Entry Date:
August 21, 2017

Collaborative Research: Causes and Effects of Shelf-edge Internal Tide Variability

Principal Investigator Pierre Lermusiaux

Internal tide generation and propagation near continental slopes is being studied using a four-dimensional numerical simulation and diagnosis approach. The purpose is to explain observed variability in internal tides and the nonlinear waves they spawn. The study concentrates on long wavelength linear internal waves (internal tides) generated from subcritical tidal flow (current speed less than wave speed), ubiquitous around the world. Three internal tide effects are being examined: variable generation, heterogeneous propagation (i.e. focusing), and conversion to nonlinear waveform. The first two effects, largely unexplored thus far, will create wave energy density structure, and may give spatial/temporal structure to the nonlinear conversion process. A set of simulations are being performed with the MIT Multidisciplinary Simulation, Estimation, and Assimilation System (MSEAS), mostly with the hydrostatic primitive equation model already tuned at mesoscales via comparison with data. Modeled configurations range from idealized bathymetric, stratification, and flow conditions to realistic conditions obtained via data-driven modeling. Inter-comparisons of the collected results will divulge the physics of variable four-dimensional internal tide generation and propagation, with the intent of describing how the process occurs in the real ocean. The new MSEAS non-hydrostatic model will then be used to study nonlinear conversion processes, forming about 20% of the project. Applicability of the results to the real ocean will be verified via comparison to remote sensing and in situ data from a one-month long experiment. The main application region is the Middle-Atlantic Bight because large constraining data sets and available tuned model, but Asian Seas areas, also with existing models and data sets, will be briefly explored to examine interregional differences.

Shallow Water 2006 (SW06) Re-Analysis -- A data-driven re-analysis of SW06 has recently been carried out. In this re-analysis a number of areas have been addressed : improved initial and boundary conditions and feature models (FM) (see below); the vertical discretization now consists of 100 optimized vertical levels; atmospheric forcing now includes improved E-P and direct fluxes from WRF/NOGAPS; amplitudes of tidal forcing have been corrected; data assimilation is weaker and more frequent with shorter space scales; and model parameters of vertical mixing (wind mixing, PP background mixing), horizontal mixing and bottom friction have been re-evaluated.

An important compenent of the re-analysis was the significant improvement of the model initial and boundary conditions. This was achieved through the inclusion of additional synoptic data (WODB, GTSPP) and pseudo profiles to bolster the shelf-break front; correcting the WOA climatology for the slope to match the 2006 conditions; better defining the shelfbreak T/S front FM (steepness and location of foot); improving the Gulf Stream T/S FM based on synoptic data and utilizing transport feature models for the Gulf Stream, slope recirculation gyre and shelfbreak front.