On the climate and climate change of Sitka, Southeast Alaska
Sitka, located in southeastern coastal Alaska, is the only meteorological station in Alaska and northern coastal British Columbia, with a long climatological record, going back to the first half of the nineteenth century. Sitka was the capital of Alaska, when it was part of the Russian Empire, to which Alaska belonged until 1867, when the American government purchased it. In 1827, the Russian established an observatory on Baranof Island, Sitka Harbor, which made 17-hourly observations, later extended to 19 and thereafter to all hours of the day. When analyzing the data, the 12-day time difference between the Russian (Julian) calendar, at which the observations were made, and ours (Gregorian) has to be considered. The climate of Sitka is maritime, with relative warm winter temperatures—there is no month with a mean temperature below freezing—and moderately warm summer temperatures with 4 months above the 10 °C level and plentiful precipitation all-year long. It is the warmest zone of Alaska. Even though there is a substantial break in observations in the late nineteenth century, these are the only observation, which started so early in the nineteenth century. Systematic US-based observations commenced much later normally in connection with the gold rush, whaling in Northern Alaska, and the fur trade, predominantly along the Yukon River. During the 186 years of observations from 1827 to 2013, the best linear fit gave a temperature increase of 1.56 °C for the whole period or 0.86 °C per century, somewhat lower than expected for the relatively high latitudes. The increase was nonlinear, with several multi-decadal variations. However, when comparing the first normal (1831–1860) to the last normal (1981–2010) and assuming a linear trend, a higher value of 1.06 °C per century was calculated. The discrepancy might be explained by nonlinearity and the fact that during the late nineteenth and early twentieth centuries, observations were sporadic. Furthermore, the observed warming is less pronounced than the values found for Interior and especially Arctic Alaska for later time period for which such a comparison was possible (Wendler et al. 2014). Significant correlation values were found with the Pacific Decadal Oscillation (PDO), the North Pacific (NP) Index, El Nino 3.4, and the 18.4 years nodal tide; the latter was previously reported in an excellent investigation by T. Royer (1993).
In 1827, the Russians started making systematic meteorological observation at that station. The temperature was measured in degree Reaumur (0° for freezing point and 80° for the boiling point), and the data were published in Russian and French annually in the Bulletin Scientifique, L’Academie Imperial des Sciences, Saint Petersbourg. After 1867, when Alaska became a territory of the USA, there was a break in the observations, and from thereon, temperature measurements were made in degree Fahrenheit.
There were two additional stations in Alaska, where the Russians carried out meteorological measurements, namely Iliuliuk (53° 52.6′ N, 166° 31.6′ W) on Unalaska Island (Aleutian Chain) and Ikogmut Mission (61° 47′ N, 161° 14′ W), close to the mouth of the Yukon River, in the proximity of today’s Bethel. These two stations had a dual purpose: (1) to spread the Russian Orthodox religion and (2) being headquarters for fur trade. Concerning the latter one, sea otters were especially valuable as were silver fox skins. However, for these two stations, the meteorological measurements were of relatively short duration; hence, they are of little value for climatology studies.
A summary of the early climatic observations in Alaska is given by C. Patterson (1879a) in the “Pacific Coast Pilot, Coasts and Island Stations of Alaska,” which discusses in great detail all early meteorological measurements. Further, Abbe (1906) published the first Atlas of Climate of Alaska, remarkable as it was based on a relatively short time period. Less than two decades later, Day (1922), the meteorologist in charge for Alaska by the Weather Bureau, at that time part of the US Department of Agriculture, published the climatological data of Alaska by section for the time period up to 1921.
In this paper, we discuss the climate and the observed climate change based on this single station, Sitka, as it is only station in Alaska with such a long data record. Systematic US-sponsored measurements started much later around the end on the nineteenth century mostly in connection with the Alaskan Gold Rush. Data of these stations were only used sporadically to fill gaps in missing data of twentieth century of Sitka. Further, it should be pointed out that the Sitka dataset was previously used by Royer (1993).
2 Climate of Sitka
Southeast Alaska is the warmest climate zone of Alaska (Searby 1968; Shulski and Wendler 2007) and the only region where the mean monthly temperatures stay above the freezing point year-round. It is a mid-latitude maritime climate (Cfb) according to Köppen’s classification (Köppen 1884, Köppen and Geiger 1940). “C” indicates that it is a temperate climate with no monthly mean below the freezing point, the second letter “f” refers to precipitation and indicates that significant precipitation is observed in all seasons, while the third letter “b” indicates the degree of summer heat. The warmest month of the year is below 22 °C, but there have to be at least 4 months with temperature above 10 ° C for this classification; for Sitka, it was exactly 4 months.
The annual snowfall is, on average, 820 mm; more than half of this occurs in the two winter months of January and February. As there is no month in which the mean temperature is below the freezing point, snow cover, when established, does not last long.
Mean climatic values of the last climatic normal (1981–2010) for Sitka, Alaska
Wind speed (m/s)
Looking at the annual course of the wind speed, the maximum is observed in November and December, during the months of minimum atmospheric pressure with 4.3 m/s, an expected result, while the minimum occurred in August (2.7 m/s), the month with the highest temperature. The mean annual wind speed could be calculated as 3.7 m/s (see Table 1).
Concerning heat island effects of a growing city with time in Alaska (e.g., Magee et al. 1999), this specific element is of little importance for Sitka, as the population has not grown substantially and is at present roughly 9000 inhabitants.
- 2.The original observing location of Sitka Magnetic was terminated in 1989, and the observational site was moved some 2 km to the southwest at Sitka Airport. However, for the time period from 1976 to 1989, both stations were operating, which allows a careful comparison. In Fig. 5, such a comparison has been carried out for the above time period and temperature.
