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Abstract In spite of the mean warming trend over the last few decades and its amplification in the Arctic, some studies have found no robust decline or even a slight increase in wintertime cold air outbreaks over North America. But fossil evidence from warmer paleoclimate periods indicates that the interior of North America never dropped below freezing even in the depths of winter, which implies that the maintenance of cold air outbreaks is unlikely to continue indefinitely with future warming. To identify key mechanisms affecting cold air outbreaks and understand how and why they will change in a warmer climate, we examine the development of North American cold air outbreaks in both a preindustrial and a roughly 8×CO₂scenario using the Community Earth System Model, version 2 (CESM2). We observe a sharp drop-off in the wintertime temperature distribution at the freezing temperature, suppressing below-freezing conditions in the warmer climate and above-freezing conditions in the preindustrial case. The disappearance of Arctic sea ice and loss of the near-surface temperature inversion dramatically decrease the availability of below-freezing air in source regions. Using an air parcel trajectory analysis, we demonstrate a remarkable similarity in both the dynamics and diabatic effects acting on cold air masses in the two climate scenarios. Diabatic temperature evolution along cold air outbreak trajectories is a competition between cooling from longwave radiation and warming from boundary layer mixing. Surprisingly, while both diabatic effects strengthen in the warmer climate, the balance remains the same, with a net cooling of about −6 K over 10 days. Significance StatementWe compare a preindustrial climate scenario to a much warmer climate circa the year 2300 under high emissions to understand the physical processes that influence the coldest wintertime temperatures and how they will change with warming. We find that enhanced warming in the Arctic, and particularly over the Arctic Ocean due to the loss of wintertime sea ice, dramatically reduces the availability of cold air to be swept into North America. By tracing these cold air masses as they travel, we also find that they experience the same total amount of cooling in the much warmer climate as they did in the preindustrial climate even though many of the individual heating and cooling processes have gotten stronger. 

Abstract Coastal upwelling, driven by alongshore winds and characterized by cold sea surface temperatures and high upper-ocean nutrient content, is an important physical process sustaining some of the oceans’ most productive ecosystems. To fully understand the ocean properties in eastern boundary upwelling systems, it is important to consider the depth of the source waters being upwelled, as it affects both the SST and the transport of nutrients toward the surface. Here, we construct an upwelling source depth distribution for parcels at the surface in the upwelling zone. We do so using passive tracers forced at the domain boundary for every model depth level to quantify their contributions to the upwelled waters. We test the dependence of this distribution on the strength of the wind stress and stratification using high-resolution regional ocean simulations of an idealized coastal upwelling system. We also present an efficient method for estimating the mean upwelling source depth. Furthermore, we show that the standard deviation of the upwelling source depth distribution increases with increasing wind stress and decreases with increasing stratification. These results can be applied to better understand and predict how coastal upwelling sites and their surface properties have and will change in past and future climates. 

Abstract Glacial‐interglacial oscillations exhibit a periodicity of approximately 100 Kyr during the late Pleistocene. Insolation variations are understood to play a vital role in these ice ages, yet their exact effect is still unknown; the 100 Kyr ice ages may be explained in two different ways. They could be purely insolation‐driven, such that ice ages are a consequence of insolation variations and would not have existed without these variations. Or, ice ages may be self‐sustained oscillations, where they would have existed even without insolation variations. We develop several observable measures that are used to differentiate between the two scenarios and can help to determine which one is more likely based on the observed proxy record. We demonstrate these analyses using two representative models. First, we find that the self‐sustained model best fits the ice volume proxy record for the full 800‐Kyr time period. Next, the same model also shows a 100 Kyr peak consistent with observations, yet the insolation‐driven model exhibits a dominant 400 Kyr spectral peak inconsistent with observations. Our third measure indicates that midpoints in ice volume during terminations do not always occur during the same phase of insolation in both observations and the self‐sustained scenario, whereas they do in the insolation‐driven scenario. While some of these results suggest that the self‐sustained ice ages are more consistent with the observed record, they rely on simple representations of the two scenarios. To draw robust conclusions, a broader class of models should be tested using this method of producing observable differences. 

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate that is projected to weaken under future anthropogenic climate change. While many studies have investigated the AMOC’s response to different levels and types of forcing in climate models, relatively little attention has been paid to the AMOC’s sensitivity to the rate of forcing change, despite it also being highly uncertain in future emissions scenarios. In this study, I isolate the AMOC’s response to different rates of CO₂increase in a state-of-the-art global climate model and find that the AMOC undergoes more severe weakening under faster rates of CO₂change, even when the magnitude of CO₂change is the same. I then propose an AMOC-ocean heat transport-sea ice feedback that enhances the decline of the circulation and explains the dependence on the rate of forcing change. The AMOC’s rate-sensitive behavior leads to qualitatively different climates (including differing Arctic sea ice evolution) at the same CO₂concentration, highlighting how the rate of forcing change is itself a key driver of global climatic change.