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The polar-to-subtropical temperature gradient in the free troposphere is a key driver of the mid-latitude jet stream response to climate change. Climate models tend to steepen this gradient in response to large greenhouse gas increases, due to very strong subtropical upper-level warming. This strengthens the simulated jets. However, multiple lines of observational evidence point to a slowing northern jet over the satellite era, driven by enhanced Arctic free-tropospheric warming and weakening of the gradient. Here, we reconcile these seemingly contradictory results by showing that the CMIP6 ensemble successfully simulates both the observed satellite-era slowdown/weakening, and the speedup/strengthening with strong global warming. Specifically, the observed gradient weakening from 1980–1997 to 1997–2014 appears inconsistent (p

Terrestrial hydroclimate change projections in full-complexity climate models are characterized by a patchwork of regional drying and wetting responses for most variables, even as temperatures warm and evaporative demand increases. Yet, certain geologic proxies suggest that during warm climates of the deep past, the continents were very wet. Recent studies also find that climate models do not reproduce the observed enhancement of tropical precipitation by the presence of land. Motivated by this gap in understanding, we explore continental hydroclimates and their responses to global warming in an atmospheric model over several different idealized land-ocean configurations, with ocean heat transport turned on and off. We find that the presence and width of Mediterranean-like subtropical seaways play key roles in wetting the continents, and that the cooling of the tropical ocean by heat transport further wets the tropical continent(s) in a thermally direct response. Critically, configurations with drier (wetter) tropical continents tend to further dry (wet) them with global warming, and these responses are strongly concave-down nonlinear. We also briefly test the roles of land parameters, astronomical choices, and topography.

Different types of methods give very conflicting impressions about whether water will become scarcer on land as Earth warms, and in what sense(s). Here, I examine how environmental records from past climate changes can be used to clarify the interpretation of these confusing results. Evidence from the last ice age and the historical era agrees that CO2-driven warming causes a runoff response dominated by regional signals of varying sign, and a vegetation response dominated by greening. This result supports comprehensive Earth system model output, while casting doubt on the interpretation of temperature-driven indices that project widespread “drying” with warming. In contrast, evidence from pre-Quaternary warm climates points to exotic features such as wet subtropics and extremely polar-amplified warming which are not found in model simulations, suggesting unknown forcings and/or feedbacks. The terrestrial eco-hydrologic response to CO2-driven warming in the recent past is consistent with comprehensive models, and not with drought indices. However, in the deeper past, it is consistent with neither.

Significance Predicting how increasing atmospheric CO2 will affect the hydrologic cycle is of utmost importance for a wide range of applications. It is typically thought that future dryness will depend on precipitation changes, i.e., change in water supply, and changes in evaporative demand due to either increased radiation or temperature. Opposite to this viewpoint, using Earth system models, we show that changes in key water-stress variables will be strongly modified by vegetation physiological effects in response to increased [CO2] at the leaf level. These results emphasize that the continental carbon and water cycles have to be studied as an interconnected system., Predicting how increasing atmospheric CO2 will affect the hydrologic cycle is of utmost importance for a range of applications ranging from ecological services to human life and activities. A typical perspective is that hydrologic change is driven by precipitation and radiation changes due to climate change, and that the land surface will adjust. Using Earth system models with decoupled surface (vegetation physiology) and atmospheric (radiative) CO2 responses, we here show that the CO2 physiological response has a dominant role in evapotranspiration and evaporative fraction changes and has a major effect on long-term runoff compared with radiative or precipitation changes due to increased atmospheric CO2. This major effect is true for most hydrological stress variables over the largest fraction of the globe, except for soil moisture, which exhibits a more nonlinear response. This highlights the key role of vegetation in controlling future terrestrial hydrologic response and emphasizes that the carbon and water cycles are intimately coupled over land.

Past cold climates are often thought to have been drier than today on land, which appears to conflict with certain recent studies projecting widespread terrestrial drying with near-future warming. However, other work has found that, over large portions of the continents, the conclusion of future drying versus wetting strongly depends on the physical property of interest. Here, it is shown that this also holds in simulations of the Last Glacial Maximum (LGM): the continents have generally wetter topsoils and higher values of common climate wetness metrics than in the preindustrial, as well as generally lower precipitation and ubiquitously lower photosynthesis (likely driven by the low CO2), with streamflow responses falling in between. Using a large existing global pollen and plant fossil compilation, it is also confirmed that LGM grasslands and open woodlands grew at many sites of present-day forest, seasonal forests at many sites of present-day rain forest, and so forth (116–144 sites out of 302), while changes in the opposite sense were very few (9–17 sites out of 302) and spatially confined. These vegetation changes resemble the model photosynthesis responses but not the hydroclimate responses, while published lake-level changes resemble the latter but not the former. Thus, confidence in both the model hydrologic and photosynthesis projections is increased, and there is no significant conflict. Instead, paleo- and modern climate researchers must carefully define “wetting” and “drying” and, in particular, should not assume hydrologic drying on the basis of vegetation decline alone or assume vegetation stress on the basis of declines in hydroclimatic indicators.

