Low frequency variability in the Southern Ocean

Metadata record for data expected ASAC Project 1207See the link below for public details on this project.---- Public Summary from Project ----Project title: 'Effects of variability in ocean surface forcing on the properties of SAMW and AAIW in the South Indian Ocean'This project will study the formation and subduction processes and the properties of Antarctic Intermediate Water and Sub-Antarctic Mode Water as simulated by an Ocean General Circulation model, with particular reference to the South Indian Ocean. The study will attempt to determine how its formation and properties are affected by interannual variations in SST and wind forcing and by differing prescriptions of mixing and convection processes occurring in mid-to high latitude oceanic frontal regions of the Southern Ocean. The investigation of the ocean response in the Indian Ocean will profit from the use of a model employing general orthogonal coordinates and efficient variable resolution grids which are global but concentrated in the Indian sector.From the abstracts of the referenced papers:This article considers how some of the measures used to overcome numerical problems near the North Pole affect the ocean solution and computational time step limits. The distortion of the flow and tracer contours produced by a polar island is obviated by implementing a prognostic calculation for a composite polar grid cell, as has been done at NCAR. The severe limitation on time steps caused by small zonal grid spacing near the pole is usually overcome by Fourier filtering, sometimes supplemented by the downward tapering of mixing coefficients as the pole is approached; however, filtering can be expensive, and both measures adversely affect the solution. Fourier filtering produces noise, which manifests itself in such effects as spurious static instabilities and vertical motions; this noise can be due to the separate and different filtering of internal and external momentum modes and tracers, differences in the truncation at different latitudes, and differences in the lengths of filtering rows, horizontally and vertically. Tapering has the effect of concentrating tracer gradients and velocities near the pole, resulting in some deformation of fields. In equilibrium ocean models, these effects are static and localised in the polar region, but with time-varying forcings or coupling to atmosphere and sea ice it is possible that they may seriously affect the global solution. The marginal stability curve in momentum and tracer time-step space should have asymptotes defined by diffusive, viscous, and internal gravity wave stability criteria; at large tracer time steps, tracer advection stability may become limiting. Tests with various time-step combinations and a flat-bottomed Arctic Ocean have confirmed the applicability of these limits and the predicted effects of filtering and tapering on them. They have also shown that the need for tapering is obviated by substituting a truncation which maintains a constant time step limit rather than a constant minimum wave number over the filtering range.Continuous and finite difference forms of the governing equations are derived for a version of the Bryan-Cox-Semtner ocean general circulation model which has been recast in orthogonal, transversely curvilinear coordinates. The coding closely follows the style of the Geophysical Fluid Dynamics Laboratory modular ocean model No. 1. Curvilinear forms are given for the tracer, internal momentum, and stream function calculations, with the options of horizontal and isopycnal diffusion, eddy-induced transport, nonlinear viscosity, and semiimplicit treatment of the Coriolis force. The model is designed to operate on a rectangular three-dimensional array of points and can accomodate reentrant boundary conditions at both 'northern' and 'east-west' boundaries. Horizontal grid locations are taken as input and need to be supplied by a separate grid generation program. The advantages of using a better behaved and more economical grid in the north polar region are investigated by comparing simulations performed on two curvilinear grids with one performed on a latitude-longitude grid and by comparing filtered and unfiltered latitude-longitude simulations. Resolution of horizontally separated currents in Fram Strait emerges as a key challenge for representing exchanges with the Arctic in global models.It is shown that a global curvilinear grid with variable resolution is an efficient way of providing a high density of grid points in a particular region. In equilibrium experiments using asynchronous time steps, this type of grid has been found to allow a better representation of smaller-scale features in the high-resolution region while maintaining contact with the rest of the World Ocean, provided that lateral mixing coefficients be scaled with grid size so as to maintain marginal numerical stability. In this study, the region of interest is the southern Indian Ocean and, in particular, that of the South Indian Ocean Current. In all experiments, decreased viscosities and diffusivities were found to control tracer gradients on isopycnals but not isopycnal slopes, while thickness diffusivities controlled isopycnal slopes but only to a small degree tracer gradients. Changes to mixing coefficients in the coarse part of the grid had hardly any influence on the frontal properties examined, although they did affect currents in the Indian Ocean to some extent via their control on size of the Antarctic Circumpolar Current and the Pacific-Indian Throughflow.

