Using sinuosity to measure the waviness of the extratropical circulation under climate change
Wednesday, 17 December 2014
Extreme weather events, such as heat and cold waves, droughts, and floods, have substantial social and economic impacts. Whether these extreme events are related to one of the prominent components of climate change --- Arctic Amplification (AA)--- is controversial. The hypothesis proposed by Francis and Vavrus (2012) is that a reduced meridional temperature gradient owing to AA will cause a weaker and wavier extratropical circulation, which will result in slower progression of weather systems and more atmospheric blocking events. To test this hypothesis we borrow the concept of "sinuosity" from geomorphology to measure the waviness of the boreal extratropical circulation. As applied here, sinuosity is defined as the ratio of the curvilinear length of a circumhemispheric geopotential height contour to the perimeter of its equivalent latitude, where the contour and the equivalent latitude enclose the same area. We use 500hPa daily geopotential heights from NCEP/NCAR Reanalysis and from the CESM climate model’s historical and RCP8.5 greenhouse simulations to calculate sinuosity. Observations and simulations exhibit similar annual cycles of sinuosity, with the maximum sinuosity occurring in summer and minimum sinuosity during winter. Although no long-term trend (1948-2013) in sinuosity is observed in winter (DJF) or summer (JJA), a positive linear trend has occurred since the 1980s and accelerated after 1995 at middle latitudes (winter) and high latitudes (summer). The 500hPa zonal wind is found to weaken at latitudes where sinuosity increases. The change of sinuosity and corresponding zonal wind in RCP8.5 simulations is also explored. Our study finds a strong negative correlation between observed daily sinuosity and the daily Arctic Oscillation (AO) index in all seasons. This negative correlation and the tendency of CMIP5 models to simulate a negative AO-like pattern aloft during winter in a warmer climate suggest a trend toward a wavier extratropical atmospheric circulation in the future.