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Abstract: At most latitudes, the seasonal cycle of zonal‐mean surface air temperature is notably asymmetric: the length of the warming season is not equal to the length of the cooling season. The asymmetry varies spatially, with the cooling season being ∼40 days shorter than the warming season in the subtropics and the warming season being ∼100 days shorter than the cooling season at the poles. Furthermore, the asymmetry differs between the Northern Hemisphere and the Southern Hemisphere. Here, we show that these observed features are broadly captured in a simple model for the evolution of temperature forced by realistic insolation. The model suggests that Earth's orbital eccentricity largely determines the hemispheric contrast, and obliquity broadly dictates the meridional structure. Clouds, atmospheric heat flux convergence, and time‐invariant effective surface heat capacity have minimal impacts on seasonal asymmetry. This simple, first‐order picture has been absent from previous discussions of the surface temperature seasonal cycle.

Abstract: The Arctic Ocean is characterized by an ice-covered layer of cold and relatively fresh water above layers of warmer and saltier water. It is estimated that enough heat is stored in these deeper layers to melt all the Arctic sea ice many times over, but they are isolated from the surface by a stable halocline. Current vertical mixing rates across the Arctic Ocean halocline are small, due in part to sea ice reducing wind–ocean momentum transfer and damping internal waves. However, recent observational studies have argued that sea ice retreat results in enhanced mixing. This could create a positive feedback whereby increased vertical mixing due to sea ice retreat causes the previously isolated subsurface heat to melt more sea ice. Here, we use an idealized climate model to investigate the impacts of such a feedback. We find that an abrupt “tipping point” can occur under global warming, with an associated hysteresis window bounded by saddle-node bifurcations. We show that the presence and magnitude of the hysteresis are sensitive to the choice of model parameters, and the hysteresis occurs for only a limited range of parameters. During the critical transition at the bifurcation point, we find that only a small percentage of the heat stored in the deep layer is released, although this is still enough to lead to substantial sea ice melt. Furthermore, no clear relationship is apparent between this change in heat storage and the level of hysteresis when the parameters are varied.

AbstractPrevious studies have used coupled climate model simulations with perturbed sea ice covers to assess the impact of future Arctic sea ice loss. The results of these studies suggest that Arctic sea ice loss will cause substantial climate impacts in the Arctic and beyond. The approaches used in these simulations can be broadly categorized into three methods: adding a ghost flux to the sea ice module, nudging, and modifying the surface albedo. Here we show that all three methods ultimately add heat to the Arctic in order to melt the sea ice, and that this artificial heating causes a spurious warming signal that is added to the warming that occurs due to sea ice loss alone. We illustrate this using an idealized climate model, which provides a preliminary rough estimate of the effect. In this model, the annual-mean warming due to sea ice loss alone can be directly calculated. We compare this with the warming that would be attributed to sea ice loss using each of the three methods in the idealized model. The results suggest that each method substantially overestimates the warming due to sea ice loss alone, overestimating the surface warming throughout the Northern Hemisphere by a factor of 1.5–2 in the idealized model. Hence, these results suggest that previous coupled climate modeling studies have overestimated the climate response to sea ice loss.

Nearly half of the freshwater flux from the Antarctic Ice Sheet into the Southern Ocean occurs in the form of large tabular icebergs that calve off the continent's ice shelves. However, because of difficulties in adequately simulating their breakup, large Antarctic icebergs to date have either not been represented in models or represented but with no breakup scheme such that they consistently survive too long and travel too far compared with observations. Here, we introduce a representation of iceberg fracturing using a breakup scheme based on the "footloose mechanism." We optimize the parameters of this breakup scheme by forcing the iceberg model with an ocean state estimate and comparing the modeled iceberg trajectories and areas with the Antarctic Iceberg Tracking Database. We show that including large icebergs and a representation of their breakup substantially affects the iceberg meltwater distribution, with implications for the circulation and stratification of the Southern Ocean.

