Southern Ocean Blocking: A comparison of reanalyses

A new climatology of Southern Hemisphere blocking has been developed, based on the ERA-Interim and MERRA reanalyses. In this presentation, results will compared against those using the NCEP1 reanalysis. The main approach for identifying blocking events is based on persistent anticyclonic anomalies in the 500 hPa height field but results will be compared against a gradient-reversal approach to defining blocking. The main blocking region in the Southern Hemisphere lies across the southern Pacific Ocean, and blocking in that sector is well known to affect West Antarctic climate and sea ice extent. Blocking occurrence in this region is negatively correlated with the Southern Annular Mode (SAM) index, such that blocking occurrence is reduced when zonal winds across the southern Pacific are enhanced. relationships betweem the SAM and blocking occurrence will be compared across reanalysis data sets.