Climate dynamics

Abstract:

Recent James Webb Space Telescope observations of cool, rocky exoplanets reveal a probable lack of thick atmospheres, suggesting the prevalent escape of the “secondary” atmospheres formed after losing primordial hydrogen. Yet, simulations indicate that the hydrodynamic escape of secondary atmospheres, composed of nitrogen and carbon dioxide, requires intense fluxes of ionizing radiation (X-ray and extreme ultraviolet (XUV)) to overcome the effects of high molecular weight and efficient line cooling. This transonic outflow of hot, ionized metals (not hydrogen) presents a novel astrophysical regime ripe for exploration. We introduce an analytic framework to determine which planets retain or lose their atmospheres, positioning them on either side of the cosmic shoreline. We model the radial structure of escaping atmospheres as polytropic expansions—power-law relationships between density and temperature driven by local XUV heating. Our approach diagnoses line cooling with a three-level atom model and incorporates how ion–electron interactions reduce the mean molecular weight. Crucially, hydrodynamic escape onsets for a threshold XUV flux depend upon the atmosphere’s gravitational binding. The ensuing escape rates either scale linearly with XUV flux when weakly ionized (energy limited) or are controlled by a collisional–radiative thermostat when strongly ionized. Thus, airlessness is determined by whether the XUV flux surpasses the critical threshold during the star’s active periods, accounting for expendable primordial hydrogen and revival by volcanism. We explore atmospheric escape from the young Sun Mars and Earth, LHS 1140 b and c, and TRAPPIST-1 b. Our modeling characterizes the bottleneck of atmospheric loss on the occurrence of observable Earth-like habitats and offers analytic tools for future studies.

Abstract:

Abstract. This study uses an ensemble of climate model experiments coordinated by the Quasi-Biennial Oscillation initiative (QBOi) to analyze the Madden-Julian Oscillation (MJO) in the presence of either perpetual El Niño or La Niña sea surface temperatures during boreal winter. In addition to the prescribed El Niño Southern Oscillation (ENSO) conditions, the nine models internally generate QBOs, meaning each may influence the MJO. Objectives of our analyses are to assess the response of the MJO to strong idealized ENSO forcing and look for evidence of a QBO influence on the MJO in a multi-model context. The diagnostics used include wavenumber-frequency spectra of tropical convective and dynamical fields, measures of MJO lifetime, an evaluation of MJO diversity and visualization of MJO vertical structure, as well as an assessment of QBO morphology and the QBO's impact on tropical convection. Kelvin wave spectral power increases in the El Niño simulations whereas equatorial Rossby waves power is stronger in the La Niña simulations. All models simulate faster MJO propagation under El Niño conditions. This change in speed is corroborated by the MJO diversity analysis, which reveals that models better reproduce the observed “fast propagating” and “standing” MJO archetypes given perpetual El Niño and La Niña, respectively. Regardless of ENSO, QBO descent into the lower stratosphere is underestimated and we detect little QBO influence on tropical tropopause stability and MJO activity. With little influence from the QBO on the MJO activity in these runs, we can be confident that the aforementioned changes in the MJO indeed arise from the different ENSO boundary conditions.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>In contrast to global warming trends, much of Eurasia experienced a winter cooling trend over 1990–2014. Some studies have proposed a causal link between this regional cooling, particularly strong over Siberia, to coincident reductions in Arctic sea-ice extent. However, free-running historical climate models overwhelmingly simulate a forced Eurasian warming signal, leading other studies to suggest that internal variability explains the observed cooling. Here, we use retrospective seasonal climate predictions to highlight a robust dynamical link between Siberian cooling and upstream north-east Atlantic atmospheric circulation changes. Examining the interannual predictability of these circulation patterns, we find spuriously weak but skilful model signals. When these weak dynamical signals are corrected, stronger low-frequency variability in downstream Siberian temperature also emerges, with half of the observed 1990–2014 cooling simulated. Our results suggest that Eurasian decadal climate variability is at least partly driven by a predictable atmospheric circulation response to slowly evolving boundary conditions.</jats:p>

Abstract:

Plain Language Summary: Understanding the environments that support spatial growth of storms is crucial for predicting their impacts. This study uses a 6.5‐month‐long high‐resolution model simulation to examine the environments of simulated storm systems separated based on their rate of spatial growth. Results show that environments with a faster growth rate have greater moisture and instability. While vertical wind shear, defined as the change in wind speed and/or direction with height, is a well‐known factor in storm organization, its magnitude does not vary strongly with growth rate when shear is averaged over a large area, likely due to the large spread in values present. However, the direction of wind shear is likely an important factor in the rate of growth, and faster spatial growth is found for storms growing near the mountains when a favorable shear direction (parallel to the mountain range) is present. A low‐level jet, an area of stronger wind speeds elevated off of the surface, is found more frequently in environments with a faster growth rate and is likely an important mechanism that facilitates differences in thermodynamics and wind shear.

Abstract:

Ultrashort-period (USP) exoplanets—with Rp ≤ 2R⊕ and periods ≤1 day—are expected to be stripped of volatile atmospheres by intense host star irradiation, which is corroborated by their nominal bulk densities and previous eclipse observations, consistent with bare-rock surfaces. However, a few USP planets appear anomalously underdense relative to an Earth-like composition, suggesting an exotic interior structure (e.g., coreless) or a volatile-rich secondary atmosphere increasing their apparent radius. Here, we present the first dayside emission spectrum of the low-density (4.3 ± 0.4 g cm−3) USP planet TOI-561 b, which orbits an iron-poor, alpha-rich, ∼10 Gyr old thick-disk star. Our 3–5 μm JWST/NIRSpec observations demonstrate the dayside of TOI-561 b is inconsistent with a bare-rock surface at high statistical significance, suggesting instead a thick volatile envelope that is cooling the dayside to well below the ∼3000 K expected in the bare-rock or thin-atmosphere case. These results reject the popular hypothesis of complete atmospheric desiccation for highly irradiated exoplanets and support predictions that planetary-scale magma oceans can retain substantial reservoirs of volatiles, opening up the geophysical study of ultrahot super-Earths through the lenses of their atmospheres.