Phase Of Interdecadal Pacific Oscillation Research Articles

Unraveling the Role of the Interdecadal Pacific Oscillation in Recent Tropical Expansion via Large‐Ensemble Simulations

AbstractObservational evidence has shown that the Earth's tropics have widened since 1980. However, climate models underestimate the observed tropical expansion rate, with a large spread among individual models. The proposal of internal variability to account for model–observation differences is hindered by the limited availability of sufficient realizations from models in the Coupled Model Intercomparison Project (CMIP), restricting the accuracy of quantitative contribution estimation. The emergence of a single model initial‐condition large ensemble provides a new opportunity to quantify the role of internal variability. Here, using large‐ensemble simulations from two individual models complemented with CMIP Phase 6 (CMIP6) simulations, we show evidence that the recent widening of the tropics is mainly caused by internal variability related to the Interdecadal Pacific Oscillation (IPO). The positive‐to‐negative phase transition of the IPO from 1980 to 2014 reduced the meridional tropospheric temperature gradient, resulting in poleward shifts in tropical edges. After adjusting the IPO trends simulated by individual realizations to ensure consistency with the observations, the IPO phase transition can account for at least 73% (66%) of the observed tropical expansion rate in the Northern Hemisphere based on the metric of the meridional stream function (surface zonal wind). The IPO is also essential for shaping tropical expansion‐related precipitation changes. Our results underscore the significance of considering internal variability when explaining model–observation differences and understanding intermodel uncertainty.

Near-term projection of Amazon rainfall dominated by phase transition of the Interdecadal Pacific Oscillation

The Amazon basin experienced a prolonged drought condition during the 2010s, leading to a large-scale forest degradation destructive to ecosystems and human society. Elusive are issues as to whether the decadal drought is driven by external forcing or internal variability, and whether the drought will continue or recover soon. Using large ensemble simulations from a state-of-the-art climate model, here we find a negative-to-positive phase transition of the Interdecadal Pacific Oscillation (IPO) explains ~45% (~40–49%) of the observed decadal drought of Amazon rainfall since 2010, much greater than the role of external forcing (~12%). Constraining future IPO phase transition reduces the uncertainty by ~38% from a range of −0.73 to + 0.31 mm day−1 decade−1 to a range of −0.42 to + 0.23 mm day−1 decade−1, of the near-term Amazon rainfall projection before 2040 under a mid-intensity emission scenario. Thus, the IPO plays a crucial role in the post-2010 drying and the near-term rainfall projection.

Statistical relationships between the Interdecadal Pacific Oscillation and El Niño–Southern Oscillation

AbstractThe climate of the Pacific Ocean varies on interannual, decadal, and longer timescales. This variability is dominated by the El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO), both of which have profound impacts on countries within and well beyond the Pacific. To date, previous studies have only examined a small subset of the possible links between ENSO, its diversity, and the IPO. Here we focus on the statistical relationship between decadal variability in ENSO properties and the IPO, testing the null hypothesis that the IPO arises from random decadal changes in ENSO activity, including ENSO diversity. We use observed sea surface temperature (SST) records since 1920 to investigate how the timing, structure, frequency, duration, and magnitude of El Niño and La Niña events differ between IPO phases. We find that using the relative frequency of El Niño and La Niña events and either the mean event duration or SST magnitude can reproduce up to 60% of the IPO Tripole Index timeseries. While the spatial SST patterns that represent the IPO and ENSO are similar, the IPO is meridionally broader in the central to eastern Pacific, which may be caused by a lagged relationship with low-frequency SST variability in the equatorial Pacific. In addition, North Pacific SST anomalies of opposite sign to the tropical Pacific SST anomalies is a unique feature of the IPO that cannot be explained by decadal ENSO variability. This suggests a clear IPO and ENSO relationship, but also independence in some of the IPO’s characteristics.

