Life cycle dynamics of Greenland blocking from a potential vorticity perspective
Abstract. Blocking over Greenland has substantial impacts on surface weather in particular over Europe and North America, and can increase melting of the Greenland Ice Sheet. Climate models notoriously underestimate the frequency of blocking over Greenland in historical periods, but the reasons for this are not entirely clear, as we are still lacking a full dynamical understanding of Greenland blocking from formation through maintenance to decay. This study investigates the dynamics of blocking life cycles over Greenland based on ERA5 reanalysis data from 1979–2021. A year-round weather regime definition allows us to identify Greenland blocking as consistent life cycles with an objective onset, maximum, and decay stage. By applying a new quasi-Lagrangian potential vorticity (PV) perspective, following the negative, upper-tropospheric PV anomalies (PVAs-) associated with the block, we examine and quantify the contribution from different physical processes, including dry and moist dynamics, to the evolution of the PVA- amplitude.
We find that PVAs- linked to blocking do not form locally over Greenland but propagate into the region along two distinct pathways (termed "upstream" and" retrogression") during the days before the onset. Remarkably, the development of PVAs- differs more between the pathways than between seasons. Moist processes play a key role in the amplification of PVAs- before the onset and are linked to midlatitude warm conveyor belts. Interestingly, we find moist processes supporting the westward propagation of retrograding PVAs- from Europe, too, previously thought to be a process dominated by dry-barotropic Rossby wave propagation. After onset, moist processes remain the main contribution to PVA- amplification and maintenance. However, moist processes weaken markedly after the maximum stage and dry processes, i.e. barotropic, non-linear wave dynamics, dominate the decay of the PVAs- accompanied by a general decrease in blocking area. Our results corroborate the importance of moist processes in the formation and maintenance of Greenland blocking, and suggest that a correct representation of moist processes might help reducing forecast errors linked to blocking in numerical weather prediction models and blocking biases in climate models.
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