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Arctic warming under increased CO₂peaks in winter, but is influenced by summer forcing via seasonal ocean heat storage. Yet changes in atmospheric heat transport into the Arctic have mainly been investigated in the annual mean or winter, with limited focus on other seasons. We investigate the full seasonal cycle of poleward heat transport modeled with increased CO₂or with individually applied Arctic sea‐ice loss and global sea‐surface warming. We find that a winter reduction in dry heat transport is driven by Arctic sea‐ice loss and warming, while a summer increase in moist heat transport is driven by sub‐Arctic warming and moistening. Intermodel spread in Arctic warming controls spread in seasonal poleward heat transport. These seasonal changes and their intermodel spread are well‐captured by down‐gradient diffusive heat transport. While changes in moist and dry heat transport compensate in the annual‐mean, their opposite seasonality may support non‐compensating effects on Arctic warming.

The impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, version 6 (WACCM6). A suite of experiments explores the roles of both resolved orography and the parameterized effects of unresolved orographic drag (hereafterparameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extratropical tropospheric and stratospheric zonal mean zonal windby up to 80%; this is substantially greater than previous estimates. Ultimately, parameterized orography accounts for 60%–80% of this reduction; however, away from the surface most of the forcing ofby parameterized orography is accomplished byresolvedplanetary waves. We propose that a catalytic wave–mean-flow positive feedback in the stratosphere makes the stratospheric flow particularly sensitive to parameterized orography. Orography and land–sea contrast contribute approximately equally to the strength of the midlatitude stationary waves in the free troposphere, although orography is the dominant cause of the strength of the Siberian high and Aleutian low at the surface and of the position of the Icelandic low. We argue that precisely quantifying the role of orography on the observed stationary waves is an almost intractable problem, and in particular should not be approached with linear stationary wave models in whichis prescribed. We show that orography has less impact on stationary waves, and therefore on, on a backward-rotating Earth. Last, we show that atmospheric meridional heat transport shows remarkable constancy across our simulations, despite vastly different climates and stationary wave strengths.

The increasing frequency of very high temperatures driven by global warming has motivated growing interest in how the probability distribution of summertime temperatures will evolve in the future. Climate models forced by increasing CO₂simulate increasing monthly‐averaged temperature variance across the midlatitudes. In this study we present evidence that these projections are credible and driven primarily by the magnitude of local warming. A first‐principles analytic theory reproduces the increased midlatitude summertime temperature variance in climate models extremely well by considering only the warming‐induced change in the climatological vapor pressure deficit. The impacts of local warming on saturation specific and relative humidity are shown to have roughly equal contributions to increases in summertime temperature variance. The vegetation response to increasing CO₂is found to be an important contributor to the uncertainty in modeled temperature variance change, highlighting the role of plants in shaping the summertime temperature distribution.

We examine the evidence for large‐scale tropical hydroclimate changes over the Common Era based on a compilation of 67 tropical hydroclimate records from 55 sites and assess the consistency between the reconstructed hydroclimate changes and those simulated by transient model simulations of the last millennium. Our synthesis of the proxy records reveals several regionally coherent patterns on centennial time scales. From 800 to 1000 CE, records from the eastern Pacific and parts of Mesoamerica indicate a pronounced drying event relative to background conditions of the Common Era. In addition, 1400–1700 CE is marked by pronounced hydroclimate changes across the tropics, including dry and/or isotopically enriched conditions in South and East Asia, wet and/or isotopically depleted conditions in the central Andes and southern Amazon in South America, and fresher and/or isotopically depleted conditions in the Maritime Continent. We find notable dissimilarities between the regional hydroclimate changes and global‐scale and hemispheric‐scale temperature reconstructions, indicating that more work needs to be done to understand the mechanisms of the widespread tropical hydroclimate changes during the LIA. Apropos to previous interpretations of large‐scale reorganization of tropical Pacific climate during the LIA, we do not find support for a large‐scale southward shift of the Pacific Intertropical Convergence Zone, while evidence for a strengthened Pacific Walker Circulation and/or an equatorward contraction of the monsoonal Asian‐Australian rain belt exists from limited geographic regions but require additional paleoclimate constraints. Transient climate model simulations exhibit weak forced long‐term tropical rainfall changes over the last millennium but provide several important insights to the proxy reconstructions.

Climate models show that soil moisture and its subseasonal fluctuations have important impacts on the surface latent heat flux, thus regulating surface temperature variations. Using correlations between monthly anomalies in net absorbed radiative fluxes, precipitation, 2-m air temperature, and soil moisture in the ERA-Interim reanalysis and the HadCM3 climate model, we develop a linear diagnostic model to quantify the major effects of land–atmosphere interactions on summertime surface temperature variability. The spatial patterns in 2-m air temperature and soil moisture variance from the diagnostic model are consistent with those from the products from which it was derived, although the diagnostic model generally underpredicts soil moisture variance. We use the diagnostic model to quantify the impact of soil moisture, shortwave radiation, and precipitation anomalies on temperature variance in wet and dry regions. Consistent with other studies, we find that fluctuations in soil moisture amplify temperature variance in dry regions through their impact on latent heat flux, whereas in wet regions temperature variability is muted because of high mean evapotranspiration rates afforded by plentiful surface soil moisture. We demonstrate how the diagnostic model can be used to identify sources of temperature variance bias in climate models.

The lapse rate feedback is the dominant driver of stronger warming in the Arctic than the Antarctic in simulations with increased CO₂. While Antarctic surface elevation has been implicated in promoting a weaker Antarctic lapse rate feedback, the mechanisms in which elevation impacts the lapse rate feedback are still unclear. Here we suggest that weaker Antarctic warming under CO₂forcing stems from shallower, less intense climatological inversions due to limited atmospheric heat transport above the ice sheet elevation and elevation‐induced katabatic winds. In slab ocean model experiments with flattened Antarctic topography, stronger climatological inversions support a stronger lapse rate feedback and annual mean Antarctic warming comparable to the Arctic under CO₂doubling. Unlike the Arctic, seasonality in warming over flat Antarctica is mainly driven by a negative shortwave cloud feedback, which exclusively dampens summer warming, with a smaller contribution from the winter‐enhanced lapse rate feedback.