Variability of Climate and Related Physical Factors

The 11-year climate record for the Toolik Lake site (table 5.1) indicates a mean daily air temperature of -8.8°C and a total annual precipitation of 315 mm. Monthly means are above freezing for 3 months, and most of the precipitation occurs from June through September. In figure 5.2, the year-to-year variability for two biologically important indices, the annual degree-days above 0°C and the summer rainfall, illustrate the nearly twofold difference from year to year.

Solar radiation is a very important physical factor that affects characteristics such as the stratification and heating of the surface layers of lakes as well as the depth of thaw of soils. Biological processes are also affected, particularly photosynthesis in terrestrial plants and plankton. Solar radiation varies at different timescales from the minute-to-minute variation caused by passing clouds to the in-terannual changes between summers differing in cloudiness.

Although the air temperatures and their sum, the degree-days shown in figure 5.2, are useful indicators of possible effects on the aboveground parts of plants, a better indicator of soil temperatures and its effect on soil roots and microbes is the depth of thaw of the active layer of the soil (figure 5.3). The thaw depth is mainly affected by the amount of insulating plant material on the surface of the tundra, by the air temperature, and by the soil moisture. For example, 10 years after a single

Climate Forcing at the Arctic LTER Site 77 Table 5.1 Eleven-year summary of climate at Toolik Lake, 1989-2000

Maximum Minimum

Mean Daily Air Average Average Total Monthly

Month Temperature, °C Temperature, °C Temperature, °C Precipitation, mm

Maximum Minimum

Mean Daily Air Average Average Total Monthly

Month Temperature, °C Temperature, °C Temperature, °C Precipitation, mm

1

-24.2

-20.3

-31.7

8.2

2

-21.4

-8.1

-31.3

13.3

3

-19.9

-12.5

-26.6

9.4

4

-11.4

-4.5

-17.5

9.2

5

-0.9

4.3

-9.6

16.7

6

8.5

9.6

6.2

44.6

7

11.6

14.1

9.2

67.8

8

7.3

11.3

3.7

67.1

9

-0.8

3.3

-8.8

37.3

10

-12.7

-6.5

-16.7

18.3

11

-19.7

-12.3

-29.2

9.8

12

-22.6

-17.1

-29.7

13.0

Long-term average or sum

-8.8

314.7

dose of fertilizer, the amount of plant litter was greater and the soil temperature was significantly lower in a treated tundra plot than in a nearby untreated plot (LTER data of G. Shaver and J. Laundre). Despite these complications, the degree-days (figure 5.2) are roughly correlated with the thickness of the active layer (figure 5.3). The two periods with the thickest depth of thaw, 46 cm in 1993 and 1997, occurred during warm summers but not the warmest.

Another indicator of the effect of climate variability is the 26-year record of July temperatures of the surface waters of Toolik Lake (figure 5.4). These temperatures are affected by the air temperature but most of all by the amount and timing of the

Figure 5.2 The annual degree-days (sum of daily average temperatures above 0°C) and the June through August rainfall at Toolik Lake, 1989-2000.

Figure 5.2 The annual degree-days (sum of daily average temperatures above 0°C) and the June through August rainfall at Toolik Lake, 1989-2000.

Figure 5.3 The mean thickness of the active layer at the end of the summer season at the Toolik Lake LTER site (68° 37' N, 149° 36' W) and the Barrow CRREL site (71° 19' N, 156°35' W), Alaska (data from the Circumpolar Active Layer Monitoring network) (Brown et al. 2000).

H 13

Jl 3

1975 1980 1985 1990 1995 2000

Year

Figure 5.4 Toolik Lake average temperatures during July at 1 m depth.

Figure 5.5 Average temperature at 20 meters in an 80-m-deep borehole near Galbraith Lake, Alaska (68° 28' N, 149° 29' W) (pers. Comm. T. Osterkamp and V. Romanovsky, University of Alaska, Fairbanks. 4/2/02).

solar radiation. When the ice stays on the lake until 1 July, then the monthly mean water temperature will be cooler than a summer with ice out in mid June. Given the variation in the dates of the ice melt on the lake, it is remarkable that there is any trend at all. The one obvious trend is the long-term increase of about 2°C. Other studies (e.g., Chapman and Walsh 1993) have pointed out that there is a continuing 30-year warming of air temperature in northern Alaska. A possible integrator of air temperature is the temperature in the upper levels of the permafrost. T. E. Osterkamp and V. Romanovsky of the University of Alaska Fairbanks (pers. comm., 4/2/02) found that at Galbraith Lake, 20 km south of Toolik, the temperatures at a depth of 20 m in a borehole driven into the soil showed an impressive warming of 0.8°C since 1991 (figure 5.5). Unfortunately for the perfect integrator theory, recent analysis (Marc Stieglitz, Lamont Doherty Earth Observatory, Columbia University, pers. comm., 1/8/03) points out that the change is very likely caused by two factors: a warming of air temperatures and an increase in the amount of snow during the winter. Only about half of the permafrost warming is due to an increase in air temperature. While the permafrost temperatures may not tell us about the air temperatures, the analysis illustrates changes in snow cover can be just as important as changes in air temperature in regulating soil temperatures and can even amplify below-ground temperatures. For long-term predictions, the alteration in winter precipitation must be better understood.

The year-to-year variability is especially important for stream ecology. Figure 5.6 illustrates two extremes in the Kuparuk River. In 1990 the flow was very low, with only one high flow event (or spate) after the spring runoff; temperatures fell mostly between 10 and 15°C. In 1995 there were some nine spates during the summer, and temperatures fell between 5 and 10°C. The inlet stream to Toolik Lake showed similar variability in the number and temperature of spates.

-Discharge Temperature

31-May 15-Jun 30-Jun 15-Jul 30-Jul 14-Aug

31-May 15-Jun 30-Jun 15-Jul 30-Jul 14-Aug

Figure 5.6 The Kuparuk River discharge (m3/s) and water temperature (°C) during (A) a low flow year (1990) and (B) a high flow year (1995).

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