Temporal and Spatial Surface Heat Source Variation in the Gurbantunggut Desert from 1950 to 2021

Based on data from the Gurbantunggut Desert, the largest fixed/semi-fixed desert in China, and ERA5-Land reanalysis data, the long-term variations and spatial surface heat source (SHS) differences in the Gurbantunggut Desert are discussed herein. The results show the following: (1) The hourly SHS at the Kelameili station during the 2013–2021 period was a weak heat source at night; contrastingly, it was a strong heat source during the day. The duration of the hourly SHS increased gradually from January to July, but it decreased gradually from July to December. The daily SHS showed obvious seasonal variation, reaching the maximum in summer and the minimum in winter. The ERA5-Land reanalysis can reproduce all the variation characteristics of the SHS well. (2) The climatology (i.e., multi-year mean) of the monthly SHS intensity was lower than 50 W/m2 during the January–March and September–December periods in the Gurbantunggut Desert, indicating a weak heat source. On the other hand, the climatology recorded in April–August was higher than 50 W/m2, with a strong heat source. From the perspective of spatial distribution, the eastern and western regions of the Gurbantunggut Desert show strong heat sources, while the central region shows weak heat sources. The spatial distribution of the first and second modes of the empirical orthogonal function (EOF) decomposition reflected the consistent spatial variability and a north–south (or east–west) polarity variation of the monthly SHS in the Gurbantunggut Desert, respectively. (3) The yearly SHS showed negative anomalies during the 1950–1954, 1964–1982 and 2004–2015 periods, and positive anomalies during the 1955–1963, 1983–2003 and 2016–2021 periods in the Gurbantunggut Desert. Additionally, the time series of the SHS anomalies was positively correlated with the Interdecadal Pacific Oscillation (IPO) index. During the negative IPO phase, the yearly SHS showed a negative anomaly in the Gurbantunggut Desert, while the yearly SHS showed a positive anomaly during the positive IPO phase in most regions of the Gurbantunggut Desert.

The influence of large-scale climate modes on tropical cyclone tracks in the southwest Pacific

Tropical cyclones (TCs) impact the economy, properties, lives and infrastructure of island nations and territories of the southwest Pacific (SWP), accounting for three in four regional disasters each year. To increase the resilience of the SWP to the destructive impacts of TCs, improved TC track forecasts are needed since a high degree of uncertainty exists around the likely path a TC will take in this region post-formation. This requires better comprehension of the factors contributing to TC track variability occurring at different timescales. Therefore, we examine the modulating impact of key Indo-Pacific climate drivers: the El Niño-Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO), Southern Annular Mode (SAM) and the Indian Ocean Dipole (IOD), on SWP TC track variability. We present new insights into the spatial (i.e. prevailing trajectories) and temporal (i.e. track length, average speed and duration) components of TC tracks, being modulated by both individual and combined climate modes. Overall, TC tracks tend to shift northeast during El Niño, IPO positive, IOD east positive and/or SAM negative phases (with a southwest shift observed during the opposite climate phases). Further, we show that when two of these climate modes are in their positive phase (e.g. El Niño with the positive phases of IPO or eastern pole of IOD and SAM), TC track length and average speed are enhanced. However, for cases where either one (e.g. El Niño/negative phase of IPO and IOD east) or two (La Niña/negative phase of IPO, IOD east and SAM) climate modes were in the negative phase, an increase in TC track duration was observed. The findings of this study may be used to improve TC forecasting and better quantify TC-related risks.

Impact of the Interdecadal Pacific Oscillation on tropical Indian Ocean sea surface height: An assessment from CMIP6 models

