198
K. Myrberg and T. Soomere
The winds in the region surrounding the Gulf of Finland are governed by the
large-scale circulation patterns over the whole Baltic Sea. This domain belongs to
the zone of westerlies (where typically a strong zonal circulation type exists with
cyclones moving to the east) controlled by the NAO 3 index. A positive value of the
NAO index (as for most of the winters in the recent past) favours the predominance
of westerly winds. During winters characterized by negative values of the NAO
index (like 2010, 2011 and 2012) the winds blow mostly from northerly and easterly
directions. In the summertime winds are weaker and the role of local features (like
the land-sea breeze) is larger.
The wind regime of the open part of the gulf reflects a superposition of the
most frequent south-western and north-north-western winds (predominant in the
entire Baltic Sea basin) and more local but also relatively frequent western and
eastern winds (east-north-eastern winds on the southern coast) blowing along the
gulf. On the northern coast, moderate (6–10 m/s) and strong winds (>10 m/s) come
mostly from south or south-west, but on the southern coast they blow more often
from south-west or west. South-eastern winds are infrequent and relatively weak
(Soomere and Keevallik 2003). The average wind speed has been reported to exhibit opposite trends on the opposite coasts (Keevallik and Soomere 2004), probably because of long-term changes in the land surface roughness in the vicinity of
some measurement sites. It may also be a reflection of the considerable changes in
the overall air flow patterns (Keevallik and Rajasalu 2001; Keevallik and Soomere
2008) (Fig. 6.11).
Measurements from vessels and at lighthouses reveal that wind speeds on the
open sea are considerable greater than at or near coasts (Niros et al. 2002). The
probability of strong storms (wind speeds >20 m/s) was dramatically greater in
the 1990s than previously. Wind speeds exceeded 10 m/s with a probability of 24 %
(13.6 % for the period 1977–1982) and 20 m/s with a probability of 0.2 % (0.008 %).
The mean wind speed during the most severe, long-lasting storms in the Gulf of
Finland is by 2–3 m/s less than in the open part of the Baltic Sea. Southern or southwestern winds are the strongest: the 3-hour mean wind speed may reach 24–25 m/s
once in a century. Eastern winds are confined to a narrow direction span and may
reach 23 m/s (Soomere and Keevallik 2003). Further estimates of extreme wind
properties in the eastern part of the gulf are presented in Lopatukhin et al. (2006a,
2006b).
The local winds have a relatively large role in the summertime when the largescale winds are relatively weak. The frequent, relatively strong afternoon surface
winds observed along the northern coast of the gulf in typical summer conditions
may reflect the interplay of the basic flow, sea breeze and the geometry of the gulf
(Savijärvi et al. 2005). The meanders of the sea breeze, amplified by the unidirectional basic flow, may become evident as relatively strong winds, at times located just above the opposite coast—the Finnish coast—for the predominant southwestern winds.
3 The North Atlantic oscillation (NAO) is a climatic phenomenon in the northern Atlantic Ocean.
It is characterized by the NAO index, which is the difference of atmospheric pressure at sea level
between the Icelandic low and the Azores high.
K. Myrberg and T. Soomere
The winds in the region surrounding the Gulf of Finland are governed by the
large-scale circulation patterns over the whole Baltic Sea. This domain belongs to
the zone of westerlies (where typically a strong zonal circulation type exists with
cyclones moving to the east) controlled by the NAO 3 index. A positive value of the
NAO index (as for most of the winters in the recent past) favours the predominance
of westerly winds. During winters characterized by negative values of the NAO
index (like 2010, 2011 and 2012) the winds blow mostly from northerly and easterly
directions. In the summertime winds are weaker and the role of local features (like
the land-sea breeze) is larger.
The wind regime of the open part of the gulf reflects a superposition of the
most frequent south-western and north-north-western winds (predominant in the
entire Baltic Sea basin) and more local but also relatively frequent western and
eastern winds (east-north-eastern winds on the southern coast) blowing along the
gulf. On the northern coast, moderate (6–10 m/s) and strong winds (>10 m/s) come
mostly from south or south-west, but on the southern coast they blow more often
from south-west or west. South-eastern winds are infrequent and relatively weak
(Soomere and Keevallik 2003). The average wind speed has been reported to exhibit opposite trends on the opposite coasts (Keevallik and Soomere 2004), probably because of long-term changes in the land surface roughness in the vicinity of
some measurement sites. It may also be a reflection of the considerable changes in
the overall air flow patterns (Keevallik and Rajasalu 2001; Keevallik and Soomere
2008) (Fig. 6.11).
Measurements from vessels and at lighthouses reveal that wind speeds on the
open sea are considerable greater than at or near coasts (Niros et al. 2002). The
probability of strong storms (wind speeds >20 m/s) was dramatically greater in
the 1990s than previously. Wind speeds exceeded 10 m/s with a probability of 24 %
(13.6 % for the period 1977–1982) and 20 m/s with a probability of 0.2 % (0.008 %).
The mean wind speed during the most severe, long-lasting storms in the Gulf of
Finland is by 2–3 m/s less than in the open part of the Baltic Sea. Southern or southwestern winds are the strongest: the 3-hour mean wind speed may reach 24–25 m/s
once in a century. Eastern winds are confined to a narrow direction span and may
reach 23 m/s (Soomere and Keevallik 2003). Further estimates of extreme wind
properties in the eastern part of the gulf are presented in Lopatukhin et al. (2006a,
2006b).
The local winds have a relatively large role in the summertime when the largescale winds are relatively weak. The frequent, relatively strong afternoon surface
winds observed along the northern coast of the gulf in typical summer conditions
may reflect the interplay of the basic flow, sea breeze and the geometry of the gulf
(Savijärvi et al. 2005). The meanders of the sea breeze, amplified by the unidirectional basic flow, may become evident as relatively strong winds, at times located just above the opposite coast—the Finnish coast—for the predominant southwestern winds.
3 The North Atlantic oscillation (NAO) is a climatic phenomenon in the northern Atlantic Ocean.
It is characterized by the NAO index, which is the difference of atmospheric pressure at sea level
between the Icelandic low and the Azores high.