  Several recent studies have linked the exceptional North Atlantic and
Eurasian atmospheric evolution during late February and March 2018 to the
Sudden Stratospheric Warming (SSW) that took place a few weeks earlier.
February 2018 was characterized by an abrupt transition from the positive to
the negative phase of the North Atlantic Oscillation (NAO) and a subsequent
persistence of the negative NAO for several weeks. This paper investigates the
contribution of atmospheric and oceanic phenomena to both the 2018 event and a
set of 19 identified analogues (including the former) for the period 1959-2022.
Evidence is given that La Ni\~na conditions in the tropical Pacific and
upstream North Atlantic cyclones play an important role as a trigger for these
events. Ensuing two-way tropospheric-stratospheric coupling and eddy feedbacks
provide extended-range persistence for negative NAO conditions. These results
may help improve the prediction of such exceptional events.
