Relating Atmospheric blocking to Jet changes in a future climate

Post date: Oct 18, 2014 6:59:45 PM

Thursday, February 28, 2013. Together with colleagues from KNMI, University of Reading and Oxford University, I tried to relate changes of atmospheric blocking to changes of the jet. The paper was published in Climate Dynamics. (doi:10.1007/s00382-013-1699-7). Abstract follows below.

Blocking frequency at 60N, diagnosed using 500mb geostrophic zonal wind reversal. The black and red lines indicate present and future frequencies. The difference is denoted by the dashed line. After correcting for differences in mean and standard deviation, the patterns become virtually identical (thin dotted line)

Relating central blocking longitude and jet strength (for definitions see the paper). The contours denote a 2-dimensional pdf. The ellipse is tilted, indicating that a stronger jet promotes blocking to occur further east. The shading shows the climate change signal: it is in the same direction as the axis of the ellipse of current climate, suggesting that the mechanisms are similar.

Abstract The future changes of atmospheric blocking over the Euro-Atlantic sector, diagnosed from an ensemble of 17 global-climate simulations obtained with the ECHAM5/MPI-OM model, are shown to be largely explainable from the change of the 500 hPa mean zonal circulation and its variance. The reduction of the blocking frequency over the Atlantic and the increased frequency of easterly upper-level flow poleward of 60°N are well explained by the changes of mean zonal circulation. In winter and autumn an additional downstream shift of the frequency maximum is simulated. This is also seen in a subset of the CMIP5 models with RCP8.5. To explain this downstream shift requires the inclusion of the changing variance. It is suggested that the increased downstream variance is caused by the stronger, more eastward extending future jet, which promotes Rossby wave breaking and blocking to occur further downstream. The same relation between jet-strength and central-blocking longitude is found in the variability of the current climate.