Atmospheric Physics 
System Nordsee 
25 
pendence of the future on the present - form notable parallels to numerical weather 
forecasting from the laws of physics. 
The strong inclination of weather types to stay alive makes persistence forecasts (to- 
morrow-as-today) with success probabilities from 39 (NENE) to 59% (AA) the best op 
tion. Disregarding stagnation periods of about 2 days on average, the most probable 
sequence of weather types under arbitrary initial states (except NE & SE) is the peri 
odic succession of A-SW-C-NW-A, which describes a typical-of-the-region passage of 
a low pressure system preceded and followed by interim highs. As with their cousins 
(SW & NW), the rare weather types SE and NE are most likely embedded in sequenc 
es A-SE-C and C-NE-A, respectively. The two-way transitions CoA, NEoSW, and 
NWoSE cannot be realized from one day to the next, or rarely so. 
Compared to climatology, directional weather types NW/SW occurred noticeably 
less/more frequently in 2006. Primarily, these departures arose from less/more fre 
quent self-transitions, more specifically, from the rare (1)/frequent (7) occurrence of 
long-lived episodes (> 5 days). Virtually inverse conditions as to NW and SW were 
a notable characteristic of 2007. In addition, NE types were more abundant for the 
same reason, while SE types dropped by half (vis-à-vis 2006) overall, as well as to 
self-transitions and proper transitions to other weather types. As noted earlier, the total 
number of weather type episodes - equivalent of the total frequency of proper transi 
tions - was subject to minor fluctuations, only. Apparently, this finding widely applies 
to individual numbers of episodes as well, such that anomalies in total occurrence of 
individual types primarily reflect anomalies in the occurrence of long-lived episodes. 
In contrast to 2006, the periodic cycle in 2007 was limited to the sequence A-SW- 
NW-A. Moreover, this cycle was less stable, since transitions ASW and ANW were 
about equally frequent, which also holds for SWNW and SWC transitions. The causes 
thereof were explained on the basis of reduced weather-type calendars. 
Atmospheric pressure distribution (p. 68sqs.) 
In signifying geostrophic vector winds, mean sea level pressure (MSLP) distributions 
represent the large-scale atmospheric circulation in the greater North Sea region. 
The general climatological cycle is characterized by intense SW-ly flow through fall 
and winter, an attenuated flow in spring, turning NW in early summer, finally backing 
W and intensifying when blending into its cold-season mode. This course of change 
reflects the seasonally varying dominance of the Islandic low and the Azores high and 
is coupled to a meridional migration of the frontal zone on the order of 10 degrees. The 
shifts go along with the weakening of the Icelandic low and the NE-ward expansion 
of the Azores high in the course of spring and, conversely, the re-strengthening of the 
subpolar low and the retraction of the Azores high to subtropical latitudes towards the 
end of summer. 
Using the full weather-typing scheme, monthly and seasonal pressure patterns in 
2006 and 2007 were classified in the same fashion as daily MSLP distributions and 
compared to corresponding climatologies for the period 1971 -2000. Unlike synoptic 
distributions, temporal means do not represent weather types but compound flow con 
ditions, the anomalies of which may be conceived as manifestations of the chaotic- 
dynamic evolution of a memoryless atmosphere. As anomalies on these time scales 
embody uncorrelated or disjointed states, they can be little more than documented. 
Seasonal MSLP distributions are frequently composed of complementary shorter- 
term circulation anomalies, which tend to cancel out one another on this time scale. In