Collaborative Research: Examining a New Paradigm for Eighteen Degree Water Formation

在CLIVAR MODE水实验（CLIMODE）期间获得的观测结果表明，18度水（EDW）的形成中有很大一部分发生在向东流动的分离墨西哥湾流（GS）中。这是因为进入北大西洋风暴轴下西经70度附近形成区域的水在EDW源沃茨中具有温暖的温度、相对高的盐度、高的位涡（PV）和低的氧饱和度百分比，而离开西经50度附近区域的EDW具有较低的温度、盐度PV和较高的氧饱和度。所有正在讨论的水都是在100公里的反气旋区域内发现的，就在最大下游墨西哥湾流的南部。据估计，50%-90%的所需新EDW量在该锋区内形成，表明可能需要EDW形成的新范例：一个明显偏离准一维的想法，纯粹的非绝热形成在北方马尾藻海，并涉及非绝热和风应力驱动的生产新的EDW内的墨西哥湾流锋面区域和有力的交叉-与中尺度不稳定性和近惯性波的剪切弥散有关的水柱的锋面混合和增鲜。本研究建议通过创新的CLIMODE结合亚中尺度解决数值模拟嵌套在全球，涡分辨混合坐标海洋模式（HYCOM）同化运行的观测分析，检查这些结果的鲁棒性。特别是这个项目将调查的重要性，亚中尺度运动和锋面动力学的大规模预算的PV和盐度，因为它们涉及到EDW的形成在GS附近。将使用广泛的船上观测、地下剖面浮标测量和嵌套模型模拟来研究2006年和2007年冬季EDW产生的其他案例研究。就知识价值而言，该项目将严格研究具有活跃的中尺度下不稳定性的锋面尺度过程对EDW形成的重要性以及惯性剪切弥散驱动的混合。虽然有新的证据表明，在冬季墨西哥湾流上发生了强烈的间歇性热量和浮力交换，背景海洋涡度可能是新模式水形成的基本要素，但应力驱动形成过程的作用仍然是一个悬而未决的问题，将使用现有数据和数值模拟进行评估。亚热带模态水的形成总是与强纬向气流的侧翼联系在一起，在粗糙的复杂气候模式中不能充分理解，这些模式不能正确地解决或参数化这些锋面动力学。这一研究项目有很大的潜力，有助于在强海洋锋面区域确定海气耦合参数的基本新方法。该项目的研究人员均来自环境科学研究生院，其中一名研究生和一名博士后将接受培训和指导。该项目是对美国CLIVAR（CLImate Variability and Predictability）项目的贡献。

The observations obtained during the CLIVAR MODE Water experiment (CLIMODE) suggest that a significant fraction of Eighteen Degree Water (EDW) formation occurs within the eastward-flowing, separated Gulf Stream (GS). This is because water entering the formation area near 70 degree West under the North Atlantic storm track has warm temperatures, relatively high salinity, high potential vorticity (PV) and low percent oxygen saturation in the EDW source waters, while EDW exiting the region near 50 degree West has lower temperatures, salinity & PV, and higher oxygen saturation. All of the water being discussed is found within the 100 km anticyclonic region just to the south of the maximum downstream Gulf Stream flow. Estimates that 50%-90% of the needed amount of new EDW is formed within this frontal region indicate that a new paradigm of EDW formation may be needed: one that departs significantly from the quasi-one dimensional ideas of purely diabatic formation in the Northern Sargasso Sea and that involves diabatic and wind stress-driven production of new EDW within the Gulf Stream frontal region and vigorous cross-frontal mixing and freshening of the water column associated with sub-mesoscale instabilities and shear dispersion by near inertial waves. This study proposes to examine the robustness of these results through innovative analyses of the observations available from CLIMODE combined with sub-mesoscale resolving numerical simulations nested within the global, eddy-resolving hybrid coordinate ocean model (HYCOM) run with assimilation. In particular this project will investigate the importance of sub-mesocale motions and frontal dynamics on the large-scale budgets of PV and salinity as they relate to EDW formation in the proximity of the GS. Additional case studies will be examined of EDW production during the winters of 2006 and 2007 using extensive shipboard observations, subsurface profiling float measurements, and nested model simulations.In terms of intellectual merit, this project will critically examine the importance to EDW formation of frontal-scale processes with active submesocale instabilities and mixing driven by inertial shear dispersion. While there is emerging evidence that strong episodic heat and buoyancy exchange occurs over the wintertime Gulf Stream and that background oceanic vorticity may be an essential element in the new mode water formation, the role of the stress driven formation process remains an open question which will be evaluated using both existing data and numerical simulations.In terms of broader impacts, if this new paradigm is valid, sub-tropical mode water formation, which is invariably tied to the flanks of strong zonal flows, cannot be adequately understood in coarse, Complex climate models that do not properly resolve or parameterize these frontal dynamics. This research project has a high potential to contribute to fundamental new ways of parameterizing air-sea coupling in regions of strong oceanic fronts. The investigators on this project are all members of academic institutions with strong graduate programs in environmental sciences and one graduate student and a post-doc will be trained and mentored as part of the project.This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program.