The poleward extent of Earth's zonal-mean Hadley cells varies across seasons and years, which would be nice to capture in a simple theory. A plausible, albeit diagnostic, candidate from Hill et al. combines the conventional two-layer, quasigeostrophic, baroclinic instability-based framework with a less conventional assumption that each cell's upper-branch zonal winds are suitably captured by a single, cell-wide Rossby number, with meridional variations in the local Rossby number neglected. We test this theory against ERA5 reanalysis data, finding that it captures both seasonal and interannual variations in the Hadley cell zonal winds and poleward extent fairly well. For the seasonal cycle of the Northern Hemisphere (NH) cell poleward edge only, this requires empirically lagging the prediction by 1 month, for reasons unclear to us. In all cases, the bulk Rossby number value that yields the most accurate zonal wind fields is approximately equal to the actual, diagnosed cell-mean value. Variations in these cell-mean Rossby numbers, in turn, predominantly drive variations in each cell's poleward extent. All other terms matter much less including the subtropical static stability, which, by increasing under global warming, is generally considered the predominant driver of future Hadley cell expansion. These results argue for developing a predictive theory for the cell-mean Rossby number and for diagnosing its role in climate model projections of future Hadley cell expansion.

Abstract Congestus clouds, characterized by their vertical extent into the middle troposphere, are widespread in tropical regions and play an important role in Earth's climate system. However, fundamental questions regarding their formation and prevalence remain unanswered. Here, we endeavor to answer how congestus cloud tops form by detraining preferentially at altitudes between 5 and 6 km and why this detraining outflow is invigorated by drier mid‐tropospheric conditions. We construct a clear‐sky radiative‐convective framework of congestus cloud‐top formation that is grounded in the discovery of an important spectroscopic property of water vapor. In this mass‐ and energy‐conserving framework, convective detrainment maximizes at a height of 5 and 6 km due to a swift decline in radiative cooling in clear‐sky regions. This decline is, in turn, a consequence of water vapor spectroscopy: more specifically, a drop in the number of strong absorption lines in the water vapor rotation band. In a simple spectral model, we link this spectroscopic property to the shape of the rotation band, which can be approximated as the product of a power law and a sine wave representing the band's deviation from statistical log‐linearity. The characteristic “C”‐shaped relative humidity profile in the tropics further strengthens the outflow in drier mid‐level conditions by amplifying vertical decreases in the clear‐sky cooling rate. Essential to this process are strong RH gradients, which are most pronounced under the driest conditions and induce a vertical decrease in the optical depth lapse rate across the mid‐troposphere.

Earth's tropics are characterized by quasi-steady precipitation with small oscillations about a mean value, which has led to the hypothesis that moist convection is in a state of quasi-equilibrium (QE). In contrast, very warm simulations of Earth's tropical convection are characterized by relaxation-oscillator-like (RO) precipitation, with short-lived convective storms and torrential rainfall forming and dissipating at regular intervals with little to no precipitation in between. We develop a model of moist convection by combining a zero-buoyancy model of bulk-plume convection with a QE heat engine model, and we use it to show that QE is violated at high surface temperatures. We hypothesize that the RO state emerges when the equilibrium condition of the convective heat engine is violated, that is, when the heating rate times a thermodynamic efficiency exceeds the rate at which work can be performed. We test our hypothesis against one- and three-dimensional numerical simulations and find that it accurately predicts the onset of RO convection. The proposed mechanism for RO emergence from QE breakdown is agnostic of the condensable, and can be applied to any planetary atmosphere undergoing moist convection. To date, RO states have only been demonstrated in three-dimensional convection-resolving simulations, which has made it seem that the physics of the RO state requires simulations that can explicitly resolve the three-dimensional interaction of cloudy plumes and their environment. We demonstrate that RO states also exist in single-column simulations of radiative-convective equilibrium with parameterized convection, albeit in a different surface temperature range and with much longer storm-free intervals.

