Nordseezustand 2004
21
Physical Oceanography
4.0 m/s from W (261°), which hardly differed from the climatological vector wind (3.7
m/s from WSW, 256°).
The annual cycle of global radiation on the island of Norderney was generally in line
with climatological data. Minor surplus radiation in March, April and September was
attributed to high-pressure weather. The seasonal evolution of air temperature on Nor
derney was normal as well. Major positive departures occurred in April and August due
to continental warm air advection from SE. At 16.7 °C, the mean temperature in sum
mer (JJA) would appear cool in comparison to the »mediterranean« extremes of pre
ceding years (2002: 17.9 °C; 2003: 18.3 °C), but in fact was close to normal, statisti
cally.
Physical Oceanography
The atmospheric circulation in the North Sea region imprints itself on patterns and in
tensities of surface currents. The characteristic current pattern is large scale cyclonic
(counterclockwise) rotation due to prevailing winds from SW to NW. In 2004, this pat
tern was not restricted to winter and fall, but was a prominent feature of the summer
circulation as well. Stronger wind forcing than in the preceding year led to enhanced
current speeds and a reduced directional variability. Unusually frequent NW winds re
sulted in a strong northerly inflow of saline Atlantic water, while net transports through
the Channel and the Kattegat, at 0.015 and 0.044 Sv/a, respectively, were in the nor
mal range.
The sequence of daily current patterns in the German Bight is documented in the cur
rent calendar. In 2004 (2003), cyclonic circulation was recorded on 160 (150) days, an-
ticyclonic states on 71 (50) days, and variable circulation patterns on 77 (109) days.
The reduced frequency of variable (diffuse) circulation states resulted from a sharp fall
in high-pressure weather types, usually associated with weak winds. The increase in
anticyclonic current patterns, on the other hand, corresponded to an enhanced fre
quency of NW weather types. Other close associations between current and weather
calendar were identified. SW weather types, for instance, almost exclusively generate
cyclonic current states. Both calendars are important tools in the interpretation of
anomalous events in dependent state variables.
Waves are wind-induced perturbations of the sea surface. The wave climate in 2004
was characterized by seasonal geographic distributions of significant wave height and
direction of wind sea and swell. In winter, summer, and fall, mean wave heights were
up to 0.5 m higher than in the same seasons of the preceding year, when weak winds
prevailed. Nonetheless did mean and maximum wave heights at representative loca
tions fall short of climatological reference values. Both, wind and wind sea, on average
came from westerly directions, except in spring. Swell was directed NW in all seasons.
During the NW storm >Pia< of November 18, significant wave heights of about 8 m oc
curred in the central and southern North Sea.
Regional differences in wind forcing and wind sea were pointed out for two represen
tative positions in the German Bight and the central North Sea. In the German Bight,
wind and wave direction were found to have bimodal distributions (double peak); a dis
tinct secondary peak in the E-SE sector was observed only here. The main peak at
either position varied seasonally between SW and NW and was associated with high
est wind speeds and wind seas. The mean wave height, at 1.4 m, was 0.5 m lower than