Collaborative Research: Forcing and the North Atlantic Spring Bloom

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The goal of this proposal is to determine whether easily observed physical processes control phytoplankton bloom initiation in the North Atlantic. The hypothesis is that blooms are triggered by a reduction in mixing in late winter/early spring when heat loss and winds weaken. For a simple phytoplankton-nutrient system with a wide range of biological parameters, blooms develop in realistic models of convective turbulence in the upper ocean when the surface cooling changes into heating in spring. This work hypothesizes that the change in turbulence is readily predicted from observable properties and is strong enough to spark a bloom in any biological model that admits them. Unlike previous explanations which are very sensitive to details of the biological interactions which are both poorly understood and difficult to quantify, this atmospheric forcing hypothesis is testable with remote and in-situ data which can constrain physical parameters much better than biological ones. This project will extend our existing criterion for bloom initiation to account for wind forcing, evaporation and precipitation. The range of biological models and parameters for which the criterion works well will be assessed. The theory will be tested using numerical simulations, new glider data (to be collected), an established UK time-series station, and available satellite data. The proposed project is a timely, unique and cost-effective opportunity to address one of the longest-running and most intensively studied problems in oceanography.The spring bloom, an annual population explosion that dominates the seasonal cycle of phytoplankton abundance in much of the global ocean, exemplifies the interplay between ocean biology and physics. The continuing lively debate on the processes controlling bloom initiation is driven largely by the paucity of testable hypothesis and suitable in-situ data. After 60 years of research into this topic by the biological oceanography community, theory and technology has now advanced sufficiently to the point where we can resolve fundamental outstanding questions on the interactions between physical processes and bloom timing. By coupling modern turbulence and biological models, driving them by observed atmospheric forcings, and comparing results with observations, we will determine the degree to which aspects of the bloom can be predicted simply by knowing the forcing and gain a deeper understanding of the phenomenon.Phytoplankton blooms in the North Atlantic are important for driving ocean carbon uptake and supporting ecosystems, including the fisheries of the North Americas and Europe. Hence this proposal targets topics of relevance for society both in the short term (fisheries) and long term (climate change). The proposal has a strong educational component through the training of a graduate student and a postdoc in interdisciplinary research on topics of growing importance for our understanding of the climate system. The international collaboration with UK colleagues to collect relevant data is another important aspect.