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Temperatures of north Norwegian fjords and coastal waters: Variability, significance of local processes and air–sea heat exchange
Estuarine, Coastal and Shelf Science 67 (2006) 530e538 www.elsevier.com/locate/ecss
Temperatures of north Norwegian fjords and coastal waters: Variability, significance of local processes and airesea heat exchange Hans Chr. Eilertsen a,*, Jofrid Skarðhamar b a
Department of Aquatic Biosciences, NFH, University of Tromsø, N-9037 Tromsø, Norway b Akvaplan-niva AS, N-9296 Polar Environmental Centre, Tromsø, Norway Received 28 January 2005; accepted 19 December 2005 Available online 2 February 2006
Abstract Sea and air temperature data from the period 1980e2003, representing fjordeouter coast transects at three locations in north Norway (Balsfjord, Altafjord and Porsangerfjord), have been examined. Air and sea surface temperatures were well inter-correlated between all stations, indicating that the coastline in question is a coherent climatic region. This conclusion is strengthened since no correlations were found between the transect data and the northern FugløyaeBjørnøya transect. Neither did our data sets bear any clear resemblance to the variation in the NAO index. Generally there was a delay of four months before the surface temperature signals reached the deepest bottom waters. The cooling period (heat loss from sea to air) lasts longer than further south, leading to a prolonged period with unstratified waters in the north. This strengthens the effect of sea temperatures to follow the air temperatures, i.e. sea temperatures in the region are controlled by local climatic processes. The mean annual heat loss for the measured stations was 31 W m2. The largest mean surface heat loss was in Porsangerfjord, that has no sill (45 W m2). Balsfjord, the only fjord with a sill, had the lowest annual heat loss (21 W m2). This difference can be explained by transport of inflowing coastal water being restricted by the sills in Balsfjord. No clear heating or cooling trends could be detected for the period 1980e2003, and comparison with data from the period 1930e1979 revealed anomalies well within the ones observed by us for 1980e2003. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: fjord; coast; sea temperatures; heat flux; north Norway
1. Introduction The fjords and shelf areas along the north Norwegian coast hosts large fisheries and aquaculture activities. Since the last period of the 19th century, when researchers hypothesized that variability in physical parameters influenced marine life (Sars, 1879), much effort has been put into research on physicalebiological regulatory mechanisms (Cushing, 1978; Larraneta, 1988). The 17th century had long periods with extremely low fish catches (Sandvik and Winge, 1987). This had massive impacts on the northern communities, and it is estimated that during this period the child population was reduced to ca. 50% due to starvation. These declines in fisheries are
* Corresponding author. E-mail address: [email protected] (H.Chr. Eilertsen). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.12.006
claimed to have had climatic reasons, i.e. a significant drop in sea-temperatures due to low air temperatures. This may also have occurred during the period 1785e1810 (Hanssen, 1990). According to Ellertsen et al. (1987) there is a clear relationship between temperature and year class strength of Arcto Norwegian cod. It is therefore obvious that understanding and predicting sea temperature variation is of crucial importance. There exists several monographs on inter-annual variability in sea temperatures globally (MacKay and Ko, 2001), in the North Atlantic (Dickson et al., 2000), the Norwegian and Barents Sea (Ottersen et al., 2000; Furevik, 2001) and the Arctic Ocean (Steele and Boyd, 1998). Several papers deal with hydrography from the area around Tromsø (Soot-Ryen, 1934; Sælen, 1950; Eilertsen et al., 1981; Svendsen, 1995). The lack of published material from Finnmark, the northernmost part of the Norwegian coastline adjacent to the Barents Sea, is though apparent. The coastal areas and fjords of North
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Norway are under influence of the northward flowing Norwegian Coastal Current (NCC) containing Norwegian Coastal Water (NCW) that has its origin in the Baltic Ocean. Northern coastal waters can therefore be influenced by climatic processes taking place further south, but also by interactions between NCC and the outer lying Atlantic Water (AW) in the Norwegian Atlantic Current (NWAC). It is assumed that temperature anomalies may be advected into the region, but may also be caused by local variations in heat flux processes (Furevik, 2001). One of the main mechanisms believed to cause telecommunication variations is that during periods with prevailing westerlyesouth westerly winds the water in Skagerak will pile up and the flow northwards along the Norwegian coast is hence retarded (Aure and Sætre, 1981). When the winds then either slackens or changes direction the water masses will be let loose and move fast up along the Norwegian coast, thus changing temperatures in coastal waters off Norway (Aure and Sætre, 1981; Nilsen and Hansen, 1980). During the winter 1978/1979 temperatures were exceptionally low in the Norwegian fjords, and it is suggested that this was mainly caused by cold water piled up in Skagerak let loose by changing wind directions (Nilsen and Hansen, 1980). This may well have been the cause for the southern fjords due to the close vicinity to Skagerak. NCW is continuously being mixed with AW while travelling northwards and we therefore question the validity of the ‘‘Skagerak hypothesis’’ for the northern areas. When investigating the causes of the 1978/1979 ‘‘lows’’ (Nilsen and Hansen, 1980), local heat flux was not considered, even if climatic data showed air temperatures well below normal. Our hypothesis is that northern coastal areas and fjords are much less influenced by seaborne telecommunication mechanisms than the southern temperate areas due to a prolonged period with winter overturning of the water masses (Sælen, 1950; Eilertsen et al., 1981; Svendsen and Thompson, 1978). The summer stratification is also generally weaker in the north since solar heating is less effective and surface salinities are higher due to less runoff compared to southern Norway (Skofteland, 1985), making transportation of heat from deeper waters more predominant. This makes us expect that ambient air temperature plays a significant role in temperature regulation in northern waters, enhancing the mixing regime characteristic of temperate shelf seas (Prandle and Lane, 1995). The present work attempts to clarify some central issues related to the hydrophysical mechanisms of northern coastal areas and fjords, i.e. the actual ranges of spatial, seasonal and inter-annual temperature variations and the relative degree of regulation by local or distant oceanic/climatic processes. 2. Methodology The data sets, representing the period 1980e2003, were collected as part of the Sea Environmental sampling programme managed by the University of Tromsø. The research vessels used were ‘‘Johan Ruud’’ (100 ft) and occasionally after 1984 ‘‘Jan Mayen’’ (184 ft). The applied CTD systems were Neil Brown Instruments Mark III and Seabird 911
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Plus. From 1999 on the CTD systems were calibrated prior to and after each cruise. For the earlier period temperature and conductivity sensors were checked or calibrated at infrequent intervals and the data sets, if necessary, corrected for the observed drift. The sea temperature data sets were normally sampled at 1e4 monthly intervals, while for the periods 1985e1986 and 1990e1991 we performed 2e4 samplings each year. Daily sea surface temperatures from Skrova in Lofoten (Fig. 1) and meteorological data (air temperature, wind, cloud cover) are provided by The Norwegian Meteorological Institute. All hydrographical data can be downloaded from http://lupus.nfh.uit.no. The present analysis is based on sea temperature data from 5, 10, 25, 50, 75 m and 5e10 m above the bottom. For the periods 1930e1931 and 1971e 1979, temperature data from Balsfjorden and Malangen are from Gaarder (1938), Skreslet (1973), Eilertsen et al. (1981), Eilertsen and Taasen (1984), Aure et al. (1997). The applied North Atlantic Oscillation (NAO) Index (Lisbon minus Stykkisholmur, normalized DecembereMarch Average SLP Anomalies) was an update by Hurrell of time series published in Hurrell (1995). Values are normalized and ascribed to the year of the month of January. The southernmost of the outer stations, Malangen (M, N 69 30.0eE 18 21.4, 201 m) is outside the Balsfjord sills (Fig. 1). Outside the station lies the Malangen Deep (477 m). The essential part of freshwater supply to this fjord comes from the Malangen River, and though there is some effect of this river on salinity at the station, it is also heavily influenced by communication with the outer coastal waters (Sælen, 1950). Meteorological data representing the station are from the nearby Hekkingen lighthouse (Fig. 1). The station in Balsfjord (B, N 69 21.8eE 19 06.4, 180 m) is inside three shallow sills (8, 9, 30 m) separating the fjord system from the outer coastal waters. Run-off is from several small rivers, and there is a typical estuarine circulation taking place during summer. The chosen station is representative for the hydrography of the fjord, and the station depth represents maximum basin depth (Sælen, 1950; Eilertsen et al., 1981). Meteorological data are from Tromsø airport (Fig. 1). The Altafjord station (A, N 70 06.4eE 23 02.4, 330 m) is in the middle of the fjord, ca. 300 north to the stations in Malangen and Balsfjord. Maximum depth is 450 m, and sill is 190 m. The main source of freshwater is the Alta River and the innermost part of the fjord is normally ice-covered during winter. Meteorology is from Alta airport. Outer station Skipsholmen (S, N 70 54.1eE 23 50.3, 220 m) is representative of the outer coastal water adjacent to Altafjord. Meteorology is from Fruholmen lighthouse close to the station. Porsangerfjord is by far the largest fjord in Northern Norway, ca. 100 km long and 15e20 km wide. Since the station Porsanger (P, N 70 43.1eE 25 44.8, 210 m) is outside of the 60 m sill (Fig. 1), the outer part of the fjord can communicate freely with open water. The major part of runoff to Porsangerfjord comes from the rivers Lakselv and Børselv, both situated in the inner part. Meteorology is from Banak (Lakselv) airport.
