Causes of the Record-Breaking Pacific Northwest Heatwave, Late June 2021

The extreme heat event that hit the Pacific Northwest (Oregon, Washington, southern British Columbia) at the end of June 2021 was 3 °C greater than the previous Seattle record of 39 °C; larger extremes of 49 °C were observed further inland that were 6 °C above previous record. There were hundreds of deaths over the region and loss of marine life and forests. At the large scale prior to the event, the polar vortex was split over the Arctic. A polar vortex instability center formed over the Bering Sea and then extended southward along the west coast of North America. The associated tropospheric trough (low geopotential heights) established a multi-day synoptic scale Omega Block (west-east oriented low/high/low geopotential heights) centered over the Pacific Northwest. Warming was sustained in the region due to subsidence/adiabatic heating and solar radiation, which were the main reasons for such large temperature extremes. The seasonal transition at the end of spring suggests the possibility of a southern excursion of a polar vortex/jet stream pair. Both the Pacific Northwest event in 2021 and the Siberian heatwave climax in June 2020 may be examples of crossing a critical state in large-scale atmospheric circulation variability.

Projections of northern hemisphere extratropical climate underestimate internal variability and associated uncertainty

Internal climate variability will play a major role in determining change on regional scales under global warming. In the extratropics, large-scale atmospheric circulation is responsible for much of observed regional climate variability, from seasonal to multidecadal timescales. However, the extratropical circulation variability on multidecadal timescales is systematically weaker in coupled climate models. Here we show that projections of future extratropical climate from coupled model simulations significantly underestimate the projected uncertainty range originating from large-scale atmospheric circulation variability. Using observational datasets and large ensembles of coupled climate models, we produce synthetic ensemble projections constrained to have variability consistent with the large-scale atmospheric circulation in observations. Compared to the raw model projections, the synthetic observationally-constrained projections exhibit an increased uncertainty in projected 21st century temperature and precipitation changes across much of the Northern extratropics. This increased uncertainty is also associated with an increase of the projected occurrence of future extreme seasons.

Linking air stagnation in Europe with the synoptic- to large-scale atmospheric circulation

The build-up of pollutants to harmful levels can occur when meteorological conditions favour their production or accumulation near the surface. Such conditions can arise when a region experiences air stagnation. The link between European air stagnation, air pollution and the synoptic- to large-scale circulation is investigated in this article across all seasons and the 1979–2018 period. Dynamical indices identifying atmospheric blocking, Rossby wave breaking, subtropical ridges, and the North Atlantic eddy-driven and subtropical jets are used to describe the synoptic- to large-scale circulation as predictors in statistical models of air stagnation and pollutant variability. It is found that the large-scale circulation can explain approximately 60 % of the variance in monthly air stagnation, ozone and wintertime particulate matter (PM) in five distinct regions within Europe. The variance explained by the model does not vary strongly across regions and seasons, apart from for PM when the skill is highest in winter. However, the dynamical indices most related to air stagnation do depend on region and season. The blocking and Rossby wave breaking predictors tend to be the most important for describing air stagnation and pollutant variability in northern regions, whereas ridges and the subtropical jet are more important to the south. The demonstrated correspondence between air stagnation, pollution and the large-scale circulation can be used to assess the representation of stagnation in climate models, which is key for understanding how air stagnation and its associated climatic impacts may change in the future.

Monthly Streamflow Forecasting Using Convolutional Neural Network

Monthly streamflow forecasting is vital for the management of water resources. Recently, numerous studies have explored and evidenced the potential of artificial intelligence (AI) models in hydrological forecasting. In the current study, the feasibility of a relatively new AI model, namely the convolutional neural network (CNN), is explored for forecasting monthly streamflow. The CNN is a method of deep learning, the unique convolution-pooling mechanism in which creates its superior attribute of automatically extracting critical features from input layers. Hydrological and large-scale atmospheric circulation variables including rainfall, streamflow, and atmospheric circulation factors (ACFs) are used to establish models and forecast streamflow for Huanren Reservoir and Xiangjiaba Hydropower Station, China. The ANN and ELM with inputs identified based on cross-correlation analysis (CC) and mutual information analysis (MI) are established for comparative analysis. The performances of these models are assessed with several statistical metrics and graphical evaluation methods. The results show that CNN performs better than ANN and ELM across all the statistical measures. Moreover, CNN shows better stability in forecasting accuracy.

Climatic features of the 2020/2021 winter season and the air temperature and precipitation outlook for the summer of 2021 over Northern Eurasia

Climatic features of the 2020/2021 winter season and the air temperature and precipitation outlook for the summer of 2021 over Northern Eurasia / Khan V.M., Vilfand R.M., Emelina E.V., Kaverina E.S., Kulikova I.A., Sumerova K.A., Tischenko V.A. // Hydrometeorological Research and Forecasting, 2021, no. 2 (380), pp. 6-19. The main features of the Northern Hemisphere large-scale atmospheric circulation are analyzed for the past 2020/2021 winter. The accuracy of consensus forecasts of air temperature and precipitation compiled during the work of the 19th session of the North Eurasian Climate Outlook Forum (NEACOF-19) is presented, with the skill scores of consensus forecasts for Northern Eurasia. The main features of the thermal state of the ocean and large-scale atmospheric circulation for the coming summer of 2021 are considered and analyzed. A forecast of surface air temperature and precipitation anomalies for the summer of 2021 agreed with the NEACOF-20 experts is formulated. Keywords: North Eurasian Climate Outlook Forum, North Eurasian Climate Center, consensus forecast, air temperature, precipitation, large-scale atmospheric circulation, hydrodynamic models, sea surface temperature