This study investigates the mechanisms behind the Pacific Meridional Mode (PMM) in influencing the development of El Niño-Southern Oscillation (ENSO) event and its seasonal predictability. To examine the relative importance of various factors that may modulate the efficiency of the PMM influence, a series of experiments are conducted for selected ENSO events with different intensity using the Community Earth System Model, in which ensemble predictions are made from slightly different ocean initial states but under a common prescribed PMM surface heat flux forcing. Overall, the matched PMM forcing to ENSO, i.e., a positive (negative) PMM prior to an El Niño (a La Niña), plays an enhancing role, while a mismatched PMM forcing plays a damping role. For the matched cases, a positive PMM event enhances an El Niño more strongly than a negative PMM event enhances a La Niña. This asymmetry in influencing ENSO largely originates from the asymmetry in intensity between the positive and negative PMM events in the tropics, which can be explained by the nonlinearity in the growth and equatorward propagation of the PMM-related anomalies of sea surface temperature (SST) and surface zonal wind through both wind-evaporation-SST feedback and summer deep convection response. Our model results also indicate that the PMM acts as a modulator rather than a trigger for the occurrence of ENSO event. Furthermore, the response of ENSO to an imposed PMM forcing is modulated by the preconditioning of the upper-ocean heat content, which provides the memory for the coupled low-frequency evolution in the tropical Pacific.

The Pacific Meridional Mode (PMM) has been suggested to play an important role in modulating the development of the El Niño-Southern Oscillation (ENSO). In this study, we examine the projected changes in the PMM and its impact on the ENSO under greenhouse gas forcing using the models of the Coupled Model Intercomparison Project Phase 6. These models can properly reproduce the characteristics of PMM patterns, but reveal discrepant PMM-ENSO relationships owing to different Wind-Evaporation-Sea-surface Temperature (SST) (WES) feedback efficiency and different magnitude of atmospheric convection response to SST anomalies. We select the models that show good performances in simulating the PMM and its impact on the ENSO for investigation of future projections. Results show potential increases in both PMM amplitude and its impact on the ENSO under the SSP585 warming scenario with great inter-model consensus. Diagnosis of the WES feedback indicates increasing sensitivity of latent heat flux to zonal wind speed in a warming climate, which seems to be the main reason for the projected strengthening PMM and its impact on the ENSO. In addition, a slightly intensified response of atmospheric convection to SST anomalies in the subtropical Pacific may partially contribute to a stronger PMM-ENSO relationship. The results from this study highlight the increasing importance of the PMM for ENSO development, which calls for more attention to be paid to the PMM for ENSO prediction.

Model warming projections, forced by increasing greenhouse gases, have a large inter-model spread in both their geographical warming patterns and global mean values. The inter-model warming pattern spread (WPS) limits our ability to foresee the severity of regional impacts on nature and society. This paper focuses on uncovering the feedbacks responsible for the WPS. Here, we identify two dominant WPS modes whose global mean values also explain 98.7% of the global warming spread (GWS). We show that the ice-albedo feedback spread explains uncertainties in polar regions while the water vapor feedback spread explains uncertainties elsewhere. Other processes, including the cloud feedback, contribute less to the WPS as their spreads tend to cancel each other out in a model-dependent manner. Our findings suggest that the WPS and GWS could be significantly reduced by narrowing the inter-model spreads of ice-albedo and water vapor feedbacks, and better understanding the spatial coupling between feedbacks.

The Pacific Meridional Mode (PMM) can modulate El Niño-Southern Oscillation (ENSO), and is also affected by ENSO-related tropical Pacific sea-surface temperature anomalies (SSTAs). Two tropical feedbacks on the PMM have been proposed: the positive one of central tropical Pacific SSTAs and the negative one of eastern tropical Pacific (ETP) SSTAs, the latter of which is suggested to be active only during strong eastern Pacific (EP) El Niño events like 1982/1983 and 1997/1998. However, we find that no strong negative PMM-like SSTAs appeared although the PMM indices (PMMIs) were strongly negative in spring of 1983 and 1998. Observation and model experiments show that tropical warming in 1983 and 1998 not only occurred in the ETP, but also extended to the dateline, thus inducing wind anomalies unfavorable for establishing the wind-evaporation-SST feedback for negative PMM in the subtropics. To understand the discrepancy between the large negative PMMIs and weak PMM-related subtropical cooling during strong EP El Niño events, we isolate the relative contributions of subtropical and tropical SSTAs to the PMMIs by calculating their spatial projections on the PMM. Analysis combinedly using observation and CMIP6 models shows that despite the large contribution from subtropical SSTAs, the large tropical SSTAs, especially the extreme ETP warming, during strong EP El Niño events could cause large negative PMMIs even without strong negative subtropical SSTAs. Our study clarifies the impact of ETP warming in causing negative PMM and indicates the overstatement of negative PMMIs by tropical SSTAs during strong EP El Niño events.