Lozier, M.: Duke Univ. (Project Lead)
Project Funding:
2013 - 2015
NRA: 2012 NASA: Ocean Biology and Biogeochemistry
Funded by NASA
Abstract:
The nutrient-limited oligotrophic regions of the subtropics are an ecologically significant part of the global ocean. As such, the interannual variability in primary productivity within these regions is of prime importance to our understanding of how ecosystems might respond to climate change. Explanations for this variability have almost uniformly emphasized the role of local buoyancy changes in determining nutrient supply to the euphotic zone: changes in local stratification associated with anthropogenic warming and/or climate variability are expected to drive productivity variability by impacting the upward fluxes of deep nutrients needed to fuel primary production. Analyses of the global satellite chlorophyll record have seemingly confirmed this linkage by correlating spatially-averaged variability in subtropical chlorophyll/productivity fields with spatially- averaged stratification or SST. Recent comparisons of contemporaneous and co-located measures of subtropical productivity and stratification, however, do not suggest a strong correlative relationship between the two on interannual time scales. This apparent

contradiction suggests that non-local, basin-scale forcing plays a role in controlling the supply of nutrients to the subtropical euphotic zone.
In this interdisciplinary proposed work, variability in non-local wind and buoyancy forcing is hypothesized to impact interannual changes in productivity within subtropical oceans. Four possible mechanisms by which wind and buoyancy changes across the basin can impact nutrient supply and, consequently, productivity at a given location will be investigated: 1) wind-forced changes in the size and intensity of the downwelling subtropical gyre, 2) wind-forced changes in horizontal Ekman nutrient fluxes into the gyre, 3) wind-and buoyancy-forced changes in mode water formation and advection, which would impact the subsurface nutrient reservoir, and 4) lateral eddy fluxes of nutrients across the subtropical gyre boundary. Preliminary evidence and past studies support the viability of each of these mechanisms and also introduce new questions regarding their exact role in nutrient delivery. To test the proposed hypothesis, an analysis of observational data coupled with a modeling study is proposed. The main focus of this work is on the well-sampled North Atlantic, but because the biological response to physical forcing is expected to vary between ocean basins, the proposed analysis will also characterize interbasin differences via an application to the North Pacific.
The goal of this work is compatible with the overarching programmatic goals of NASA's Ocean Biology and Biogeochemistry program. Specifically, this work directly addresses OBB’s goal of understanding and quantifying the impacts and feedbacks of Earth System processes, particularly oceanographic mechanisms, on the global and regional spatial and temporal variability of ocean biology, including phytoplankton and organisms from other trophic levels . This goal will be met via observational and modeling analyses that aim to elucidate the role of physical mechanisms on nutrient delivery to the subtropical euphotic zone. Such elucidation will lead to an improved understanding of the spatial and temporal variability of marine productivity. Finally, NASA remote sensing data is of fundamental importance to the conduct of this proposed work. Because the goal is to link productivity response to physical mechanisms, SeaWiFS ocean color, SST, SSH and wind data are all critical to this study.
More details may be found in the following project profile(s):
