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Humid heatwaves, characterized by high temperature and humidity combinations, challenge tropical societies. Extreme wet-bulb temperatures (TW) over tropical land are coupled to the warmest sea surface temperatures by atmospheric convection and wave dynamics. Here, we harness this coupling for seasonal forecasts of the annual maximum of daily maximum TW (TWmax). We develop a multiple linear regression model that explains 80% of variance in tropical mean TWmax and significant regional TWmax variances. The model considers warming trends and El Niño and Southern Oscillation indices. Looking ahead, the strong-to-very-strong El Niño at the end of 2023, with an Oceanic Niño Index of ∼2.0, suggests a 2024 tropical land mean TWmax of 26.2°C (25.9–26.4°C), and a 68% chance (24%–94%) of breaking existing records. This method also predicts regional TWmax in specific areas.

The ozone layer is often noted to exhibit self-healing, whereby depletion of ozone aloft induces ozone increases below, explained as resulting from enhanced ozone production due to the associated increase in ultraviolet (UV) radiation below. Similarly, ozone enhancement aloft can reduce ozone below (reverse self-healing). This paper considers self-healing and reverse self-healing to manifest a general mechanism we call photochemical adjustment, whereby ozone perturbations lead to a downward cascade of anomalies in UV and ozone. Conventional explanations for self-healing imply that photochemical adjustment is stabilizing, damping perturbations towards the surface. However, photochemical adjustment can be destabilizing if the enhanced UV disproportionately increases the ozone sink, as can occur if the enhanced UV photolyzes ozone to produce atomic oxygen, which speeds up catalytic destruction of ozone. We analyze photochemical adjustment in two linear ozone models (Cariolle v2.9 and LINOZ), finding that (1) photochemical adjustment is destabilizing above 40 km in the tropical stratosphere and (2) self-healing often represents only a small fraction of the total photochemical stabilization. The destabilizing regime above 40 km is reproduced in a much simpler model: the Chapman cycle augmented with destruction of O and O 3 by generalized catalytic cycles and transport (the Chapman+2 model). The Chapman+2 model reveals that photochemical destabilization occurs where the ozone sink is more sensitive than the source to perturbations in overhead column ozone, which is found to occur when the window of overlapping absorption by O 2 and O 3 is optically unsaturated, i.e., when overhead slant column ozone is below approximately 10 18 molec. cm -2. [ABSTRACT FROM AUTHOR]

Ozone is the most significant radiatively-active gas whose number density maximizes in the interior of the atmosphere, at an altitude of around 26 km in the tropics. Textbook explanations for this interior maximum begin by invoking the Chapman Cycle, a photochemical system that reproduces the altitude of maximum ozone despite omitting leading-order sinks from catalytic cycles and transport. Yet, these textbook explanations subsequently fragment into (1) a source-controlled paradigm, explaining ozone to maximize where its production rate maximizes, between abundant photons aloft and abundant O2 below, and (2) a source/sink competition paradigm explaining ozone to maximize due to competition between the photolytic source and photolytic sink. Augmenting the Chapman Cycle with destruction by generalized catalytic cycles and transport, we demonstrate that these paradigms correspond to different regimes of ozone destruction, distinguished by whether photolysis of O3 contributes at leading order to the sink. The tropical stratosphere is estimated to occupy a photolytic sink regime above 26 km and a non-photolytic sink regime below. Paradoxically, each paradigm predicts ozone to maximize outside its altitude range of applicability, motivating a new explanation, the regime transition paradigm: the interior maximum of ozone occurs at the transition from the photolytic sink regime aloft to the non-photolytic sink regime below. An explicit solution is derived for ozone under gray radiation, which produces an interior maximum at an endogenously-determined regime transition, and elucidates the ozone response to top-of-atmosphere UV perturbations. [ABSTRACT FROM AUTHOR]

Global storm‐resolving models (GSRMs) that can explicitly resolve some of deep convection are now being integrated for climate timescales. GSRMs are able to simulate more realistic precipitation distributions relative to traditional Coupled Model Intercomparison Project 6 (CMIP6) models. In this study, we present results from two‐year‐long integrations of a GSRM developed at Geophysical Fluid Dynamics Laboratory, eXperimental System for High‐resolution prediction on Earth‐to‐Local Domains (X‐SHiELD), for the response of precipitation to sea surface temperature warming and an isolated increase in CO2 and compare it to CMIP6 models. At leading order, X‐SHiELD's response is within the range of the CMIP6 models. However, a close examination of the precipitation distribution response reveals that X‐SHiELD has a different response at lower percentiles and the response of the extreme events are at the lower end of the range of CMIP6 models. A regional decomposition reveals that the difference is most pronounced for midlatitude land, where X‐SHiELD shows a lower increase at intermediate percentiles and drying at lower percentiles. Plain Language Summary: The main tool to investigate and make projections of the response of the climate system to warming is global climate models. These models usually have ∼100 km horizontal resolution and as a result need parameterizations in order to account for small‐scale processes. These parameterizations are a large source of uncertainty. Advancements in computational capabilities and technology allow us to run global models with high enough resolutions, called global storm‐resolving models (GSRMs), that some important small‐scale processes are explicitly resolved. In this study, we use two‐year‐long integrations of an atmosphere GSRM in present‐day and warmer climates and compare the precipitation response to more traditional global climate models. We find that the precipitation response of the GSRM lies within the range of the traditional models, except for the low percentiles of precipitation. A closer examination indicates that the main source of differences between the GSRM and traditional models occurs in midlatitude land regions, where the GSRM predicts more extensive drying of low precipitation percentiles compared to the traditional models. The GSRM also has an increase in precipitation extremes with warming that is somewhat weaker than that of traditional global climate models. Key Points: We compare precipitation changes between Coupled Model Intercomparison Project 6 (CMIP6) models and eXperimental System for High‐resolution prediction on Earth‐to‐Local Domains (X‐SHiELD), a global storm resolving model developed at Geophysical Fluid Dynamics LaboratoryX‐SHiELD response is mostly within the range of CMIP6 models, except for more drying of low precipitation percentilesX‐SHiELD precipitation distribution changes over midlatitude land differs significantly from the CMIP6 models [ABSTRACT FROM AUTHOR]