Monday, December 1, 2014

Changing Köppen Climate Classifications in Alaska

How has the climate of Alaska changed in the last 110 years? This is not so easy of a question to answer. In this analysis, we evaluate all stations in Alaska during 10-year increments beginning in 1901 and continuing through 2010. This allow for non-overlapping, temporally homogeneous comparisons. The results are a series of maps set into motion via YouTube.

Introduction:

There are many ways to describe the climate of a place. The descriptions can range from quite technical to quite informal. Earlier this year we discussed the venerable Köppen Climate Classification System that is described in every climate textbook written in the last 100 years. The system uses monthly and annual temperature and precipitation to classify all portions of the earth into one of 5 major categories and 30 minor categories. There are many critics of the system and many alternative classification systems have been developed but nothing has come close to the widespread acceptance of the Köppen Climate Classification System. Therefore, we will utilize this well established system.

The five main categories of the Köppen Climate Classification System as as follows:

A – Topical climate: All months have a temperature greater than 64.4°F.
B – Dry, arid, or semiarid climate: Potential evapotranspiration exceeds precipitation. Criteria on a sliding scale based on average annual temperature.
C – Mesothermal climate: Warmest month above 50°F and all months between 32°F and 64.4°F.
D – Microthermal climate: Warmest month above 50°F, all months below 64.4°F, and at least one month less than 32°F.
E – Tundra climate: All months below 50°F.

There are many subcategories based on a variety of temperature and precipitation factors. The Encyclopedia Britannica entry for the Köppen Climate Classification System has an excellent description of the major and sub categories. (Note: they use 26.6°F as the cutoff between C and D climate types whereas the traditional cutoff is 32°F).

Methodology:

A recent paper by Chen and Chen (2013) looked at the change in spatial coverage of  the major and minor Köppen categories across the entire earth as a metric for measuring climate change between 1900 and 2010. They combined all station data within 0.5° grid cells to determine a Köppen category for those cells. My methodology is similar to the Chen and Chen methodology except that the focus is on individual stations and not a continuous surface.

Using the Global Historical Climate Network (GHCN) database, we extracted daily temperatures and precipitation for all WBAN "first-order" and Cooperative stations in Alaska. No RAWS or SNOTEL stations were incorporated into the analysis.

Data were obtained by decade (e.g., 1901-1910, 1911-1920, etc.) and only those stations with valid observations for at least 75% of the days during the decade were included. Unlike Chen and Chen, data are non-overlapping in consecutive decades. For example, data for the 2001-2010 period only utilize data during those 10 years. The traditional method is to use 30-years of data to compute normal for a 10-year period. However, I decided that to map the changing patterns effectively, overlapping data were to be eliminated.

Monthly and annual averages were computed for all stations meeting the selection criteria using a Java parsing routine. A decision tree was constructed in the Java program to assign each station into a major Köppen category and a minor Köppen subcategory or subcategories.

Results:

This YouTube video shows the results of the analysis for each 10-year period. The 5 major Köppen types are represented by different colors (of course there are no Tropical climates in Alaska). The minor Köppen subcategories are represented by different shapes and shades of the major category color. Early time periods have relatively few stations meeting the selection criteria, so spatially meaningful analysis is limited. To view the video clearly, expand it into full screen mode and make sure the resolution is set to 1080p HD from the settings (gear-shaped) button. This is the only way to read the text in the legend.




Much of Alaska is in the D climate type. This is not surprising given that nearly everywhere at least one month is below freezing on average. However, some areas in Southeast do not experience the minimum criteria for a D climate.

Along the west coast and North Slope, a number of coastal stations meet the E (Tundra) criteria. However, the number of E stations is shrinking.

In the Interior, the most notable pattern is the shifting of criteria defined by precipitation patterns. However, this is partially an artifact of the definitions of summer (April through September) and winter (October through March).

Interestingly, in the most recent decade, several Interior station qualified as type B climates. Those stations were Nuiqsut, Deering, and Delta Junction. This too is an artifact of their overwhelming prevalence of summer precipitation (>80% of annual total) and low potential evapotranspiration. Note: the B climate type supersedes all other types.

Conclusion:

Why should anyone care what the climates type are for a collection of stations? It is not an overstatement to say that the climate of an area is the single most important variable in describing the ecological regime, the agricultural potential, and the geomorphological processes of a region. Measuring shifting climate patterns will give politicians and the public an additional tool for developing public policy adaptations for the benefit of all.

© Brian Brettschneider, 2014

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