The timing of sea ice melt onset (MO) strongly influences the length of the melt season and the subsequent summer ice evolution of Arctic sea ice. However, the atmospheric mechanisms governing the pronounced interannual and regional variability in MO remain incompletely understood. Here, we investigate the atmospheric contribution to the spatiotemporal variability of MO dates across the Arctic sea ice during spring (April–June) over 1980–2017. Using passive microwave satellite-derived MO dates and ERA5 reanalysis, we identify regions of extreme positive surface energy budget (SEB) anomalies (SEB events) as markers of episodic atmospheric influence and relate their occurrence and airmass origin to locally anomalous MO timings in a pixel-based framework. Very early MO occurs preferentially in the Beaufort, Kara, and Barents Seas and is associated with strongly enhanced frequency of SEB events during the 24 days preceding melt, with peak occurrence both near the melt date and about 3 weeks earlier. Earlier melt is further characterized by larger-than-usual anomalies in downward longwave and turbulent fluxes and predominantly associated with SEB events of Pacific and Atlantic airmass origin. In contrast, very late MO is located more widespread across the eastern Arctic Ocean and coincides with a significant absence of SEB events before melt, weaker downward longwave fluxes, positive downward shortwave anomalies under clear-sky conditions, and continental airmass inflows. These results demonstrate that synoptic-scale atmospheric processes are a key driver of regional MO variability and highlight the need for improved representation of springtime atmosphere–ice interactions in climate models to properly capture the seasonal transition in the Arctic.

The winter-long merging of the African and Atlantic jets during 2009–2010 was associated with extreme winter weather across the Northern Hemisphere. Past studies have shown that merging of these two jets is linked to weaker Atlantic eddy activity and stronger tropical heating, and is strongly correlated with a negative North Atlantic Oscillation (NAO) state. Here, we examine the relationship between jet merging and extreme weather, taking care to separate out the effects of the NAO and El Niño, in order to be left with the added influence of Atlantic–African jet merging. Our analysis, which examines percentile exceedance and anomaly composites of surface temperature, surface wind, and precipitation, identifies distinct weather signatures of merged-jet winters, notably affecting the Iberian Peninsula, North Africa, the southern Mediterranean, southwest Greenland, and Northern Europe. Additionally, we analyze the relationship between merged jets and shifts in cyclone track orientation contributing to the observed extreme weather patterns over these regions. Furthermore, once we remove the NAO effect from the merged-jet surface temperature anomaly signal, we find that winter-long jet merging coincides with anomalously warm Arctic, cold Eurasian, and strong El Niño conditions. The resulting weaker high-latitude temperature gradient is consistent with a weakening of eddy activity and, alongside stronger tropical heating, is thought to ultimately lead to persistent jet merging. This is consistent with previous theoretical work suggesting that reduced midlatitude baroclinicity and stronger tropical heating result in a transition from coexisting thermally driven subtropical and eddy-driven midlatitude jets to one with a single mixed eddy–thermally driven jet. This provides further evidence that Atlantic–African jet merging constitutes a dynamical regime transition of the Atlantic jet.

Anthropogenic climate change has a disproportionate effect on the Arctic, with the Arctic warming at approximately 2–4 times the rate of the global average, a phenomenon known as Arctic amplification. The greater rate of warming in the Arctic is not only having profound local effects on ecosystems and Indigenous and other communities in the far North, but may also be causing remote effects on weather and climate at lower latitudes. While much has been learned about the climate feedbacks that drive Arctic amplification in response to increasing atmospheric carbon dioxide concentrations, there remain outstanding questions about the evolution of and interactions between climate feedbacks, the relative roles of different climate forcings and feedbacks and local versus remote processes. This focus collection includes 17 articles which contribute novel research findings on (1) the mechanisms driving Arctic amplification with new insights into the time-dependent nature of Arctic amplification and feedback interactions, (2) Arctic amplification across a wide range of CO₂ and non-CO₂ forcings and (3) new approaches to assessing the climate response of amplified Arctic warming and the role of sea ice loss.

