Surface Chlorophyll Analyses for Standard Areas
NIWA receives and analyses all Moderate Resolution Imaging Spectroradiometer (MODIS) ocean colour granules – 5 minute segments of MODIS Aqua (formerly EOS PM) satellite passes – that view New Zealand. This satellite provides regular data coverage (up to 2 times daily for any particular location) over a large area. Each weekday around 10:00 NZLT, the surface chlorophyll data retrieved from the most recent passes are temporally composited (to remove cloud) and an analysis generated for our standard coverage region at 1 km spatial resolution.
Two different temporal compositing periods are used. On Tuesday and Friday, a four-day time window is employed, while on the other days a three-day period is used. Despite this compositing approach, long spells of inclement weather may still leave some regions affected by cloud. Ocean colour products such as chlorophyll cannot be estimated for such areas. Accordingly, there will be fuller data coverage on Tuesdays and Fridays.
Standard Chlorophyll Analysis Areas
As for the SST data products, the composited data are mapped onto smaller areas around the New Zealand landmass, and uploaded to the web (usually by 13:00 NZST) for use by fishers and other end-users.
These areas (Fig. 1) are suitable for fishers and other end-users who may be working far from shore.
Area Maps provide important information
These “cloud cover” area maps provide the first access point to the chlorophyll data and indicate two key pieces of information:
- The compositing period over which the latest available chlorophyll analyses were generated.
- Areas where cloud (coloured white) prevented the construction of chlorophyll analysis.
Chlorophyll Analysis Availability
The Chlorophyll data is only updated during the week days, Monday to Friday excluding national and local public holidays (mainly Wellington).
No update is available during the holiday break between Christmas and the New Year (25 December to 2 January inclusive).
Normally, updated data will be on the web site at about half past noon. The most recent images will always be about 3 days old.
Chlorophyll Analysis Imagery Details
All surface chlorophyll analyses include two types of data: the composited chlorophyll shown as a colour image, and an isochlor analysis of that image. The latter is especially powerful for identifying areas of the sea surface where chlorophyll concentration is the same or similar, and for identifying chlorophyll “fronts” – these occur where the isochlors bunch together.
This example analysis (Fig. 2) is for the Brett area and shows these features:
- The data compositing period was 25–29 August 2005 expressed in NZ local time (NZLT).
- The colour bar at the top left shows the relationship between the colour in the image and the surface chlorophyll concentration. Note: as the season progresses, this relationship will change, so that as many colours as are possible are used to match the range of chlorophyll in the area. Similarly, and for the same reason, the relationship between “image colour” and “chlorophyll” may change from area to area.
- The isochlor analysis (i.e. the solid and dotted lines) connects together all locations having the same chlorophyll concentration. Regions of high gradient in chlorophyll (e.g. near 36.2°S, 175°E at the entrance to the Hauraki Gulf) indicate the presence of ocean colour fronts. Note, however, the caveat below regarding accuracy of MODIS AQUA chlorophyll in the coastal ocean.
- Where there is no data, the image is coloured grey. When isochlor lines appear over grey areas they should be ignored.
- All latitude and longitude labels are expressed in decimal degrees (e.g. 35.5°S = 35°30'S)
This image (Fig. 3) shows a sample analysis for the Mahia area. As noted above the colours used to represent chlorophyll in both this image and the following one are the same, but the values of chlorophyll that they represent differ, as can be seen from looking at the scales in the top left of each image.
Whole NZ Region Analysis
From the Commercial Areas page it is also possible to obtain a chlorophyll analysis for the whole New Zealand region (Fig. 4), as shown in the sample here.
Note: Residual cloudy areas are coloured grey. Isochlor lines over grey areas should be ignored.
Users who subscribe (Gold and Silver levels) to our Satellite Data Services will be able to review the last ten analyses from any (and all) areas. Pay-per-view users have access to only the most recent analyses.
The surface chlorophyll values represented by these data have yet to be independently validated in the NZ region. NIWA shipboard validation of the surface chlorophyll product from a similar ocean colour satellite sensor (SeaWiFS) and processing system showed it to be within mission target of ±35% in the open ocean, with regionally-dependent negative biases of 20-30% in the open ocean. However, regional and seasonal differences have been observed between chlorophyll data derived from MODIS/AQUA and SeaWiFS instruments which are not yet completely understood and are the subject of on-going research. The time series below (Fig. 5) shows MODIS AQUA/SeaWiFS chlorophyll ratios for carefully selected 4-day composites in a region to the east of NZ (30-40°S, 170-150E)1, using data from the latest reprocessings (MODIS/AQUA r1.1, SeaWIFS r5.1). This suggests that current MODIS chlorophyll products are often negatively biased with respect to SeaWiFS (i.e., give lower values than would be inferred from SeaWiFS data) in waters close to those NIWA used in its SeaWiFS chlorophyll validation research. Other (i.e., non-calibration) sources of error in the composited products include:
- If the chlorophyll concentrations change during the compositing period – perhaps as a result of strong winds from the passage of a weather front stirring the upper layer of the ocean, and/or in-situ growth/decline in phytoplankton (the microscopic marine plants that contain chlorophyll, and other coloured photosynthetic pigments).
- Sampling variability in the product as a result of cloud temporal and spatial variability in cloud cover during the compositing period.
Note Well: Chlorophyll close to the coast is also likely to be in error, due to the presence of non-phytoplanktonic coloured material in the surface ocean (e.g., sediments, coloured dissolved organic matter) derived from land/riverine inputs). Shallow water may also be contaminated by bottom reflectance.
1. See oceancolor.gsfc.nasa.gov/REPROCESSING/Aqua/R1.1 for other regions and an explanation of the methodology.