Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F10: Statistical and Nonlinear Physics of Earth and Its ClimateFocus Recordings Available
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Sponsoring Units: GPC Chair: Mary Silber, University of Chicago Room: McCormick Place W-181A |
Tuesday, March 15, 2022 8:00AM - 8:36AM |
F10.00001: An Analytical Model for CO2 Radiative Forcing Invited Speaker: Nadir Jeevanjee CO2 radiative forcing is a central quantity in climate science and is accurately modeled by state-of-the-art radiative transfer codes. But, the combined intricacies of molecular spectroscopy as well as radiative transfer have rendered a chalkboard understanding of this quantity elusive. Here, building on the work of Gea-Banacloche and Wilson, we construct an analytical model for CO2 forcing which accounts for spectral variations in absorption and reproduces the results from comprehensive radiative transfer codes with surprising accuracy. Moreover, the analytical model allows for a fairly accurate back-of-the-envelope estimate of CO2 forcing via evaluation of the Planck function, provides insight into how and why CO2 forcing varies over the globe (strongest in the tropics and weakest at the poles), and shows how the presence of water vapor weakens CO2 forcing, particularly in the tropics. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F10.00002: Seasonality and spatial dependence of meso- and submesoscale ocean currents from satellite altimetry Albion Lawrence, Joern Callies Submesoscale flows have been seen in simulations and shipboard data to exhibit qualitative seasonal changes, consistent with submesoscale baroclinic instabilities being activated in deep winter mixed layers. To further test and understand this, we use Jason-2/OSTM satellite data to compute along-track sea surface height (SSH) spectra across the global ocean, for each calendar month. The balanced flow is modeled by a low-wavenumber plateau transitioning to a submesoscale power-law falloff. When tides are weak, we find statistically significant variation with location and season in the parameters describing balanced motion. Our model of the signal and altimeter noise produces a power-law falloff which is closer to the results of numerical models and in situ measurements as compared to prior altimetry-based studies. The power law exponent decreases in the winter and increases in the summer, and there is evidence of kinetic energy moving upwards to larger scales in the months following the month of maximum mixed layer depth. This correlates with the deepening and shoaling of the mixed layer (computed from climatology) and is consistent with shipboard ADCP data. |
Tuesday, March 15, 2022 8:48AM - 9:24AM |
F10.00003: Instabilities, thresholds and tipping points in the ice sheet system: from the marine ice sheet to the marine ice cliff instability Invited Speaker: Jeremy N Bassis Observations and theory are increasingly pointing to the possibility that ice sheets can rapidly collapse. For example, over the past two decades we have witnessed the explosive disintegration of ice shelves on the Antarctic Peninsula, collapse of portions of the Pine Island and Thwaites glaciers in West Antarctica and sustained retreat of marine terminating glaciers surrounding the Greenland Ice Sheet. Few of these events were predicted by models. Part of the difficulty in projecting ice sheet changes stems from uncertainty related to the presence of "tipping points" and thresholds in the ice sheet system. Here, we review past and present evidence that suggests our ice sheets could be edging ever closer to tipping points. We show that, despite ongoing retreat, much of the retreat we have observed so far has been driven by atmospheric and oceanic forcing and we have yet to breach the tipping point. However, continued warming will likely push portions of the ice sheets into an unstable regime where the ice sheet is subject to the "marine ice cliff instability". The marine ice cliff instability is based on the idea that the finite strength of ice places a limit on the maximum ice cliff height possible at the ice sheet calving cliff. We show that sustained collapse is possible when retreating ice shelves expose a thick calving cliff. However, viscous thinning of the ice combined with small bed rock protrusions—called pinning points— provide a stabilizing forcing that reduces retreat rates. Overall, advances in modeling the flow and fracture of ice sheets is allowing us to begin to simulate punctuated ice sheet decay providing the potential for actionable sea level projections. |
Tuesday, March 15, 2022 9:24AM - 9:36AM |
F10.00004: Oceanic Eddy-killing by Wind from Global Satellite Observations Hussein Aluie, Shikhar Rai, Matthew Hecht, Mathew Maltrud While wind is the primary driver of the oceanic general circulation, we find that it kills the ocean's most energetic motions --its mesoscale eddies-- at an average rate of 50 GW. We use satellite observations and a recent method to disentangle multi-scale processes on the sphere. A length-scale analysis of air-sea energy transfer on the entire globe had not been undertaken before, to our knowledge. In fact, we show that the temporal mean-eddy decomposition (i.e. Reynolds averaging) commonly used in oceanography fails to unravel eddy-killing. Our results present the first evidence that eddy-killing is a major seasonal sink for the oceanic eddies, peaking in winter. We find that eddy-killing removes a substantial fraction (up to 90%) of the wind power input in western boundary currents such as the Gulf Stream and Kuroshio. This process, often overlooked in analyses and models, is a major dissipation pathway for mesoscales, the ocean's most energetic scales. