Uncovering the Lagrangian template of the stirring on the Martha's Vineyard inner-shelf using a combination of surface drifters, high-resolution HF radar systems, and techniques from the dynamical systems theory

Irina Rypina, Physical Oceanography


Funded Project 2010:

Stirring and water mass exchange processes that occur across the inner part of the continental shelf maintain the balance between the near-shore ocean circulation and the larger coastal ocean off-shore. These processes have a major influence on the sediment distribution, nutrients concentrations, and the hydrographic structure of the inner shelf. However, the spatially-variable, depth-dependent dynamics of the inner shelf is exceptionally complex, which makes it difficult to characterize and quantify inner shelf water mass exchanges using conventional techniques. Dynamical systems theory (which lies halfway between the deterministic and the stochastic description of the flow) provides a way to reduce this complexity by picking out specific structures from the body of complicated underlying fluid parcel motions, which guide water dispersal over time. These structures, which are called Lagrangian Coherent Structures (LCSs), form a natural template (sometimes referred to as a “Lagrangian skeleton”) for stirring in the ocean. LCSs can be used to map out transport pathways and barriers, and to identify regions with enhanced or inhibited stirring. 
This proposal requests funds to study water mass exchange processes that occur on the inner shelf south from Martha’s Vineyard. I plan to perform both conventional and dynamical systems analysis of: 1) velocity fields measured by the high-resolution HF radar system (installed recently by WHOI at the Martha’s Vineyard Coastal Observatory (MVCO)); and 2) a set of 100 measured near-surface drifter tracks (purchased using my WHOI startup funds and deployed near MVCO using a companion Access to the Sea proposal).  The dynamical systems analysis will focus on uncovering the Lagrangian template of stirring comprised of LCSs. The comparison between the radar-based estimates of LCSs (that are limited by the radar’s spatial and temporal resolution) and the drifter-based estimates (that do not have these limitations but instead are limited by the duration of drifter tracks) will validate the utility of radar-based LCSs for understanding, visualizing, monitoring and predicting transport events on the inner shelf. A unique combination of the high-resolution MVCO HF radar system and a set of 100 high-quality, low-error near-surface drifter tracks would allow such a comparison to be made for the very first time. The constructed LCSs will be used to identify regions with enhanced and inhibited stirring and to map out time-varying transport pathways and barriers that occur on the inner part of the Martha’s Vineyard shelf.  
This work will help to establish the utility of the dynamical systems approach to studying transport and stirring in coastal flows, will motivate more detailed studies of water mass exchange processes on the inner shelf, and will stimulate collaborative work on the effects of LCSs on the hydrographic structure of the shelf, and redistribution of nutrients and sediments in the region. Note also that the work proposed presents the first comprehensive drifter study of an inner shelf. Thus, this research is consistent with the Institute theme and is relevant to the Institute mission.
Dynamical systems analyses will be complemented by conventional analyses of drifter tracks and HF radar velocities. Specifically, I plan to: 1) validate (evaluate accuracy of) HF radar velocity measurements using measured drifter trajectories and study the effects of the radar resolution on resulting simulated drifter tracks; 2) investigate the circulation pattern in the area; 3) estimate fluid particle dispersion rates in the region; and 4) study the connection between stirring and mixing (advection and diffusion) in coastal zones.  
This work would add to the capabilities of MVCO by providing high-quality drifter data for studying circulation, stirring and fluid exchanges on the inner shelf and by providing validation and analyses of the MVCO HF radar (the newest addition to MVCO) data. With the proposed study complete, I envision submitting proposals to NSF to carry out more extensive analyses of the MVCO HF radar and drifter data, perform subsequent experiments with the same drifter set over a larger domain and for longer duration, and apply the dynamical systems approach to study stirring and water mass exchanges in other regions.