Atlantico Missing: aprile...

[Blizzard]

ps: aspetto la carta sulla ripresa di zonalità nella seconda parte del mese

Secondo ECMWF siamo di fronte ad un PAM (Polar Annular Mode) negativo. Il PAM (a differenza del NAM) risulta essere il "leading pattern" in caso di SFW.

In particolare:

In the stratosphere each winter season concludes with a rather abrupt transition from
circumpolar westerly winds to easterlies. This annual breakdown of the polar vortex is
known as the stratospheric final warming [SFW]. A recent observational study of SFW
events found that these events strongly organize the large-scale circulation of the
stratosphere and troposphere [Black et al., 2006]. Specifically, SFW events are associated
with a vertically coherent north-south dipole pattern in the zonal wind anomaly field at mid
to high latitudes. However, this pattern is distinct from the canonical NAM structure as the
primary centers in the north-south anomaly dipole are retracted northward compared to the
NAM. The results of Black et al. [BMR - 2006] indicate that SFW events may be associated
with distinct and previously unrecognized annular modes of variability at high latitudes.

Although negative NAM events develop more rapidly, they remain below -0.5F until
30 days after onset. In contrast to NAM events, PAM events develop and decay relatively
quickly, with index magnitudes remaining above 0.5F for less than two weeks. Thus, a key
result is that the characteristic time scale for PAM events is on the order of 1-2 weeks, which
is considerably shorter than that of the stratospheric NAM.

Both the NAM
and PAM indices exhibit positive tendencies after SSW onset, during which time the
stratospheric polar vortex recovers in strength. In contrast to SSW events, both NAM and
PAM play approximately equal roles in the development, onset, and decay of SFW events.
In particular, we note that SFW events have a precursor evolution qualitatively mimicking
the behavior of the simultaneous positive NAM and PAM events in Fig. 2c. It is evident that
the newly identified PAM pattern plays a first order role in SFW events, consistent with the
observation by BMR that SFW events have important distinctions from the [stratospheric
or tropospheric] NAM.

Confronto: