Å·ÃÀ¿Ú±¬ÊÓƵ

Keen, Richard ¹

¹ ATOC, University of Å·ÃÀ¿Ú±¬ÊÓƵ

In a study commissioned by the National Park Service, data from 99 climate stations and 61 snow survey sites within and near the three units of the NPS Central Alaska Network (CAKN), Wrangell-St. Elias, Denali, and Yukon-Charley Rivers, provide a record of past climate changes and fluctuations. The period of record begins in 1899, allowing insights into the causes of Alaskan climate change not accessible from more recent shorter records.

Six stations and three merged combinations of stations yield temperature records longer than 87 years. Yearly z-values are combined into a CAKN regional average. Snow water equivalent (SWE) records since 1958, expressed as a percentage of the long term average, are taken to be the total cold season precipitation.

Time series of temperature and SWE are presented in figures 1, 2, and 3, along with climate indices. Climate indices are measures of processes within the ocean-atmosphere system, or in some cases external to the system, that can be considered large scale influences on climate variations. Among the indices examined: Total Solar Irradiance (TSI), Carbon Dioxide (CO2), Northern Annular Mode (NAM), Arctic Oscillation (AO), North Atlantic Oscillation (NAO), North Pacific (NP), Pacific Decadal Oscillation (PDO), East Pacific Oscillation (EP), Atlantic Multidecadal Oscillation (AMO), and Multidecadal ENSO Index (MEI).

The North Pacific (NP) has the highest correlation with annual temperature (r= -0.68); the correlation improves to r= -0.77 for 5-year means of temperature and NP index. The NP is the barometric pressure within the Aleutian Low that inhabits the North Pacific; when the pressure is lower (negative NP), southerly winds invade Alaska, bringing mild maritime air and raising the temperature. When the index is high, the low is weaker, and arctic air may engulf the state. The NP index is the atmospheric component of the Pacific Decadal Oscillation, an air-sea process that has been shown to have a major influence on Alaska’s climate.

The NP is responsible for 61 percent of the variance of central Alaska’s decadal-scale temperature variations (5-year means). The Arctic Oscillation (AO), which in Alaska involves the location and intensity of the jet stream and storm track, explains another 5 percent. Together, the NP and AO explain all but 0.1 degree of the observed trend since 1899. After subtracting the NP and AO, no other index explains more than 4 percent of the residual, or about 1 percent of the total variance.

SWE is poorly correlated with the NP. However, very high AO indices (when the jet stream and storm track move northward over interior Alaska) bring above average SWE, as in 1989-1993 and 2000, while low AO indices are dry years (1969, 1970, and 2010). Strong el Niño events (MEI) bring below average SWE to the region.

Climate Indices at NOAA and NCAR: http://www.esrl.noaa.gov/psd/data/climateindices/list/ and http://www.cgd.ucar.edu/cas/jhurrell/indices.info.html

Hartmann, B., and Wendler, G. 2005. The significance of the 1976 Pacific climate shift on the climatology of Alaska. Journal of Climate 18: 4824–4839. http://climate.gi.alaska.edu/researchprojects/hartmann%20and%20wendler%202005.pdf

Keen, R.A., 2008. Climate Data Analysis of Existing Weather Stations in and around the Central Alaska Network (CAKN), http://science.nature.nps.gov/im/units/cakn/Documents/2008reports/CAKN_Climate_Data_%20Analysis_%20Keen_2008.pdf