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In August 2017, a smoke plume from wildfires in British Columbia and the Northwest Territories recirculated and persisted over northern Canada for over two weeks. We compared a full‐factorial set of NASA Goddard Institute for Space Studies ModelE simulations of the plume to satellite retrievals of aerosol optical depth and carbon monoxide, finding that ModelE performance was dependent on the model configuration, and more so on the choice of injection height approach, aerosol scheme and biomass burning emissions estimates than to the choice of horizontal winds for nudging. In particular, ModelE simulations with free‐tropospheric smoke injection, a mass‐based aerosol scheme and comparatively high fire NOx emissions led to unrealistically high aerosol optical depth. Using paired simulations with and without fire emissions, we estimated that for 16 days over an 850,000 km2 region, the smoke decreased planetary boundary layer heights by between 253 and 547 m, decreased downward shortwave radiation by between 52 and 172 Wm−2, and decreased surface temperature by between 1.5°C and 4.9°C, the latter spanning an independent estimate from operational weather forecasts of a 3.7°C cooling. The strongest surface climate effects were for ModelE configurations with more detailed aerosol microphysics that led to a stronger first indirect effect. Plain Language Summary: Smoke from biomass burning is known to have effects on surface weather. We used the NASA GISS ModelE to estimate these effects for a large 2017 smoke plume over northern Canada that persisted for two weeks. We first found that the height of the smoke release at the source was the most important factor influencing agreement between ModelE and satellite retrievals of aerosols and carbon monoxide, and that specific, plausible configurations of the model led to unrealistically high aerosol amounts. By comparing simulations with and without fire, we estimated a 16‐day cooling over a 850,000 km2 region of between 1.5°C and 4.9°C, depending on the model configuration. Key Points: We captured the overall pattern and magnitude of a large 2017 smoke plume over Canada with the NASA GISS ModelEOf the sixteen plausible model configurations tested under a full‐factorial design, two with higher NOx emissions, free‐tropospheric smoke release and mass‐based aerosols led to unrealistically high aerosol optical depthOver an 850,000 km2 region, we estimated a 16‐day surface cooling of between 1.5°C and 4.9°C [ABSTRACT FROM AUTHOR]

This study uses a combined research approach based on remote-sensing and numerical modeling to quantify the effects of burned areas on the surface climate in the two Brazilian biomes most affected by fires: the tropical savanna and the Amazon rainforest. Our estimates indicate that between 2007 and 2020, approximately 6% of the savanna and 2% of the rainforest were burned on average. Non-parametric regressions based on 14-year climate model simulations indicate that latent heat flux decreases on average by approximately 0.17 W m−2 in the savanna and 0.60 W m−2 in the rainforest per each 1 km2 burned, with most of the impacts registered during the onset of the wet season. Sensible and ground heat fluxes are also impacted but at less intensity. Surface air is also warmer and drier, especially over rainforest burned sites. On average, fire reduced gross primary production in the savanna and rainforest by 12% and 10%, respectively, in our experiments. [ABSTRACT FROM AUTHOR]

Northern high latitudes are experiencing some of the greatest increases in air temperatures on Earth. Air temperatures (along with other modulating variables including precipitation and the onset of snowmelt) are influenced by atmospheric–oceanic circulation patterns, some of which are persistent and recurrent. One pattern in particular, the Arctic Dipole (AD) anomaly, is a persistent sea‐level pressure teleconnection pattern between the Canadian Archipelago and Barents Sea that has unknown impacts on local climate variability. These patterns may be important, especially in hydro‐ecologically sensitive areas such as Northwest Territories (NWT), Canada where permafrost thaw and ecosystem changes are influenced by interannual climate variability. The goal of this research is to determine the impacts of the AD on local climate (air temperature, precipitation, snowmelt) for a 66‐year period (1950–2015) spanning both latitudinal and longitudinal gradients across NWT from north to south and foothills to plains. Deviations during strong positive and negative modes of the AD index were calculated in reference to the complete 66‐year record. Results showed considerable year‐to‐year variability in the AD pattern, with more frequent strong negative modes during the 2000s. During 1950–2015, there were 64 and 56 occurrences of strong positive and strong negative AD modes, respectively, across all seasons. Spring and summer strong AD modes led to local air temperature anomalies of greater than 0.8°C compared with the long‐term (66 years) mean. Earlier onset of snowmelt, by an average of 3–5 days, was also noted during positive AD modes. Despite strong connectivity between the AD and local air temperature, we found less correspondence between the AD and seasonal precipitation. These findings improve understanding of the impacts of the AD on local weather and climate in NWT and suggest implications for ecosystem change, such as drying and shrubification of northern boreal peatlands and possible connectivity to teleconnection impacts on wildland fire. [ABSTRACT FROM AUTHOR]

