Pacific Decadal Variability Research Articles

Why Is Decadal Climate Variability Predominantly Observed in the Niño4 Region?

AbstractThis 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.

Accelerated warming of High Mountain Asia predicted at multiple years ahead

High-Mountain Asia (HMA) is an important source of freshwater since it holds the largest reservoir of frozen water outside the polar regions. HMA feeds ten great rivers, ultimately supporting more than 2 billion people. However, the threat of accelerated glacier melt, which is a consequence of unprecedented global warming since the early 1950s, threatens water resources in the surrounding countries. Accurate predictions of the near-term temperature change and synergistic mass loss of glaciers are essential but challenging to implement because of the impacts of internal climate variability. Here, based on large ensembles of state-of-the-art decadal climate prediction experiments, we provide evidence that the internally generated surface air temperature variations in HMA can be predicted multiple years in advance, and the model initialization has robust added value to the decadal prediction skill. Real-time decadal forecasts suggest that the HMA will experience accelerated warming in 2025–2032, where the surface warming will increase by 0.98 °C (0.67 to 1.33 °C; 5 % to 95 % range) relative to the reference period 1991–2020, which is equivalent to 1.75 times the observed warming during 2016–2023. The decadal predictability originates from extratropical Pacific decadal variability modes, which modulate the convective heating in the tropical Pacific and influence HMA via the eastward-propagating atmospheric Kelvin waves. Accelerated warming in the coming decade will likely increase the shrinkage of the glacier volume over the HMA by 1.4 %. This change poses unprecedented challenges, including potential water scarcity, ecosystem disruption, and increased risk of natural disasters, to HMA and surrounding regions.

Evaluation of tropical cyclone genesis frequency in FGOALS-g3 large ensemble: mean state and interannual variability

The tropical cyclone genesis frequency (TCGF) is an essential metric for gauging the performance of climate models. Previous evaluations on CMIP family models usually employ one realization for each model and show their diversities in performance. The single model initial condition large ensemble experiments provide a unique opportunity to quantify how internal variability may affect the model evaluation skill. Here, taking the TCGF in the Western North Pacific (WNP) as an example, we use two genesis potential indices as proxies to evaluate the performance of the FGOALS-g3 large ensemble simulation with 110 members. We show that while internal variability does not have a significant influence on the TCGF mean state evaluation, the TCGF-ENSO (El Niño–Southern Oscillation) relationship is significantly modulated by the decadal scale internal variability. For mean state simulation, the FGOALS-g3 large ensembles show reasonable performance in the simulation of TCGF spatial pattern but have differences compared with ERA5 in magnitude. Physical process analysis indicates that compared with ERA5, nearly all dynamic terms are more unfavorable for tropical cyclogenesis due to the cold sea surface temperature anomalies in the midlatitude, while the thermodynamic terms are more conducive to more TCs. For interannual variability, the ENSO-TCGF connection is significantly modulated by the tropical Pacific decadal variability (TPDV) mode by influencing the vertical wind shear in the WNP. Particularly, the model simulation skill depends on the choice of genesis potential indices. Our finding highlights the importance of considering decadal-scale internal variability in the evaluation of interannual ENSO-TCGF variability.

Evaluating CMIP6 models in simulating the North Pacific decadal variability in sea surface salinity

Evaluating CMIP6 models in simulating the North Pacific decadal variability in sea surface salinity

Formation Mechanisms of the Decadal Indian Ocean Dipole Driven by Remote Forcing from the Tropical Pacific

