A Triggering Mechanism for the Indian Ocean Dipoles Independent of ENSO

Abstract

Although the Indian Ocean dipole (IOD) and ENSO are significantly correlated, there are indeed some IODs independent of ENSO. In this research, the characteristics of independent IOD are investigated and a new triggering mechanism is proposed based on case study and statistical analysis. Results show that the independent IODs peak in an earlier season and have a weaker intensity compared with the IODs associated with ENSO. The wind anomaly associated with the independent IOD is very unique and shows a monsoonlike pattern, in addition to the equatorial easterly wind anomaly (EEWA) common to all IODs. The evolution of the EEWA associated with the independent IOD is well captured by the second EOF mode of the equatorial zonal wind interannual variability, suggesting that the independent IOD is an important climate mode inherent to the tropical Indian Ocean. The EEWA associated with the independent IOD is tightly linked to Indian summer monsoon activities in spring, and the convection anomalies associated with early summer monsoon onset in the Bay of Bengal plays a key role in inducing the EEWA. The EEWA can persist through spring and summer and causes a series of processes similar to those related to the IODs associated with ENSO. The correlation between the independent IOD and Indian summer monsoon activities increases dramatically after the 1980s, which is probably due to the mean state change in the tropical Indian Ocean climate system.

Indian Ocean Dipole in CMIP5 and CMIP6: characteristics, biases, and links to ENSO

“…ENSO is connected to the Indian Ocean via an atmospheric bridge and can trigger an IOD event and modulate its evolution 19 – 21 . Recent research has indicated that the IOD is a combination of internally forced and ENSO driven phenomenon 20 , 22 . Two of the strongest pIOD events in recent history have occurred with and without El Niño being present; the 1997 pIOD co-occurred with El Niño, and the 2019 pIOD occurred independently.…”

“…Some studies have suggested that like ENSO, which exhibits different flavours of events 5 , 23 , IOD also appears to exhibit event diversity. For instance, IOD has been classified into three different event types based on their timing and the influence of ENSO 20 , 22 , 24 : (i) internally forced IOD events peaking in SON, (ii) early-onset IOD unrelated to ENSO events, typically peaking in JJA (before the IOD usual peak season in SON) 20 ; and, (iii) those triggered by ENSO 20 , 22 that typically last longer than an independent IOD event. The unseasonable IOD type (ii) has been proposed to be IODs aborted by downwelling warm Kelvin waves, excited by intraseasonal disturbances 25 .…”

An Observational Perspective of Sea Surface Salinity in the Southwestern Indian Ocean and Its Role in the South Asia Summer Monsoon

“…As water density is controlled by both temperature and salinity, evidence suggests that Sea Surface Salinity (SSS) could be an indicator of abrupt changes in ocean dynamic and air-sea interaction [5]. Salinity affects many aspects of ocean stability [6,7], dynamic ocean variability [8][9][10][11][12] and complicates air-sea interactions [13][14][15][16][17]. In previous studies [6,18], the variability of SSS is attributed to a series of complex mechanisms, mainly including the freshwater flux, horizontal advection, vertical entrainment as well as some turbulence of mixing and diffusion but the latter in the mixed layer has little influence on the seasonal variability of salinity [19][20][21].…”

Impacts of April snow cover extent over Tibetan Plateau and the central Eurasia on Indian Ocean Dipole

“…Fischer et al (2005) removed the ENSO activities in a coupled model and found that without the ENSO influences, IOD is mainly triggered by an anomalous Hadley circulation in spring over the eastern tropical Indian Ocean and Maritime Continent. Sun et al (2015) analysed the observed IOD and claimed that those independent of ENSO can be triggered by the monsoon-like wind anomalies in April-May probably related to the BOB summer monsoon. However, they have not clarified where the anomalous Hadley circulation or monsoon-like wind pattern comes from.…”

“…However, not all IOD co‐occur with ENSO. Some IOD events are triggered by the ENSO‐independent surface wind forcings (e.g., Fischer et al ., ; Schott et al ., 2009; Sun et al ., ), and some are triggered by the oceanic waves propagation from the off‐equatorial oceans. For example, the positive IOD in 2006 was triggered by the downwelling Rossby wave from the southern Indian Ocean that propagated westwards, reflected to the downwelling Kelvin wave at the western boundary, then propagated eastwards along the Equator and triggered the positive IOD (Luo et al ., ).…”