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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 precipitation in the south-central United States and dried the southeastern United States. Mechanism denial experiments performed 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 thermodynamic effect, smaller eastward phase shifting of stationary waves, and weaker jet stream anomalies. Significance Statement Our study is motivated by the impact of climate change on hydroclimate in North America, especially in the semiarid areas near the southwestern United States, and the persistent problem of significant disagreements among CMIP model projections. Yet modern models and observations alone cannot tell us the likeliest climate change outcome in this region. Here we analyze precipitation during the LGM reconstructed from proxies and as simulated by the PMIP3, which reveal a precipitation trend with wetting in the southwestern United States and drying in the southeastern United States. Our interpretation of the PMIP3 simulations involves application of simple theories for midlatitude stationary waves, and indicates that the LGM precipitation is generated by the phase shift of stationary waves from the enhancement of jet speeds. Our model–data comparison also suggests that in the PMIP3 simulations, weak LGM jet stream anomalies, due to weak polar amplification, compare more favorably to the proxies.

Atmospheric rivers (ARs) are filamentary conduits of intense water vapour transport in the extratropics, accounting for the majority of poleward moisture transport in the mid-latitudes and acting as a key precipitation source for coastal regions. How ARs have responded to climate change nevertheless remains uncertain. Here we use a series of coupled model experiments to show that there was little to no change in mean AR characteristics in 1920–2005 due to opposite but equal influences from industrial aerosols, which weaken ARs, and greenhouse gases (GHGs), which strengthen them. Despite little historical change, the simulations project steep intensification of ARs in the coming decades, including mean AR-driven precipitation increases of up to ~20 mm per month, as the influence of GHGs greatly outpaces that of industrial aerosols. We also investigate the extent to which future AR changes are dynamically and thermodynamically driven, highlighting the need to conceptualize AR change beyond the scaling of humidity with warming. Atmospheric rivers, concentrated plumes of moisture important to extratropical precipitation, have not changed with historical warming. Model simulations suggest that this is due to the competing weakening effect of aerosols and strengthening effect of greenhouse gases, which dominates with future warming.

Annular modes explain much of the internal variability of Earth’s atmosphere but have never been identified as influential on other planets. Using data assimilation datasets for Mars and a general circulation model for Titan, we demonstrate that annular modes are prominent in the atmospheres of both worlds, capturing a larger fraction of their respective variabilities than Earth’s. One mode describes latitudinal shifts of the jet on Mars, as on Earth, and vertical shifts of the jet on Titan. Another describes pulses of mid-latitude eddy kinetic energy on all three worlds, albeit with somewhat different characteristics. We demonstrate that this latter mode has predictive power for regional dust activity on Mars, revealing its usefulness for understanding Martian weather. The similarity of annular variability in dynamically diverse worlds suggests its ubiquity across the Solar System, potentially extending to exoplanets. Annular (zonally symmetric) atmospheric modes, common on Earth, are found on both Mars and Titan via observations and general circulation models. On Mars, they have considerable impact on dust activity and could be used to predict dust storms. On Titan, they could be linked to sporadic cloud outbursts.

Titan’s stratospheric polar vortex is a prominent phenomenon of Titan’s global wind field, consisting of a westerly jet achieving speeds of nearly 200 m s-1 [1, 2]. The strong westerly winds have been hypothesized to serve as a mixing barrier to molecules transported to the high winter latitudes by Titan’s stratospheric meridional overturning circulation [2]. Similar to Earth’s stratospheric polar vortex, the vortex on Titan is primarily driven by diabatic cooling at high winter latitudes, resulting in a steep meridional temperature gradient and, via the thermal wind relation, a strong vertical wind shear [3]. While the existence and evolution of the vortex has been constrained for one half of a Titan year by observations from the Cassini spacecraft, a rigorous analysis of the potential mechanisms that give rise to the strong stratospheric jet has not yet been performed. Here, using simulations from the Titan Atmospheric Model [4], which has recently been updated to better simulate Titan’s stratosphere [5], we study the temporal evolution of processes proposed to be responsible for the evolution of Titan’s stratospheric polar vortex, including: solar shortwave heating (largely controlled by the presence of stratospheric aerosols), adiabatic heating from the descending branch of the meridional overturning circulation, and molecular longwave cooling [3]. References [1] Sharkey, J. et al, (2021) Icar 354, 114030 [2] Schultis, J., et al., (2022) PSJ 3, 73 [3] Teanby, N. A., et al, (2017), Nat Comm 8, 1586 [4] Lora, J. M., et al., (2015), Icar 250, 516 – 528 [5] Lombardo, N. A., et al., (in review), Icar