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F10.00005: Spectral Analysis of Oceanic Surface Temperature Variance Drivers in a High-Resolution Atmosphere-Ocean Model Avik B Mondal, Brian K Arbic, Dimitris Menemenlis, Patrice Klein, Andrea M Molod, Ehud Strobach, Hector G Torres Ocean-atmosphere coupling affects the variability of Earth’s climate and weather on a wide range of temporal and spatial scales. However, the specific interactions that contribute to this coupling and the scales on which they act are not yet fully understood. The development of high-resolution coupled atmosphere-ocean models provides useful outputs to study weather and climate variability, particularly on shorter time and spatial scales. Here, a high-resolution configuration of the coupled GEOS-MITgcm atmosphere-ocean model is used to study heat fluxes in the atmospheric and oceanic boundary layers. The model comprises a cubed-sphere configuration of GEOS with 6.9 km horizontal grid spacing to a latitude-longitude-polar cap configuration of MITgcm with nominal grid spacing of 1/24o. Hourly model outputs over western boundary current regions (Gulf Stream, Kuroshio, etc.) are analyzed over the course of one model year to capture the effects of ocean mesoscale eddies and high-velocity currents on the atmosphere-ocean boundary layer temperature variance budget. Spectra of the temperature variance budget terms are computed in order to identify the different scales at which atmosphere-ocean interactions modulate heat fluxes at the air-sea interface. Studies of these spectral terms in the ocean and atmosphere will provide insight into the drivers of variability in the atmosphere-ocean system at a range of spatial and temporal scales. |
Tuesday, March 15, 2022 9:48AM - 10:24AM |
F10.00006: Arctic Soil Patterns as Large, Exceedingly Slow Fluid Instabilities Invited Speaker: Rachel Glade Slow-moving arctic soils commonly organize into striking large-scale spatial patterns called solifluction terraces and lobes. While these features are viewed as hallmarks of freeze-thaw processes, no mechanistic explanation exists for their formation. Everyday fluids—such as paint dripping down walls—produce markedly similar fingering patterns resulting from competition between viscous and cohesive forces. We use a scaling analysis to show that soil cohesion and hydrostatic effects alone can lead to similar large-scale patterns in arctic soils. A rich dataset of high-resolution solifluction lobe spacing and morphology across Norway supports theoretical predictions. Our findings provide a quantitative explanation of a common pattern on Earth and Mars, illuminating the importance of cohesive forces in landscape dynamics. These patterns operate at length and time scales previously unrecognized, with implications toward understanding fluid–solid dynamics in particulate systems with complex rheology. Yet fundamental questions remain. If solifluction patterns are analogous to classic fluid instabilities, then why do we only see them in cold places on Earth? Annual temperature data from Norway point toward a broad climate control on solifluction lobe morphology. What are the necessary and sufficient ingredients to initiate solifluction instabilities, and how will these conditions control landscape response to climate change? To get at these questions, we are using a combination of theory, satellite imagery analysis, fieldwork, and physical experiments in a walk-in climate chamber. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F10.00007: Dryland Vegetation Pattern Formation: Modeling Possible Annihilation Under Changing Rainfall Patterns Mary Silber, Punit Gandhi, Lily Liu A beautiful example of spontaneous pattern formation appears in the distribution of vegetation in some dry-land environments. Examples from Africa, Australia and the Americas reveal that vegetation, at a community scale, can respond to aridity stress by forming into stripe-like bands, that alternate with striking regularity with bands of bare soil. These bands form on gentle slopes that lead to overland water flow during the rare storms, and are transverse to the grade. The bands harvest the water from the upslope bare regions. A typical length scale for such patterns is 100 m; they are readily surveyed by modern satellites. Ecologists have proposed that characteristics of the vegetation patterns may provide early warning signs of eco-system collapse. We develop a mathematical model that allows us to investigate the possible collapse of banded vegetation under changes in the precipitation patterns, including not only changes to mean annual precipitation, but also possible changes in seasonality, storm frequency and storm depth, which are parameters of our stochastic rainfall model. We also propose that the spacing of the vegetation bands is set by the properties of the overland water transport and its infiltration rate into the soil, both of which depend on the amount of biomass on the surface. Our modeling framework parsimoniously exploits the disparity of timescales between the rain events, typically hours, to the biomass growth over decades. It emphasizes the possible importance of storm properties for the patterns to form and remain robust. |
Tuesday, March 15, 2022 10:36AM - 10:48AM |
F10.00008: Scaling properties of wildfire frequency and intensity in the western United States Jatan Buch The total annual area burned due to wildfires in the western United States (US) has dramatically increased (~300%) over the past four decades. In the same period, anthropogenic climate change has increased the likelihood of warm temperature extremes, high aridity, and prolonged drought conditions, all of which have been shown to play a key role in increasing fire risk and intensity. While there has been some progress in the literature toward understanding equilibrium climate-fire relationships at large spatiotemporal scales, there is currently no unified framework to model the scale dependence of wildfire activity in a non-stationary climate with dynamic vegetation and human variables. In this talk, I will a) provide a brief overview of our novel machine learning (ML) approach based on hybrid Mixture Density Networks coupled by a Random Forest classifier, b) discuss our results for the scaling properties of wildfire frequency and sizes, c) and highlight ongoing work that seeks to disentangle the role of anthropogenic forcing from natural variability in future climate-fire relationships using a scale aware generative ML fire model and climate simulations. |
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