AbstractThis study examined the sea level (SL) changes on the interdecadal timescale in the tropical Indian Ocean (TIO) in response to the Interdecadal Pacific Oscillation (IPO) in 20 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The analysis suggests that the interdecadal sea level pattern variation in the Thermocline Ridge of the Indian Ocean (TRIO) area coincides with the IPO phases and is mainly caused by fluctuations of surface forcing over the TIO. Changes in tropical wind stress curl (WSC) mainly induce the SL variability on the interdecadal timescale associated with IPO in the TRIO region in both observations and models. The stochastic wind forcing and its interaction with the ocean's internal variability produce significant interdecadal SL fluctuation in the southeast Indian Ocean. We evaluated the ability of 20 CMIP6 models to capture interdecadal variations in sea level teleconnections. We found that about 75% of models reproduce decadal fluctuations related to IPO pretty well. The interdecadal variations of the SL in the western TIO emerge primarily in response to westward propagating downwelling Rossby waves induced by the WSC in the central‐south TIO in MMM, and models as in observations. Pacific IPO‐related anomalies primarily spread through the oceanic pathways of the Maritime Continent in the eastern Indian Ocean. Further, the IPO induces changes in the western Indian Ocean WSC through atmospheric pathways, which in turn causes Rossby waves to move westward, affecting the southwest Indian SL. Almost all the models and MMM show interdecadal SL variations in TIO associated with IPO. This study is beneficial as interdecadal prediction research on SL has a significant societal need.

Asymmetry of Salinity Variability in the Tropical Pacific during Interdecadal Pacific Oscillation Phases

Asymmetry of Salinity Variability in the Tropical Pacific during Interdecadal Pacific Oscillation Phases

Interdecadal Pacific Oscillation modulation of ENSO teleconnections in its decaying stages: Relations with Indian Ocean basin‐wide mode and South American precipitation

AbstractThis study examines the Interdecadal Pacific Oscillation (IPO) modulation of the El Niño–Southern Oscillation (ENSO) teleconnections in its decaying stages with the tropical ocean by focusing on the Indian Ocean Basin‐Wide (IOBW) mode and the precipitation over South America (SA) in the 1901–2012 period. Composite analyses revealed that the ENSO teleconnections are IPO‐modulated due to the differential ENSO decaying speed, which is slower during the positive than negative IPO phase, for both El Niño (EN) and La Niña (LN) cases. Negative precipitation anomalies related to EN persist over northeastern SA until austral winter for the positive IPO phase (POS IPO), while significant opposite sign anomalies occur in this region for the negative IPO phase (NEG IPO). These results are associated with the Walker circulation's reversal during NEG IPO which is, in turn, accompanied by negative IOBW. During the POS IPO, the positive IOBW causes upward movements over there and, by continuity, downward movements over SA. In the NEG IPO, for LN events the wave train originating in the north of Australia propagates toward subtropical SA, which, coupled with the surface circulation, causes dryness in this region. In addition, the rapid decay of LN in the NEG IPO, followed by the emergence of EN, caused changes in the Walker circulation, such that enhanced upward movements occurred over the Pacific and SA region, and downward movements over the Indian Ocean until austral winter. In turn, the slower decay of the LN in the POS IPO maintains strong subsidence over the central Pacific and weak upward motions over western SA. So, the EN (LN) and positive (negative) IOBW during the POS (NEG) IPO prolong the scarcity of precipitation over equatorial (subtropical) SA. Persistent dry periods over these regions during the ENSO decaying stage might have important implications for the seasonal forecasts.

Agricultural drought over water-scarce Central Asia aggravated by internal climate variability

A severe agricultural drought swept Central Asia in 2021, causing mass die-offs of crops and livestock. The anthropogenic contribution to declines in soil moisture in this region over recent decades has remained unclear. Here we show from analysis of large ensemble simulations that the aggravation of agricultural droughts over southern Central Asia since 1992 can be attributed to both anthropogenic forcing and internal variability associated with the Interdecadal Pacific Oscillation (IPO). Although the negative-to-positive phase transition of IPO before 1992 offset human-induced soil moisture decline, we find that the positive-to-negative phase transition thereafter has doubled the externally forced rate of drying in the early growing season. Human-induced soil moisture loss will probably be further aggravated in the following century due to warming, albeit with increasing precipitation, and our simulations project that this trend will not be counterbalanced by the IPO phase change. Instead, this internal variability could modulate drying rates in the near term with an amplitude of −2 (+2) standard deviation of the IPO trend projected to amplify (weaken) the externally forced decrease in surface soil moisture by nearly 75% (60%). The findings highlight the need for the interplay between anthropogenic forcing and the natural variability of the IPO to be considered by policymakers in this climate-sensitive region.

