Mid-Latitude Controls on Monsoon Onset and Progression (MiLCMOP)

印度夏季季风对10亿人的福祉至关重要，因为它为饮用水、卫生设施、工业和农业提供了大部分水。社会对降雨的适应是如此精细，以至于降雨时间、强度和持续时间的任何变化都会产生深远的影响。季风性降雨通常在6月1日开始在印度南部开始，覆盖整个国家需要长达6周的时间。与此同时，在季风期间，一周或更长时间尺度的变化会产生降雨过剩和减少的时期，即所谓的活跃期和间歇期。能够更好地预测降雨的开始、发展以及季风的活跃和中断事件将会带来很大的好处，特别是在农业方面，可以改善种植和使用灌溉和肥料的决策。要取得这样的进展，就需要进一步了解季风如何在全国范围内发展，以及它在用于预报和气候预测的计算机模型中的行为。人们已经知道，季风发生的时间受到热带地区被称为马登-朱利安涛动（Madden-Julian Oscillation）的变化的影响。新的研究表明，中纬度地区也发挥着强大的控制作用。但是，与热带驱动因素相比，温带因素在季风开始进程和活跃期和间歇期的时间上的全部作用是难以量化和理解的。MiLCMOP小组早些时候带领INCOMPASS实地考察到印度，进行了新的测量并提出了关于如何控制季风的新想法，包括季风进程可以被描述为热带和温带气团之间的“拔河”的概念。这种“拉锯战”是一个不稳定的过程，在潮湿的热带气流接管整个季节之前，两个气团会来回移动。MiLCMOP团队汇集了季风系统和温带过程的专业知识，全球和对流尺度的最先进模型，以及最新的机器学习和因果推理技术。通过对正在起作用的中纬度过程的详细调查，确定温带动力在印度季风发生和发展中的作用的时机已经成熟。MiLCMOP将使用已建立的技术并开发新的指标来量化季风进程与温带强迫之间的相互作用。这些方法将包括使用涡度预算（检查环流变化的来源）和拉格朗日特征跟踪（确定所涉及的不同气团的起源和性质）。涡度预算和拉格朗日特征跟踪工具将应用于基于观测的“再分析”和案例研究中快速和缓慢开始行为的模型数据，以确定控制季风进展的主要机制。为IPCC第六次评估报告做出贡献的最新全球气候模型将被测试，看是否能代表观测到的开始过程，而初始化的亚季节和季节预测（包括来自英国气象局和印度NCMRWF的项目合作伙伴）将被询问，以测试是否能做出熟练的预测。将设计和进行新的模式试验，以分离出温带驱动因素影响季风发生及其发展的机制。最后，新的因果推理技术将被用于从热带地区的驱动因素中分离出温带驱动因素的影响。MiLCMOP将回答以下关键问题：(1)在观测和模式中，印度季风进程的速度和稳定性如何受到与温带气旋相互作用的影响？(2)温带控制对ISM发展和变率的影响机制是什么？(3) ISM进程的温带和热带驱动因素如何相互抵消或加强，热带和温带过程的竞争作用是否可以推广到其他季风？

The Indian summer monsoon is critical to the well-being of a billion people since it supplies most of the water for drinking, sanitation, industry and agriculture. Society is so finely tuned to the rains that any variation in their timing, intensity and duration has profound impacts. The onset of monsoon rains typically starts in southern India by 1 June, taking up to 6 weeks to cover the country. Meanwhile, during the monsoon, variations on time scales of a week or more give rise to periods of excess and reduced rainfall, known as active and break events. Being able to better predict the onset of the rains, their progression, and of active and break events in the monsoon would be of great benefit particularly in agriculture, for improved decision-making for planting and use of irrigation and fertilizer. Such advances need improved understanding of how the monsoon progresses across the country and of its behaviour in the computer models used for forecasting and climate prediction. The timing of the monsoon onset is already known to be influenced by variability in the tropics known as the Madden-Julian Oscillation. New research has shown that the mid-latitudes also exert a powerful control. But compared to tropical drivers, the full extent of the extratropical role in monsoon onset progression and in the timing of active and break periods is poorly quantified and understood. The MiLCMOP team earlier led the INCOMPASS field campaign to India, taking new measurements and putting forward new ideas on how the monsoon is controlled, including the concept that monsoon progression can be described as a "tug-of-war" between tropical and extratropical airmasses. This "tug-of-war" is an unsteady process, with a back and forth of the two airmasses before the moist tropical flow takes over for the rest of the season. The MiLCMOP team brings together expertise in monsoon systems and extratropical processes, state-of-the-art modelling at global and convective scales, and in the latest machine learning and causal inference techniques. The time is ripe for MiLCMOP to establish the role of extratropical dynamics on Indian monsoon onset and progression through a detailed investigation of the mid-latitude processes at play. MiLCMOP will use established techniques and develop new metrics to quantify the interactions between monsoon progression and extratropical forcing. These methods will include use of vorticity budgets (examining the sources of changes in circulation) and Lagrangian feature tracking (determining the origin and properties of the different airmasses involved). The vorticity budget and Lagrangian feature tracking tools will be applied to observation-based "reanalysis" and model data in case study years of fast and slow onset behaviour, to determine the dominant mechanisms controlling monsoon progression. The latest global climate models contributing to the IPCC 6th Assessment Report will be tested to see if observed onset processes are represented, while initialised subseasonal and seasonal forecasts (including from project partners at the UK Met Office and India's NCMRWF) will be interrogated to test whether skilful forecasts can be made. New model experiments will be designed and performed to isolate the mechanisms by which extratropical drivers affect monsoon onset and its progression. Finally, novel causal inference techniques will be used to disentangle the effects of extratropical drivers from those in the tropics. MiLCMOP will answer the following key questions: (1) How are the pace and steadiness of Indian monsoon progression affected by interactions with the extratropics in observations and models? (2) What are the mechanisms of extratropical control on ISM progression and variability? (3) In what way do the causal extratropical and tropical drivers of ISM progression offset or reinforce each other and can the competing roles of tropical and extratropical processes be generalised to other monsoons?