Vesivirrat

Vesivirrat, or water currents, are organized flows of water that move in a preferred direction over a region. The term is used in Finnish hydrology and oceanography to describe the movement of water masses in oceans, seas, rivers and estuaries, as well as smaller-scale flows within channels and lakes. They arise from a combination of wind, density differences, tides, gravity and the shape of coastlines and seabeds.

The dynamics of vesivirrat are driven by several processes. Wind stress on the surface generates surface currents through friction and the Coriolis effect, which deflects moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Temperature and salinity differences create density-driven (thermohaline) currents that can sink or rise, contributing to deep-ocean circulation. Tidal forces, freshwater input from rivers, and submarine topography further shape current patterns. In rivers and estuaries, river plumes produce distinct currents that mix with coastal waters.

Major categories include surface currents, governed primarily by wind and Ekman transport; and deep or thermohaline currents, driven by density contrasts and gravity. Localized phenomena such as turbidity currents, coastal upwelling, and river outflows also form important vesivirrat in particular regions.

Measurement and modelling rely on satellites, drifting buoys, floats, and coastal instruments, combined with numerical models to simulate circulation patterns. Vesivirrat influence climate, nutrient transport, marine ecosystems, and navigation, making them a central concept in physical oceanography and hydrology. Notable examples include the Gulf Stream, the Kuroshio Current, and the Antarctic Circumpolar Current.