Ytströmmarna

Ytströmmarna, or the surface currents, denote the horizontal flow of seawater within the ocean's upper layer. They are primarily driven by wind stress from the atmosphere and modified by Earth's rotation (Coriolis effect), sea-floor topography, and density stratification. The surface layer is well mixed by wind, and the resulting flows extend from tens to a few hundred meters depth depending on stratification and season.

The dynamics of Ytströmmarna include Ekman transport, which deflects surface flow at an angle to the wind, and geostrophic balance, which causes currents to follow contours of constant sea surface height. This combination leads to large-scale gyres, western boundary currents (for example, Gulf Stream and Kuroshio) and numerous smaller coastal and equatorial currents. They transport heat, nutrients, and momentum, influencing regional climate and marine ecosystems.

Measurement and study of Ytströmmarna rely on satellite radar altimetry, drifting buoys and floats, and coastal high-frequency radar. Contemporary models integrate these observations to forecast weather, climate, and ocean conditions. The term is used in Swedish oceanography to refer to surface currents in general and is central to understanding ocean circulation patterns and their role in global climate.