Abstract. The tropical tropopause layer (TTL) is the main gateway for air transiting from the troposphere to the stratosphere and therefore impacts the chemical composition of the stratosphere. In particular, the cold-point tropopause, where air parcels encounter their final dehydration, effectively controls the water vapor content of the lower stratosphere. Given the important role of stratospheric water vapor for the global energy budget, it is crucial to understand the long-term changes in cold-point temperature and their impact on water vapor trends. Our study uses Global Navigation Satellite System – Radio Occultation (GNSS-RO) data to show that there has been no overall cooling trend of the TTL over the past 2 decades, in contrast to observations prior to 2000. Instead, the cold point is warming, with the strongest trends of up to 0.7 K per decade during boreal winter and spring. The cold-point warming shows longitudinal asymmetries, with the smallest warming over the central Pacific and the largest warming over the Atlantic. These asymmetries are anticorrelated with patterns of tropospheric temperature trends, and regions of strongest cold-point warming are found to show slight cooling trends in the upper troposphere. Overall, the here-identified warming of the cold point is consistent with model predictions under global climate change, which attribute the warming trends to radiative effects. The seasonal signals and zonal asymmetries of the cold-point temperature and height trends might be related to dynamical responses to enhanced upper-tropospheric heating, changing convection, or trends in the stratospheric circulation. Water vapor observations in the TTL show mostly positive trends consistent with cold-point warming for 2004–2021. We find a decrease in the amplitude of the cold-point temperature seasonal cycle by ∼ 7 % driving a reduction in the seasonal cycle in 100 hPa water vapor by 5 %–6 %. Our analysis shows that this reduction in the seasonal cycle is transported upwards together with the seasonal anomalies and has reduced the amplitude of the well-known tape recorder over the last 2 decades.

中文翻译：

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热带冷点温度和水汽的变率和长期变化


摘要。热带对流层顶层 (TTL) 是空气从对流层传输到平流层的主要门户，因此影响平流层的化学成分。特别是，冷点对流层顶，空气团在这里最终脱水，有效地控制了平流层下部的水蒸气含量。鉴于平流层水蒸气在全球能源预算中的重要作用，了解冷点温度的长期变化及其对水蒸气趋势的影响至关重要。我们的研究使用全球导航卫星系统 – 无线电掩星 (GNSS-RO) 数据表明，与 2000 年之前的观测相比，过去 20 年来 TTL 并没有整体变冷的趋势。相反，冷点正在变暖。 ，在北方冬季和春季，最强烈的趋势高达每十年 0.7 K。冷点变暖呈现出纵向不对称性，太平洋中部变暖幅度最小，大西洋变暖幅度最大。这些不对称性与对流层温度趋势模式反相关，并且发现冷点变暖最强的区域在对流层上层表现出轻微的冷却趋势。总体而言，这里确定的冷点变暖与全球气候变化下的模型预测一致，将变暖趋势归因于辐射效应。冷点温度和高度趋势的季节信号和纬向不对称性可能与对流层上层加热增强、对流变化或平流层环流趋势的动态响应有关。 TTL 中的水蒸气观测显示，大部分积极趋势与 2004-2021 年冷点变暖一致。 我们发现冷点温度季节循环的幅度减少了〜7％，导致100hPa水汽的季节循环减少了5％–6％。我们的分析表明，季节性周期的减少与季节性异常一起向上传播，并降低了过去 20 年著名录音机的振幅。