Abstract. Ancient air trapped in ice core bubbles has been paramount to developing our understanding of past climate and atmospheric composition. Before air bubbles become isolated in ice, the atmospheric signal is altered in the firn column by transport processes such as advection and diffusion. However, the influence of low-permeability layers and barometric pumping (driven by surface pressure variability) on firn air transport is not well understood and is not readily captured in conventional one-dimensional (1-D) firn air models. Here we present a two-dimensional (2-D) trace gas advection–diffusion–dispersion model that accounts for discontinuous horizontal layers of reduced permeability. We find that layering or barometric pumping individually yields too small a reduction in gravitational settling to match observations. In contrast, when both effects are active, the model's gravitational fractionation is suppressed as observed. Layering focuses airflows in certain regions in the 2-D model, which acts to amplify the dispersive mixing resulting from barometric pumping. Hence, the representation of both factors is needed to obtain a realistic emergence of the lock-in zone. In contrast to expectations, we find that the addition of barometric pumping in the layered 2-D model does not substantially change the differential kinetic fractionation of fast- and slow-diffusing trace gases. Like 1-D models, the 2-D model substantially underestimates the amount of differential kinetic fractionation seen in actual observations, suggesting that further subgrid-scale processes may be missing in the current generation of firn air transport models. However, we find robust scaling relationships between kinetic isotope fractionation of different noble gas isotope and elemental ratios. These relationships may be used to correct for kinetic fractionation in future high-precision ice core studies and can amount to a bias of up to 0.45 °C in noble-gas-based mean ocean temperature reconstructions at WAIS Divide, Antarctica.

Abstract. Ancient air trapped in ice core bubbles has been paramount to developing our understanding of past climate and atmospheric composition. Before air bubbles become isolated in ice, the atmospheric signal is altered in the firn column by transport processes such as advection and diffusion. However, the influence of impermeable layers and barometric pumping (driven by surface pressure variability) on firn air transport is not well understood and cannot be captured in conventional 1-dimensional firn air models. Here we present a 2-dimensional (2D) trace gas advection-diffusion-dispersion model that accounts for discontinuous horizontal layers of reduced permeability. We find that layering and barometric pumping individually yield too small a reduction in gravitational settling to match observations. In contrast, a combination of both effects more strongly suppresses gravitational fractionation. Layering locally focuses airflows in the 2D model and thus amplifies the dispersive mixing resulting from barometric pumping. Hence, the representation of both factors is needed to obtain a more natural emergence of the lock-in zone. Moreover, we find that barometric pumping in the layered 2D model does not substantially change the differential kinetic fractionation of fast and slow diffusing trace gases, which is observed in nature. This suggests that further subgrid-scale physics may be missing in the current generation of firn air transport models. However, we find robust scaling relationships between kinetic isotope fractionation of different noble gas isotope and elemental ratios. These relationships may be used to correct for kinetic fractionation in future high precision ice core studies.

Although iceberg models have been used for decades, they have received far more widespread attention in recent years, due in part to efforts to explicitly represent icebergs in climate models. This calls for increased scrutiny of all aspects of typical iceberg models. An important component of iceberg models is the representation of iceberg capsizing, or rolling. Rolling occurs spontaneously when the ratio of iceberg width to height falls below a critical threshold. Here we examine previously proposed representations of this threshold, and we find that there have been crucial flaws in the representation of rolling in many modeling studies to date. We correct these errors and identify an accurate model representation of iceberg rolling. Next, we assess how iceberg rolling influences simulation results in a hierarchy of models. Rolling is found to substantially prolong the lifespan of individual icebergs and allow them to drift farther offshore. However, rolling occurs only after large icebergs have lost most of their initial volume, and it thus has a relatively small impact on the large-scale freshwater distribution in comprehensive model simulations. The results suggest that accurate representations of iceberg rolling may be of particular importance for operational forecast models of iceberg drift, as well as for regional changes in high-resolution climate model simulations.

