Understanding surface temperature is important for habitability. Recent work on Mars has found that the dependence of surface temperature on elevation (surface lapse rate) converges to zero in the limit of a thin $\mathrm{CO_2}$ atmosphere. However, the mechanisms that control the surface lapse rate are still not fully understood. It remains unclear how the surface lapse rate depends on both greenhouse effect and surface pressure. Here, we use climate models to study when and why “mountaintops are cold”. We find the tropical surface lapse rate increases with the greenhouse effect and with surface pressure. The greenhouse effect dominates the surface lapse rate transition and is robust across latitudes. The pressure effect is important at low latitudes in moderately opaque ($\tau \sim 0.1$) atmospheres. A simple model provides insights into the mechanisms of the transition. Our results suggest that topographic cold-trapping may be important for the climate of arid planets.

A document by Nora Loose. Click on the document to view its contents.

Energy exchanges between large-scale ocean currents and mesoscale eddies play an important role in setting the large-scale ocean circulation but are not fully captured in models. To better understand and quantify the ocean energy cycle, we apply along-isopycnal spatial filtering to output from an isopycnal 1/32$^\circ$ primitive equation model with idealized Atlantic and Southern Ocean geometry and topography. We diagnose the energy cycle in two frameworks: (1) a non-thickness-weighted framework, resulting in a Lorenz-like energy cycle, and (2) a thickness-weighted framework, resulting in the Bleck energy cycle. This paper shows that (2) is the more useful framework for studying energy pathways when an isopycnal average is used. Next, we investigate the Bleck cycle as a function of filter scale. Baroclinic conversion generates mesoscale eddy kinetic energy over a wide range of scales, and peaks near the deformation scale at high latitudes, but below the deformation scale at low latitudes. Away from topography, an inverse cascade transfers kinetic energy from the mesoscales to larger scales. The upscale energy transfer peaks near the energy-containing scale at high latitudes, but below the deformation scale at low latitudes. Regions downstream of topography are characterized by a downscale kinetic energy transfer, in which mesoscale eddies are generated through barotropic instability. The scale- and flow-dependent energy pathways diagnosed in this paper provide a basis for evaluating and developing scale- and flow-aware mesoscale eddy parameterizations.

Long term climate change on early Mars is characterized by a shift in the spatial distribution of rivers and lakes. Geological datasets suggest earlier paleo-rivers prefer higher surface elevations compared to rivers that formed later (Kite, 2019). On the other hand, modeling work also suggest a transition of surface lapse rate that comes with atmospheric escape throughout the Martian history (Wordsworth, 2016). The surface lapse rate follows the atmospheric lapse rate, which is close to dry adiabatic, when the CO2 atmosphere is thick, but decouples when the atmosphere is thin. Figuring out the surface temperature distribution on early Mars is critical, because it tells us where the water sources from ice/snowmelt would have been during warming episodes. We use the MarsWRF GCM to explore the transition of river-forming climates. We assume the atmosphere is CO2-only, but allow additional greenhouse warming by a gray gas scheme. To simplify the relation between elevation and surface temperature, we set 0 obliquity and include simulations with both idealized topography and real topography. The range of surface pressure is between 0.01 bar and 2 bar. We use a surface energy budget framework to analyze outputs (Fig. 2). Under the framework, variations in surface emission LWs correspond to surface temperature variations. We find greenhouse heating LWa is the only term that scales with surface temperature under high PCO2, in contrast to predictions from the previous literature that sensible heat SH was the cause of the regime transition (Wordsworth, 2016). This conclusion does not change with switching to realistic topography or switching CO2 radiation to a gray gas scheme. Under the low Ps but high-optical-depth κ gray gas case, the surface lapse rate still follows the atmosphere, so the regime transition can be attributed to the evolution of greenhouse gases other than CO2. In future, we will add a surface liquid water potential algorithm to link the surface energy balance to paleo-river observations. Assuming surface liquid water is formed during transient ice-melting period, surface liquid water potential can be calculated from the Ts and Ps distributions. The output will be compared with different historical epochs to find the best-fit scenario with both CO2 and non-CO2 greenhouse forcing.

\justifying The wet-to-dry transition of Mars is recorded by a changing distribution of rivers, which was interpreted as the result of a decorrelation between surface temperature and elevation with a thinner atmosphere. Here we use a climate model to re-examine the interpretation. We find that the weakening of the greenhouse effect, rather than atmospheric pressure, accounts for the decorrelation. The decorrelation happens near surface pressure $ \sim 0.1$ bar for a pure $\mathrm{CO_2}$ atmosphere, or longwave optical depth $\sim$ 1 for a gray gas. Under the weak greenhouse limit, the surface is mainly heated by insolation and thus surface temperature decorrelates with elevation. Under the strong greenhouse limit, the surface is mainly heated by the atmosphere, whose temperature is controlled by advection and convection over the lowlands. Our results suggest that the correlation between river distribution and elevation should be re-examined with different greenhouse forcings and a low atmospheric pressure.

