François Ascani Research¶
Since 2008, I have been a postdoctoral fellow at the Department of Oceanography at the University of Hawai`i at Mānoa and in 2012, I became an Associate Affiliate Researcher at the same department. My PhD thesis was about the deep mean east-west currents observed below the thermocline (below about 500 m/1500 feet the ocean surface) in the Pacific and Atlantic oceans. I developed a theory suggesting that some of the currents are generated from a dissipated beam of equatorial waves (Ascani et al., J. Phys. Ocean., 2010). One of the key concepts I described is the modification of the potential vorticity of parcels flowing inside the equatorial wave field. I am now working on extending this view of the dynamics of the eddy-driven transport to other components of the general ocean circulation especially the meridional overturning circulation (see section Eddy-driven Mean Circulation).
I am also involved with Eric Firing and Julia Hummon in acquiring measurements of ocean velocity over the full water column using Lower Acoustic Doppler Current Profilers (LADCPs) mounted on a CTD rosette as part of the US CLIVAR & CO2 Programs Repeat Hydrography. This program monitors the climate variability of the oceans, in particular its thermodynamic structure, carbon dioxide and tracer contents. So far, I have crossed the Indian Ocean from west to east and the South Atlantic Ocean from north to south. I have also trained several graduate students and postdoctoral fellows to LADCP operations for four other cruises. The data can be found here.
I am also interested in the dynamics of ecosystems in the open ocean. One first issue is the effect of mesoscale eddies and submesoscale features and processes on the injection of nitrate from the deep ocean into the euphotic zone. This is particularly relevant to oligotrophic oceans, such as the North Pacific subtropical gyre around Hawai`i, because they are depleted in nitrate over the upper 125 m. It is also relevant to the climate as it concerns the efficiency of the ocean in sequestering atmospheric carbon dioxide, the so-called “biological carbon pump”. Between September 2009 and June 2011, I analyzed float observations, satellite data as well as global and regional high-resolution numerical simulations to explore the issue (see sections Submesoscale Processes, OFES Analysis, HYCOM Analysis). I have submitted a manuscript reporting these results in June 2012 and I will provide here the reference of the paper once it is published. This work has prompted me to re-visit the vertical turbulent flux of nitrate using a relatively novel combination of instruments (a microstructure profiler and an optical nitrate sensor) as well as to quantify the overall effect played by small-scale turbulence on the efficiency of the biological carbon pump. I am hoping to have soon a funded project to address these issues.
Another theme that I have started to study with Kelvin Richards is the role of viruses. Viruses are the most abundant entity of the ocean and contains a large proportion of the genetic diversity of the ocean. Viruses have also an important impact on marine ecosystems: they kill bacteria at a high rate effectively arresting marine plankton blooms, they redistribute genes amongst bacteria and they are engaged in an arms race with their hosts which is responsible in part for the high genetic diversity in the oceans. My goal is to apply mathematical modelling to explore the impact of viruses in current ecosystem models as well as in exploratory individual-based models that take into account evolutionary mechanisms (see section Microbial Modelling). I am at the preliminary stage at this moment (September 2012).
Finally, I have been helping biological oceanographers to give sense to the hydrographic conditions and the effect of physics on the ecosystem during two observations programs, BIOLINCS and HOE-DYLAN.
If you want more information on my work or would like to collaborate, you can contact me at fascani at hawaii dot edu.