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Denali – Mountain Weather and Climate

The world’s 3rd most prominent and 3rd most isolated mountain, Denali, in the Alaska Range, has attracted climbers since the early 20th century. Denali is known to “create its own weather” – this is due to its geographic isolation and large elevation difference between the base and summit, which affects atmospheric flow at the micro- and mesoscales. The unfortunate side effect of this, for climbers, is high wind speeds and cold temperatures. Many a climber who has been up there will attest to this, even in the “best” of conditions during the “warm” climbing season of the Northern Hemisphere’s late spring/early summer.

Much unfavorable climbing weather is attributed to the mountain’s latitude: at 63 degrees North, Denali lies within the influence of the polar jet stream, the polar front, and thus, the associated storm tracks. As climbers well know, weather and overall climatic factors play a big role in whether or not they are successful in getting to the top – and in the most extreme cases, whether they make it off the mountain alive or not. Little is known about how specific synoptic-scale weather patterns in the North Pacific affect conditions on the mountain – and regional forecasting is challenging due to the extreme environment, overall data sparsity and comparatively poor model resolution of the already existing weather forecast models.

In ACRC research team member’s Hartl et al.’s 2020 paper, “The Mountain Weather and Climate of Denali, Alaska – An Overview”, the authors explore in-depth the climatological and meteorological conditions and seasonal variability in the Denali summit region, with data analysis accomplished using an NCEP-NCAR reanalysis 1 dataset spanning a 70-year time period (1948 – 2018). So, what are the trends? Over the last 70 years, an equivalent of 1.4 degrees C warming has been seen, while the number of very cold days during the climbing season (April – July) has decreased by about a day/decade. The number of very windy days (greater than 20 m/s) during the climbing season also showed a decreasing trend. However, the extreme wind speeds that Denali is known for still occur in all seasons and are mainly associated with the polar jet passing directly over Denali or due to cyclogenesis in the Bering Sea.

Extreme wind gusts are thought to have played a role in a number of climbing falls. After a fatal accident involving Japanese climbers in 1989, an automatic weather station was installed on Denali Pass at a height of 5715 meters (the summit is 6190 meters). In Hartl et al. (2020), History and Data Records of the Automatic Weather Station on Denali Pass (5715 m), 1990 – 2007data records and station history are presented, highlighting the harsh meteorological conditions at the site. Air temperatures were recorded down to -60 degrees C and wind speeds in excess of 60 m/s. Looking at critical wind speed thresholds and a reanalysis-based reconstruction of meteorological conditions during the 1989 accident reveal and confirm that the climbers did, indeed, face extremely hazardous wind speeds and very low temperatures during that fateful day.

Data from the Denali Pass weather station provides a unique record – for those interested, it can be referenced when looking at other fateful climbs. But hopefully it will also serve as a basis for future monitoring efforts. There are at least 1000 climbers during a given climbing season, which may increase in the future. The climate is changing – while it may potentially make conditions less harsh on the mountain, in terms of temperature and wind, other hazards may prevail, like avalanches and unstable snow bridges over crevasses. The more information we can gather, the more we can pass on the knowledge to the boots-on-the-ground which are facing the hazards.