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Tropical convection that overshoots the cold point tropopause can impact the climate by directly influencing water vapor, temperatures, and thin cirrus in the upper troposphere‐lower stratosphere (UTLS) region. The distribution of cold point overshoots between land and ocean may help determine how the overshoots will affect the UTLS in a changing climate. Using 4 years of satellite and reanalysis data, we test a brightness temperature proxy calibrated by radar/lidar data to identify cold point‐overshooting convection across the global tropics. We find evidence of cold point‐overshooting convection throughout the tropics, though other cirrus above the cold point cover an area 100 times larger than overshooting tops. Cold point‐overshooting convection occurs 30%–40% more often over convectively active land areas than over the warmest oceans. This proxy can be generalized to evaluate the fidelity of cold point overshoots simulated by storm‐resolving models. Plain Language Summary Extremely deep convection in the tropics that overshoots the cold point, the coldest temperature level between the upper troposphere and lower stratosphere, influences the vertical temperature structure of this region and water vapor in the lower stratosphere, where it acts as a greenhouse gas. Overshooting cloud tops appear “cold” in infrared satellite imagery, so they can be identified from the difference between their brightness temperature and the nearby cold point temperature. We calibrate this brightness temperature proxy using satellite measurements of cloud ice. Cold point overshoots occur almost as often over the warmest oceans as over moist tropical land areas. Overshooting tops comprise only 1% of satellite‐detectable cloud above the cold point, most of which is very thin ice cloud. Our proxy can be used as a real‐world observational test of cold point overshoots simulated by the most realistic global atmospheric models, which resolve individual thunderstorm systems. Key Points We identify likely tropical cold point overshoots using a radar/lidar calibrated cold point‐relative brightness temperature proxy In a 4‐year climatology, cold point overshoots only modestly favor convectively active land areas over the Indo‐Pacific warm pool Thin cirrus above the cold point covers over 100‐fold more tropical area than cold point overshoots

Global positioning system (GPS) radio occultation (RO) observations, first made of Earth’s atmosphere in 1995, have contributed in new ways to the understanding of the thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS), an important component of the climate system. The UTLS plays an essential role in the global radiative balance, the exchange of water vapor, ozone, and other chemical constituents between the troposphere and stratosphere, and the transfer of energy from the troposphere to the stratosphere. With their high accuracy, precision, vertical resolution, and global coverage, RO observations are uniquely suited for studying the UTLS and a broad range of equatorial waves, including gravity waves, Kelvin waves, Rossby and mixed Rossby–gravity waves, and thermal tides. Because RO measurements are nearly unaffected by clouds, they also resolve the upper-level thermal structure of deep convection and tropical cyclones as well as volcanic clouds. Their low biases and stability from mission to mission make RO observations powerful tools for studying climate variability and trends, including the annual cycle and intraseasonal-to-interannual atmospheric modes of variability such as the quasi-biennial oscillation (QBO), Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO). These properties also make them useful for evaluating climate models and detection of small trends in the UTLS temperature, key indicators of climate change. This paper reviews the contributions of ROobservations to the understanding of the three-dimensional structure of tropical UTLS phenomena and their variability over time scales ranging from hours to decades and longer.

By analyzing observational reanalysis datasets since the late 20th century, in this study we investigated the monthly variability of East Asian ozone at the upper troposphere–lower stratosphere (UTLS, 250 hPa), from a perspective of atmospheric stationary waves. To identify the primary modes of East Asian UTLS ozone variability, an empirical orthogonal function (EOF) analysis was conducted, revealing that the first three EOF modes account for approximately two-thirds of the total variability. The first EOF mode, characterized by a monopole pattern, is associated with the Polar–Eurasian-like teleconnection. Additionally, the second and third EOF modes, featuring meridional and zonal dipole patterns, are linked to the West Pacific-like teleconnection and the British–Baikal Corridor-like teleconnection, respectively. It was found that UTLS ozone concentrations increase over troughs and decrease over ridges of these atmospheric stationary waves in the mid-latitudes. The increase in UTLS ozone concentration caused by these stationary atmospheric waves is expected to create favorable background conditions for a stronger intrusion of stratospheric ozone into the mid-troposphere. Additionally, our results indicate that stratospheric ozone can sustain these stationary atmospheric waves by modulating geopotential height through solar radiation absorption. These findings highlight the crucial role of dynamic-chemical interactions in advancing our understanding of the UTLS system.

The South Asian summer monsoon (SASM) is of considerable scientific and social importance to the densely populated South Asia. Existing literature signified that the interannual variation of the SASM can be reflected by multiple dynamical and radiative processes. However, quantifying their relative contributions remains inadequate, particular for the contribution of aerosol process. Here, the land-sea thermal contrast index (LSTCI) is employed to represent the large-scale thermal driving at the mid–upper troposphere associated with the SASM, which is defined as temperature difference between the southern Eurasia (SE) and the tropical Indian Ocean (TIO) at the mid–upper troposphere. Based on the coupled atmosphere-surface climate feedback-response analysis method, this study linearly decomposes the total temperature change associated with the LSTCI into several partial temperature changes associated with individual dynamical and radiative processes. Our result demonstrates that the LSTCI is mainly explained by the positive contributions of atmospheric dynamic (69%), water vapor (35%), and aerosol (11%) processes, which are partially offset by a negative contribution of cloud process (-18%). Surface dynamic process play a neglectable role in the LSTCI, because it exerts similar effects on the temperature anomalies over the SE and the TIO. Further analysis indicates that the total effect of aerosols is dominated by change in black carbon. As two important components, the temperature anomalies over the SE and the TIO separately account for about 55% and 45% to the LSTCI. Our finding provides a new insight onto quantitatively understanding the relevant processes involved in the SASM variation.

