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).