The impact of the quasi‐biennial oscillation (QBO) on the surface air temperature (SAT) in the Northern Hemisphere extratropics is investigated. It is found that the QBO, defined as 70‐hPa zonal wind in the deep tropics, is negatively correlated with the SAT over the western North Pacific in February and March. Cold temperature anomaly appears during the QBO westerly phase. Such relationship is likely mediated by the subtropical jet. During the QBO westerly phase, a horseshoe‐shaped zonal wind anomaly forms in the upper troposphere and lower stratosphere and is connected to the equatorward shift of the Asia‐Pacific jet. This equatorward jet shift is accompanied by a cyclonic circulation anomaly in the subtropical North Pacific and an anticyclonic circulation anomaly over northern Eurasia in the troposphere. The resultant temperature advection brings cold air to East Asia and the western North Pacific. This regional downward coupling in February and March, which is not sensitive to El Niño‐Southern Oscillation, has become statistically significant in recent decades. [ABSTRACT FROM AUTHOR]

The boreal summer climate is of significant societal importance and is trending toward increased risks of extreme climate events such as heatwaves. The summer North Atlantic Oscillation, as the primary mode of atmospheric variability in the northern hemisphere, has been long considered lacking predictability on seasonal time scales. Here we show that the summer North Atlantic Oscillation is predictable with a 2‐month lead for the recent decades. The primary predictor is the March North Atlantic jet strength, which is correlated with the summer North Atlantic Oscillation index at a correlation coefficient of 0.66 over 1979–2018. Spring stratosphere‐troposphere coupling plays a critical role in this extended predictability from spring to summer, in contrast to the common knowledge that this dynamical coupling is relatively inactive outside the winter season. These results may bring sound prospects for summer seasonal prediction of boreal climate that benefits the energy and public health sectors. Plain Language Summary: The summer climate in the northern hemisphere is of significant societal importance. Extreme climate events such as heatwaves occur more and more frequently under global warming. The summer North Atlantic Oscillation is a good representation of atmospheric motions in the North Atlantic region and can influence the atmospheric flow of the entire northern hemisphere. Scientists have not made accurate seasonal forecast of this mode and hence boreal summer climate. In this study, we discovered a key predictor in March whose signal can be transmitted by the stratosphere back to the summer surface to make the summer North Atlantic Oscillation predictable 2 months in advance. Our findings can help the society better prepared for the extreme summer climate in the northern hemisphere. Key Points: The summer North Atlantic Oscillation is found to become predictable in the recent decadesMarch North Atlantic jet strength is the key predictor for the seasonal forecast of the summer North Atlantic OscillationStratosphere‐troposphere dynamical coupling leads to this extended seasonal predictability of the summer North Atlantic Oscillation [ABSTRACT FROM AUTHOR]