There are records of past Earth climates that were ice-free all the way to the poles (Barron 1983), which can be described as “hothouse” climates. These hothouse climates can be contrasted with an “all-tropics” planet, where the tropics are defined by the atmospheric dynamics, i.e. the Hadley Cell extent (Faulk et al. 2017). This classification is thus primarily dependent on a planet’s rotation, rather than its ice-free extent or surface temperatures. We investigate the parameter space between Earth and an all-tropics world using the open-source GCM Isca, developed by Vallis et al (2018). We take an Earth analog and perform a parameter sweep in three dimensions: global reservoir depth (1000m, 100m, 10m, 1m, 1cm); global saturation vapor pressure (1.5x current, 1.4x, 1.3x, 1.2x, 1.1x, 1x); and rotation rate (16 days, 8 days, 1 day). The sweep will allow us to explore the effects of surface liquid coverage, atmospheric moisture content, and large-scale atmospheric circulation on an Earth-like climate. In this presentation we provide a status report and analysis of initial findings.

This work is a direct continuation of McKinney et al., who attempted to create a planet with Earth-like temperatures and physical properties but with precipitation and circulation patterns that were Titan-like. McKinney et al. attempted to do so by changing only three basic planetary parameters: the ratio of dry land to ocean on the surface, the rotation period, and the volatility of the condensable. Each of these parameters is varied from an Earth-like value to a Titan-like one to analyze the climate transition between these two planetary archetypes. In this work, we expand on McKinney et al. by including a seasonal cycle and increasing the number of diagnostic criteria for determining Titan-like dynamics. The simulations use Earth-like obliquity and an Earth-like solar constant. We fi nd that the presence of a dry land strip extending to at least 55 degrees N/S degrees N/S is most effective at creating Titan-like climatic conditions on an otherwise Earth-like planet, such as high-latitude summer precipitation maxima and a low-humidity equator. In contrast, slow rotation and high atmospheric vapor abundance have minimal climatic impacts despite being characteristic features of Titan. Our experiments show that it is not difficult fi cult to produce distinctly Titan-like features in an Earth-like GCM with minimal changes to its fundamental parameters. This suggests that Earth-like planets could have a large range of global climate states throughout their history just through changes in topography. Similarly, Titan may have experienced more Earth-like climate states periods where its tropics were wetter.

ABSTRACTExamining the morphologies of Titan’s clouds can provide a general physical interpretation of observed storms and their relation to atmospheric dynamics of the moon. Through a combined analysis of observations of images collected by Cassini ISS during Titan flybys, we search for cloud phenomena to identify various types of storms. Several of the observed cloud features also give us both spatial and temporal information that reveals how the clouds evolve in time. We employ the cloud activity of the observed clouds to describe their characteristics and search for time evolution patterns to try to identify the dynamics behind them, for instance Rossby and gravity waves.Observations of a predominant example of a mid-latitude cloud system was captured by Cassini ISS cameras over a period of about 24 hours from Dec. 13 to 14, 2009, after the onset of a new season of the Saturn system. The images show methane clouds in the troposphere concentrated in a band between 45o and 63o south latitude, a streak-shaped mid-latitude cloud system extending across half the globe, traveling several hundred kilometers during the observation period. The sequence of images obtained throughout this flyby allowed us to create maps (see image below) that were made into movies of clouds moving across the moon's surface background.We present the analysis of these “streamer clouds”, as we have dubbed them, and a handful of other mid-latitude cloud system events based on observations of the movies produced from their ISS-mapped images. The results of the analysis and the implications for Titan's atmospheric instabilities will be discussed.

