Climatological Investigations of Atmospheric Blocking

Croci-Maspoli Mischa, Diss ETH No. 16151

Abstract

In recent years, there has been an abundance of research regarding the dynamics of atmospheric blocking. Even with this concerted effort, however, there still remains a fundamental lack of understanding regarding their formation, maintenance and breakdown mechanisms. A major complicating factor when it comes to evaluating different proposed blocking mechanisms, is the large observed variation in the length of the life cycle of an atmospheric block, ranging from several days up to two weeks. This time range spans the bridge between synoptic and short-term climate processes and is of particular interest for monthly to seasonal weather forecasting.

An atmospheric block in the extratropics takes the form of a vertically coherent, quasi-stationary, synoptic-scale high-pressure system. Its presence is associated with major modifications of the prevailing westerly flow as well as the in-situ temperature and precipitation distributions. Sustained and large-scale blocking episodes have been shown to play an integral role in several historical cases of extreme weather and short-term climate events.

In this thesis, atmospheric blocking is examined from a dynamical standpoint: the entire life cycle of individual blocks are investigated, beginning with genesis and ending with lysis. This event-based approach allows for a deeper understanding of the dynamical mechanisms that are crucial for blocking formation, maintenance and decay. The analysis technique is applied to the ERA-40 reanalysis data set over the Northern Hemisphere from 1958 to 2002, making extensive use of the potential vorticity (PV) perspective.

In this work, a novel dynamical blocking indicator is introduced: one that is based upon the vertical averaging of PV in the tropopause region. This procedure is motivated by the pronounced negative PV anomaly that is observed beneath the elevated dynamical tropopause during a blocking event. The two-dimensional blocking indicator identifies these strong negative PV anomalies and tracks them for temporal persistence. This methodology is used to derive a seasonal long-term blocking climatology.

The results indicate two preferred regions of enhanced atmospheric blocking occurrence: one in the Euro-Atlantic and the other in the Pacific basin. Blocking in these two ocean basins is subsequently investigated in relation to large-scale climate modes such as the North Atlantic Oscillation (NAO) and the Pacific North American (PNA) pattern. A statistical composite analysis shows that strong changes in the blocking distribution occur during opposed NAO/PNA phases. In particular the Euro-Atlantic region exhibits significantly more blocking episodes over the northwestern Atlantic during NAO-. In contrast European blocking occurrence is significantly increased during NAO+. These results are robust for both monthly and seasonal based NAO indices. The dynamical approach investigates the relationship between the life time of individual blocking events and the corresponding pattern (NAO/PNA) index value, treating the two ocean basins separately. It is shown that long-lasting blocking events in the Euro-Atlantic sector play a significant role in the formation of the negative NAO phase. The situation is considerably different for Pacific blockings, where the negative PNA phase is a necessary precondition for the formation of blocking.

The ERA-40 data set, due to its coherence and length, is well suited for a blocking trend analysis, a topic of particular interest with respect to recent climate change. The results of such an analysis indicate that southwestern Greenland exhibits a significant negative trend (towards less blocking days) of wintertime blocking. It is demonstrated that this trend can be accompanied by a shift of the blocking pattern from western Greenland towards Iceland. The trend pattern changes dramatically during the spring season, with an overall positive blocking trend over the North Atlantic.

Blocking episodes are further examined as to their occurrence, either in the Euro-Atlantic and Pacific separately or both simultaneously. Simultaneous blocking accounts for approximately 13% of wintertime blocking, and more than 25% in a single ocean basin respectively. In addition synoptic scale positive PV anomalies in the vicinity of the blocking center are further examined during the blocking genesis, maturity and lysis phase. They can be related to the proposed eddy feedback processes for blocking formation. It is shown that these positive PV anomalies form predominantly in the
Euro-Atlantic sector during all blocking phases. Little to no difference in positive PV anomaly occurrence between only Atlantic/Pacific and simultaneous blocking can be observed.