High Resolution Applications of the NCOM-4DVAR in Ocean Western Boundary Current Systems

The NCOM-4DVAR is an ocean assimilation system that employs the weak-constraint representer method to assimilate a wide variety of temperature, salinity, velocity, and sea surface height observations over a specified time period (anywhere between 2-10 days).  This assimilation system has been tested and validated in a number of regional and coastal applications, and has shown to outperform, in terms of analysis and forecast skill, the currently operational 3dvar assimilation system.  So far, these applications that this system has been applied to have had relatively low horizontal resolution and contained only moderate currents.  In this presentation, we will examine the ability of the NCOM-4DVAR to operate at high resolution (3km or less) in regions containing western boundary currents (The Kuroshio extension, the Agulhas, and the US East Coast). 

Applications of 4dvar with realistic ocean models are hindered by two main factors: 1) the computational cost and 2) the nonlinear interactions that develop in the model limiting the validity of the tangent linear approximation, especially in the presence of strong nonlinear flows (e.g. eddies and meanders of the western boundary currents separation). The linear approximation limitations are mostly observed in the strong constraints version of the 4dvar.  This presentation aims to demonstrate that given enough computational resources, 4dvar can be successfully applied to high resolution models in strong nonlinear flows.  We will determine how well the NCOM-4DVAR is able to fit assimilated observations in strong nonlinear flows at high resolution by comparing the ensuing forecasts with temperature and salinity observations and unassimilated SSH maps.  We will also use an eddy tracking software to compute statistics that show the accuracy with which the 4dvar predicts the locations of meandering eddies shedding off of the western boundary currents.