International audience; There is increasing concern with anthropogenic greenhouse gas emissions that ocean warming, in concert with summer and winter precipitation changes, will induce anoxia in multiple ocean basins, such as in the Mediterranean Sea. Although the hydrological changes in the eastern Mediterranean are quite well constrained, quantitative evidence of changes in sea surface temperature (SST) and winter rainfall in the western Mediterranean across the past interglacials is relatively scarce. In this study, we use a combination of trace element (Ba/Ca and Mg/Ca) and stable oxygen isotope composition of planktonic foraminifera from a sediment core located off the Golo River, Corsica (northern Tyrrhenian Sea) to reconstruct variations in SSTs and sea surface salinities (SSS) during the Holocene (MIS 1) and warm periods of the past two interglacials (MIS 5, 7). We also analyse PMIP3 model simulations for the mid-Holocene to investigate the mechanism for moisture transport in the western Mediterranean. Our Mg/Ca-SSTs, Ba/Ca-salinity and derived δ18O-seawater records suggest that the warm periods of the past interglacials were characterized by high river discharge and lower SSS in the northern Tyrrhenian Sea. Since this region is ideally located on the trajectory of wintertime storm tracks across the North Atlantic into the Mediterranean Sea and is also outside the influence of the ITCZ-controlled summer monsoon rains, we suggest enhanced winter rainfall during the past interglacials. Our analysis of PMIP3 model simulations for mid-Holocene also support increased south-westerly moisture transport into the western Mediterranean originating from the North Atlantic. We also find evidence that long-term amplitude of the salinity decrease tightly follows eccentricity. We suggest that these hydrologic changes in the western Mediterranean, and the northern Mediterranean borderlands as a whole, were a contributing factor, together with local cyclogenesis and African summer monsoon rainfall, to basin-wide anoxia in the past. Our findings offer new constraints to the amplitude and cause of winter rainfall changes in the Mediterranean during past warm periods.

During the Last Glacial Maximum (LGM) and subsequent deglaciation, the Great Basin in the southwestern United States was covered by numerous extensive closed-basin lakes, in stark contrast with the predominately arid climate observed today. This transition from lakes in the Late Pleistocene to modern aridity implies large changes in the regional water balance. Whether these changes were driven by increased precipitation rates due to changes in atmospheric dynamics, decreased evaporation rates resulting from temperature depression and summer insolation changes, or some combination of the two remains uncertain. The factors contributing to these large-scale changes in hydroclimate are critical to resolve, given that this region is poised to undergo future anthropogenic-forced climate changes with large uncertainties in model simulations for the 21st century. Furthermore, there are ambiguous constraints on the magnitude and even the sign of changes in key hydroclimate variables between the Last Glacial Maximum and the present day in both proxy reconstructions and climate model analyses of the region. Here we report thermodynamically derived estimates of changes in temperature, precipitation, and evaporation rates, as well as the isotopic composition of lake water, using clumped isotope data from an ancient lake in the northwestern Great Basin, Lake Surprise (California). Compared to modern climate, mean annual air temperature at Lake Surprise was 4.7 °C lower during the Last Glacial Maximum, with decreased evaporation rates and similar precipitation rates to modern. During the mid-deglacial period, the growth of Lake Surprise implied that the lake hydrologic budget briefly departed from steady state. Our reconstructions indicate that this growth took place rapidly, while the subsequent lake regression took place over several thousand years. Using models for precipitation and evaporation constrained from clumped isotope results, we determine that the disappearance of Lake Surprise coincided with a moderate increase in lake temperature, along with increasing evaporation rates outpacing increasing precipitation rates. Concomitant analysis of proxy data and climate model simulations for the Last Glacial Maximum are used to provide a robust means to understand past climate change, and by extension, predict how current hydroclimates may respond to expected future climate forcings. We suggest that an expansion of this analysis to more basins across a larger spatial scale could provide valuable insight into proposed climate forcings, and aid in climate model process depiction. Ultimately, our analysis highlights the importance of temperature-driven evaporation as a mechanism for lake growth and retreat in this region.

The impact of methane convection on the circulation of Titan is investigated in the Titan Atmospheric Model (TAM), using a simplified Betts–Miller (SBM) moist convection parameterization scheme. We vary the reference relative humidity ( R H S B M ) and relaxation timescale of convection ( τ ) parameters of the SBM scheme. Titan’s atmosphere is mostly insensitive to changes in τ , but convective instability and precipitation are highly impacted by changes in R H S B M . Convection behavior changes from infrequent ( R H S B M to near-continuous precipitation at the poles during summer at high R H S B M (85%). The intermediate regime ( R H S B M =70%–80%) consists of frequent events ( ∼ 10 per Titan year) of moderate intensity that are limited in meridional extent to their respective hemisphere. Using results from the Titan Regional Atmospheric Modeling System (TRAMS) and observations, we tune the parameters of the SBM parameterization with optimum values of RH=80% and τ =28800 s. We present a simulated decadal climatology that qualitatively matches observations of Titan’s humidity and cloud activity and generally resembles previous results with TAM. Comparing this simulation to one without moist convection demonstrates that convection strengthens the meridional circulation, warms the mid-levels and cools the surface at the poles, and magnifies zonal-mean global moisture anomalies.