Abstract On decadal time scales, a zonal SST dipole dominates the tropical Indian Ocean in boreal late summer and fall, called the decadal Indian Ocean dipole (D-IOD). The D-IOD has a spatial pattern different from the traditional interannual IOD, with its eastern pole located off Java, rather than the whole Sumatra–Java coasts as the latter. Here, we show that the D-IOD is generated by both the remote tropical Pacific decadal variability (TPDV) forcing and the decadal modulation of interannual IODs, but with its distinctive spatial pattern and seasonality mainly shaped by the former. In August–September (AS), due to the seasonal strengthening of trade winds, the descending branch of TPDV-induced Walker circulation moves westward into the eastern Indian Ocean relative to June–July, which stimulates equatorial easterly anomalies and oceanic upwelling Kelvin waves, causing subsurface cooling off Java. The subsurface cooling just occurs within the time window of climatological coastal upwelling so that subsurface cold anomalies are brought into the surface by mean upwelling and further transported offshore by mean flows, forming the D-IOD eastern pole. The subsurface cooling is only generated near Java but not Sumatra, because the former is closer to the exit of the Indonesian Throughflow (ITF). Weakened ITF during positive TPDV inhibits the growth of subsurface warming off Java prior to the establishment of AS equatorial easterly anomalies, whereas this ITF effect is not observed off Sumatra. Moreover, warming of the D-IOD western pole might be associated with off-equatorial Rossby waves induced by TPDV-related wind stress curls. Significance Statement The decadal Indian Ocean dipole (D-IOD) is one of the leading decadal modes of the tropical Indian Ocean SST variations. Its formation mechanisms, especially those related to its spatiotemporal characteristics, are not well understood. Based on observations and reanalysis, we show that the tropical Pacific decadal variability (TPDV) mainly accounts for the distinctive spatial pattern and seasonality of the D-IOD via the seasonal migration of the anomalous Walker circulation and ensuing oceanic subsurface dynamics. Our results highlight that the TPDV is an important source of D-IOD’s predictability, and it might be beneficial for operational decadal predictions for the tropical Indian Ocean.

The role of the tropical Atlantic in tropical Pacific climate variability

Interactions between Atlantic and Pacific Oceans can affect tropical Pacific variability and its global impacts at both interannual and decadal timescales. Thus, a deepened understanding of the coupled Atlantic-Pacific interactions is needed. While possible dynamical mechanisms by which the Atlantic can influence the Pacific have been identified, the effectiveness of those mechanisms is difficult to establish using climate model simulations where Atlantic sea surface temperatures (SSTs) are prescribed and Pacific feedbacks cannot be realistically included. As an alternative approach, here we use a Linear Inverse Model (LIM) trained on observations and capable of correctly reproducing the observed statistics, to assess the relative role of the Atlantic-to-Pacific and Pacific-to-Atlantic influences on tropical Pacific variability. Our results indicate that Atlantic internal variability can enhance interannual SST anomalies in the eastern equatorial Pacific, and decadal SST anomalies in the central equatorial Pacific, while Pacific influences on the Atlantic significantly damp tropical Pacific decadal variability. This methodological framework could also be used to assess climate model fidelity in representing tropical basin interactions, helping to reconcile existing differences among models’ results.

The response of Pacific decadal variability to global warming

Pacific Decadal Variability (PDV) plays a pivotal role in understanding climate shifts. This study focuses on investigating the primary components of PDV, namely, the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO). These components are defined as the first and second principal modes of the North Pacific sea surface temperature anomaly field. Our analysis indicates that under the influence of global warming, both PDO and NPGO exhibit a weakening of the amplitude and a shortening of the period. Furthermore, NPGO demonstrates a more pronounced response to a 2C warming scenario compared to a 1.5C increase. This heightened sensitivity is attributed to the accelerated propagation of Rossby waves, a consequence of enhanced ocean stratification. The findings of this research contribute to the establishment of a scientific foundation for informed environmental policy development, facilitating the promotion of environmental conservation and sustainable development.

