Extreme weather events (e.g., heatwaves, heavy precipitation, drought) have significant socioeconomic costs by, for example, threateningpublic safety, destabilizing supply chains, and damaging infrastructure. Skillful predictions of such events at the subseasonal-to-seasonal (S2S) timescale (i.e., forecasts with a lead of 2 to 8 weeks) present enormous value for preparation and loss mitigation. One factor affecting skillful S2S forecasts of extreme weather is accurate representation of mid-latitude atmospheric blocking patterns – i.e., stagnant, anomalous circulation patterns (typically high pressure systems) that persist for days to weeks and enhance probabilities of abnormal and extreme weather over large areas of the Northern Hemisphere.Several processes are important for blocking initiation and maintenance, including large-scale wave breaks in the upper troposphere (i.e., similar to ocean waves breaking on the shore)and remote effects from tropical and high-latitude climate variability. Unfortunately, S2S forecasting models and climate models poorly capture such processes important for predicting these blocking patterns, leaving considerable uncertainty in their predictions.

In this project, we will focus on (a) the dynamical and thermodynamical processes that lead to the development and maintenance of blocking patterns, (b) how the ability of a model to simulate those processes affects that model's forecast skill of extreme weather events on the S2S timescale, and (c) quantifying the relative contributions of different tropical and high-latitude atmospheric processes to simulating atmospheric blocking patterns. The work will use multiple sources of data and techniques, including analysis with atmospheric reanalysis data, operational S2S forecast models of past weather (called reforecasts), and performing targeted experiments with the latest version of the Energy Exascale Earth System Model (E3SM). By the end of the project, we will provide a comprehensive assessment of mechanisms that are most important for forming and maintaining Northern Hemisphere blocking episodes, with the aim of making more skillful forecasts and better future climate projections of extreme weather. The proposed research will consist of three (3) research activities.

• Research Activity #1 will use the ERA5 dataset (an atmospheric reanalysis product that reproduces global atmospheric conditions from 1950 to near present) to study the observed evolution of blocking patterns and relate them to modes of climate variability, tropical convection patterns, and upper-level vortices in the polar atmosphere (called tropopause polar vortices, which have important implications for extreme weather). We will identify blocking episodes across the Northern Hemisphere using established algorithms and perform statistical analyses with several variables (e.g., geopotential heights, jet-stream winds, mid-tropospheric heating rates). We will characterize the frequency, intensity, and duration of these blocking episodes in the 20th and early 21st centuries. We will repeat these analyses with existing runs of the E3SM over a similar time period (i.e., the historical experiment) to benchmark its performance in simulating blocking patterns and associated processes.
• Research Activity #2 will focus on examining the S2S operational model reforecasts for these blocking episodes and how the models represent them. We will perform a sensitivity analysis onselect cases of extreme weather events associated with blocking from Research Activity #1 within the forecast models to quantify the dependencies of blocking formation and maintenance in the forecast models to preceding conditions in the Arctic, mid-latitude, and tropical regions. We will document the skill and performance of these models for these events to understand better which precursor features need to be better represented in the models for more skillful forecasts of blocking episodes and associated extreme weather.
• Research Activity #3 will test the findings of the previous activities through customized E3SM experiments. We will isolate the impacts of tropical and high-latitude weather systems on extratropical blocking episodes through so-called nudging experiments in which particular features in the tropics and the Arctic are selectively enhanced or suppressed in the model, the model is then run, and the resulting simulated blocking patterns (or lack thereof) are assessed. Doing this will help us isolate which processes and features are critical to simulate correctly for capturing blocking episodes. If time allows, we will also perform future climate change simulations to understand how the relationship between the Arctic and tropical features and mid-latitude blocking might change.

Our proposed research addresses the RGMA topics "Modes of Variability and extreme events." Our objectives align with the call “to enhance understanding of predictability of the Earth system by simulating, evaluating, and analyzing modes of climate variability...and extreme events." We will also satisfy the RGMA goal “to enhance a predictive and process- and system-level understanding of the modes of variability and change within the earth system." Finally, conducting our model experiments using E3SM and comparing the results with reanalysis will enable us to assess model performance and identify potential pathways toward improvement.