Teleconnection Pathways of ENSO and the IOD and the Mechanisms for Impacts on Australian Rainfall

Abstract

Impacts of El Niñ o-Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD) on Australian rainfall are diagnosed from the perspective of tropical and extratropical teleconnections triggered by tropical sea surface temperature (SST) variations. The tropical teleconnection is understood as the equatorially trapped, deep baroclinic response to the diabatic (convective) heating anomalies induced by the tropical SST anomalies. These diabatic heating anomalies also excite equivalent barotropic Rossby wave trains that propagate into the extratropics. The main direct tropical teleconnection during ENSO is the Southern Oscillation (SO), whose impact on Australian rainfall is argued to be mainly confined to near-tropical portions of eastern Australia. Rainfall is suppressed during El Niñ o because near-tropical eastern Australia directly experiences subsidence and higher surface pressure associated with the western pole of the SO. Impacts on extratropical Australian rainfall during El Niñ o are argued to stem primarily from the Rossby wave trains forced by convective variations in the Indian Ocean, for which the IOD is a primary source of variability. These equivalent-barotropic Rossby wave trains emanating from the Indian Ocean induce changes to the midlatitude westerlies across southern Australia, thereby affecting rainfall through changes in mean-state baroclinicity, west-east steering, and possibly orographic effects. Although the IOD does not mature until austral spring, its impact on Australian rainfall during winter is also ascribed to this mechanism. Because ENSO is largely unrelated to the IOD during austral winter, there is limited impact of ENSO on rainfall across southern latitudes of Australia in winter. A strong impact of ENSO on southern Australia rainfall in spring is ascribed to the strong covariation of ENSO and the IOD in this season. Implications of this pathway from the tropical Indian Ocean for impacts of both the IOD and ENSO on southern Australian climate are discussed with regard to the ability to make seasonal climate predictions and with regard to the role of trends in tropical SST for driving trends in Australian climate.

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

“…These AGCM experiments seem to indicate that western Indian Ocean SSTs are not crucial in varying Australian climate, as seen mediated through the Rossby wave train in observations (Cai et al 2011). The emphasis on eastern Indian Ocean SST, as included in the N w /N c cases, instead agrees with Ummenhofer et al (2009), who showed that rainfall across southern regions of Australia is modulated by the meridional gradient in eastern Indian Ocean SST, with little effect in response to western Indian Ocean SST anomalies.…”

“…The role of anomalous SST in the western Indian Ocean for Australian climate has been raised previously as a means of transmitting ENSO and Indian Ocean dipole impacts onto extratropical Australian rainfall, mediated through Rossby wave trains (Cai et al 2011). Here, we explore the effect of western Indian Ocean SST on Rossby wave trains and blocking in the AustraliaNew Zealand sector further: in the 100-yr simulations WIO w and WIO c , warm/cold SST anomalies were superimposed over the western Indian Ocean within 58N-128S, 408-908E (Fig.…”

“…For the SH, Renwick (1998) linked variations in the blocking activity in the South Pacific to tropical SST anomalies associated with ENSO, giving rise to anomalous convection and divergence in the tropics and transmitted to the extratropics through Rossby wave propagation (Renwick and Revell 1999). This way, variations in atmospheric blocking might be the conduit for remote transmission of tropical influences in the Indo-Pacific region, mediated by Rossby waves, on midlatitude precipitation in Australia (Cai et al 2011;Timbal and Hendon 2011).…”

Seasonal prediction of global sea level anomalies using an ocean–atmosphere dynamical model

“…It is interesting to see that the IOD signal still has a strong sea level anomaly in DJF despite the fact that it disappears from the SST in December (Saji et al 1999;Hendon et al 2012). Whilst the IOD covaries strongly with ENSO in austral spring (Lim et al 2009b;Cai et al 2011) and rapidly terminates as the Australian monsoon develops in the austral early summer season , it evidently still persists in the subsurface, affecting the SLA at the eastern and western boundaries of the Indian Ocean. In addition, EOF1 has a strong signature of a variable strength Leeuwin current along the West Australian coastline associated with ENSO.…”

Seasonal dependence of the predictable low-level circulation patterns over the tropical Indo-Pacific domain

“…The large-scale atmospheric circulation over this region plays an important role in not only influencing the variability of the tropical climate systems but also transferring their impacts remotely through teleconnections (Wang and Zhang 2002;Cai et al 2011;Sun and Zhou 2013). Within the tropical Indo-Pacific domain, the low-level atmospheric circulation plays a major role in facilitating the intensive ocean-atmosphere interactions.…”