Elevational and Seasonal Dependence in Climate Change Within a Mid-latitude High Mountain System, Colorado Front Range, USA

We contrasted 59-year (1952-2010) climate records of two high-elevation sites in the Colorado Rocky Mountains Front Range, USA; one a subalpine forest (3021 m asl) and the other high alpine tundra (3739 m asl), to evaluate the degree of synchrony versus decoupling in their long-term climate trends. The sites, separated 6 km horizontally and 700 m vertically, exhibited significant annual and seasonal differences in the long-term trends in air temperature and precipitation. Annual average maximum air temperature (T_max) increased in the subalpine (+0. 4ºC/decade), but did not change significantly in the high alpine, nor did annual air temperature minima (T_min) at both sites. Monthly T_max trends in the subalpine were positive throughout most of the year. The high alpine site’s seasonal T_max and T_min trends were more complex, with muted warm-season positive trends, switching to negative in early winter (October-December). While annual total precipitation did not change significantly in the subalpine, it increased in the high alpine (+59 mm·yr^-1·decade^-1); this increase came nearly entirely in the cold season (October through April). Over the cold season period, these changes:

1. Steepened air temperature and precipitation lapse rates, increasing the alpine-subalpine climatological contrast
2. Altered air temperature and precipitation seasonal cycles, increasing warm-to-cold season contrasts
3. Widened the diurnal temperature range, increasing day-to-nighttime temperature contrasts.

We hypothesize that, in the high-alpine, wintertime increases in snowfall altered the surface energy fluxes in ways that led to early-winter through early-summer air cooling tendencies which countered a regional warming signal.

On the other hand, subalpine T_max and T_min trends were poorly related to each other and to precipitation trends; we hypothesize instead that the subalpine T_max amplified the regional signal, while subalpine T_min trends were coupled to those of the high alpine via nocturnal downslope flows. We offer a conceptual model for climate change in winter-snow mountain regions, specifically for their highest reaches well above the snow meltline throughout the snowfall season; the model integrates the roles of regional and hemispheric forcing with surface energy budget responses and landscape linkages in creating contrasting and coupled responses across these high elevation domains.