Baltic Sea

3. A case study of thermal variability following deep
water intrusions in the Eastern Gotland Basin
3.1 The deep inow event
3.1.1 Mooring data
The deep warm-water inow was recorded at 170 m depth by the two temperature sensors
at the moorings NE and SW shown in Fig. 1.2. Resulting records illustrate that exceptional
warm deep water reached mooring NE on 28 November 1997 after a quiescent period with little
temporal variability (Fig. 3.1 a). This cyclonically travelling thermal signal was not observed
before 27 December 1997 at moorings SW, i.e. it was visible with a delay of approximately
30 days. Superimposed uctuations suggest that the warm-water inow was not continuous.
Both temperature records exhibit a total of eight large pulse-like events, each with an average
duration of about 20 days. Associated temperature changes were somewhat stronger at
position NE than at SW, reaching peak temperatures of 6.7 ◦ C on 13 March 1998, and 6.6 ◦ C
on 23 March 1998, respectively (Fig. 3.1 a). Following Lehmann and Hinrichsen (2000)
and assuming that such temperature anomalies travel in some geostrophically adjusted way
along isobaths between both positions, the observed time-lag suggest an overall propagation
speed of approximately 3 cm/s. This estimate is consistent with the averaged along-slope
inow velocities as discussed in the following.
Obtained daily current records from mooring NE illustrate that pre-inow velocities were directed
towards the northwest, while those of mooring SW mainly pointed to the south, cf.
Fig. 3.1 b and c and Table 3.1. At 170 m depth, both moorings also recorded an increase in
velocity during the inow period, just after the arrival of thermal fronts. Consequently, the
accelerated deep water circulation was observed with some delay after the temperature fronts
have passed the mooring positions. During the main inow period, the current directions ex-
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