Spring may be a critical season with respect to climate change because the return of the sun at high latitudes initiates potential positive feedbacks. These include effects related to interactions between atmospheric dynamics and chemistry in the stratosphere, and between albedo and the melt of snow and sea ice. Regarding the stratosphere, Pawson and Naujokat (1999) found that the 1990s included years that had stratospheric temperatures cold enough to support polar stratospheric clouds, which had not been characteristic of the Arctic. The presence of these clouds tends to exacerbate the destruction of stratospheric ozone in the spring, which in turn tends to reduce the local absorption of UV radiation, and hence promotes cold anomalies. Regarding surface albedo effects, an 8-day advancement of the timing of snowmelt in the Barrow region has occurred since the 1960s. This absence of snow cover has caused the net radiative heat flux at the surface to increase by more than 100 W m locally over that 8-day period (Stone et al. 2002).
In this paper we conduct a diagnostic study of changes that have occurred in the western Arctic near the surface and in the lower stratosphere. A primary aspect of this study is to examine whether the characteristic signature of the Arctic Oscillation (AO) carries over into spring. Special attention is devoted to comparing the 1980s with the 1990s, and how this decadal change was manifested in individual years. We pursue the dynamics behind the observed changes through analysis of the principal contributions to the low-level thermodynamic energy budget.
We begin by discussing the data sets used in our analysis, the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis and the Television Infrared Observational Satellite (TIROS-N) Operational Vertical Sounder (TOVS) Polar Pathfinder (Path-P) product. We illustrate decadal and monthly variability in the western Arctic using time series at Barrow, Alaska, and Eureka, Canada. These time series show striking differences during spring between the 1990s and the previous four decades. We then explore the hemispheric patterns of temperature differences between these decades as a function of season, both aloft and near the surface. Finally, we document the mechanisms responsible for the anomalous lower-tropospheric warming observed in the western Arctic during spring in the 1990s.
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