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  1. Abstract

    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.

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  2. Abstract

    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.

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  3. null (Ed.)
  4. 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.

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  5. Abstract

    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.

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