Role of local and remote forcing on the decadal variability of Mixed Layer Depth in the Bay of Bengal

The Mixed Layer Depth (MLD) of an ocean acts as a storage of the incoming heat and releases via air-sea exchange processes. The present study tries to understand the decadal variability of MLD over the Bay of Bengal (BoB) and its association with local as well as external forcing. The time series and wavelet analysis confirm the significant decadal signals for the period from 1980 to 2015. The BoB MLD is found to have opposing trend prior to and post of 1998. In the decadal timescale, the BoB MLD is found to be comparably deep for regime-1 (1980–1998) and shallow for regime-2 (1999–2014) with stronger variation in the western bay. The local forcing like wind, wind-related processes, and Evaporation minus Precipitation (E-P) are found to have a prominent role to modulate the decadal variability of BoB MLD. The leading mode of the decadal Empirical Orthogonal Function (EOF-1) of MLD over the BoB shows a strong spatial pattern with the strongest in the western part of the Bay. The corresponding principal component shows a similar variability to Interdecadal Pacific Oscillation (IPO) signal. The enhancement of descending (ascending) branch of the Indian branch of Walker circulation (IWC) in the positive (negative) phase of IPO influences the changes over the equatorial Indian Ocean remotely modulate MLD of the coastal region of the BoB via the propagating coastal Kelvin waves. The decadal variability of MLD in the coastal region of the BoB is caused mainly by remote effect but the interior BoB is by the local factors.

Interdecadal wind stress variability over the tropical Pacific causes ENSO diversity in an intermediate coupled model

The role of interdecadal wind stress variability in the genesis of ENSO diversity is examined by using an intermediate coupled model (ICM) in the tropical Pacific; two types of experiments are performed, one with the original ICM, and the other with interdecadal wind stress ( $${\uptau }_{interde}$$ ) effect being explicitly represented. The $${\uptau }_{interde}$$ component is derived from NCEP/NCAR reanalysis dataset as follows. First, the ensemble empirical mode decomposition (EEMD) is used to extract the interdecadal component of wind stress anomalies on about a 10–40 yr timescale. Next, an idealized interdecadal cycle of $${\uptau }_{interde}$$ is reconstructed by a principal oscillation pattern (POP) analysis based on the EEMD-extracted interdecadal wind component. A 110-yr model integration is then performed by explicitly incorporating the reconstructed $${\uptau }_{interde}$$ cycle into the ICM. Compared with the regular interannual oscillation in the original ICM, the simulated ENSO events become highly irregular with interdecadal variations in the amplitude and asymmetry when the $${\uptau }_{interde}$$ effect is included. Especially, the model reproduces two types of El Niño with different spatial distribution and temporal evolution of SST anomalies, namely Eastern-Pacific (EP) and Central-Pacific (CP) types. Further attribution analyses are performed to understand the modulating effects of $${\uptau }_{interde}$$ in the tropical Pacific using the ocean component of the ICM, forced by the added $${\uptau }_{interde}$$ effect. Two different roles of the Interdecadal Pacific Oscillation (IPO) in modulating different types of El Niño are illustrated. On the one hand, the warm phase of IPO favors for the emergence of EP-El Niño events, in association with the initial warming signals occurring in the eastern equatorial Pacific, which are absent in the original ICM. On the other hand, the cold phase of IPO tends to shift the El Niño (i.e., the single type of El Niño in the original ICM) to an CP type, with which the SST anomalies propagate eastward along the equator but cannot extend into the eastern boundary. This simple modeling study highlights the significant contributions of interdecadal wind variability to the genesis of ENSO irregularity and diversity theoretically.

