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The Aleutian Low drives decadal variability in North Pacific sea surface temperatures (SST), but its role in basin-wide Pacific SST variability is less clear owing to the difficulty of disentangling coupled atmosphere-ocean processes. We apply local atmospheric nudging to isolate the effects of an intense winter Aleutian Low using an intermediate complexity climate model. An intensified Aleutian Low produces a basin-wide SST response with a similar pattern to internally-generated Pacific Decadal Oscillation (PDO). The amplitude of the SST response in the North Pacific is comparable to PDO, but in the tropics and southern subtropics the anomalies induced by the intense Aleutian Low are a factor of 3 weaker. The tropical Pacific warming peaks in boreal spring, though anomalies persist year-round. A heat budget analysis shows the northern subtropical Pacific SST response is predominantly driven by anomalous surface heat fluxes in boreal winter, while in the equatorial Pacific the response is mainly due to meridional heat advection in boreal spring. The propagation of anomalies from the extratropics to the tropics can be explained by the seasonal footprinting mechanism, involving the wind-evaporation-SST feedback. The results show that low frequency variability and trends in the Aleutian Low could contribute to basin-wide anomalous Pacific SST, but the magnitude of the effect cannot explain the full amplitude of the PDO. This finding suggests that external forcing of the Aleutian Low is unlikely to explain observed shifts in the phase of PDO in the late 20th and early-21st centuries. [ABSTRACT FROM AUTHOR]

The 18.6-year lunar nodal cycle arises from variations in the angle of the Moon's orbital plane. Previous work has linked the nodal cycle to climate but has been limited, either by the length of observations analysed, or geographical regions considered in model simulations of the pre-industrial period. Here we examine the global effect of the lunar nodal cycle in multi-centennial climate model simulations of the pre-industrial period. We find cyclic signals in global and regional surface air temperature having amplitudes of O (0.1 K), ocean heat uptake and ocean heat content. The timing of anomalies of global surface air temperature and heat uptake are consistent with the so-called slowdown in global warming in the first decade of the 21st century, also displaying warmer than average Arctic surface temperatures at the same time. The lunar nodal cycle causes variations in mean sea level pressure exceeding 0.5 hPa in the Nordic seas region, thus affecting the North Atlantic Oscillation Index during boreal winter. Our results suggest that the contribution of the lunar nodal cycle to global temperature should be negative in the mid-2020s before becoming positive again in the early-2030s, reducing the uncertainty in time at which projected global temperature reaches 1.5C above pre-industrial levels. [ABSTRACT FROM AUTHOR]

FORTE 2.0 is an intermediate-resolution coupled atmosphere–ocean general circulation model (AOGCM) consisting of the Intermediate General Circulation Model 4 (IGCM4), a T42 spectral atmosphere with 35 σ layers, coupled to Modular Ocean Model – Array (MOMA), a 2 ∘ × 2 ∘ ocean with 15 z -layer depth levels. Sea ice is represented by a simple flux barrier. Both the atmosphere and ocean components are coded in Fortran. It is capable of producing a stable climate for long integrations without the need for flux adjustments. One flexibility afforded by the IGCM4 atmosphere is the ability to configure the atmosphere with either 35 σ layers (troposphere and stratosphere) or 20 σ layers (troposphere only). This enables experimental designs for exploring the roles of the troposphere and stratosphere, and the faster integration of the 20 σ layer configuration enables longer duration studies on modest hardware. A description of FORTE 2.0 is given, followed by the analysis of two 2000-year control integrations, one using the 35 σ configuration of IGCM4 and one using the 20 σ configuration. [ABSTRACT FROM AUTHOR]

FORTE 2.0 is an intermediate resolution coupled Ocean Atmosphere General Circulation Model (AOGCM) consisting of IGCM4, a T42 spectral atmosphere with 35 σ layers, coupled to MOMA, a 2° x 2° ocean with 15 z layer depth levels. It is capable of producing a stable climate for long integrations without the need for flux adjustments. One flexibility afforded by the IGCM4 atmosphere is the ability to configure the atmosphere with either 35 σ layers (troposphere and stratosphere) or 20 σ layers (troposphere only). This enables experimental designs for exploring the roles of the troposphere and stratosphere, and the faster integration of the 20 σ layer configuration enables longer duration studies on modest hardware. A description of FORTE 2.0 is given, followed by analysis of a 2000 year long control integration. [ABSTRACT FROM AUTHOR]

