Signal of CP El Niño propagation in Southern hemispheric stratosphere

Abstract:

By using ERA-Interim and MERRA reanalysis, we firstly point out the Southern Hemisphere (SH) stratosphere warming is most significant during September, rather than during boreal winter of CP El Niño years. The SH stratospheric temperature anomalies become significant since July and reach the peak around September with a maximum above 4K, indicates the suppressed southern polar vortex and the strengthening of BD circulation in the Southern Hemisphere. After October, the southern polar positive signal decreases and the temperature anomalies becomes not significant. The E-P flux and the E-P flux divergence variations in SH mid-latitudes are most significant in August of CP El Niño, which lead SH stratospheric temperature anomalies by 1 month. The anomalously enhanced westward planetary waves at SH middle latitudes can propagate upward and poleward through the stratosphere westerly wind during SH winter (July-September). Thus, enhanced dissipation of westward planetary waves in the upper stratosphere of SH can cause anomalous warming in the polar region and cooling in the tropical stratosphere. The propagation of planetary waves in SH stratosphere is depressed and not significant since the westerly wind in SH stratosphere weakens rapidly from October to December. The significant anomalous eddy heat flux (EHF) are also found over mid- and high- latitude of SH stratosphere during August and weaken after September. The EHF at 100hPa (40°-80°S) suggest that the planetary wave activity is strengthened during July-September rather than during October-November in most CP El Niño events.