Contenido de humedad

3.1.1

Climatic conditions in NW Svalbard

The NW part of Svalbard is strongly influenced by a branch of the Gulf Stream, which leads to a maritime climate with cool summers, while the winter tempera- tures are comparatively mild compared to locations at similar northern latitudes. The warmest month is July with an average air temperature of around +5◦_C,

while February is on average coldest with mean air temperatures of −14◦_C.

Since the 1970s, the air temperatures in Ny-˚Alesund have increased by about 2 K, which can be attributed to warmer winter temperatures (Fig. 3.3). The

Kongsfjorden

Ny-Ålesund

Bayelva site

Bayelva river

Bayelva

climate

station

Eddy

covariance

system

Thermal camera

temperature profile

Figure 3.1: Photos of the Kongsfjorden area and the study area at the Bayelva river taken from Schetelig mountain (740 m a.s.l.) on 8 September 2007. As can be seen in the upper image, the “lowland” permafrost areas comprise a 2 to 4 km wide strip between the Kongsfjorden and the partly glaciated mountain chain of the Brøgger peninsula to the right. A closer view of the study area, which covers roughly 0.5 km2_{, is shown in the lower image with the locations of}

Kongsfjorden

N

10 50 40 30 20 BS, EC TC 500 m Bayelva airport Ny- lesundÅ BSRN station

Figure 3.2: Location of the study area (red) and the BSRN site in the village of Ny-˚Alesund, where routine meteorological measurements are conducted; BS: Bayelva station; EC: Eddy covariance system; TC: Thermal imaging system; thick black lines: roads; contour lines in meters above sea level.

increased winter temperatures can be largely explained by to changes in atmo- spheric circulation patterns, which lead to more frequent advection of warm maritime air masses to Svalbard (Hanssen-Bauer and Førland, 1998). In the long-term mean, the area around Ny-˚Alesund receives an annual precipitation of around 400 mm (Førland et al., 1997), with on average 75% of the annual precip- itation occurring during the “winter” months October to May (www.eklima.no, 2010). While the precipitation features a strong interannual variability, it has not increased significantly since the 1970s (Fig. 3.3).

Due to the influence of warm ocean currents, the NW part of Svalbard receives the least sea ice of all areas of the archipelago. During summer, the sea off Ny- ˚

Alesund is generally ice-free, and a permanent ice cover on Kongsfjorden does not form before December (Gerland and Renner, 2007). While strong variations in the ice conditions on decadal timescales have been observed in the past, sea ice concentrations around NW Svalbard have gradually diminished since the mid of the 20th century. In the past five years, the Kongsfjorden has only been sporadically frozen even during winter, which has not been recorded previously (Gerland and Renner, 2007; Cottier et al., 2007).

The budget of solar radiation is naturally determined by the annual cycle of polar night and day, but is strongly modulated by albedo changes due to the seasonality of the snow cover. In Ny-˚Alesund, the polar night lasts approxi- mately from mid of October until mid of February, while the sun does not set below the horizon from mid April to mid August. The average duration of the snow-free period is three months, typically from July to September, but it can vary from 50 to 150 days (Winther et al., 2002). During summer, the west coast of Svalbard is predominantly influenced by moist Atlantic air masses, which leads to a high percentage of cloudy days and thus effectively reduces the avail- able solar radiation.

-30 -20 -10 0 10 1969 1977 1985 1993 2002 2010 0 200 400 600 800 T_air/ C P/ mm

Figure 3.3: Average monthly air temperatures and total annual precipitation at Ny-˚Alesund from 1969 to 2009 from the records of the Norwegian Meteorological service (www.eklima.no, 2010). The warming trend for the depicted period is on the order of 2 K.

dry polar air masses, which are typically associated with comparatively warm air temperatures with overcast skies and cold air temperatures with clear skies, respectively (e.g. Førland et al., 1997). As a result of the exchange of the air masses during winter, the incoming long-wave radiation is found to vary over a wide range, while its range is much smaller during summer (Yamanouchi and Ørbaek, 1995).

