Seasonal climate forecasts are increasingly vital across various sectors, yet their accuracy remains limited. El Niño-Southern Oscillation (ENSO) is a key factor in these forecasts, making its predictability paramount. While ENSO originates in the tropical Pacific, extra-tropical climate variability influences its behavior. Studies have shown that Atlantic Niño (or Atlantic Zonal Mode), an equatorial Atlantic phenomenon resembling ENSO but with weaker amplitude and sporadic occurrence, can remotely induce a La Niña-like response in the Pacific. The 2021/22 La Niña event supports this Atlantic-forcing-Pacific premise, following a strong Atlantic Niño in the boreal summer of 2021. However, the significance of this Atlantic remote influence on ENSO is still debated, highlighting the need for a deeper understanding of the mechanisms involved. Existing descriptions of the mechanism focus on anomalous Walker Circulation with ascending branches over the warm equatorial Atlantic and descending branches over the western and central Pacific. These descriptions however do not fully explain how the anomalous Walker Circulation initiates and develops outside the equatorial Atlantic. This study aims to address this knowledge gap using observations and large-ensemble climate simulations to identify the role of the Maritime Continent in the teleconnection between Atlantic Nino and Pacific La Nina.

Previous research has explored the remote forcing of La Niña-like conditions in the Pacific by Atlantic Niño, often describing it through an anomalous Walker Circulation. However, these descriptions are based on a fully developed response, lacking an explanation of the anomalous Walker Circulation's initial development outside the equatorial Atlantic. Studies have proposed different mechanisms such as anomalous Walker Circulation, the role of Matsuno-Gill response to diabatic heat source in the EA, and the Kelvin wave mechanism, but these require further validation and a comprehensive understanding of the role of the Maritime Continent. This study builds upon these previous findings by using a large-ensemble model to further test and refine the understanding of how Atlantic Niño affects the Pacific.

The study employed observational analysis and large-ensemble climate model simulations to investigate the Atlantic-Pacific teleconnection. For observational analysis, a lag-regression analysis was performed on datasets including sea surface temperature (SST) from the HadISST dataset and winds from three reanalysis datasets (ERA5, ERA-Interim, and JRA55) during 1959-2021. The boreal summer ATL3 SST time series was used after removing the influence of El Niño-Southern Oscillation (ENSO) using linear regression with the preceding boreal winter Niño3.4 SST index. The large-ensemble simulations were conducted using the Community Earth System Model (CESM) version 1.1.2 with a nominal -1° horizontal resolution. Three ensemble experiments were performed: CTRL (control), NTOP (topography removal over the Maritime Continent), and NFRC (topography removal and land friction reduction over the Maritime Continent). Each ensemble included 60 members, with observed Atlantic Niño SST anomalies prescribed in both positive and negative forcing simulations for each ensemble. The difference between positive and negative forcings for each ensemble was then analyzed. The study also examined the role of the Maritime Continent by comparing simulations with realistic topography and those with flattened topography. In addition, it assessed the impact of land friction by reducing friction in one experiment. Finally, the analysis included the calculation of zonal mass stream function for quantitative analysis of Walker Cell behavior.

Observational analysis revealed that the initial atmospheric response to Atlantic Niño SST forcing manifests as anomalous easterlies over the western Pacific and Maritime Continent (WP/MC), followed by the development of a local Walker Cell over the WP/MC. Large-ensemble CESM simulations confirmed this, demonstrating that an eastward-propagating atmospheric Kelvin wave carries easterly wind anomalies from the Atlantic across the Indian Ocean to the Pacific. This Kelvin wave initiates the development of the anomalous Walker Cell. The simulations showed that the Maritime Continent's orography plays a crucial role in the generation of the local Walker Cell through orographic moisture convergence. Furthermore, land friction over the Maritime Continent dissipates Kelvin wave energy, influencing the strength of the Bjerknes feedback and the development of the La Niña-like response. Removing the Maritime Continent's topography resulted in a slower build-up and eastward shift of the local Walker Cell. Reducing land friction over the Maritime Continent led to a stronger La Niña-like response in the Pacific. The study demonstrated that atmosphere-land-ocean interactions over the Maritime Continent significantly regulate the position and strength of the La Niña-like response to Atlantic Niño forcing.

The findings highlight the crucial role of the Maritime Continent in mediating the teleconnection between Atlantic Niño and Pacific ENSO. The eastward-propagating Kelvin wave is the primary pathway, but its interaction with the Maritime Continent's orography and land friction determines the strength and location of the subsequent La Niña-like response. The results demonstrate that accurate representation of atmosphere-land-ocean interactions over the Maritime Continent is essential for improving ENSO prediction. Poorly resolved orography in current climate models might underestimate the interaction between the Kelvin wave and the Maritime Continent, thus affecting the accuracy of ENSO simulations.

This study advances our understanding of the Atlantic-Pacific teleconnection by demonstrating the importance of the Maritime Continent in regulating the Pacific response to Atlantic Niño. The interaction of the eastward-propagating Kelvin wave with the Maritime Continent's orography and land friction is crucial in initiating and modulating the La Niña-like response. Improving the representation of these interactions in climate models is key for enhancing ENSO prediction. Future research should focus on increasing model resolution to better capture the complex atmosphere-land-ocean interactions over the Maritime Continent and conducting multi-model intercomparison studies to better understand Atlantic Niño's overall impact on ENSO predictability.

The study uses an idealized forcing pattern for Atlantic Niño, which might not perfectly capture the natural variability. The CESM1.1.2 model used has inherent biases that, although partly mitigated by the experimental design, could potentially influence the results. The focus is on the mechanistic understanding of the teleconnection, not the overall significance of Atlantic Niño's impact on ENSO predictability, which requires further investigation using a multi-model approach.