Tuesday, 14 May 2002: 11:00 AM
Trends in the 1949–2000 northern hemisphere circumpolar vortex and climate change implications
The 700-hPa, 500-hPa, and 300-hPa Northern Hemisphere circumpolar vortex is investigated for the period 1949-2000. The circumpolar vortex has never been analyzed for such a long data period, nor for three levels in the atmosphere. Further, the vortex is quantified for the higher, middle, and lower latitudes, at a 5° longitude resolution, making this the most comprehensive vortex study to date. The overall and seasonal vortex trends for the Northern Hemisphere are determined for the high, middle, and lower latitudes at each of the three levels in the atmosphere. The vortex is found to be statistically significantly contracting over time at the 300-hPa, 500-hPa, and 700-hPa level. This contraction is dominated by strong contraction over Asia and Europe, as well as contraction over North America. Over the Atlantic, Europe, and Asia, the lower latitudes are found to be contracting more than the higher latitudes, suggesting an increase in the meridional height gradient; over the Pacific and North America, the height gradient is found to be weakening. The only circulation region of the Northern Hemisphere that is consistently expanding is over the Pacific. Three to four different circulation regimes are also evident in the vortex time series. The period before the late 1950s is characterized by a contracted vortex, and the 1960s and 1970s are characterized by an expanded vortex. At the time of the Pacific Climate Shift the vortex shifted into a slightly contracted position, until the late 1980s. The 1990s are characterized by a highly variable circulation, with 2-3 year alternating periods of expansion and contraction. Since the circumpolar vortex is contracting at every level of the atmosphere, the climate change implications are that the atmosphere is warming at every level in the atmosphere. Comparisons with the MSU temperature history indicate that the Northern Hemisphere circulation as a whole accounts for roughly half of the variability in MSU temperature, indicating that the vortex is a good measure of hemispheric temperature. On a latitude by latitude and level by level basis, the lower latitudes are associated most strongly with MSU temperature, as is the upper atmosphere (300-hPa). The vortex trends are also qualitatively reconcilable with observed surface warming trends.