Abstract The fate of icebergs in the polar oceans plays an important role in Earth’s climate system, yet a detailed understanding of iceberg dynamics has remained elusive. Here, the central physical processes that determine iceberg motion are investigated. This is done through the development and analysis of an idealized model of iceberg drift. The model is forced with high-resolution surface velocity and temperature data from an observational state estimate. It retains much of the most salient physics, while remaining sufficiently simple to allow insight into the details of how icebergs drift. An analytical solution of the model is derived, which highlights how iceberg drift patterns depend on iceberg size, ocean current velocity, and wind velocity. A long-standing rule of thumb for Arctic icebergs estimates their drift velocity to be 2% of the wind velocity relative to the ocean current. Here, this relationship is derived from first principles, and it is shown that the relationship holds in the limit of small icebergs or strong winds, which applies for typical Arctic icebergs. For the opposite limit of large icebergs (length > 12 km) or weak winds, which applies for typical Antarctic tabular icebergs, it is shown that this relationship is not applicable and icebergs move with the ocean current, unaffected by the wind. The latter regime is confirmed through comparisons with observed iceberg trajectories near the Antarctic Peninsula.

Uncovering universal early warning signals for critical transitions has become a coveted goal in diverse scientific disciplines, ranging from climate science to financial mathematics. There has been a flurry of recent research proposing such signals, with increasing autocorrelation and increasing variance being among the most widely discussed candidates. A number of studies have suggested that increasing autocorrelation alone may suffice to signal an impending transition, although some others have questioned this. Here we consider variance and autocorrelation in the context of sea ice loss in an idealized model of the global climate system. The model features no bifurcation, nor increased rate of retreat, as the ice disappears. Nonetheless, the autocorrelation of summer sea ice area is found to increase in a global warming scenario. The variance, by contrast, decreases. A simple physical mechanism is proposed to explain the occurrence of increasing autocorrelation but not variance when there is no approaching bifurcation. Additionally, a similar mechanism is shown to allow an increase in both indicators with no physically attainable bifurcation. This implies that relying on autocorrelation and variance as early warning signals can raise false alarms in the climate system, warning of "tipping points" that are not actually there.

Abstract Record lows in Arctic sea ice extent have been making frequent headlines in recent years. The change in albedo when sea ice is replaced by open water introduces a nonlinearity that has sparked an ongoing debate about the stability of the Arctic sea ice cover and the possibility of Arctic “tipping points.” Previous studies identified instabilities for a shrinking ice cover in two types of idealized climate models: (i) annual-mean latitudinally varying diffusive energy balance models (EBMs) and (ii) seasonally varying single-column models (SCMs). The instabilities in these low-order models stand in contrast with results from comprehensive global climate models (GCMs), which typically do not simulate any such instability. To help bridge the gap between low-order models and GCMs, an idealized model is developed that includes both latitudinal and seasonal variations. The model reduces to a standard EBM or SCM as limiting cases in the parameter space, thus reconciling the two previous lines of research. It is found that the stability of the ice cover vastly increases with the inclusion of spatial communication via meridional heat transport or a seasonal cycle in solar forcing, being most stable when both are included. If the associated parameters are set to values that correspond to the current climate, the ice retreat is reversible and there is no instability when the climate is warmed. The two parameters have to be reduced by at least a factor of 3 for instability to occur. This implies that the sea ice cover may be substantially more stable than has been suggested in previous idealized modeling studies.

Introduction: In the treatment of borderline personality disorder (BPD), there is empirical support for both dialectical behavior therapy (DBT) and schema therapy (ST); these treatments have never been compared directly. This study examines whether either of them is more effective than the other in treating patients with BPD. Methods: In this randomized, parallel-group, rater-blind clinical trial, outpatients aged between 18 and 65 years with a primary diagnosis of BPD were recruited in a tertiary outpatient treatment center (Lübeck, Germany). Participants were randomized to DBT or ST with one individual and one group session per week over 1.5 years. The primary outcome was the BPD symptom severity assessed with the mean score of the Borderline Personality Disorder Severity Index at 1-year naturalistic follow-up. Results: Between November 26, 2014, and December 14, 2018, we enrolled 164 patients (mean age = 33.7 [SD = 10.61] years). Of these, 81 (49.4%) were treated with ST and 83 (50.6%) with DBT, overall, 130 (79.3%) were female. Intention-to-treat analysis with generalized linear mixed models did not show a significant difference at 1-year naturalistic follow-up between DBT and ST for the BPDSI total score (mean difference 3.32 [95% CI: –0.58–7.22], p = 0.094, d = −24 [–0.69; 0.20]) with lower scores for DBT. Pre-to-follow-up effect sizes were large in both groups (DBT: d = 2.45 [1.88–3.02], ST: d = 1.78 [1.26–2.29]). Conclusion: Patients in both treatment groups showed substantial improvements indicating that even severely affected patients with BPD and various comorbid disorders can be treated successfully with DBT and ST. An additional non-inferiority trial is needed to show if both treatments are equally effective. The trial was retrospectively registered on the German Clinical Trials Register, DRKS00011534 without protocol changes. [ABSTRACT FROM AUTHOR]

