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A Climatology of 500 hPa Closed Lows in the Northeast Pacific, 1948-2011
AuthorOakley, Nina S.
AdvisorRedmond, Kelly T
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The Northeast Pacific is a preferential location for the formation of closed low pressure systems. These slow moving, quasi-‐barotropic systems have the potential to produce or affect sustained precipitation episodes along the West Coast of the United States. They do so because they affect vertical stability and sustain a moist environment favoring cloudiness. In concert with other circulation features they can lead to uncommon airflow trajectories that may impact fire and wind events. As a class, these features have not been addressed as attentively in research as have been open wave disturbances and cyclones. This study uses the NCEP/NCAR Reanalysis dataset at 6-‐hour and 2.5°x2.5° resolution for 1948-‐2011 to develop an objective climatological description of 500 hPa closed lows in the Northeast Pacific. Time series for seasonal and annual event counts were found to exhibit strong inter-‐ annual variability. Composites of landfalling closed low tracks revealed a preferential track as they move inland over the western United States. The features travel southeast as they travel onshore, and then make a pronounced cyclonic turn as they move over southern California. Precipitation at 30 U.S. Cooperative Observer stations was associated with closed low events, suggesting 20-‐60% of 1948-‐2011 precipitation along the U.S. West Coast may be driven by closed lows. A latitudinal dependence on closed low precipitation was also expressed, with stations south of 40°N receiving a greater percentage of their October-‐March precipitation from closed lows. Values of metrics of various modes of atmospheric variability (ENSO, PDO, AO, PNA, etc.) were assessed for relationships with closed low events.iCorrelations with ENSO indices, PDO, and PNA were found to be of significance, favoring more events in the warm ENSO phase and positive PDO and PNA phases. In the period between water years 1998 and 2008, 43% of Atmospheric Rivers were found to be concurrent with identified closed lows.