ORCID Profile
0000-0001-7509-7650
Current Organisation
The University of Edinburgh
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Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-1448
Abstract: The expectations of the public and policymakers for accurate climate projections have grown with improvements in climate models. Internal variability, however, poses an inherent limit on climate predictability and, thus, accurate future climate projections of temperature and precipitation. This challenge is further lified at a regional scale where internal variability can even dominate over forced anthropogenic climate change.& In this study, we focused on the contribution of decadal climate variability and anthropogenic forcing (greenhouse gases and aerosols) on past precipitation changes over Australia since the 1970s. Using observational data, we find that the variance explained on decadal to multi-decadal timescales is comparable to that on sub-decadal scales across Australia, underlining the importance of examining Australian trends in the context of variability. While decadal and longer precipitation trends over Australia& #8217 s east coast are dominated by internal variability, significant drying trends in the austral winter (June to August) over southwest Western Australia and wettening trends in summer (December to February) over northwest Australia are evident. We further disentangle the influence of internal variability from that of different anthropogenic forcing agents on these trends using simulations from the CESM2 Large Ensemble and idealised anthropogenic aerosol simulations from PDRMIP (Precipitation Driver Response Model Intercomparison Project). Our findings provide additional evidence for the significant role of internal variability on regional climate change and also underline the importance of a focused dialogue between scientists, policymakers and the public to ensure realistic expectations for regional future climate projections.
Publisher: IOP Publishing
Date: 04-2023
Abstract: The North Atlantic Oscillation (NAO) plays a leading role in modulating wintertime climate over the North Atlantic and the surrounding continents of Europe and North America. Here we show that the observed evolution of the NAO displays larger multi-decadal variability than that simulated by nearly all CMIP6 models. To investigate the role of the NAO as a pacemaker of multi-decadal climate variability, we analyse simulations that are constrained to follow the observed NAO. We use a particle filter data-assimilation technique that sub-selects members that follow the observed NAO among an ensemble of simulations, as well as the El Niño Southern Oscillation and Southern Annular Mode in a global climate model, without the use of nudging terms. Since the climate model also contains external forcings, these simulations can be used to compare the simulated forced response to the effect of the three assimilated modes. Concentrating on the 28 year periods of strongest observed NAO trends, we show that NAO variability leads to large multi-decadal trends in temperature and precipitation over Northern Hemisphere land as well as in sea-ice concentration. The Atlantic subpolar gyre region is particularly strongly influenced by the NAO, with links found to both concurrent atmospheric variability and to the Atlantic Meridional Overturning Circulation (AMOC). Care thus needs to be taken to account for impacts of the NAO when using sea surface temperature in this region as a proxy for AMOC strength over decadal to multi-decadal time-scales. Our results have important implications for climate analyses of the North Atlantic region and highlight the need for further work to understand the causes of multi-decadal NAO variability.
Publisher: American Meteorological Society
Date: 04-2017
Abstract: Record-breaking summer heat waves were experienced across the contiguous United States during the decade-long “Dust Bowl” drought in the 1930s. Using high-quality daily temperature observations, the Dust Bowl heat wave characteristics are assessed with metrics that describe variations in heat wave activity and intensity. Despite the sparser station coverage in the early record, there is robust evidence for the emergence of exceptional heat waves across the central Great Plains, the most extreme of which were preconditioned by anomalously dry springs. This is consistent with the entire twentieth-century record: summer heat waves over the Great Plains develop on average ~15–20 days earlier after anomalously dry springs, compared to summers following wet springs. Heat waves following dry springs are also significantly longer and hotter, indicative of the importance of land surface feedbacks in heat wave intensification. A distinctive anomalous continental-wide circulation pattern accompanied exceptional heat waves in the Great Plains, including those of the Dust Bowl decade. An anomalous broad surface pressure ridge straddling an upper-level blocking anticyclone over the western United States forced substantial subsidence and adiabatic warming over the Great Plains, and triggered anomalous southward warm advection over southern regions. This prolonged and lified the heat waves over the central United States, which in turn gradually spread westward following heat wave emergence. The results imply that exceptional heat waves are preconditioned, triggered, and strengthened across the Great Plains through a combination of spring drought, upper-level continental-wide anticyclonic flow, and warm advection from the north.
Publisher: American Geophysical Union (AGU)
Date: 20-07-2021
DOI: 10.1029/2020JD034342
Abstract: Monsoon precipitation affects nearly half of the world's population, but monsoon biases are a long‐standing problem in climate simulations. We apply dynamical nudging either globally or regionally to demonstrate the role of regional and remote circulation in generating Asian monsoon biases in an atmospheric general circulation model. Monsoon precipitation biases are substantially reduced in response to global nudging but may also be exacerbated over the warm oceanic equatorial areas because of unconstrained sub‐grid convection. Regional nudging over Asia appears to be more efficient than nudging outside Asia in reducing seasonal precipitation biases over eastern China and India. This suggests a predominant role of local circulation anomalies in generating monsoon precipitation errors in these regions. An exception is the summer precipitation bias over eastern China, which is more strongly controlled by remote circulation. Besides seasonal mean rainfall, nudging can also improve the simulated interannual and intraseasonal precipitation variability over the subtropics. This results in a better skill in reproducing the observed El Niño teleconnections to India and China and the monsoon onset date. Improved understanding of the origin of Asian monsoon biases and the contribution from regional and remote circulation advances our knowledge of the interplay between the Asian monsoon and large‐scale circulation, which can be beneficial to the simulation and interpretation of monsoon projections.
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Massimo Bollasina.