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BackgroundAs atmospheric greenhouse gas concentrations continue to rise, temperature and humidity will increase further, causing potentially dire increases in human heat stress. On physiological and biophysical grounds, exposure to higher levels of humidity should worsen heat stress by decreasing sweat evaporation. However, population-scale epidemiological studies of heat exposure and response often do not detect associations between high levels of humidity and heat-related mortality or morbidity. These divergent, disciplinary views regarding the role of humidity in heat-related health risks limit confidence in selecting which interventions are effective in reducing health impacts and in projecting future heat-related health risks.ObjectivesVia our multidisciplinary perspective we seek to a) reconcile the competing realities concerning the role of humidity in heat-related health impacts and b) help ensure robust projections of heat-related health risks with climate change. These objectives are critical pathways to identify and communicate effective approaches to cope with present and future heat challenges.DiscussionWe hypothesize six key reasons epidemiological studies have found little impact of humidity on heat-health outcomes: a) At high temperatures, there may be limited influence of humidity on the health conditions that cause most heat-related deaths (i.e., cardiovascular collapse); b) epidemiological data sets have limited spatial extent, a bias toward extratropical (i.e., cooler and less humid), high-income nations, and tend to exist in places where temporal variations in temperature and humidity are positively correlated; c) analyses focus on older, vulnerable populations with sweating, and thus evaporative, impairments that may be further aggravated by dehydration; d) extremely high levels of temperature and humidity (seldom seen in the historical record) are necessary for humidity to substantially impact heat strain of sedentary individuals; e) relationships between temperature and humidity are improperly considered when interpreting epidemiological model results; and f) sub-daily meteorological phenomena, such as rain, occur at high temperatures and humidity, and may bias epidemiological studies based on daily data. Future research must robustly test these hypotheses to advance methods for more accurate incorporation of humidity in estimating heat-related health outcomes under present and projected future climates. https://doi.org/10.1289/EHP11807.

AbstractSouthern Ocean (SO) surface winds are essential for ventilating the upper ocean by bringing heat and CO2 to the ocean interior. The relationships between mixed-layer ventilation, the Southern Annular Mode (SAM), and the storm tracks remain unclear because processes can be governed by short-term wind events as well as long-term means.In this study, observed time-varying 5-day probability density functions (PDFs) of ERA5 surface winds and stresses over the SO are used in a singular value decomposition to derive a linearly independent set of empirical basis functions. The first modes of wind (72% of the total wind variance) and stress (74% of the total stress variance) are highly correlated with a standard SAM index (r = 0.82) and reflect SAM’s role in driving cyclone intensity and, in turn, extreme westerly winds. This Joint PDFs of zonal and meridional wind show that southerly and less westerly winds associated with strong mixed-layer ventilation are more frequent during short and distinct negative SAM phases. The probability of these short-term events might be related to mid-latitude atmospheric circulation. The second mode describes seasonal changes in the wind variance (16% of the total variance) that are uncorrelated with the first mode.The analysis produces similar results when repeated using 5-day PDFs from a suite of scatterometer products. Differences between wind product PDFs resemble the first mode of the PDFs. Together, these results show a strong correlation between surface stress PDFs and the leading modes of atmospheric variability, suggesting that empirical modes can serve as a novel pathway for understanding differences and variability of surface stress PDFs.

Convective clustering, the spatial organization of tropical deep convection, can manifest itself in two ways: through a decrease in the total area covered by convection and/or through a decrease in the number of convective areas. Much of our current understanding of convective clustering comes from simulations in idealized radiative convective equilibrium (RCE) configurations. In these simulations the two forms of convective clustering tend to covary, and their individual effects on the climate are thus hard to disentangle. This study shows that in aquaplanet simulations with more realistic boundary conditions, such as meridional gradients of surface temperature and rotational forces, the two aspects of convective clustering are not equivalent and are associated with different impacts on the large-scale climate. For instance, reducing the convective area in the equatorial region in the aquaplanet simulations results in broader meridional humidity and rain distributions and in lower tropospheric temperatures throughout the tropics. By contrast, the number of convective regions primarily impacts the zonal variance of humidity-related quantities in the aquaplanet simulations, as the distribution of convective regions affects the size of the subsidence regions and thereby the moistening influence of convective regions. The aquaplanet simulations confirm many other qualitative results from RCE simulations, such as a reduction of equatorial tropospheric humidity when the area covered by convection diminishes.

The impact of large-scale orography on wintertime near-surface (850 hPa) temperature variability on daily and synoptic time scales (from days to weeks) in the Northern Hemisphere is investigated. Using a combination of theory, idealized modeling work, and simulations with a comprehensive climate model, it is shown that large-scale orography reduces upstream temperature gradients, in turn reducing upstream temperature variability, and enhances downstream temperature gradients, enhancing downstream temperature variability. Hence, the presence of the Rockies on the western edge of the North American continent increases temperature gradients over North America and, consequently, increases North American temperature variability. By contrast, the presence of the Tibetan Plateau and the Himalayas on the eastern edge of the Eurasian continent damps temperature variability over most of Eurasia. However, Tibet and the Himalayas also interfere with the downstream development of storms in the North Pacific storm track, and thus damp temperature variability over North America, by approximately as much as the Rockies enhance it. Large-scale orography is also shown to impact the skewness of downstream temperature distributions, as temperatures to the north of the enhanced temperature gradients are more positively skewed while temperatures to the south are more negatively skewed. This effect is most clearly seen in the northwest Pacific, off the east coast of Japan.

