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Supplementary material to “Role of the Seasonal Cycle in Coupling Climate and Carbon Cycling in the Subantarctic Zone”
12 July 2011
Pedro M. S. Monteiro, Ocean Systems and Climate, Council for Scientific and Industrial Research, Stellenbosch, South Africa
Philip Boyd, National Institute for Water and Atmospheric Research (NIWA) Centre for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, Dunedin, New Zealand
Richard Bellerby, Bjerknes Centre for Climate Research, University of Bergen, Bergen, Norway
Citation:
Monteiro, P. M. S., P. Boyd, and R. Bellerby (2011), Role of the seasonal cycle in coupling climate and carbon cycling in the Subantarctic Zone, Eos Trans. AGU, 92(28), 235–236, doi:10.1029/2011EO280007. [Full Article (pdf)]
Figure S1. A summary of key regional and seasonal aspects in the Southern Ocean identified at the workshop as being important in improving our understanding of climate-carbon-ecology interactions. The temporal scale bar denotes the regional characteristics of the timing of phytoplankton bloom initiation date. The overlayed colored symbols represent hot spots, which may play disproportionately important roles in exporting local mixed layer changes to the globe through Mode Water formation (brown ellipses and red arrows), upwelling (yellow). Included are also the location of western boundary currents where mesoscale instabilities across the sub-tropical front may be important for both production and mode water formation [adapted from Sallée et al., 2010; Thomalla et al., 2011, submitted to Biogeosciences]
Methodology of bloom initiation-date analysis (from Thomalla et al., 2011) Satellite-derived Surface chlorophyll concentrations
Ocean color data are used to examine seasonal, intraseasonal and inter-annual dynamics of phytoplankton blooms in the Southern Ocean. SeaWiFS (Sea-viewing Wide Field of view Sensor [McClain et al., 1998]) data used in this study cover the period from January 1998 to December 2007. Eight-day mean level 3 standard mapped images of chlorophyll (mg Chl m−3) on a global 9 km equidistant cylindrical grid from SeaWiFS were obtained from the Goddard Space Flight Center (GSFC) [http://oceancolor.gsfc.nasa.gov]. While technically an estimate of pigment concentrations, we use SeaWiFS surface chlorophyll as a proxy for phytoplankton biomass [Sullivan et al., 1993; Comiso et al., 1993; Moore et al., 1999; Moore and Abbott, 2000, 2002]. We extracted the Southern Ocean domain south of 30°S and interpolated the original data set onto a regular 1/4 degree grid, identical to the Mean Absolute Dynamic Topography data set resolution (see below). Chlorophyll concentrations in the Ocean tend to be log normally distributed [Campbell, 1995]. When computing parametric statistical analyses on the original 8 d composite data set and in some cases for display purposes, the decimal logarithm of chlorophyll concentration has been used. Note that in most cases comparison with raw time series do not lead to any substantial difference.
Bloom initiation date
We use "bloom" to refer to events of elevated chlorophyll concentration, without reference to a particular threshold. The initiation of the bloom (or the date of bloom onset) (Figure S1) is understood here as the period of the year registering a relative increase in chlorophyll concentration, irrelevant of the actual value. The chlorophyll bloom is considered a key phase of the seasonal cycle of phytoplankton biomass. It is here defined statistically as in other studies [Henson and Thomas, 2007, Follows and Dutkiewicz, 2002; Siegel et al., 2002]. Given the presence of missing values and the large degree of variability in some areas (e.g., Figure S1), extra care should be taken to allow the algorithm to accommodate for "aberran" cases and avoid false detections of the bloom initiation date.
The mean bloom initiation dates were obtained for each pixel (on the 1/4 degree grid) as follows
1) The time series running from the 1st week of May 1998 to the last week of April 2007 is extracted.
2) The aberrant values (isolated spikes over the 99th percentile) are masked and discarded in the subsequent calculations.
3) The mean seasonal cycle is computed over the 9 years analyzed.
4) A 1D Gaussian filter (with sigma = 1) is applied, effectively reducing the degree of intraseasonal variability.
5) The median is calculated.
6) The filtered mean seasonal cycle is repeated and wrapped around itself. The bloom initiation date is subsequently constrained to fall between the seasonal minima's.
7) The date of the bloom initiation is determined as the first week of the year where the chlorophyll concentration reaches + 5% above the median and stays above this value for at least 2 consecutive weeks.