The Atmospheric River (AR) Tracking Method Intercomparison Project (ARTMIP) is a community effort to systematically assess how the uncertainties from AR detectors (ARDTs) impact our scientific understanding of ARs. This study describes the ARTMIP Tier 2 experimental design and initial results using the Coupled Model Intercomparison Project (CMIP) Phases 5 and 6 multi-model ensembles. We show that AR statistics from a given ARDT in CMIP5/6 historical simulations compare remarkably well with the MERRA-2 reanalysis. In CMIP5/6 future simulations, most ARDTs project a global increase in AR frequency, counts, and sizes, especially along the western coastlines of the Pacific and Atlantic oceans. We find that the choice of ARDT is the dominant contributor to the uncertainty in projected AR frequency when compared with model choice. These results imply that new projects investigating future changes in ARs should explicitly consider ARDT uncertainty as a core part of the experimental design.

One of the first large cloud systems ever observed on Titan was a stationary event at the southern pole that lasted almost two full Titan days. Its stationary nature and large extent are puzzling given that low-level winds should transport clouds eastward, pointing to a mechanism such as atmospheric waves propagating against the mean flow. We use a composite of 47 large convective events across 15 Titan years of simulations from the Titan Atmospheric Model to show that Rossby waves trigger polar convection—which halts the waves and produces stationary precipitation—and then communicate its impact globally. In the aftermath of the convection, forced waves undergo a complicated evolution, including cross-equatorial propagation and tropical-extratropical interaction. The resulting global impact from convection implies its detectability anywhere on Titan, both via surface measurements of pressure and temperature and through remote observation of the outgoing longwave radiation, which increases by ~0.5% globally.

NASA’s Dragonfly mission will send a rotorcraft lander to the surface of Titan in the mid-2030s. Dragonfly's science themes include investigation of Titan’s prebiotic chemistry, habitability, and potential chemical biosignatures from both water-based “life as we know it” (as might occur in the interior mantle ocean, potential cryovolcanic flows, and/or impact melt deposits) and potential “life, but not as we know it” that might use liquid hydrocarbons as a solvent (within Titan’s lakes, seas, and/or aquifers). Consideration of both of these solvents simultaneously led to our initial landing site in Titan’s equatorial dunes and interdunes to sample organic sediments and water ice, respectively. Ultimately, Dragonfly's traverse target is the 80 km diameter Selk Crater, at 7° N, where we seek previously liquid water that has mixed with surface organics. Our science goals include determining how far prebiotic chemistry has progressed on Titan and what molecules and elements might be available for such chemistry. We will also determine the role of Titan’s tropical deserts in the global methane cycle. We will investigate the processes and processing rates that modify Titan’s surface geology and constrain how and where organics and liquid water can mix on and within Titan. Importantly, we will search for chemical biosignatures indicative of past or extant biological processes. As such, Dragonfly, along with Perseverance, is the first NASA mission to explicitly incorporate the search for signs of life into its mission goals since the Viking landers in 1976.

Author(s): Rutz, JJ; Shields, CA; Lora, JM; Payne, AE; Guan, B; Ullrich, P; O’Brien, T; Leung, LR; Ralph, FM; Wehner, M; Brands, S; Collow, A; Goldenson, N; Gorodetskaya, I; Griffith, H; Kashinath, K; Kawzenuk, B; Krishnan, H; Kurlin, V; Lavers, D; Magnusdottir, G; Mahoney, K; McClenny, E; Muszynski, G; Nguyen, PD; Prabhat, M; Qian, Y; Ramos, AM; Sarangi, C; Sellars, S; Shulgina, T; Tome, R; Waliser, D; Walton, D; Wick, G; Wilson, AM; Viale, M | Abstract: Atmospheric rivers (ARs) are now widely known for their association with high-impact weather events and long-term water supply in many regions. Researchers within the scientific community have developed numerous methods to identify and track of ARs—a necessary step for analyses on gridded data sets, and objective attribution of impacts to ARs. These different methods have been developed to answer specific research questions and hence use different criteria (e.g., geometry, threshold values of key variables, and time dependence). Furthermore, these methods are often employed using different reanalysis data sets, time periods, and regions of interest. The goal of the Atmospheric River Tracking Method Intercomparison Project (ARTMIP) is to understand and quantify uncertainties in AR science that arise due to differences in these methods. This paper presents results for key AR-related metrics based on 20+ different AR identification and tracking methods applied to Modern-Era Retrospective Analysis for Research and Applications Version 2 reanalysis data from January 1980 through June 2017. We show that AR frequency, duration, and seasonality exhibit a wide range of results, while the meridional distribution of these metrics along selected coastal (but not interior) transects are quite similar across methods. Furthermore, methods are grouped into criteria-based clusters, within which the range of results is reduced. AR case studies and an evaluation of individual method deviation from an all-method mean highlight advantages/disadvantages of certain approaches. For example, methods with less (more) restrictive criteria identify more (less) ARs and AR-related impacts. Finally, this paper concludes with a discussion and recommendations for those conducting AR-related research to consider.

