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Understanding and forecasting Tropical Pacific Decadal‐scale Variability (TPDV) strongly rely on climate model simulations. Using a Linear Inverse Modeling (LIM) diagnostic approach, we reveal Coupled Model Intercomparison Project Phase 6 models have significant challenges in reproducing the spatial structure and dominant mechanisms of TPDV. Specifically, while the models' ensemble mean pattern of TPDV resembles that of observations, the spread across models is very large and most models show significant differences from observations. In observations, removing the coupling between extratropics and tropics reduces TPDV by ∼60%–70%, and removing the tropical thermocline variability makes the central tropical Pacific a key center of action for TPDV and El Niño Southern Oscillation variability. These characteristics are only confirmed in a subset of models. Differences between observations and simulations are outside the range of natural internal TPDV noise and pose important questions regarding our ability to model the impacts of natural internal low‐frequency variability superimposed on long‐term climate change. Plain Language Summary Tropical Pacific Decadal‐scale Variability (TPDV) has been shown to impact global‐scale climate fluctuations, weather regimes, and temperature trends such as the 1998–2012 global warming hiatus. Understanding and predicting TPDV's impacts strongly rely on climate model simulations and projections. However, the models show key deficiencies in reproducing the observed structure of TPDV. Although the arithmetic mean of TPDV's spatial pattern in each individual model is similar to observations, there is a large spread among models, and most models show significant differences from observations. Using an empirical dynamical model to decompose the mechanisms of the climate models reveals that more than 50% of the models fail to reproduce the roles of extratropics‐tropics coupling and the tropical thermocline variability play in TPDV. These differences show significant challenges exist across the models for modeling the decadal‐scale climate variability in the tropical Pacific. Improving the simulation of TPDV in climate models is vital for understanding the impacts of natural internal low‐frequency variability alongside long‐term climate change. Key Points A Linear Inverse Model is used to decompose and compare tropical Pacific decadal variability dynamics in observations and climate models Coupled Model Intercomparison Project Phase 6 models have significant challenges in reproducing the observed dynamics and spatial pattern of tropical Pacific decadal variability Many models (>50%) cannot reproduce the roles of extratropics‐tropics coupling and thermocline variability in tropical Pacific variability

The tropical cyclone (TC) genesis number in the western North Pacific (WNP) exhibits a pronounced decadal decrease around the mid-1990s, with prominent seasonal and spatial inhomogeneity. This decadal shift of TC activity is mostly confined to the southeastern part of the WNP and occurs mainly during the second half of the calendar year. Accordingly, westward and northeastward TC recurving movements strongly decreased in recent decades after 1995 compared with TC tracks in the earlier period (1979–1994). We find that this TC activity decadal change is associated with tropical Pacific decadal variability, which is measured here by a low-pass filtered Niño3.4 index. In contrast to the earlier period, the anomalous cold mean state in the tropical Pacific during recent decades favored the enhancement of zonal vertical wind shear (UVWS) and suppressed TC activity. This tropical Pacific mean state change is possibly related to decadal changes of El Niño–Southern Oscillation (ENSO) properties (i.e., more La Niña events occurred during recent decades). This relationship between tropical Pacific mean state change and the UVWS in the southeastern WNP on decadal timescales is further validated based on longer observations (1951–2017) and control simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The statistical relationships between TC activity and the Pacific Decadal Oscillation (PDO) or Atlantic Multidecadal Oscillation (AMO) are weaker and insignificant, both for the observations and for simulations. Our results imply that decadal variations of the tropical Pacific mean state should be taken into account when predicting WNP TC activities on decadal timescales.

This study investigates why observed decadal‐scale climate variability is predominantly pronounced in the Niño4 region compared to other equatorial Pacific areas using both observation and model sensitivity experiments. The initial shift to the negative phase of Tropical Pacific Decadal Variability (TPDV) is primarily driven by the upward and eastward migration of isopycnal negative temperature anomalies along the equator. Subsequently, the wind fields associated with the negative phase of the Pacific Meridional Mode (PMM) induce anomalous vertical currents in the equatorial Pacific. This leads to anomalous upwelling and downwelling of mean temperature in the Niño4 and Niño3 regions, respectively, thereby strengthening and weakening the corresponding subsurface‐produced sea surface temperature anomalies. Our findings clarify the roles of subsurface temperature anomalies in the phase reversal of TPDV and PMM in amplifying decadal variance, specifically in the equatorial central Pacific. Plain Language Summary Observations have consistently highlighted prominent decadal climate variability in the Niño4 region, yet the underlying cause of this distinct pattern remains largely elusive. In this study, we use composite analysis during the phase transition of Tropical Pacific Decadal Variability (TPDV) and modeling experiments to elucidate the mechanisms governing the observed decadal climate variability in the Niño4 region compared to other equatorial areas. Our findings reveal that the eastward and upward propagation of negative subsurface temperature anomalies primarily drives the phase reversal of TPDV. Following this transition from positive to negative phase, the Pacific Meridional Mode (PMM) plays a crucial role. Specifically, PMM‐associated wind forcing induces anomalous upwelling and downwelling in the Niño4 and Niño3 regions, respectively. This results in anomalous vertical advection of mean temperature, contributing to the strengthening and weakening of decadal variances in these regions. Key Points Subsurface temperature anomalies initiate the phase reversal of TPDV while PMM plays a key role in equatorial SSTAs post‐transition Vertical heat advection is crucial in reinforcing/weakening decadal variance in the Niño4/Niño3 region PMM‐associated wind fields induce anomalous vertical advection after the TPDV phase transition

