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，因为它提供了大部分饮用水、卫生、工业和农业用水。社会对降雨的适应是如此之好，以至于降雨的时间、强度和持续时间的任何变化都会产生深远的影响。季风降雨通常在6月1日开始在印度南部，需要长达6周的时间才能覆盖整个国家。与此同时，在季风期间，一周或更长时间尺度的变化会导致降雨过多和减少的时期，称为活跃和中断事件。能够更好地预测降雨的开始、进展以及季风中的活跃和中断事件将非常有益，特别是在农业方面，有助于改善种植和灌溉及肥料使用的决策。这些进展需要更好地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。季风爆发的时间已经被认为是受热带地区的变化影响的，称为马登-朱利安振荡。新的研究表明，中纬度地区也发挥着强大的控制作用。但与热带驱动因素相比，热带气旋在季风爆发过程中的作用以及活跃期和间歇期的时间安排方面的作用程度很小。MiLCMOP团队早些时候领导了印度的INCOMPASS实地活动，进行了新的测量，并就如何控制季风提出了新的想法，包括季风发展可以被描述为热带和热带外气团之间的“拔河”的概念。这场“拔河”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对流过程的专业知识，全球和对流尺度的最先进建模，以及最新的机器学习和因果推理技术。时机已经成熟，MiLCMOP通过详细调查中纬度过程在发挥作用，以建立印度季风爆发和进展的热带外动力学的作用。MiLCMOP将使用已建立的技术并开发新的度量标准来量化季风进程和热带外强迫之间的相互作用。这些方法将包括使用涡度收支（检查环流变化的来源）和拉格朗日特征跟踪（确定所涉不同气团的起源和性质）。涡度收支和拉格朗日特征跟踪工具将应用于基于观测的“再分析”和案例研究年的快速和缓慢开始行为的模型数据，以确定控制季风发展的主要机制。将测试为IPCC第六次评估报告做出贡献的最新全球气候模型，以确定是否代表了观测到的开始过程，同时将询问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），以测试是否可以进行熟练的预测。将设计和进行新的模型实验，以分离出热带气旋驱动因素影响季风爆发及其发展的机制。最后，新的因果推理技术将被用来解开那些在热带地区的热带外驱动程序的影响。MiLCMOP将回答以下关键问题：（1）印度季风发展的速度和稳定性如何受到观测和模式中与热带外相互作用的影响？(2)激素外控制对ISM进展和变异性的作用机制是什么？(3)ISM进程中的热带和温带驱动因素以何种方式相互抵消或加强，热带和热带外过程的竞争作用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?