Predictability of Marine Heatwaves off Western Australia Using a Linear Inverse Model

Marine heatwaves (MHWs) off Western Australia (110 & DEG;-116 & DEG;E, 22 & DEG;-32 & DEG;S; herein, WA MHWs) can cause devastating ecological impacts, as was evidenced by the 2011 extreme event. Previous studies suggest that La Nifia is the major large-scale driver of WA MHWs, while the Indian Ocean dipole (IOD) may also play a role. Here, we investigate historical WA MHWs and their connections to these large-scale climate modes in an ocean model (ACCESS-OM2) simulation driven by a prescribed atmosphere from JRA-55-do over 1959-2018. Rather than analyzing sea surface temperature, the WA MHWs and climate mode indices were characterized and investigated in vertically averaged temperature (VAT) to -300-m depth to afford the longer ocean dynamic time scales, including remote oceanic connections. We develop a cyclostationary linear inverse model (CS-LIM; from 35 & DEG;S to 10 & DEG;N, across the Indo-Pacific Ocean), to investigate the relative contributions of La Nifia VAT and positive IOD VAT to the predictability of WA VAT MHWs. Using a large ensemble of CS-LIM simulations, we found that -50% of WA MHWs were preceded about 5 months by La Nifia, and 30% of the MHWs by positive IOD about 20 months prior. While precursor La Nifia or positive IOD, on their own, were found to correspond with increased WA MHW likelihood in the months following (-2.7 times or -1.5 times more likely than by chance, respectively), in combination these climate mode phases were found to produce the largest enhancement in MHW likelihood (-3.2 times more likely than by chance). Additionally, we found that stronger and longer La Nifia and/or positive IOD tend to lead stronger and longer WA MHWs.

Automated summary

By analyzing the ocean model simulation, it is found that La Nifia and the positive IOD are the major drivers of WA MHWs. The combination of these two climate modes leads to the highest likelihood of WA MHWs, which is 3.2 times higher than by chance.