Ice shelf calving constitutes roughly half of the total mass loss from the Antarctic ice sheet. Although much attention is paid to calving of giant tabular icebergs, these events are relatively rare. More frequent, smaller-scale calving events likely play an important role in the ice shelf frontal dynamics. Here, we investigate the role of bending stresses at the ice shelf front in driving calving on the scale 100 m – 1 km, perpendicular to the ice edge. We focus in particular on how buoyant underwater 'feet' that protrude beyond the above-water ice cliff may cause tensile stresses at the base of the ice and ultimately lead to fracture. Indirect and anecdotal observations of such feet at the Ross Ice Shelf front suggest that this process may be widespread. We consider satellite observations, together with an elastic beam model and a parameterization of frontal wave erosion to estimate the size and frequency of such calving events. Our results suggest that foot-induced mass loss at Ross Ice Shelf may cause up to 25 % of the total frontal ablation. However, stresses induced through this process are likely not sufficient to initiate crevassing but rather act to propagate existing crevasses. In addition, the relatively strong ice thickness dependence of the frontal uplift suggests an important role for internal bending moments due to temperature gradients in the ice. The highly variable environment, irregularity of pre-existing crevasse spacing, and complex rheology of the ice continue to pose challenges in better constraining the drivers behind the observed deformations and resulting calving rates. [ABSTRACT FROM AUTHOR]

Frontal ablation at tidewater glaciers, which comprises iceberg calving, submarine and subaerial melting, is a key boundary condition for numerical ice sheet models but remains difficult to measure directly in-situ. Many previous studies have quantified frontal ablation over varying spatio-temporal scales, however most use ice discharge as an approximation for frontal ablation, thereby neglecting the influence of terminus location change. Frontal ablation estimates that do account for terminus location change are spatio-temporally limited by the availability of observational data. Here, we present a processing chain to quantify frontal ablation using open-source observational data. We apply the processing chain to 49 tidewater glaciers in Greenland with reliable near-terminus bathymetry data in the BedMachine V4 dataset. Nearterminus volume change over the time period 1987 - 2020 is determined using a previously published dataset of terminus positions (TermPicks), ice thicknesses from ArcticDEM and AeroDEM, adjusted for surface elevation change over time, and bathymetry data from BedMachine v4. Assuming a vertical terminus geometry and uniform ice density, we estimate frontal ablation as the difference between mass flux towards the terminus (Mankoff et al., 2020) and mass change between consecutive observation. The frontal ablation dataset offers exciting opportunities for developing new insights into ice dynamics, including helping to improve numerical model hindcasting and projections. Lastly, we provide a processing chain that may serve as a community standard for determining frontal ablation from observational data for any tidewater glacier. [ABSTRACT FROM AUTHOR]

Phytoplankton primary production in the Arctic Ocean has been increasing over the last two decades. In 2019, a record spring bloom occurred in Fram Strait, characterized by a peak in chlorophyll that was reached weeks earlier than in other years and was larger than any previously recorded May bloom. Here, we consider the conditions that led to this event and examine drivers of spring phytoplankton blooms in Fram Strait using in situ, remote sensing, and data assimilation methods. From samples collected during the May 2019 bloom, we observe a direct relationship between sea ice meltwater in the upper water column and chlorophyll a pigment concentrations. We place the 2019 spring dynamics in context of the past 20 years, a period marked by rapid change in climatic conditions. Our findings suggest that increased advection of sea ice into the region and warmer surface temperatures led to a rise in meltwater input and stronger near‐surface stratification. Over this time period, we identify large‐scale spatial correlations in Fram Strait between increased chlorophyll a concentrations and increased freshwater flux from sea ice melt. [ABSTRACT FROM AUTHOR]