A subtropical continent is added to two aquaplanet atmospheric general circulation models (AGCMs) to better understand the influence of land on tropical circulation and precipitation. The first model, the gray-radiation moist (GRaM) AGCM, has simplified physics, while the second model, the GFDL Atmospheric Model version 2.1 (AM2.1), is a fully comprehensive AGCM. Both models have a continent that is 60° wide in longitude from 10° to 30°N, in an otherwise slab-ocean-covered world. The precipitation response varies with cloudy- and clear-sky feedbacks and depends on continental albedo. In GRaM simulations with a continent, precipitation in the Northern Hemisphere decreases mostly as a result of decreased evaporation. In AM2.1 simulations, precipitation also shifts southward via Hadley circulation changes due to increasing albedo, but the radiative impact of clouds and moisture creates a more complex response. Results are similar when a seasonal cycle of insolation is included in AM2.1 simulations. The impact of a large, bright subtropical continent is to shift precipitation to the opposite hemisphere. In these simulations, the hemisphere of greater tropical precipitation is better predicted by the hemisphere with greater atmospheric energy input, as has been shown in previous literature, rather than the hemisphere that has higher surface temperature.

© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Global constraints on momentum and energy govern the variability of the rainfall belt in the intertropical convergence zone and the structure of the zonal mean tropical circulation. The continental-scale monsoon systems are also facets of a momentum- and energy-constrained global circulation, but their modern and palaeo variability deviates substantially from that of the intertropical convergence zone. The mechanisms underlying deviations from expectations based on the longitudinal mean budgets are neither fully understood nor simulated accurately. We argue that a framework grounded in global constraints on energy and momentum yet encompassing the complexities of monsoon dynamics is needed to identify the causes of the mismatch between theory, models and observations, and ultimately to improve regional climate projections. In a first step towards this goal, disparate regional processes must be distilled into gross measures of energy flow in and out of continents and between the surface and the tropopause, so that monsoon dynamics may be coherently diagnosed across modern and palaeo observations and across idealized and comprehensive simulations. Accounting for zonal asymmetries in the circulation, land/ocean differences in surface fluxes, and the character of convective systems, such a monsoon framework would integrate our understanding at all relevant scales: from the fine details of how moisture and energy are lifted in the updrafts of thunderclouds, up to the global circulations.

The aridity of a terrestrial climate is often quantified using the dimensionless ratio of annual precipitation (P) to annual potential evapotranspiration (PET). In this study, the climatological patterns and greenhouse warming responses of terrestrial P, Penman–Monteith PET, and are compared among 16 modern global climate models. The large-scale climatological values and implied biome types often disagree widely among models, with large systematic differences from observational estimates. In addition, the PET climatologies often differ by several tens of percent when computed using monthly versus 3-hourly inputs. With greenhouse warming, land P does not systematically increase or decrease, except at high latitudes. Therefore, because of moderate, ubiquitous PET increases, decreases (drying) are much more widespread than increases (wetting) in the tropics, subtropics, and midlatitudes in most models, confirming and expanding on earlier findings. The PET increases are also somewhat sensitive to the time resolution of the inputs, although not as systematically as for the PET climatologies. The changes in the balance between P and PET are also quantified using an alternative aridity index, the ratio , which has a one-to-one but nonlinear correspondence with . It is argued that the magnitudes of changes are more uniformly relevant than the magnitudes of changes, which tend to be much higher in wetter regions. The ratio and its changes are also found to be excellent statistical predictors of the land surface evaporative fraction and its changes.

For years, theory and model simulations have strongly disagreed on whether global warming will lead to scarcer or more plentiful water supplies. An elegant study now supplies the missing theoretical piece, strengthening the case that global water resources will increase in a warmer world.

Potential evapotranspiration (PET) is a supply-independent measure of the evaporative demand of a terrestrial climate—of basic importance in climatology, hydrology, and agriculture. Future increases in PET from greenhouse warming are often cited as key drivers of global trends toward drought and aridity. The present work computes recent and “business as usual” future Penman–Monteith PET fields at 3-hourly resolution in 13 modern global climate models. The percentage change in local annual-mean PET over the upcoming century is almost always positive, modally low double-digit in magnitude, usually increasing with latitude, yet quite divergent between models. These patterns are understood as follows. In every model, the global field of PET percentage change is found to be dominated by the direct, positive effects of constant-relative-humidity warming (via increasing vapor deficit and increasing Clausius–Clapeyron slope). This direct-warming term accurately scales as the PET-weighted (warm-season daytime) local warming, times 5%–6% °C−1 (related to the Clausius–Clapeyron equation), times an analytic factor ranging from about 0.25 in warm climates to 0.75 in cold climates, plus a small correction. With warming of several degrees, this product is of low double-digit magnitude, and the strong temperature dependence gives the latitude dependence. Similarly, the intermodel spread in the amount of warming gives most of the spread in this term. Additional spread in the total change comes from strong disagreement on radiation, relative humidity, and wind speed changes, which make smaller yet substantial contributions to the full PET percentage change fields.

