https://doi.io-warnemuende.de/10.12754/msr-2012-0088
doi:10.12754/msr-2012-0088
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Spatiotemporal Scales of the Deep Circulation in the Eastern Gotland Basin / Baltic Sea
Abstract. The Baltic Sea is surrounded by land, thus exchanges with the open ocean only take place through the North Sea. The Baltic Sea is divided into different deep basins connected by narrow sills and channels. Compared to the open ocean and the North Sea the salinity in the Baltic Sea is generally low due to large amounts of fresh water provided by river discharges. In flowing saline water from the North Sea travels along the bottom and therefore produces a permanent halocline, separating the surface water from the deep water in the basins. Saline and also often oxygen-rich inflows are essential for the deep water renewal in the largest basin of the Baltic Sea, the Eastern Gotland Basin (EGB). These inflows occur only under certain meteorological conditions and thus so-called stagnation periods (periods without inflows) can occur for several years, oxygen depletion can lead to the formation of hydrogen sulfide in the Baltic deep water. In this work two different inflows and their effects on the deep water in the Baltic Proper were investigated. First, the major warm and saline deep inflow into the Eastern Gotland Basin, lasting from the end of November 1997 until the beginning of May 1998, was investigated. Temporal fluctuations of the deep circulation were monitored at 170 m depth by two current meter moorings deployed at the north-eastern and south-western rim of the EGB between August 1997 - September 1998. Three-dimensional structures of stratification parameters were inferred from comparisons of two high-resolution hydrographic surveys carried out before and during the end of the 1997/1998 inflow. Results indicated a lifting of near-bottom isopycnals by more than 50 m during the inflow up to a depth of 170 m. Since the basin is enclosed at 170 m depth this implies a complete deep water renewal. Detected warm-water intrusions enhanced the temperature variability on isopycnal surfaces at intermediate depth, creating strong inverse temperature gradients below the halocline. Corresponding Turner angles suggest a significant contribution of diffusive convection to diapycnal mixing in this region. In contrast, for the deepest near-bottom layers which were only marginally affected by intrusions, basin-scale budgets of heat and salinity suggest comparable diffusivities ([...]), implying that double-diffusion does not play a major role in mixing there. For intermediate layers between 90-170 m the basin is not closed, thus the budget method could not be adopted. Instead an idealised model for the decay of temperature variance associated with the intrusions was applied. Results indicate that the intrusions decay within a few weeks, corresponding to vertical diffusivities of a few times 10−5 m2/s. Secondly, a cold, oxygen-rich inflow in intermediate waters was captured in May 2006 in the Stolpe Channel and later in September of the same year in the EGB. Temporal means of the zonal velocity of two ADCPs, deployed between September and December 2006 at the outlet of the Stolpe Channel, reveal a separation of the water column, where the eastward fl ow at the northern side dominates depths between 65 - 71 m and depths of 52 - 71 m at the southern side of the channel. The layer above on the other hand shows a westward flow. The deep, mainly eastward flow exhibits fluctuations of about 2 - 4 days. Hourly and daily volume transports estimated from ADCP measurements for this period agree well with modelled volume transports from the high spatial resolution (1 nautical mile) MOM4 model. All signi cant peaks in estimated transports were also captured by modelled volume transports. The mean volume transport through the Stolpe Channel for estimated volumes is 0.75 ± 2.32 km3/d and slightly higher for modelled volumes 0.81 ± 3.15 km3/d, both are well correlated with R=0.79. In the Stolpe Channel the following mechanisms can be derived from correlations. The sea level is directly steered by the regional wind and surface waters in the same direction as the wind. The deep current is mainly steered by changes in the sea level and is counteracting to the surface flow, i.e. during winds from the west the deep currents travel towards west and during winds from the east currents travel eastwards. Although, in times of weak westerly winds deep eastward currents still prevail, but then originate from a geostrophic flow. Hence, observed pulse-like currents in the Stolpe Channel are partly driven by the regional wind, i.e. up to 50 % and partly driven by sloping density gradients of dense bottom waters and therefore resulting geostrophic currents. Comparisons of the wind's frequency spectrum with the frequency spectra of the along-slope current of the two ADCPs reveal that the 2 - 4 day long current fluctuations originate from changes in the regional wind. Each of these fluctuations transports a volume of around 1.78 ± 1.15 km3/d eastwards. These 2 - 4 day long fluctuations transport nearly the same volume in a single event as the 3-monthly mean.
Citation
Gunda Wieczorek: Spatiotemporal Scales of the Deep Circulation in the Eastern Gotland Basin / Baltic Sea. Meereswiss. Ber., Warnemünde, 88 (2012), doi:10.12754/msr-2012-0088