We investigate how climate, clouds, and convection change as the amount of water vapor in the atmosphere is varied by altering the saturation vapor pressure (SVP) by a constant in a one-dimensional climate model. We identify four effects of altering SVP on clouds in an Earthlike climate with distinct layers of low and high clouds. First, the anvils of high clouds get higher as SVP is increased (and vice versa) because they are bound by radiative constraints to occur at a lower temperature. The vapor pressure path above the cold anvils does not change in Earthlike climates. Second, low clouds get lower as SVP increases (and vice versa) because they are coupled to a convective boundary layer (CBL) that shallows primarily from an increase in the tropospheric static stability. The third and fourth effects follow from the first two, namely, that single-layer cloud states exist both in vapor-poor states with a merged cloud deck and vapor-rich states with an elevated cloud deck. We identify two cloud instability parameters that determine the transitions between single- and double-layer cloud regimes. Qualitatively, sufficiently vapor-poor states have a deep, diffusive layer that overlaps with a weaker convective layer (topping out at the tropopause) that cannot maintain low relative humidity in the midtroposphere through the drying of descending air, thus causing the cloud layers to merge. Sufficiently vapor-rich states lose their low clouds as the shallowing CBL drops below the lifting condensation level.

Data and code for Spaulding-Astudillo and Mitchell (2023c), "Clear-sky convergence and the origin of tropical congestus clouds". The Python code used to create Figures 1, 3-6 and supplementary Figure 1 are included in separate directories. Within each directory, we include experimental data generated by the Reference Forward Model (RFM; Dudhia 2017) that is needed to reproduce the figures. To create the figures, run the provided code in a Jupyter notebook or from the command line. Helper functions for data processing within the figure scripts have the nomenclature ..._data.py. Note that the provided code only allows you to visualize RFM data, not produce it yourself. To do so, you must download and run RFM yourself (https://eodg.atm.ox.ac.uk/RFM/). In addition, Figure 1 requires spatio-temporally averaged data from MERRA-2, ERA5, and CloudSat/CALIPSO (Betrand et al. 2023), which we provide in pickle files. See the main text for data citations, which will allow you to access the original, non-averaged climate data. RFM data directories have the nomenclature /ctrl-..... For example, ctrl-RHmid-75-zmid-7-uniform-1 where:RHmid=75% is the relative humidity at a mid-tropospheric height zmid=7.5 km. Uniform=True indicates that the tropospheric relative humidity is constant with height. When uniform=False, the relative humidity follows a C-shaped distribution between the surface and the tropopause. The helper functions in ..._data.py allow you to read the RFM output from .asc files. The nomenclature of the RFM output files is standardized: https://eodg.atm.ox.ac.uk/RFM/.

Paleo-proxy reconstructions reveal a moistening of the American Southwest during the Last Glacial Maximum (LGM; 21 ka). However, the primary mechanisms driving the moistening trend are still debated, with relatively few studies focused on hypotheses related to synoptic changes in precipitation. Analysis of the Paleoclimate Intercomparison Model Project (PMIP3) simulations shows enhancement of precipitation in the southwest and south-central United States during the winter and summer. Here, we suggest that summertime eastward phase shifting of stationary waves at the LGM enhanced pre-cipitation in the south-central United States and dried the southeastern United States. Mechanism denial experiments per-formed with version 3 of the Hadley Centre Coupled Model (HadCM3) indicate that the thermodynamic effect of the Laurentide Ice Sheet forced eastward phase shifting of stationary waves. By comparing a synthesis of LGM proxies to the PMIP3 ensemble, we find models that compare more favorably to the reconstructions simulate a weaker Laurentide ice ther-modynamic effect, smaller eastward phase shifting of stationary waves, and weaker jet stream anomalies.