Planetary surfaces beyond Earth’s are impacted by surface hydrology, and exhibit fluvial and lacustrine features. Titan in particular harbours a rich hydroclimate replete with valley networks, lakes, seas and putative wetlands, all of which are pronounced in the lower-elevation polar regions. However, understanding of Titan’s global climate has heretofore neglected the hydraulic influence of Titan’s large-scale topography. Here we add a surface hydrology model to an existing Titan atmospheric model, and find that infiltration, groundmethane evaporation, and surface and subsurface flow are fundamental to simultaneously reproducing Titan’s observed surface liquid distribution and other aspects of its climate system. We propose that Titan’s climate features infiltration into unsaturated low- and mid-latitude highlands and surface or subsurface flow into high-latitude basins, producing the observed polar moist climes and equatorial deserts. This result implies that a potentially massive unobserved methane reservoir participates in Titan’s methane cycle. It also illustrates the importance of surface hydrology in Titan climate models, and by extension suggests the influence of surface hydrology in idealized models of other planetary climates, including the climates and palaeoclimates of Earth, Mars and exoplanets. Coupling a global surface hydrology model to an existing atmospheric model of Titan reproduces the observed variable climate and distribution of surface liquid, with possible implications for an unobserved methane reservoir on Titan.

Cassini-Huygens provided a wealth of data with which to constrain numerical models of Titan. Such models have been employed over the last decade to investigate various aspects of Titan's atmosphere and climate, and several three-dimensional general circulation models (GCMs) now exist that simulate Titan with a high degree of fidelity. However, substantial uncertainties persist, and at the same time no dedicated intercomparisons have assessed the degree to which these models agree with each other or the observations. To address this gap, and motivated by the proposed Dragonfly Titan lander mission, we directly compare three Titan GCMs to each other and to in situ observations, and also provide multi-model expectations for the low-latitude environment during the early northern winter season. Globally, the models qualitatively agree in their representation of the atmospheric structure and circulation, though one model severely underestimates meridional temperature gradients and zonal winds. We find that, at low latitudes, simulated and observed atmospheric temperatures closely agree in all cases, while the measured winds above the boundary layer are only quantitatively matched by one model. Nevertheless, the models simulate similar near-surface winds, and all indicate these are weak. Likewise, temperatures and methane content at low latitudes are similar between models, with some differences that are largely attributable to modeling assumptions. All models predict environments that closely resemble that encountered by the Huygens probe, including little or no precipitation at low latitudes during northern winter. The most significant differences concern the methane cycle, though the models are least comparable in this area and substantial uncertainties remain. We suggest that, while the overall low-latitude environment on Titan at this season is now fairly well constrained, future in situ measurements and monitoring will transform our understanding of regional and temporal variability, atmosphere-surface coupling, Titan's methane cycle, and modeling thereof.

The Last Glacial Maximum (LGM, ∼ 21 000 years ago) has been a major focus for evaluating how well state-of-the-art climate models simulate climate changes as large as those expected in the future using paleoclimate reconstructions. A new generation of climate models has been used to generate LGM simulations as part of the Paleoclimate Modelling Intercomparison Project (PMIP) contribution to the Coupled Model Intercomparison Project (CMIP). Here, we provide a preliminary analysis and evaluation of the results of these LGM experiments (PMIP4, most of which are PMIP4-CMIP6) and compare them with the previous generation of simulations (PMIP3, most of which are PMIP3-CMIP5). We show that the global averages of the PMIP4 simulations span a larger range in terms of mean annual surface air temperature and mean annual precipitation compared to the PMIP3-CMIP5 simulations, with some PMIP4 simulations reaching a globally colder and drier state. However, the multi-model global cooling average is similar for the PMIP4 and PMIP3 ensembles, while the multi-model PMIP4 mean annual precipitation average is drier than the PMIP3 one. There are important differences in both atmospheric and oceanic circulations between the two sets of experiments, with the northern and southern jet streams being more poleward and the changes in the Atlantic Meridional Overturning Circulation being less pronounced in the PMIP4-CMIP6 simulations than in the PMIP3-CMIP5 simulations. Changes in simulated precipitation patterns are influenced by both temperature and circulation changes. Differences in simulated climate between individual models remain large. Therefore, although there are differences in the average behaviour across the two ensembles, the new simulation results are not fundamentally different from the PMIP3-CMIP5 results. Evaluation of large-scale climate features, such as land–sea contrast and polar amplification, confirms that the models capture these well and within the uncertainty of the paleoclimate reconstructions. Nevertheless, regional climate changes are less well simulated: the models underestimate extratropical cooling, particularly in winter, and precipitation changes. These results point to the utility of using paleoclimate simulations to understand the mechanisms of climate change and evaluate model performance.