In this study, we examine the impact of stratospheric ozone on the Northern Hemisphere (NH) surface climate during boreal winter by analyzing the two experiments using Global/Regional Integrated Model system‐Chemistry Climate Model (GRIMs‐CCM) (i.e., LINOZ‐on/‐off experiments) and eight Atmosphere Model Inter‐comparison Project (AMIP) climate models. In LINOZ‐on, ozone concentration in the upper troposphere and lower stratosphere (UTLS) varies with latitudes, being low in the tropics and high in the mid‐to‐high‐latitudes, by adopting a linearized ozone scheme. The LINOZ‐off, however, prescribes constant ozone concentration throughout all latitudes. Cooler surface temperatures over the Eurasian and Asian continents in boreal winter are simulated in LINOZ‐on than that in LINOZ‐off. A reduced mean UTLS temperature gradient in LINOZ‐on, due to the difference of climatological mean ozone concentration, causes a weakened mean zonal wind in the lower stratosphere compared to the LINOZ‐off. This mean wind difference is extended all the way to the surface, resulting in a negative Arctic Oscillation (AO)‐like mean state in the mid‐to‐high latitudes in the LINOZ‐on. Subsequently, it causes cool surface temperatures over the Eurasian and Asian continents by altering horizontal winds. This result is also supported by the comparison between AMIP models with a relatively high and low stratospheric ozone concentration in the mid‐to‐high latitudes. Therefore, a careful attention should be paid to the climatology of stratospheric ozone concentration in climate models to correctly simulate surface climate in the NH in the past and future climate. Key Points: We examined the impact of stratospheric ozone on the NH surface climate during boreal winter using the Global/Regional Integrated Model system‐Chemistry Climate Model and eight Atmosphere Model Inter‐comparison Project climate modelsDifferences in imposed ozone concentrations lead to reduced UTLS temperature gradient, causing a weak polar vortex in the lower stratosphereThis wind change associated with a weak polar vortex extended all the way to the surface, resulting in negative AO‐like anomalies [ABSTRACT FROM AUTHOR]

Accurate and high spatial resolution (<100 m) surface climate information is crucial for process‐based modelling in hydrology, ecology, agriculture, urban studies etc, especially in complex terrain landscapes where coarse grid resolution information (∼10 km) is inadequate to represent pronounced local variability. We used a machine learning‐based workflow to predict high resolution (30 m) and sub‐daily atmospheric variables fields of near‐surface air temperature and humidity, and wind speed. The method used the Principal Component Analysis (PCA) decomposition applied on ground stations observations or Global Climate Model (GCM) residual error, in a sequence with bias correction and statistical models (Linear Regression‐LR, Artificial Neural Network model‐ANN and Empirical Quantile Mapping‐EQM) to provide downscaling from large scale atmospheric conditions to complex terrain variability. The predictions described relationships of Principal Component (PC) scores dependent on GCM temporal variability on 6‐hourly basis (with LR or ANN or EQM), and PC loadings dependent on topographic indexes to help providing horizontal sub‐grid extrapolation. The methods were validated with a 1‐year dataset from a dense weather stations network deployed in a complex terrain basin in tropical climate of Southeast Brazil. We present an exhaustive description of the PC modes daily/seasonal variability for each variable, and their spatial variability associated to the topography and thermal driven circulations. The predictions in general substantially improved accuracy when compared to GCM outputs, especially near the valley and in sheltered area where local effects are mandatories. Specially, ANN and EQM significantly improved the predictions at the variability of extreme events, such as the formation of strong cold air pooling or wetting in the valley. [ABSTRACT FROM AUTHOR]

The impact of the solar wind on sea level pressure (SLP), sea surface temperature (SST), zonal mean zonal wind (U) and air temperature (T) was examined using multiple linear regression analysis. Our analysis of the December–January–February (DJF) mean fields indicates that significant links between the solar wind speed (SWS) and the North Atlantic oscillation (NAO), SST tripolar structure and polar stratospheric temperature. The monthly reanalysis data (November to March) show that high SWS is associated with a poleward‐ and downward‐ propagating solar wind signal from December to February. The response of the Eliassen‐Palmer (EP) flux shows that more planetary wave activity is refracted equatorward in the upper stratosphere under higher SWS conditions, corresponding to an enhanced EP flux convergence in early winter. Enhanced EP flux divergence occurs in the stratosphere starting in January and propagates poleward and downward from January to February. For the coupling mechanism between the stratosphere and troposphere, in addition to chemical‐dynamical processes, cloud microphysical processes associated with the global electric circuit (GEC) might play a role in the downward propagation of the solar wind signal and the modulation of the NAO. [ABSTRACT FROM AUTHOR]

