Collaborative Research: Marginal instability and deep cycle turbulence in the equatorial oceans

合作研究：赤道海洋的边缘不稳定和深循环湍流

• 批准号: 1355768
• 负责人: William Smyth
• 金额: $ 48.2万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-03-01 至 2020-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1355768&HistoricalAwards=false
• 关键词: Collaborative; Research; Marginal; instability; deep

Overview: Thirty years after its discovery, the deep cycle of equatorial turbulence remains unexplained. Climate forecast models are unable to represent it accurately, leading to errors in ocean heat uptake. This project brings together two research groups who have made recent progress on the problem to pursue a unified understanding. Combining existing observations and new large-eddy simulations (LES), they will explore deep cycle physics over the range of seasonal and ENSO variability and in all three equatorial oceans. The previously-unexploited deep cycle property of marginal instability will be used together with numerical simulations to (1) document the history of the deep cycle over the past 25 years, (2) describe the mechanics of the deep cycle, and (3) develop improved parameterizations for use in climate models.Intellectual merit: The broad variability of mixing in the upper equatorial Pacific is exemplified by the turbulent diapycnal heat flux sampled at the same location and in the same season, but coinciding with different phases of the El Niño/ Southern Oscillation (ENSO) cycle. Early in the 1991 El Niño, currents were slow and the heat flux was relatively weak. In the 2008 cruise, which coincided with La Niña, currents were much more energetic. The resulting heat flux was stronger by an order of magnitude, and heat was transported to 100 m depth. Despite the extreme difference in magnitude, these mixing regimes exhibited a striking commonality: strong turbulence coincided with a distinct layer in which the gradient Richardson number (Ri) remained within a factor of two of the value of a quarter. Persistent clustering of Ri near 1/4 over a range of depths signifies the state of marginal instability. This near-critical state is maintained in the mean by a balance between large-scale forcing (which reduces Ri) and turbulence (which increases it). Deep cycle mixing is crucial to the climate via its effects on both the zonal current system and the sea surface temperature. In this observational data analyses and large eddy simulations, the investigators will explore the physics of the deep cycle with particular attention to the property of marginal instability. Using marginal instability as a proxy, they will document the history and longitudinal dependence of the deep cycle from existing data. This will lead to both a deeper understanding of the phenomenon and an improved ability to parameterize its effects in large-scale models.Broader Impacts: The causes of long-term Sea Surface Temperature variations will be examined and the results will contribute to ENSO and interdecadal climate prediction. The project will also foster a new collaboration, support a junior researcher, and complement an existing NSF observational project. Two presentations on equatorial oceanography will be developed and presented at high schools and community colleges.

概述：赤道湍流的深层循环被发现三十年后仍然无法解释，导致海洋热量吸收出现错误。该项目汇集了两个在该问题上取得进展的研究小组。为了寻求统一的理解，他们将结合现有的观测和新的大涡模拟（LES），探索季节性和 ENSO 变化范围以及所有三个赤道海洋的深循环物理学。以前未开发的深循环边际不稳定特性将与数值模拟一起使用，以 (1) 记录过去 25 年深循环的历史，(2) 描述深循环的力学，以及 (3) 开发改进的深循环机制。用于气候模型的参数化。智力价值：赤道上太平洋混合的广泛变化可以通过在同一地点和同一季节采样的湍流二重热通量来例证，但同时发生1991年厄尔尼诺/南方涛动（ENSO）周期的早期，洋流缓慢，热通量相对较弱，而2008年的巡航，恰逢拉尼娜现象，洋流更加活跃。由此产生的热通量增强了一个数量级，并且热量传输至 100 m 深度，尽管数量级存在极大差异，但这些混合状态表现出惊人的共性：强烈的湍流与强烈的湍流同时发生。梯度理查森数 (Ri) 保持在四分之一值的两倍以内的不同层在一定深度范围内持续聚集在 1/4 附近，表示这种接近临界状态。通过大规模强迫（减少 Ri）和湍流（增加 Ri）之间的平衡来维持平均水平。深循环混合通过其对纬向流系统和海面温度的影响而对气候至关重要。观测数据分析和大通过涡流模拟，研究人员将探索深循环的物理学，特别关注边缘不稳定的特性，他们将根据现有数据记录深循环的历史和纵向依赖性。更深入地了解这一现象，并提高在大型模型中参数化其影响的能力。更广泛的影响：将研究长期海面温度变化的原因，其结果将有助于 ENSO 和年代际气候预测项目。也会促进新的合作，支持初级研究人员，并补充现有的国家科学基金会观测项目，将开发并在高中和社区大学进行两场关于赤道海洋学的演讲。

项目成果

Marginal Instability and the Efficiency of Ocean Mixing

• DOI: 10.1175/jpo-d-20-0083.1
• 发表时间: 2020-08
• 期刊: Journal of Physical Oceanography
• 影响因子: 3.5
• 作者: W. Smyth