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Bohua Huang and J. Shukla

probably associated with the thermocline deepening in preceding months. The possibility of strong oceanic dynamical influence in the southwestern Indian Ocean has been suggested by previous studies ( Xie et al. 2002 ; Huang and Kinter 2002 ; Shinoda et al. 2004 ). Using observational data, Xie et al. (2002) have shown this long persistence into summer of copropagating anomalies from the thermocline to the surface (their Fig. 11). They also observed a cyclonic anomalous surface circulation over the

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Suryachandra A. Rao, Sebastien Masson, Jing-Jia Luo, Swadhin K. Behera, and Toshio Yamagata

the western Indian Ocean is anomalously warm (cool). In this study, particular attention is paid on positive IOD events. We refer to these events simply as IOD events. The coupled nature of this phenomenon is well documented in the literature (see Yamagata et al. 2004 and references therein). Several coupled general circulation models (CGCM) have been successful in simulating this phenomenon ( Iizuka et al. 2000 ; Gualdi et al. 2003 ; Lau and Nath 2004 ; Yu and Lau 2004 ; Yamagata et al

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Bohua Huang and J. Shukla

outside the tropical Indian Ocean, we have shown that the major dynamical response to the El Niño–Southern Oscillation (ENSO) forced anomalous atmospheric circulation over the Indian Basin is a pattern of opposite thermocline fluctuations between the eastern equatorial and southwestern Indian Ocean, with the former preceding the latter. This ENSO-related pattern of fluctuation explains a significant part of the Indian Ocean interannual variability during the studied period. In particular, it explains

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Roxana C. Wajsowicz

(equivalent to forecasting simple persistence of the initial anomaly) of the Southern Oscillation index ( Webster and Yang 1992 ). A persistence/predictability barrier may be expected in boreal spring, as the Walker circulation and zonal equatorial gradient in sea surface temperature are at their weakest, so the coupled ocean–atmosphere system over the Pacific is least stable and most susceptible to external influences and noise. Lagged correlations of observed SSTAs over the tropical Indian Ocean suggest

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Tommy G. Jensen

circulation is intensified during El Niño, and in particular during IOD. Valsala and Ikeda are identifying the major pathways of water from the Indonesian Throughflow using particle trajectories, passive tracers, temperature, and salinity from an OGCM. They found three main routes: A clockwise circulation cell in the thermocline confined to the south of the equator, a near-surface branch entering the Arabian Sea via the Somali Current, and a cross-equatorial subsurface branch at depths between the two

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Gabriel A. Vecchi and Matthew J. Harrison

) and Schott et al. (2004) for discussion of these upwelling zones]. These regions are of significant climate interest (e.g., Saji et al. 1999 ; Webster et al. 1999 ; Harrison and Vecchi 2001 ; Xie et al. 2002 ; Annamalai and Murtugudde 2004 ; Schott et al. 2004 ; Vecchi and Harrison 2004 ; Yamagata et al. 2004 ), in part because the thermocline tends to be close to the surface. Ironically, the climate significance of these regions results from the very circulation that tends to remove the

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H. Annamalai, H. Okajima, and M. Watanabe

that Rossby wave forcing has to be in mean westerly flow while the observed forcing during El Niño years is in mean easterlies; this and other related issues are addressed in subsequent studies and the details can be found in the review article by Trenberth et al. (1998) . Many atmospheric general circulation models (AGCMs) forced by ENSO-related SST anomalies have simulated the PNA pattern with moderate success ( Shukla and Wallace 1983 ; Blackmon et al. 1983 ; Geisler et al. 1985 ; Lau 1981

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Qian Song, Gabriel A. Vecchi, and Anthony J. Rosati

). Associated with the high SLP anomalies are anticyclonic atmospheric circulation features in each hemisphere ( Fig. 9m ). The anomalous southeasterly surface winds along Java–Sumatra act to elevate the thermocline there ( Fig. 9n ), in a similar fashion to the evolution of the ETIO during the developing phase of IODZM events in the IODZM-Only group. Meanwhile, a warm SSTA develops in the northwestern tropical Indian Ocean principally due to anomalous zonal advection. In boreal winter, the El Niño

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Vinu K. Valsala and Motoyoshi Ikeda

signals of ENSO ( Murtugudde et al. 1998 ) and Indian Ocean dynamics ( Yamagata et al. 1996 ) are also responsible for the changes in this net volume transport. Such a large water injection modifies both water properties and ambient circulation over the Indian Ocean. To identify such effects, we would need a proper understanding of the trajectory of the ITF. Several studies have been done on the spreading pathways of the ITF over the Indian Ocean, although none of them provide a detailed three

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Tomoki Tozuka, Jing-Jia Luo, Sebastien Masson, and Toshio Yamagata

general circulation model with mixed layer physics. J. Phys. Oceanogr. , 23 , 1363 – 1388 . Boccaletti , G. , R. C. Pacanowski , S. G. H. Philander , and A. V. Fedorov , 2004 : The thermal structure of the upper ocean. J. Phys. Oceanogr. , 34 , 888 – 902 . Bray , N. A. , S. Hautala , J. C. Chong , and J. Pariwono , 1996 : Large-scale sea level, thermocline, and wind variations in the Indonesian throughflow region. J. Geophys. Res. , 101 , 12239 – 12254 . Cai , W

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