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.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Atmospheric rivers (ARs) are an important driver of surface mass balance over today's Greenland and Antarctic ice sheets. Using paleoclimate simulations with the Community Earth System Model, we find ...ARs also had a key influence on the extensive ice sheets of the Last Glacial Maximum (LGM). ARs provide up to 53% of total precipitation along the margins of the eastern Laurentide ice sheet and up to 22%–27% of precipitation along the margins of the Patagonian, western Cordilleran, and western Fennoscandian ice sheets. Despite overall cold conditions at the LGM, surface temperatures during AR events are often above freezing, resulting in more rain than snow along ice sheet margins and conditions that promote surface melt. The results suggest ARs may have had an important role in ice sheet growth and melt during previous glacial periods and may have accelerated ice sheet retreat following the LGM.
Plain Language Summary
During the Last Glacial Maximum (∼21,000 years ago), ice sheets covered much of northern North America, Fennoscandia, and the Patagonian Andes. Using climate model simulations, we find that much of the precipitation that fell on the margins of these ice sheets came from transient, narrow corridors of atmospheric moisture known as atmospheric rivers. The atmospheric rivers were important in driving ice sheet accumulation during cold seasons, and ice sheet melt during warm seasons. The results suggest that atmospheric rivers may have had a role in driving the movement of ice sheets during Earth's past.
Key Points
Atmospheric rivers were less frequent and supplied less precipitation globally during the Last Glacial Maximum (LGM)
Over land, atmospheric river precipitation peaked along the margins of the extratropical ice sheets
Atmospheric rivers were important contributors to the surface mass balance of LGM ice sheets
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 ice sheet (LIS) and Cordilleran ice sheet separate (∼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 with a weak and meridionally tilted jet stream and storm track.
Key Points
The North Atlantic jet axis tilt is controlled by the size and spatial extent of the Laurentide ice sheet
The jet abruptly changes from a zonal to a meridional configuration in response to the North American ice sheet retreat
There were only two main climatological atmospheric circulation regimes in the North Atlantic sector over the last 21,000 years
•We present simulations of Titan’s middle and lower atmosphere with the Titan Atmospheric Model (TAM) GCM.•Vertical and latitudinal temperature profiles from the surface through the stratopause are ...reproduced.•Superrotation develops naturally in the model.•Comparison to observations indicates the prevalence of dry conditions at low latitudes.•Polar and equatorial precipitation are consistent with observed clouds, but mid-latitude cloudiness remains a puzzle.
Simulation results are presented from a new general circulation model (GCM) of Titan, the Titan Atmospheric Model (TAM), which couples the Flexible Modeling System (FMS) spectral dynamical core to a suite of external/sub-grid-scale physics. These include a new non-gray radiative transfer module that takes advantage of recent data from Cassini–Huygens, large-scale condensation and quasi-equilibrium moist convection schemes, a surface model with “bucket” hydrology, and boundary layer turbulent diffusion. The model produces a realistic temperature structure from the surface to the lower mesosphere, including a stratopause, as well as satisfactory superrotation. The latter is shown to depend on the dynamical core’s ability to build up angular momentum from surface torques. Simulated latitudinal temperature contrasts are adequate, compared to observations, and polar temperature anomalies agree with observations. In the lower atmosphere, the insolation distribution is shown to strongly impact turbulent fluxes, and surface heating is maximum at mid-latitudes. Surface liquids are unstable at mid- and low-latitudes, and quickly migrate poleward. The simulated humidity profile and distribution of surface temperatures, compared to observations, corroborate the prevalence of dry conditions at low latitudes. Polar cloud activity is well represented, though the observed mid-latitude clouds remain somewhat puzzling, and some formation alternatives are suggested.
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.
Key Points
Atmospheric rivers at the Last Glacial Maximum were shifted southeast and increased southwesterly moisture transport to southwestern North America
Model‐data comparisons show higher model skills in models that simulate more southeastward changes near the North American coast
Magnitude of precipitation differences due to atmospheric rivers are consistent with reconstructed regional patterns
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.
Plain Language Summary
Saturn's moon Titan hosts a methane hydrologic cycle with occasional large cloud events that are sometimes stationary and last for up to 30 days at a time. These events have previously been speculated to be caused by convective thunderstorms, but for the first time, we show that their formation is reliant on the interaction between a particular type of high‐latitude atmospheric wave, a Rossby wave, and instability caused by increased surface heating during the summer. Convectively forced growth of the Rossby wave accounts for the lack of movement as waves in the summer hemisphere interact with waves that have been forced in the winter hemisphere. The resulting global impact from the forced convection may be detected both from Earth as changes in outgoing longwave radiation and on the surface, which may have relevance for the Dragonfly mission.