Water availability is fundamental to societies and ecosystems, but our understanding of variations 55 in hydroclimate (includingextreme events, flooding, and decadal periods of drought) is limited because of a paucity of modern instrumental observationsthat are distributed unevenly across the globe and only span parts of the 20th and 21st centuries. Such data coverage isinsufficient for characterizing hydroclimate and its associated dynamics because of its multidecadal-to-centennial variability andhighly regionalized spatial signature. High-resolution (seasonal to decadal) hydroclimatic proxies that span all or parts of the60 Common Era (CE) and paleoclimate model simulations are therefore important tools for augmenting our understanding ofhydroclimate variability. In particular, the comparison of the two sources of information is critical for addressing theuncertainties and limitations of both, while enriching each of their interpretations. We review the principal proxy data availablefor hydroclimatic reconstructions over the CE and highlight contemporary understanding of how these proxies are interpreted ashydroclimate indicators. We also review the available last-millennium simulations from fully-coupled climate models and65 discuss several outstanding challenges associated with simulating hydroclimate variability and change over the CE. A specificreview of simulated hydroclimatic changes forced by volcanic events is provided, as well as a discussion of expectedimprovements in estimated forcings, models and their implementation in the future. Our review of hydroclimatic proxies andlast-millennium model simulations is used as the basis for articulating a variety of considerations and best practices for how toperform proxy-model comparisons of CE hydroclimate. This discussion provides a framework for how best to evaluate70 hydroclimate variability and its associated dynamics using these comparisons, as well as how they can better informinterpretations of both proxy data and model simulations. We subsequently explore means of using proxy-model comparisonsto better constrain and characterize future hydroclimate risks. This is explored specifically in the context of several examplesthat demonstrate how proxy-model comparisons can be used to quantitatively constrain future hydroclimatic risks as estimatedfrom climate model projections. Fil: Smerdon, Jason E.. Columbia University; Estados Unidos Fil: Luterbacher, Jürg. University Of Giessen; Alemania Fil: Phipps, Steven J.. University of Tasmania; Australia Fil: Anchukaitis, Kevin J.. University of Arizona; Estados Unidos Fil: Ault, Toby. Cornell University; Estados Unidos Fil: Coats, Sloan. State University of Colorado at Boulder; Estados Unidos Fil: Cobb, Kim M.. Instituto Tecnológico de Georgia; Estados Unidos Fil: Cook, Benjamin I.. Nasa Goddard Institute For Space Studies; Estados Unidos Fil: Colose, Chris. Nasa Goddard Institute For Space Studies; Estados Unidos Fil: Felis, Thomas. Universitat Bremen; Alemania Fil: Gallant, Ailie. Monash University; Australia Fil: Jungclaus, Johann H.. Max-Planck-Institute for Meteorology; Alemania Fil: Konecky, Bronwen. State University of Colorado at Boulder; Estados Unidos Fil: LeGrande, Allegra. NASA Goddard Institute for Space Studie; Estados Unidos Fil: Lewis, Sophie. The Australian National University; Australia Fil: Lopatka, Alex S.. University of Maryland; Estados Unidos Fil: Man, Wenmin. Chinese Academy of Sciences; República de China Fil: Mankin, Justin S.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos. NASA Goddard Institute for Space Studies; Estados Unidos Fil: Maxwell, Justin T.. Indiana University; Estados Unidos Fil: Otto-Bliesner, Bette L.. National Center for Atmospheric Research; Estados Unidos Fil: Partin, Judson W.. University of Texas at Austin; Estados Unidos Fil: Singh, Deepti. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Stevenson, Samantha. National Center for Atmospheric Research; Estados Unidos Fil: Tierney, Jessica E.. Arizona State University; Estados Unidos Fil: Zanchettin, Davide. Ca’ Foscari University of Venice; Italia Fil: Zhang, Huan. Justus Liebig University; Alemania Fil: Atwood, Alyssa R.. Georgia Institute of Technology; Estados Unidos. Brown University; Estados Unidos Fil: Andreu Hayles, Laia. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Baek, Seung H.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Buckley, Brendan. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Cook, Edward. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: D'Arrigo, Rosanne. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Dee, Sylvia G.. Brown University; Estados Unidos Fil: Griffiths, Michael L.. William Paterson University; Estados Unidos Fil: Kulkarni, Charuta. City University of New York; Estados Unidos Fil: Kushnir, Yochanan. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Lehner, Flavio. National Center for Atmospheric Research; Estados Unidos Fil: Leland, Caroline. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Linderholm, Hans W.. University of Gothenburg; Suecia Fil: Okazaki, Atsushi. University of Tokyo; Japón Fil: Palmer, Jonathan. University of New South Wales; Australia Fil: Piovano, Eduardo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina Fil: Raible, Christoph C.. University of Bern; Suiza Fil: Rao, Mukund P.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Scheff, Jacob. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Schmidt, Gavin A.. NASA Goddard Institute for Space Studie; Estados Unidos Fil: Seager, Richard. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Widmann, Martin. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Park Williams, A.. Lamont-Doherty Earth Observatory of Columbia University; Estados Unidos Fil: Xoplaki, Elena. Justus Liebig University; Alemania