Characterized by a salinity minimum in the mid-depth, the Antarctic Intermediate Water (AAIW) is an important component of global ocean water mass. The simulation of the AAIW in current climate models, however, has remained deficient, especially in the Atlantic sector. Here, we evaluate the simulation of the South Atlantic AAIW in eleven state-of-the-art coupled climate models. It is found that all the models show a common AAIW bias relative to the observation, with a saltier, warmer and lighter core located at a shallower depth. This AAIW bias seems to contribute to a deficient freshwater export by the Atlantic Meridional Overturning Circulation (AMOC), potentially overstabilizing the AMOC. The causes of the bias are investigated in sensitivity experiments using an ocean alone model. It is found that the AAIW bias is caused neither by the surface climate bias nor the North Atlantic bias, although is weakly affected by the inter-basin exchange. This left the conclusion that the AAIW bias is caused predominantly by the deficient model representation of ocean dynamics and mixing processes in the AAIW region. [ABSTRACT FROM AUTHOR]

China has experienced intense land use and land cover changes during the past several decades, which have exerted significant influences on climate change. Previous studies exploring related climatic effects have focused mainly on one or two specific land use changes, or have considered all land use and land cover change types together without distinguishing their individual impacts, and few have examined the physical processes of the mechanism through which land use changes affect surface temperature. However, in this study, we considered satellite-derived data of multiple land cover changes and transitions in China. The objective was to obtain observational evidence of the climatic effects of land cover transitions in China by exploring how they affect surface temperature and to what degree they influence it through the modification of biophysical processes, with an emphasis on changes in surface albedo and evapotranspiration (ET). To achieve this goal, we quantified the changes in albedo, ET, and surface temperature in the transition areas, examined their correlations with temperature change, and calculated the contributions of different land use transitions to surface temperature change via changes in albedo and ET. Results suggested that land cover transitions from cropland to urban land increased land surface temperature (LST) during both daytime and nighttime by 0.18 and 0.01 K, respectively. Conversely, the transition of forest to cropland tended to decrease surface temperature by 0.53 K during the day and by 0.07 K at night, mainly through changes in surface albedo. Decreases in both daytime and nighttime LST were observed over regions of grassland to forest transition, corresponding to average values of 0.44 and 0.20 K, respectively, predominantly controlled by changes in ET. These results highlight the necessity to consider the individual climatic effects of different land cover transitions or conversions in climate research studies. This short-term analysis of land cover transitions in China means our estimates should represent local temperature effects. Changes in ET and albedo explained

Atmospheric icing became a primary concern due to the significant impact and hazardous conditions of its accretion on structures. The objective of this study is to provide a map of icing events over 32years (1979 to 2010) that describes the severity of winter icing. This information will prove useful to prevent damages and economical losses due to icing events by documenting the risk factor. To validate the icing climatology method, two case studies involving two topographically contrasting sites were selected: a simple terrain site which is the airport of Bagotville, near Saguenay (Canada) and a complex terrain site located in Mt Bélair, near Quebec City (Canada). Ice accumulation calculated by the use of reanalysis data was quantified using ice accretion on a cylinder model. Comparison between measurement and the model over Bagotville revealed insignificant differences in ice accumulation less than 0.3mm, and in duration of icing events less than 0.2day. On the other hand, during winter months, the calculation that showed a maximum of 60mm in January 1999 over Mt Bélair site also had an underestimation of ice accumulation that varies from 5mm to 16mm. The horizontal resolution of NARR imposes a challenge on the calculation of icing events over complex terrains, especially during the months of November and March when air temperature is near freezing point. Taking into account the liquid water content, the duration of icing events and the classes of icing events as weighting factors, the icing severity index based on reanalysis data was introduced to assess the severity level of icing events, covering the north-east of Quebec including Quebec City, Sept-Iles, the east of Saguenay, the lower St Lawrence River and the Gaspe region. Consequently, an icing severity index mapping that represents the climatology of in-cloud atmospheric icing was produced. [ABSTRACT FROM AUTHOR]

