Wave-Breaking Characteristics of Northern Hemisphere Winter Blocking: A Two-Dimensional Approach

Abstract

his paper generalizes and applies recently developed blocking diagnostics in a two-dimensional (2D) latitude?longitude context, which takes into consideration both mid- and high-latitude blocking. These diagnostics identify characteristics of the associated wave breaking as seen in the potential temperature ? on the dynamical tropopause, particularly the cyclonic or anticyclonic direction of wave breaking (?DB index?) and the relative intensity (?RI index?) of the air masses that contribute to blocking formation. The methodology is extended to a 2D domain and a cluster technique is deployed to classify mid- and high-latitude blocking according to the wave-breaking characteristics. Midlatitude blocking is observed over Europe and Asia, where the meridional gradient of ? is generally weak, whereas high-latitude blocking is mainly present over the oceans, to the north of the jet stream, where the meridional gradient of ? is much stronger. They occur on the equatorward and poleward flank of the jet stream, respectively, where the horizontal shear ?u/?y is positive in the first case and negative in the second case. A regional analysis is also conducted. Warm-cyclonic blocking over the Pacific and cold-anticyclonic blocking over Europe are identified as the most persistent types and are associated with large amplitude anomalies in temperature and precipitation. Finally, the high-latitude cyclonic events seem to correlate well with low-frequency modes of variability over the Pacific and Atlantic Oceans.