A multifaceted climatology of atmospheric blocking and its recent linear trend

A dynamically based climatology is derived for Northern Hemisphere atmospheric blocking events. Blocks are viewed as large amplitude, long-lasting, and negative potential vorticity ( PV) anomalies located beneath the dynamical tropopause. The derived climatology [ based on the 40- yr European Centre for Medium-Range Weather Forecasts ( ECMWF) Re- Analysis ( ERA- 40)] provides a concise, coherent, and illuminating description of the main physical characteristics of blocks and the accompanying linear trends. The latitude - longitude distribution of blocking frequency captures the standard bimodal geographical distribution with major peaks over the North Atlantic and eastern North Pacific in all four seasons. The accompanying pattern for the age distribution, the genesis - lysis regions, and the track of blocks reveals that 1) younger blocks ( 1 - 4 days) are more prevalent at lower latitudes whereas significantly older blocks ( up to 12 days) are located at higher latitudes; 2) genesis is confined predominantly to the two major ocean basins and in a zonal band between 40 degrees and 50 degrees N latitude, whereas lysis is more dispersed but with clear preference to higher latitudes; and 3) the general northeastward - west- northwest movement of blocks in the genesis - lysis phase also exhibits subtle seasonal and intra- and interbasin differences. Examination of the intensity and spatial- scale changes during the blocking life cycle suggests that in the mean a block's evolution is independent of the genesis region and its eventual duration. A novel analysis of blocking trends reveals significant negative trends in winter over Greenland and in spring over the North Pacific. It is shown that the changes over Greenland are linked to the number of blocking episodes, whereas a neighboring trend signal to the south is linked to higher- frequency anticyclonic systems. Furthermore, evidence is adduced that changes in blocking frequency contribute seminally to tropopause height trends.