Atmospheric jet variability: linking STRucture, Evolution and Mechanisms

Jet streams are a key feature of the global scale wind systems that shape Earth's climate. The low-frequency variability of jet streams reflects shifting weather patterns and regimes. Understanding jet variability is thus essential for understanding year-to-year variations in regional climate and weather extremes, and projecting how these will change in a warming world. Though we have learned much about low-frequency jet variability, there remain many open questions about what causes and controls it. An ongoing challenge is connecting the theoretical mechanisms behind jet variability to observed jet variability. What are the basic recipes for jet variability? What is the most dynamically meaningful way to describe and characterize it in the real world ? How are these two linked? jetSTREAM aims to identify the dynamical origins of jet variability by bridging the gap between existing theoretical and statistical (observational) views of the extratropical atmospheric circulation. It will achieve this goal by combining new analyses of observed, three-dimensional jet structure with simulations using a hierarchy of models, from idealized atmospheric models to fully coupled climate models. The project has yielded new insight into North Atlantic jet variability. Norway is very sensitive to these changes, so understanding the mechanisms behind them is critical for narrowing uncertainties in future impacts (e.g., extreme precipitation). A concrete link has been found between jet stream configurations and weather regimes, the large-scale patterns that are tightly connected to European weather conditions. The tropical Pacific has been identified as an important player, providing a remote influence on the North Atlantic jet. Finally, a northward shift of the jet exit has been found to emerge around 2 degrees Celsius of global warming in a "large ensemble" experiment using several different climate models. The results from jetSTREAM contribute to better understanding statistically derived climate patterns (e.g., the North Atlantic Oscillation) and predicting regional climate changes under global warming. Project webpage: http://jetstream.b.uib.no

Development of jet detection method that accounts for 3-D variability of winds. Data set is available, algorithm is implemented to run automatically for ECMWF forecasts. Jet clusters framework. Has clarified relationships between European weather patterns and the North Atlantic jet; is now being applied to seasonal climate signals in Europe, and the dynamics behind shifts of the North Atlantic jet from its usual state to a less eddy-driven state. Idealized experiments helped identify a mechanism by which fast jets exhibit less north-south shifting variability. Processes influencing the North Atlantic jet and storm track: 2 factors found to be important are ocean temperatures in the tropical Pacific (remote), and latent heating within weather systems (local). Large-ensemble simulations helped constrain the relative size of climate change vs. natural variability in midlatitudes; results fed into IPCC Special Report on 1.5C global warming.

Jet streams are a key feature of the global scale wind systems that shape Earth's climate. The low-frequency variability of jet streams reflects shifting weather patterns and regimes. Understanding jet variability is thus essential for understanding year -to-year variations in regional climate and weather extremes, and projecting how these will change in a warming world. Though we have learned much about low-frequency jet variability, there remain many open questions about the fundamental mechanisms unde rlying the dominant variability patterns. An ongoing challenge is connecting the theoretical, process-based view of jet variability to observations. What are the basic recipes for jet variability? What is the most dynamically meaningful way to character ize jet variability in the real world? How are these linked? We will establish an innovative collaboration in extratropical atmospheric dynamics by combining key and complementary expertise from the Universities of Bergen, Oslo, Oxford (UK) and Oregon State (USA). The central goal of the collaboration is to elucidate the mechanisms responsible for low frequency jet variability by combining new analyses of observed, three-dimensional jet structure with simulations using a hierarchy of models, from idea lized atmospheric models to fully coupled climate models. The novel integrated framework is grounded in fundamental dynamics but exploits high-resolution reanalysis data sets and climate model output that have only become available in recent years. We ex pect jetSTREAM to yield exceptional new insight into jet stream dynamics, which in turn will contribute to better understanding statistically derived climate variability patterns (e.g., NAO, PNA) and predicting regional climate changes under global warmin g.