Titan, the largest moon of Saturn, is characterized by gigantic linear dunes and an active dust cycle. Much like on Earth, these and other aeolian processes are caused by the wind-driven mobilization of surface grains, known as saltation. To date, very little is known about the conditions that allow for the occurrence of saltation on Titan. In fact, Titan saltation may be fundamentally different from Earth saltation given the denser atmosphere, the lower gravity, and the cohesion of its surface grains. Here, we draw on experiments, theory, and modeling to progress towards a comprehensive understanding of saltation on Titan. We find that aerodynamic lifting of surface grains requires strong wind speeds due to the high cohesion of the grains. However, saltation may be sustained through granular splash at wind speeds much smaller than those required to initiate grain motion. This suggests that most saltation transport on Titan is intermittent rather than continuous. We account for these insights by proposing a saltation mass flux parameterization specific for Titan conditions that accounts for transport intermittency, and use it to quantify yearly sediment transport with a general circulation model. The results show that Titan's prevailing atmospheric circulation is capable of generating highly intermittent yet significant saltation, yielding yearly transport rates similar to those on the most active dunes of Mars. Furthermore, we find that accounting for surface topography might be critical to answering open questions related to Titan's landscape evolution, including the formation of linear dunes in opposite direction to the prevailing circulation.

Thanks to the Cassini-Huygens mission, Titan, the pale orange dot of Pioneer and Voyager encounters has been revealed to be a dynamic, hydrologically-shaped, organic-rich ocean world offering unparalleled opportunities to explore prebiotic chemistry. And while Cassini-Huygens revolutionized our understanding of each of the three layers of Titan--the atmosphere, the surface, and the interior--we are only beginning to hypothesize how these realms interact. In this paper, we summarize the current state of Titan knowledge and discuss how future exploration of Titan would address some of the next decade's most compelling planetary science questions. We also demonstrate why exploring Titan, both with and beyond the Dragonfly New Frontiers mission, is a necessary and complementary component of an Ocean Worlds Program that seeks to understand whether habitable environments exist elsewhere in our solar system., Submitted to the PSJ Focus Issue on Ocean World Exploration

In response to ESA Voyage 2050 announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a "heavy" drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan northern latitudes with an orbiter and in situ element(s) would be highly complementary with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan equatorial regions in the mid-2030s., Comment: arXiv admin note: substantial text overlap with arXiv:1908.01374

We outline a flagship-class mission concept focused on studying Titan as a global system, with particular emphasis on the polar regions. Investigating Titan from the unique standpoint of a polar orbit would enable comprehensive global maps to uncover the physics and chemistry of the atmosphere, and the topography and geophysical environment of the surface and subsurface. The mission includes two key elements: (1) an orbiter spacecraft, which also acts as a data relay, and (2) one or more small probes to directly investigate Titan's seas and make the first direct measurements of their liquid composition and physical environment. The orbiter would carry a sophisticated remote sensing payload, including a novel topographic lidar, a long-wavelength surface-penetrating radar, a sub-millimeter sounder for winds and for mesospheric/thermospheric composition, and a camera and near-infrared spectrometer. An instrument suite to analyze particles and fields would include a mass spectrometer to focus on the interactions between Titan's escaping upper atmosphere and the solar wind and Saturnian magnetosphere. The orbiter would enter a stable polar orbit around 1500 to 1800 km, from which vantage point it would make global maps of the atmosphere and surface. One or more probes, released from the orbiter, would investigate Titan's seas in situ, including possible differences in composition between higher and lower latitude seas, as well as the atmosphere during the parachute descent. The number of probes, as well as the instrument complement on the orbiter and probe, remain to be finalized during a mission study that we recommend to NASA as part of the NRC Decadal Survey for Planetary Science now underway, with the goal of an overall mission cost in the "small flagship" category of ~$2 bn. International partnerships, similar to Cassini-Huygens, may also be included for consideration., 13 pages, white paper submitted to the NRC Decadal Survey for Planetary Science and Astrobiology

It is virtually certain that the mean surface temperature of the Earth will continue to increase under realistic emission scenarios. Yet comparatively little is known about future changes in climate variability. We explore changes in climate variability over the large range of climates simulated in the framework the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5/6) and the Paleoclimate Modeling Intercomparison Project Phases 3 and 4 (PMIP3/4). This consists of time slice simulations for the Pliocene, Last Interglacial, Last Glacial Maximum, the Mid Holocene and idealized warming experiments (1% CO2 and abrupt 4xCO2), and encompasses climates with a range of 12°C of global mean temperature change. We examine climate variability from different perspectives: from local interannual change, to coherent climate modes and through compositing extremes. The change in the interannual variability of precipitation is strongly dependent upon the local change in the total amount of precipitation. Meanwhile only over tropical land is the change in the interannual temperature variability positively correlated to temperature change, and then weakly. In general, temperature variability is inversely related to mean temperature change - with analysis of power spectra demonstrating that this holds from intra-seasonal to multi-decadal timescales. We systematically investigate changes in the standard deviation of modes of climate variability. Overall, no generalisable pattern emerges. Several modes do show, sometimes weak, increasing variability with global mean temperature change (most notably the Atlantic Zonal Mode), but also the El Niño/Southern Oscillation indices (NINO3.4 and NINO4). The annular modes in the Northern (Southern) hemisphere show only weakly increasing (decreasing) relationships. By compositing extreme precipitation events across the ensemble, we demonstrate that the atmospheric drivers dominating rainfall variability in Mediterranean climates persist throughout palaeoclimate and future simulations. The robust nature of the response of climate variability in model simulations, between both cold and warm climates and across multiple timescales, suggests that observations and proxy reconstructions could provide a meaningful constraint on climate variability in future projections.

