A fundamental aspect of vertical velocities in the atmosphere is their asymmetric nature, with stronger upward than downward motions. Here we study this asymmetry from a synoptic‐scale perspective, by employing a storm‐tracking algorithm in observations and climate change simulations. We show that midlatitude cyclones and anticyclones are both skewed toward stronger upward motions, and that this asymmetry intensifies in a warmer climate. Downward motions are generally influenced by changes in the dry static stability, which increases in a warmer climate, and therefore weaken. However, upward motions are influenced by a reduced static stability, which takes into account the influence of latent heating on saturated ascent. The latter decreases locally in regions of upward motions, especially for strong cyclones, leading to an intensification of the upward velocity. The projected increase in the upward velocity of strong storms has potential implications for extreme midlatitude precipitation events.

The midlatitude storm tracks are of the most prominent features of extratropical climate. Despite the theoretical expectation, based on baroclinic instability theory, that baroclinic eddies strengthen with jet intensification, there is evidence that this relation breaks when the jet is particularly strong. The most known case is the Pacific midwinter minimum in storm track activity. To isolate the effect of jet strength on storm activity, we conduct a series of GCM experiments systematically varying jet intensity. The simulations are analyzed using Lagrangian tracking to understand the response from a single-eddy perspective. The Lagrangian analysis shows that while the response of upper-level eddies is dominated by a reduction in the amount of tracked features, the lower-level eddies’ response is also affected by a reduction in their lifetime. Analyzing the jet strength effect on the pairing between the upper- and lower-level eddies, we find that the jet intensification increases the relative speed of the upper-level eddies, breaking the baroclinic wave structure and limiting its growth. We show that the Lagrangian response correlates with a shift in the midlatitude spectrum to low wavenumbers. The shift settles these results with linear baroclinic instability theory, as under the stronger jet conditions synoptic-scale eddies are predicted to have a sub-optimal growth rate. These results can potentially explain the midwinter suppression of storm activity over the Pacific and the difference from the Atlantic response.

Clouds are one of the most influential components of Earth’s climate system. Specifically, the midlatitude clouds play a vital role in shaping Earth’s albedo. This study investigates the connection between baroclinic activity, which dominates the midlatitude climate, and cloud-albedo and how it relates to Earth’s existing hemispheric albedo symmetry. We show that baroclinic activity and cloud-albedo are highly correlated. By using Lagrangian tracking of cyclones and anticyclones and analyzing their individual cloud properties at different vertical levels, we explain why their cloud-albedo increases monotonically with intensity. We find that while for anticyclones, the relation between strength and cloudiness is mostly linear, for cyclones, in which clouds are more prevalent, the relation saturates with strength. Using the cloud-albedo strength relationships and the climatology of baroclinic activity, we demonstrate that the observed hemispheric difference in cloud-albedo is well explained by the difference in the population of cyclones and anticyclones, which counter-balances the difference in clear-sky albedo. Finally, we discuss the robustness of the hemispheric albedo symmetry in the future climate. Seemingly, the symmetry should break, as the northern hemisphere’s storm track response differs from that of the southern hemisphere due to Arctic amplification. However, we show that the saturation of the cloud response to storm intensity implies that the increase in the skewness of the southern hemisphere storm distribution toward strong storms will decrease future cloud-albedo in the southern hemisphere. This complex response explains how albedo symmetry might persist even with the predicted asymmetric hemispheric change in baroclinicity under climate change.

Clouds are one of the most fascinating, important, and complex components of Earth's climate system (Siebesma et al., 2020). Despite their importance, theoretical understanding of what controls planetary-wide cloudiness is largely absent. For example, while we have a good understanding of how clouds form and interact with radiation (Cotton et al., 2014;Houze, 2014;Siebesma et al., 2020), it is difficult to use these theories to make claims about global cloudiness. Earth System Models (ESMs) and other bottom-up approaches do couple simple models of cloud formation to the global circulation. However, so far they have not been proven effective in constraining global cloudiness variability (Sherwood et al., 2020;Zelinka et al., 2020). This makes it difficult to transparently establish links between variability in global cloudiness and Earth's energy balance, or how such a link would change in a changing climate.Conceptual models could be useful in elucidating how clouds, circulation, and energy balance, are tied together. Existing theoretical work has linked cloudiness to circulation, and most examples of such work focus on particular circulation systems, like the tropical overturning circulation (Betts & Ridgway, 1989;Pierrehumbert & Swanson, 1995), or the Walker cell (Peters & Bretherton, 2005), or individual cyclones (Carlson, 1980). What is missing is a conceptual framework that both closes the top-of-atmosphere energy budget (and hence by necessity considers the planet as a whole), but also includes clouds. A suitable candidate for such a framework would be an energy balance model (Budyko, 1969;Ghil, 1981;North & Kim, 2017;Sellers, 1969) that explicitly represents dynamic cloudiness, likely as an implicit function of circulation measures or other state variables.