In relaxation-oscillator (RO) climate states, short-lived convective storms with torrential rainfall form and dissipate at regular, periodic intervals. RO states have been demonstrated in two- and three-dimensional simulations of radiative-convective equilibrium (RCE), and it has been argued that the existence of the RO state requires explicitly resolving moist convective processes. However, the exact nature and emergence mechanism of the RO state have yet to be determined. Here, we show that (1) RO states exist in single-column-model simulations of RCE with parameterized convection, and (2) the RO state can be understood as one that has no steady-state solutions of an analytical model of RCE. As with model simulations with resolved convection, these simpler, one-dimensional models of RCE clearly demonstrate RO states emerge at high surface temperatures and/or very moist atmospheres. Emergence occurs when atmospheric instability quantified by the convective available potential energy can no longer support the latent heat release of deep, entraining convective plumes. The proposed mechanism for RO emergence is general to all moist planetary atmospheres, is agnostic of the condensing component, and naturally leads to an understanding of Titan's bursty methane weather.

We present a theory for the latitudinal extents of both Hadley cells throughout the annual cycle by combining our recent scaling for the ascending edge latitude based on low-latitude supercriticality with the theory for the poleward, descending edge latitudes of Kang and Lu based on baroclinic instability and a uniform Rossby number (Ro) within each cell's upper branch. The resulting expressions for all three Hadley cell edges are predictive except for diagnosed values of Ro and two proportionality constants. Thermal inertia-which damps and lags the ascent latitude relative to the insolation-is accounted for semianalytically through the Mitchell et al. model of an "effective" seasonal forcing cycle. Our theory, given empirically an additional similar to 1-month lag for the descending edge, captures the climatological annual cycle of the ascending and descending edges in an Earthlike simulation in an idealized aquaplanet general circulation model (GCM). In simulations in this and two other idealized GCMs with varied planetary rotation rate (omega), the winter, descending edge of the solsticial, cross-equatorial Hadley cell scales approximately as omega(-1/2) and the summer, ascending edge as omega(-2/3), both in accordance with our theory. Possible future refinements and tests of the theory are discussed.

Present-day environmental conditions on Titan, the largest moon of Saturn, may do active geomorphic work on its surface. On Earth, the hydrologic water cycle erodes and weathers its continents. Deluges over elevated terrain create debris flows and sheetfloods that spread into alluvial fans as the topographic slope decreases. Mars also shows evidence of past fluvial erosion, but fluvial activity cannot be ongoing in the present. On Titan, however, fluvial erosion is likely ongoing. In this study, we focus on understanding the environmental controls on the spatial distributions of alluvial fans, a type of fluvial depositional feature observed globally on the surface of Titan. To do this, we utilize probabilistic models to determine the strength of spatial correlations between spatial distributions of alluvial fans and present-day environmental factors. We find that the spatial distribution of alluvial fans on Titan correlates well with several present-day environmental conditions, including average precipitation, precipitation variability, and elevation. Based on our model, we also provide predictions of the likelihood of alluvial fan occurrences for areas of Titan not mapped with Cassini, which may be of interest for future missions to Titan.

Weak-temperature-gradient influences from the tropics and quasigeostrophic influences from the extratropics plausibly constrain the subtropical-mean static stability in terrestrial atmospheres. Because mean descent acting on this static stability is a leading-order term in the thermodynamic balance, a state-invariant static stability would impose constraints on the Hadley cells, which this paper explores in simulations of varying planetary rotation rate. If downdraft-averaged effective heating (the sum of diabatic heating and eddy heat flux convergence) too is invariant, so must be vertical velocity—an “omega governor.” In that case, the Hadley circulation overturning strength and downdraft width must scale identically—the cell can strengthen only by widening or weaken only by narrowing. Semiempirical scalings demonstrate that subtropical eddy heat flux convergence weakens with rotation rate (scales positively) while diabatic heating strengthens (scales negatively), compensating one another if they are of similar magnitude. Simulations in two idealized, dry GCMs with a wide range of planetary rotation rates exhibit nearly unchanging downdraft-averaged static stability, effective heating, and vertical velocity, as well as nearly identical scalings of the Hadley cell downdraft width and strength. In one, eddy stresses set this scaling directly (the Rossby number remains small); in the other, eddy stress and bulk Rossby number changes compensate to yield the same, ~Ω −1/3 scaling. The consistency of this power law for cell width and strength variations may indicate a common driver, and we speculate that Ekman pumping could be the mechanism responsible for this behavior. Diabatic heating in an idealized aquaplanet GCM is an order of magnitude larger than in dry GCMs and reanalyses, and while the subtropical static stability is insensitive to rotation rate, the effective heating and vertical velocity are not.