The mechanisms by which clouds impact the variability of the mid‐latitude atmosphere are poorly understood. We use an idealized, dry atmospheric model to investigate the relationship between Atmospheric Cloud Radiative Effects (ACRE) and annular mode persistence. We force the model with time‐varying diabatic heating that mimics the observed ACRE response to the Southern Annular Mode (SAM). Realistic ACRE forcing reduces annular mode persistence by 5 days (−16%), which we attribute to a weakening of low‐frequency eddy forcing via modified low‐level temperature gradients, though this effect is partly compensated by reduced frictional damping due to zonal wind anomalies becoming more top‐heavy. The persistence changes are nonlinear with respect to the amplitude of ACRE forcing, reflecting nonlinearities in the response of the eddy forcing. These results highlight the ACRE's impact on low‐frequency eddy forcing as the dominant cause of changes in annular mode persistence. Plain Language Summary: In this study, we conduct novel modeling experiments aimed at understanding how clouds affect the unforced variability of jet streams—concentrated bands of eastward wind in the mid‐latitude atmosphere. We focus on direct heating and cooling of the atmosphere by clouds, called Atmospheric Cloud Radiative Effects (ACRE), and how these impact the annular mode—the most significant type of jet stream variability, which consists of north‐south shifts of the jet's position. Using a simplified atmospheric model, we investigate how the persistence and structure of the annular mode are impacted by heatings and coolings that mimic the observed ACRE response to the Southern Hemisphere Annular Mode (SAM). We find that the ACRE reduces the persistence of the annular mode (i.e., deviations of the jet stream from its mean position tend to persist for less time), which we attribute to a weakening of the feedback mechanism by which atmospheric waves strengthen the jet where it is already strongest. We identify some compensating effects, but, overall, our results highlight that ACRE impacts jet streams via low‐altitude temperature gradients and atmospheric wave generation. Key Points: The response of Atmospheric Cloud Radiative Effects (ACRE) to the Southern Annular Mode (SAM) reduces SAM's persistenceThe decrease in SAM's persistence is attributed to weakened low‐frequency eddy forcing via disruption of low‐level temperature gradientsThe change in persistence is nonlinear with respect to the amplitude of ACRE forcing [ABSTRACT FROM AUTHOR]

Mock‐Walker simulations have the potential to play a key role in a tropical model hierarchy, bridging small‐scale Radiative‐Convective Equilibrium simulations and global models of tropical circulations. We demonstrate that mock‐Walker simulations transition from single‐ to double‐celled overturning circulations as mean Sea Surface Temperature (SST) is increased, with the transition occurring near 300 K. The transition is robust to domain geometry and microphysical scheme, and is favored by larger SST gradients. The transition is associated with the development of a mid‐tropospheric minimum in the radiative‐subsidence velocity over the cold pool of the simulations, and is likely reinforced by zonal moisture and temperature fluxes between the warm and cold pools. Several methods of suppressing the transition are investigated, but all set‐ups produce a double‐cell at sufficiently warm mean SSTs. The striking dynamical transition of mock‐Walker simulations dominates their response to warming, though its relevance for observed tropical climate change is unclear. Plain Language Summary: Untangling the coupled interactions between clouds and large‐scale atmospheric flows is one of the "Grand Challenges" of climate science. Large‐scale flows are the main control on the spatial distribution of cloud‐types in the tropics, but clouds in turn play a key role in setting the strengths and spatial structures of these flows. Here, we investigate "mock‐Walker" simulations as a potential idealized modeling set‐up for investigating the two‐way interactions between clouds and tropical circulations. Mock‐Walker simulations include the zonal sea‐surface temperature (SST) gradient needed to generate realistic tropical circulations, while using grid resolutions sufficient to partially resolve clouds. However, in this study we document a robust transition in the large‐scale flow in mock‐Walker simulations from a single overturning cell to two vertically‐stacked overturning cells (a double‐celled flow) as the mean SST is increased. This transition occurs for a mean SST close to the observed SSTs in the equatorial Pacific, and dominates the response of mock‐Walker simulations to warming. The transition is robust to domain geometry, microphysical scheme and to fixing the radiation. While we are unable to provide a complete explanation of the transition, it does seem to be associated with extreme dryness over the cold pools of the simulations. Key Points: Mock‐Walker simulations transition from single‐to a double‐celled circulation as the mean SST is increasedThis is associated with a mid‐tropospheric minimum in the cold pool radiative‐subsidence velocityElevated stability maxima cause transitions in mock‐Walker simulations with fixed radiative cooling and artificial moisture sources [ABSTRACT FROM AUTHOR]