The sensitivity of surface energy and water fluxes to recent land cover changes is simulated for a small region in northern Mato Grosso, Brazil. The Simple Biosphere Model (SiB2) is used, driven by biophysical parameters derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) at 250-m resolution, to compare the effects of different land conversion types. The mechanisms through which changes in vegetation alter surface fluxes of energy, momentum, water, and carbon are analyzed for both wet and dry seasons. It is found that morphological changes contribute to warming and drying of the atmosphere while physiological changes, particularly those associated with a plant¿s photosynthetic pathway, counterbalance or exacerbate the warming depending on the type of conversion and the season. Furthermore, this study¿s results indicate that initial clearing of evergreen and transition forest to bare ground increases canopy temperature by up to 1.7°C. For subsequent land use such as pasture or cropland, the largest effect is seen for the conversion of evergreen forest to C3 cropland during the wet season,with a 21% decrease of the latent heat flux and 0.4°C increase in canopy temperature. The secondary conversion of pasture to cropland resulted in slight warming and drying during the wet season driven mostly by the change in carbon pathway from C4 to C3. For all conversions types, the daily temperature range is amplified, suggesting that plants replacing forest clearing require more temperature tolerance than the trees they replace. The results illustrate that the effect of deforestation on climate depends not only on the overall extent of clearing but also on the subsequent land use type. [ABSTRACT FROM AUTHOR]

To adequately assess the effects of global warming it is necessary to address trends and impacts at the local level. This study examines phenological changes in the start-of-season (SOS) derived from satellite observations from 1982–2008 in the US High Plains region. The surface climate-based SOS was also evaluated. The averaged profiles of SOS from 37° to 49°N latitude by satellite- and climate-based methods were in reasonable agreement, especially for areas where croplands were masked out and an additional frost date threshold was adopted. The statistically significant trends of satellite-based SOS show a later spring arrival ranging from 0.1 to 4.9 days decade ^−1 over nine Level III ecoregions. We found the croplands generally exhibited larger trends (later arrival) than the non-croplands. The area-averaged satellite-based SOS for non-croplands (i.e. mostly grasslands) showed no significant trends. We examined the trends of temperatures, precipitation, and standardized precipitation index (SPI), as well as the strength of correlation between the satellite-based SOS and these climatic drivers. Our results indicate that satellite-based SOS trends are spatially and primarily related to annual maximum normalized difference vegetation index (NDVI, mostly in summertime) and/or annual minimum NDVI (mostly in wintertime) and these trends showed the best correlation with six-month SPI over the period 1982–2008 in the US High Plains region.

Abstract Human modification of the landscape potentially affects exchanges of energy and water between the terrestrial biosphere and the atmosphere. This study develops a possible scenario for land cover in the year 2050 based on results from the IMAGE 2 (Integrated Model to Assess the Greenhouse Effect) model, which projects land-cover changes in response to demographic and economic activity. We use the land-cover scenario as a surface boundary condition in a biophysically-based land-surface model coupled to a general circulation model for a 15-years simulation with prescribed sea surface temperature and compare with a control run using current land cover. To assess the sensitivity of climate to anthropogenic land-cover change relative to the sensitivity to decadal-scale interannual variations in vegetation density, we also carry out two additional simulations using observed normalized difference vegetation index (NDVI) from relatively low (1982–83) and high (1989–90) years to describe the seasonal phenology of the vegetation. In the past several centuries, large-scale land-cover change occurred primarily in temperate latitudes through conversion of forests and grassland to highly productive cropland and pasture. Several studies in the literature indicate that past changes in surface climate resulting from this conversion had a cooling effect owing to changes in vegetation morphology (increased albedo). In contrast, this study indicates that future land-cover change, likely to occur predominantly in the tropics and subtropics, has a warming effect governed by physiological rather than morphological mechanisms. The physiological mechanism is to reduce carbon assimilation and consequently latent relative to sensible heat flux resulting in surface temperature increases up to 2 °C and drier hydrologic conditions in locations where land cover was altered in the experiment. In addition, in contrast to an observed decrease in diurnal temperature... [ABSTRACT FROM AUTHOR]