In the past 40 years, the global annual mean surface temperature has experienced a nonuniform warming, differing from the spatially uniform warming simulated by the forced responses of large multimodel ensembles to anthropogenic forcing. Rather, it exhibits significant asymmetry between the Arctic and Antarctic, with intermittent and spatially varying warming trends along the Northern Hemisphere (NH) midlatitudes and a slight cooling in the tropical eastern Pacific. In particular, this "wavy" pattern of temperature changes over the NH midlatitudes features strong cooling over Eurasia in boreal winter. Here, we show that these nonuniform features of surface temperature changes are likely tied together by tropical eastern Pacific sea surface temperatures (SSTs), via a global atmospheric teleconnection. Using six reanalyses, we find that this teleconnection can be consistently obtained as a leading circulation mode in the past century. This tropically driven teleconnection is associated with a Pacific SST pattern resembling the interdecadal Pacific oscillation (IPO), and hereafter referred to as the IPO-related bipolar teleconnection (IPO-BT). Further, two paleo-reanalysis reconstruction datasets show that the IPO-BT is a robust recurrent mode over the past 400 and 2000 years. The IPO-BT mode may thus serve as an important internal mode that regulates high-latitude climate variability on multidecadal time scales, favoring a warming (cooling) episode in the Arctic accompanied by cooling (warming) over Eurasia and the Southern Ocean (SO). Thus, the spatial nonuniformity of recent surface temperature trends may be partially explained by the enhanced appearance of the IPO-BT mode by a transition of the IPO toward a cooling phase in the eastern Pacific in the past decades.

We present theories for the latitudinal extents of both Hadley cells throughout the annual cycle by combining our recent scaling for the ascending edge latitude (Hill et al. 2021) with the uniform Rossby number (Ro), baroclinic instability-based theory for the poleward, descending edge latitudes of Kang and Lu 2012. The resulting analytic expressions for all three Hadley cell edges are predictive except for diagnosed values of Ro and two proportionality constants. The theory captures the climatological annual cycle of the ascending and descending edges in an Earth-like simulation in an idealized aquaplanet general circulation model (GCM), provided the descending edge prediction is lagged by one month. In simulations in this and two other idealized GCMs with varied planetary rotation rate ($\Omega$), the winter, descending edge of the solsticial, cross-equatorial Hadley cell scales approximately as $\Omega^{-1/2}$ and the summer, ascending edge as $\Omega^{-2/3}$, both in accordance with our theory.

How far the Hadley circulation’s ascending branch extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth’s climate. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of supercritical forcing. Supercriticality sets the minimum extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. We explicitly simulate this latitude-by-latitude radiative-convective equilibrium (RCE) state. Its depth-averaged temperature profile is suitably captured by a simple analytical approximation that increases linearly with sin φ , where φ is latitude, from the winter to the summer pole. This, in turn, yields a one-third power-law scaling of the supercritical forcing extent with the thermal Rossby number. In moist and dry idealized GCM simulations under solsticial forcing performed with a wide range of planetary rotation rates, the ascending edge latitudes largely behave according to this scaling.

