A process-based understanding of Greenland Blocking regime life cycle dynamics in ERA-5 reanalysis from a potential vorticity perspective

Atmospheric blocking describes a flow configuration that is associated with the meridional reversal of the large-scale atmospheric circulation in the midlatitudes. Due to the blocking of the mean westerly flow and the disruption of the eastward propagation of embedded synoptic-scale weather systems, such situations can lead to many kinds of extreme weather - within the block itself but also in adjacent regions. One blocking type of particular interest is the Greenland Blocking as it is more long-lived than blocking over other regions and strongly related to the negative phase of the North Atlantic Oscillation. Greenland Blocking has a strong surface impact, as the associated temperature anomalies can lead to ice sheet melting. It is therefore crucial to understand the underlying processes and dynamics of blocking in this region.

Here we look at Greenland Blocking from a year-round weather regime perspective based on ERA-5 reanalysis data from 1979-2021 focusing on the dynamical life cycle of the block. A diagnostic, quasi-Lagrangian potential vorticity (PV) framework has been developed and is applied to all Greenland Blocking weather regime life cycles. The framework allows the tracking of negative, upper-tropospheric PV anomalies in the Northern Hemisphere and assigns them to regime life cycles. With piecewise PV inversion, we partition the PV tendency equation into different contributions – separated into advective and diabatic PV tendencies – to disentangle different physical and dynamical processes that affect the amplitude evolution of the negative PV anomalies. This complements the quasi-Lagrangian framework and enables us here to distinguish the roles of dry and moist dynamics in the life cycle of the Greenland blocking regime.

Our results show that most negative, upper-tropospheric PV anomalies that are associated with Greenland Blocking regime life cycles are not generated locally over Greenland. We present different pathways by which the PV anomalies reach the area of the incipient block and analyze their seasonal dependence. A detailed investigation of the individual pathways and their associated negative PV anomalies reveals that the amplitude evolution of the PV anomalies is characterized by different proportions of dry and moist processes. Seasonal differences between the occurrence of these pathways emphasize the complexity of the underlying processes.