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Fig. 1. Map of investigated area with sampling sites. M denotes meteorological stations. The meteorological and hydrographical station at Skrova is the same.
The northernmost outer station Helnes (H, N 71 07.5eE 26 18.0, 205 m) is located in open coastal water close to the North Cape, i.e. inside and adjacent to the North Cape bank area. Meteorology is from Helnes lighthouse. Surface heat flux was computed as, Qt ¼ Qh þ Qe þ Qb þ Qs . Sensible heat (Qh) was Qh ¼ Ch rCp V10 ðTair Tsea Þ (Brown, 1990), where Ch ¼ heat flux coefficient (1.1 103 for neutral stratification); r ¼ air density (1.2 kg m3); Cp ¼ heat capacity (air) at constant pressure (1004 J kg1 K1); V10 ¼ wind in m s1 10 m above sea surface; T ¼ temperature in C. Latent heat (Qe) is evaporation energy of sea water, Qe ¼ rLv Ce V10 ðqair qsea Þ (Smith et al., 1983). Here Lv ¼ latent heat of evaporation (2.5 106 J kg1) and Ce ¼ Ch while q denotes specific humidity at 10 m altitude and at the sea surface. Further qair=sea ¼ 3eair=sea =ðph ð1 3Þeair=sea Þ, and 3 ¼ 0.622; ratio between molecular weight of water vapour and dry air,
eair=sea ¼ r611:0 10ð7:5Tair=sea =ðTsea þ273:1535:86ÞÞ , where r is the relative humidity of the air, 0e1, and ph ¼ mean air pressure in the northern hemisphere; 101 400 Pa). Since we lacked data on humidity we used a constant value of 0.6. Net long-wave radiation (Qb, black-body radiation) can be expressed as, Qb ¼ 3g dðfair ð273:15 þ Tair Þ4 ð273:15 þ Tsea Þ4 Þ (Henderson-Sellers, 1986), where 3g ¼ long-wave emissivity for sea (0.97), d ¼ Stefan Boltzmann’s constant (5.67 108 4 2 J s1 K4 m2), fair ¼ ð1 0:261 e7:7710 Tair Þ ð1 þ 0:275fc Þ where fc ¼ relative proportion of cloud cover (scale 0e1). Short-wave solar irradiance (Qs) in W m2 for clear sky was computed as described in Frouin et al. (1989), after input of surface visibility, regression coefficients for maritime atmospheres and solar zenith angle computed at given geographical position and time according to the equations in Iqbal (1983). In all the computations we used monthly means of all meteorological and sea temperature data. A more detailed presentation/analysis
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of the present data sets can be found on http://nfh.uit.no/ phaeocystis/env. 3. Results 3.1. Air and sea temperatures The lowest (9; 7.4 C) and the highest (12.7; 13.5 C) mean monthly air temperatures were in January and July at Porsanger and Altafjord (Table 1). The highest annual mean was in Malangen (4.1 C) while Porsanger had the lowest (0.85 C). All the outer stations (Malangen, Skipsholmen and Helnes) had higher minimum sea surface temperatures than the fjord stations (Table 1) while the lowest temperatures were in February in Balsfjord. The northernmost station, Helnes, had the highest minimum temperatures, while Skipsholmen (between Helnes and Malangen) had second highest minimum temperatures. The highest monthly mean was in August in Altafjord while Balsfjord was coldest (Table 1). The coldest fjords during winter/early spring were Balsfjord, Porsanger and Altafjord while during winter the outer stations were warmest (Malangen, Helnes and Skipsholmen). The sea temperature amplitude was gradually reduced with depth (not shown), but the tendency prevailing at the surface was observed at all depths, also in the bottom waters (Fig. 2). During autumn, winter and spring (SeptembereApril)
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the sea surface temperatures were always higher than the air temperatures (Table 1) while during summer (MayeAugust) this was reversed. For all locations the annual mean sea temperatures were higher than the air temperatures (Table 1). When analyzing the variance of the meteorological data and sea temperatures for the period, the deviation from the mean when 75% of the data were accounted for was at maximum 385% for air temperatures, 22.4% for sea temperatures, 18% for wind and 12% for cloud cover. 3.2. Seaeair temperature correlations and anomalies When daily air temperatures from the stations were correlated linearly, all correlations were significant at p < 0.05 (Table 2). All stations, except Skrova, had correlation coefficient values between 0.88 and 0.97 (Table 2). The air temperatures at the Skrova station differed from the rest at all instances by showing significantly lower correlations with the other stations (0.61 < R < 0.66). Correlation tests between means of air temperatures 3, 5, 10, 20 and 30 prior to the date of sampling and sea temperatures, showed that the best correlations were always between sea temperatures at all depths and the 30 and 20 day means (not shown). The R values decreased downwards, and for the bottom water there were no significant correlations. All sea temperatures were also well correlated between the fjords (Table 2). In sum 82% of the correlation analyses performed between air and 5 m sea temperature had
Table 1 Monthly mean of cloud cover (0e9), air and sea surface temperatures and modelled heat flux (Qt) for each month (W m2). Positive Qt means heating of sea. At bottom of table is shown Qt when cloud cover, wind and sea temperatures were increased to values comprising 75% of the data for the period 1980e2003, i.e. ca. þ10% CC, þ20% wind and 20% increased sea temperature
Malangen cloud cover Malangen, sea temperature Malangen, air temperature Malangen Qt Balsfjord cloud cover Balsfjord Qt Balsfjord, sea temperature Balsfjord, air temperature Skipsholmen cloud cover Skipsholmen, sea temperature Fruholmen, air temperature Skipsholmen Qt Altafjord cloud cover Altafjord, sea temperature Altafjord, air temperature Altafjord Qt Porsanger, cloud cover Porsanger, sea temperature Porsanger, air temperature Porsanger Qt Helnes cloud cover Helenes, sea temperature Helnes, air temperature Helnes Qt Helnes Qt þ10% cloud cover Helnes Qt þ20% wind Helnes Qt þ20% sea temp.
