The Paris agreement was adopted to hold the global average temperature increase to well below 2 °C and pursue efforts to limit it to 1.5 °C. Here, we investigate the event-to-event hydroclimatic ...intensity, where an event is a pair of adjacent wet and dry spells, under future warming scenarios. According to a set of targeted multi-model large ensemble experiments, event-wise intensification will significantly increase globally for an additional 0.5 °C warming beyond 1.5 °C. In high latitudinal regions of the North American continent and Eurasia, this intensification is likely to involve overwhelming increases in wet spell intensity. Western and Eastern North America will likely experience more intense wet spells with negligible changes of dry spells. For the Mediterranean region, enhancement of dry spells seems to be dominating compared to the decrease in wet spell strength, and this will lead to an overall event-wise intensification. Furthermore, the extreme intensification could be 10 times stronger than the mean intensification. The high damage potential of such drastic changes between flood and drought conditions poses a major challenge to adaptation, and the findings suggest that risks could be substantially reduced by achieving a 1.5 °C target.
The Canadian Earth System Model version 5 (CanESM5) is a global
model developed to simulate historical climate change and variability, to
make centennial-scale projections of future climate, and to ...produce
initialized seasonal and decadal predictions. This paper describes the model
components and their coupling, as well as various aspects of model
development, including tuning, optimization, and a reproducibility strategy.
We also document the stability of the model using a long control simulation,
quantify the model's ability to reproduce large-scale features of the
historical climate, and evaluate the response of the model to external
forcing. CanESM5 is comprised of three-dimensional atmosphere (T63 spectral
resolution equivalent roughly to 2.8∘) and ocean (nominally 1∘) general
circulation models, a sea-ice model, a land surface scheme, and explicit
land and ocean carbon cycle models. The model features relatively coarse
resolution and high throughput, which facilitates the production of large
ensembles. CanESM5 has a notably higher equilibrium climate sensitivity
(5.6 K) than its predecessor, CanESM2 (3.7 K), which we briefly discuss, along
with simulated changes over the historical period. CanESM5 simulations
contribute to the Coupled Model Intercomparison Project phase 6 (CMIP6)
and will be employed for climate science and service applications in Canada.
The Canadian Centre for Climate Modelling and Analysis (CCCma) has developed the fourth generation of the Canadian Atmospheric Global Climate Model (CanAM4). The new model includes substantially ...modified physical parameterizations compared to its predecessor. In particular, the treatment of clouds, cloud radiative effects, and precipitation has been modified. Aerosol direct and indirect effects are calculated based on a bulk aerosol scheme. Simulation results for present-day global climate are analyzed, with a focus on cloud radiative effects and precipitation. Good overall agreement is found between climatological mean short- and longwave cloud radiative effects and observations from the Clouds and Earth's Radiant Energy System (CERES) experiment. An analysis of the responses of cloud radiative effects to variations in climate will be presented in a companion paper.
Traduit par la rédaction Le Centre canadien de la modélisation et de l'analyse climatique (CCmaC) a mis au point la quatrième génération du modèle canadien de circulation générale de l'atmosphère (CanAM4). Le nouveau modèle comprend des paramétrisations physiques passablement modifiées comparativement à son prédécesseur. En particulier, le traitement des nuages, des effets radiatifs des nuages et des précipitations a été modifié. Les effets directs et indirects des aérosols sont calculés à l'aide d'un schéma d'aérosols en bloc. Nous analysons des résultats de simulation pour le climat général du jour présent en mettant l'accent sur les effets radiatifs des nuages et les précipitations. Nous trouvons un bon accord général entre la moyenne climatologique des effets radiatifs des nuages pour les courtes et les grandes longueurs d'onde et les observations de l'expérience CERES (Clouds and Earth's Radiant Energy System). Une analyse de la réponse des effets radiatifs des nuages aux variations du climat sera présentée dans un article connexe.