The influence of El Niño–Southern Oscillation (ENSO) in the Asian monsoon region can persist through the post-ENSO summer, after the sea surface temperature (SST) anomalies in the tropical Pacific have dissipated. The long persistence of coherent post-ENSO anomalies is caused by a positive feedback due to interbasin ocean–atmospheric coupling, known as the Indo-western Pacific Ocean capacitor (IPOC) effect, although the feedback mechanism itself does not necessarily rely on the antecedence of ENSO events, suggesting the potential for substantial internal variability independent of ENSO. To investigate the respective role of ENSO forcing and non-ENSO internal variability, we conduct ensemble "forecast" experiments with a full-physics, globally coupled atmosphere–ocean model initialized from a multidecadal tropical Pacific pacemaker simulation. The leading mode of internal variability as represented by the forecast-ensemble spread resembles the post-ENSO IPOC, despite the absence of antecedent ENSO forcing by design. The persistent atmospheric and oceanic anomalies in the leading mode highlight the positive feedback mechanism in the internal variability. The large sample size afforded by the ensemble spread allows us to identify robust non-ENSO precursors of summer IPOC variability, including a cool SST patch over the tropical northwestern Pacific, a warming patch in the tropical North Atlantic, and downwelling oceanic Rossby waves in the tropical Indian Ocean south of the equator. The pathways by which the precursors develop into the summer IPOC mode and the implications for improved predictability are discussed. [ABSTRACT FROM AUTHOR]

Solar radiation modification (SRM) reduces the CO2‐induced change to the mean global hydrological cycle disproportionately more than it reduces the CO2‐induced increase in mean surface temperature. Thus, if SRM were used to offset all CO2‐induced mean warming, global‐mean precipitation would be less than in an unperturbed climate. Here, we show that the mismatch between the mean hydrological effects of CO2 and SRM may partly be alleviated by spectrally tuning the SRM intervention (reducing insolation at some wavelengths more than others). By concentrating solar dimming at near‐infrared wavelengths, where H2O has strong absorption bands, the direct effect of CO2 on the tropospheric energy budget can be offset, which minimizes perturbations to the mean hydrological cycle. Idealized cloud‐resolving simulations of radiative‐convective equilibrium confirm that spectrally tuned SRM can simultaneously maintain mean surface temperature and precipitation at their unperturbed values even as large quantities of CO2 are added to the atmosphere. Plain Language Summary: It may be possible to partly counteract CO2‐driven climate change by solar radiation modification (SRM) that intentionally reduces the amount of sunlight absorbed by the Earth. But different wavelengths of the solar spectrum are absorbed at different altitudes within the surface‐atmosphere system, so different climatic effects are to be expected depending on which wavelengths of sunlight are affected by an SRM intervention. Here, we show that if the goal is to minimize perturbations to the mean hydrological cycle, the ideal spectrally tuned SRM intervention may need to focus on near‐infrared wavelengths. This study clarifies the basic physics underlying the effects of SRM on atmospheric energetics and the mean hydrological cycle. Key Points: Spectrally flat solar geoengineering reduces CO2‐induced change in mean rainfall disproportionately more than mean temperatureA spectrally tuned sunshade restores mean temperature and rainfall simultaneously in an idealized modelEmerging technologies could preferentially scatter sunlight in the near‐infrared, providing a spectral sunshade [ABSTRACT FROM AUTHOR]

Solar radiation modification (SRM) reduces the CO2‐induced change to the mean global hydrological cycle disproportionately more than it reduces the CO2‐induced increase in mean surface temperature. Thus, if SRM were used to offset all CO2‐induced mean warming, global‐mean precipitation would be less than in an unperturbed climate. Here, we show that the mismatch between the mean hydrological effects of CO2 and SRM may partly be alleviated by spectrally tuning the SRM intervention (reducing insolation at some wavelengths more than others). By concentrating solar dimming at near‐infrared wavelengths, where H2O has strong absorption bands, the direct effect of CO2 on the tropospheric energy budget can be offset, which minimizes perturbations to the mean hydrological cycle. Idealized cloud‐resolving simulations of radiative‐convective equilibrium confirm that spectrally tuned SRM can simultaneously maintain mean surface temperature and precipitation at their unperturbed values even as large quantities of CO2 are added to the atmosphere. Plain Language Summary: It may be possible to partly counteract CO2‐driven climate change by solar radiation modification (SRM) that intentionally reduces the amount of sunlight absorbed by the Earth. But different wavelengths of the solar spectrum are absorbed at different altitudes within the surface‐atmosphere system, so different climatic effects are to be expected depending on which wavelengths of sunlight are affected by an SRM intervention. Here, we show that if the goal is to minimize perturbations to the mean hydrological cycle, the ideal spectrally tuned SRM intervention may need to focus on near‐infrared wavelengths. This study clarifies the basic physics underlying the effects of SRM on atmospheric energetics and the mean hydrological cycle. Key Points: Spectrally flat solar geoengineering reduces CO2‐induced change in mean rainfall disproportionately more than mean temperatureA spectrally tuned sunshade restores mean temperature and rainfall simultaneously in an idealized modelEmerging technologies could preferentially scatter sunlight in the near‐infrared, providing a spectral sunshade [ABSTRACT FROM AUTHOR]