Examining the Relationship Between the Pacific / North American Teleconnection Pattern and the Great Plains Low-Level Jet

Tuesday, 16 December 2014
Keith J Harding and Peter K Snyder, University of Minnesota Twin Cities, Minneapolis, MN, United States
The Pacific / North American (PNA) teleconnection pattern is a prominent low frequency mode of variability in the Northern Hemisphere that is most pronounced in the boreal winter. However, the PNA can exert significant daily to weekly variability and influence weather patterns over the Midwest during the spring and summer months. In this study, we use the NCEP-NCAR Reanalysis to demonstrate the link between the PNA teleconnection pattern and the Great Plains Low-Level Jet (GPLLJ). The negative phase of the PNA, which is associated with lower heights over the Great Plains and ridging in the Southeast U.S., enhances the GPLLJ by increasing the pressure gradient within the GPLLJ on daily to monthly timescales. We show that strong GPLLJ events predominantly occur when the PNA index is negative. Strong GPLLJ events with a very negative PNA index (< -1) are associated with longer wavelength planetary waves that are more persistent and increase the duration of GPLLJ events. As a result, strong GPLLJ events associated with a very negative PNA index contribute to a greater number of heavy precipitation events over the North Central U.S. by enhancing the meridional transport of moisture into the Upper Midwest and driving greater moisture convergence north of the GPLLJ maximum over extended periods. A significant majority of the greatest 5-day North Central precipitation events in the Climate Prediction Center Daily Precipitation dataset occur when the PNA index is negative, with most strong precipitation events having a very negative PNA. The PNA is typically the most negative 1-4 days in advance of heavy precipitation events. This finding may help improve short- to medium-range predictability of Upper Midwest heavy rainfall events.