We consider upwelling and downwelling dynamics in an idealized ocean model configuration of the Western Gotland Basin in the Baltic Sea, featuring a gently sloping bottom in the west and a steep bathymetry in the east. Typical transient wind conditions and seasonally variable stratification are examined. Upwelling and downwelling jets develop at the coastal boundaries and interact through cross-shore boundary-layer flows. Initial evolution of the coastal jets is consistent with linear theory. The front position and the onset of instability is governed by the wind forcing, with a weak dependence on seasonal stratification. The unstable growth rates and wavelengths over the slope depend on the relative orientation of the slope and isopycnals, consistent with theory. The upwelling jets become baroclinically unstable during the wind-forced phase, whereas instability onset for downwelling on the slope is after 2–3 weeks (during the relaxation phase). The downwelling on the steep side is consistently stable. The regime with unstable upwelling on the slope side with concurrent stable downwelling on the steep side is more frequent (southwesterly winds: 30% occurrence) and leads to strong cross-shore transport. Unstable downwelling on the slope with upwelling on the steep side is a rarer event (northwesterly winds: 10% occurrence) and generates strong vertical mixing on the slope, with implications for oxygen and nutrient fluxes on the inner shelf along the Swedish coast. Baroclinic eddies contribute to elevated vertical mixing in the surface layer.

We present an analysis of coastal hazards associated with the passage of an extreme storm named Hans in the Baltic Sea in August 2023. The storm resulted in disturbance of drinking water production at the desalination plants on Gotland, the largest island of the Baltic Sea. The limited ground water resources combined with increased demand during the warm tourist season lead to recurring seasonal water stress on Gotland. Thus, drinking water production through desalination of sea water is needed to complement the municipal water supply. The storm Hans triggered extreme water and organic material transport to the intake stations of the desalination plants clogging the filters, and coinciding with cold sea temperature spells, that collectively disturbed the water treatment process. We analyze the ocean-dynamical drivers of these coastal hazards and their impacts, and present a pathway toward a tailored forecast system to the desalination plants combining available observations, operational ocean model outputs, and statistical models. The linear coastal response to southwesterly winds of storm Hans was found to be the primary driver, either directly or indirectly, of the coastal hazards impacts. The linear regression models building upon this finding show a potential for future development of forecast framework to inform the water management on Gotland, if continued reporting and observational efforts can be secured.

The Baltic Sea is a semi-enclosed, salinity-stratified sea featuring hypoxic and anoxic bottom areas that have been expanding due to the anthropogenic pressures and are predicted to continue to do so under the regional climate change. Recent observations from a coastal site on the Swedish east coast  reported on the occurrence of both, hypoxic pulses and reoxygenation events, with hypothesized driver being the transient wind-driven ocean circulation. This study gives a regional context to these observations by analyzing 115 upwelling events and 53 downwelling events and their impacts on the bottom salinity and bottom oxygen in coastal areas during 2010-2024 using sea state reanalysis products. The composites reveal that the circulation associated with coastal upwelling and downwelling along the Swedish east coast is largely consistent with the theoretical expectation and idealized simulations, featuring geostrophically balanced coastal jets and return flows confined to the bottom boundary layer with expected consequences for bottom salinity and oxygen. The bottom oxygen is anticorrelated with the bottom salinity, consistent with the hypothesis that the wind-driven circulation is a primary driver of the coastal oxygen variability, at least on time and space scales resolved by the reanalysis. The oxygen impacts of upwelling and downwelling are more pronounced during autumn when the bottom oxygen is at the minimum after the summer bloom. The complex coastal bathymetry modulates the basin-scale bottom oxygen impacts. The circulation and oxygen response to upwelling and downwelling favorable winds on the Western Gotland coast is reversed due to the basin scale recirculation driven by the bathymetry.