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| Titel |
The impact of ENSO on wave breaking and Southern Annular Mode events |
| VerfasserIn |
Tingting Gong, Steven Feldstein, Dehai Luo |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035910
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| Zusammenfassung |
| This study examines the relationship between Southern Annular Mode (SAM) events and the
El Niño–Southern Oscillation (ENSO) using daily ERA-40 data. The data coverage spans the
years 1979 through 2002, for the austral spring and summer seasons. The focus of this study
is on the question of why positive SAM events dominate during La Niña and negative SAM
events during El Niño. A composite analysis is performed on the zonal-mean zonal
wind, Elliassen-Palm fluxes, and two diagnostic variables, the meridional potential
vorticity gradient, a quantity that is used to estimate the likelihood of wave breaking,
and a wave breaking index, which is used to evaluate the frequency of the wave
breaking.
On the equatorward side of the eddy-driven jet, positive SAM events are associated with
strong anticyclonic wave breaking, and negative SAM events with weak anticyclonic wave
breaking. On the poleward side of the eddy-driven jet, positive SAM events coincide with
little wave breaking and negative SAM events by weak cyclonic wave breaking. These wave
breaking events are found to occur far from the critical latitudes. With the exception of the El
Nino and La Nina years, these SAM events decayed within 7 to 10 days of their
establishment through mixing.
The results of this investigation suggest that the background zonal-mean flow associated
with La Niña (El Niño) is preconditioned for strong (weak) anticyclonic wave breaking on the
equatorward side of the eddy-driven jet, the type of wave breaking that is found to drive
positive (negative) SAM events. A probability density function analysis indicates that strong
(weak) anticyclonic wave breaking takes place with a much higher frequency during La Niña
(El Niño). It is suggested that these wave breaking characteristics, and their dependency on
the background flow, can explain the strong preference for SAM events of one phase during
ENSO.
The analysis also showed that austral spring SAM events that coincide with ENSO are
preceded by strong stratospheric SAM anomalies, and then followed by a prolonged
period of wave breaking that lasts for about 30 days. These findings suggest that the
ENSO background flow also plays a role in the excitation of stratospheric SAM
anomalies, and that the presence of these stratospheric SAM anomalies in turn excites
and then maintains the tropospheric SAM anomalies via a positive eddy feedback. |
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