Regime shifts in the surface-level average air flow over the Gulf of Finland during 1981–2010; pp. 428–437Full article in PDF format | doi: 10.3176/proc.2014.4.08
Abrupt changes in large-scale wind patterns are often masked by local features and are not visible in classical properties of surface-level winds. We explore the potential of the average air flow to reveal such changes from wind data, recorded in the region of the Gulf of Finland during 1981–2010. The monthly average air flow speed is very small in April and does not exceed 2.8 m/s during the rest of the year. The monthly mean wind directional persistency factor has a similar annual course. The direction of the average air flow does not always coincide with the direction of the most frequent winds. In summer, autumn, and winter the air flow direction is relatively stable from SW or WSW to NE or ENE over the whole gulf whereas in spring the direction varies at different measurement sites. Time series analysis by means of the Rodionov regime shift detection technique reveals that changes in the wind speed are mainly caused by changes in the measurement conditions, first of all relocation of the measurement site. Significant shifts in the air flow speed are not related to the changes in the wind speed. The changes are concentrated in the windy months. A significant increase in the air flow speed occurred in January 1988 and a comparable decrease in January 1994–1996 at all measurement sites except for Kotka. In October an abrupt decrease occurred in 1988–1989 at all stations except for Hanko. The identified shifts may be associated with a major change in the geostrophic air flow over the southern Baltic Sea in 1988 and a relaxation of the system back to the pre-1980s situation after a few years. These events do not become evident in the average wind speed.
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