The Indian Ocean Dipole: A Monopole in SST

Abstract

The claim for a zonal-dipole structure in interannual variations of the tropical Indian Ocean (IO) SSTs-the Indian Ocean dipole (IOD)-is reexamined after accounting for El Niño-Southern Oscillation's (ENSO) influence. The authors seek an a priori accounting of ENSO's seasonally stratified influence on IO SSTs and evaluate the basis of the related dipole mode index, instead of seeking a posteriori adjustments to this index, as common.Scant observational evidence is found for zonal-dipole SST variations after removal of ENSO's influence from IO SSTs: The IOD poles are essentially uncorrelated in the ENSO-filtered SSTs in both recent and century-long (1900-2007) periods, leading to the breakdown of zonal-dipole structure in surface temperature variability; this finding does not depend on the subtleties in estimation of ENSO's influence. Deconstruction of the fall 1994 and 1997 SST anomalies led to their reclassification, with a weak IOD in 1994 and none in 1997.Regressions of the eastern IOD pole on upper-ocean heat content, however, do exhibit a zonal-dipole structure but with the western pole in the central-equatorial IO, suggesting that internally generated basin variability can have zonal-dipole structure at the subsurface.The IO SST variability was analyzed using the extended-EOF technique, after removing the influence of Pacific SSTs; the technique targets spatial and temporal recurrence and extracts modes (rather than patterns) of variability. This spatiotemporal analysis also does not support the existence of zonal-dipole variability at the surface. However, the analysis did yield a dipole-like structure in the meridional direction in boreal fall/ winter, when it resembles the subtropical IOD pattern (but not the evolution time scale).

Long-term stationarity of El Niño–Southern Oscillation teleconnections in southeastern Australia

“…Although debate continues about the existence or non-existence of a dipole mode in the tropical Indian Ocean (e.g. Allan et al 2001;Dommenget 2011;Zhao and Nigam 2015), many studies have found that SST variations in the eastern Indian Ocean at least have an impact on SEA rainfall that is independent of ENSO (e.g. Meyers et al 2007;Ummenhofer et al 2011;Pepler et al 2014).…”

Indo-Pacific Variability on Seasonal to Multidecadal Time Scales. Part I: Intrinsic SST Modes in Models and Observations

“…Recently, Dommenget (2011) and Zhao and Nigam (2015) undertook an effort to verify previously applied methodologies and argued that upon careful analysis no IOD pattern emerges independently of ENSO. Moreover, Izumo et al (2010Izumo et al ( , 2014 and Yuan et al (2011Yuan et al ( , 2013 reported that the IOD is a good predictor of ENSO more than one year ahead.…”

Sensitivity of MJO propagation to a robust positive Indian Ocean dipole event in the superparameterized CAM

“…With the exception of 1991 and 1994, the remaining +IOD events had contemporaneous Niño3.4 indices [ Trenberth , ] greater than supporting the known ENSO influence on Indian Ocean SSTs [see Schott et al ., review]. Recent analysis by Zhao and Nigam [] indicates that the IOD is manifested more clearly in upper ocean heat content rather than SST. Those authors find that the +IOD events of 1994 and 2006 exhibit clear dipole variability in subsurface ocean temperatures even when ENSO influences are removed, suggesting that these two events are manifested largely by processes internal to the Indian Ocean region and are representative of the canonical +IOD.…”

“…A secondary motivation is to discriminate the local Indian Ocean SST dipole effects from remote tropical Pacific SST anomalies that can also associate with the +IOD phase. This is relevant to an ongoing debate about the dependence [ Dommenget , ; Zhao and Nigam , ] or independence [ Saji and Yamagata , ; Fischer et al ., ; Meyers et al ., ] between the IOD and ENSO. This study is not aimed at fully disentangling the potential IOD‐ENSO link; however, we do demonstrate sensitivities in the simulated MJO response to perturbed SST conditions in different geographic sectors referenced from a single but representative +IOD event.…”