International audience; This paper introduces the Tropical Rain belts with an Annual cycle and a Continent Model Inter-comparison Project (TRACMIP). TRACMIP studies the dynamics of tropical rain belts and their response to past and future radiative forcings through simulations with 13 comprehensive and one simplified atmosphere models coupled to a slab ocean and driven by seasonally varying insolation. Five idealized experiments , two with an aquaplanet setup and three with a setup with an idealized tropical continent, fill the space between prescribed-SST aquaplanet simulations and realistic simulations provided by CMIP5/6. The simulations reproduce key features of present-day climate and expected future climate change, including an annual-mean intertropical convergence zone (ITCZ) that is located north of the equator and Hadley cells and eddy-driven jets that are similar to present-day climate. Quadrupling CO2 leads to a northward ITCZ shift and preferential warming in Northern high latitudes. The simulations show interesting CO2-induced changes in the seasonal excursion of the ITCZ and indicate a possible state dependence of climate sensitivity. The inclusion of an idealized continent modulates both the control climate and the response to increased CO2 ; for example, it reduces the northward ITCZ shift associated with warming and, in some models , climate sensitivity. In response to eccentricity-driven seasonal insolation changes, seasonal changes in oceanic rainfall are best characterized as a meridional dipole, while seasonal continental rainfall changes tend to be symmetric about the equator. This survey illustrates TRACMIP's potential to engender a deeper understanding of global and regional climate and to address questions on past and future climate change.

Declines in subtropical precipitation are a robust response to modeled twenty-first-century global warming. Two suggested mechanisms are the “dry-get-drier” intensification of existing subtropical dry zones due to the thermodynamic increase in vapor transport and the poleward expansion of these same dry zones due to poleward shifts in the modeled general circulation. Here, subtropical drying in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report multimodel archive is compared to each of these two mechanisms. Each model’s particular, biased, and seasonally and zonally varying mean state is considered relative to the location of that model’s predicted changes, and these relationships are recorded in a common framework that can be compared across models. The models have a strong tendency to reduce precipitation along the subtropical flanks of their existing midlatitude cyclonic precipitation belts. This broad result agrees with the poleward expansion mechanism and with a poleward storm-track shift in particular. In contrast, the models have no clear tendency to reduce precipitation in the central nor equatorward portions of their subtropical dry zones, implying that the thermodynamic mechanism is broadly unimportant for the precipitation reductions. This is unlike the response of precipitation minus evaporation, which robustly declines in large portions of these regions, especially over the oceans. The models also tend to increase precipitation in their wet deep tropical areas, but this is not as robust as the above reduction in the subtropical midlatitudes. High-latitude precipitation increases are the most robust precipitation changes of all in this framework.

The original version of this article, unfortunately, contained an error. In the recently published paper, “Drought Indices, Drought Impacts, CO2, and Warming: a Historical and Geologic Perspective,” there are erroneous inconsistencies in the referencing for Table 1.

[1] Robust subtropical precipitation declines have been a prominent feature of general circulation model (GCM) responses to future greenhouse warming. Recent work by the authors showed that for the models making up the Coupled Model Intercomparison Project phase 3 (CMIP3), this drying was found mainly in the midlatitude-driven precipitation poleward of the model subtropical precipitation minima. Here, using more comprehensive diagnostics, we extend that work to 36 new CMIP5 models, and find that CMIP5 robust precipitation declines are also found mainly between subtropical minima and midlatitude precipitation maxima, implicating dynamic poleward expansion of dry zones rather than thermodynamic amplification of dry-wet contrasts. We also give the full seasonal cycle of these projected declines, showing that they are much more widespread in local spring than in local fall, and that for most of the year in the Northern Hemisphere they are entirely confined to the Atlantic side of the globe.