Sum mary
16
Nordseezustand 2003
stantial differences in directional wind forcing in fall 2002 (SE) and 2003 (W) caused
notable departures in corresponding seasonal wave height distributions. While mean
significant wave heights > 2 m occurred as far south as 54° N in the stormy winter of
2002, such wave heights were restricted to sea areas north of 56° N during the winter
season of 2003. As in 2002, local wind and windsea distributions from the German
Bight showed 2 modes in the sectors NW-SW and E-SE. This bimodal structure was
more pronounced closer to the coast and during the cold seasons of the year. High
and extreme wind speeds were associated with NW and SW winds, while largest wave
heights in excess of 8 m came from NW on account of the larger fetch.
Monthly high-water levels at Cuxhaven did not show notable deviations from the climatological
annual cycle. The only storm surge occurred Dec, 21 2003 together with
the year's most violent storm.
The annual mean SST (as derived from BSH's weekly North Sea SST-analyses) fell
short of the record of 11.0 °C set in 2002 by 0.1 K. Significant positive departures from
the climatological cycle occurred from June through September and in December. The
warm regime, which in fall 1987 replaced a decade-long cold regime, has continued
(cf. AnhangQS. 195). The regime character of the North Sea SST apparently is maintained
through a hybrid wind climate, the properties of which undergo monsoon-like
seasonal shifts. An alternation between maritime winds in winter (summer) and continental
winds in summer (winter) was found typical of the recent warm regime (preceding
cold regime).
The heat content of the North Sea was estimated from model data. Annual means
were strongly associated with observational North Sea SST (r 2 = 0.87) during the period
1969 - 1993. This association also showed in parallel regime lengths and shifts.
Since about 1994, however, this relationship has been severely disturbed. At the same
time, the statistical dependence of both variables on the North Atlantic Oscillation began
to fade. The seasonal maximum in heat content occurs in September and thus
lags maximum SST by about one month. While heat content in August 2003 exceeded
that of the preceding year the converse was true for September.
The increased frequency of high-pressure weather in winter was untypical of the warm
regime. Associated cool air temperatures produced a sea-ice winter of moderate
strength along the coasts of the German Bight. A temporary disruption of the cold spell
lasted about three weeks from mid-January and was associated with an enhanced
storm frequency. As a consequence, the accumulated areal ice volume amounted to
no more than 0.53 m.
In 2003 large parts of the northern and central North Sea were filled with Atlantic water.
The fractional North Sea area with near-bottom salinities exceeding 35 increased
since the winter 2002 within about one year's time from 23 to 37 % and thus to highest
coverage since 1998. The strong inflow of Atlantic water during 2002 is consistent with
strong westerly winds and enhanced storm activity in the North Sea region in that
year's winter season. Runoff rates of Elbe river were below normal since April 2003.
This was caused in part by unusually low precipitation rates but foremost by persistently
below average precipitation during the dry-year 2003. The seasonal salinity cycle
at Helgoland Roads showed positive but statistically insignificant departures from
the climatological cycle almost throughout the year.