3.1.2

The study period

Daily means of air temperatures and monthly totals of precipitation are dis- played in Fig. 3.4 for the period, when the field measurements for this thesis have been conducted. The period integrates well in the warming trend of the past 40 years (compare to Fig. 3.3), but does not represent exceptionally warm conditions, which have been recorded previously on Svalbard (e.g. Isaksen et al., 2007). The summer air temperatures are usually in the range between 0◦_{C and}

10◦_{C, with the highest temperatures recorded in July in both 2008 and 2009.}

The winter periods are characterized by rapid changes between cold periods with air temperatures below -20◦_{C and much warmer periods with temperatures}

Sep-07 Mar-08 Oct-08 Apr-09 Nov-09 -40 -30 -20 -10 0 10 0 20 40 60 80 100 Tair/ C P/ mm

Figure 3.4: Daily average air temperatures and total monthly precipitation at Ny-˚Alesund for the study period from the records of the Norwegian Meteoro- logical service (www.eklima.no, 2010).

is situated at the boundary between polar air masses and warm maritime air masses during winter.

The monthly precipitation is highly variable during the presented period (Fig. 3.4). The highest precipitation rates are recorded in fall and early winter from Septem- ber to February, which is the typical precipitation pattern for NW Svalbard (www.eklima.no, 2010). With almost 100 mm of precipitation each, the months of January, September and December 2008 stand out. Although these months feature more than twice the amount of precipitation of the long-term averages for these months, similar precipitation rates have been observed in a number of occasions in fall and early winter since 2000 (www.eklima.no, 2010). Therefore, the high precipitation rates rather reflect the strong variability of the precipi- tation in NW Svalbard than exceptional events. From March to August, only little precipitation is recorded compared to fall and early winter. The Kongs- fjorden off Ny-˚Alesund has been free or almost free of sea ice during the winter 2007/2008. In the consecutive winter 2008/2009, the Kongsfjorden has been covered with sea ice from March to May.

3.1.3

The Bayelva study area

In the area around Ny-˚Alesund, “lowland” permafrost is restricted to an about 2 to 4 km wide strip along the coast of the Kongsfjorden, which is bordered by the inland glaciers and mountain chains of the Brøgger peninsula (Fig. 3.1). All field measurements have been conducted in a tundra area in the catchment of the Bayelva river at 78◦_{, 55}′_{N, 11}◦_{, 50}′_{E (Fig. 3.1), which is located approximately}

2 km SW of the village of Ny-˚Alesund (Fig. 3.2). The study area is situated about half-way between the glacier Brøggerbreen, from which the Bayelva river originates, and the Kongsfjorden. The area roughly comprises the catchment of a small tundra stream (Fig. 3.1), while the gravel floodplain of the Bayelva river borders to the area to the W and S. It includes Leirhaugen hill in a bend of the Bayelva river, which is elevated by about 10 m above the floodplain of the Bayelva River.

The study area is characterized by hilly tundra at elevations between 10 m and 45 m above sea level. Sparse vegetation alternates with exposed soil and rock fields, and frost features, such as mud boils (Boike et al., 2008) and sorted circles, are found in many parts of the study area. The soil at the study site features a high mineral content, while the organic content is low, with volumetric fractions below 10%. The soil texture comprises a wide range, from gravel close to the Bayelva river to clayey sediments on the slopes of Leirhaugen hill (Boike et al., 2008). A wide variety of soil moisture conditions is found in the study area: close to the small tundra stream, the ground is permanently water-saturated, and in the streambed itself, areas with permanent standing water and even a small lake with a diameter of about 30 m exist. The ridge slopes adjacent to the stream and Leirhaugen hill feature intermediate soil moisture conditions, which strongly depend on the precipitation and the synoptic conditions. Very dry conditions can occur on the well-drained crests of the ridges, which dry out rapidly after rain events. However, these areas only comprise a very small fraction of the total area.

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