Upwelling along the Swedish west coast during the 20th century

A time series of geostrophic winds from 1892 until present is used to force a simple upwelling model. The model is tuned against a long time series (1930–1989) of daily hydrographic observations at Bornö station inside Gullmar fjord at the Swedish west coast. It is shown that a large part of the short-term variability at Bornö is controlled by upwelling events that can be predicted from the model. Statistical properties of upwelling events for the entire forcing period are presented as well as time series of upwelling and downwelling. Distinctive feature of the upwelling in the time series are: a declining trend between the late 1890s and 1920, an increasing trend between 1920 and 1940s and another declining trend between 1950s and 1990. After 1990 no trend is obvious. Another feature is that upwelling during summer (May–September) is much less intense after 1963. The up/downwelling is related in a qualitative way to the North Atlantic oscillation index during January–April but there is no correlation during summer. Some possible effects on the coastal marine environment due to long-term changes of the upwelling intensity are discussed.