Understanding ENSO modulation of regional sea surface temperatures and marine heatwaves

Marine heatwaves (MHWs) - prolonged and extreme oceanic warm water events – have brought severe and long-lasting impacts to marine ecosystems and economies in many regions of the world. While local short-timescale processes (e.g., synoptic weather) typically cause MHW onset and/or decay locally, large-scale teleconnection mechanisms and remote drivers can modulate MHW likelihood, influencing the overall predictability on longer time scales. El Niño-Southern Oscillation (ENSO), the dominant global interannual mode of climate variability, is known as a significant modulator of sea surface temperature (SST) variations and MHW events. Dynamical understanding of how ENSO modulates SST variations via different teleconnection mechanisms has been progressing well. However, our knowledge is far from complete for many regions including the oceans off Australia’s southeast and west coasts, as well as in the eastern Pacific rim – where severe MHWs have been occurring in recent years.
This thesis presents comprehensive investigations of ENSO modulation mechanisms of MHWs and SST variations in these regions, including relationships with other large-scale remote drivers and teleconnection mechanisms. To minimise the effects of local short-timescale processes, while exploring the remote drivers and large-scale teleconnections, interannual SST variations are examined in lieu of MHWs in the selected regions. For each selected region, how ENSO modulates regional SST variations is studied by taking account of: (1) the contributions of different ENSO forcings (e.g., ENSO-driven wind stress forcing in the tropical Pacific) that are related to different teleconnection pathways; and (2) the superposition of non-ENSO forcings (i.e., forcings generated by climate variabilities other than ENSO) on ENSO forcings.
First, the large-scale forcing mechanisms that connect remote drivers with the MHWs and SST variations regionally off Australia’s southeast coast in the Tasman Sea are investigated. The region experienced a record-breaking MHW during the austral summer of 2015/16. By using an upper-ocean heat budget analysis, positive SST anomalies during about 50% of the historical MHWs including the 2015/16 event were identified to be primarily due to an intensified East Australian Current (EAC) Extension. A lagged correlation analysis and simple Rossby wave models connected variations in the EAC Extension transport with the South Pacific wind forcing via contributions from westward-propagating Rossby waves. The results reveal that South Pacific wind forcing contributes deterministically to modulating interannual SST variations and regional MHWs off Australia’s southeast coast.
Second, the mechanistic role of ENSO on SST variations off Australia’s southeast coast is investigated. Compared to the strong and certain role of ENSO on SST variations off Australia’s west coast, which is another MHW “hotspot”, Australia’s southeast-coast SST variations are much weaker, though still significantly correlated with ENSO. A novel modelling framework was developed in this study by combining tropical Pacific pacemaker ensemble simulations with ocean general circulation model (OGCM) perturbation experiments. Using this modelling framework, the less-understood ENSO modulation off Australia’s southeast coast and the associated underpinning mechanisms were compared with the west coast. The results show that the strong SST response to ENSO off the west coast of Australia is dominated by tropical Pacific ENSO-driven wind forcing with small contributions from non-ENSO climate variabilities. However, off Australia’s southeast coast, the large contribution of tropical Pacific ENSO-driven wind forcing – which was found to be achieved via anticlockwise wave propagation around Australia excited in the tropical Pacific – is offset by ENSO-driven buoyancy forcing from the extratropical South Pacific including the Tasman Sea, leading to a weak SST response to ENSO in the region. This weak ENSO response was found to be further weakened by non-ENSO climate variabilities, resulting in an even weaker ENSO effect on SST variations off Australia’s southeast coast.
By comparing the ENSO and non-ENSO large-scale forcing mechanisms discussed above, we explored why the ENSO modulation is weak off Australia’s southeast coast. The offset among different ENSO-driven forcings leads to a relatively small net effect of ENSO on the regional SST variability. More importantly, the ENSO effect is further weakened by non-ENSO SST variability imprinted by forcings from the South Pacific, as indicated by different teleconnection pathways between the ENSO and non-ENSO remote drivers.
Finally, the ENSO modulation mechanisms of SST variations in the California Current System (CCS) and Humboldt Current System (HCS) – the eastern boundary current systems in North and South Pacific – are investigated and compared. A recent MHW that occurred off California and Baja California was reported to be primarily driven by the 2015/16 El Niño via equatorial and coastal wave propagation. While ENSO is hypothesised to exert similar and somewhat symmetric influences on the CCS and HCS off the west coasts of North and South America via equatorial and coastal waveguides, mismatches of SST anomalies are apparent among the CCS, HCS, and tropical Pacific. A contrast study was performed between SST responses to ENSO in the two latitudinally-symmetric regions in the CCS and HCS together with their underpinning mechanisms, by applying the same modelling framework introduced above together with composite and lagged regression analyses based on tropical Pacific pacemaker ensemble simulations. The results show that while the SST changes due to ENSO oceanic teleconnections by equatorial and coastal wave propagation are indeed similar and relatively symmetric, different ENSO-driven atmospheric forcings in the far eastern North and South Pacific – which can arise from several atmospheric mechanisms and their interactions - tend to induce asymmetric SST responses in the selected latitudinally-symmetric regions. Furthermore, non-ENSO large-scale atmosphere variabilities over the North and South Pacific behave differently in the far eastern Pacific, weakening or enhancing ENSO responses in the selected regions with varying degrees and phases. These lead to different ENSO modulations and mismatches of SST anomalies in the CCS and HCS.
Overall, this thesis deepens our dynamical understanding of how ENSO modulates SST variations and MHWs in several representative regions, together with the relationships between ENSO and non-ENSO large-scale forcing mechanisms. These regions are the west and southeast coasts of Australia, as well as North and South Pacific eastern boundary current system regions. Additionally, this thesis develops a valuable modelling framework for identifying ENSO responses and inferring their underpinning teleconnection mechanisms. This modelling framework can be applied more generally to any region of interest to study regional ENSO modulation mechanisms.