Arctic sea ice melting processes in summer due to internal atmospheric variability have recently received considerable attention. A regional barotropic atmospheric process over Greenland and the Arctic Ocean in summer (June-July-August), featuring either a year-to-year change or a low-frequency trend toward geopotential height rise, has been identified as an essential contributor to September sea ice loss, in both observations and the CESM1 Large Ensemble (CESM-LE) of simulations [1-2]. This local melting is further found to be sensitive to remote sea surface temperature (SST) variability in the East Central Pacific [3]. Here, we utilize five available single-model large ensembles and 31 CMIP5 models’ pre-industrial control simulations to show that the same atmospheric process, resembling the observed one and the one found in the CESM-LE, also dominates internal sea ice variability on interannual to interdecadal time scales in pre-industrial, historical and future scenarios, regardless of the modeling environment. However, all models exhibit limitations in replicating the correct magnitude of the observed local atmosphere-sea ice coupling and its sensitivity to remote tropical SST variability. These biases cast a shadow over models’ credibility in simulating interactions of sea ice variability with the Arctic and global climate systems. Further efforts toward identifying possible causes of these model limitations may provide profound implications for alleviating the biases and improving interannual and decadal time scale sea ice prediction and future sea ice projection. [1] Ding, Q., and Coauthors, (2017): Influence of high-latitude atmospheric circulation changes on summertime Arctic sea ice. Nat. Climate Change, 7, 289-295. [2] Ding, Q., and Coauthors, (2019): Fingerprints of internal drivers of Arctic sea ice loss in observations and model simulations. Nat. Geosci., 12, 28–33. [3] Baxter, I., and Coauthors, (2019): How tropical Pacific surface cooling contributed to accelerated sea ice melt from 2007 to 2012 as ice is thinned by anthropogenic forcing. J. Climate, 32, 8583–8602 https://doi.org/10.1175/JCLI-D-18-0783.1

How far the ascending branch of the Hadley circulation extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth's solsticial general circulation. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of so-called supercritical forcing. Supercriticality sets the minimal extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. Despite its origins in axisymmetric theory, supercriticality constitutes a conceptually valid predictor for the ascending edge even in zonally varying, macroturbulent atmospheres like Earth's, provided certain empirical conditions are met. Numerical simulations of latitude-by-latitude radiative-convective equilibrium (RCE) under Earth's solsticial forcing show that the supercritical forcing extent aligns well with the observed climatological boreal summer ascending edge. A simple analytical approximation to the solsticial RCE temperature profile, from Lindzen and Hou (1988) but with the temperature maximum located far off-equator, accurately approximates the RCE state and thereby the supercritical forcing extent. The accuracy of the resulting analytical predictor for the solsticial ascending edge is confirmed in moist and dry idealized GCMs under solsticial forcing with varying planetary rotation rate, $\Omega$. In particular, in the small-angle limit appropriate for Earth, the simulated ascending edges exhibit the theory's predicted $\Omega^{-2/3}$ scaling.

Arctic sea ice melting processes in summer due to internal atmospheric variability have recently received considerable attention. A regional barotropic atmospheric process over Greenland and the Arctic Ocean in summer (June–August), featuring either a year-to-year change or a low-frequency trend toward geopotential height rise, has been identified as an essential contributor to September sea ice loss, in both observations and the CESM1 Large Ensemble (CESM-LE) of simulations. This local melting is further found to be sensitive to remote sea surface temperature (SST) variability in the east-central tropical Pacific Ocean. Here, we utilize five available large “initial condition” Earth system model ensembles and 31 CMIP5 models’ preindustrial control simulations to show that the same atmospheric process, resembling the observed one and the one found in the CESM-LE, also dominates internal sea ice variability in summer on interannual to interdecadal time scales in preindustrial, historical, and future scenarios, regardless of the modeling environment. However, all models exhibit limitations in replicating the magnitude of the observed local atmosphere–sea ice coupling and its sensitivity to remote tropical SST variability in the past four decades. These biases call for caution in the interpretation of existing models’ simulations and fresh thinking about models’ credibility in simulating interactions of sea ice variability with the Arctic and global climate systems. Further efforts toward identifying the causes of these model limitations may provide implications for alleviating the biases and improving interannual- and decadal-time-scale sea ice prediction and future sea ice projection.