Significant future changes in bloom phenology over the high latitudes identified with a large ensemble suite of simulations

While changes in biogeochemical properties under a changing climate from the perspective of emergent annual mean trends are relatively well studied, there are still only few descriptions and estimates of changes in the seasonal cycle itself. Interest stems from the expectation that future changes in phytoplankton bloom phenology (initiation, end, duration, etc.) should have significant impacts on higher trophic levels and even on fisheries. However, it is challenging to estimate the forced long-term phenological changes as they are embedded in natural interannual and/or decadal variability. To deconvolve the forced trend from intrinsic variability in bloom timing we consider projections of daily mean surface chlorophyll concentrations with a 30-member ensemble simulations from Geophysical Fluid Dynamics Laboratory Earth System Model 2 (GFDL-ESM2M) under a historical/RCP8.5 pathway over 1950-2100. Emergence is define using a signal-to-noise ratio of two over the time interval 1990-2100.

Over global scales, bloom initiation reveals large structured shifts over the 21st century, with large emergent shifts towards earlier blooms (of order one month by century’s end) north of 30°N, and zonal heterogeneity of the sign of the shift over the Southern Ocean (later in the Pacific and Atlantic sectors and earlier in the Indian Ocean sector). A range of variables are considered for attribution of the changes, including not only mixed layer depth and SST but also cloud cover impacting irradiance and surface nutrients. Also of interest are the decidedly non-emergent signals over the tropical regions spanning much of 30°S-30°N, where despite strong trends the model reveals substantial natural variability in the timing of bloom initiation.