ASAC项目1207预期数据的元数据记录

有关该项目的公开详情请参见以下链接。

---- 项目公开摘要 ----

项目标题：《海洋表面强迫变率对南印度洋南极中层水（AAIW）与亚南极模态水（SAMW）性质的影响》

本项目将基于海洋环流模式（Ocean General Circulation Model, OGCM）的模拟结果，研究南极中层水（Antarctic Intermediate Water, AAIW）与亚南极模态水（Sub-Antarctic Mode Water, SAMW）的形成、俯冲过程及性质，重点关注南印度洋区域。研究将尝试阐明：中高纬度南大洋锋区的混合与对流过程参数化方案差异，以及海表温度（Sea Surface Temperature, SST）和风强迫的年际变化，如何影响上述水团的形成与性质。

---- 项目相关摘要 ----

本文探讨了北极点附近数值问题的若干解决方案对海洋模式结果及计算时间步长限制的影响。如NCAR所采用的方法，通过复合极地网格单元的预报计算，可避免极地岛屿导致的流场与示踪剂等值线扭曲。极地附近纬向网格间距过小会严重限制时间步长，通常通过傅里叶滤波解决，有时还辅以向极地靠近时混合系数的向下衰减；然而，滤波计算成本较高，且两种方法均会对结果产生不利影响。傅里叶滤波会引入噪声，表现为虚假静力不稳定与垂直运动等现象；此类噪声源于内、外动量模态及示踪剂的单独滤波差异、不同纬度的截断差异，以及水平与垂直方向滤波行长度的差异。衰减会使示踪剂梯度与速度在极地附近集中，导致场的变形。在平衡海洋模式中，这些效应是静态且局限于极地区域的，但在时变强迫或与大气、海冰耦合的情况下，它们可能严重影响全球结果。动量与示踪剂时间步长空间中的边际稳定曲线应具有由扩散、粘性及内重力波稳定判据定义的渐近线；当示踪剂时间步长较大时，示踪剂平流稳定性可能成为限制因素。通过不同时间步长组合及平底北冰洋的测试，证实了这些限制的适用性及滤波与衰减对其的预期影响。测试还表明，若在滤波范围内采用维持恒定时间步长限制而非恒定最小波数的截断方式，可无需衰减。

本文推导了Bryan-Cox-Semtner海洋环流模式正交横向曲线坐标系版本的控制方程连续及有限差分形式。代码风格与地球物理流体动力学实验室（Geophysical Fluid Dynamics Laboratory, GFDL）模块化海洋模式1号高度一致。文中给出了示踪剂、内动量及流函数计算的曲线坐标形式，包含水平与等密度面扩散、涡旋诱导输运、非线性粘性及科里奥利力（Coriolis Force）半隐式处理等选项。该模式设计用于矩形三维点阵列，可适应“北”边界及东西边界的重入条件。水平网格位置作为输入，需由独立的网格生成程序提供。通过对比两种曲线网格与经纬度网格的模拟结果，以及滤波与未滤波的经纬度模拟结果，研究了在北极区域使用性能更优且更经济的网格的优势。弗拉姆海峡中水平分离洋流的分辨率，是全球模式中准确表征与北极交换过程的关键挑战。

研究表明，可变分辨率的全球曲线网格是在特定区域提供高密度网格点的高效方式。在采用异步时间步长的平衡实验中，若侧向混合系数随网格大小缩放以维持边际数值稳定性，则此类网格可在保留与世界大洋其余部分联系的同时，更好地表征高分辨率区域的小尺度特征。本研究的关注区域为南印度洋，尤其是南印度洋流区域。所有实验均显示，降低的粘性与扩散系数可控制等密度面上的示踪剂梯度，但不影响等密度面坡度；而厚度扩散系数可控制等密度面坡度，但对示踪剂梯度的影响较小。网格粗分辨率部分混合系数的变化对所研究的锋区性质几乎无影响，但通过控制南极绕极流与太平洋-印度洋贯穿流的规模，在一定程度上影响了印度洋的洋流。