The Controlling Mechanisms of the Recent Global Warming Hiatus: A Focus on the Internal Variabilities

A flattening trend of global surface temperature from 1998 to 2013 is generally referred to global warming hiatus. In this review, its basics and controlling mechanisms are the focuses with the latter receiving more attention. The mechanisms for the hiatus are largely derived from internal climate variabilities which could be divided into sea surface temperature (SST) and energy variabilities. The major SST variabilities are Interdecadal Pacific Oscillation (IPO) and Atlantic Multi-decadal Oscillation (AMO). The negative phase of IPO during the hiatus is associated with strengthened Pacific trade winds that enhance subsurface heat uptake, which is generally thought to be the predominant mechanism for the hiatus. Furthermore, AMO influences the global surface temperature at decadal time scales, or even the IPO, which are considered to be the main drivers of hiatus. As for the energy variabilities, vertical uptake and interbasin redistribution of heat are also suggested to be capable of leading to a hiatus period. There was significant warming of global ocean below 700 m during the hiatus as an evidence for the storage of excessive heat in deep ocean. And various heat redistribution patterns like those through ITF or AMOC could also play a significant role in regulating the hiatus.

The Influence of Interannual and Decadal Indo-Pacific Sea Surface Temperature Variability on Australian Monsoon Rainfall

Abstract Monsoonal rainfall in northern Australia (AUMR) varies substantially on interannual, decadal, and longer time scales, profoundly impacting natural systems and agricultural communities. Some of this variability arises in response to sea surface temperature (SST) variability in the Indo-Pacific linked to both El Niño–Southern Oscillation (ENSO) and the interdecadal Pacific oscillation (IPO). Here we use observations to investigate unresolved issues regarding the influence of the IPO and ENSO on AUMR. Specifically, we show that during negative IPO phases, central Pacific (CP) El Niño events are associated with below-average rainfall over northeast Australia, an anomalous anticyclonic pattern to the northwest of Australia, and eastward moisture advection toward the date line. In contrast, CP La Niña events (distinct from eastern Pacific La Niña events) during negative IPO phases drive significantly wet conditions over much of northern Australia, a strengthened Walker circulation, and large-scale moisture flux convergence. During positive IPO phases, the impact of CP El Niño and CP La Niña events on AUMR is weaker. The influence of central Pacific SSTs on AUMR has been stronger during the recent (post-1999) negative IPO phase. The extent to which this strengthening is associated with climate change or merely natural internal variability is not known.

Interannual Variability of the Indonesian Rainfall and Air–Sea Interaction over the Indo–Pacific Associated with Interdecadal Pacific Oscillation Phases in the Dry Season

Interannual Variability of the Indonesian Rainfall and Air–Sea Interaction over the Indo–Pacific Associated with Interdecadal Pacific Oscillation Phases in the Dry Season

A very likely weakening of Pacific Walker Circulation in constrained near-future projections

The observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979. However, whether the observed change is forced by external forcing or internal variability remains inconclusive, a solid answer to more societal relevantly question of how the PWC will change in the near future is still a challenge. Here we perform a quantitative estimation on the contributions of external forcing and internal variability to the recent observed PWC strengthening using large ensemble simulations from six state-of-the-art Earth system models. We find the phase transition of the Interdecadal Pacific Oscillation (IPO), which is an internal variability mode related to the Pacific, accounts for approximately 63% (~51–72%) of the observed PWC strengthening. Models with sufficient ensemble members can reasonably capture the observed PWC and IPO changes. We further constrain the projection of PWC change by using climate models’ credit in reproducing the historical phase of IPO. The result shows a high probability of a weakened PWC in the near future.

