The maximum temperature in the summer
Between 1973 and 2010, the mean maximum daily air temperature in the summer (June–August) in Northern Europe was 18.4 °C, and it decreased from the south-east to north-west (Fig. 2b). Within the investigated area, the warmest summer on average was recorded in 1997, and the Tmax fluctuated from 16.9 °C (Alta) to 23.9 °C (Oslo). Equally warm summer seasons were recorded in 2002 and 2006. However, the coldest summer season was observed in 1987, with the mean Tmax fluctuating from 12.8 °C (Alta) to 18.2 °C (Oslo). Cold summer seasons were also noted in 1993 and 1981. The course of the mean Tmax showed considerable year-to-year fluctuations; however, the variability was similar within the investigated area, which was noted by the standard deviation values residing between 1 and 1.6 °C. Within the majority of the investigated area, a statistically significant increase was noted in the maximum daily air temperature, fluctuating from 0.07 °C/10 years (Kvikkjokk) to 0.69 °C/10 years (Bergen) (Figs. 2c, 3). This increase was considerably influenced by the Tmax changes in the first decade of the twenty-first century, when Tmax generally exceeded the norm from the 1973–2010 multiannual period. In the analysed period, the range of deviation fluctuated from −3.8 °C (Uppsala, 1987) to 4.3 °C (Orland, 2002). The highest maximum temperature was recorded on 8 August 1975 in Uppsala at 35.0 °C.
Relatively warm days
In Northern Europe, the observed increase in Tmax translated into an increased number of relatively warm days and, consequently, into the frequency of the occurrence of warm spells. At each station, the correlation coefficient between the mean Tmax in the summer and the number of relatively warm days in summer fluctuated from approximately 0.8 to 0.9. In the analysed region, 18 relatively warm days were annually recorded on average. In the particular years, the mean number of relatively warm days for the analysed region fluctuated from six (1987) to 37 (1997). In 1987, the number of relatively warm days fluctuated from 2 (Jönköping, Ronneby) to 10 (Ivalo, Muonio). A comparatively rare occurrence of relatively warm days was observed in 1998 (8 days on average) and in 1993 (9 days on average). However, in 1997, the number of relatively warm days fluctuated from 22 (Alta) to 55 (Lindesnes). An equally high number of relatively warm days was observed in 2002 (35 days on average) and in 2006 (34 days on average). In Northern Europe during the analysed period, a mean increase in the number of relatively warm days to 1.7 per 10 years was noted. This increase was not statistically significant. Additionally, this increase was not evenly distributed in the analysed area. The highest increase in the number of relatively warm days was recorded in south-western Norway, namely in Orland (4.4 days/10 years) and Bergen (4.2 days/10 years) (Fig. 4). Additional statistically significant changes were also found in Helsinki, Lindesnes and Muonio. A decrease (−0.3 days/10 years) was noted only in Jönköping. The course of the annual number of relatively warm days showed considerable year-to-year fluctuations. The variability was similar within the investigated area; the standard deviation values fluctuated from 6 to 12 days. Relatively warm days occurred from April to October. The maximum of their occurrence was recorded in July (approximately 43 % of all relatively warm days). In October, only two relatively warm days were recorded (1 day in Bergen and 1 day in Jönköping). In September, relatively warm days were most numerous at seaside stations (e.g. Bergen, Bodo, Orland), which is specific to a marine climate. The earliest and the latest relatively warm days in the discussed multiannual period occurred in Jönköping and occurred on 22 April 1996 and 24 October 1977, respectively.
