Seasonal and ENSO Variations in Southern Hemisphere Westerly Jets

The Southern Hemisphere (SH) contains a subtropical jet (STJ), polar night jet (PNJ), and a subpolar jet (SPJ). We employ 30 years of ERA-40 data (1984-2013) to investigate variations in the structure and position of these jets at seasonal and ENSO (El Nino –Southern Oscillation) time scales. The seasonality of “split” and “spiral” jet structures seen on 150 hPa charts are described in terms of 3D superposition of these jets. During SH winter (JJA), outflow from Southeast Asian convection creates the STJ which extends from the Eastern Indian to the Eastern Pacific Ocean. Continuous vertical alignment of the PNJ and SPJ is observed from the surface into the stratosphere during JJA, but with notable zonal asymmetry. Superposition of these jet systems yields the climatological “split jet” structure seen near 150 hPa centered near New Zealand. The “spiral jet” structure may be viewed as the east end of the STJ blending with a relative weakness of the PNJ/SPJ over South America. The STJ is nearly absent during SH summer (DJF), but the PNJ/SPJ is present in all seasons, resulting in changes in their superposition for each season.

The influence of ENSO on the structure of the STJ, SPJ, and its vertical coherence with the PNJ is highlighted. El Nino favors a more extended STJ. La Nina favors a stronger, more zonally asymmetric PNJ, with greater downward penetration of the PNJ, a stronger SPJ, and stronger westerlies in Drake Passage. The influences of changes in Asian monsoon outflow on the STJ, radiation of planetary wave trains from tropical convective centers to foster high latitude zonal asymmetries, and processes governing the PNJ/SPJ complex are explored.