Tropical decadal variability in nutrient supply and phytoplankton community in the Central Equatorial Pacific during the late Holocene

We have reconstructed baseline δ15N and δ13C of export production at Kingman Reef in the Central Equatorial Pacific (CEP) at sub-decadal resolution, nearly continuously over the last 2000 years. The changes in δ15N reflects the strength of the North Equatorial Counter Current (NECC) relative to the South Equatorial Current (SEC), and to a lesser extent, the North Equatorial Current (NEC). Seasonal to multi-decadal variation in the strength of these currents, through the redistribution of heat, have global climate impacts and influence marine and terrestrial ecosystems. We use modern El Niño-La Nina dynamics and the Tropical Pacific Decadal Variability (TPDV) pattern, which is defined in the CEP, as a framework for analyzing the isotopic data. The CEP δ15N and δ13C records exhibit multi-decadal (50–60 year) variability consistent with TPDV. A large multi-centennial feature in the CEP δ15N data, within age-model uncertainties, is consistent with one of the prolonged dry-pluvial sequences in the American west at the end of the Medieval Climate Anomaly, where low TPDV is correlated with drier conditions. This unique record shows that the strength of the NECC, as reflected in baseline δ15N and δ13C, has at quasi-predictable intervals throughout the late Holocene, toggled the phytoplankton community between prokaryotes and picoplankton versus eukaryotes.

SPEEDY-NEMO: performance and applications of a fully-coupled intermediate-complexity climate model

A fully-coupled general circulation model of intermediate complexity is documented. The study presents an overview of the model climatology and variability, with particular attention to the phenomenology of processes that are relevant for the predictability of the climate system on seasonal-to-decadal time-scales. It is shown that the model can realistically simulate the general circulation of the atmosphere and the ocean, as well as the major modes of climate variability on the examined time-scales: e.g. El Niño-Southern Oscillation, North Atlantic Oscillation, Tropical Atlantic Variability, Pacific Decadal Variability, Atlantic Multi-decadal Variability. Potential applications of the model are discussed, with emphasis on the possibility of generating sets of low-cost large-ensemble retrospective forecasts. We argue that the presented model is suitable to be employed in traditional and innovative model experiments that can play a significant role in future developments of seasonal-to-decadal climate prediction.

Anthropogenic Aerosols Contribute to the Recent Decline in Precipitation Over the U.S. Southwest

AbstractThe winter‐spring precipitation over the Southwestern United States (SWUS) decreased since 1980. It is frequently attributed to Pacific internal decadal variability, but recent studies found anthropogenic aerosols (AA) can also induce a transition to a negative Pacific Decadal Variability (PDV) phase. We revisit the attribution of SWUS drying by quantifying the contributions of anthropogenically forced decadal Pacific Sea Surface Temperatures (SSTs). Applying a low‐frequency component analysis to observations, Community Earth System Model version 2 (CESM2) all‐forcings and single‐forcing large ensembles, we find up to 42% of the observed precipitation trend to be related to the AA‐induced negative PDV‐like pattern, which is driven by the emission shift from the Western to the Eastern Hemisphere. In CESM2, other radiative forcings counteract the influence of AA, but it remains unclear whether the model correctly simulates this balance. This implies that the near‐future trajectories of these forcings, in particular Asian aerosols, are important for projections of SWUS precipitation.

Responses of the Pacific and Atlantic decadal variabilities under global warming by using CMIP6 models

Decadal variability in the ocean is an important indicator of climate system shifts and has considerable influences on marine ecosystems. We investigate the responses of decadal variability over the global ocean regions using nine CMIP6 models (BCC-CSM2-MR, CESM2-WACCM, CMCC-ESM2, EC-Earth3-Veg-LR, FGOAL-f3-L, INM-CM5-0, MIROC6, MPI-ESM1-2-LR, and NorESM2-MM). Our results show that climate models can capture the Pacific Decadal Oscillation, Tropical Pacific Decadal Variability, South Pacific Decadal Oscillation, and Atlantic Multidecadal Variability under present-day conditions. The ocean decadal variabilities are becoming weaker and their periods are decreasing, especially under the strong global warming scenario. However, there is a discrepancy between the Tropical Pacific Decadal Variability and the other three modes of climate variability. This might be caused by the nearly unchanged atmospheric forcing in the equatorial region, which is decreasing in the higher latitude regions.

