Dr Scott Osprey FRMetS

Abstract:

Abstract Motivated by the strong Antarctic sudden stratospheric warming (SSW) in 2019, a survey on the similar Antarctic weak polar vortex events (WPVs) is presented, including their life cycle, dynamics, seasonality, and climatic impacts. The Antarctic WPVs have a frequency of about four events per decade, with the 2002 event being the only major SSW. They show a similar life cycle to the SSWs in the Northern Hemisphere but have a longer duration. They are primarily driven by enhanced upward-propagating wavenumber 1 in the presence of a preconditioned polar stratosphere (i.e., a weaker and more contracted Antarctic stratospheric polar vortex). Antarctic WPVs occur mainly in the austral spring. Their early occurrence is preceded by an easterly anomaly in the middle and upper equatorial stratosphere in addition to the preconditioned polar stratosphere. The Antarctic WPVs increase the ozone concentration in the polar region and are associated with an advanced seasonal transition of the stratospheric polar vortex by about one week. Their frequency doubles after 2000 and is closely related to the advanced Antarctic stratospheric final warming in recent decades. The WPV-resultant negative phase of the southern annular mode descends to the troposphere and persists for about three months, leading to persistent hemispheric-scale temperature and precipitation anomalies. Significance Statement The Antarctic weak polar vortex events (WPVs) are similar to the sudden stratospheric warming (SSW), but many of their characteristics remain unclear. Their climatology is presented as a benchmark based on high-quality reanalysis datasets. WPVs have a life cycle that is similar to that of Arctic SSWs but has a longer duration. They occur due to the amplified tropospheric wave forcing in the presence of a preconditioned polar stratosphere. Its seasonality is partly controlled by the equatorial stratospheric easterly in addition to the polar stratosphere. Its occurrence is closely related to the advanced breakdown of the Antarctic polar vortex and can reduce the size of the Antarctic ozone hole. Moreover, it further causes persistent hemispheric-scale climate anomalies in the troposphere, which provides a prediction potential for surface weather and climate.

Abstract:

Summer 2010 saw two simultaneous extremes linked by an atmospheric wave train: a record-breaking heatwave in Russia and severe floods in Pakistan. Here, we study this wave event using a large ensemble climate model experiment. First, we show that the circulation in 2010 reflected a recurrent wave train connecting the heatwave and flooding events. Second, we show that the occurrence of the wave train is favored by three drivers: (1) 2010 sea surface temperature anomalies increase the probability of this wave train by a factor 2-to-4 relative to the model’s climatology, (2) early-summer soil moisture deficit in Russia not only increases the probability of local heatwaves, but also enhances rainfall extremes over Pakistan by forcing an atmospheric wave response, and (3) high-latitude land warming favors wave-train occurrence and therefore rainfall and heat extremes. These findings highlight the complexity and synergistic interactions between different drivers, reconciling some seemingly contradictory results from previous studies.