Observations of low‐frequency current fluctuations in deep water of the Eastern Gotland Basin/Baltic Sea

Abstract

In the deep Eastern Gotland Basin of the Baltic Sea, multiday fluctuations could be analyzed based on hourly sampled current and temperature measurements from August 31, 1999, to October 23, 2002. Data were continuously recorded in layers beneath the perennial pycnocline over a stagnation period of the hydrographic regime which lasted more than 3 years. At a water depth of 204 m, records were realized at 20 m above the seabed in the northeast of this basin (57°23′N, 20°19.6′E). Overall averages show vanishing east‐west motions, but a permanent northward velocity of 3 cm/s. The results obtained from daily means suggest that current fluctuations with periods longer than about 18 days are energetically decoupled from those of shorter timescales. Quasi‐cycles of 100–150 days characterize along‐slope currents while fluctuations typically occur in two period ranges of 40–65 and 25–37 days in both current components. Corresponding fluctuations also occur in sea level anomalies recorded at the coastal station Landsort/Sweden. Shorter timescales involve periods around 10 and 3.5 days at the synoptic scale. There is a positive correlation between daily values of the along‐slope current and corresponding variances with strengthening during the winter season. This suggests an enhanced flux of kinetic energy from the daily current field toward more turbulent fluctuations to maintain mixing processes via enhanced vertical current shear within thin near‐bottom layers. Results are discussed in the context of previous observations and existing working hypotheses.