There is a general lack of information about the potential effects of 1.5, 2 or more degrees of global warming on the regional climates within Africa, and most studies that address this use data from ...coarse resolution global models. Using a large ensemble of CORDEX Africa simulations, we present a pan-African overview of the effects of 1.5 and 2 °C global warming levels (GWLs) on the African climate. The CORDEX simulations, consistent with their driving global models, show a robust regional warming exceeding the mean global one over most of Africa. The highest increase in annual mean temperature is found over the subtropics and the smallest one over many coastal regions. Projected changes in annual mean precipitation have a tendency to wetter conditions in some parts of Africa (e.g. central/eastern Sahel and eastern Africa) at both GWLs, but models' agreement on the sign of change is low. In contrast to mean precipitation, there is a consistent increase in daily precipitation intensity of wet days over a large fraction of tropical Africa emerging already at 1.5 °C GWL and strengthening at 2 °C. A consistent difference between 2 °C and 1.5 °C warmings is also found for projected changes in annual mean temperature and daily precipitation intensity. Our study indicates that a 0.5 °C further warming (from 1.5 °C-2 °C) can indeed produce a robust change in some aspects of the African climate and its extremes.
The Canadian Seasonal to Interannual Prediction System (CanSIPS) became operational at Environment Canada's Canadian Meteorological Centre (CMC) in December 2011, replacing CMC's previous two-tier ...system. CanSIPS is a two-model forecasting system that combines ensemble forecasts from the Canadian Centre for Climate Modeling and Analysis (CCCma) Coupled Climate Model, versions 3 and 4 (CanCM3 and CanCM4, respectively). Mean climate as well as climate trends and variability in these models are evaluated in freely running historical simulations. Initial conditions for CanSIPS forecasts are obtained from an ensemble of coupled assimilation runs. These runs assimilate gridded atmospheric analyses by means of a procedure that resembles the incremental analysis update technique, but introduces only a fraction of the analysis increment in order that differences between ensemble members reflect the magnitude of observational uncertainties. The land surface is initialized through its response to the assimilative meteorology, whereas sea ice concentration and sea surface temperature are relaxed toward gridded observational values. The subsurface ocean is initialized through surface forcing provided by the assimilation run, together with an offline variational assimilation of gridded observational temperatures followed by an adjustment of the salinity field to preserve static stability. The performance of CanSIPS historical forecasts initialized every month over the period 1981-2010 is documented in a companion paper. The CanCM4 model and the initialization procedures developed for CanSIPS have been employed as well for decadal forecasts, including those contributing to phase 5 of the Coupled Model Intercomparison Project.
Employing a comprehensive atmospheric general circulation model, the authors have shown in a previous study that the time-mean Northern Hemisphere (NH) winter circulation response to a CO₂ doubling ...perturbation depends significantly on parameterized orographic gravity wave drag (OGWD) parameter settings, which are essentially related to the strength of OGWD. A possible implication is that aspects of the greenhouse gas–induced circulation response could depend directly on the formulation and internal parameters settings of the OGWD scheme. Such a result would further heighten the importance of OGWD parameterizations for climate studies and have far-reaching implications for modeled projections of future climate change.
In this study the causal relationship between OGWD and changes in time-mean NH wintertime circulation response to CO₂ doubling is investigated. This is accomplished by introducing a methodology that allows one to hold the OGWD forcing fixed to its 1 × CO₂ value when CO₂ is doubled. Employing this methodology for perturbation experiments with different strengths of OGWD, the authors find that the changes in OGWD forcing due to CO₂ doubling have essentially no impact on the time-mean zonal-mean zonal wind response. The primary conclusion is that the OGWD influence is limited to its impact on the 1 × CO₂ basic-state climatology, which defines the propagation characteristics of resolved waves. Different strengths of OGWD result in control basic states with different refractive properties for the resolved waves. It is shown that the action of resolved waves, as well as their sensitivity to such differences in the control climatology, explains essentially all of the NH wintertime circulation sensitivity identified here and in a previous study. Implications for climate change projections and climate-model development are discussed.