A long-standing and pervasive problem in climate modeling is the proper representation of cloud albedo over the Southern Ocean (SO). In this study, we investigate the causes of SO cloud albedo biases using phase 6 of the Coupled Model Intercomparison Project (CMIP6) simulations and a cloud-controlling factor approach on daily time scales. Cloud albedo, computed from upwelling and downwelling shortwave radiation at the surface and top of the atmosphere, is averaged into bins of vertical velocity, surface wind, and sea surface temperature. The performance of 15 models in both atmosphere-only and ocean-coupled configurations is evaluated against Clouds and the Earth’s Radiant Energy System (CERES) satellite retrievals in combination with ERA5 reanalysis for the 2000–14 period. We find that the SO cloud biases maximize in the 508–658 oceanic band and that models tend to underestimate SO cloud reflectivity for cold conditions and weak surface winds. In turn, descent (ascent) conditions are consistently underestimated (overestimated) across models for both hemispheres in the same latitude band. With a very similar approach, we evaluate how models represent the observed cloud albedo hemispheric asymmetry over oceans, which also maximizes in the 508–658 band. The sign of the asymmetry is consistently predicted by all models, many of which also predict a similar magnitude to observations. However, this is also a consequence of compensating global biases as individually most models tend to either overpredict or underpredict cloud albedo in both hemispheres. We propose that surface temperatures less than 48C are important in explaining SO bias in cloud albedo and that they also partly explain the observed hemispheric asymmetry. Further, we hypothesize that the 48C threshold sets a hemispheric asymmetry in cloud phase.

Teegarden’s Star b, a nearby terrestrial world receiving an Earth-like instellation, is a prime candidate for next-generation observatories targeting temperate exoplanets in their habitable zones. We employ a suite of three-dimensional global climate model simulations to (1) map the inner boundary of the habitable zone of Teegarden’s Star b and (2) characterize its surface climate under the assumption of an Earth-analog atmosphere. Our simulations show that, with its most recently estimated instellation of 1481 W m⁻², Teegarden’s Star b remains below the runaway greenhouse threshold for both low (α_s = 0.07, ocean-dominated) and moderate (α_s = 0.30, land-dominated) surface albedos. However, a different estimate of 1565 W m⁻² places it beyond the runaway threshold. The result that Teegarden’s Star b is habitable under the most recent instellation measurement reinforces its status as one of the most compelling targets for future habitability and biosignature searches. Given the planet’s proximity to the runaway threshold, it would benefit from a comparative study done with other models using different parameterizations.

We develop a novel single-column model of clear-sky radiative–advective equilibrium where advective heating is internally determined by relaxing the column temperature and humidity toward fixed midlatitude profiles, consistent with an airmass transformation perspective. The model reproduces observed polar temperature and advective heating rate profiles, and also captures many of the climate-change responses found in climate models. Exploring the model’s physics, we show that the surface-based temperature inversion develops by ceding energy downward to the surface, which then radiates this energy to space; we name this the “surface radiator fin” effect. We use the model to address three outstanding questions regarding polar climate change: (i) What mechanisms control polar lapse-rate change? (ii) What determines the known compensation between changes in dry and moist energy transport? (iii) What is the most physically consistent way to decompose forcing and feedbacks at the poles? Within the model, the answers to these questions are (i) three mechanisms control the lapse-rate response to warming: weakening of the surface radiator fin, increased radiative cooling by free-tropospheric water vapor emission, and relaxation toward the external profile anomaly; all three increase the lapse rate as climate warms. (ii) Compensation between dry and moist advective heating results from a delicate balance between changes in the boundary layer and the free troposphere, with no constraints imposing precise compensation. (iii) Remote advective influence on the poles should be considered a forcing, while lapse-rate and advective heating changes jointly contribute to the temperature feedback.

While the Southern Ocean is known to have the strongest annual-mean surface winds globally, it remains unclear whether surface wind extremes are stronger there than over the Northern Hemisphere basins. We address this question by analyzing reanalysis and satellite data sets and employing feature tracking to associate cyclones with surface winds. Consistent with previous work, we find the highest annual-mean and median winds over the Southern Hemisphere. However, we find a statistically distinguishable hemispheric asymmetry in extreme surface windspeeds, with the Northern Hemisphere having stronger extremes. The stronger extremes in the Northern Hemisphere are driven primarily by extreme windspeeds occurring during winter and in proximity to cyclones (within a 1,000 km radius around objectively tracked cyclone centers). Large-scale differences between basins likely play a role in shaping hemispheric asymmetries, as the Northern Hemisphere has higher extreme windspeeds above the boundary layer (700 hPa) and higher extremes of midtropospheric Eady growth rates.

