Improving Our Understanding Of Ocean Circulation: A Modeling Study

The Gulf Stream is not a rigidly contained ocean current; it curves and meanders in a manner that causes portions of it to go unstable, breaking off to form warm core rings. These rings travel west – due to the earth’s rotation - through the Slope Sea and onto the southern New England shelfbreak, affecting various physical parameters of our coastal waters as well as the biogeochemical and ecological properties (i.e. the ‘health’) of the those waters. Where a warm core ring lands along the east coast and the time of year it appears varies with each ring, causing each ring to have a different affect on the southern New England coastal waters. Guided by observations, a numerical model can examine multiple scenarios of ring/coastal current interactions to determine how the affect of a warm core ring on our coastal waters changes with varying parameters. The scientific understanding gained from a well-cast hierarchy of these numerical experiments can be used as an important component for informing sustainable management of the health of our coastal waters. The numerical modeling system chosen for this study is the Regional Ocean Modeling System (ROMS). It is designed to represent realistic ocean physics by using Newton’s equations of motion, in the form of the Navier-Stokes equations, to derive the relationship between velocity, pressure, temperature and salinity for the constantly moving ocean. ROMS is first used in an attempt to recreate the seasonal cycle of circulation along the Atlantic coast of the United States, which is then further focused on the southern New England coast. Important properties of the New England coastal circulation are well represented by the model, as determined by a detailed comparison with all available observations. This realistic coastal circulation is then used as the ‘background physics’ into which a warm core ring is numerically introduced, thus accomplishing the objectives of the study – the numerical modeling of the dynamics of warm core ring/coastal current interactions. Since ROMS is based on the Navier-Stokes equations any anomaly introduced into ROMS, such as a warm core ring, will also need to be a solution to this set of equations. Because of this the warm core ring is assumed to be in thermal wind (geostrophic and hydrostatic) balance, which is an known to be an exact solution of the Navier-Stokes equations, i.e. the numerically imposed warm core ring will be stable. A specified temperature field, using parameters based upon the observations, is used to derive the velocity, pressure and density of the warm core ring, providing a solution that ROMS recognizes and thus creating an idealized, yet reasonably realistic warm core ring to be used in the numerical study. The warm core ring is then embedded in the ROMS seasonal cycle of circulation in multiple places to determine how the different locations of warm core rings have different effects of the physical properties of the southern New England coastal shelf