The South Asian summer monsoon (SASM) circulation in 2015 is the weakest since 2000s, which results in severe drought over broad regions of the Indian peninsula. The 2015 SASM is closely related to the weakened summer meridional thermal contrast between southern Eurasia (SE) and the tropical Indian Ocean (TIO) at the mid–upper troposphere. Based on an updated climate feedback-response analysis method, this study conducts a quantitative attribution analysis of the thermal contrast anomalies associated with the 2015 SASM to multiple dynamical and radiative processes, particular for aerosol process. Result shows that the 2015 weak SASM is mainly attributed to the effect of water vapor (58%), followed by the effects of atmospheric dynamics (18%), clouds (15%), and aerosols (15%), respectively. These positive effects are partially offset by the negative contribution from surface dynamic process (-14%). As the most pronounced factor, the water vapor process weakens the SASM circulation via inducing SE cooling and TIO warming, which is closely linked to the decreased (increased) specific humidity over SE (TIO). Further analysis indicates that the total effect of aerosols is dominated by the changes in black carbon and sea salt. As two important components, the SE cooling and TIO warming separately account for about 51% and 49% to the 2015 SASM. The former is mainly attributed to the cooling effect of clouds, while the latter is mainly induced by the warming effect of atmospheric dynamics. Our result provides a new insight into the 2015 weak SASM from a quantitative perspective.

Ozone in the upper troposphere–lower stratosphere (UTLS) is primarily regulated by tropospheric dynamics. Understanding mechanisms driving ozone variability at the UTLS is crucial to evaluate the transport of mass to and from the lower stratosphere. The El Niño-Southern Oscillation (ENSO) is the primary coupled mode acting on interannual timescales modulating tropospheric circulation worldwide. ENSO teleconnections can depend on the phases of the Pacific Decadal Oscillation (PDO) and on the characteristics of the warming over central and eastern tropical Pacific. This study investigates the role of ENSO on UTLS ozone variability with focus on South America and examines patterns of teleconnections in the two recent warm (1980–1997) and cool (1998–2012) PDO phases. The dominant mode of ozone variability is identified by applying a principal component analysis (PCA) to modern-era retrospective analysis for research and applications, Version 2 (MERRA-2) ozone data from September–November (SON). SON is the season with the largest UTLS ozone variance over South America. The first mode resembles a Rossby wave train across South America with spatial patterns dependent on PDO phase. We show that the ENSO teleconnections and respective influences on SON UTLS ozone are stronger during the cool PDO when ENSO and PDO are mostly in phase. Additionally, the strength of the ENSO teleconnection appears to depend on patterns of SST anomalies over tropical Pacific. The decadal variability in the ENSO-PDO relationships and teleconnections with the Southern Hemisphere resulted in a shift in upper tropospheric circulation in tropical and subtropical regions of South America.

The vertical and spatial structure of the atmospheric El Niño‐Southern Oscillation (ENSO) signal is investigated using radio occultation (RO) data from August 2006 to December 2010. Due to their high vertical resolution and global coverage, RO data are well suited to describe the full 3‐dimensional ENSO structure in the troposphere and lower stratosphere. We find that interannual temperature anomalies in the equatorial region show a natural decomposition into zonal‐mean and eddy (deviations from the zonal‐mean) components that are both related to ENSO. Consistent with previous studies, we find that during the warm phase of ENSO, zonal‐mean temperatures increase in the tropical troposphere and decrease in the tropical stratosphere. Maximum warming occurs above 8 km, and the transition between warming and cooling occurs near the tropopause. This zonal‐mean response lags sea surface temperature anomalies in the eastern equatorial Pacific by 3 months. The atmospheric eddy component, in contrast, responds rapidly (within 1 month) to ENSO forcing. This signal features a low‐latitude dipole between the Indian and Pacific Oceans, with off‐equatorial maxima centered around 20° to 30° latitude in both hemispheres. The eddy response pattern attains maximum amplitude in the upper troposphere near 11 km and (with opposite polarity) in a shallow layer near the tropopause at approximately 17 km. The eddy ENSO signal tends to be out‐of‐phase between low and middle latitudes in both the troposphere and lower stratosphere. Key Points Utilize GPS radio occultation (RO) data to detect the 3‐dim ENSO structure Coherent upper tropospheric and lower stratospheric ENSO signals Differences in zonal‐mean and eddy ENSO components

