Modeling Studies for EXPORTS in a Dynamic Ocean Environment
Particulate organic mater (POM) in the upper ocean is subject to advection by near- surface velocities related to the dynamics of eddies, fronts and mixed layer instabili- ties. This advection influences both the horizontal and vertical distributions of POM. When advective motion is coupled to the differential sinking rates of various classes of POM, a sorting effect emerges. Slower sinking or neutrally buoyant particles are advected with the water, while faster sinking particles are quicker to descend from the surface ocean that typically has higher velocities. In order to relate the surface pro- duction of POM to its export flux at depth, we will consider the interaction between a submesoscale-resolving flow field and particles characterized by a spectrum of sinking velocities. As the particles sink, their mass and sinking velocity may be transformed by remineralization, aggregation and disaggregation.
Our objectives are: (1) To characterize the vertical transport of POM as a function of the flow field, the POM mass and sinking distributions, and the patchiness of produc- tivity; and (2) To assess the ability of various configurations of an observing system (autonomous and ship-based) to characterize these fluxes. Our results will improve our ability to interpret observational data and to suggest strategies for employing a system observing assets for the EXPORTS field campaigns.
Autonomous Investigation of Export Pathways from Hours to Seasons
We propose to implement a multi-platform, multi-scale, multi-month autonomous array at two EXPORTS sites to measure upper ocean community structure, sinking cells, aggregates and fecal pellets, physical export, and migrating zooplankton. These observations will complement ship-based programs by spanning a wider range of ecosystem states and providing a diverse dataset for EXPORTS modeling. The measurements will resolve evolution of dominant pathways and quantify fluxes for carbon export from the euphotic zone to the upper twilight zone, as well as attenuation flux in the upper twilight zone. This effort will also demonstrate the capability to measure export fluxes using a combination of in situ measurements and satellite remote sensing, thus serving as a prototype for future operational systems.
We hypothesize that dominant export pathways and efficiencies differ between Atlantic and Pacific sites based on relative phasing of net community production (NCP) and carbon export (EZ) from the euphotic zone. In the Pacific, where NCP and EZ vary in phase, export will be dominated by zooplankton diel migration and slow-sinking pellet fluxes, with a smaller role for physical export and a weaker export efficiency. In contrast, decoupling of NCP and EZ in the Atlantic will produce both more variable and more efficient export, associated with fast-sinking aggregates, physical transport and low
respiration rates. Small-scale physical and ecosystem variations in the Atlantic will enhance submesocale eddy flux, which may constitute up to half of total export during springtime restratification. We propose that similar in situ measurements will permit improved export predictions in other oceanic regions based on remote-sensing observations.
A system of heavily-instrumented Lagrangian floats and Seagliders, leveraging satellite data and ship-based calibration and proxy measurements, will sample euphotic and twilight zone properties and rates, export pathways and fluxes over a 6 mo period at each site. Observations collected by OOI (Pacific) and a coarse array of Bio-Argo floats (Atlantic) will place the float and glider measurements in context of the annual cycle. The Lagrangian float will drift along isopycnals just below the euphotic zone, measuring sinking export flux at the top of the twilight zone with an optical sediment trap and characterizing types of sinking particles with a camera system. Seagliders will profile to 1000m in close proximity to and in a 10-50 km region surrounding the float, measuring optics and zooplankton acoustic biomass. NO3 and O2 measurements will be used to compute productivity, particulate carbon, O2 and NO3 budgets. The budgets will define NCP and total carbon loss, e.g. respiration rate, across the euphotic and in the upper mesopelagic zones and, in particular, the export efficiency at the top of the euphotic zone and its decay with depth below this. Ongoing observing system simulation experiments (OSSEs) will be used to further refine sampling plans.
We will address EXPORTS Science Question I (SQ 1) by optically characterizing the euphotic zone community and types of sinking particles, thereby studying how (SQ1a) ecosystem characteristics determine the vertical transfer of organic carbon. We will distinguish between the 5 export pathways (SQ1b) by measuring the magnitude and type of sinking particle flux, by measuring zooplankton characteristics and migration rates, and by computing submesoscale eddy flux using models and parameterizations. We will measure both physical and ecological processes on the same space and time scales (SQ1d). We will quantify (SQ2) the vertical structure of export efficiency from the top of the twilight zone, differentiating between the efficiency of the 5 export pathways (SQ2a) and resolving variations in efficiency with a changing euphotic zone community (SQ2b). We will work with all the EXPORTS team to bring satellite, cruise and autonomous data to bear on these questions.
WHOI: A. Mahadevan
U. Rhode Island: M.M. Omand
CalTech: A.F. Thompson
U. Maine: Mary Jane Perry
NOC/Southampton: A.P. Martin
UW APL: Craig Lee, Eric D’Asaro
NASA OBB: Modeling Studies for EXPORTS in a Dynamic Ocean Environment
NASA OBB: Autonomous Investigation of Export Pathways from Hours to Seasons