Jan
Feb
Mar
5.68 3.50 1.51 99 5.76 112 2.66 3.74 5.80 4.52 1.89 116 5.61 3.56 7.43 163 5.18 3.28 9.03 177 4.90 4.67 3.05 123 119 129 132
5.22 5.06 3.05 3.19 1.43 0.40 94 67 6.00 5.85 90 64 1.33 1.46 3.56 2.26 6.52 6.89 3.67 3.44 1.78 0.97 97 72 6.13 5.64 2.59 2.19 6.60 4.24 137 96 5.95 5.08 2.46 1.95 7.89 5.36 149 106 6.65 6.20 4.25 3.61 3.01 1.92 96 69 91 67 102 73 101 74
Apr
May
Jun
Jul
Aug
Sept
Oct
4.56 3.69 2.09 5 5.70 6 2.48 0.66 6.50 3.63 0.84 26 5.14 2.66 0.06 13 4.37 2.30 0.81 10 5.28 3.80 0.28 10 14 11 13
4.82 4.97 5.71 78 6.74 37 4.54 4.91 7.39 4.22 3.77 13 6.53 4.33 4.84 43 6.41 3.45 4.12 47 7.06 4.52 3.53 15 2 15 9
3.57 6.64 9.06 155 4.22 135 7.17 9.29 4.98 5.84 6.84 99 4.73 7.18 10.0 136 4.38 6.15 9.37 148 5.20 5.88 6.99 77 66 78 63
3.93 8.32 11.4 148 5.49 120 8.56 11.8 6.26 7.85 9.71 79 6.00 9.27 13.5 131 5.60 8.41 12.7 136 5.95 7.38 10.0 69 57 71 56
3.90 9.38 11.4 86 5.49 74 8.56 11.0 5.36 8.93 9.97 52 5.48 10.5 12.2 65 4.72 10.4 11.2 57 4.51 9.30 9.99 38 32 38 23
3.46 4.26 9.30 7.32 8.35 4.40 24 85 3.75 5.96 27 84 8.28 6.53 7.09 2.65 4.40 5.90 8.71 7.58 7.63 4.15 25 80 4.42 5.66 8.67 7.40 7.60 2.05 26 107 4.52 5.63 8.24 7.05 6.90 1.41 29 110 4.13 5.81 8.44 7.77 7.37 3.29 30 90 29 86 31 94 42 102
Nov
Dec
Annual mean
5.02 5.67 1.11 103 5.75 115 5.13 1.06 6.91 6.63 0.75 106 6.23 6.06 3.76 155 5.67 5.58 4.77 163 6.70 6.26 0.21 111 106 118 120
5.88 5.46 0.92 117 6.42 120 4.14 3.11 6.71 5.91 1.05 119 5.53 5.24 6.38 176 5.57 4.71 7.67 181 6.41 5.69 2.10 124 119 132 131
4.6 5.87 4.11 9.8 5.6 21 5.07 2.81 6.1 5.91 3.16 33 5.6 5.81 1.82 42 5.3 5.33 0.85 45 5.7 5.96 2.59 38 39.5 42 52
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Fig. 2. Mean monthly 5 m temperatures for the investigated stations. For station depths see Fig. 1.
R > 0.6, while inter-correlation analysis between depths and similar depths between stations showed the same trend, i.e. 84% had R > 0.6. For both cases the correlations between the data from Skrova and the other localities were in another class of correlation in that R values generally were of an order of 0.2 below the others. Air temperatures at all stations from Tromsø and northwards were therefore significantly correlated, as was also air vs. sea surface temperatures. At all stations the temperature anomalies indicated that there were four clearly recognizable cold periods, i.e. 1981e 1982; 1986e1988; 1994 and 1998, while 1990e1993 and 1995 were warm periods. This tendency persisted at almost all depths, i.e. if the surface anomalies showed warm years the bottom waters followed as well (Figs. 3e5). The deviations from this general picture was that in Balsfjord in 1995 and in Malangen 2002 the surface temperatures were above normal while bottom waters were slightly below normal. The overall coldest and warmest years in the region with respect to sea temperatures were 1981 and 1992. 3.3. Heat flux At all stations the annual mean of sea temperature minus air temperature was positive (Table 1) The mean heat loss for the
measured stations was 31 W m2 while the largest mean net surface heat loss was in Porsanger with 45 W m2 (Table 1). The northernmost fjord/inland stations had the highest loss, while Balsfjord had less than half of this (21 W m2). 4. Discussion Our analysis of sea and air temperature data demonstrate that the area from Tromsø and northwards, represented by three pairs of adjacent ‘‘fjordecoastal’’ stations (Balsfjorde Malangen; AltafjordeSkipsholmen; PorsangereHelnes), is a coherent climatic region with respect to both air and sea temperatures. This is also demonstrated by the much weaker correlation between temperatures from Vestfjorden (Skrova) and the northern areas, as well as the significant correlation between the northern locations themselves. The inner fjord stations had lower air and sea temperatures than the outer stations during winter, while during summer this was reversed. This is a typical feature of inland climate where the moderation of air temperature from sea is less pronounced than for outer coastal areas (Taylor and Stephens, 1983). The winter sea surface temperatures were lowest in Balsfjord while Porsanger and Altafjord had higher temperatures (Table 1). The winter bottom temperatures were substantially lower in
Table 2 Linear correlation coefficients for daily air temperature measurements and sea bottom temperatures at investigated stations. Sea bottom temperatures for Skrova not available. All values are significant at p < 0.05
Air Skrova Balsfjord Malangen Alta Skipsholmen Porsanger Helnes Sea bottom Balsfjord Malangen Alta Skipsholmen Porsanger Helnes
Skrova
Balsfjord
Malangen
Alta
Skipsholmen
Porsanger
Helnes
1.00 0.66 0.68 0.62 0.60 0.61 0.61
0.66 1.00 0.93 0.95 0.92 0.93 0.93
0.68 0.93 1.00 0.90 0.88 0.88 0.89
0.62 0.95 0.90 1.00 0.91 0.98 0.94
0.60 0.92 0.88 0.91 1.00 0.91 0.97
0.61 0.93 0.88 0.98 0.91 1.00 0.94
0.61 0.93 0.89 0.94 0.97 0.94 1.00
1.00 0.65 0.77 0.53 0.40 0.51
0.65 1.00 0.68 0.87 0.81 0.81
0.77 0.68 1.00 0.45 0.33 0.40
0.53 0.87 0.45 1.00 0.90 0.92
0.40 0.81 0.33 0.90 1.00 0.79
0.51 0.81 0.40 0.92 0.79 1.00
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Fig. 3. Temperature anomalies for 5 m at station Balsfjord.
Porsanger and Balsfjord than in Altafjord (ca. 2 C). Balsfjord is the only fjord with shallow sill areas (9e30 m), i.e. a true fjord (Fig. 1), while Altafjord and especially Porsangefjord have deep sills (190e200 m). Since the winter bottom water in Altafjord was only ca. 1 C lower than at the coastal station Skipsholmen, we conclude that it is Altafjord that has the best exchange of water. In narrow fjords the exchange with adjacent waters takes place as a two or multilayered current system. In wider fjords, such as Porsangerfjord, rotational effects influence the dynamics significantly (Svendsen, 1995; Cushman-Roisin et al., 1994), and may slow down the exchange of fjord water relative to more narrow fjords. Since in addition current measurements in Porsangerfjorden indicates this (Svendsen, 1995), we believe that this makes up for the large part of the explanation why the Altafjord bottom water was warmer than in Porsanger, even if the winter surface temperatures were comparable (Table 1). This also coincides
well with observations that, during the excessively cold winter 1978/1979, temperatures registered in Balsfjord (0e50 m) were considerably lower than for Altafjord (Nilsen and Hansen, 1980). Though, taking into account these main differences, it is clear that the relative changes in air and sea temperatures were well inter-correlated. This means that if it was a cold year it was cold all over, and if warm this was true for the entire area from Tromsø northwards. This is demonstrated by the high degree of correlation between air temperatures, between depths in each fjord and between depths in fjords and between the bottom waters (Table 2). Since the temperature variations at Skrova in Vestfjorden were significantly different from all the northern stations and anomalies at the northern stations followed the same pattern (Figs. 3e5), we feel confident that our conclusion is solid. Further this conclusion is not surprising taken into account that the Vestfjorden basin, situated
Fig. 4. Temperature anomalies for 5 m at station Malangen.
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Fig. 5. Temperature anomalies for 5 m at station Porsanger.
more than 1 of latitude south to Tromsø (Fig. 1), traps and slows down north flowing NCC water influencing both the atmospheric and ocean climate in the region making it unique (Furnes and Sundby, 1981). The bottom temperatures in our investigation were at almost all instances uncorrelated with the shallow water temperatures (5e50 m). The apparent reason for this was that the amplitudes were much weaker and out of phase with the overlying water masses. While surface temperatures peaked in July, corresponding well with the highest air temperatures (Table 1), the bottom waters peaked in Novembere December while minimum was in May. This is in the same range as depicted earlier for Balsfjord and Malangen (Sælen, 1950; Eilertsen et al., 1981) and suggests that there is typically
a delay of ca. four months in the seasonal temperature signal from surface to bottom. In the Barents Sea (Bjørnøyae Fugløya section) the mean delay in the signal is also ca. four months from surface to 400 m (Furevik, 2001). For the North Sea, i.e. a more shallow shelf area, it is suggested that the delay is up to three months (Prandle and Lane, 1995). They also concluded that the enclosed area of the North Sea is essentially localized with respect to airesea thermal balance, with only minor effects of advection and dispersion on temperature. A simplified expression for the time in days for complete vertical mixing of a surface exchange in well mixed areas is Tv ¼ D2 =E where D is the depth in meters and E is the eddy diffusion coefficient (Prandle and Lane, 1995). By assuming
Fig. 6. Temperatures (left y-axis, solid line, C is mean of 10e200 m, updated data from Furevik, 2001) in the FugløyaeBjørnøya transect and NAO winter index (right y-axis, dotted line).
