The recent increase in the occurrence of a boreal summer teleconnection and its relationship with temperature extremes

This study has investigated the relationship between temperature extremes and a subseasonal hemispheric teleconnection pattern over the Northern Hemisphere during boreal summer. By applying self-organizing map (SOM) analysis to 200-hPa geopotential fields from the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) for the period 1979-2012, a teleconnection pattern is identified that increased dramatically in its occurrence after the late 1990s. This pattern is characterized by a zonal wavenumber-5 pattern with anomalous high pressure cores over eastern Europe, northeastern Asia, the eastern North Pacific, the eastern United States, and Greenland. These high pressure centers coincide with regions of increasingly frequent temperature extremes in recent decades. To investigate the temporal evolution of the identifiedSOMpattern, time-lagged composites were performed relative to the days in which the 200-hPa geopotential field most closely resembled the SOM pattern. From day 210 to day 0, a wave train propagated from the central tropical Pacific to the Canadian Arctic Archipelago and Greenland. This poleward wave propagation was followed by the establishment of quasi-stationary high pressure centers over Greenland, Europe, and Asia. This study suggests that more frequent occurrence of the hemispheric teleconnection is linked to more severe and longer extreme weather events over the Northern Hemisphere since the late 1990s.