The bloom initiation dates for each year (from 1998 to 99 to 2006–07) have also been calculated. In this case steps 3 and 6 are not applied. It has been verified that the bloom initiation date obtained from the average of the 9 years is generally comparable to the one obtained from the mean seasonal cycle, albeit presenting a more noisy field given the sensitivity of the mean to extreme values and the short period considered.
References
Campbell, J.W.: The lognormal distribution as a model for bio-optical variability in the sea, J. Geophys. Res., 100, 13237-13254, 1995.
Comiso, J.C., McClain, C.R., Sullivan, C.W., Ryan, J.P., and Leonard, C.L.: Coastal Zone Color Scanner pigment concentrations in the Southern Ocean and relationships to geophysical surface features, J. Geophys. Res., 98, 2419–2451, 1993.
Follows, M., and Dutkiewicz, S.: Meteorological modulation of the North Atlantic spring bloom, Deep Sea Res., Part II, 49, 321–344, 2002.
Henson, S.A. and Thomas, A.C.: Interannual variability in timing of bloom initiation in the California Current System, J. Geophys. Res., 112, C08007, doi:10.1029/2006JC003960, 2007.
Moore, J.K., Abbott, M.R., and Richman, J.G.: Location and dynamics of the Antarctic Polar Front from satellite sea surface temperature data, J. Geophys. Res., 104, 3059–3073, 1999.
Moore, J.K. and Abbott, M.R.: Phytoplankton chlorophyll distributions and primary production in the Southern Ocean, J. Geophys. Res., 105(C12), 28, 709–28, 722, doi:10.1029/1999JC000043, 2000.
Moore, J.K. and Abbott, M.R.: Surface chlorophyll concentrations in relation to the Antarctic Polar Front: seasonal and spatial patterns from satellite observations, J. Mar. Syst., 37, 69–86, 2002.
Sallée, Jean-Baptiste, Kevin Speer, Steve Rintoul, S. Wijffels, 2010: Southern Ocean Thermocline Ventilation. J. Phys. Oceanogr., 40, 509–529. doi: 10.1175/2009JPO4291.1
Siegel, D.A., Doney, S.C., and Yoder, J.A.: The North Atlantic spring phytoplankton bloom and Sverdrup's critical depth hypothesis, Science, 296(5568), 730–733, 2002.
Sullivan, C.W., Arrigo, K.R., McClain, C.R., Comiso, J.C. and Firestone, J.: Distributions of phytoplankton blooms in the Southern Ocean, Science, 262, 1832–1837, doi:10.1126/Science.262.5141.1832, 1993.
Thomalla, S. J., N. Fauchereau, S. Swart, and P. M. S. Monteiro (2011), 2010, The Regional scale characteristics of the Seasonal Cycle of Chlorophyll in the Southern Ocean, Biogeosciences Discuss., 8, 4763–4804, doi:10.5194/bgd-8-4763-2011.
Workshop Participants
Alakendra Roychoudhury Stellenbosch University, South Africa Alessandro Tagliabue LSCE, University of Paris Andrew Lenton CSIRO, Australia Erika Keen University of Cape Town George Philander UCT, SA & Princeton University, USA Gisle Nondal BCCR, Bergen, Norway Howard Waldron University of Cape Town, South Africa Luke Gregor University of Cape Town Marina Levy L'Ocean, University of Paris (Jussieu),France Michael Bender Princeton University, USA Mike Lucas University of Cape Town, South Africa MJ Gibberd University of Cape Town Nicolas Fauchereau CSIR, South Africa Nicolette Chang CSIR, South Africa Pedro MS Monteiro (pmonteir@csir.co.za) CSIR, South Africa Phillip Boyd (pboyd@chemistry.otago.ac.nz) Otago University, New Zealand Ray Barlow DEA: Oceans & Coasts, South Africa Richard Bellerby (richard.bellerby@bjerknes.uib.no) BCCR, Berger, Norway Sandy Thomalla UCT/CSIR, South Africa Scott Nodder NIWA, New Zealand Sebastiaan Swart UCT/CSIR, South Africa Stephanie Rainier University of Cape Town Thato Mtshali CSIR, South Africa Tom Trull (ttrull@utas.edu.au) University of Tasmania, Australia Warren Joubert CSIR, South Africa The workshop was hosted by the Southern Ocean carbon – Climate Observatory Programme, South Africa, and financially supported by the National Research Foundation, South Africa, Norwegian Research Council, Norway, Africa Centre for Climate and Earth Systems Science a Centre of Excellence hosted by CSIR.