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

CLIVAR模式水实验(CLIMODE)期间获得的观测结果表明，18度水(EDW)形成的很大一部分发生在向东流动的分离墨西哥湾流(GS)内。这是因为北大西洋风暴路径下进入西经70度附近地层区域的水具有温暖的温度、相对较高的盐度、较高的位涡(PV)和较低的氧饱和度百分比，而离开西经50度附近的EDW区域的水具有较低的温度、盐度和盐度以及较高的氧饱和度。所有讨论的水都在100公里的反气旋区域内，就在最大的下游墨西哥湾气流以南。据估计，在这一锋区内形成的新EDW占所需总量的50%-90%，这表明可能需要一种新的EDW形成模式：这种模式与马尾藻海北部纯粹非绝热形成的准一维概念有很大不同，涉及在墨西哥湾流锋区内由非绝热和风应力驱动的新EDW的产生，以及与次中尺度不稳定和近惯性波的切变扩散有关的强烈的跨锋面混合和水柱的淡化。这项研究建议通过对CLIMODE提供的观测数据进行创新分析，结合嵌套在全球涡旋分辨混合坐标海洋模式(HYCOM)内的次中尺度分辨数值模拟，来检验这些结果的稳健性。特别是，该项目将调查次中尺度运动和锋面动力学对PV和盐度的大尺度预算的重要性，因为它们与GS附近的EDW形成有关。2006年和2007年冬季的EDW生产的更多案例研究将使用广泛的船上观测、次表层廓线浮标测量和嵌套模式模拟来检验。就知识价值而言，该项目将关键地检验具有活跃的次中尺度不稳定的锋面尺度过程和由惯性切变弥散驱动的混合对EDW形成的重要性。虽然有新的证据表明，冬季墨西哥湾气流发生了强烈的幕式热量和浮力交换，背景海洋涡度可能是新模式水形成的一个重要因素，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来评估。从更广泛的影响来看，如果这种新模式是有效的，副热带模式水形成，总是与强纬向气流侧翼联系在一起的，不能用粗略、复杂的气候模式来充分理解，因为它不能正确地解决这些锋面动力或将其参数化。这一研究项目有很大的潜力为海洋锋面较强区域的海-气耦合参数化的基本新方法作出贡献。该项目的研究人员都是在环境科学方面拥有强大研究生课程的学术机构的成员，作为该项目的一部分，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(气候可变性和可预测性)计划的贡献。

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.