Statistical teasing or response to global warming: Implications from a 11-year time series of bio-optical properties in the South Pacific gyre

A 11-year (1998-2008) monthly time series of the primary optical properties (non-molecule backscattering coefficient (bbnm, at 551 nm), and absorption coefficients (at 443 nm) of phytoplankton (aph) and the combination of detritus and gelbstoff (adg)) in the surface center of the South Pacific gyre was derived from the remote sensing reflectance measured by the SeaWiFS/MODIS. The seasonal and inter-annual variations of aph (a proxy of phytoplankton biomass) and adg were analyzed. These results show that 1) It is more appropriate to use semi-analytical ocean color inversion algorithms that separate the optical contributions of phytoplankton and gelbstoff than to use an empirical band-ratio algorithm even for this ocean desert; 2) In the past 11 years, there is no universal trend in aph or adg, and 3) The “trends” are sensitive to the selected time windows. In particular, if we focus on the background (summer low) and intensity of aph (difference between winter high and summer low), a more pronounced decreasing trend is found for the background aph but not for the intensity. This is because that both the background and the intensity of aph show inter-annual oscillations (with a period of ~6 years). Correlation studies with concurrent sea surface temperature suggest a more interesting and intriguing picture about the status and response of phytoplankton to environmental forcing, instead of a decline “trend” of phytoplankton due to global warming.