Us Forest Service Northern Research Station Lansing

United States

Us Forest Service Northern Research Station Lansing

United States
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Yu L.,Michigan State University | Zhong S.,Michigan State University | Winkler J.A.,Michigan State University | Doubler D.L.,Michigan State University | And 2 more authors.
International Journal of Climatology | Year: 2016

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 (1979-2009) 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. © 2016 Royal Meteorological Society.


Yu L.,Michigan State University | Zhong S.,Michigan State University | Bian X.,Us Forest Service Northern Research Station Lansing
International Journal of Climatology | Year: 2016

Valleys can trap cold air to form Valley Cold Pools (VCPs). Characterized by stable stratification and weak winds, VCPs, especially those that last over multiple days, can produce adverse effects such as poor visibility and severe air pollution. Using the gridded data set of the North American Regional Reanalysis (NARR) for the period of 1979 through 2012, this study examines the climatology of VCPs in the western United States with a focus on spatial and temporal variability. The results reveal a widespread occurrence of short-lived (1-2days) VCPs over both the mountainous areas of the West and the Northern Plains. Longer VCP episodes, however, tend to be limited to large basins/valleys in the Northwest and the Intermountain West. The leading mode of variability in the annual number of cold-season multi-day VCP event anomalies appears to be linked to a sea-surface temperature anomaly pattern typically found during the warm phase of Pacific Decadal Oscillation. Corresponding to the positive phase of the leading mode of variability is an anomalous 500-hPa ridge over the western United States induced by a Rossby wave train, which, by blocking cold-air intrusions and producing mid-level subsidence warming, leads to more persistent VCP episodes in the western United States. In addition to large-scale wintertime circulation anomalies, local surface temperature anomalies also contribute to the variability. © 2016 Royal Meteorological Society.

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