The Last Glacial Maximum (LGM, ~ 21,000 years ago) has been a major focus for evaluating how well state-of-the-art climate models simulate climate changes as large as those expected in the future using paleoclimate reconstructions. A new generation of climate models have been used to generate LGM simulations as part of the Palaeoclimate Modelling Intercomparison Project (PMIP) contribution to the Coupled Model Intercomparison Project (CMIP). Here we provide a preliminary analysis and evaluation of the results of these LGM experiments (PMIP4-CMIP6) and compare them with the previous generation of simulations (PMIP3-CMIP5). We show that the PMIP4-CMIP6 are globally less cold and less dry than the PMIP3-CMIP5 simulations, most probably because of the use of a more realistic specification of the northern hemisphere ice sheets in the latest simulations although changes in model configuration may also contribute to this. There are important differences in both atmospheric and ocean circulation between the two sets of experiments, with the northern and southern jet streams being more poleward and the changes in the Atlantic Meridional Overturning Circulation being less pronounced in the PMIP4-CMIP6 simulations than in the PMIP3-CMIP5 simulations. Changes in simulated precipitation patterns are influenced by both temperature and circulation changes. Differences in simulated climate between individual models remain large so, although there are differences in the average behaviour across the two ensembles, the new simulation results are not fundamentally different from the PMIP3-CMIP5 results. Evaluation of large-scale climate features, such as land-sea contrast and polar amplification, confirms that the models capture these well and within the uncertainty of the palaeoclimate reconstructions. Nevertheless, regional climate changes are less well simulated: the models underestimate extratropical cooling, particularly in winter, and precipitation changes. The spatial patterns of increased precipitation associated with changes in the jet streams are also poorly captured. However, changes in the tropics are more realistic, particularly the changes in tropical temperatures over the oceans. Although these results are preliminary in nature, because of the limited number of LGM simulations currently available, they nevertheless point to the utility of using paleoclimate simulations to understand the mechanisms of climate change and evaluate model performance.

The deep convective cloud–environment feedback loop is likely important to Titan's global methane, energy, and momentum cycles, just as it is for Earth's global water, energy, and momentum budgets. General circulation models of Titan's atmosphere are unable to explicitly simulate deep convection and must instead parameterize the impact of this important subgrid-scale phenomenon on the model-resolved atmospheric state. The goal of this study is to better quantify through cloud resolving modeling the effects of deep convective methane storms on their environment and to feed that information forward to improve parameterizations in global models. Dozens of atmospheric profiles unstable with respect to deep moist convection are extracted from the global Titan Atmospheric Model (TAM) and used to initialize the cloud-resolving Titan Regional Atmospheric Modeling System (TRAMS). Mean profiles of heating/cooling and moistening/drying of the large-scale environment in TRAMS indicate that Titan's deep convection forces the environment in a manner analogous to Earth: Large-scale subsidence of the environmental air warms and dries the environment, but clouds can also moisten the environment through the detrainment and evaporation of condensate near cloud top. Relative humidity profiles and characteristic convective time scales are derived to guide the tuning of the deep convective parameterization implemented in TAM, as described in a companion paper. The triggering of convection, the dry convective mixing of the planetary boundary layer, and the entrainment of environmental air into rising air parcels are found to be critical to determining whether a deep convective cloud will form. Only profiles with relatively large convective available potential energy (CAPE) and well mixed planetary boundary layers with high relative humidity were found to produce storms. Environments with low-level thermal inversions and planetary boundary layers with low relative humidity or rapidly decreasing moisture with height failed to generate deep convection in TRAMS despite positive CAPE.

The Western U.S. is vulnerable to hydrological stress, and insights from past climate periods are helpful for providing historical benchmarks for future climate projections. Myriad evidence from coupled models and paleoclimatic proxies suggests a major reorganization of west coast hydroclimate during the Last Glacial Maximum (LGM, ∼17–25 ka), such that the Southwest U.S. was wetter than modern day and the Pacific Northwest was drier. Yet the fundamental mechanisms underlying these hydroclimatic shifts remain unclear. Here, we employ a suite of targeted model simulations to probe the influence of LGM Northern Hemisphere ice sheets on west coast atmospheric dynamics. Whereas previous modeling studies have suggested that the southward shift of LGM west coast precipitation was driven only by the mechanical steering of atmospheric circulation by elevated ice sheet topography, we find this to be an artifact of earlier simulations that neglected realistic air-sea interaction. Instead, our simulations indicate that ice sheet albedo induced a pattern of North Pacific sea surface temperatures, reinforced by ocean-atmosphere feedbacks, that shifted the large-scale atmospheric circulation as well as the latitudinal distribution of west coast precipitation southward during the LGM. Crucially, we find that atmosphere-ocean feedbacks that sustained this ice sheet albedo-induced temperature pattern in the LGM could drive similar hydroclimatic changes today.

