Keywords:Large ensemble, Phytoplankton bloom, phenology, future change
While future changes in annual mean ocean biogeochemical properties (Net Primary Production, Export Production, Chlorophyll concentration etc.) has been studied, there are still only few descriptions and estimates of changes in the seasonal cycle itself. Changes in phytoplankton bloom phenology (initiation and peak timing) should have significant impacts on higher trophic levels, even on fisheries, and on the oceanic carbon cycle, since it is one of largest biogeochemical seasonal signal and could characterize the seasonal biological activity. However, it is challenging to estimate the forced long-term phenological changes as they are embedded in natural variability. In present study, we use a 30-member ensemble simulated by Geophysical Fluid Dynamics Laboratory Earth System Model 2 (GFDL-ESM2M) to deconvolve the forced signal from intrinsic variability in bloom phenology. Using daily mean surface chlorophyll concentrations, we detect the bloom timing shift precisely and estimate the emergence time-scale under a historical/RCP8.5 pathway over 1990-2100.
Over global scales, bloom initiation reveals large structured shifts over the 21st century, with large emergent shifts towards earlier blooms (~few days/decade) north of 30°N, and zonal heterogeneity of the sign of the shift over the Southern Ocean. On the other hand, bloom peak timing is expected to shift more than the initiation, indicating the bloom duration will also change significantly by the end of this century. As the results from budget analysis and the taylor decomposition, a range of variables are considered for attribution of the changes, including not only growth rate (that is, temperature, light, and nutrient) but also loss rate (grazing pressure as a function of temperature and biomass).