Double-diffusive convection, often referred to as semi-convection in astrophysics, occurs in thermally and compositionally stratified systems which are stable according to the Ledoux-criterion but unstable according to the Schwarzchild criterion. This process has been given relatively little attention so far, and its properties remain poorly constrained. In this paper, we present and analyze a set of three-dimensional simulations of this phenomenon in a Cartesian domain under the Boussinesq approximation. We find that in some cases the double-diffusive convection saturates into a state of homogeneous turbulence, but with turbulent fluxes several orders of magnitude smaller than those expected from direct overturning convection. In other cases the system rapidly and spontaneously develops closely-packed thermo-compositional layers, which later successively merge until a single layer is left. We compare the output of our simulations with an existing theory of layer formation in the oceanographic context, and find very good agreement between the model and our results. The thermal and compositional mixing rates increase significantly during layer formation, and increase even further with each merger. We find that the heat flux through the staircase is a simple function of the layer height. We conclude by proposing a new approach to studying transport by double-diffusive convection in astrophysics.

Observations indicate that the Arctic sea ice cover is rapidly retreating while the Antarctic sea ice cover is steadily expanding. State-of-the-art climate models, by contrast, typically simulate a moderate decrease in both the Arctic and Antarctic sea ice covers. However, in each hemisphere there is a small subset of model simulations that have sea ice trends similar to the observations. Based on this, a number of recent studies have suggested that the models are consistent with the observations in each hemisphere when simulated internal climate variability is taken into account. Here we examine sea ice changes during 1979-2013 in simulations from the most recent Coupled Model Intercomparison Project (CMIP5) as well as the Community Earth System Model Large Ensemble (CESM-LE), drawing on previous work that found a close relationship in climate models between global-mean surface temperature and sea ice extent. We find that all of the simulations with 1979-2013 Arctic sea ice retreat as fast as observed have considerably more global warming than observations during this time period. Using two separate methods to estimate the sea ice retreat that would occur under the observed level of global warming in each simulation in both ensembles, we find that simulated Arctic sea ice retreat as fast as observed would occur less than 1% of the time. This implies that the models are not consistent with the observations. In the Antarctic, we find that simulated sea ice expansion as fast as observed typically corresponds with too little global warming, although these results are more equivocal. We show that because of this, the simulations do not capture the observed asymmetry between Arctic and Antarctic sea ice trends. This suggests that the models may be getting the right sea ice trends for the wrong reasons in both polar regions.

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The Last Interglacial (LIG), a warmer period 130-116 ka before present, is a potential analog for future climate change. Stronger LIG summertime insolation at high northern latitudes drove Arctic land summer temperatures 4-5 • C higher than the preindustrial era. Climate model simulations have previously failed to capture these elevated temperatures, possibly because they were unable to correctly capture LIG sea-ice changes. Here, we show the latest version of the fully-coupled UK Hadley Center climate model (HadGEM3) simulates a more accurate Arctic LIG climate, including elevated temperatures. Improved model physics, including a sophisticated sea-ice melt-pond scheme, result in a complete simulated loss of Arctic 1 sea ice in summer during the LIG, which has yet to be simulated in past generations of models. This ice-free Arctic yields a compelling solution to the longstanding puzzle of what drove LIG Arctic warmth and supports a fast retreat of future Arctic summer sea ice. Both land air temperatures and sea surface temperatures in high northern latitudes were considerably warmer during the LIG (≈ 130 000-116 000 years before present) 1-5 and global sea level was likely 6-9 m higher than present 6, 7. Previous climate model simulations of the LIG, forced by appropriate greenhouse gas (GHG) and orbital changes, have failed to capture the observed high temperatures 8-11. This suggests that these models may not have accurately captured Arctic key climate processes in warmer climates. Whilst knowledge of past Arctic temperatures is robust thanks to the available observations 2, 10 , interpretation of Arctic sea ice changes during the LIG has previously been afflicted by uncertainty 8, 10, 12, 13. Water-isotope measurements from ice cores have been interpreted to suggest that, alongside the Arctic warming, there was a reduction in mean annual sea ice area 8. Microfauna in LIG marine sediments recovered from boreholes on the Beaufort Sea Shelf have been interpreted as implying a lack of perennial Arctic sea ice cover 14 , as have planktonic foraminifera recovered from some Arctic marine cores 15, 16. Similarly, ostracodes on the Lomonosov and Mendeleyev Ridges and Morris Jesup Rise have been interpreted as indicative of minimum sea ice coverage during peak LIG warmth 17. On the other hand, measurements of the recently-developed sea ice proxy IP25 (a carbon-25 highly-branched isoprenoid lipid), when combined with terrestrial and open-water phytoplankton biomarkers, have been interpreted as evidence of perennial LIG ice cover in the central

