PNNL

Abstract

Ural blocking (UB) in winter has been shown to play a crucial role in cold extremes over Eurasia. In this study, the response of UB to early winter Barents-Kara Seas (BKS) warming is studied using a set of experiments with a simple atmospheric general circulation model. A nonlinearity is found in the UB response to linearly increased heating over the BKS, and the duration of individual blocking events and total blocked days first increase from weak to moderate heating but decrease from moderate to strong heating. Nonlinear phase speed and energy dispersion formula of blocking are used to explain the nonlinear response of UB to BKS warming. With a weak early winter BKS warming, UB tends to have a longer duration because it is more quasi-stationary and less dispersive owing to the increased upstream background zonal flow and meridional gradient of potential vorticity (PVy) over the North Atlantic mid-to-high latitudes, corresponding to an intensification of the North American trough. However, when the BKS warming is sufficiently strong, a positive stratospheric height anomaly appears in the early winter and its subsequent downward influence to Greenland leads to a negative Arctic Oscillation associated with reduced zonal wind and PVy in the mid-to-high latitudes from the North Atlantic to Europe, the upstream of UB. In this case, a shorter duration of UB is found as a result of its stronger retrogression and energy dispersion attributed to its largely negative phase speed and largely energy dispersion.