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that D is 277 m and E is constant with depth and solving the expression for a time period of 115 days (3.5 months) we find E ¼ 7.7 103 m2 s1. An approximated E for stratified areas may be 2.0 104 m2 s1 (Malacic, 1991; Lane and Prandle, 1996), indicating that either the mixing was large in our area and/or that the stratification was weak. The stratified period in Balsfjord, around Tromsø island and Malangen lasts from the second week of May until late Septembereearly October (Sælen, 1950; Skreslet, 1973; Eilertsen et al., 1981), i.e. ca. four months. The stratification is weak (max. Dst 0e50 m ¼ 1.0e2.0) and is mainly caused by lowered salinity due to freshwater runoff and to a lesser degree by increased temperatures. Further north offshore this period is shorter and the pyncolines are even weaker. Thus coastal waters and fjords of northern Norway are, for natural climatic reasons, less stratified for a shorter period of time than the fjords of western and southern Norway (Sælen, 1950; Nilsen and Hansen, 1980). This emphasizes the potentially localized nature of the airesea thermal balance in the area, and that the mixing period lasts longer. At all the localities the annual mean sea temperature was higher than annual means for air (Table 1), demonstrating the consistent net release of heat transported northwards into the region with the AC and NCC. The mean heat loss for the measured stations was 31 W m2 while the largest mean net surface heat loss was at Porsanger (Table 1). Typically it was the northern fjord/inland stations that had the highest loss (Porsanger 45 W m2 and Alta 42 W m2), while Balsfjord had half of this (21 W m2), even though it also penetrates into inland climate with low winter temperatures. The reason for this is that Balsfjord is a ‘‘true’’ fjord with shallow sills, and that exchange of deeper lying bottom water is hindered by the sills, thus limiting advection of heat into the fjord. The deep water exchange normally takes place during spring and early summer. NCC lies as a wedge on top of AW and oscillations of this wedge is needed to lift the more dense AW high enough to cross the sill, thus advecting heat to the fjord basin (Sælen, 1950; Eilertsen et al., 1981). The other two fjord locations have much deeper (Altafjord) or no (Porsangefjorden) sills, so exchange of deeper water will take place either continually or at frequent intervals (Svendsen, 1995). The most cloudy climate was at the outer station Skipsholmen (CC annual mean ¼ 6.1), while Malangen had the clearest skies (4.6). Cloud cover blocks transfer of solar radiation to the sea, and it is clear that this plays an important role in determining the local climate since it is also indirectly linked to sensible heat transfer. The sensitivity tests (bottom of Table 2) where we increased cloud cover, wind and sea temperature to values comprising 75% of the data sets, clearly indicate that it is advected heat into the fjords that cause the largest heat flux variations. Since all the bottom temperatures were so well correlated this also stresses the importance of fjordecoast communication, and that the annual mean difference between sea and air temperatures in Altafjord and Porsangefjorden possibly can be used as a reasonably reliable measure of long term changes in heat transported northwards with NCC.
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Our temperature anomalies indicated four cold periods (1981e1982; 1986e1988; 1994 and 1998). The temperature of the coastal water is influenced by the outer lying AW, that again is believed to be linked (though weakly) to the NAO winter index (Furevik, 2001). As Fig. 6 indicates, by computing 10e200 m means from the data given by Furevik (2001) there was only one clearly cold period (1985e1988), but also possibly 1996, plus two warm periods (1983e1985; 1992e1995). From our data we identify the warm periods as 1990e1993 and 1995. Thus the sea temperatures in the fjords, and hence the air temperatures, are not well correlated to either AW temperatures or the NAO index. This again strengthens the conclusion of the localized nature of the hydrophysical variations in the region. A drawback of our data sets is that for some of the years sampling was scarce (e.g. Helnes last part of period), and that we only have single point measurements. Also may modulation of the seasonal air temperature by mixed water (Prandle, 1998) have played a role, i.e. the inflowing seawater may have forced air temperatures somewhat. Considering the clarity of our results, we though conclude that since the variations in sea temperatures in the entire north Norwegian coastal region (north to Tromsø) is highly interlinked this should be taken into account when planning further sampling. A possible strategy would also be to sample at selected stations at frequent intervals, also taking into account the assumed short-term (hours) variability of sea temperatures due to wind and tidal excursion (Eilertsen and Taasen, 1981). By analyzing our data sets we could not observe any warming or cooling trends, and analysis of data from periods prior to 1980 (see references in Section 2) showed no cold or warm trends, i.e. all the data were close to the means and deviations smaller than the maximum anomalies observed for the period 1980e2003.
References Aure, J., Sætre, R., 1981. Wind effects on the Skagerak outflow. In: Sætre, R., Mork, M. (Eds.), The Norwegian Coastal Current, vol. I, pp. 263e293. Aure, J., Føyn, L., Pettersen, R., 1997. Miljøundersøkelser i Norske fjorder 1975e1995. Fisken og Havet 13, 1e70. Brown, R.A., 1990. Meteorology. In: Smith Jr., Walker O. (Ed.), Polar Oceanography, Part A, Physical Science. Academic Press, Inc., San Diego, pp. 1e47. Cushing, D.H., 1978. Biological effects of climatic change. Rapports et Proce’s-Verbaux des Re´unions du Conseil International pour l’Exploration de la Mer 173, 107e186. Cushman-Roisin, B., Asplin, L., Svendsen, H., 1994. Upwelling in broad fjords. Continental Shelf Research 14 (15), 1701e1721. Dickson, R.R., Osborne, T.J., Hurrell, J.W., Meincke, J., Blindheim, J., ˚ landsvik, B., Vinje, T., Alekseev, G., Maslowski, W., 2000. The Arctic A Ocean response to the North Atlantic Oscillation. Journal of Climate 13 (15), 2671e2696. Eilertsen, H.C., Falk-Petersen, S., Hopkins, C.C.E., Tande, K., 1981. Ecological investigations on the plankton community of Balsfjord, Northern Norway. Program for the project, study area, topography, and physical environment. Sarsia 66, 25e34. Eilertsen, H.C., Taasen, J.P., 1981. Diel variations in some important phytoplankton parameters during a 24 hours sampling period in Balsfjord, Troms and Skjomenfjorden, Nordland, North Norway. Kieler Meeresforschung Sonderheft 5, 68e81.
