Increasing biomass in the global and warm oceans? Unexpected new insights from SeaWiFS data: 1997-2010
Irina Marinov1, David J Shields2, Anna Cabre3, Priya Sharma2, Danica Fine2 and Tihomir S Kostadinov4, (1)University of Pennsylvania, Department of Earth and Environmental Sciences, Philadelphia, PA, United States, (2)University of Pennsylvania, Philadelphia, PA, United States, (3)Instituto de Ciencias del Mar, Barcelona, Spain, (4)University of Richmond, Richmond, VA, United States
Abstract:
It is commonly assumed that warming of the nutrient-depleted subtropical gyres intensifies their oligotrophy, as it decreases vertical mixing of the surface and nutrient-rich deep waters, increasing the nutrient limitation of phytoplankton. Previous researchers have claimed expansions of the oligotrophic gyres and decreasing Chlorophyll (Chl) concentrations since the advent of satellite color measurements, purportedly due to climate warming. We review this topic via a reanalysis of Chl and the backscattering-based phytoplankton functional group (PFT) biomass timeseries (Kostadinov et al. 2009, 2010) over the 1997-2010 SeaWIFS period. We find that globally biomass and the percent of large phytoplankton increase while Chl decreases over the 1999-2010 period. The Chl decrease can be ascribed to changes in the Pacific tropical tongue, while the biomass increase is driven by the subtropical gyres. Globally we see a weak reduction in the total size of oligotrophic gyres. We show here that all these tropical and subtropical biological signals - which drive the global biological signals - can be explained to a first order by ENSO-driven physical variability in the Pacific, and are therefore not a signature of climate change.
We find that the 1999-2010 period can be divided into a period with increasing Multivariate El Nino index (MEI) (1999-2002) and a period with decreasing MEI (2002-2010). The trends in tropical/subtropical Chl, biomass and oligotrophic gyre sizes over these two periods are opposite and can be ascribed to ENSO-driven trends in stratification and winds. Importantly, these trends almost cancel out, resulting in weak overall trends over the entire SeaWIFS period in both Chl and biomass. We analyze the complex spatial differences in biological variability across the subtropical gyres, the Pacific warm pool and equatorial tongue via EOF and composite analysis for both physical (SST, MLD, winds) and biological (Chl, total biomass, PFT biomass) fields. The interannual variability and the trends in physical and biological fields show clear signatures of the Modoki or Warm Pool El Nino.
