The North American Regional Reanalysis (NARR) was used to develop an expanded, long-term climatology of meridional (southerly and northerly) low-level jets over North America and surrounding coastal environs. NARR has greater spatial coverage and finer temporal (3 hourly) and horizontal (32 km) resolutions than do routine rawinsonde wind measurements. The NARR climatology focuses on jet frequency and average speed and elevation by month and 3-hourly time step. To evaluate the plausibility of the climatology, jet characteristics were compared with those obtained from prior climatological analyses, case studies, field campaigns, and numerical simulations. Strong agreement was found with many of the previously documented characteristics of well-known jets, including the northerly Pacific coast jet and southerly Great Plains jet. The NARR climatology provides additional insights into the spatial extent and seasonal shifts of large jet frequencies and into diurnal fluctuations in frequency, speed, and elevation. Weaker and/or less spatially extensive jets are also well depicted in the NARR climatology, including the southerly Gulf of California jet, summertime southerly jets and autumn northerly jets off the mid-Atlantic coast, and northerly jets in the high plains. Furthermore, several new areas of relatively frequent jet occurrence, most of which align with shallow thermal gradients, are seen in the NARR climatology. The NARR climatology supplements and enhances our understanding of North American low-level jets and points to the need for additional research on both the climatological characteristics of these jets and on the processes contributing to their formation.

The southerly Great Plains low-level jet (GPLLJ) is one of the most significant circulation features of the central U.S. linking large-scale atmospheric circulation with the regional climate. GPLLJs transport heat and moisture, contribute to thunderstorm and severe weather formation, provide a corridor for the springtime migration of birds and insects, enhance wind energy availability, and disperse air pollution. We assess future changes in GPLLJ frequency using an eight member ensemble of dynamically-downscaled climate simulations for the mid-21st century. Nocturnal GPLLJ frequency is projected to increase in the southern plains in spring and in the central plains in summer, whereas current climatological patterns persist into the future for daytime and cool season GPLLJs. The relationship between future GPLLJ frequency and the extent and strength of anticyclonic airflow over eastern North America varies with season. Most simulations project a westward shift of anticyclonic airflow in summer, but uncertainty is larger for spring with only half of the simulations suggesting a westward expansion. The choice of regional climate model and the driving lateral boundary conditions have a large influence on the projected future changes in GPLLJ frequency and highlight the importance of multi-model ensembles to estimate the uncertainty surrounding the future GPLLJ climatology.

ABSTRACT:The inter-annual variability of southerly low-level jets (SLLJs) over North America during the warm (April-September) and cool (October-March) seasons is investigated. SLLJ occurrences over a 31-year period were identified from the North American Regional Reanalysis (NARR) vertical wind profiles. The first empirical orthogonal function (EOF) modes of the SLLJ frequency during the warm and cool seasons account for about 30 and 20% of the total variance, respectively. Both modes can be interpreted as a strengthening or weakening of the core area of SLLJ anomalies. The principal component (PC) time series display significant positive trends, suggesting an increase in SLLJ activity during both seasons on inter-decadal time scales and are significantly correlated to the summertime Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) for the warm season and the wintertime PDO, AMO and El Niño Modoki for the cool season. The second modes account for about 20 and 15% of the total variance for the warm and cool seasons, respectively, and are interpreted as primarily a subseasonal latitudinal shift in SLLJ activity between the central Great Plains and the western Gulf of Mexico and southern Texas during the warm season and a longitudinal shift between the western Gulf of Mexico and the Caribbean during the cool season. The second mode appears to be significantly correlated to El Niño Modoki for the warm season and to Niño 3.4 for the cool season.

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