Interdecadal variability in the salinity of the Baltic Sea is dominated by a 30-year cycle with a peak-to-peak amplitude of around 0.4 g kg⁻¹ at the surface. Such changes may have substantial consequences for the ecosystem, since species are adapted to a suitable salinity range and may experience habitat shifts. It is therefore important to understand the drivers of such changes. We use both analysis of empirical data and a numerical model reconstruction for the period of 1850–2008 to explain these interdecadal changes. The model explains 93 % and 52 % of the variance in the observed interdecadal salinity changes at the surface and the bottom, respectively, at an oceanographic station at Gotland Deep. It is known that the 30-year periodicity coincides with a variability in river runoff. Periods of enhanced runoff are followed by lower salinities. We demonstrate, however, that the drop in mean salinity cannot be understood as a simple dilution of the Baltic Sea water by freshwater. Rather, the 30-year periodicity in river runoff occurs synchronously with a substantial variation in salt water import across Darss Sill. Fewer strong inflow events occur in periods of enhanced river runoff. This reduction in the import of high-salinity water is the main reason for the freshening of the water below the permanent halocline. In the bottom waters, the variation in salinity is larger than at the surface. As a consequence, the surface layer salinity variation is caused by a combination of both effects: a direct dilution by river water and a reduced upward diffusion of salt as a consequence of reduced inflow activity. Our findings suggest that the direct dilution effect is responsible for 27 % of the salinity variations only. It remains unclear whether the covariation in river runoff and inflow activity are only a coincidental correlation during the historical period or whether a mechanistic link exists between the two quantities, e.g. whether both are caused by the same atmospheric patterns.

中文翻译：

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在1850-2008年的后预报模拟中研究波罗的海年代际盐度变化

波罗的海盐度的年代际变化主要由30年周期决定，其峰峰值振幅约为0.4  g kg ^-1在表面。这种变化可能会对生态系统产生重大影响，因为物种适应了适当的盐度范围，并且可能经历生境变化。因此，了解这种变化的驱动因素很重要。我们使用经验数据分析和1850-2008年期间的数值模型重构来解释这些年代际变化。该模型解释了在哥特兰深部海洋站观测到的年代际盐度变化的93％和52％的方差。众所周知，这30年的周期与河流径流量的变化相吻合。径流增加的时期之后是较低的盐度。但是，我们证明平均盐度的下降不能理解为淡水对波罗的海水的简单稀释。相反，河流径流的30年周期与整个Darss Sill咸水进口量的巨大变化同步发生。在径流增加的时期，较少的强流入事件发生。高盐度水进口量的减少是永久性盐卤下水质清新的主要原因。在底部水域，盐度的变化大于在地表的变化。结果，表层盐度的变化是由以下两种作用共同引起的：河水的直接稀释和由于入流活动的减少而使盐的向上扩散减少。我们的发现表明，直接稀释作用仅占盐度变化的27％。