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The Northern and Southern Hemispheres reflect on average almost equal amounts of sunlight due to compensating hemispheric asymmetries in clear-sky and cloud albedo. Recent work indicates that the cloud albedo asymmetry is largely due to clouds in extratropical oceanic regions. Here, we investigate the proximate causes of this extratropical cloud albedo asymmetry using a cloud-controlling factor (CCF) approach. We develop a simple index that measures the skill of CCFs, either individually or in combination, in predicting the asymmetry. The index captures the contribution to the asymmetry due to interhemispheric differences in the probability distribution function of daily CCF values. Cloud albedo is quantified using daily MODIS satellite retrievals, and is related to range of CCFs derived from the ERA5 reanalysis product. We find that sea-surface temperature is the CCF that individually explains the largest fraction of the asymmetry, followed by surface wind. The asymmetry is predominantly due to low clouds, and our results are consistent with prior local-scale modelling work showing that marine boundary-layer clouds become thicker and more extensive as surface wind increases and surface temperature cools. The asymmetry is consistent with large-scale control of storm track intensity and surface winds by meridional temperature gradients: persistently cold and windy conditions in the Southern Hemisphere keep cloud albedo high year-round. Our results have important implications for global-scale cloud feedbacks and contribute to efforts to develop a theory for planetary albedo and its symmetry.

Clouds are one of the most fascinating, important, and complex components of Earth's climate system (Siebesma et al., 2020). Despite their importance, theoretical understanding of what controls planetary-wide cloudiness is largely absent. For example, while we have a good understanding of how clouds form and interact with radiation (Cotton et al., 2014;Houze, 2014;Siebesma et al., 2020), it is difficult to use these theories to make claims about global cloudiness. Earth System Models (ESMs) and other bottom-up approaches do couple simple models of cloud formation to the global circulation. However, so far they have not been proven effective in constraining global cloudiness variability (Sherwood et al., 2020;Zelinka et al., 2020). This makes it difficult to transparently establish links between variability in global cloudiness and Earth's energy balance, or how such a link would change in a changing climate.Conceptual models could be useful in elucidating how clouds, circulation, and energy balance, are tied together. Existing theoretical work has linked cloudiness to circulation, and most examples of such work focus on particular circulation systems, like the tropical overturning circulation (Betts & Ridgway, 1989;Pierrehumbert & Swanson, 1995), or the Walker cell (Peters & Bretherton, 2005), or individual cyclones (Carlson, 1980). What is missing is a conceptual framework that both closes the top-of-atmosphere energy budget (and hence by necessity considers the planet as a whole), but also includes clouds. A suitable candidate for such a framework would be an energy balance model (Budyko, 1969;Ghil, 1981;North & Kim, 2017;Sellers, 1969) that explicitly represents dynamic cloudiness, likely as an implicit function of circulation measures or other state variables.

<p>The midlatitude storm tracks are one of the most prominent features of extratropical climate. Despite the theoretical expectation, based on baroclinic instability theory that baroclinic eddy strength correlates with jet intensity, there is a decrease in storm-track activity during midwinter over the Pacific compared to the shoulder seasons. Recent studies suggest this phenomenon is a result of the general circulation effect on the storm-track through interaction with the jet-stream. To isolate the effect of jet strength, we conduct a series of GCM experiments with a systematically varied jet intensity. The simulations are analyzed using Lagrangian tracking to understand the response from a single eddy perspective. The results of the Lagrangian analysis show that while the response of upper-level eddies is dominated by a reduction in the amount of tracked features, the lower-level eddies' response is also affected by a reduction in their lifetime. Analyzing the effect of the jet strength on the pairing between the upper- and lower-level eddies, we show how the jet intensification break the baroclinic wave structure and limits its growth. Furthermore, we show that these results can be settled with linear baroclinic instability models if the eddies' spatial scale is considered. The intensification of the jet and increase in the deformation radius shift the preferred scale for growth from the synoptic-scale toward the planetary-scale, consistent with the reduction in storm activity. This mechanism potentially explains the midwinter suppression of storm activity over the Pacific and the difference from the response over the Atlantic.</p>