Winter Mid-Tropospheric Weather Regimes in the Eastern North Pacific

Background: The eastern North Pacific (ENPac) is a region of climatologically significant cyclone activity, often associated with extreme weather in North America. Regions of high (ridges) and low (troughs) 500- hPa height typically drive this activity. We identify 500-hPa height time variability extremes as “regimes.” Our objectives are to determine the regimes’ characteristics, predictability, and relationships to North American extreme weather. Methods: We define weather regimes, separating them into two types based on whether the 500-hPa height variance is extremely low or high. We analyze their general characteristics during the winter (December, January, and February) and relationships to extreme North American weather. To analyze the regimes’ predictability, we define forecast discontinuities as significantly improved extreme 500-hPa height variability model forecasts compared with model forecasts verifying at the same time, but initialized 24 hours earlier. We analyze their effects on anticipated weather. Results: ENPac low variance regimes are usually dominated by one or two large, slow-moving features, usually a trough with an associated surface cyclone 200-700 km to the west and a ridge with an associated surface anticyclone 200-700 km to the east. This pattern leads to anomalous southerly winds and moisture transport. Low variance regimes are generally associated with anomalous wetness in northwestern Canada, warmth in western North America, and dryness in the southwest U.S. High variance regimes are usually dominated by smaller, faster-moving features that alter the 500-hPa heights substantially. These regimes are more varied, but there is a tendency to have a ridge 200-700 km to the west and a trough 200-700 km to the east, leading to anomalous northerly winds and transport of drier polar air into the ENPac region. High variance regimes are generally associated with anomalously cold air in western North America and wetness in the western U.S. Some forecast discontinuities are associated with changes in anticipated weather locally in the ENPac region, while other discontinuities are associated with changes in anticipated weather on a much larger scale, extending to North America. Limitations: Limitations include the small sample of regimes found during the period of record (18 low variance and 10 high variance), the metric being limited to the 500-hPa level, and the study of only the ENPac winter. Conclusion: Low and high variance regimes generally lead to different ENPac weather patterns and North American extreme weather. Forecast discontinuities differ significantly from each other in their spatial extents. Further work is necessary to identify their causes and characteristics.

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