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Arctic warming under increased CO2 peaks 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 CO2 or 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. Plain Language Summary: The Arctic is warming much faster than the rest of the planet in response to rising greenhouse gas concentrations. Because Arctic warming peaks in winter, many studies have focused on the wintertime processes amplifying Arctic warming. However, others have found that summer atmospheric heating also contributes to winter warming by melting sea ice and storing heat in the ocean until it is released to the atmosphere in winter. Here we study changes in all seasons for one source of atmospheric heating in the Arctic—atmospheric heat transport from lower latitudes. Using climate model simulations, we find that heat and moisture are transported away from the regions that warm and moisten the most in response to rising greenhouse gas concentrations. The Arctic warms more than lower latitudes in winter, which reduces heat transport to the Arctic in winter. Atmospheric moisture increases most in late summer at lower latitudes, driving increased moisture transport in late summer from lower latitudes to the Arctic. We suggest that changes in heat and moisture transport may impact Arctic warming differently due to their opposite seasonality: by producing a larger change in surface solar absorption, summer changes in moisture transport may outweigh winter changes in heat transport. Key Points: Sea‐ice loss reduces dry heat transport to the Arctic in winter; sub‐Arctic warming increases latent heat transport to the Arctic in summerIntermodel spread in Arctic warming controls intermodel spread in seasonal heat transport changesThe seasonal pattern of poleward heat transport change is well‐captured by down‐gradient diffusion of temperature and moisture anomalies [ABSTRACT FROM AUTHOR]

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 CO2 simulate 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 CO2 is 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. Plain Language Summary: Extreme summertime temperatures are a focal point for the impacts of climate change. Climate models driven by increasing CO2 emissions project increasing summertime temperature variability by the end of the 21st century. If credible, these increases imply that extreme summertime temperatures will become even more frequent than a simple shift in the contemporary probability distribution would suggest. Given the impacts of extreme temperatures on public health, food security, and the global economy, it is of great interest to understand whether the projections of increased temperature variance are credible. In this study, we use a theoretical model of the land surface to demonstrate that the large increases in summertime temperature variance projected by climate models are credible, predictable from first principles, and driven by the effects of warmer temperatures on evapotranspiration. We also find that the response of plants to increased CO2 and mean warming is important to the projections of increased temperature variability. Key Points: Summertime temperature variance over land increases with local mean temperature in contemporary global climate modelsA theoretical model captures these increases using only projected changes in temperature and relative humidity from global climate modelsUncertainties in climate sensitivity and plant processes control the spread of climate model summertime temperature variance change [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Arctic warming under increased CO2 peaks 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 CO2 or 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. Plain Language Summary: The Arctic is warming much faster than the rest of the planet in response to rising greenhouse gas concentrations. Because Arctic warming peaks in winter, many studies have focused on the wintertime processes amplifying Arctic warming. However, others have found that summer atmospheric heating also contributes to winter warming by melting sea ice and storing heat in the ocean until it is released to the atmosphere in winter. Here we study changes in all seasons for one source of atmospheric heating in the Arctic—atmospheric heat transport from lower latitudes. Using climate model simulations, we find that heat and moisture are transported away from the regions that warm and moisten the most in response to rising greenhouse gas concentrations. The Arctic warms more than lower latitudes in winter, which reduces heat transport to the Arctic in winter. Atmospheric moisture increases most in late summer at lower latitudes, driving increased moisture transport in late summer from lower latitudes to the Arctic. We suggest that changes in heat and moisture transport may impact Arctic warming differently due to their opposite seasonality: by producing a larger change in surface solar absorption, summer changes in moisture transport may outweigh winter changes in heat transport. Key Points: Sea‐ice loss reduces dry heat transport to the Arctic in winter; sub‐Arctic warming increases latent heat transport to the Arctic in summerIntermodel spread in Arctic warming controls intermodel spread in seasonal heat transport changesThe seasonal pattern of poleward heat transport change is well‐captured by down‐gradient diffusion of temperature and moisture anomalies [ABSTRACT FROM AUTHOR]