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

在当前的气候系统中，北大西洋形成了深水，但北太平洋没有形成深水，这导致了经向翻转环流（MOC），在大西洋向北到处输送热量。北大西洋相对于北太平洋较高的表面盐度控制了MOC下沉分支的位置。大西洋盐度升高的部分原因是大西洋MOC （AMOC）向北的盐平流。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构型中没有观察到这一点。此外，当前的气候模式不能准确地再现MOC，其原因也不清楚。因此，关于MOC的动力学，以及“为什么有AMOC而没有PMOC”等许多关键问题仍有待回答。该项目假设，与大西洋相比，印度太平洋的宽度更大，不利于海洋向北输送盐度。具体来说，对于一个狭窄的盆地，MOC上分支的北向速度更大，有利于印度洋-太平洋上空大西洋的盐平流反馈。这项研究将有助于确定控制MOC强度的因素，而MOC强度反过来又对高纬度地区的热量和冰含量有很大影响。研究生将丰富自己在物理海洋学、气候动力学和科学方法方面的知识，并在数学、数值模拟和现代科学计算方面进行训练。一场公开的电视演讲将向市民介绍气候科学的成就、局限性和未来前景。“越窄的盆地，越北的MOC速度越大”的假设是基于以下论点：1)MOC总输运与下沉位置无关；(2)在较窄的区域内，MOC的上分支受风力环流的控制。检验这一假设和量化对PMOC不利的过程的方法是在跨越一系列复杂性的理想几何形状中使用海洋模型：(i)体积和盐运输预算的零维模型（“箱形模型”）；（ii）盐作为被动示踪剂的二维平流扩散方程；（iii）三维海洋原始方程模型，包括正演和伴随。该套件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。除了确定在理想情况下有利于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耦合海气交换的众多研究。在理想情况下，使用伴随方法将有效地探索高纬度盐度在长达300年的时间尺度上对强迫（风应力、地表温度、淡水通量）的敏感性，这比以前使用的时间尺度大一个数量级。

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.