One has to be careful insofar that the temperature for the 40-year period from 1827 to 1867 was measured as 5.1°Reaumur (6.2 °C), as the incorrect assumption that these early measurements were carried out in the now commonly used scale of degrees centigrade would add 1.1 °C to the observed warming to the nearly two centuries of observations. Further, the difference in the calendar (12 days) between the Russian and ours, as well as the conversion from degree Reaumur into Fahrenheit, was already carried out by Patterson (1879b).
It can be seen that the temperatures correlate excellently (r = 0.99), but that the Sitka Magnetic was, on average, 0.9 °C colder than Sitka Airport.
3 Observed climate change
The graph shows that the first half of the nineteenth century was cold, for the following time period, the general trends are difficult to determine due to frequently missing data, but years with higher temperatures were occasionally observed. The 1920s brought high temperatures, the same phenomenon being observed in other parts of Alaska (Wendler and Shulski 2009, Wendler et al. 2010). In the following years, the temperature varied widely but was relatively cool. In 1975, a strong warming was observed in connection with the change of sign of the Pacific Decadal Oscillation, which changed from dominantly negative to dominantly positive values. Mantua et al. (1997) were the first to show the strong influence of the phase of the Pacific Decadal Oscillation (PDO) on the climate of Alaska by examining the relationship between climate variability and salmon production in Alaska and the US Pacific Northwest. Monthly anomalies in the sea surface temperature (SST) field of the NP, poleward of 20° N, constitute the basis of the PDO index. Other studies of the PDO, with emphasis on its effect on Alaska, were carried out by Papineau (2001) and Hartmann and Wendler (2005). The latter studies showed clearly that in 1975/1976, when the PDO value changed from dominantly negative to dominantly positive values, a sudden temperature increase across Alaska was observed. At the start of the twenty-first century, the PDO became negative and the temperatures decreased again, which could be seen for most of Alaska with the exception of Arctic Alaska and led to an increase in the sea ice in the Bering Sea, as the intensity of the Aleutian Low decreased, advecting less warm air from the south into Alaska (e.g., Wendler et al. 2014).
The calculated temperature increase of Fig. 7 is 0.86 °C per century. The confidence in this value is somewhat compromised by several periods of missing data. Hence, we also calculated the linear trend between the first normal of 1831–1860 and for the last normal of 1981–2010, as complete climatological datasets are available for both time periods. Taking the midpoints of these two climate normals, a slightly higher value of 1.06 °C per century was found.
Ancient thermometers can suffer from a secular rise of the zero point (Mitchell 1953). In a substantial publication, Winkler (2009) made corrections to the long-term temperature series of Hohenpeißenberg, which goes back to 1781. It is the most famous long-term temperature series in Germany, as the location is not affected by urbanization. This secular rise of the zero point can be of the order of 0.5–1.0 °C. We were not able to locate the old thermometer nor do we know which type was used. We know only that the Russians used a thermometer measuring in degree Reaumur, while the Americans, after the purchase of Alaska in 1867, used thermometers calibrated in degree Fahrenheit. Such effect, if it existed, should result in a more warming than values calculated above. It might be assumed that the American observations were free of such effect, as systematic observation started only at the very end of the nineteenth century, by which time, this has been recognized and thermometers were mostly free of this effect.
4 Correlations of temperature with atmospheric indices
Wilson et al. (2006) carried out a study on the temperatures for the Gulf of Alaska based on tree rings, going back 1300 years. Looking at climatic indices, he found also the best correlation with the PDO (r = 0.53, var. = 0.28), again significant for this longer time period at the 95 % confidence level. Further, Royer (1989) investigated the upper ocean temperature variability in the Northeast Pacific Ocean.
Since the beginning of the twentieth century, monthly values of the PDO are available (Zhang et al 1997, Mantua et al. 1997) and we were able to correlate seasonal temperatures with the PDO. Spring had the highest correlation coefficient (r = 0.704, var. = 0.50), followed by winter (r = 0.648, var. = 0.42), while for summer (r = 0.546, var. = 0.30) and autumn (r = 0.507, var. = 0.26), lower values were found. This is understandable, as at these times of the year, local heating plays a more significant role. All values are significant at the 95 % confidence level.
Looking for cycles, we carried out a Fourier transform on the temperatures (not shown). Besides the normal and always observed annual variation, the solar cycle with 11 years was not statistically significant. However, Royer (1993) and McKinnell and Crawford (2007) observed previously the 18.6-year lunar nodal cycle.
5 Air–water temperature comparison
6 Discussion and conclusion
In a very recent and highly interesting paper, Johnstone and Mantua (2014) analyzed the temperature variability and change for coastal California, Oregon, and Washington. While the area is to the south of Sitka, it lies in the same climatic area of the Northeast Pacific Coast. They found a temperature increase of about 1 °C since 1900, similar to our values. As in our case, they find a strong temperature dependence on the PDO, which responds to regional atmospheric dynamics. They furthermore demonstrate that the observed temperature increase can be explained without anthropogenic greenhouse forcing. Using several independent data sources, they demonstrated that the century-long warming can be primarily attributed to changes in atmospheric circulation. We were, with our much more limited dataset, unable to demonstrate this.
We are thankful to T. Royer for temperature data as well as very useful discussions about the historic Sitka climatology. B. Moore and M. Shulski helped in various ways, for which we thank them. Further, we acknowledge Robert McCoy, Director of the Geophysical Institute, who financially supported this investigation, with State Funds directed to the Alaska Climate Research Center. Finally, we thank the two reviewers unknown to us, who pointed out some shortcomings in the manuscript, which improved this study.
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