Fall warming events on the Arctic Slope of Alaska

In this presentation we summarize five years (1994-1998) of continuous surface meteorological data taken along a transect from near Prudhoe Bay on the Arctic Coast to the foothills of the Brooks Range as part of the NSF Arctic Transitions in Land-Atmosphere System (ATLAS) program. The transect spans an environment from the coastal plain near sea level to the uplands region with elevations over 900 m. This unique data set is the first to contain contemporary multi-year cold season measurements in upland as well as coastal plain locations of the Arctic Slope and the first to include both wind speed and direction measurements throughout the period of record.

The observed patterns of temperature and wind have important ramifications for the winter snow cover of the Kuparuk Basin, which plays a central role in the hydrology, ecology and surface energy balance of the region. A picture emerges of the North Slope as a region dominated by subfreezing temperatures for most of the annual cycle, the brief warm and snow-free season in June, July and August even shorter than that of Arctic Alaska south of the Brooks Range. The surprisingly low wind speeds experienced across the network in October and November, particularly in the southern parts of the region, help to explain the presence of thick but low density layers in the base of the snowpack throughout this area. These layers show signs of having been deposited in low wind conditions and their presence retards heat loss from the ground through out the winter.

A notable feature of the annual temperature curve is a pronounced "bump" or short term warming trend in the otherwise steady decline of temperatures across the Arctic Slope during the fall snow accumulation period. Typically, low density snow layers are at risk for erosion during high wind events later in the winter, but the unusual thaw observed each winter in November creates an icy cap at the top of the snow pack that apparently is responsible for preventing localized wind erosion and consequent thinning or elimination of the snow cover.

Focusing on one representative site, it is found that the "temperature bump" occurs to some extent in all years of the period of record. On at least two of the five years considered here, this was the result of a nearly stationary or slowly propagating vertically stacked low pressure system over southern Alaska that brought warm air northward through along the Alaska-Canada border and back westward along the Arctic Coast. These warming events typically bring from several hours to several days of above freezing temperatures to the Arctic Slope after a significant amount snow has accumulated. While of brief duration, these events have a profound impact on the evolution of the snowy land surface during the following six months.