Climate Change

Climate change in Alaska

Click on a station in the map to view annual and seasonal temperature anomalies.

Anomlies are based on the 1981-2010 normal. Seasonal time series can be added an removed by clicking on the name of the graph below the plot. Seasonal means refer to the meteorological seasons (Spring = March, April, May; Summer = June, July, August; Fall = Septmeber, October, November; Winter = December, January, February).

Large-scale climate drivers in Alaska

Global atmospheric circulation is significantly influenced by the oceans. Whether or not an area sees regional sea ice has a strong impact on local and regional climate, as does proximity to the ocean in general, as noted above. In addition, there are several other factors that can influence regional climate on a synoptic scale, such as the position of the polar jet stream and cyclical shifts in patterns of sea surface temperatures. In Alaska, both the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) are important climate drivers influencing storm tracks as well as temperature and precipitation patterns.

Pacific Decadal Oscillation

The PDO is a robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. It consists of low frequency oscillations in North Pacific sea level pressure and the strength of low pressure systems frequenting the southwestern regions of Alaska (Aleutian Low), which drives broad patterns in northwestern North America surface temperatures. The PDO index is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N.

Like with ENSO, a negative PDO index indicates the cold phase and a positive index indicates the warm phase. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales. A positive phase of the PDO implies warmer and wetter conditions for the glaciers of Alaska and northwestern Canada. This climate pattern affects coastal sea and continental surface air temperatures from Alaska to California.

There is evidence of reversals in the prevailing polarity of the PDO occurring around 1925, 1947, and 1977; the last two reversals corresponded with dramatic shifts in salmon production regimes in the North Pacific Ocean and the 1977 phase shift coincides with a significant shift of mean annual air temperatures in Alaska. (See section on climate change in Alaska. Superimposed on the PDO phase shifts ocurring on a mostly decadal time scale are 0.5 - 1.5 year duration ENSO events. Depending on the phases either oscillation is in, the effects of ENSO and PDO may amplify or dampen each other. It is very difficult to definitely attribute specific climatic variations in Alaska to either PDO or ENSO, especially on small spatial and temporal scales. More information can be found in these publications:

The Significance of the 1976 Pacific Climate Shift in the Climatology of Alaska (Hartmann and Wendler, 2005.)

Disentangling Global Warming, Multidecadal Variability, and El Niño in Pacific Temperatures (Willis et al., 2018.)

PDO, ENSO

Between 1945 and 1976 a largely negative PDO index coincides with largely negative temperature anomalies in Alaska. The temperature anomalies in the lower plot are statewide values from the NCEI ClimDiv database.

El Niño Southern Oscillation

ENSO is a periodically recurring pattern of shifting seas surface temperatures, air surface pressure and winds in the tropical regions of the Pacific Ocean. It shifts irregularly between a cold phase (La Niña), a warm phase (El Niño), and neutral conditions. The duration of a particular phase is typically on the scale of a few months to a season and warm and cold phases can vary significantly in intensity.

El Niño is the warm phase of ENSO. In an El Niño winter, mid-latitude low pressure systems tend to be more vigorous than normal in the region of the Gulf of Alaska. These systems pump abnormally warm air into western Canada, Alaska with increased precipitation in southern Alaska in particular.

During La Niña winters, there is generally a blocking ridge set up in the pacific which keeps storms and moisture to the south of Alaska, affecting precipitation patterns especially in southern Alaska.

Image source: NOAA

Temperature change in Alaska

The station data in the previous section as well as gridded data for the entire state clearly show us that considering just a linear trend can mask some important variability characteristics in the time series. There are large variations from year to year and the majority of the timeseris show a large increase in 1976. A linear trend might have been expected from the fairly steady observed increase of CO2 during this time period. The period 1949 to 1976 was substantially colder than the period from 1976 to 2017. The stepwise shift appearing in the temperature data in 1976 corresponds to a phase shift of the Pacific Decadal Oscillation from a negative phase to a positive phase. Synoptic conditions during the positive phase tend to consist of increased southerly flow and warm air advection into Alaska during the winter, resulting in positive temperature anomalies.

The plots on the right show state-wide departures of seasonal and mean annual air temperature from a long-term normal (1925-2000). The data shown can be accessed through the NOAA Climate Division (nClimDiv) database. Climate divisions are subdivisions of states that have roughly consistent climatological behaviour. For these subdivisions, monthly means are computed based on station data that is gridded and superimposed on a gridded climatological baseline. For Alaska this baseline is derived from the 1971-2000 mean developed from the PRISM climatological database. The interpolation of gridded data from point data for single stations reduces the bias introduced by irregular spacing of stations throughout the state and allows for adjustments related to topographic factors such as elevation. The nClimDiv dataset has been adjusted to account for the artificial effects introduced into the climate record by factors such as instrument changes, station relocation, observer practice changes and urbanization.

If a linear trend is taken for the statewide nClimDiv data, the trend in mean annual air temperature (MAAT) for the period 1949 - 2017 amounts to 0.58°F/decade, or about 4 degrees Fahrenheit per 70 years. For the period 1976 - 2017 it is 0.48°F/decade over the entire state and 1.13°F/decade for the North Slope. Analyzing the trends for the four seasons since 1949, it can be seen that most of the change has occurred in winter and spring, with the least amount of change in fall. The North Slope, as exemplified by the weather station in Utqiaġvik, is again the exeption to the rule: here, fall is the season in which warming is strongest.