Search

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 weak temperature gradients in the tropical free troposphere due to the vanishing Coriolis force near the equator lead to a strong dynamical coupling over the entire tropics. Using theory and a suite of targeted model experiments, we show that the weak temperature gradients further weaken under global warming. We show that the temperature gradient is set by the circulation strength, with a weaker circulation being associated with weaker gradients. Thus, the known scaling difference between atmospheric radiative cooling and static stability that leads to a slowdown of the circulation under warming also leads to a weakening of the temperature gradients in the tropical free troposphere. The impact from the weakening circulation on the weakening of temperature gradients is shown to dominate over the impact of masked CO2 forcing and the El Niño–like tropical Pacific warming pattern in model projections. Key to the result is the nonlinear zonal momentum advection term. Using the well-known Matsuno–Gill model with the correct scaling of heating and static stability may give the correct sign of the response in the temperature gradients, but inaccurate scaling, due to the linear momentum damping in that model. The robust scaling of the magnitude of the tropical quasi-stationary structure with temperature opens possibilities for theoretical advances on questions of societal relevance, ranging from changes in tropical cloudiness to heat stress under climate change. [ABSTRACT FROM AUTHOR]

Global storm‐resolving model (GSRM) simulations (kilometer‐scale horizontal resolution) of the atmosphere can capture the interaction between the scales of deep cumulus convection and the large‐scale dynamics and thermodynamic properties of the atmosphere. Here, we assess the vertical structure of tropical temperature change in Geophysical Fluid Dynamics Laboratory's GSRM X‐SHiELD, perturbed by a uniform sea surface temperature (SST) warming and/or increased CO2 ${\text{CO}}_{2}$ concentration. The simulated warming from an SST increase is weakly amplified relative to the surface through the mid‐troposphere before increasing to a factor of about 2.5 in the upper troposphere. This combination of muted warming in the mid‐troposphere and amplified warming aloft is within the range of CMIP6 models at individual pressure levels but, taken together, is distinctive behavior. The response to CO2 ${\text{CO}}_{2}$ increase with unchanged SST is an approximately vertically uniform warming, comparable to CMIP6 models, and is linearly additive with the SST‐induced warming in X‐SHiELD.Plain Language Summary: The vertical structure of tropical temperature change plays an important role in determining the magnitude of surface warming and modulates the strength of atmospheric convection and large‐scale circulations in response to climate changes. How much enhancement of the tropical upper troposphere compared to the surface has been a longstanding question in climate science. Here, we assess this in climate timescale global storm‐resolving model simulations of climate change with kilometer‐scale horizontal resolution (3.25km $3.25\,\mathrm{k}\mathrm{m}$, roughly 20 times higher resolution than the atmospheric component of conventional climate models). Revisiting this question in this class of models is interesting because at kilometer‐scale horizontal resolution, the atmospheric convection responsible for the enhanced heating—from the condensation of water vapor—of the upper troposphere is explicitly simulated. For the response to sea surface temperature warming, there is enhanced warming near the tropopause that is larger magnitude than typical climate models, while the middle of the troposphere (near 5km $5\,\mathrm{k}\mathrm{m}$ altitude) has relatively muted warming compared to typical climate models. For the response to increased carbon dioxide concentration, this storm‐resolving model has somewhat more tropospheric warming than typical climate models.Key Points: The vertical structure of tropical temperature change is assessed in global storm‐resolving model (GSRM) simulations of climate changeThe GSRM analyzed, X‐SHiELD, has a response to SST increase and CO2 increase that are broadly similar to CMIP6 simulation resultsGSRM response to SST has muted warming through the mid‐troposphere, but a large degree of amplification in the upper troposphere [ABSTRACT FROM AUTHOR]

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]