This study investigates the role of large-scale atmospheric processes in the development of cyclones causing extreme surface winds over the central North Atlantic basin (30 to 60° N, 10 to 50° W), focusing on the extended winter period (October–March) from 1950 until 2020 in the ERA5 reanalysis product. Extreme surface wind events are identified as footprints of spatio-temporally contiguous 10 m wind exceedances over the local 98th percentile. Cyclones that cause the top 1 % most intense wind footprints are identified. After excluding 16 (14 %) of cyclones that originated as tropical cyclones, further analysis is done on the remaining 99 extratropical cyclones (“top extremes”). These are compared to a set of cyclones yielding wind footprints with exceedances marginally above the 98th percentile (“moderate extremes”). Cyclones leading to top extremes are, from their time of cyclogenesis, characterised by the presence of pre-existing downstream cyclones, a strong polar jet, and positive upper-level potential vorticity anomalies to the north. All these features are absent or much weaker in the case of moderate extremes, implying that they play a key role in the explosive development of top extremes and in the generation of spatially extended wind footprints. There is also an indication of cyclonic Rossby wave breaking preceding the top extremes. Furthermore, analysis of the pressure tendency equation over the cyclones' evolution reveals that, although the leading contributions to surface pressure decrease vary from cyclone to cyclone, top extremes have on average a larger diabatic contribution than moderate extremes.

Previous studies have shown that low-level mixed-phase clouds that form during idealized moist intrusions into the Arctic can exist in either a stable (stratus) or a convective (stratocumulus) state. Here, we examine the conditions that promote a transition from the stable to the convective state through idealized simulations using a three-dimensional large-eddy simulation model coupled with a one-dimensional multilayer sea ice model. We find that the vertical distribution of the initial dew point temperature (Td) profile fundamentally influences whether a transition between the two states occurs or not. If the initial moisture content of the advected airmass decreases rapidly with height, then a turbulent transition is likely to occur and a stratocumulus cloud can form. However, the availability and properties of aerosols as well as the cloud ice content can delay or even prevent stratocumulus formation, regardless if the conditions in terms of the initial Td profile are favorable. A low cloud ice water content promotes a stably stratified cloud layer and delays the transition. Furthermore, if no cloud condensation nuclei are available at the base of the cloud when a cloud-layer instability forms, then there is no new droplet formation, the buoyancy remains low and the cloud remains as a stratus. Our results suggest that the low-level mixed-phase cloud evolution and the thermodynamic transition of an airmass during a moist intrusion into the Arctic are closely linked to the aerosol processing by the cloud, that is, a chemical transformation, and that the two processes should be considered simultaneously.

Nontransiting terrestrial planets will be accessible by upcoming observatories of which LIFE is an example. Planet b orbiting Teegarden’s Star is one of the optimal targets for such missions. We use a one-dimensional atmospheric model with real gas radiation, a multispecies pseudo-adiabatic convection-condensation scheme, and a water cloud scheme to estimate the impact of the cloud coverage on the emission spectrum of the target, as well as to assess how sensitive LIFE could be to changes in outgoing flux caused by these clouds. Though the emergent flux decreases with a higher cloud coverage, it does not decrease by more than 1 order of magnitude as the coverage increases from 0% to 90%. This allows LIFE to retain a high sensitivity to the cloud cover fraction for wavelengths longer than 7 μm. In this spectral range, with at least 1 bar of N2, LIFE is able to distinguish cloud cover fractions as small as 10% given an integration time of 24 hr, and yields much better precision with a week-long integration. An integration time of 1 week also allows the resolution of local variations in spectral flux, which can lead to easier molecule identification. This ability remains if the planet is a CO2-dominated Venus analog. Partial pressures of N2 lower than 1 bar may create a degeneracy with the cloud cover fraction. LIFE shows promising potential for detecting and characterizing atmospheres even with a high cloud coverage, and retaining a fine sensitivity to relatively small differences in cloud cover fractions.