Forecasting the effects of a developing El Niño event on chlorophyll and phytoplankton composition in the Equatorial Pacific

Current model predictions forecast the biggest El Niño events this winter since 1997. Using a global biogeochemical model combined with a forecast of ocean and atmospheric variables from the NASA Global Modeling and Assimilation Office, we provide a chlorophyll forecast for the Equatorial Pacific. For validation of the model, the NASA Ocean Biogeochemical Model, we have established that monthly model-derived chlorophyll in the Equatorial Pacific were significantly (p<0.05) correlated to those from SeaWIFS (R=0.69), MODIS-Aqua (R=0.46) and VIIRS (R=0.66) for the period 1997-2015. Our winter forecast suggests that the annual chlorophyll concentration by the end of 2015 will have reached the lowest annual mean (0.15 μg chl l^-1) since 1997 (0.14 μg chl l^-1) in the Equatorial Pacific (10°S-10°N). This coincides with the warmest conditions encountered in the Equatorial Pacific since the beginning of the modern ocean color era (27.9°C in 1997 compared to 28.14°C by the end of 2015). Relative abundance of diatoms were the lowest since 1997 (34.8% in 2015 compared to 27.6% in 1997) and cyanobacteria reached their highest relative abundance in 2015 (17.4%) after 1997 and 1998 (22.6%). We assess the uncertainties of the chlorophyll forecast by comparing previous model runs with the most recent satellite ocean chlorophyll data. A first test indicated that the model predicted an increase between July and August 2015 of ~7.7% and the updated VIIRS satellite data is showing an increase of 6%. We will present the updated forecast for spring and summer 2016 of the total chlorophyll as well as the phytoplankton composition. These results will be put in the context of historical and forecasted changes in physical and atmospheric conditions.