The Hadley circulation is one of the most important atmospheric circulations. Widening of the Hadley circulation has drawn extensive studies in the past decade. The key concern is that widening of the Hadley circulation would cause poleward shift of the subtropical dry zone. Various metrics have been applied to measure the widening of the tropics. What are responsible for the observed widening trends of the Hadley circulation? How anthropogenic and natural forcings caused the widening? How the widening results in regional climatic effects? These are the major questions in studies of the Hadley circulation. While both observations and simulations all show widening of the Hadley circulation in the past few decades, there are no general agreements of changes in the strength of the Hadley circulation. Although some reanalysis datasets show strengthening of the Hadley circulation, it was shown that the strengthening trend could be due to artificial reasons, and simulations show weakening of the Hadley circulation for global greenhouse warming. In the present paper, we shall briefly review the major progresses of studies in trends in width and strength of the Hadley circulation. We address answers to these questions, clarify inconsistent results, and propose ideas for future studies.

哈德利环流是最重要的大气环流之一。在过去十年中，哈德利环流的扩大已经进行了广泛的研究。关键问题是，哈德利环流的扩大将引起亚热带干旱区的极移。已经应用了各种度量来度量热带的扩展。是什么导致观察到的哈德利环流不断扩大的趋势？人为和自然强迫如何造成了这种扩大？扩大如何对区域气候产生影响？这些是哈德利环流研究中的主要问题。尽管观察和模拟都显示了过去几十年来哈德利环流的扩大，但对于哈德利环流强度的变化尚无普遍共识。尽管一些再分析数据集显示哈德利环流的加强，但表明加强趋势可能是由于人为原因造成的，而模拟表明全球温室气体变暖的哈德利环流的减弱。在本文中，我们将简要回顾哈德利环流的宽度和强度趋势研究的主要进展。我们解决了这些问题的答案，澄清了不一致的结果，并提出了未来研究的想法。