Key Points
Development of convection on Titan involves the coordinated interaction of Rossby waves with convective instability
Convection temporarily halts the movement of waves, which may explain stationary cloud features in the middle and high latitudes
Global impacts imply that observables like surface pressure and outgoing longwave radiation may reveal instances of convection
Abstract Using an idealized climate model incorporating seasonal forcing, we investigate the impact of rotation rate on the abundance of clouds on an Earth-like aquaplanet, and the resulting impacts ...upon albedo and seasonality. We show that the cloud distribution varies significantly with season, depending strongly on the rotation rate, and is well explained by the large-scale circulation and atmospheric state. Planetary albedo displays nonmonotonic behavior with rotation rate, peaking at around 1/2Ω E . Clouds reduce the surface temperature and total precipitation relative to simulations without clouds at all rotation rates, and reduce the dependence of total precipitation on rotation rate, causing nonmonotonic behavior and a local maximum around 1/8Ω E ; these effects are related to the impacts of clouds on the net atmospheric and surface radiative energy budgets. Clouds also affect the seasonality. The influence of clouds on the extent of the winter Hadley cell and the intertropical convergence zone is relatively minor at slow rotation rates (<1/8Ω E ), but becomes more pronounced at intermediate rotation rates, where clouds decrease their maximum latitudes. The timing of seasonal transitions varies with rotation rate, and the addition of clouds reduces the seasonal phase lag.
With the discovery of ever smaller and colder exoplanets, terrestrial worlds with hazy atmospheres must be increasingly considered. Our solar system's Titan is a prototypical hazy planet, whose ...atmosphere may be representative of a large number of planets in our Galaxy. As a step toward characterizing such worlds, we present simulations of exoplanets that resemble Titan but orbit three different stellar hosts: G, K, and M dwarf stars. We use general circulation and photochemistry models to explore the circulation and chemistry of these Titan-like planets under varying stellar spectra, in all cases assuming a Titan-like insolation. Due to the strong absorption of visible light by atmospheric haze, the redder radiation accompanying later stellar types produces more isothermal stratospheres, stronger meridional temperature gradients at mbar pressures, and deeper and stronger zonal winds. In all cases, the planets' atmospheres are strongly superrotating, but meridional circulation cells are weaker aloft under redder starlight. The photochemistry of hydrocarbon and nitrile species varies with stellar spectra, with variations in the FUV/NUV flux ratio playing an important role. Our results tentatively suggest that column haze production rates could be similar under all three hosts, implying that planets around many different stars could have similar characteristics to Titan's atmosphere. Lastly, we present theoretical emission spectra. Overall, our study indicates that, despite important and subtle differences, the circulation and chemistry of Titan-like exoplanets are relatively insensitive to differences in the host star. These findings may be further probed with future space-based facilities, like WFIRST, LUVOIR, HabEx, and OST.
The Climate of Titan Mitchell, Jonathan L; Lora, Juan M
Annual review of earth and planetary sciences,
01/2016, Letnik:
44, Številka:
1
Journal Article
Recenzirano
Over the past decade, the Cassini-Huygens mission to the Saturn system has revolutionized our understanding of Titan and its climate. Veiled in a thick organic haze, Titan's visible appearance belies ...an active, seasonal weather cycle operating in the lower atmosphere. Here we review the climate of Titan, as gleaned from observations and models. Titan's cold surface temperatures (∼90 K) allow methane to form clouds and precipitation analogously to Earth's hydrologic cycle. Because of Titan's slow rotation and small size, its atmospheric circulation falls into a regime resembling Earth's tropics, with weak horizontal temperature gradients. A general overview of how Titan's atmosphere responds to seasonal forcing is provided by estimating a number of climate-related timescales. Titan lacks a global ocean, but methane is cold-trapped at the poles in large seas, and models indicate that weak baroclinic storms form at the boundary of Titan's wet and dry regions. Titan's saturated troposphere is a substantial reservoir of methane, supplied by deep convection from the summer poles. A significant seasonal cycle, first revealed by observations of clouds, causes Titan's convergence zone to migrate deep into the summer hemispheres, but its connection to polar convection remains undetermined. Models suggest that downwelling of air at the winter pole communicates upper-level radiative cooling, reducing the stability of the middle troposphere and priming the atmosphere for spring and summer storms when sunlight returns to Titan's lakes. Despite great gains in our understanding of Titan, many challenges remain. The greatest mystery is how Titan is able to retain an abundance of atmospheric methane with only limited surface liquids, while methane is being irreversibly destroyed by photochemistry. A related mystery is how Titan is able to hide all the ethane that is produced in this process. Future studies will need to consider the interactions between Titan's atmosphere, surface, and subsurface in order to make further progress in understanding Titan's complex climate system.
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 (RHSBM) 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 RHSBM. Convection behavior changes from infrequent (<1 per Titan year), intense events at summer solstice that quickly encompass the entire globe at low RHSBM to near-continuous precipitation at the poles during summer at high RHSBM (85%). The intermediate regime (RHSBM=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.
•Changing parameters in a simplified convective parameterization alters the nature of convection in the Titan Atmospheric Model (TAM).•Using results from a cloud-resolving model and observations, we tune the scheme in TAM.•The resulting decadal climatology broadly matches observations of humidity and cloud activity.•Moist convection warms 1400–600 hPa, cools the surface, and strengthens the ascending branch of the solstitial Hadley cell.
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