Demonstrations using rotating tanks of fluid can help demystify otherwise counterintuitive behaviors of atmospheric, oceanic, and planetary interior fluid motions. But the expense and complicated assembly of existing rotating table platforms limit their appeal for many schools, especially those below the university level. Here, we introduce Do-It-Yourself Dynamics (DIYnamics), a project developing extremely low-cost rotating tank platforms and accompanying teaching materials. The devices can be assembled in a few minutes from household items, all available for purchase online. Ordering, assembly, and operation instructions are available on the DIYnamics website. Videos using these and other rotating tables to teach specific concepts such as baroclinic instability are available on the DIYnamics YouTube channel—including some in Spanish. The devices, lesson plans, and demonstrations have been successfully piloted at multiple middle schools, in a university course, and at public science outreach events. These uses to date convince us of the DIYnamics materials’ pedagogical value for instructors from well-versed university professors to K–12 science teachers with little background in fluid dynamics.

This study examines the differences in the moisture budget over North America between the Last Glacial Maximum (LGM) and modern climate, as simulated by nine models from Paleoclimate Modelling Intercomparison Project phase 3. The results help elucidate the components and mechanisms of the LGM hydrologic cycle. The models predict substantial increases in winter precipitation minus evaporation ( P − E) over the ice-free parts of western North America with respect to the modern climate, primarily because of increases in moisture convergence by the mean flow. In summer they predict P − E increases from the Great Plains to the southeastern margin of the ice sheet—driven by large decreases in E—that are due to a combination of increased convergence by the mean flow and transient eddies. In both seasons, the LGM–modern changes in P − E are dominated by changes in the circulation, rather than by changes in atmospheric water vapor. Compared to a proxy reconstruction of LGM–modern changes in P, the simulated P responses show modest skill. They generally reproduce the reconstruction in the western part of North America but underestimate the indicated drying of the eastern part. The models that score best tend to simulate more drying of the eastern part as a result of increased moisture divergence by the mean flow. In various regions, there are trade-offs between contributions from the mean flow and transient eddies, pointing to changes in variability during the LGM; however, further work to examine such changes requires higher-frequency model output.

While it has long been known that Titan’s haze and atmosphere are dynamic on seasonal timescales, recent results have revealed that they also exhibit significant subseasonal variations. Here, we report on observations of Titan acquired over an eight-month period between 2014 April and 2015 March with the Spectrograph for Integral Field Observations in the Near Infrared instrument on the Very Large Telescope using adaptive optics. These observations have an average five-day cadence, permitting interrogation of the short-period variability of Titan’s atmosphere. Disk-resolved spectra in the H and K bands (1.4–2.4 μm) were analyzed with the PyDISORT radiative transfer model to determine the spatial distribution and variation of stratospheric haze opacity over subseasonal timescales. We observed a uniform decrease in haze opacity at 20°N and an increase in haze opacity at 250–300°E and ∼40°N over the span of our observations. Globally, we found variations on the order of 5%–10% on timescales of weeks, as well as a steady, global increase in the amount of haze over timescales of months. The observed variations in haze opacity over the short timescales of our observations were of similar magnitude to long-period variations attributed to seasonal variation, suggesting rapid dynamical processes that may take part in the distribution of hazes in Titan’s atmosphere.

The mid-Pliocene warm period (~3.3–3 million years ago) sustained similar atmospheric CO2 concentrations to today, and is thus considered one of the best recent paleoclimate analogs for a future warmer climate. Using model simulations following the Pliocene Model Intercomparison Project Phase 2 experimental design, we investigate the mid-Pliocene mid-latitude hydroclimate with a focus on Pacific atmospheric rivers and related changes to mean and extreme precipitation over the Americas. We find distinct spatial patterns of change in precipitation compared to the pre-industrial control simulation in the northern and southern hemispheres; specifically, the Pacific coasts of North America and Chile, two regions where atmospheric rivers strongly influence local weather and climate today, experience opposite signs of change in precipitation. The precipitation changes are linked to dynamical shifts in atmospheric rivers, especially in the Northern Hemisphere. There, a wintertime stationary wave train present in the pre-industrial control nearly disappears in the mid-Pliocene simulation, resulting in fewer atmospheric rivers making landfall over western North America. Sensitivity simulations reveal that model boundary conditions for mid-Pliocene ice sheets and topography, applied in isolation or in combination, cause the relevant stationary wave response independently of CO2 forcing. The stationary wave response to surface boundary conditions weakens the analogy between mid-Pliocene and future regional midlatitude hydroclimate, and informs our interpretation of the apparent discrepancy between modeled precipitation and hydroclimate proxies in western North America.

