As Earth warms, the tropopause is expected to rise, but predictions of its temperature change are less certain. One theory ties tropopause temperature to outgoing longwave radiation (OLR), but this contradicts simulations that exhibit a Fixed Tropopause Temperature (FiTT) even as OLR increases. Another theory ties tropopause temperature to upper tropospheric moisture, but is not precise enough to make quantitative predictions. Here, we argue that tropopause temperature, defined by where radiative cooling becomes negligible, is set by water vapor’s maximum spectroscopic absorption and Clausius-Clapeyron scaling. This “thermospectric constraint’ makes quantitative predictions for tropopause temperature that are borne out in single column and general circulation model experiments where the spectroscopy is modified and the tropopause changes in response. This constraint underpins the FiTT hypothesis, shows how tropopause temperature can decouple from OLR, suggests a way to relate the temperatures of anvil clouds and the tropopause, and shows how spectroscopy manifests in Earth’s general circulation.

Tropical high cloud cover decreases with surface warming in most general circulation models. This reduction, according to the “stability-iris” hypothesis, is thermodynamically controlled and linked to a decrease in the radiatively-driven clear-sky convergence, when the peak anvil clouds rise because of the rising isotherms. The influence of the large-scale dynamical changes on the tropical high cloud fraction remains difficult to disentangle from the local thermodynamic influence, given that the mean meridional circulation remains inextricably tied to the local thermodynamic structure of the atmosphere. However, using idealized general circulation model (GCM) simulations, we propose a novel method to segregate the dynamical impact from the thermodynamic impact on the tropical high cloud fraction. To this end, our investigation primarily focuses on the mechanisms underpinning changes in the high cloud cover in the deep tropics in response to extratropical surface warming, when tropical sea surface temperatures remain invariant. We find that the relative importance of the net convective detrainment of ice cloud condensates to the cloud microphysical processes, such as the net depositional growth of ice aggregates, in controlling the tropical high cloud fraction is altitude-dependent.

Climate sensitivity peaks around 310K in a wide variety of climate models, ranging from idealized single column models to fully comprehensive climate models. Although an explanation for this peak has been developed using single column models with fixed relative humidity and line-by-line radiation, the relevance of this theory for explaining the peak in comprehensive climate models is unclear. In this paper we increase CO2 using a clear-sky 3-dimensional climate model with a radiation scheme that maintains accuracy for high CO2 and temperature levels. The Equilibrium Climate Sensitivity (ECS) of our model shows a plateau around 310K with the moistening of the subtropical regions caused by a slowdown in atmospheric circulation increasing the ECS at very high CO2 values. Though the changes in CO2 and temperature presented here are extreme, this study shows the potential importance of changes in atmospheric circulation and relative humidity in quantitative assessments of climate sensitivity.

Future projections indicate the AMOC will weaken and shoal in response to global warming, but models disagree widely over the amount of weakening. We analyse projected AMOC weakening in 27 CMIP6 climate models, in terms of changes in three return pathways of the AMOC. The branch of the AMOC that returns through diffusive upwelling in the Indo-Pacific, but does not later upwell in the Southern Ocean, is particularly sensitive to warming, in part, because shallowing of the deep flow prevents it from entering the Indo-Pacific via the Southern Ocean. The present-day strength of this Indo-Pacific pathway provides a strong constraint on the projected AMOC weakening. However, estimates of this pathway using four observationally-based methods imply a wide range of AMOC weakening under the SSP5-8.5 scenario of 29% to 61% by 2100. Our results suggest that improved observational constraints on this pathway would substantially reduce uncertainty in 21st century AMOC decline.

Asian monsoon rainfall impacts one third of the global population and predicting its variability and future change is of clear importance. However, the dynamics of even the climatological monsoon are not fully understood; seemingly unconnected behaviors and abrupt jumps in rainfall location occur in different regions through the year. Three independent subsystems have traditionally been considered: the East Asian, South Asian, and Western North Pacific monsoons. These are generally viewed as passive stationary-wave responses to insolation, but this picture cannot explain the abrupt jumps in rainfall location. Using model simulations, reanalysis and observations, we show that the complex behavior of all three subsystems in fact results from active propagation of the summertime ‘stationary’ wave. A continent-scale cyclone first expands northwestwards and then propagates eastwards via advective and evaporative feedbacks. We propose that the monsoon’s response to forcings may be understood by considering how this wave interacts with the background state.

Understanding how the outgoing long-wave radiation (OLR) depends on surface temperature is a climate problem of long standing. Various studies have suggested that clear-sky OLR varies linearly with surface temperature, with a longwave clear-sky feedback that is independent of surface temperature and relative humidity. However, this uniformity lies in tension with the notion that humidity controls tropical stability (e.g. the “furnace” and “radiator fins” of Pierrehumbert, 1995). Here we explore the state-dependence of longwave clear-sky feedback as a function of both surface temperature and column relative humidity. We find that a strong relative humidity-dependence in the feedback emerges above 275 K, stemming from the closing of the H2O window. We construct a simple model for estimating the all-sky feedback and find that although clouds lower the zonal-mean longwave feedback, the dependence on humidity remains significant.

Asian monsoon rainfall impacts one third of the global population and predicting its variability and future change is of clear importance. However, the dynamics of even the climatological monsoon are not fully understood. Three independent subsystems are traditionally considered: the East Asian, South Asian, and Western North Pacific monsoons. Here we use idealized model simulations with east-west thermal contrast to explore the complex observed onset behavior of these subsystems. Our results suggest that the summertime ‘stationary wave’ monsoon circulation is not simply a passive response to insolation, but instead expands northwestwards and then eastwards via advective and evaporative feedbacks. In particular, our simulations indicate that onset over the Western North Pacific results from eastward extension of the summertime continental low via atmospheric feedbacks. We propose that the regional monsoons’ responses to forcings may be understood by considering how these feedbacks are influenced by the background state.