Between 1973 and 2010, the total number of WSs in Northern Europe oscillated between 24 (Kallax) and 53 (Oslo) (Fig. 5a). The total number of WS days during the entire studied period ranged from 183 days in Orland to 404 days in Oslo (Fig. 5b). At approximately 70 % of the stations, the lowest number of WSs was recorded between 1981 and 1990, and their number oscillated between 3 (Kallax) and 10 (Ivalo) (Fig. 6). In 86 % of the stations of the analysed area, the highest number of WS occurred in the first decade of the twenty-first century—their number fluctuated from 9 (Alta, Ivalo, Kajaani, Oulu) to 18 (Bergen, Oslo, Pori). In the analysed multiannual period, the average duration of WSs oscillated from 6.5 days in Orland to 9.7 days in Lindesnes. The most frequent were 5- and 6-day WSs, constituting on average 27 and 20 % of all spells, respectively, whereas WSs lasting more than 10 days constituted 15 %. Only at four stations (Kvikkjokk, Lindesnes, Ronneby, Sundsvall) did WSs lasting more than 10 days occur more frequently than 5-day spells. Within the discussed period, only Alta noted no WSs lasting more than 10 days. The longest WS lasted as many as 32 days and was recorded in Lindesnes from 29 July to 29 August 2002. A similarly long WS occurred in Kvikkjokk and lasted 27 days, from 19 June to 15 July 1973. However, at seven stations (mainly in the western part of the region) on average, the longest WS occurred in 2003 and occurred from 16 July to 2 August 2003 (18 days).
In the analysed multiannual period, the WSs occurred from May to September. However, they were most frequently recorded in July (approximately 47 % of all WSs). Only Lindesnes recorded the highest number of WSs in August (48.8 % of all warm spells). In most of the stations, the first WS was most frequently recorded at the end of May and the beginning of June. The earliest and the latest WSs in the analysed multiannual period were recorded in Bergen, and they occurred from 5 to 9 May 2006 and from 8 to 12 September 2002, respectively. In Northern Europe, the potential period for WSs in the analysed multiannual period was 131 days, occurring from 5 May to 12 September.
The mean Tmax during the analysed WSs was 25.3 °C, whereas the Tmin was 12.9 °C. The highest mean Tmax was observed during WSs in Jönköping (3–11 August 1975) at 31.4 °C, whereas the highest mean Tmin occurred in Lindesnes (1–8 August 1982) at 19.1 °C. During the longest WS, that is, the one that occurred from 29 July to 29 August 2002 (Lindesnes), the mean Tmax was 21.7 °C, whereas the mean Tmin was 17.8 °C. In the analysed multiannual period, at three stations (Joensuu, Kuusamo, Pori) statistically significant changes of Tmin during WSs were observed.
Impact of the circulation on the occurrence of warm spells
The mean sea level pressure in the Euro-Atlantic sector between 1973 and 2010 in the summer (June–August) reached the highest value in the area of the Azores Islands (>1024 hPa) (Fig. 7a). The pressure drop occurred in the northerly direction, and the centre of the low pressure was located in the southern west of Iceland (<1009 hPa). Between these pressure centres over the ocean, the considerable horizontal gradient of pressure was observed and a lower gradient was found over the continent. In the summer season, the averaged 500-hPa isobaric surface was inclined towards the north-west. The maximum height for the z500 hPa was recorded over the Mediterranean Sea (>5880 m), and the minimum was found over the northern Atlantic (<5500 m). The air temperature on the 850-hPa isobaric surface decreased from the south (>20 °C) to the north-west (<0 °C) (Fig. 7b). The pressure system caused the westerly circulation typical of Europe both in the middle and bottom troposphere.
The occurrence of WSs in Northern Europe in the analysed multiannual period, on average, was connected with a ridge of high pressure lying across Europe. Within this ridge, a high-pressure area was formed with a centre over southern Finland and north-western Russia (>1018 hPa) (Fig. 8a). The contour lines of the 500-hPa isobaric surface over the majority of the continent bent northward, creating a clear elevation over Northern Europe and confirming the presence of warm air masses. Warm air masses are characterised by a lower density than cool masses, thus dropping the pressure faster than the height. During the occurrence of WSs, the pressure over the analysed area was higher than in the average summer season pressure, confirmed in the sketches of maps of SLP anomalies which oscillated between 2 and >6 hPa over the analysed area (Fig. 8b). The z500-hPa isobaric surface settled at a higher elevation over the analysed area than usually during the summer season, and the positive anomalies in the centre exceeded 120 m. The occurrence of WSs was also connected with T850 positive anomalies (in the centre, these were >4 °C) (Fig. 8c). The system described above caused an inflow of warm and dry continental air masses from the north-east in the bottom troposphere. However, tropical air masses advection from the south-west occurred in higher troposphere layers.
