The lower troposphere in the western Arctic during spring was considerably warmer in the 1990s than in the previous four decades. This warming is consistent with reductions in sea ice extent and snow cover, permafrost warming, and with the perceptions of residents. Our analysis has sought to elucidate the causes for the springtime warming, with a focus on comparisons between the cold decade of the 1980s and the warm decade of the 1990s. The analysis is based on two sources: the NCEP-NCAR reanalysis and the TOVS Path-P datasets.
Time series of 925-hPa temperatures for March and April at Barrow, Alaska, and Eureka, Canada, show that the 1990s were warm, and the 1980s were cold, due to an increase in the frequency of warm and cold months, respectively, rather than due to an increase in the magnitude of the temperature anomalies. There was a tendency for springtime temperature anomalies to be out of phase between the lower troposphere and lower stratosphere; for example, out of the six years in the 1990s with a warm spring at Barrow at 925 hPa (1990, 1993, 1995, 1998, and to a lesser extent, 1991 and 1997), four of these years (1990, 1993, 1995, and 1997) also featured cold 200-hPa temperature anomalies.
The mechanisms responsible for the warm springs of the 1990s relative to the cool springs of the 1980s were examined via thermodynamic energy budgets at the 850-hPa level for the region of Barrow, Alaska. Budgets were estimated for March and April during four particularly cool years in the 1980s (1980, 1982, 1984, and 1987) and four particularly warm years in the 1990s (1990, 1993, 1995, and 1997). The amount of warming over the March–April period averaged 7 K for the selected years in the 1980s, and 15 K for the 1990s. The primary difference in the heat budgets between the 1980s and 1990s was the sense of the horizontal advection of heat. This term was a cooling effect (–28 K over the 2-month period) during the 1980s, but contributed toward warming (47 K) in the 1990s. The heat advection was found to be consistent with mean lower-tropospheric circulation changes accompanying the AO, but also of a highly episodic nature. Because the western Arctic is a region of large sea level pressure gradient for the AO pattern, we expect northern Alaska to be influenced by AO variability.
In essence, spring tended to come earlier to the western Arctic in the 1990s than in previous decades. Based on our analysis, the typical scenario can be described as follows. At the end of winter in the 1990s, that is, in March, relatively cold temperatures and low geopotential heights tended to be present in the stratosphere. These anomalies persisted into April, with associated geopotential height anomalies in the lower troposphere that promoted warm air advection. Subsequent warm anomalies near the surface lasted well into May. We interpret this scenario as a signature of the systematic shift of the AO that occurred at the end of the 1980s. We speculate that the springtime stratospheric manifestation of the AO may be separate from the decadal changes in the NAO that have occurred for the winter. A conjecture that the NAO and the stratospheric component of the AO are separable has been made previously by Kodera et al. (1999).
Our results clearly show large changes occurred near the end of the 1980s in the western Arctic during spring, but this does not necessarily imply a permanent shift in the climate. The recent report on global climate change by the Intergovernmental Panel on Climate Change (IPCC) lists relatively cold stratospheric temperatures as one of the more robust examples of global change. Whether the results documented here represent more an interdecadal fluctuation, or a long-term trend, has yet to be determined. Since the stratospheric vortex and AO appear to at least partially influence surface conditions in the western Arctic during spring, they represent an important and perhaps crucial indicator of the local climate.
Acknowledgments. We appreciate the support from the Arctic Research Initiative (ARI) and the North Pacific Marine Research (NPMR) Initiative. This publication was supported by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA67RJO155.
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