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

CLIVAR MODE水实验（CLIMODE）中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的墨西哥湾流（GS）中。这是因为在北大西洋风暴路径下西经70度附近进入地层的水在EDW源水中温度较高，盐度较高，位涡（PV）高，氧饱和度低，而在西经50度附近离开的EDW则温度较低，盐度和位涡（PV）较低，氧饱和度较高。所有正在讨论的水都是在墨西哥湾流最大下游流以南100公里的反气旋区域内发现的。据估计，新EDW所需量的50%-90%是在该锋面区域内形成的，这表明可能需要一种新的EDW形成模式：这与北马尾藻海纯非绝热形成的准一维观点有很大的不同，它涉及到墨西哥湾流锋面区域内的非绝热和风应力驱动的新EDW的产生，以及与亚中尺度不稳定性和近惯性波切变色散相关的强烈的交叉锋面混合和水柱的新鲜。本研究提出，通过对CLIMODE观测资料进行创新性分析，并结合采用同化运行的全球涡旋解析混合坐标海洋模式（HYCOM）内的亚中尺度解析数值模拟，来检验这些结果的稳健性。特别是，该项目将研究中低层运动和锋面动力学对PV和盐度大尺度收支的重要性，因为它们与GS附近的EDW形成有关。在2006年和2007年冬季，将使用广泛的船上观察、水下剖面浮子测量和嵌套模型模拟来检查EDW生产的其他案例研究。就知识价值而言，该项目将严格检查锋面尺度过程对EDW形成的重要性，这些过程具有活跃的中低层不稳定性和由惯性剪切色散驱动的混合。虽然越来越多的证据表明，冬季墨西哥湾流发生了强烈的间歇性热和浮力交换，并且背景海洋涡度可能是新模式水形成的重要因素，但应力驱动的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。就更广泛的影响而言，如果这种新模式是有效的，那么亚热带模式的水形成，总是与强纬向流的侧翼联系在一起，不能在粗糙的、复杂的气候模式中充分理解，因为这些气候模式不能正确地解决或参数化这些锋面动力学。该研究项目具有很大的潜力，有助于在强海洋锋区参数化海气耦合的基本新方法。该项目的研究人员都是在环境科学方面有很强的研究生课程的学术机构的成员，一名研究生和一名博士后将作为项目的一部分接受培训和指导。该项目是对美国CLIVAR（气候变率和可预测性）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.