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Eilertsen, H.C., Taasen, J.P., 1984. The phytoplankton 1976e1978. Ecological investigations on the plankton community of Balsfjord, Northern Norway. Environmental factors, dynamics of growth and primary production. Sarsia 69, 1e15. Ellertsen, B., Fossum, P., Solemdal, P., Sundby, S., Tilseth, S., 1987. The effect of biological and physical factors on the survival of arcto-Norwegian cod and the influence on recruitment variability. In: Loeng, H. (Ed.), Proceedings of the Third Soviet-Norwegian Symposium, Murmansk, 26e28 May 1986. Institute of Marine Research, Bergen, pp. 101e126. Frouin, R., Ligner, D.W., Gautier, C., Baker, K.S., Smith, R.C., 1989. A simple analytical formula to compute clear sky total and photosynthetically active solar irradiance at the ocean surface. Journal of Geophysical Research 94 (7), 9731e9742. Furevik, T., 2001. Annual and interannual variability of Atlantic water temperatures in the Norwegian and Barents Seas. Deep Sea Research 48, 383e 404. Furnes, G.K., Sundby, S., 1981. Upwelling and wind induced circulation in Vestfjorden. In: Sætre, R., Mork, M. (Eds.), The Norwegian Coastal Current, vol. I, pp. 152e177. Gaarder, K.R., 1938. Phytoplankton studies from the Tromsø district 1930e ˚ rshefter 55 (1), 1e159. 1931. Tromsø Museums A Hanssen, R., 1990. Nordkapp en fiskerikommune fra de eldste tider til i dag. Nordkapp Kommune, ISBN 82-992263-0-9, 528 pp. Henderson-Sellers, B., 1986. Calculating the surface energy balance for lake and reservoir modeling, a review. Reviews of Geophysics 24 (3), 625e649. Hurrell, J.W., 1995. Decadal trends in the North Atlantic Oscillation and relationships to regional temperature and precipitation. Science 269, 676e679. Iqbal, M., 1983. An Introduction to Solar Radiation. Academic Press, New York, 390 pp. Larraneta, M.G., 1988. Fish recruitment and environment. In: Wyatt, T., Larraneta, M.G. (Eds.), Long Term Changes in Marine Fish Populations, pp. 21e36 (Vigo, Spain). Lane, A., Prandle, D., 1996. Inter-annual variability in the temperature of the North Sea. Continental Shelf Research 16 (11), 1489e1507. Malacic, V., 1991. Estimation of vertical eddy diffusion coefficient for heat in the Gulf of Trieste (Northern Adriatic). Oceanologica Acta 14, 23e32. MacKay, R.M., Ko, M.K.W., 2001. An analysis of simulated and observed global mean near-surface air temperature anomalies from 1979 to 1999,
trends and attributed causes. Chemosphere e Global Change Science 3, 393e411. Nilsen, J., Hansen, E., 1980. Temperatur og salholdighetsforhold i en rekke norske fjorder den kalde vinteren 1978/79. VHL Rapport, Sintef Rapportnummer SFT60 A80030, ISBN 82-595-2022-2, 160 pp. ˚ landsvik, B., Loeng, H., 2000. Predicting the temperature of the Ottersen, G., A Barents Sea. Fisheries Oceanography 9 (2), 121e135. Prandle, D., Lane, A., 1995. The annual temperature cycle in shelf seas. Continental Shelf Research 15 (6), 681e704. Prandle, D., 1998. Global expressions for seasonal temperatures of the sea surface and ambient air e the influence of tidal currents and water depth. Oceanologica Acta 21 (3), 419e428. Sandvik, H., Winge, H., 1987. Tingbok for Finnmark 1620e1633. Norsk lokalhistorisk institutt, Oslo, ISBN 82-90176-27-9, 385 pp. Sars, G.D., 1879. Undersøkelser over loddefisket i Finmarken. Indberetning til Departementet for det indre, Christiania, pp. 17e32. Skofteland, E., 1985. Ferskvannstilførsel til kysten før og etter regulering. Vassdragsreguleringers innvirkning pa˚ fjorder. In: Andreassen, L., Asvall, R.P. (Eds.), Norsk Hydrologisk Komite´, pp. 47e72 (Rapporter 19). Skreslet, S., 1973. Water transport in the sill area of Balsfjord, North Norway, during vernal meltwater discharge. Astarte 6, 1e6. Smith, S.D., Anderson, R.J., Hartog, G.D., Topham, D.R., Perkins, R.G., 1983. Air investigations of a polynya in the Canadian archipelagos, structure of turbulence and heat flux. Journal of Geophysical Research 88, 2900e2910. Soot-Ryen, T., 1934. Hydrographical investigations in the Tromsø district ˚ rshefter 52, 1e78. 1930. Tromsø Museums A Steele, M., Boyd, T., 1998. Retreat of the cold halocline layer in the Arctic Ocean. Journal of Geophysical Research 103 (C5), 10419e10435. Svendsen, H., 1995. Physical oceanography of coupled fjordecoast system in northern Norway with special focus on frontal dynamics and tides. In: Skjoldal, H.R., Hopkins, C., Erikstad, K.E., Leinaas, H.P. (Eds.), Ecology of Fjords and Coastal Waters. Elsevier Science B.V. Svendsen, H., Thompson, R., 1978. Wind-driven circulation in a fjord. Journal of Physical Oceanography 8 (4), 703e713. Sælen, O.H., 1950. The hydrography of some fjords in Northern Norway. Balsfjord, Ullsfjord, Grøtsund, Vengsøyfjord and Malangen. Tromsø Mu˚ rshefter Naturhistorisk avdeling 38, 1e102. seums A Taylor, A.H., Stephens, J.A., 1983. Seasonal and year to year changes in the temperature of the English Channel and the southern North Sea 1961e 76, a budget. Oceanologica Acta 6 (1).
Temperatures of north Norwegian fjords and coastal waters: Variability, significance of local processes and airesea heat exchange Hans Chr. Eilertsen a,*, Jofrid Skarðhamar b a
Department of Aquatic Biosciences, NFH, University of Tromsø, N-9037 Tromsø, Norway b Akvaplan-niva AS, N-9296 Polar Environmental Centre, Tromsø, Norway Received 28 January 2005; accepted 19 December 2005 Available online 2 February 2006
Abstract Sea and air temperature data from the period 1980e2003, representing fjordeouter coast transects at three locations in north Norway (Balsfjord, Altafjord and Porsangerfjord), have been examined. Air and sea surface temperatures were well inter-correlated between all stations, indicating that the coastline in question is a coherent climatic region. This conclusion is strengthened since no correlations were found between the transect data and the northern FugløyaeBjørnøya transect. Neither did our data sets bear any clear resemblance to the variation in the NAO index. Generally there was a delay of four months before the surface temperature signals reached the deepest bottom waters. The cooling period (heat loss from sea to air) lasts longer than further south, leading to a prolonged period with unstratified waters in the north. This strengthens the effect of sea temperatures to follow the air temperatures, i.e. sea temperatures in the region are controlled by local climatic processes. The mean annual heat loss for the measured stations was 31 W m2. The largest mean surface heat loss was in Porsangerfjord, that has no sill (45 W m2). Balsfjord, the only fjord with a sill, had the lowest annual heat loss (21 W m2). This difference can be explained by transport of inflowing coastal water being restricted by the sills in Balsfjord. No clear heating or cooling trends could be detected for the period 1980e2003, and comparison with data from the period 1930e1979 revealed anomalies well within the ones observed by us for 1980e2003. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: fjord; coast; sea temperatures; heat flux; north Norway
1. Introduction The fjords and shelf areas along the north Norwegian coast hosts large fisheries and aquaculture activities. Since the last period of the 19th century, when researchers hypothesized that variability in physical parameters influenced marine life (Sars, 1879), much effort has been put into research on physicalebiological regulatory mechanisms (Cushing, 1978; Larraneta, 1988). The 17th century had long periods with extremely low fish catches (Sandvik and Winge, 1987). This had massive impacts on the northern communities, and it is estimated that during this period the child population was reduced to ca. 50% due to starvation. These declines in fisheries are
* Corresponding author. E-mail address: [email protected] (H.Chr. Eilertsen). 0272-7714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2005.12.006
claimed to have had climatic reasons, i.e. a significant drop in sea-temperatures due to low air temperatures. This may also have occurred during the period 1785e1810 (Hanssen, 1990). According to Ellertsen et al. (1987) there is a clear relationship between temperature and year class strength of Arcto Norwegian cod. It is therefore obvious that understanding and predicting sea temperature variation is of crucial importance. There exists several monographs on inter-annual variability in sea temperatures globally (MacKay and Ko, 2001), in the North Atlantic (Dickson et al., 2000), the Norwegian and Barents Sea (Ottersen et al., 2000; Furevik, 2001) and the Arctic Ocean (Steele and Boyd, 1998). Several papers deal with hydrography from the area around Tromsø (Soot-Ryen, 1934; Sælen, 1950; Eilertsen et al., 1981; Svendsen, 1995). The lack of published material from Finnmark, the northernmost part of the Norwegian coastline adjacent to the Barents Sea, is though apparent. The coastal areas and fjords of North
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Norway are under influence of the northward flowing Norwegian Coastal Current (NCC) containing Norwegian Coastal Water (NCW) that has its origin in the Baltic Ocean. Northern coastal waters can therefore be influenced by climatic processes taking place further south, but also by interactions between NCC and the outer lying Atlantic Water (AW) in the Norwegian Atlantic Current (NWAC). It is assumed that temperature anomalies may be advected into the region, but may also be caused by local variations in heat flux processes (Furevik, 2001). One of the main mechanisms believed to cause telecommunication variations is that during periods with prevailing westerlyesouth westerly winds the water in Skagerak will pile up and the flow northwards along the Norwegian coast is hence retarded (Aure and Sætre, 1981). When the winds then either slackens or changes direction the water masses will be let loose and move fast up along the Norwegian coast, thus changing temperatures in coastal waters off Norway (Aure and Sætre, 1981; Nilsen and Hansen, 1980). During the winter 1978/1979 temperatures were exceptionally low in the Norwegian fjords, and it is suggested that this was mainly caused by cold water piled up in Skagerak let loose by changing wind directions (Nilsen and Hansen, 1980). This may well have been the cause for the southern fjords due to the close vicinity to Skagerak. NCW is continuously being mixed with AW while travelling northwards and we therefore question the validity of the ‘‘Skagerak hypothesis’’ for the northern areas. When investigating the causes of the 1978/1979 ‘‘lows’’ (Nilsen and Hansen, 1980), local heat flux was not considered, even if climatic data showed air temperatures well below normal. Our hypothesis is that northern coastal areas and fjords are much less influenced by seaborne telecommunication mechanisms than the southern temperate areas due to a prolonged period with winter overturning of the water masses (Sælen, 1950; Eilertsen et al., 1981; Svendsen and Thompson, 1978). The summer stratification is also generally weaker in the north since solar heating is less effective and surface salinities are higher due to less runoff compared to southern Norway (Skofteland, 1985), making transportation of heat from deeper waters more predominant. This makes us expect that ambient air temperature plays a significant role in temperature regulation in northern waters, enhancing the mixing regime characteristic of temperate shelf seas (Prandle and Lane, 1995). The present work attempts to clarify some central issues related to the hydrophysical mechanisms of northern coastal areas and fjords, i.e. the actual ranges of spatial, seasonal and inter-annual temperature variations and the relative degree of regulation by local or distant oceanic/climatic processes. 2. Methodology The data sets, representing the period 1980e2003, were collected as part of the Sea Environmental sampling programme managed by the University of Tromsø. The research vessels used were ‘‘Johan Ruud’’ (100 ft) and occasionally after 1984 ‘‘Jan Mayen’’ (184 ft). The applied CTD systems were Neil Brown Instruments Mark III and Seabird 911