During austral summer, the majority of precipitation over the Pacific Ocean is concentrated in the South Pacific Convergence Zone (SPCZ). The surface boundary conditions required to support the diagonally (northwest-southeast) oriented SPCZ are determined through a series of experiments with an atmospheric general circulation model. Continental configuration and orography do not have a significant influence on SPCZ orientation and strength. The key necessary boundary condition is the zonally asymmetric component of the sea surface temperature (SST) distribution. This leads to a strong subtropical anticyclone over the southeast Pacific that, on its western flank, transports warm moist air from the equator into the SPCZ region. This moisture then intensifies (diagonal) bands of convection that are initiated by regions of ascent and reduced static stability ahead of the cyclonic vorticity in Rossby waves that are refracted toward the westerly duct over the equatorial Pacific. The climatological SPCZ is comprised of the superposition of these diagonal bands of convection. When the zonally asymmetric SST component is reduced or removed, the subtropical anticyclone and its associated moisture source is weakened. Despite the presence of Rossby waves, significant moist convection is no longer triggered; the SPCZ disappears. The diagonal SPCZ is robust to large changes (up to ±6 °C) in absolute SST (i.e. where the SST asymmetry is preserved). Extreme cooling (change <−6 °C) results in a weaker and more zonal SPCZ, due to decreasing atmospheric temperature, moisture content and convective available potential energy. [ABSTRACT FROM AUTHOR]

State-of-the-art climate models now include more climate processes simulated at higher spatial resolution than ever. Nevertheless, some processes, such as atmospheric chemical feedbacks, are still computationally expensive and are often ignored in climate simulations. Here we present evidence that the representation of stratospheric ozone in climate models can have a first-order impact on estimates of effective climate sensitivity. Using a comprehensive atmosphere-ocean chemistry-climate model, we find an increase in global mean surface warming of around 1 °C (20%) after 75 years when ozone is prescribed at pre-industrial levels compared with when it is allowed to evolve self-consistently in response to an abrupt 4×CO2 forcing. The difference is primarily attributed to changes in long-wave radiative feedbacks associated with circulation-driven decreases in tropical lower stratospheric ozone and related stratospheric water vapour and cirrus cloud changes. This has important implications for global model intercomparison studies in which participating models often use simplified treatments of atmospheric composition changes that are consistent with neither the specified greenhouse gas forcing scenario nor the associated atmospheric circulation feedbacks. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence causes most weather-related aircraft incidents. Commercial aircraft encounter moderate-or-greater turbulence tens of thousands of times each year worldwide, injuring probably hundreds of passengers (occasionally fatally), costing airlines tens of millions of dollars and causing structural damage to planes. Clear-air turbulence is especially difficult to avoid, because it cannot be seen by pilots or detected by satellites or on-board radar. Clear-air turbulence is linked to atmospheric jet streams, which are projected to be strengthened by anthropogenic climate change. However, the response of clear-air turbulence to projected climate change has not previously been studied. Here we show using climate model simulations that clear-air turbulence changes significantly within the transatlantic flight corridor when the concentration of carbon dioxide in the atmosphere is doubled. At cruise altitudes within 50-75° N and 10-60° W in winter, most clear-air turbulence measures show a 10-40% increase in the median strength of turbulence and a 40-170% increase in the frequency of occurrence of moderate-or-greater turbulence. Our results suggest that climate change will lead to bumpier transatlantic flights by the middle of this century. Journey times may lengthen and fuel consumption and emissions may increase. Aviation is partly responsible for changing the climate, but our findings show for the first time how climate change could affect aviation. [ABSTRACT FROM AUTHOR]

Aeolian dust modelling has improved significantly over the last ten years and many institutions now consistently model dust uplift, transport and deposition in general circulation models (GCMs). However, the representation of dust in GCMs is highly variable between modelling communities due to differences in the uplift schemes employed and the representation of the global circulation that subsequently leads to dust deflation. In this study two different uplift schemes are incorporated in the same GCM. This approach enables a clearer comparison of the dust uplift schemes themselves, without the added complexity of several different transport and deposition models. The global annual mean dust aerosol optical depths (at 550 nm) using two different dust uplift schemes were found to be 0.014 and 0.023-both lying within the estimates from the AeroCom project. However, the models also have appreciably different representations of the dust size distribution adjacent to the West African coast and very different deposition at various sites throughout the globe. The different dust uplift schemes were also capable of influencing the modelled circulation, surface air temperature, and precipitation despite the use of prescribed sea surface temperatures. This has important implications for the use of dust models in AMIP-style (Atmospheric Modelling Intercomparison Project) simulations and Earth-system modelling. [ABSTRACT FROM AUTHOR]

The article examines the regional changes in rainfall. The changes in rainfall are found to be non-monotonic under a high emissions scenario which is considered to impact biodiversity and carbon cycle. The movement of the latitudinal position of the Intertropical Convergence Zone is noted to stir low-latitude regions to experience rainfall that increases in accordance with the wet get wetter paradigm. Projections of precipitation from different global climate models are used in the study.