Relatively warm days forming WSs at least five stations were grouped by the sea level pressure, and on this basis, two circulation types conducive to the occurrence of WSs in Northern Europe were determined. For type 1, 272 relatively warm days were recorded. On these days, Northern Europe was under the influence of a centre of high pressure (with its centre >1021 hPa) (Fig. 9a). SLP anomalies of this type were much more frequent than those of type 1. Over the analysed area, the SLP anomalies oscillated between 3 and >10 hPa (Fig. 9b). The distribution of z500 hPa anomalies was similar. The entire system shifted to the west. Over the analysed area, the z500 hPa settled at a higher elevation than usual during the summer season, from 75 to >150 m. The described pressure conditions were also accompanied by T850 positive anomalies, which varied from 2.5 to >5.0 °C over the analysed area (Fig. 9c). This barometric situation caused an inflow of warm and dry continental air masses from the east. Using the developed calendar of circulation types, the probability of occurrence of WSs was noted to be higher with anticyclonic circulations (13 % on average) than with cyclonic circulations (2.9 % on average) (Table 1). The occurrence of anticyclonic circulations from the east and the south-east is connected with the highest probability of WS occurrence within the analysed area. The probability of the occurrence of WSs in region 1 with an eastern anticyclonic circulation is 48.5 %, whereas in region 6 with a southeastern anticyclonic circulation, the probability is 47.1 %.
For type 2, 213 relatively warm days were classified. On average, their occurrence was connected with a ridge of high pressure covering the European continent, within which a local high-pressure area was formed (>1015 hPa), with its centre over Latvia (Fig. 10a). Over the analysed area, the SLP in this type was higher than usual during the summer season, and anomalies oscillated between 0 and >3 hPa (Fig. 10b). A similar course concerned z500 hPa anomalies, which exceeded 100 m in the centre. The described conditions were accompanied by T850 positive anomalies, which oscillated between 3.0 and >4.0 °C over the analysed area (Fig. 10c). This barometric situation caused an inflow of warm and dry continental air masses from the south and south-west. For this type, a higher probability of WS occurrence with anticyclonic circulation (8.2 % on average) than with cyclonic circulation (4.2 %) was also recorded (Table 2). The predominance of anticyclonic circulation over cyclonic is less considerable when compared to type 1. The greatest probability of WS occurrence was connected with the occurrence of anticyclonic circulations from the south-west and the west in summer. In region 1, as opposed to the rest of the area, the highest probability of WS occurrence was connected with the inflow of air masses from the south.
Case studies of the 1975 and 2002 warm spells
Detailed analyses were performed for two selected WSs, namely, for the warmest WS during which the Tmax in Jönköping (3–11 August 1975) was 31.4 °C and for the longest WS lasting 32 days in Lindesnes (29 July–29 August 2002).
During the warmest WS, a strong high pressure system settled over Northern Europe with its centre lying over the southern part of the Scandinavian Peninsula (>1023 hPa) (Fig. 11a, left column). The research area stayed within the reach of the SLP positive anomalies, with the centre >11 hPa (Fig. 11a, middle). Contour lines of the isobaric surface located over the northern part of the continent bent northward, creating a clear elevation and confirming the presence of warm air masses. Over the southern part of the Scandinavian Peninsula, z500 hPa settled at 5860 m. Therefore, this level was higher than usual in summer by >220 m. During the analysed WS, T850 positive anomalies were also recorded with a centre (>9 °C) characterised by a similar location to the centres of SLP and z500 hPa anomalies (Fig. 11a, right column).
The longest WS was connected with a high pressure system with its centre over the Gulf of Finland (1019 hPa) (Fig. 11b, left column). Similar to the warmest WS, the z500 hPa contour lines bent northward, creating a clear elevation over the analysed region. The SLP anomalies over the research area oscillated between 2 and >7 hPa, whereas the z500 hPa anomalies varied from 80 to >130 m (Fig. 11b, middle). During the analysed WS, T850 positive anomalies were recorded (in the centre >5 °C) (Fig. 11a, right column). This barometric situation caused an inflow of warm air masses from the south-east.