Energy balance regimes (e.g., Radiative Convective Equilibrium or RCE) qualitatively characterize the low, mid, and high latitudes of Earth’s modern climate. Currently we do not have a complete quantitative understanding of the spatio-temporal structure of energy balance regimes. Here we use the vertically-integrated moist static energy budget to define a nondimensional number that quantifies when and where RCE is approximately satisfied in Earth’s modern climate. We find RCE exists yearround in the tropics and also in the Northern midlatitudes during summertime. Decomposing the seasonal RCE regime transition in the midlatitudes over land and ocean shows that RCE occurs predominantly over land. We use idealized models (energy balance and aquaplanet models) to test the hypothesis that the RCE regime occurs during midlatitude summer for land-like (small heat capacity) surface conditions. Consistent with the hypothesis, an aquaplanet model configured with a shallow mixed layer depth transitions to RCE in the midlatitudes during summertime whereas it does not for a deep mixed layer depth.

We quantify the volume transport and watermass transformation rates of the global ocean circulation using the Estimating the Circulation and Climate of the Ocean version 4 release 4 (ECCOv4r4) reanalysis product. Our results support large rates of intercell exchange between the mid-depth and abyssal cells, in agreement with modern theory and observations. However, the present-day circulation in ECCO cannot be interpreted as a near-equilibrium solution. A dominant portion of the apparent diapycnal transport of watermasses within the deep ocean is not associated with irreversible watermass transformation. Instead up- and down-welling is associated with isopycnal volume changes, reflecting trends in the deep ocean density structure. Our results reveal disagreement between ECCO’s representation of the overturning circulation and associated watermass transformations, and prevailing equilibrium theories of the overturning circulation.

We examine the role of transient and standing eddies in an idealized model with a Southern Ocean analog and two basins in the Southern hemisphere. We use the new MOM6 code base in an entirely adiabatic configuration, which allows for full equilibrium integration in only 100 years of simulation time. Fine resolution eddy-resolving simulations at 1/16- and 1/8-degree resolutions are performed in cases with and without continental barriers and/or bottom roughness. A time- and zonal-Reynolds averaging approach is used to decompose eddy fluxes into transient eddies and standing meanders. For a flat bottom channel the wind driven overturning circulation is balanced by transient eddies, consistent with previous studies. However, when continental barriers or bottom roughness are present, we find that standing meanders are dominant in the Southern Ocean.

We quantify the volume transport and watermass transformation rates of the global ocean circulation using data from the Estimating the Circulation and Climate of the Ocean version 4 release 3 (ECCOv4r3) reanalysis product. Our results support large rates of intercell exchange between the mid-depth and abyssal cells, in agreement with modern theory and observations. However, the present-day circulation in ECCO cannot be interpreted as a near-equilibrium solution. Instead, a dominant portion of the apparent diapycnal transport of watermasses within the deep ocean is associated with isopycnal volume change, rather than diabatic processes, reflecting trends in the deep ocean density structure. Our results imply two possibilities: either such trends in ECCOv4r3 are unrealistic, implying that ECCO’s representation of the overturning circulation and watermass transformations are inconsistent, or the trends in ECCOv4r3 are realistic and equilibrium theories of the overturning circulation cannot be applied to the present-day ocean.

We describe an idealized primitive equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable. The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans, and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography. The model is perfectly adiabatic and spans the equator, and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally have diabatic processes and buoyancy forcing. We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest, and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence.

Climate models project that tropical warming is amplified aloft in response to increased CO$_2$. Amplification aloft is expected following moist adiabatic adjustment and the Clausius-Clapeyron relation. Here, we show that moist adiabatic adjustment overpredicts the multi-model mean temperature response at 300 hPa by 12.9–25.3\% across the model hierarchy. We show that overprediction is influenced by at least three mechanisms: large-scale circulation, direct effect of CO$_2$, and convective entrainment. Accounting for the large-scale circulation and the direct effect of CO$_2$ reduces overprediction by 5.7\% and 3.8\% respectively, but does not eliminate it. To test the influence of entrainment, we vary the Tokioka parameter in aquaplanet simulations with and without a large-scale circulation. When varying the climatological entrainment rate in the aquaplanet, overprediction varies from 6.7–17.9\%. The sensitivity of overprediction to climatological entrainment rate in the aquaplanet configured in radiative-convective equilibrium agrees well with the predictions of zero-buoyancy bulk-plume models.