Atmospheric forcing of an extreme Baltic Sea-level depression and post-minimum exchange in winter 2025/2026

Abstract

Introduction During winter 2025/2026, the Baltic Sea experienced a pronounced basin-wide sea-level depression, with regional sea surface height (SSH) anomalies reaching approximately −60 cm or lower. We examine its atmospheric forcing, associated volume changes and subsequent hydrographic conditions. Methods We combine Copernicus Marine SSH, ERA5 atmospheric fields, Landsort/Landsort–Norra observations, modelled net transport across a Kattegat section, and CTD, Argo and glider measurements. Historical analyses assess the rarity of the atmospheric forcing and sea-level response. Results The minimum on 7 February 2026 followed persistent high pressure over the Baltic, easterly winds over the southern Baltic and North Sea, and an outflow-favourable Baltic–North Sea pressure difference. The Baltic–North Sea pressure-gradient anomaly peaked approximately five days before the Landsort–Norra sea-level minimum. The SSH-derived volume anomaly reached approximately −290 km³ relative to 1 November–15 December 2025 and increased to approximately −61 km³ by 6 April, a recovery of 229 km³, or 76% of the preceding 302 km³ decrease. The North Sea and southern Baltic easterly wind components ranked first among 87 winters, while the compound forcing index ranked second among 78 valid winters. The Landsort/Landsort–Norra sea-level minimum ranked fourth among 139 winter seasons. April CTD sections show relatively saline and oxygenated lower-layer water in the Bornholm Basin and Słupsk Furrow, while oxygen-depleted deep water persisted in the Gdańsk Basin. Glider observations show marked but spatially heterogeneous lower-layer oxygenation from late April into May. Discussion Transport-derived volume change broadly followed the SSH-derived evolution, supporting a substantial contribution from exchange. Differences in domain, atmospheric corrections and unquantified freshwater terms prevent quantitative budget closure. Hydrographic observations are consistent with inflow-related renewal but do not uniquely attribute all features to one post-minimum event. The preceding depression is interpreted as a possible preconditioning factor; volume recovery alone does not determine saline-water import or Major Baltic Inflow classification.