Life cycle dynamics of Greenland blocking from a potential vorticity perspective

Blocking over Greenland stands out in comparison to blocking in other regions, as it favors accelerated Green-
land Ice Sheet melting and has substantial impacts on surface weather in adjacent regions, particularly in Europe and
North America. Climate models notoriously underestimate the frequency of blocking over Greenland in historical pe-
riods, 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−) as-
sociated 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 lo-
... mehr
cally over Greenland but propagate into the region along two distinct pathways (termed “upstream” and “retrogres-
sion”) during the days before the onset. 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 midlati-
tude 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. Af-
ter onset, moist processes remain the main contribution to PVA− amplification and maintenance. However, moist pro-
cesses weaken markedly after the maximum stage, and dry processes, i.e., barotropic, nonlinear wave dynamics, domi-
nate the decay of the PVAs− accompanied by a general decrease in blocking area. Our results corroborate the impor-
tance of moist processes in the formation and maintenance of Greenland blocking and suggest that a correct represen-
tation of moist processes might help reduce forecast errors linked to blocking in numerical weather prediction models
and blocking biases in climate models.