This study investigates the impacts of global warming of 1.5 °C and 2.0 °C above pre-industrial conditions (Paris Agreement target temperatures) on the South Asian and East Asian monsoon rainfall ...using five atmospheric global climate models participating in the 'Half a degree Additional warming, Prognosis and Projected Impacts' (HAPPI) project. Mean and extreme precipitation is projected to increase under warming over the two monsoon regions, more strongly in the 2.0 °C warmer world. Moisture budget analysis shows that increases in evaporation and atmospheric moisture lead to the additional increases in mean precipitation with good inter-model agreement. Analysis of daily precipitation characteristics reveals that more-extreme precipitation will have larger increase in intensity and frequency responding to the half a degree additional warming, which is more clearly seen over the South Asian monsoon region, indicating non-linear scaling of precipitation extremes with temperature. Strong inter-model relationship between temperature and precipitation intensity further demonstrates that the increased moisture with warming (Clausius-Clapeyron relation) plays a critical role in the stronger intensification of more-extreme rainfall with warming. Results from CMIP5 coupled global climate models under a transient warming scenario confirm that half a degree additional warming would bring more frequent and stronger heavy precipitation events, exerting devastating impacts on the human and natural system over the Asian monsoon region.
Peak runoff in streams and rivers of the western United States is strongly influenced by melting of accumulated mountain snowpack. A significant decline in this resource has a direct connection to ...streamflow, with substantial economic and societal impacts. Observations and reanalyses indicate that between the 1980s and 2000s, there was a 10-20% loss in the annual maximum amount of water contained in the region's snowpack. Here we show that this loss is consistent with results from a large ensemble of climate simulations forced with natural and anthropogenic changes, but is inconsistent with simulations forced by natural changes alone. A further loss of up to 60% is projected within the next 30 years. Uncertainties in loss estimates depend on the size and the rate of response to continued anthropogenic forcing and the magnitude and phasing of internal decadal variability. The projected losses have serious implications for the hydropower, municipal and agricultural sectors in the region.
Abstract
The quasi-biennial oscillation (QBO) of tropical stratospheric zonal winds is simulated in an atmospheric general circulation model and its sensitivity to model parameters is explored. ...Vertical resolution in the lower tropical stratosphere finer than ≈1 km and sufficiently strong forcing by parameterized nonorographic gravity wave drag are both required for the model to exhibit a QBO-like oscillation. Coarser vertical resolution yields oscillations that are seasonally synchronized and driven mainly by gravity wave drag. As vertical resolution increases, wave forcing in the tropical lower stratosphere increases and seasonal synchronization is disrupted, allowing quasi-biennial periodicity to emerge. Seasonal synchronization could result from the form of wave dissipation assumed in the gravity wave parameterization, which allows downward influence by semiannual oscillation (SAO) winds, whereas dissipation of resolved waves is consistent with radiative damping and no downward influence. Parameterized wave drag is nevertheless required to generate a realistic QBO, effectively acting to amplify the relatively weaker mean-flow forcing by resolved waves.
The separate effects of ozone depleting substances (ODSs) and greenhouse gases (GHGs) on forcing circulation changes in the Southern Hemisphere extratropical troposphere are investigated using a ...version of the Canadian Middle Atmosphere Model (CMAM) that is coupled to an ocean. Circulation-related diagnostics include zonal wind, tropopause pressure, Hadley cell width, jet location, annular mode index, precipitation, wave drag, and eddy fluxes of momentum and heat. As expected, the tropospheric response to the ODS forcing occurs primarily in austral summer, with past (1960–99) and future (2000–99) trends of opposite sign, while the GHG forcing produces more seasonally uniform trends with the same sign in the past and future. In summer the ODS forcing dominates past trends in all diagnostics, while the two forcings contribute nearly equally but oppositely to future trends. The ODS forcing produces a past surface temperature response consisting of cooling over eastern Antarctica, and is the dominant driver of past summertime surface temperature changes when the model is constrained by observed sea surface temperatures. For all diagnostics, the response to the ODS and GHG forcings is additive; that is, the linear trend computed from the simulations using the combined forcings equals (within statistical uncertainty) the sum of the linear trends from the simulations using the two separate forcings. Space–time spectra of eddy fluxes and the spatial distribution of transient wave drag are examined to assess the viability of several recently proposed mechanisms for the observed poleward shift in the tropospheric jet.
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