Interplay between Boreal Summer Intraseasonal Oscillation and Southern Hemisphere Stratospheric Polar Vortex Warming

The boreal summer intraseasonal oscillation (BSISO) features more distinctive and complex propagation characteristics than its wintertime counterpart, the Madden-Julian oscillation (MJO). While the relationship between the MJO and the Arctic stratosphere during boreal winter has been widely documented, the linkage between the BSISO and the Antarctic stratosphere during austral winter has not been extensively discussed. Here, after identifying the Southern Hemisphere (SH) stratospheric polar vortex warming (SPVW) events, we reveal the bidirectional connection between BSISO and SPVW. Before onset of the SPVW events, the occurrence frequency and amplitude of BSISO phase 5 (P5) shows a significant increase. The most significant responses of the SH polar stratospheric temperature to the BSISO are found about 10 days after BSISO P5. Thus, to some extent, BSISO P5 can be regarded as a precursor to the SH SPVW event, which is attributed to the enhanced upward propagation and dissipation of planetary waves in the SH stratosphere induced by the BSISO P5. After onset of the SPVW events, a significant increase in the occurrence and amplitude of BSISO P6 is ob-served, corresponding to the enhanced convection over the South China Sea and southern Philippine Sea. Tropical upwelling associated with the strengthened Brewer-Dobson circulation (BDC) induced by the SPVW tends to result in unstable circum-stances in the tropical upper troposphere. Then the high correlation between static stability at 150 hPa and outgoing longwave radiation (OLR) anomalies over the South China Sea and southern Philippine Sea provides robust evidence that the intensity of convective activity in the tropics can indeed be modulated by the variability in SH stratospheric polar vortex.

Automated summary

This study reveals the bidirectional connection between the boreal summer intraseasonal oscillation (BSISO) and the Southern Hemisphere (SH) stratospheric polar vortex warming (SPVW) events. The occurrence frequency and amplitude of BSISO phase 5 (P5) significantly increase before the onset of SPVW events. The response of the SH polar stratospheric temperature to BSISO is the most significant about 10 days after BSISO P5. After the onset of SPVW events, there is a significant increase in the occurrence and amplitude of BSISO phase 6 (P6), corresponding to enhanced convection over the South China Sea and southern Philippine Sea. These findings suggest that BSISO can be considered as a precursor to SH SPVW events and influences convective activity in the tropics.