531
Plus. From 1999 on the CTD systems were calibrated prior to and after each cruise. For the earlier period temperature and conductivity sensors were checked or calibrated at infrequent intervals and the data sets, if necessary, corrected for the observed drift. The sea temperature data sets were normally sampled at 1e4 monthly intervals, while for the periods 1985e1986 and 1990e1991 we performed 2e4 samplings each year. Daily sea surface temperatures from Skrova in Lofoten (Fig. 1) and meteorological data (air temperature, wind, cloud cover) are provided by The Norwegian Meteorological Institute. All hydrographical data can be downloaded from http://lupus.nfh.uit.no. The present analysis is based on sea temperature data from 5, 10, 25, 50, 75 m and 5e10 m above the bottom. For the periods 1930e1931 and 1971e 1979, temperature data from Balsfjorden and Malangen are from Gaarder (1938), Skreslet (1973), Eilertsen et al. (1981), Eilertsen and Taasen (1984), Aure et al. (1997). The applied North Atlantic Oscillation (NAO) Index (Lisbon minus Stykkisholmur, normalized DecembereMarch Average SLP Anomalies) was an update by Hurrell of time series published in Hurrell (1995). Values are normalized and ascribed to the year of the month of January. The southernmost of the outer stations, Malangen (M, N 69 30.0eE 18 21.4, 201 m) is outside the Balsfjord sills (Fig. 1). Outside the station lies the Malangen Deep (477 m). The essential part of freshwater supply to this fjord comes from the Malangen River, and though there is some effect of this river on salinity at the station, it is also heavily influenced by communication with the outer coastal waters (Sælen, 1950). Meteorological data representing the station are from the nearby Hekkingen lighthouse (Fig. 1). The station in Balsfjord (B, N 69 21.8eE 19 06.4, 180 m) is inside three shallow sills (8, 9, 30 m) separating the fjord system from the outer coastal waters. Run-off is from several small rivers, and there is a typical estuarine circulation taking place during summer. The chosen station is representative for the hydrography of the fjord, and the station depth represents maximum basin depth (Sælen, 1950; Eilertsen et al., 1981). Meteorological data are from Tromsø airport (Fig. 1). The Altafjord station (A, N 70 06.4eE 23 02.4, 330 m) is in the middle of the fjord, ca. 300 north to the stations in Malangen and Balsfjord. Maximum depth is 450 m, and sill is 190 m. The main source of freshwater is the Alta River and the innermost part of the fjord is normally ice-covered during winter. Meteorology is from Alta airport. Outer station Skipsholmen (S, N 70 54.1eE 23 50.3, 220 m) is representative of the outer coastal water adjacent to Altafjord. Meteorology is from Fruholmen lighthouse close to the station. Porsangerfjord is by far the largest fjord in Northern Norway, ca. 100 km long and 15e20 km wide. Since the station Porsanger (P, N 70 43.1eE 25 44.8, 210 m) is outside of the 60 m sill (Fig. 1), the outer part of the fjord can communicate freely with open water. The major part of runoff to Porsangerfjord comes from the rivers Lakselv and Børselv, both situated in the inner part. Meteorology is from Banak (Lakselv) airport.
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Fig. 1. Map of investigated area with sampling sites. M denotes meteorological stations. The meteorological and hydrographical station at Skrova is the same.
The northernmost outer station Helnes (H, N 71 07.5eE 26 18.0, 205 m) is located in open coastal water close to the North Cape, i.e. inside and adjacent to the North Cape bank area. Meteorology is from Helnes lighthouse. Surface heat flux was computed as, Qt ¼ Qh þ Qe þ Qb þ Qs . Sensible heat (Qh) was Qh ¼ Ch rCp V10 ðTair Tsea Þ (Brown, 1990), where Ch ¼ heat flux coefficient (1.1 103 for neutral stratification); r ¼ air density (1.2 kg m3); Cp ¼ heat capacity (air) at constant pressure (1004 J kg1 K1); V10 ¼ wind in m s1 10 m above sea surface; T ¼ temperature in C. Latent heat (Qe) is evaporation energy of sea water, Qe ¼ rLv Ce V10 ðqair qsea Þ (Smith et al., 1983). Here Lv ¼ latent heat of evaporation (2.5 106 J kg1) and Ce ¼ Ch while q denotes specific humidity at 10 m altitude and at the sea surface. Further qair=sea ¼ 3eair=sea =ðph ð1 3Þeair=sea Þ, and 3 ¼ 0.622; ratio between molecular weight of water vapour and dry air,
eair=sea ¼ r611:0 10ð7:5Tair=sea =ðTsea þ273:1535:86ÞÞ , where r is the relative humidity of the air, 0e1, and ph ¼ mean air pressure in the northern hemisphere; 101 400 Pa). Since we lacked data on humidity we used a constant value of 0.6. Net long-wave radiation (Qb, black-body radiation) can be expressed as, Qb ¼ 3g dðfair ð273:15 þ Tair Þ4 ð273:15 þ Tsea Þ4 Þ (Henderson-Sellers, 1986), where 3g ¼ long-wave emissivity for sea (0.97), d ¼ Stefan Boltzmann’s constant (5.67 108 4 2 J s1 K4 m2), fair ¼ ð1 0:261 e7:7710 Tair Þ ð1 þ 0:275fc Þ where fc ¼ relative proportion of cloud cover (scale 0e1). Short-wave solar irradiance (Qs) in W m2 for clear sky was computed as described in Frouin et al. (1989), after input of surface visibility, regression coefficients for maritime atmospheres and solar zenith angle computed at given geographical position and time according to the equations in Iqbal (1983). In all the computations we used monthly means of all meteorological and sea temperature data. A more detailed presentation/analysis
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of the present data sets can be found on http://nfh.uit.no/ phaeocystis/env. 3. Results 3.1. Air and sea temperatures The lowest (9; 7.4 C) and the highest (12.7; 13.5 C) mean monthly air temperatures were in January and July at Porsanger and Altafjord (Table 1). The highest annual mean was in Malangen (4.1 C) while Porsanger had the lowest (0.85 C). All the outer stations (Malangen, Skipsholmen and Helnes) had higher minimum sea surface temperatures than the fjord stations (Table 1) while the lowest temperatures were in February in Balsfjord. The northernmost station, Helnes, had the highest minimum temperatures, while Skipsholmen (between Helnes and Malangen) had second highest minimum temperatures. The highest monthly mean was in August in Altafjord while Balsfjord was coldest (Table 1). The coldest fjords during winter/early spring were Balsfjord, Porsanger and Altafjord while during winter the outer stations were warmest (Malangen, Helnes and Skipsholmen). The sea temperature amplitude was gradually reduced with depth (not shown), but the tendency prevailing at the surface was observed at all depths, also in the bottom waters (Fig. 2). During autumn, winter and spring (SeptembereApril)
533
the sea surface temperatures were always higher than the air temperatures (Table 1) while during summer (MayeAugust) this was reversed. For all locations the annual mean sea temperatures were higher than the air temperatures (Table 1). When analyzing the variance of the meteorological data and sea temperatures for the period, the deviation from the mean when 75% of the data were accounted for was at maximum 385% for air temperatures, 22.4% for sea temperatures, 18% for wind and 12% for cloud cover. 3.2. Seaeair temperature correlations and anomalies When daily air temperatures from the stations were correlated linearly, all correlations were significant at p < 0.05 (Table 2). All stations, except Skrova, had correlation coefficient values between 0.88 and 0.97 (Table 2). The air temperatures at the Skrova station differed from the rest at all instances by showing significantly lower correlations with the other stations (0.61 < R < 0.66). Correlation tests between means of air temperatures 3, 5, 10, 20 and 30 prior to the date of sampling and sea temperatures, showed that the best correlations were always between sea temperatures at all depths and the 30 and 20 day means (not shown). The R values decreased downwards, and for the bottom water there were no significant correlations. All sea temperatures were also well correlated between the fjords (Table 2). In sum 82% of the correlation analyses performed between air and 5 m sea temperature had
Table 1 Monthly mean of cloud cover (0e9), air and sea surface temperatures and modelled heat flux (Qt) for each month (W m2). Positive Qt means heating of sea. At bottom of table is shown Qt when cloud cover, wind and sea temperatures were increased to values comprising 75% of the data for the period 1980e2003, i.e. ca. þ10% CC, þ20% wind and 20% increased sea temperature
Malangen cloud cover Malangen, sea temperature Malangen, air temperature Malangen Qt Balsfjord cloud cover Balsfjord Qt Balsfjord, sea temperature Balsfjord, air temperature Skipsholmen cloud cover Skipsholmen, sea temperature Fruholmen, air temperature Skipsholmen Qt Altafjord cloud cover Altafjord, sea temperature Altafjord, air temperature Altafjord Qt Porsanger, cloud cover Porsanger, sea temperature Porsanger, air temperature Porsanger Qt Helnes cloud cover Helenes, sea temperature Helnes, air temperature Helnes Qt Helnes Qt þ10% cloud cover Helnes Qt þ20% wind Helnes Qt þ20% sea temp.