To investigate the stratosphere-troposphere exchange (STE) process induced by the gravity waves (GWs) caused by Typhoon Molave (2020) in the upper troposphere and lower stratosphere, we analyzed the ERA5 reanalysis data provided by the European Centre for Medium-Range Weather Forecasts and the CMA Tropical Cyclone Best Track Dataset. We also adopted the mesoscale forecast model Weather Research and Forecasting model V4.3 for numerical simulation. Most of the previous studies were about typhoon-induced STE and typhoon-induced GWs, while our research focused on the STE caused by typhoon-induced gravity waves. Our analysis shows that most of the time, the gravity wave signal of Typhoon Molave appeared below the tropopause. It was stronger on the east side of the typhoon center (10°20°N, 110°–120°E) than on the west side, suggesting an eastward tilted structure with height increase. When the GWs in the upper troposphere and lower stratosphere region on the west side of the typhoon center broke up, it produced strong turbulence, resulting in stratosphere-troposphere exchange. At this time, the average potential vorticity vertical flux increased with the average ozone mass mixing ratio. The gravity wave events and STE process simulated by the WRF model were basically consistent with the results of ERA5 reanalysis data, but the time of gravity wave breaking was different. This study indicates that after the breaking of the GWs induced by typhoons, turbulent mixing will also be generated, and thus the STE. Key words: gravity wave; typhoon; stratosphere-troposphere exchange; STE; upper troposphere and lower stratosphere; UTLS

The cold point tropopause, the minimum temperature within the tropical upper troposphere‐lower stratosphere region (UTLS), significantly impacts Earth's climate by influencing the amount of water vapor entering the lower stratosphere. Understanding which mechanisms are most important in setting the cold point temperature and height may help us better predict how it will change in a future warmed climate. In this analysis we evaluate two mechanisms that may influence the cold point—cold point‐overshooting convection and the radiative lofting of thin cirrus near the cold point—during boreal winter by comparing 30‐day global storm‐resolving model (GSRM) simulations from the winter phase of the DYAMOND initiative to satellite observations. GSRMs have explicit deep convection and sufficiently fine grid spacings to simulate convective overshoots and UTLS cirrus, making them promising tools for this purpose. We find that the GSRMs reproduce the observed distribution of cold point‐overshooting convection but do not simulate enough cirrus capable of radiative lofting near the cold point. Both the models and observations show a strong relationship between areas of frequent cold point overshoots and colder cold points, suggesting that cold point‐overshooting convection has a notable influence on the mean cold point. However, we find little evidence that the radiative lofting of cold point cirrus substantially influences the cold point. Cold point‐overshooting convection alone cannot explain all variations in the cold point across different GSRMs or regions; future studies using longer GSRM simulations that consider longer‐term UTLS processes are needed to fully understand what sets the cold point. Plain Language Summary The cold point is the coldest level between the tropical upper troposphere and lower stratosphere. Its temperature determines how much water vapor, a greenhouse gas, can enter the lower stratosphere, and thus can influence how much the Earth warms in the future. The processes that determine the height and temperature of the cold point are not well understood, but two of these may include cooling and moistening by very deep convection that reaches past the cold point itself, called cold point‐overshooting convection, and the heating of thin cirrus clouds near the cold point, which causes the surrounding area to gradually lift up. By comparing very high‐resolution global climate models that can represent both processes with satellite observations, we find that cold point‐overshooting convection has a much stronger influence on the cold point than the lifting from cirrus clouds. However, cold point‐overshooting convection does not explain all of the differences between models, suggesting that other processes in this part of the atmosphere, particularly those occurring on time scales longer than the 30‐day period of these model simulations, are also important for setting the cold point. Key Points Global storm‐resolving models (GSRMs) can reproduce the observed geographic distribution of cold point overshoots Radiatively active cirrus near the cold point are underestimated in GSRMs compared to radar/lidar observations Cold point‐overshooting convection influences the mean cold point but does not entirely explain its variations in temperature or height

Aviation is widely recognised to have global-scale climate impacts through the formation of ozone (O3) in the upper troposphere and lower stratosphere (UTLS), driven by emissions of nitrogen oxides (NOX). Ozone is known to be one of the most potent greenhouse gases formed from the interaction of aircraft emission plumes with atmospheric species. This paper follows up on previous research, where a Photochemical Trajectory Model was shown to be a robust measure of ozone formation along flight trajectories post-flight. We use a combination of a global Lagrangian chemistry-transport model and a box model to quantify the impacts of aircraft NOX on UTLS ozone over a five-day timescale. This work expands on the spatial and temporal range, as well as the chemical accuracy reported previously, with a greater range of NOX chemistry relevant chemical species. Based on these models, route optimisation has been investigated, through the use of network theory and algorithms. This is to show the potential inclusion of an understanding of climate-sensitive regions of the atmosphere on route planning can have on aviation’s impact on Earth’s Thermal Radiation balance with existing resources and technology. Optimised flight trajectories indicated reductions in O3 formation per unit NOX are in the range 1–40% depending on the spatial aspect of the flight. Temporally, local winter times and equatorial regions are generally found to have the most significant O3 formation per unit NOX; moreover, hotspots were found over the Pacific and Indian Ocean.