Persistent, multi-year shifts in atmospheric circulations and their associated influence on regional climates have profound impacts on physical, biological, and socioeconomic systems. The Pacific Decadal Precession (PDP), an atmospheric mode of variability consisting of a lower tropospheric height dipole which rotates counterclockwise over several years in the North Pacific, describes a series of such shifts in atmospheric circulations. One phase of the PDP, the north-south (N-S) phase, is hypothesized to be partially driven by central tropical Pacific (CP) sea surface temperature (SST) variability, but robust assessment of this dynamical connection in climate models remains to be done. In this study, we investigate this hypothesis with analyses in both reanalysis and selected models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) archive. We show that the emergence of the N-S phase is both related to and influenced by tropical Pacific decadal SST variability, specifically variability associated with CP El Niño-Southern Oscillation (ENSO) events. When examining the pre-industrial runs of the CMIP6 models, we find that most models cannot recover the characteristic cyclonic precession of the dipoles of the PDP, instead featuring only amplitude and sign changes of the N-S phase, Moreover, the models do not replicate the dynamical connections between the tropical Pacific and this North Pacific mode of climate variability. Our results suggest that primary reasons for this inconsistency are that models inaccurately simulate both the SST pattern associated with the PDP, shared low-frequency power associated with CP ENSO events, and incorrect Rossby wavetrains emanating from the tropical Pacific into the North Pacific on quasi-decadal timescales. Taken together, our analyses offer another benchmark by which to test the fidelity of the climate model simulations in capturing Pacific decadal climate variability in order to improve decadal-to-centennial climate projections.

Pacific quasi-decadal oscillation (PQDO) is one component of the multi-time-scale tropical Pacific decadal variability, with a variability center in the equatorial central Pacific (ECP). PQDO has emerged since the 1950s and has a significant impact on decadal climate variability over Asia and North America and Pacific storms. However, why it has intensified since the 1950s remains unknown. Here we test two competing hypotheses, (1) 11-year solar cycle forcing and (2) internal variability arising from El Niño–Southern Oscillation (ENSO) asymmetry, by analyzing simulation results, including one fixed-forcing control (CTRL) experiment and four sensitive experiments with millennial spectral solar irradiance (SSI), obtained from the Community Earth System Model–Last Millennium Ensemble modeling project. The four-member ensemble-averaged SSI experiments suggest that 11-year solar irradiance forcing cannot excite PQDO without stratospheric amplification of solar forcing. By analyzing 144 years of observations and the CTRL experiment, we find that the PQDO is nonstationary, and consecutive La Niña-induced decadal variability can boost PQDO in the ECP. El Niño could induce decadal ENSO signals in the NINO3.4 region but not in PQDO regions. The negative phase of PQDO tends to follow the occurrence of multi-year La Niña. We suggest that the emergence of PQDO since the 1950s is mainly due to the increase in multi-year La Niña events.

The El Niño–Southern Oscillation (ENSO) is the most consequential mode of interannual climate variability on the planet, yet its prediction has become complex due to the inability of classical paradigms to explain the observed co-evolution of the tropical mean state and interannual variability on decadal timescales. This article synthesizes the extensive research on ENSO rectification, exploring a paradigm that resolves this causality problem by recasting ENSO as an active architect of its own mean state. Tracing the intellectual development of this theory, starting from fundamental concepts such as the “dynamical thermostat” and “heat pump” hypotheses, modern analysis has identified the core physical mechanism as nonlinear dynamical heating (NDH), which is rooted in nonlinear heat advection during asymmetric ENSO cycles. The convergence of evidence from forced ocean models and observational diagnostics confirms a rectified signal characterized by an off-equatorial spatial pattern, providing a primary mechanism for tropical Pacific decadal variability (TPDV). By establishing a coherent framework linking high-frequency asymmetry with low-frequency variations, this review lays the foundation for future research and emphasizes the critical role of the rectification effect in improving decadal climate prediction.