Jan
Feb
Mar
5.68 3.50 1.51 99 5.76 112 2.66 3.74 5.80 4.52 1.89 116 5.61 3.56 7.43 163 5.18 3.28 9.03 177 4.90 4.67 3.05 123 119 129 132
5.22 5.06 3.05 3.19 1.43 0.40 94 67 6.00 5.85 90 64 1.33 1.46 3.56 2.26 6.52 6.89 3.67 3.44 1.78 0.97 97 72 6.13 5.64 2.59 2.19 6.60 4.24 137 96 5.95 5.08 2.46 1.95 7.89 5.36 149 106 6.65 6.20 4.25 3.61 3.01 1.92 96 69 91 67 102 73 101 74
Apr
May
Jun
Jul
Aug
Sept
Oct
4.56 3.69 2.09 5 5.70 6 2.48 0.66 6.50 3.63 0.84 26 5.14 2.66 0.06 13 4.37 2.30 0.81 10 5.28 3.80 0.28 10 14 11 13
4.82 4.97 5.71 78 6.74 37 4.54 4.91 7.39 4.22 3.77 13 6.53 4.33 4.84 43 6.41 3.45 4.12 47 7.06 4.52 3.53 15 2 15 9
3.57 6.64 9.06 155 4.22 135 7.17 9.29 4.98 5.84 6.84 99 4.73 7.18 10.0 136 4.38 6.15 9.37 148 5.20 5.88 6.99 77 66 78 63
3.93 8.32 11.4 148 5.49 120 8.56 11.8 6.26 7.85 9.71 79 6.00 9.27 13.5 131 5.60 8.41 12.7 136 5.95 7.38 10.0 69 57 71 56
3.90 9.38 11.4 86 5.49 74 8.56 11.0 5.36 8.93 9.97 52 5.48 10.5 12.2 65 4.72 10.4 11.2 57 4.51 9.30 9.99 38 32 38 23
3.46 4.26 9.30 7.32 8.35 4.40 24 85 3.75 5.96 27 84 8.28 6.53 7.09 2.65 4.40 5.90 8.71 7.58 7.63 4.15 25 80 4.42 5.66 8.67 7.40 7.60 2.05 26 107 4.52 5.63 8.24 7.05 6.90 1.41 29 110 4.13 5.81 8.44 7.77 7.37 3.29 30 90 29 86 31 94 42 102
Nov
Dec
Annual mean
5.02 5.67 1.11 103 5.75 115 5.13 1.06 6.91 6.63 0.75 106 6.23 6.06 3.76 155 5.67 5.58 4.77 163 6.70 6.26 0.21 111 106 118 120
5.88 5.46 0.92 117 6.42 120 4.14 3.11 6.71 5.91 1.05 119 5.53 5.24 6.38 176 5.57 4.71 7.67 181 6.41 5.69 2.10 124 119 132 131
4.6 5.87 4.11 9.8 5.6 21 5.07 2.81 6.1 5.91 3.16 33 5.6 5.81 1.82 42 5.3 5.33 0.85 45 5.7 5.96 2.59 38 39.5 42 52
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534
Fig. 2. Mean monthly 5 m temperatures for the investigated stations. For station depths see Fig. 1.
R > 0.6, while inter-correlation analysis between depths and similar depths between stations showed the same trend, i.e. 84% had R > 0.6. For both cases the correlations between the data from Skrova and the other localities were in another class of correlation in that R values generally were of an order of 0.2 below the others. Air temperatures at all stations from Tromsø and northwards were therefore significantly correlated, as was also air vs. sea surface temperatures. At all stations the temperature anomalies indicated that there were four clearly recognizable cold periods, i.e. 1981e 1982; 1986e1988; 1994 and 1998, while 1990e1993 and 1995 were warm periods. This tendency persisted at almost all depths, i.e. if the surface anomalies showed warm years the bottom waters followed as well (Figs. 3e5). The deviations from this general picture was that in Balsfjord in 1995 and in Malangen 2002 the surface temperatures were above normal while bottom waters were slightly below normal. The overall coldest and warmest years in the region with respect to sea temperatures were 1981 and 1992. 3.3. Heat flux At all stations the annual mean of sea temperature minus air temperature was positive (Table 1) The mean heat loss for the
measured stations was 31 W m2 while the largest mean net surface heat loss was in Porsanger with 45 W m2 (Table 1). The northernmost fjord/inland stations had the highest loss, while Balsfjord had less than half of this (21 W m2). 4. Discussion Our analysis of sea and air temperature data demonstrate that the area from Tromsø and northwards, represented by three pairs of adjacent ‘‘fjordecoastal’’ stations (Balsfjorde Malangen; AltafjordeSkipsholmen; PorsangereHelnes), is a coherent climatic region with respect to both air and sea temperatures. This is also demonstrated by the much weaker correlation between temperatures from Vestfjorden (Skrova) and the northern areas, as well as the significant correlation between the northern locations themselves. The inner fjord stations had lower air and sea temperatures than the outer stations during winter, while during summer this was reversed. This is a typical feature of inland climate where the moderation of air temperature from sea is less pronounced than for outer coastal areas (Taylor and Stephens, 1983). The winter sea surface temperatures were lowest in Balsfjord while Porsanger and Altafjord had higher temperatures (Table 1). The winter bottom temperatures were substantially lower in
Table 2 Linear correlation coefficients for daily air temperature measurements and sea bottom temperatures at investigated stations. Sea bottom temperatures for Skrova not available. All values are significant at p < 0.05
Air Skrova Balsfjord Malangen Alta Skipsholmen Porsanger Helnes Sea bottom Balsfjord Malangen Alta Skipsholmen Porsanger Helnes
Skrova
Balsfjord
Malangen
Alta
Skipsholmen
Porsanger
Helnes
1.00 0.66 0.68 0.62 0.60 0.61 0.61
0.66 1.00 0.93 0.95 0.92 0.93 0.93
0.68 0.93 1.00 0.90 0.88 0.88 0.89
0.62 0.95 0.90 1.00 0.91 0.98 0.94
0.60 0.92 0.88 0.91 1.00 0.91 0.97
0.61 0.93 0.88 0.98 0.91 1.00 0.94
0.61 0.93 0.89 0.94 0.97 0.94 1.00
1.00 0.65 0.77 0.53 0.40 0.51
0.65 1.00 0.68 0.87 0.81 0.81
0.77 0.68 1.00 0.45 0.33 0.40
0.53 0.87 0.45 1.00 0.90 0.92
0.40 0.81 0.33 0.90 1.00 0.79
0.51 0.81 0.40 0.92 0.79 1.00
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535
Fig. 3. Temperature anomalies for 5 m at station Balsfjord.
