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，深水形成于北大西洋，但不是在北太平洋，导致赤道翻转环流（MOC），将热量向北输送到大西洋各地。相对于北太平洋而言，北大西洋较高的表面盐度控制着MOC下沉分支的位置。大西洋盐度增加的部分原因是大西洋MOC（AMOC）向北平流盐。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结构中没有观察到。此外，目前的气候模式不能准确地再现MOC，原因尚不清楚。因此，关于主运行中心的动态，还有许多关键问题有待回答，因此“为什么有主运行中心，而不是主运行中心？". 该项目假设，与大西洋相比，印度洋-太平洋的宽度更大，不利于向北的盐度海洋运输。具体而言，向北的速度在上分支的MOC是一个狭窄的盆地较大，有利于盐平流反馈在大西洋的印度太平洋。这项研究将有助于确定控制MOC强度的因素，这反过来又对高纬度地区的热量和冰含量有很大的影响。研究生将丰富他/她在物理海洋学，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。一个公开的电视演示将告知市民气候科学的成就，局限性和未来的前景。“在较窄的盆地中出现较大的MOC向北的速度”的假设是基于这样的论点：1）总的MOC传输是独立的下沉位置; 2）MOC的上分支是由风驱动的环流在一个较窄的区域。检验这一假设和量化影响PMOC的过程的方法是使用理想化几何形状的海洋模型，这些模型涵盖了一系列复杂性：（一）体积和盐输运预算的0维模型（“箱模型”）;（二）作为被动示踪剂的盐的二维平流扩散方程;（三）三维海洋原始方程模型，包括正向和伴随模型。该套件将确定高纬度盐度和越赤道体积输送对外部强迫的敏感性。除了确定在理想环境中有利于AMOC而不是PMOC的基本海洋过程外，这项工作还将补充现实几何形状和海气耦合交换的众多研究。在理想环境中使用伴随方法将有效地探索高纬度盐度对300年时间尺度上的强迫（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.