Modeling study on submesoscale impacts on horizontal and vertical transport in the Gulf of Mexico for April 1, 2005 to May 9, 2005

Impacts of submesoscale processes on transport are investigated numerically in an energetic mesoscale flow with an ocean model run at two horizontal resolutions, 1 and 5 km. The focus is the northwestern Gulf of Mexico, where the Loop Current eddies are surrounded by smaller vortices. By increasing the horizontal resolution, the number and strength of submesoscale eddies and vorticity filaments within the mixed layer increase dramatically and with them the vertical velocities. Inside the coherent eddies and at their peripheries, increased vertical velocities for increasing resolution are associated to near inertial motions and they are not limited to the mixed layer, but are found at all depths. Horizontal velocities, on the contrary, are similar. Lagrangian isobaric tracers are deployed close to the surface and at 100 m, and three-dimensional, neutrally buoyant particles are released close to the surface, at the base of the mixed layer and at 100 m. The modeled horizontal dispersion curves for each deployment depth are independent of the kind of particles and of horizontal resolution. Close to the ocean surface, however, convergence zones, generated by submesoscale ageostrophic motions and resolved at 1 km resolution, influence the details of the tracer distributions. Vertical dispersion increases by several folds for increasing resolution at all depths explored, with the largest differences found close to the surface.本研究依托两套水平分辨率分别为1 km与5 km的海洋模式(ocean model)，在强中尺度流场(energetic mesoscale flow)中数值探究亚中尺度过程(submesoscale processes)对物质输运的影响。研究区域聚焦于墨西哥湾西北部，该区域内的环流涡旋(Loop Current eddies)被更小尺度的涡旋所环绕。随着水平分辨率提升，混合层(mixed layer)内的亚中尺度涡旋与涡度丝(vorticity filaments)的数量、强度均显著增加，垂直速度也随之同步大幅提升。在相干涡(coherent eddies)内部及其外围，分辨率提升所带来的垂直速度增强与近惯性运动(near inertial motions)密切相关，且该现象并非仅局限于混合层，而是贯穿全水深。与之相对，水平速度的分布特征几乎不受分辨率变化的影响。本研究部署了两类示踪粒子：一类是布置在近表层与100 m深度处的拉格朗日等压示踪剂(Lagrangian isobaric tracers)，另一类是分别设置在近表层、混合层底部与100 m深度处的三维中性浮子粒子(three-dimensional, neutrally buoyant particles)。各部署深度的模拟水平弥散曲线，均与粒子类型及水平分辨率无关。不过在海洋近表层，由亚中尺度非地转运动(submesoscale ageostrophic motions)产生、仅在1 km分辨率下可被解析的辐合区(convergence zones)，会对示踪剂的分布细节产生显著影响。所有探查水深下的垂直弥散均随分辨率提升有数倍增长，其中近表层区域的差异最为显著。