Do ocean currents stick together? How ocean current coherence impact European climate

大西洋经向翻转环流（AMOC）是一个大规模的海洋环流系统，由洋流组成，洋流向北携带温暖的浅水，向南返回寒冷的深水。AMOC对于维持欧洲西北部相对温和的冬季气候至关重要。AMOC的关闭将强烈影响欧洲的温度和降水变化。经向相干性是指大西洋涡旋环流作为一股贯穿大西洋盆地的相干流发挥作用（例如，在一个纬度观测到的大西洋涡旋环流的变化在其他纬度也会被观测到）。人们普遍认为，AMOC在长（年代际+）时间尺度上是经向相干的。在较短的时间尺度上，人们还不太清楚经向相干性在何处以及如何中断，相关的驱动机制如何变化，或这些机制如何影响气候。该项目的主要目标是利用结合现场和卫星数据的新技术来调查这些问题。然后，您将使用这些观测结果来评估最先进的气候模型预测。这将有助于更好地了解AMOC对欧洲气候的影响。方法：RAPID是一项国际运动，自2004年以来一直在监测AMOC，使用沿26N线的系泊仪器阵列。最近已经开发了利用卫星测高和重力测量来估计RAPID线AMOC强度的技术[2,3]。您将使用26N开发的技术从卫星获得直接估计[2,3]，为将AMOC扩展到RAPID线的北部和南部提供基础。这将使您能够仅从卫星数据重建第一个横跨34S至58N的整个盆地AMOC时间序列。您将学习如何使用网格和沿轨道海平面产品。网格化产品（测高和重力）将用于提供跨不同纬度的基于卫星的AMOC的初步估计。更高分辨率的沿轨卫星数据（例如Jason、Sentinel-3A/B、Sentinel-6、CryoSat2）将用于将SSH数据与RAPID系泊进行比较，特别是在海岸附近（<50公里），在那里，SSH信号不能很好地用网格化卫星产品[2]分辨。该方法将包括标准的测高产品以及更高分辨率的SAR测高，并且在研究的后一部分有可能包含来自创新SWOT任务的数据。高分辨率的沿航迹测高将显著改进现有估算AMOC强度的方法，因为在这些地区，沿海变率一直无法得到很好的解决。其他系泊阵列，如MOVE （16N）、ocsnap （58N）和SAMBA (34S)，将用于验证卫星对AMOC的估计。对于系泊阵列之间的纬度，Argo浮标（自主海洋剖面仪）的密度剖面将用于填补数据/知识空白，并为卫星衍生的AMOC估计提供进一步验证。你将使用得到的AMOC的纬度估计来研究AMOC从年到年代际时间尺度变化的经向一致性。总体目标将是评估AMOC经向一致性的性质和演变：它在哪里保持，在哪里破裂，其驱动因素以及这些结果对北大西洋气候系统的更广泛影响。基于这一新的认识，你将评估气候模拟（例如CMIP6）充分代表观测到的AMOC特征及其与欧洲气候的联系的能力。

The Atlantic meridional overturning circulation (AMOC) is a large-scale ocean circulation system comprised of currents that carry warm, shallow water northwards and return cold deep-water southwards. The AMOC is crucial in maintaining the relatively mild winter climate of Northwest Europe. A shutdown of the AMOC would strongly impact European temperature and precipitation variability.Meridional coherence is the idea that the AMOC functions as a coherent flow across the Atlantic basin (e.g. changes observed in the AMOC at one latitude will also be observed at other latitudes). It is generally accepted that the AMOC is meridionally coherent on long (decadal+) timescales [1]. On shorter timescales, it is not well understood where and how the meridional coherence breaks down, how the associated driving mechanisms change, or how these influence climate. The primary objective of this project is to use new techniques combining in situ and satellite data to investigate these questions. You will then use these observations to assess state-of-the-art climate model projections. This will lead to a better understanding of the influence of the AMOC on European climate.Methodology: RAPID is an international campaign that has been monitoring the AMOC since 2004 using a moored instruments array along a line at 26N. Techniques have been recently developed using satellite altimetry and gravimetry to estimate the AMOC strength at the RAPID line [2,3]. You will use the techniques developed at 26N to obtain direct estimates from satellite [2,3], providing a basis for extending the AMOC to the north and south of the RAPID line. This will enable you to reconstruct the first whole basin AMOC time series spanning 34S to 58N from satellite data alone. You will learn how to use both gridded and along-track sea level products. Gridded products (altimetry and gravimetry) will be used to provide a first estimate of a satellite-based AMOC across different latitudes. Higher resolution along-track satellite data (e.g. Jason, Sentinel-3A/B, Sentinel-6, CryoSat2) will be used to compare SSH data with RAPID moorings particularly near the coast (<50km), where the SSH signal is not very well resolved with gridded satellite products [2]. The approach will incorporate both standard altimetry products as well as higher resolution SAR altimetry and there is the potential in the latter part of the studentship for inclusion of data from the innovative SWOT mission. The incorporation of higher resolution along-track altimetry will provide significant improvements on existing methods to estimate AMOC strength, where coastal variability has been poorly resolved [2]. Other mooring arrays such as MOVE (16N), OSNAP (58N) and SAMBA (34S) will then be used to provide validation for the satellite-derived estimates of the AMOC. For latitudes in between the mooring arrays, density profiles from Argo floats (autonomous ocean profilers) will be used to fill data/knowledge gaps and provide further validation for the satellite derived AMOC estimates. You will use the resulting latitudinal estimates of the AMOC to investigate meridional coherence of the AMOC variability from annual to decadal time-scales. The overarching objective will be to assess the nature and evolution of the AMOC meridional coherence: where it holds, where it falls apart, its drivers and wider implications these results have for the North Atlantic climate system. Based on this new understanding, you will evaluate the ability of climate simulations (e.g. CMIP6) to adequately represent the observed AMOC characteristics and their connection to European climate.