Porsanger and Balsfjord than in Altafjord (ca. 2 C). Balsfjord is the only fjord with shallow sill areas (9e30 m), i.e. a true fjord (Fig. 1), while Altafjord and especially Porsangefjord have deep sills (190e200 m). Since the winter bottom water in Altafjord was only ca. 1 C lower than at the coastal station Skipsholmen, we conclude that it is Altafjord that has the best exchange of water. In narrow fjords the exchange with adjacent waters takes place as a two or multilayered current system. In wider fjords, such as Porsangerfjord, rotational effects influence the dynamics significantly (Svendsen, 1995; Cushman-Roisin et al., 1994), and may slow down the exchange of fjord water relative to more narrow fjords. Since in addition current measurements in Porsangerfjorden indicates this (Svendsen, 1995), we believe that this makes up for the large part of the explanation why the Altafjord bottom water was warmer than in Porsanger, even if the winter surface temperatures were comparable (Table 1). This also coincides
well with observations that, during the excessively cold winter 1978/1979, temperatures registered in Balsfjord (0e50 m) were considerably lower than for Altafjord (Nilsen and Hansen, 1980). Though, taking into account these main differences, it is clear that the relative changes in air and sea temperatures were well inter-correlated. This means that if it was a cold year it was cold all over, and if warm this was true for the entire area from Tromsø northwards. This is demonstrated by the high degree of correlation between air temperatures, between depths in each fjord and between depths in fjords and between the bottom waters (Table 2). Since the temperature variations at Skrova in Vestfjorden were significantly different from all the northern stations and anomalies at the northern stations followed the same pattern (Figs. 3e5), we feel confident that our conclusion is solid. Further this conclusion is not surprising taken into account that the Vestfjorden basin, situated
Fig. 4. Temperature anomalies for 5 m at station Malangen.
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H.Chr. Eilertsen, J. Skarðhamar / Estuarine, Coastal and Shelf Science 67 (2006) 530e538
Fig. 5. Temperature anomalies for 5 m at station Porsanger.
more than 1 of latitude south to Tromsø (Fig. 1), traps and slows down north flowing NCC water influencing both the atmospheric and ocean climate in the region making it unique (Furnes and Sundby, 1981). The bottom temperatures in our investigation were at almost all instances uncorrelated with the shallow water temperatures (5e50 m). The apparent reason for this was that the amplitudes were much weaker and out of phase with the overlying water masses. While surface temperatures peaked in July, corresponding well with the highest air temperatures (Table 1), the bottom waters peaked in Novembere December while minimum was in May. This is in the same range as depicted earlier for Balsfjord and Malangen (Sælen, 1950; Eilertsen et al., 1981) and suggests that there is typically
a delay of ca. four months in the seasonal temperature signal from surface to bottom. In the Barents Sea (Bjørnøyae Fugløya section) the mean delay in the signal is also ca. four months from surface to 400 m (Furevik, 2001). For the North Sea, i.e. a more shallow shelf area, it is suggested that the delay is up to three months (Prandle and Lane, 1995). They also concluded that the enclosed area of the North Sea is essentially localized with respect to airesea thermal balance, with only minor effects of advection and dispersion on temperature. A simplified expression for the time in days for complete vertical mixing of a surface exchange in well mixed areas is Tv ¼ D2 =E where D is the depth in meters and E is the eddy diffusion coefficient (Prandle and Lane, 1995). By assuming
Fig. 6. Temperatures (left y-axis, solid line, C is mean of 10e200 m, updated data from Furevik, 2001) in the FugløyaeBjørnøya transect and NAO winter index (right y-axis, dotted line).
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that D is 277 m and E is constant with depth and solving the expression for a time period of 115 days (3.5 months) we find E ¼ 7.7 103 m2 s1. An approximated E for stratified areas may be 2.0 104 m2 s1 (Malacic, 1991; Lane and Prandle, 1996), indicating that either the mixing was large in our area and/or that the stratification was weak. The stratified period in Balsfjord, around Tromsø island and Malangen lasts from the second week of May until late Septembereearly October (Sælen, 1950; Skreslet, 1973; Eilertsen et al., 1981), i.e. ca. four months. The stratification is weak (max. Dst 0e50 m ¼ 1.0e2.0) and is mainly caused by lowered salinity due to freshwater runoff and to a lesser degree by increased temperatures. Further north offshore this period is shorter and the pyncolines are even weaker. Thus coastal waters and fjords of northern Norway are, for natural climatic reasons, less stratified for a shorter period of time than the fjords of western and southern Norway (Sælen, 1950; Nilsen and Hansen, 1980). This emphasizes the potentially localized nature of the airesea thermal balance in the area, and that the mixing period lasts longer. At all the localities the annual mean sea temperature was higher than annual means for air (Table 1), demonstrating the consistent net release of heat transported northwards into the region with the AC and NCC. The mean heat loss for the measured stations was 31 W m2 while the largest mean net surface heat loss was at Porsanger (Table 1). Typically it was the northern fjord/inland stations that had the highest loss (Porsanger 45 W m2 and Alta 42 W m2), while Balsfjord had half of this (21 W m2), even though it also penetrates into inland climate with low winter temperatures. The reason for this is that Balsfjord is a ‘‘true’’ fjord with shallow sills, and that exchange of deeper lying bottom water is hindered by the sills, thus limiting advection of heat into the fjord. The deep water exchange normally takes place during spring and early summer. NCC lies as a wedge on top of AW and oscillations of this wedge is needed to lift the more dense AW high enough to cross the sill, thus advecting heat to the fjord basin (Sælen, 1950; Eilertsen et al., 1981). The other two fjord locations have much deeper (Altafjord) or no (Porsangefjorden) sills, so exchange of deeper water will take place either continually or at frequent intervals (Svendsen, 1995). The most cloudy climate was at the outer station Skipsholmen (CC annual mean ¼ 6.1), while Malangen had the clearest skies (4.6). Cloud cover blocks transfer of solar radiation to the sea, and it is clear that this plays an important role in determining the local climate since it is also indirectly linked to sensible heat transfer. The sensitivity tests (bottom of Table 2) where we increased cloud cover, wind and sea temperature to values comprising 75% of the data sets, clearly indicate that it is advected heat into the fjords that cause the largest heat flux variations. Since all the bottom temperatures were so well correlated this also stresses the importance of fjordecoast communication, and that the annual mean difference between sea and air temperatures in Altafjord and Porsangefjorden possibly can be used as a reasonably reliable measure of long term changes in heat transported northwards with NCC.
537
Our temperature anomalies indicated four cold periods (1981e1982; 1986e1988; 1994 and 1998). The temperature of the coastal water is influenced by the outer lying AW, that again is believed to be linked (though weakly) to the NAO winter index (Furevik, 2001). As Fig. 6 indicates, by computing 10e200 m means from the data given by Furevik (2001) there was only one clearly cold period (1985e1988), but also possibly 1996, plus two warm periods (1983e1985; 1992e1995). From our data we identify the warm periods as 1990e1993 and 1995. Thus the sea temperatures in the fjords, and hence the air temperatures, are not well correlated to either AW temperatures or the NAO index. This again strengthens the conclusion of the localized nature of the hydrophysical variations in the region. A drawback of our data sets is that for some of the years sampling was scarce (e.g. Helnes last part of period), and that we only have single point measurements. Also may modulation of the seasonal air temperature by mixed water (Prandle, 1998) have played a role, i.e. the inflowing seawater may have forced air temperatures somewhat. Considering the clarity of our results, we though conclude that since the variations in sea temperatures in the entire north Norwegian coastal region (north to Tromsø) is highly interlinked this should be taken into account when planning further sampling. A possible strategy would also be to sample at selected stations at frequent intervals, also taking into account the assumed short-term (hours) variability of sea temperatures due to wind and tidal excursion (Eilertsen and Taasen, 1981). By analyzing our data sets we could not observe any warming or cooling trends, and analysis of data from periods prior to 1980 (see references in Section 2) showed no cold or warm trends, i.e. all the data were close to the means and deviations smaller than the maximum anomalies observed for the period 1980e2003.
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