<p>The Last Interglacial (LIG) is a period of great importance as an analog for future climate change. Global sea level was 6-9 m higher than present. Stronger LIG summertime insolation at high northern latitudes drove Arctic land summer temperatures around 4-5 K higher than during the preindustrial era. Climate-model simulations have previously failed to capture these elevated temperatures. This may be because these models failed to correctly capture LIG sea ice changes.</p><p>Here, we show that the latest version of the UK Hadley Center coupled ocean-atmosphere climate model (HadGEM3) simulates a much improved Arctic LIG climate, including the observed high temperatures. Improved model physics in HadGEM3, including a sophisticated sea ice melt-pond scheme, results in the first-ever simulation of the complete loss of Arctic sea ice in summer during the LIG.</p><p>Our ice-free Arctic yields a compelling solution to the long-standing puzzle of what drove LIG Arctic warmth. The LIG simulation result is a new independent constraint on the strength of Arctic sea ice decline in climate-model projections, and provides support for a fast retreat of Arctic summer sea ice in the future.</p>

The downward trend in Arctic sea ice extent is one of the most dramatic signals of climate change during recent decades. Comprehensive climate models have struggled to reproduce this, typically simulating a slower rate of sea ice retreat than has been observed. However, this bias has been widely noted to have decreased in models participating in the most recent phase of the Coupled Model Intercomparison Project (CMIP5) compared with the previous generation of models (CMIP3). Here we examine simulations from both CMIP3 and CMIP5. We find that simulated historical sea ice trends are influenced by volcanic forcing, which was included in all of the CMIP5 models but in only about half of the CMIP3 models. The volcanic forcing causes temporary simulated cooling in the 1980s and 1990s, which contributes to raising the simulated 1979-2013 global-mean surface temperature trends to values substantially larger than observed. We show that this warming bias is accompanied by an enhanced rate of Arctic sea ice retreat and hence a simulated sea ice trend that is closer to the observed value, which is consistent with previous findings of an approximately linear relationship between sea ice extent and global-mean surface temperature. We find that both generations of climate models simulate Arctic sea ice that is substantially less sensitive to global warming than has been observed. The results imply that the much of the difference in Arctic sea ice trends between CMIP3 and CMIP5 occurred due to the inclusion of volcanic forcing, rather than improved sea ice physics or model resolution.

Rapid sea ice retreat has been extensively observed in the Canada Basin over the past several decades (F. McLaughlin et al., 2011). The increased sea ice melt and river runoff that has collected toward the center of the anticyclonic (convergent)

The seasonal halocline impacts the exchange of heat, energy, and nutrients between the surface and the deeper ocean, and it is changing in response to Arctic sea ice melt over the past several decades. Here, we assess seasonal halocline formation in 1975 and 2006-2012 by comparing daily, May to September, below-ice salinity profiles collected in the Canada Basin. We evaluate differences between the two time periods using a one-dimensional (1D) bulk model to quantify differences in freshwater input and vertical mixing. The 1D model metrics indicate that two separate factors contribute similarly to stronger stratification in 2006-2012 than in 1975: (1) larger surface freshwater input and (2) less vertical mixing of that freshwater. The first factor is mainly important in August-September, consistent with a longer melt season in recent years. The second factor is mainly important from June until mid-August, when similar levels of freshwater input in 1975 and 2006-2012 are mixed over a different depth range, resulting in different stratification. These results imply that decadal changes to ice-ocean dynamics, in addition to freshwater input, significantly contribute to the stronger seasonal stratification in 2006-2012 than in 1975. The findings highlight the need for near-surface process studies to elucidate the roles of lateral processes and ice-ocean momentum exchange on vertical mixing.