Unraveling Decadal Linkage and Dynamics between the Western Australian Coast and the Western-Central Tropical Pacific

Background and Study Focus

In this study published in Nature Communications, researchers explore the intricate interplay of sea surface temperature (SST) variabilities between the Western Australian coast and the western-central tropical Pacific. The Western Australian coast frequently experiences marine heatwaves, which lead to coral bleaching and impact significantly on local marine ecosystems. The western-central tropical Pacific is a key region for El Niño-Southern Oscillation (ENSO) whose impacts on weather and climate are global and significant.

 Since the 1980s, SSTs in the adjacent areas such as the tropical Indian Ocean and the Indo-Pacific warm pool not only warms rapidly, but also fluctuates on decadal timescale due to increased greenhouse gas and volcanic activities. Under such a background, this study uses a robust set of multiple observational datasets, pacemaker model experiments, and large ensemble simulations to explore how these changes have intensified those decadal climate variabilities and their implications for global climate dynamics.

Enhanced Decadal SST Variability and Linkage

The research highlights a significant enhancement in decadal SST variability off the Western Australian coast and in the western-central tropical Pacific since 1985 (Fig.1). This period marks a crucial shift in how external forcing—particularly volcanic activity and greenhouse gases—influence the Tropical Indian Ocean, driving profound decadal linkages between these regions.

Externally forced decadal SST anomalies in the tropical Indian Ocean after 1985can largely explain the observed SST anomalies between these two regions, diverging from the earlier influence of the Interdecadal Pacific Oscillation (IPO). This transition underscores a closer decadal linkage between Western Australian coast and western-central tropical Pacific, with the magnitude of decadal variability now comparable to and even exceed to those observed on interannual timescales.

Interbasin Linkage and Dynamics

The warming and decadal fluctuations in the tropical Indian Ocean, influenced by external forcings, have significantly impacted atmospheric and oceanic circulation patterns, leading to strengthened easterly winds in the western-central tropical Pacific. This change cools the SST there, enhancing the upward transport of colder water.

These dynamics are crucial for understanding the enhanced decadal linkage. SST anomalies in the tropical Indian Ocean and western Pacific are essential, as warm anomalies drive equatorial easterly wind anomalies, which in turn enhance equatorial upwelling, cooling the SST in the western-central tropical Pacific. This cooling effect strengthens the Indonesian Throughflow, which brings more warm water to western Australian coast. Moreover, colder SSTs in the western-central tropical Pacific can induce cyclonic circulation in the Southeast Indian Ocean, which, along with local air-sea feedback, further amplifies water warming off western Australian coast.

This study also notes the persistent interannual linkages that interact with the decadal changes, suggesting a complex interplay between these timescales that may amplify observed climate variabilities. The enhanced tropical Indian Ocean warming extends negative sea-level pressure (SLP) anomalies eastward to the western tropical Pacific (even to the Northeast Pacific), amplifying the zonal SLP gradient across the Pacific and intensifying anomalous easterlies that enhance the linkages.

Modeling Challenges and Future Directions

The emerging decadal linkages combined with the interannual linkages off the Western Australian coast and in the western-central tropical Pacific will lead to profound intimate interplay between the Indian and Pacific Oceans. These Indo-Pacific linkages are influenced by the incoherent warming pattern among tropical oceans since the 1980s, which is partly out of control by the IPO. This introduces complexity into tropical ocean and atmosphere dynamics. Such warming patterns cannot be captured accurately by most recent generation of fully coupled climate models. This discrepancy leads to an underestimation of the broad impacts of the Indo-Pacific linkage, particularly as these regions continue to warm.

Super large ensembles^{1, 2, 3} can enhance the separation of warming effects from internal variabilities like the IPO and the Atlantic Multidecadal Oscillation (AMO), thus refining our understanding and representation of these dynamics. Future climate models are necessary to better represent these warming patterns among tropical oceans and interactions with internal variabilities like the IPO and AMO, improving our ability to predict and manage the regional and global impacts of climate changes in a warming world.

Reference:

1. Lin P, et al. The Super-large Ensemble Experiments of CAS FGOALS-g3. Advances in Atmospheric Sciences, 1-20 (2022).
2. Maher N, et al. The Max Planck Institute Grand Ensemble: enabling the exploration of climate system variability. Journal of Advances in Modeling Earth Systems 11, 2050-2069 (2019).
3. Rodgers KB, et al. Ubiquity of human-induced changes in climate variability. Earth System Dynamics 12, 1393-1411 (2021).