Effect of Model Resolution on The Flow Structures Near Mesoscale Eddies

摘要

Three-dimensional structures of large ocean rings in the Gulf Stream region are investigated using the HYbrid Coordinate Ocean Model (HYCOM).Numerically simulated flow structures around four selected cyclonic and anticyclonic rings are compared with two different horizontal resolutions: 1/12o and 1/48o.The vertical distributions of Lagrangian Coherent Structures (LCSs) are analyzed using Finite Size Lyapunov Exponent (FSLE) and Okubo-Weiss parameters (OW).Curtain-shaped FSLE ridges are found in all four rings with extensions of surface ridges throughout the water columns,indicating that horizontal stirring is dominant over vertical motions.Near the high-resolution rings,many small-scale flow structures with size O(1~10) km are observed while these features are rarely found near the low-resolution rings.These small-scale structures affect the flow pattern around the rings as flow particles move more randomly in the high-resolution models.The dispersion rates are also affected by these small-scale structures as the relative horizontal dispersion coefficients are larger for the high-resolution models.The absolute vertical dispersion rates are,however,lower for the high-resolution models,because the particles tend to move along inclined eddy orbits when the resolution is low and this increases the magnitude of absolute vertical dispersion.Since relative vertical dispersion can reduce this effect from the orbital trajectories of particles,it gives a more reasonable magnitude range than absolute dispersion,and so is recommended in estimating vertical dispersion rates.