Natural and Anthropogenic Climate Variability Signals in a 237‐Year‐Long Coral Record From the Philippines

AbstractBoth proxy and model studies conducted to understand anthropogenic warming have revealed historical variations in sea‐surface temperature (SST) since the industrial revolution. However, because of discrepancies between observations and models in the late nineteenth century, the timing and degree of anthropogenic warming remain unclear. In this study, we reconstructed a 237‐year‐long record of SST and salinity using a coral core collected from Bicol, southern Luzon, Philippines, which is located at the northern edge of the western Pacific warm pool. The SST record showed volcanic cooling after several volcanic eruptions, including the 1815 Tambora eruption, but the pattern of change differed. Decadal SST variations at Bicol are connected to Pacific Decadal Variability (PDV). Therefore, it is suggested that the PDV conditions at the time of the eruption may have influenced marine conditions, such as the degree and duration of cooling and/or salinity, after the eruptions. Although there were discrepancies in SST variations among the modeled, observed, and proxy SST data from the late nineteenth to early twentieth centuries, SST data from the late twentieth century showed globally coherent anthropogenic warming, especially after 1976. In particular, summer SST in the northwestern Pacific has become more sensitive to anthropogenic forcing since 1976.

Coupled Feedbacks From the Tropical Pacific to the Atlantic Meridional Overturning Circulation

AbstractThe tropical Pacific Ocean is a key regulator of Earth's climate, with teleconnections that influence remote locations all around the world. Here we use partially coupled climate model experiments to show that tropical Pacific cooling related to an abrupt Atlantic Meridional Overturning Circulation (AMOC) slowdown can strengthen the AMOC by ∼25%. This tropical‐extratropical teleconnection occurs initially via atmospheric Rossby waves propagating from the tropical Pacific to the North Atlantic which alter surface climate conditions locally. These changes facilitate ocean heat loss from the subpolar gyre, favoring enhanced oceanic convection. The AMOC strengthening is subsequently enhanced by anomalous northward salt advection in the Atlantic, with a potential contribution from oceanic wave adjustment triggered by increased Southern Ocean westerly winds. These results highlight the influence of the tropical Pacific on the AMOC on multidecadal timescales and suggest that cold phases of tropical Pacific decadal variability could drive temporary strengthening of the AMOC.

Mechanisms of tropical Pacific decadal variability

Mechanisms of tropical Pacific decadal variability

A Pacific Tropical Decadal Variability Challenge for Climate Models

AbstractUnderstanding 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.

Exploring the Symmetries of Pantropical Connections between the Tropical Atlantic and Pacific Basins

Abstract Recent analysis of pantropical interactions suggests that after 1980 the tropical Atlantic Ocean’s (TAO) influence on the tropical Pacific Ocean (TPO) appears to have become much more pronounced while the tropical Indian Ocean’s (TIO) influence appears to have weakened. This study explores whether and how decadal changes in TAO and TPO SSTs modulate these pantropical connections in an attempt to explain the recent dominance of the TAO. To this end, we carry out a series of idealized atmosphere-only experiments using the ACCESS atmospheric general circulation model where the magnitude and sign of the decadal TAO SST signal are varied, presenting various warm and cool Atlantic scenarios. To understand further if these pantropical connections are influenced by changes in TPO SST, we carry out the above TAO experiments with both warm and cool phases of Pacific decadal variability (PDV). We find that an imposed TAO warming leads to increases in TPO atmospheric temperature and stability, which lead to a decrease in average TPO precipitation, with the most prominent changes occurring in June–August. These changes in TPO precipitation induced by TAO warming are largely mirrored when TAO cooling is added, whereas the TPO rainfall response to TAO anomalies remains relatively unchanged for the different phases of PDV. In contrast to the precipitation response, the wind response did display some asymmetries between different phases of TAO SST variability. Specifically, surface winds in the western half of the Niño-4 region exhibited a significantly different response to positive versus negative Atlantic multidecadal variability (AMV), whereas the surface winds in the western equatorial Pacific were significantly stronger (roughly 40% larger) in the positive phase of PDV than in the negative phase. These results suggest that the phases of PDV and AMV may modulate pantropical interactions through their effect on zonal wind stress.

