The relationship between the canonical ENSO and the phase transition of the Antarctic oscillation at the quasi-quadrenni
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The relationship between the canonical ENSO and the phase transition of the Antarctic oscillation at the quasi-quadrennial timescale LIU Changzheng1 , XUE Feng2∗ 1 2
National Climate Center, Beijing 100081, China LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
Received 6 March 2009; accepted 27 January 2010 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2010
Abstract A correlation analysis is performed to investigate the relationship between El Ni˜ no-Southern Oscillation (ENSO) and the Antarctic oscillation (AAO) at the quasi-quadrennial (QQ) timescale. It is found that the cold tongue index (CTI) and the AAO index (AAOI) are negatively correlated with about a 7-month lead-time, while they are positively correlated with about a 15-month lag-time. To further explore this relationship, complex empirical orthogonal function analysis is employed in the QQ sea level pressure (SLP) anomalies from 1951 to 2002. The results indicate that, during the ENSO cycle, there exists one kind of global tropical wave of wavenumber 1 (GTW1) propagating eastward. With the traveling of GTW1, the tropical SLP anomaly tends to intrude into the southern mid-latitudes. Accordingly, three strong signals travel synchronously along the circumSouth-Pacific path, and a relatively weak signal extends eastward and poleward over the South Ocean in the Atlantic-Indian Ocean sector. Following the propagation of these signals, the AAO phase tends to be reversed progressively. As a result, there exists an evident lead-lag correlation between CTI and AAOI. It can be concluded that ENSO plays a key role in the phase transition of AAO at the QQ timescale. It is also noticed that this regular relationship is only evident in the canonical ENSO events, for which sea surface temperature (SST) anomalies extend westward from the tropical eastern Pacific. On the other hand, the similar relationships are not found among those atypical ENSO events for which SST anomalies spread eastward from the central Pacific, such as the 1982-1983 ENSO event. Key words: ENSO, Antarctic oscillation, the quasi-quadrennial timescale, the global tropical wave of wavenumber 1
1 Introduction The El Ni˜ no-Southern Oscillation (ENSO) refers to the abnormal warming of sea surface water over the tropical central and eastern Pacific and the related atmospheric circulation anomalies with a period of 3–7 years. As a strong interannual signal, ENSO exerts a significant impact on the global climate (Rasmusson and Carpenter, 1982). The East Asian monsoon and climate anomalies in China are also associated with ENSO. For instance, in the decaying phase of ENSO, the East Asian summer monsoon tends to be weakened accompanied by more rainfall in the Yangtze River valley (Sun and Yang, 2007; Xue and Liu, 2008). Besides ENSO, the Antarctic oscillation (AAO) is an-
other primary atmospheric mode referring to the largescale alternation of atmospheric mass between midlatitudes and high latitudes over the Southern Hemisphere (Gong a
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