Modality of the Tropical Rain Belt across Models and Simulated Climates
Modality of the Tropical Rain Belt across Models and Simulated Climates
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DOI：
10.1175/jcli-d-22-0521.1
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发表时间：
2023-03-01
影响因子：
4.9
通讯作者：
Lunt, Daniel J.
中科院分区：
文献类型：
--
作者：
Adam, Ori;Farnsworth, Alexander;Lunt, Daniel J.
热带雨带在单峰和双峰经向降水分布之间变化，在区域和季节到地质时间尺度上都是如此。在这里，我们表明这种变化主要是由赤道降水抑制驱动的，并使用赤道模态指数（EMI）对其进行量化，该指数在1和2之间连续变化，用于纯单峰和双峰分布。研究表明，热带模态是热带气候的一个基本特征，我们将其定义为年平均EMI。我们研究了来自耦合模式比较项目第5和第6阶段的73个气候模式、45个古模拟（距今3亿年前至今）和观测的热带模态的大尺度方面。在现代气候模式中，热带模态的增加与热带雨带宽度的增加、经向翻转环流的变宽和变弱、赤道冷舌的变冷以及双热带辐合带偏偏的加剧密切相关。具有低热带模态的热带扇区（或全球纬向平均值）以季风季节变化为特征（即，雨带随太阳的季节性迁移）。在具有高热带模态的扇区，我们确定了三种重要的季节模态：(i)降水分布向温暖半球的迁移，（ii）半球雨带之间纬向分离的变化，以及（iii）半球雨带强度的跷跷板变化。在高热带模态扇区，由于迁移和分离模式的对比转变，与一般认识相反，热带雨带的季节性迁移通常不能假定是跟随太阳。意义说明：热带雨带是围绕热带地区的强降水带。重要的热带现象，如季风和海洋雨带的季节性变化，是由热带雨带的季节性迁移驱动的，因此，热带人口的关键社会经济方面受其影响。这项工作研究了热带降水南北剖面的变化如何在季节和地质时间尺度上影响热带气候的大尺度方面。具体来说，我们研究了热带雨带剖面从一个峰到两个峰（即从单峰到双峰）变化的趋势。我们定义了这种模态变化的客观定量测量，对于单峰和双峰剖面，它在1和2之间变化。然后我们表明，该测量的年平均值是热带气候的一个重要的一般特征，我们将其定义为热带模态。我们还表明，在热带模态低（接近1）的热带地区，雨带在其海洋季节迁移中跟随太阳，并符合热带翻转环流的典型模式，即哈德利环流，它伴随着季风季节变化。然而，在热带模态高的地区（即接近2），通常认为雨带跟随太阳（或向变暖的半球迁移）的理论预期通常是不合理的。相反，我们确定了三种重要的独立的季节变化模式：(i)降水分布向温暖半球的迁移，（ii）半球雨带之间的纬向分离（或降水剖面的宽度）的变化，以及（iii）半球雨带强度的跷跷板变化。
The tropical rain belt varies between unimodal and bimodal meridional precipitation distributions, both re-gionally and on seasonal to geological time scales. Here we show that this variation is largely driven by equatorial precipita-tion inhibition, and quantify it using an equatorial modality index (EMI) that varies continuously between 1 and 2 for purely unimodal and bimodal distributions. We show that tropical modality is a fundamental characteristic of tropical cli-mate, which we define as annual-mean EMI. We examine large-scale aspects of tropical modality across 73 climate models from phases 5 and 6 of the Coupled Model Intercomparison Project, 45 paleo simulations (;300 million years ago to present), and observations. We find increased tropical modality to be strongly related to increased width of the tropical rain belt, wider and weaker meridional overturning circulation, colder equatorial cold tongues, and more severe double inter-tropical convergence zone bias in modern climate models. Tropical sectors (or global zonal means) with low tropical mo-dality are characterized by monsoonal seasonal variations (i.e., seasonal migrations of rainbands following the sun). In sectors with high tropical modality we identify three important seasonal modes: (i) migration of the precipitation distribution toward the warmer hemisphere, (ii) variation in the latitudinal separation between hemispheric rainbands, and (iii) seesaw variation in the intensity of the hemispheric rainbands. In high tropical modality sectors, due to contrasting shifts of the migration and separation modes, counter to general wisdom, seasonal migrations of tropical rainbands cannot be generally assumed to follow the sun.SIGNIFICANCE STATEMENT: The tropical rain belt is a band of intense precipitation that encircles the tropics. Important tropical phenomena such as monsoons and seasonal shifts of marine rainbands are driven by seasonal migra-tions of the tropical rain belt, which therefore govern key socioeconomic aspects of tropical populations. This work ex-amines how changes in the north-south profile of tropical precipitation affect large-scale aspects of tropical climate, on seasonal to geological time scales. Specifically, we examine the tendency of the profile of the tropical rain belt to vary from having one to two peaks (i.e., from being unimodal to bimodal). We define an objective quantitative measure of this modality variation, which varies between 1 and 2 for unimodal and bimodal profiles. We then show that the annual mean of this measure is an important general characteristic of tropical climate, which we define as tropical modality. We also show that in tropical regions where tropical modality is low (close to 1), rainbands follow the sun in their sea-sonal migrations, and conform to the canonical model of the tropical overturning circulation, known as the Hadley circulation, which goes along with monsoonal seasonal variations. However, in regions with high tropical modality (i.e., close to 2), the common theoretical expectation that rainbands follow the sun (or migrate toward the warming hemisphere) is not generally justified. Instead, we identify three important independent seasonal modes of variation: (i) migration of the precipitation distribution toward the warmer hemisphere, (ii) variation in the latitudinal separation between hemispheric rainbands (or width of the precipitation profile), and (iii) seesaw variation in the intensity of the hemispheric rainbands.