Freshwater Contributions to Decadal Variability of the Indonesian Throughflow

AbstractThe Makassar Strait, the main passageway of the Indonesian Throughflow (ITF), is an important component of Indo‐Pacific climate through its inter‐basin redistribution of heat and freshwater. Observational studies suggest that wind‐driven freshwater advection from the marginal seas into the Makassar Strait modulates the strait's surface transport. However, direct observations are too short (<15 years) to resolve variability on decadal timescales. Here we use a series of global ocean simulations to assess the advected freshwater contributions to ITF transport across a range of timescales. The simulated seasonal and interannual freshwater dynamics are consistent with previous studies. On decadal timescales, we find that wind‐driven advection of South China Sea (SCS) waters into the Makassar Strait modulates upper‐ocean ITF transport. Atmospheric circulation changes associated with Pacific decadal variability appear to drive this mechanism via Pacific lower‐latitude western boundary current interactions that affect the SCS circulation.

Dominant Forcing Regions of Decadal Variations in the Kuroshio Extension Revealed by a Linear Rossby Wave Model

AbstractThe Kuroshio Extension (KE) is known to undergo large decadal variations, especially after the 1976/77 climate regime shift. Due to strong ocean‐atmosphere interactions over the KE, the region may be a key region for basin‐scale Pacific decadal climate variability. Although baroclinic Rossby waves are suggested to influence the KE decadal variability, it remains unclear where and how decadal atmospheric forcing anomalies that induce the Rossby waves are generated. Using a linear Rossby wave model forced by reanalysis surface wind stress, the relative contribution of wind stress at each longitude is quantified for the first time. It is shown that the contribution is largest at 160°W, and 40% of the total variance in the decadal KE variability is explained by the wind forcing around 170°–150°W. Further analyses suggest that the atmospheric forcing anomalies are generated by the El Niño/Southern Oscillation Modoki through an atmospheric teleconnection after the regime shift.

The Key Role of Cloud–Climate Coupling in Extratropical Sea Surface Temperature Variability

Abstract Cloud radiative effects have long been known to play a key role in governing the mean climate. In recent years, it has become clear that they also contribute to climate variability in the tropics. Here we build on recent work and probe the role of cloud radiative effects in extratropical sea surface temperature (SST) variability. The impact of cloud radiative effects on climate variability is explored in “cloud-locking” simulations run on an Earth System Model. The method involves comparing the output from two climate simulations: one in which clouds are coupled to atmospheric dynamic and thermodynamic processes, and another in which clouds are prescribed and thus decoupled from them. The results reveal that cloud–climate coupling leads to widespread increases in the amplitudes of extratropical SST variability from monthly to decadal time scales. Notably, it leads to ∼40%–100% increases in the amplitude of monthly to decadal variability over both the North Atlantic and North Pacific Oceans. These increases are consistent with the “reddening” of cloud shortwave radiative effects that arises when clouds respond to the dynamic and thermodynamic state of the atmosphere. The results suggest that a notable fraction of observed Northern Hemisphere SST variability—including that associated with North Pacific and North Atlantic decadal variability—is due to cloud–climate coupling.

Volcanic forcing degrades multiyear-to-decadal prediction skill in the tropical Pacific.

Volcanic aerosol forcing can affect global climate, but its role in climate prediction remains poorly understood. We isolate the impact of volcanic eruptions on multiyear-to-decadal climate prediction skill by comparing two suites of initialized decadal hindcasts conducted with and without historical volcanic forcing. Unexpectedly, the inclusion of volcanic forcing in the prediction system significantly degrades the forecast skill of detrended multiyear-to-decadal sea surface temperature (SST) variability in the central-eastern tropical Pacific. The ensemble mean hindcasts produce multiyear-to-decadal tropical Pacific SST cooling in response to large tropical volcanic eruptions through thermodynamic and El Niño-Southern Oscillation (ENSO)-like dynamic processes. However, in observations, these eruptions coincided with tropical Pacific warming, which is well predicted by the no-volcano hindcasts and, hence, is likely related to internal climate variability. Improved model representation of volcanic response and its interaction with internal climate variability is required to advance prediction of tropical Pacific decadal variability and associated global impacts.