ENSO and SAM influence on the generation of long episodes of Rossby Wave
Packets during Southern Hemisphere Summer
Abstract
This study aims at understanding the impact of low-frequency climate
modes on Rossby Wave Packets (RWPs) during the southern hemisphere
summer. In particular, we focus on long-lived RWPs (lifespan above 8
days) and determine how El Niño-Southern Oscillation (ENSO) and Southern
Annular Mode (SAM) modulate their frequency of occurrence plus the main
areas of detection and dissipation. We find that the occurrence of long
lived RWPs is maximum during El Niño years and negative SAM events.
Years with largest numbers of long-lived RWPs are characterized by a
zonally symmetric and narrow upper level jet that is shifted northward
from its climatological position. Conversely, when the jet is shifted
southward, as during positive SAM phases, particularly in the
southwestern Pacific basin, the number of long-lived RWPs detected
diminishes. El Niño sets atmospheric conditions that support the
formation of long lived RWPs whereas La Niña years presents high
interannual variability in the frequency of occurrence. Moreover, during
El Niño events the main formation area is between 61-120ºE and its main
dissipation area between 300-359ºE. During La Niña events, the main
formation area moves to 241-300ºE and no main dissipation area is
identified. During positive SAM two main formation areas appear at
61-120ºE and 241-300ºE and two main dissipation areas between 121-180º
and 301-359º, whereas in negative SAM only one formation area at
241-300º is detected and no main dissipation area is found.