Geomorphic features typically associated with extreme rainfall events in terrestrial settings, including extensive fluvial features and alluvial fans, have been detected on Titan’s surface. Methane flow from precipitation on Titan can transport sediments and potentially erode the icy bedrock, but averaged precipitation rates from prior global-scale modelling are too low by at least an order of magnitude to initiate sediment transport of observed grain sizes at low latitudes. Here, we quantify the regional magnitude, frequency and variability of extreme rainfall events from simulations of present-day Titan, with a general circulation model coupled to a land model partially covered by wetlands reservoirs that can capture Titan’s regionally varying hydroclimate. We find that the most extreme storms tend to occur in the mid-latitudes, where observed alluvial fans are most concentrated. Storms capable of sediment transport and erosion occur at all latitudes in our simulations, consistent with the observed global coverage of fluvial features. Our results demonstrate the influential role of extreme precipitation in shaping Titan’s surface. We therefore suggest that, similarly to Earth but differently from Mars, active geomorphic work may be ongoing in the present climate on Titan. Extreme methane rainstorms on Titan occur in mid-latitudes, where alluvial fans are most common, according to a general circulation model. Average precipitation rates are insufficient to actively shape Titan’s surface.

We analyze modeling results of the North Atlantic atmospheric winter circulation from a transient climate simulation over the last 21,000 years. In agreement with previous studies, we find that the midlatitude jet stream assumes a strong, stable, and zonal disposition so long as the North American ice sheets remain in their continent-wide Last Glacial Maximum (LGM) configuration. However, when the Laurentide (LIS) and Cordilleran Ice Sheets separate (ca.14,000 years ago), the jet stream abruptly changes to a tilted circulation regime, similar to modern. The proposed explanation is that the dominant stationary-wave source in the North Atlantic sector changes from the LIS to the Cordilleran mountain range during the saddle collapse. As long as the LIS dominates, the circulation retains the zonal LGM state characterized by prevalent stationary-wave reflection in the subtropical North Atlantic. When the Cordillera takes over, the circulation acquires its modern disposition.

We retrieve vertical and meridional variations of methane mole fraction in Titan's lower troposphere by re-analyzing near-infrared ground-based observations from 17 July 2014 UT (Adamkovics et al., 2016). We generate synthetic spectra using atmospheric methane profiles that do not contain supersaturation or discontinuities to fit the observations, and thereby retrieve minimum saturation altitudes and corresponding specific humidities in the boundary layer. We relate these in turn to surface-level relative humidities using independent surface temperature measurements. We also compare our results with general circulation model simulations to interpret and constrain the relationship between humidities and surface liquids. The results show that Titan's lower troposphere is undersaturated at latitudes south of 60N, consistent with a dry surface there, but increases in humidity toward the north pole indicate appreciable surface liquid coverage. While our observations are consistent with considerably more liquid methane existing at the north pole than is present in observed lakes, a degeneracy between low-level methane and haze leads to substantial uncertainty in determining the extent of the source region., Comment: Accepted for publication in Icarus

International audience; Southwestern North America was wetter than present during the Last Glacial Maximum. The causes of increased water availability have been recently debated, and quantitative precipitation reconstructions have been underutilized in model-data comparisons. We investigate the climatological response of North Pacific atmospheric rivers to the glacial climate using model simulations and paleoclimate reconstructions. Atmospheric moisture transport due to these features shifted toward the southeast relative to modern. Enhanced southwesterly moisture delivery between Hawaii and California increased precipitation in the southwest while decreasing it in the Pacific Northwest, in agreement with reconstructions. Coupled climate models that are best able to reproduce reconstructed precipitation changes simulate decreases in sea level pressure across the eastern North Pacific and show the strongest southeastward shifts of moisture transport relative to a modern climate. Precipitation increases of ∼1 mm d-1, due largely to atmospheric rivers, are of the right magnitude to account for reconstructed pluvial conditions in parts of southwestern North America during the Last Glacial Maximum.

There is increasing concern with anthropogenic greenhouse gas emissions that ocean warming, in concert with summer and winter precipitation changes, will induce anoxia in multiple ocean basins. In particular the Mediterranean Sea is susceptible to severe hydrological changes. Mediterranean hydroclimate is controlled primarily by two phenomena – the latitudinal migration of the Inter-Tropical Convergence Zone and the North Atlantic climatic processes. While the former brings about the African summer monsoon rainfall the latter drives the wintertime storm tracks into the western Mediterranean. Although the hydrological changes in the eastern Mediterranean are quite well constrained, evidence of past changes in temperature and rainfall in the western Mediterranean across the past interglacials is relatively scarce. In this study, we use trace element and stable isotope composition of planktonic foraminifera from a sediment core off Corsica at the mouth of Golo river in the western Mediterranean to reconstruct variations in sea surface temperature (SST) and sea surface salinities (SSS) during the Holocene and warm periods of the past two interglacials. Our data suggest that the warm periods of the last interglacials were characterised by high river discharge and lower SSS in the northern Tyrrhenian Sea, suggesting increased winter rainfall. We find evidence that enhanced winter rainfall during periods of precession minima and high seasonality across interglacials coincide with changes in the respective eccentricity maxima suggesting a causal link. Our model simulations for representative orbital configurations such as the mid-Holocene support increased south-westerly moisture transport into the western Mediterranean originating from the North Atlantic. We suggest that these hydrologic changes in the western and the northern Mediterranean borderlands were a contributing factor to basin-wide anoxia in the past. Our findings offer new insights into the cause and impact of winter rainfall changes in the Mediterranean during past warm periods.