Compilation of Summaries
26
System Nordsee
As annual and long-term temperature developments in the German Bight largely parallel
those in the North Sea, this region has a similar ranking.
Temperature stratification in the North Sea in August 2005 was characterised by an
unusually deep-reaching isothermal surface layer, which was unlike any other vertical
temperature distribution observed in summer from 1999 to 2008. In particular, temperatures
close to the surface (< 20 m) were up to 2 К below the mean values of the reference
period, while those in the lower part of the surface layer (20 - 40 m) were up
to 4 К higher than the reference means. The observed deep vertical mixing has been
attributed to highly variable weather from mid-July to mid-August, and, specifically, to
2 near-storms which led to the unseasonable disintegration of thermal stratification in
the shallow German Bight at the end of July. The low-pressure weather in midsummer,
combined with high insolation under anticyclonic conditions in early and late summer,
caused the North Sea SST to stagnate at a plateau of 15 °C from July to September.
In the German Bight, the September temperature even exceeded that of August, which
has been unprecedented since the beginning of temperature analyses in 1968.
Sudden temperature jumps on all time scales are among the complex behaviour of the
non-linear climate system. Regime shifts on infra- to interdecadal time scales are characteristic
of the temperature climate of the North Sea. The onset of the current warm
regime in 1987/88 was marked by a temperature jump of 0.8 K. Following another sudden
temperature rise of 0.5 К in 2001/02, an extreme level of 10.8 °C has been
reached which is 1.5 К above that of the cold regime of the 1960s. Temperature regimes
in the past used to be coupled to monsoon-like wind regimes and the seasonally
alternating temperature contrast between the North Atlantic and the European mainland.
A characteristic feature of winter (summer) seasons during the cold regime from
1977 to 1987 was continental (maritime) cold-air advection from eastern (western) directions.
The temperature jump of 1987/88 was associated with a reversal of the wind
regime and conjoined transports of maritime (continental) warm air from westerly
(easterly) directions in winter (summer). The main factor governing the persistence of
temperature regimes used to be wind conditions in winter. By contrast, the current extreme
warm regime is sustained by advective heat transports and enhanced insolation
in summer.
Interestingly, there have been synchronous, sudden shifts in the wintertime atmospheric
circulation in the Northern Hemisphere (Watanabe and Nitta 1999). In particular,
simultaneous, sometimes inverse regime shifts in ocean temperature have been
observed in the North Pacific. As with the North Sea (Weijerman 2005), these shifts
have caused dramatic changes in the marine ecosystem (Hare and Mantua 2000).
The simultaneous, quasi-global occurrence of these phenomena is indicative of the interplay
of dominant dynamic modes of the climate system, such as ENSO, PNA, PDO
and NAO.
The dependence of winter and annual North Sea SSTs on the winter NAO index,
which showed in high correlations during 1969- 1996, has become inoperative and
void thereafter. Highest temperatures which in earlier years had occurred exclusively
with strongly positive NAO modes have even been exceeded since the regime shift of
2001/02, although winter NAO since 1997 has been mostly in a neutral mode tending
to negative. Detailed analyses have shown that the NAO mechanism is still valid but
the NAO paradigm of NAO + (NAO - ) -> warm (cold) North Sea temperatures has to be
replaced by -> weak (strong) cooling in winter. The change of paradigm is a consequence
of extreme heat anomalies, which originate in summer and may persist until
