In:
Atmospheric Chemistry and Physics, Copernicus GmbH, Vol. 18, No. 9 ( 2018-05-14), p. 6749-6760
Abstract:
Abstract. One of the most prominent asymmetric features of the southern
hemispheric (SH) circulation is the split jet over Australia and New Zealand
in austral winter. Previous studies have developed indices to detect the
degree to which the upper-level midlatitude westerlies are split and
investigated the relationship between split events and the low-frequency
teleconnection patterns, viz. the Antarctic Oscillation (AAO) and the El
Niño–Southern Oscillation (ENSO). As the results were inconsistent, the
relationship between the wintertime SH split jet and the climate variability
indices remains unresolved and is the focus of this study. Until now, all split indices' definitions were based on the specific region where
the split jet is recognizable. We consider the split jet as hemispheric rather
than a regional feature and propose a new, hemispherical index that is based on
the principal components (PCs) of the zonal wind field for the SH winter. A
linear combination of PC2 and PC3 of the anomalous monthly (JAS) zonal wind is
used to identify split-jet conditions. In a subsequent correlation analysis, our newly defined PC-based split index (PSI) indicates
a strong coherence with the AAO. However, this significant relationship is
unstable over the analysis period; during the 1980s, the AAO amplitude was
higher than the PSI, and vice versa in the 1990s. It is probable that the PSI,
as well as the AAO, underlie low-frequency variability on the decadal to
centennial timescales, but the analyzed period is too short to draw these
conclusions. A regression analysis with the Multivariate ENSO Index points to
a nonlinear relationship between PSI and ENSO; i.e., split jets occur during
both strong positive and negative phases of ENSO but rarely under “normal”
conditions. The Pacific South American (PSA) patterns, defined as the second and third modes of the
geopotential height variability at 500 hPa, correlate poorly with the PSI
(rPSA−1 ≈ 0.2 and rPSA-2= 0.06), but
significantly with the individual components (PCs) of the PSI, revealing an
indirect influence on the SH split-jet variability. Our study suggests that the wintertime SH split jet is strongly associated with
the AAO, while ENSO is to a lesser extent connected to the PSI. We conclude that
a positive AAO phase, as well as both flavors of ENSO and the PSA-1 pattern
produce favorable conditions for a SH split event.
Type of Medium:
Online Resource
ISSN:
1680-7324
DOI:
10.5194/acp-18-6749-2018
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2018
detail.hit.zdb_id:
2069847-1
