What makes for a good ski season?

Ruapehu Bulletin, Volume 9, Issue 432, 14 April 1992, Page 4

What makes for a good ski season?

Snow lovers, skifield operators and people living on Ruapehu will remember the 1991 winter for a long time. The

number of snowfalls, depth of snow accumulation and length of the. ski season were all far greater than average.

Snow poles used for precise measuring of snow accumulation at reference sites at Whakapapa and Turoa skifields recorded their maximum snow buildup since records began about 10 years ago. Although seasonal snow distribution is notoriously difficult to measure or compare over decades, a variety of records for Whakapapa suggest 1991 had probably more snow than any other year during the last 30 years or so. The glacier received a healthy feed of snow for the second year in succession. Good for skiers, hard for the workers 1991 was a good year for skiing. Whakapapa skifield was open for five months and, like Turoa, closed for the season due to lack of customers, not lack of snow. It was possible to ski in more places than normal and even down to the Top of the Bruce right to midNovember. Whakapapa even reopened for skiing in the Christmas-Ncw Year period, something not seen since the early 1970s. Cross-country skiing also received a boost with people skiing into places like Waihohonu Hut and Tama Lakes, as well as Tongariro. However, skifield workers eamed a lot of

By Harry Keys, Tongariro Conservancy, Advisoiy Scientist. This article is reprinted from "Tongariro, The Journal of Tongariro National Park" overtime clearing roads and digging out lifts like the Far West, Moro and Jumbo T-bars. Drive station huts just got buried! A record amount of avalanche control work was required to stabilise all the new snowfalls, but even so a potentially serious avalanche occurred in Iwikau Village behind the public shelter. At Tukino an avalanche claimed two lives and could have taken thrce more. Factors Involved There were probably two reasons why 1991 was such a good snow year. Firstly, the Southern Oscillation Index(SOI) was substantially negative(El Nino conditions). This led to enhanced westerly and southwesterly conditions with anticyclones situated well to the west of New Zealand and, as with most E1 Ninos, this brought cooler air than average to Ruapehu. So a constant succession of fronts and depressions from the west and southwest from latc July meant snow fell on a large number of days. A lack of warm easterlies or northcrlies meant that rclativcly little melting occurred, allowing thick

snow accumulations to develop. Secondly, the 11 -year solar(sunspot) cycle peaked early in 1990 with a second peak apparently in 1991. The solar cycle is a regular variation in the nature of radiant energy emitted by the sun. Complex, poorly known processes link these variations to the Earth's atmosphere and its circulation, but precipitation has been shown to be higher in New Zealand during solar maxima. Prccipitation at the Chateau was higher than average during the 1990 and 1991 winters. This cannot be attributed to the E1 Nino as no clcar rclationship has been found betwecn SOI and precipitation at the Chateau weather rccording station. Snow and climate records from Whakapapa and the Chateau indicate that a combination of climatic paramcters are required to be present for "good" snow seasons. Cool air temperatures seem to be an important factor at Whakapapa at

least. A large number of days with snowfall is also necessary, as suggested by Frances West and Terry Healy in their recent paper. Adequate precipitation is necessary although it does not in itself produce a good snow season at Whakapapa because snow rather than rain must fall, further indicating the importance of cool temperatures. Fundamentally of coursc these parameters reflect the weather patterns (including air masses and wind directions) which are required to produce snow-form-ing weather, rather than conditions which bring rain, long dry spells or frequcnt snow melting. Forecasts and hindcasts Based on a growing but still incomplete understanding of these factors and the climate and snow records themselves, I made forecasts for the 1990 and 1991 snow seasons. Other forecasters' predictions of SOI and precipitation

were used for both these forecasts, which turned out broadly correct as regards snow season quality, wind direction and avalanche activity. However, important details such as relative snow quantity were wrong. The E1 Nino event makes it possible to make further forecasts. Most recent winters following an E1 Nino have been "poor" or "average" snow seasons so 1992 may follow that pattern. However the major eruption of Pinatubo (Philippines) and Hudson (Chile) volcanocs in 1991 should depress 1992 winter temperatures slightly while the recent solar cycle peak may rcsult in precipitation still being above average. Both factors would tcnd to offset the "post SOI effect". This could lead to an