Improving Equatorial Upper Ocean Vertical Mixing in the NOAA/GFDL OM4 Model

IF 2.9 3区 地球科学 Q2 ASTRONOMY & ASTROPHYSICS
Brandon G. Reichl, Andrew T. Wittenberg, Stephen M. Griffies, Alistair Adcroft
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Abstract

Deficiencies in upper ocean vertical mixing parameterizations contribute to tropical upper ocean biases in global coupled general circulation models, affecting their simulated ocean heat uptake and ENSO variability. To better understand these deficiencies, we develop a suite of ocean model experiments including both idealized single column models and realistic global simulations. The vertical mixing parameterizations are first evaluated using large eddy simulations as a baseline to assess uncertainties and evaluate their implied turbulent mixing. Global models are then developed following NOAA/GFDL's 0.25° nominal horizontal grid spacing OM4 (uncoupled) configuration of the MOM6 ocean model, with various modifications that target biases in the original model. We test several enhancements to the existing mixing schemes and evaluate them against observational constraints from Tropical Atmosphere Ocean moorings and Argo floats. In particular, we find that we can improve the diurnal variability of mixing in OM4 via modifications to its surface boundary layer mixing scheme, and can improve the net mixing in the upper thermocline by reducing the background vertical viscosity, allowing for more realistic, less diffuse currents. The improved OM4 model better represents the mixing, leading to improved diurnal deep-cycle variability, a more realistic time-mean tropical thermocline structure, and a better Pacific Equatorial Undercurrent.

Abstract Image

改进 NOAA/GFDL OM4 模式中的赤道上层海洋垂直混合功能
上层海洋垂直混合参数化的缺陷导致了全球耦合大气环流模式中热带上层海洋的偏差,影响了其模拟的海洋吸热和厄尔尼诺/南方涛动变率。为了更好地了解这些缺陷,我们开发了一套海洋模式实验,包括理想化的单柱模式和现实的全球模拟。首先以大涡模拟为基线,评估垂直混合参数化的不确定性,并评估其隐含的湍流混合。然后,按照 NOAA/GFDL 的 0.25°标称水平网格间距 OM4(非耦合)MOM6 海洋模式配置开发全球模式,并针对原始模式中的偏差进行各种修改。我们测试了对现有混合方案的几种改进,并根据热带大气海洋系泊和 Argo 浮漂的观测约束进行了评估。特别是,我们发现通过修改 OM4 的表层边界层混合方案,可以改善混合的昼夜变化;通过降低背景垂直粘度,可以改善上温跃层的净混合,使洋流更真实、扩散更小。改进后的 OM4 模式能更好地反映混合情况,从而改善昼夜深周期变率,使热带温跃层结构的时间均值更加逼真,并能更好地反映太平洋赤道暗流。
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来源期刊
Earth and Space Science
Earth and Space Science Earth and Planetary Sciences-General Earth and Planetary Sciences
CiteScore
5.50
自引率
3.20%
发文量
285
审稿时长
19 weeks
期刊介绍: Marking AGU’s second new open access journal in the last 12 months, Earth and Space Science is the only journal that reflects the expansive range of science represented by AGU’s 62,000 members, including all of the Earth, planetary, and space sciences, and related fields in environmental science, geoengineering, space engineering, and biogeochemistry.
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