Decadal variations of the transport and bifurcation latitude of the North Equatorial Current (NEC) in the northwestern tropical Pacific Ocean over 1959–2011 are investigated using outputs of the Ocean Analysis/Reanalysis System 3 prepared by the European Centre for Medium-Range Weather Forecasts. The results indicate that the NEC transports at different longitudes have different decadal fluctuations, which are strongest around 139°E. The NEC bifurcation latitude (NBL) has its largest decadal variations around 150 m. Extremes of the decadal NEC transport and NBL before 1975 correspond to different circulation anomalies from those after 1975. The regression map against decadal NBL exhibits negative sea surface height (SSH) anomalies and a cyclonic gyre anomaly over the northwestern tropical Pacific Ocean, while that against the decadal NEC transport exhibits a dipole structure, with positive/negative SSH anomalies to the north/south of about 13°N. Furthermore, decadal variations of the NEC transport and NBL over the whole period have different correlations with Pacific Decadal Oscillation (PDO) and Tropical Pacific Decadal Variability (TPDV). Generally, the decadal NEC transport shows higher correlations with PDO than with TPDV, while the NBL has higher correlations with TPDV than with PDO. The high correlation of decadal NEC transport with PDO mainly comes from that of its northern branch with PDO, while its southern branch shows higher correlation with TPDV.

A robust decadal Indian Ocean dipolar variability （DIOD） is identified in observations and found to be related to tropical Pacific decadal variability （TPDV）. A Pacific Ocean-global atmosphere （POGA） experiment, with fixed radiative forcing, is conducted to evaluate the DIOD variability and its relationship with the TPDV. In this experiment, the sea surface temperature anomalies are restored to observations over the tropical Pacific, but left as interactive with the atmosphere elsewhere. The TPDV-forced DIOD, represented as the ensemble mean of 10 simulations in POGA, accounts for one third of the total variance. The forced DIOD is triggered by anomalous Walker circulation in response to the TPDV and develops following Bjerknes feedback. Thermocline anomalies do not exhibit a propagating signal, indicating an absence of oceanic planetary wave adjustment in the subtropical Indian Ocean. The DIOD-TPDV correlation differs among the 10 simulations, with a low correlation corresponding to a strong internal DIOD independent of the TPDV. The variance of this internal DIOD depends on the background state in the Indian Ocean, modulated by the thermocline depth off Sumatra/Java.

The Northwest Pacific (NWP) tropical cyclone (TC) activity exhibits significant decadal variations with alternating active and inactive periods. However, it remains unknown whether such kinds of decadal variations are predictable. Here, we develop a dynamic-statistic model for the decadal predictions of the tropical cyclone genesis frequency (TCGF) and accumulated cyclone energy (ACE) index of the NWP TCs. The dynamic-statistic model is a combination of decadal prediction experiments by coupled general circulation models (CGCM) from the CMIP6 Decadal Climate Prediction Project (DCPP) and multiple linear regression models based on the correlation relationships between the NWP TCGF (ACE) and large-scale variability modes of sea surface temperature (SST) anomalies in observations. For the TCGF, we first calculate anomalous SST intensities associated with Atlantic multidecadal variability (AMV), Pacific decadal oscillation (PDO), and global mean SST (GMSST) predicted by the decadal prediction experiments. Then, they are substituted into the regression model trained by the historical observational TCs and SST to predict the NWP TCGF. For the ACE, one more predictor, viz. the anomalous SST in the NWP, is involved in its regression model. The dynamic-statistic model can be applied for both deterministic and probabilistic predictions with multi-model ensemble mean, and individual members of the decadal prediction experiments used, respectively. Retrospective predictions for the past 50 years show that the correlation skill of the deterministic predictions for the NWP TCGF (ACE) in the future 2–5 and 6–9 years reach 0.71 and 0.59 (0.59 and 0.41), respectively. The results of this dynamic-statistic model will provide decision-makers of the western Pacific Rim countries with valuable information to adapt to variations in NWP TC activity over the next 10 years.

The tropical Indian Ocean sea level displayed decadal variations in response to Pacific Decadal Oscillation (PDO). Contrasting patterns of decadal oscillation in sea level is found during the opposite phases of PDO especially in the thermocline ridge region of the Indian Ocean (TRIO; 50°E–80°E; 15°S–5°S). Epochal mean sea level rise is observed over the TRIO region during the cold phase of PDO (1958–1977), whereas epochal mean sea level fall is observed during the warm phase of PDO (1978–2002). Analysis reveals that the decadal variability in the sea level pattern in the TRIO region is in accordance with the PDO phase shifts and is primarily caused by changes in the surface forcing over the Indian Ocean as a response to PDO. The changes in the large scale Walker circulation over the tropical Indian Ocean region during the different phases of PDO support our hypothesis. The winds and wind stress curl variations associated with these large scale circulation changes are primarily inducing the observed regional decadal sea level variability over TRIO. The decadal forcing through Indonesian Through Flow (ITF; oceanic channel) however did not show any significant impact on the TRIO sea level variability. Further Ocean General Circulation Model (OGCM) sensitivity experiments are carried out to understand the mechanisms and the possible contribution of the Pacific Ocean through oceanic pathways, in the decadal variability of the TRIO sea level. It is noted that wave propagation from the Pacific Ocean to the Indian Ocean through ITF region has contributed to the sea level variations in the eastern Indian Ocean. But on the decadal time scale, tropical Indian Ocean/TRIO sea level is unaffected by decadal variability in the ITF. Moreover, the ITF contribution to the decadal sea level variability in the Indian Ocean is found to be significant only in the region south of 20°S.