Why is there a meridional overturning circulation in the Atlantic Ocean but not in the Pacific?

在目前的气候系统中，北大西洋形成了深水，但北太平洋没有形成深水，导致了经向翻转环流(MOC)，该环流将热量输送到大西洋的各个角落。北大西洋相对于北太平洋较高的表层盐度控制着MOC下沉分支的位置。大西洋盐度的增加部分是由于大西洋MOC(AMOC)向北平流的盐分。原则上，这种海洋盐平流反馈应该允许太平洋MOC(PMOC)，但在目前全球海洋的几何形状中没有观察到这一点。此外，当前一代的气候模型不能准确地再现MOC，其原因尚不清楚。因此，关于MOC的动态以及“为什么有AMOC而不是PMOC？”的许多关键问题仍有待回答。这个项目假设，与大西洋相比，印度-太平洋的宽度更大，不利于盐分向北的海洋运输。具体地说，对于狭长的海盆，MOC上支向北的速度较大，有利于大西洋对印度洋-太平洋的盐平流反馈。这项研究将有助于确定控制MOC强度的因素，而MOC强度反过来对高纬度地区的热量和冰含量有很大影响。研究生将丰富其在物理海洋学、气候动力学和科学方法方面的知识，并接受数学、数值模拟和现代科学计算方面的培训。一场公开的电视演示将向公众介绍气候科学的成就、局限性和未来前景。“较窄流域出现较大的MOC向北速度”的假设是基于这样的论点：1)MOC的总输送与下沉位置无关；2)MOC的上层分支由较窄区域上的风力涡旋控制。检验这一假设并量化惩罚PMOC的过程的办法是在一系列复杂的理想化几何中使用海洋模型：(1)体积和盐分输送预算的0维模型(“盒模型”)；(2)作为被动示踪剂的盐类的2维平流扩散方程；(3)仅含海洋的3维原始方程模型，包括正演和伴随模型。这套资料将确定高纬度盐度和跨赤道体积输送对外部强迫的敏感性。除了在理想的情况下确定有利于AMOC而不是PMOC的基本海洋过程外，这项工作还将补充在现实几何和海气耦合交换方面的大量研究。在理想情况下使用伴随方法将有效地探索高纬度盐度对长达300年的时间尺度上的强迫(风应力、地表温度、淡水通量)的敏感性，这比以前使用的要大一个数量级。

In the current climate system, deep water is formed in the North Atlantic, but not in the North Pacific, resulting in a meridional overturning circulation (MOC) which transports heat everywhere northward in the Atlantic. Higher surface salinity in the North Atlantic relative to the North Pacific controls the position of the sinking branch of the MOC. Enhanced Atlantic salinity is partially due to the northward advection of salt by the Atlantic MOC (AMOC). In principle, this oceanic salt-advection feedback should allow a Pacific MOC (PMOC), but this is not observed in the present geometrical configuration of the global ocean. Moreover, the current generation of climate models do not accurately reproduce the MOC and the causes are unclear. Thus, many key questions remain to be answered regarding the dynamics of the MOC and consequently "Why is there an AMOC and not PMOC?". This project hypothesizes that the greater width of the Indo-Pacific compared to the Atlantic disfavors oceanic transport of salinity to the north. Specifically, the northward velocity in the upper branch of the MOC is larger for a narrow basin, favoring the salt-advection feedback in the Atlantic over the Indo-Pacific. This study will help to identify the factors controlling the strength of the MOC, which in turn has a large impact on the heat- and ice-content in high latitudes. A graduate student will enrich his/her knowledge in physical oceanography, climate dynamics and the scientific method, and be trained in mathematics, numerical modeling and modern scientific computations. A public, televised presentation will inform citizens on the achievements, limitations and future prospects of climate science.The hypothesis of 'the larger MOC northward velocity in the narrower basin occurs' is based on the argument that 1) the total MOC transport is independent of the sinking location; 2) the upper branch of the MOC is steered by the wind-driven gyres over a narrower region. The approach to test this hypothesis and quantify the processes penalizing the PMOC is to use ocean models in idealized geometries spanning a range of complexities: (i) 0-dimensional models of the budget of volume and salt transport ("box-models"); (ii) 2-dimensional advection diffusion equation of salt as a passive tracer; (iii) 3- dimensional ocean-only primitive equation models, both forward and adjoint. This suite will determine the sensitivity of high-latitude salinity and cross-equatorial volume transport on the external forcing. In addition to identify the fundamental oceanic processes favoring the AMOC over the PMOC in an idealized setting, this work will also complement the numerous studies in realistic geometries and with coupled air-sea exchanges. The use of the adjoint method in an idealized setting will efficiently explore the sensitivity of high latitude salinity on the forcing (wind-stress, surface temperature, freshwater flux) on time-scales up to 300 years, which is an order of magnitude larger than previously used.