ENSO and SAM Influence on the generation of long episodes of Rossby Wave Packets during southern hemisphere summer

Abstract

This study assesses the impact of low-frequency climate modes on Rossby Wave Packets (RWPs) during southern hemisphere summer. In particular, we focus on long-lived RWPs (lifespan above 8 days) and determine how El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) influence their statistics, that is, their duration, frequency of occurrence, and activity areas. We used daily mean meridional winds at 300 hPa from December to March between 1979 and 2020 from the ERA5 and NCEP-DOE 2 reanalyses. We found that long-lived wave packets, which are a small percentage of the total number of wave packets, show large interannual variability; there are years in which these types of waves do not occur and years that present up to 9 wave packets. This suggests that large-scale circulation conditions set up by low-frequency climate modes can modulate their occurrence. Classifying years according to SAM phases reveals that the occurrence of long-lived RWPs is highest (lowest) during intense negative (positive) SAM events. ENSO influence, on the other hand, was found to be weak and not robust. Analysis of large scale circulation conditions shows that during negative SAM phases the jet shifts northward, strengthens in the Indian sector, and extends further into the Pacific basin, so that it acts as a better waveguide favoring the propagation of long-lived RWPs. Conversely, during positive phases of SAM, the jet shifts southward and an anticyclonic center develops to the southwest of Australia blocking the jet and the progression of the wave packets.