables indifferent regions and under a wide variety of hydrophysical conditions and geographical locations. On thatbasis the POC distribution from transmissometer data has been calculated and construction of sections andmaps of POC distribution for the Atlantic and Pacific Oceans has been done.

Figure A.1. Global Transmissometer data distribution and sites where POC was measured simultaneously.

A.2. Using the satellite data for the POC assessment in Southern Atlantic (see Fig. A.2).Transmissometer data were collected during six South Atlantic Ventilation Experiment (SAVE)hydrographic expeditions conducted between November 1987 and March 1989 from R/V Knorr, andMelville. A total of 361 beam attenuation profiles were made with a SeaTech transmissometer interfacedwith a CTD/rosette. In order to calculate the particulate organic carbon concentration (POC) fromtransmissometer profiles, a regression between beam attenuation and POC for open Atlantic Ocean watersderived from our previous research (North Atlantic Bloom Experiment, NABE) and enhanced by data fromthe Bermuda Time Series Station (BATS) was applied. The profiles were processed and examined asvertical sections of the surface 500 m. Although the data were not synoptic, plan view maps were made forexamination. POC concentrations were integrated for the upper 30 m with the intent of makingcomparisons with satellite optical data. No satellite optical data were available for 1987-1989, so the onlyoption was to compare our integrated POC data with several SeaWiFS-derived variables from later yearsaveraged over comparable seasons. Interannual comparisons of four SeaWiFS products from 1997 to 2001had very high spatial coherence from year to year suggesting that making comparisons of the POC fieldwith average satellite optical products from later years would be a valid approach. The highest correlationbetween POC concentration and remotely derived products was found to be with normalized water-leavingradiance at 555 nm. Other SeaWiFS-derived variables – chlorophyll concentration, diffuse attenuationcoefficient at 490 nm and integral chlorophyll integrated over the upper optical depth – were also comparedbut showed less satisfactory correlation.

A.3. El-Nino effect on beam attenuation profiles in Equatorial Pacific (see Fig. A.3).Transmissometer data collected during several WOCE expeditions in the Pacific from 1991 to 1994 havebeen analyzed and processed to investigate the spatial distribution of POC and the impact of El-Nino onthat distribution. The relationship between in-situ POC concentration and beam attenuation was determinedfrom a selected set of Hawaii Time-Series data. This relationship was used to calculate the basin-wide POCdistribution in the upper 500 m. The El-Nino (1991-1994) events can be seen clearly in the POC sections, characterized by low values ofPOC in the upper ocean layer in the equatorial region.Beam attenuation vs. POC regression was estimated based on a selected subset of data from HOT profiles.These data were collected by HOT PI's in the upper 400 m of the water column. The regression between Cp and POC shown on Fig. 1 has been used for calculating the POC concentrationalong transects during 6 WOCE cruises in the area. Figures 3 through 6 illustrate the position of the line,and sections of bottle temperature, bottle nitrite and calculated POC concentration for the upper 500 m(from top to bottom respectively).Our results show that POC calculated from transmissometer measurements of the beam attenuationcoefficient can be used for basin-wide assessment of the marine ecosystem. Although measurements ofPOC and beam attenuation were not made simultaneously for this area, the general calculated distribution isconsistent with the other data. POC concentration is a reasonably good indicator of the ocean production,especially in the upper 100 m where it closely reflects the standing stock of the plankton community. Asignificant portion of the POC is deeper than can be detected by remote sensors.The fortuitous collection of data during both El-Nino and non-El-Nino periods allows the opportunity toobserve regional differences in POC distribution under different conditions.

Integrated Chl, mg m-2 Austral Summer 1997-1998

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CHL, mg m-3 Austral Summer 1997-1998

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Calculated POC, mg m-3

SAVE Data set 1987-89

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Figure A.2. Spatial distribution of the field data and remotely sensed products for austral summer 1987-89:

a) Calculated POC concentration (mg m-3) based on beam cp data averaged for upper 30 meters usingSAVE data;

b) Normalized water-leaving radiance at 555 nm wavelength, mW cm-2 µm-1 sr-1;c) Diffuse attenuation coefficient at 490 nm wavelength, m-1;d) Chlorophyll-a concentration, mg m-3;e) Integrated Chlorophyll-a concentration, mg m-2.Note: Similar figures for 1998-1999, 1999-2000, and 2000-2001 are available at URL:

http://oceanography.tamu.edu/~pdgroup/DataDir/HTM_Files/Supplement1.htm

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Figure A.3. WOCE lines (from top to bottom): P14 (along 180°W), P16 (150°W), P17 (135°W), and P18(110°W). Data collected in Sep.92 and Aug.93 (maximum El-Nino), Aug.91 and July 91 (before El-Nino)and in March 94 (after El-Nino). Position of the POC maxima reflects the different stages of El-Ninoconditions.

B. Continuation of the Web-site development

A web site created on the TAMU Oceanography web-server for data presentation and dissemination hasbeen updated repeatedly during the second year of the project. Links to the JGOFS data were also added tothe main data portal. Historical JGOFS data obtained during Arabian Sea, Antarctic Polar Front Zone andRoss Sea expeditions were used for creation of the beam attenuation sections some of them already posted.This web site can be accessed at URL: http://oceanography.tamu.edu/~pdgroup/TAMU-SMP.html.Currently all pages have sections of Beam Attenuation Coefficient created at 0-500 and 0-6000 m scales.All Atlantic Ocean pages also have sections of Particulate Organic Carbon for the 0-500 m layer. The thumbnail image of the sections has a rollover feature, which allows the user to see the section at adifferent scale (0-6000 m) while holding the mouse cursor on it. This image is replaced by another one(section for 0-500 m layer) if the mouse cursor is moved off of the image.The high quality camera-ready images of the sections were prepared and posted as pdf (Portable DocumentFormat) files. These files can be accessed using the appropriate links. There are also links on all pagesproviding access to the corresponding cruise pages on the WOCE web site (and to the Adobe AcrobatReader web site) for those who want to access the original hydrographic data or cruise reports.Some pages are updated with a link to the appropriate data files. These data files consist of data on beamattenuation coefficient and are stored in ASCII files using simple space-separated format.