A Surface pCO2 Increasing Hiatus in the Equatorial Pacific Ocean Since 2010

AbstractWe used multiple data sets to investigate the decadal and longer time scale variability of sea surface partial pressure of CO2 (pCO2) over the equatorial Pacific Ocean from 1990 to 2019. Unlike the increasing trend in the global oceanic pCO2, both the Niño 3.4 and warm pool regions featured distinct decadal variabilities. A pCO2 increasing stagnation, a hiatus stage, was identified in the Niño 3.4 region from 2010 to 2019. Further analysis demonstrated that low‐frequency pCO2 was negatively correlated with the Interdecadal Pacific Oscillation (IPO) index. Both the 2009/10 and 2015/16 El Niño events and an increasing phase of IPO contributed to this long pCO2 stagnation. Correspondingly, a weaker upwelling was induced in the central and eastern tropical Pacific as the trade winds weakened and thus stagnated the increase in pCO2 in the Niño 3.4 region. Our results imply a significant impact of climate variabilities on sea surface pCO2 in the equatorial Pacific.

The drivers of extreme rainfall event timing in Australia

AbstractAustralia experiences some of the world's most variable rainfall. Previous studies have mostly focused on understanding rainfall variability in terms of frequency and intensity. However, understanding the timing of when extreme rainfall occurs is crucial for seasonal prediction, although it largely remains unexplored. Here we investigate the timing of extreme rainfall in Australia and the spatial variability of this timing. This study examines how some of the large‐scale drivers, such as the El Niño–Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO), determine the timing and interannual variability of the timing of extreme rainfall in Australia. Our results show that there is a clear spatial north–south delineation in the season when extreme rainfall occurs in Australia, shown by a contour diagonally extending roughly from 21°S in the west of Australia to 33°S in the east. North of this contour, extreme rainfall usually occurs in austral summer, with the smallest interannual variability in the timing of extreme rainfall in this region. In the south, extreme rainfall usually occurs in autumn/winter months; however, the timing is highly variable. In southeast Australia (SEA), extreme rainfall can fall at any time of the year, which makes seasonal prediction extremely challenging for this region. Both observation and reanalysis data show that the area where extreme rainfall occurs in summer extends further south during negative IPO years. We also find that IPO and ENSO phases, and the interaction between them, play significant roles in both determining the timing of extreme rainfall and constraining the interannual variability, especially in SEA. We focus on SEA for further analysis as this region shows the greatest shift in seasonality of extremes in response to large‐scale variability. We conclude that studying the relationship between rainfall and large‐scale drivers is important for verification and improvement of the seasonal prediction of extreme rainfall.

Reconstructing an Interdecadal Pacific Oscillation Index from a Pacific Basin–Wide Collection of Ice Core Records

AbstractUsing an assemblage of four ice cores collected around the Pacific basin, one of the first basinwide histories of Pacific climate variability has been created. This ice core–derived index of the interdecadal Pacific oscillation (IPO) incorporates ice core records from South America, the Himalayas, the Antarctic Peninsula, and northwestern North America. The reconstructed IPO is annually resolved and dates to 1450 CE. The IPO index compares well with observations during the instrumental period and with paleo-proxy assimilated datasets throughout the entire record, which indicates a robust and temporally stationary IPO signal for the last ~550 years. Paleoclimate reconstructions from the tropical Pacific region vary greatly during the Little Ice Age (LIA), although the reconstructed IPO index in this study suggests that the LIA was primarily defined by a weak, negative IPO phase and hence more La Niña–like conditions. Although the mean state of the tropical Pacific Ocean during the LIA remains uncertain, the reconstructed IPO reveals some interesting dynamical relationships with the intertropical convergence zone (ITCZ). In the current warm period, a positive (negative) IPO coincides with an expansion (contraction) of the seasonal latitudinal range of the ITCZ. This relationship is not stationary, however, and is virtually absent throughout the LIA, suggesting that external forcing, such as that from volcanoes and/or reduced solar irradiance, could be driving either the ITCZ shifts or the climate dominating the ice core sites used in the IPO reconstruction.

Decadal Modulation of the ENSO–Indian Ocean Basin Warming Relationship during the Decaying Summer by the Interdecadal Pacific Oscillation

AbstractMany previous studies have shown that an Indian Ocean basin warming (IOBW) occurs usually during El Niño–Southern Oscillation (ENSO) decaying spring to summer seasons through modifying the equatorial zonal circulation. Decadal modulation associated with the interdecadal Pacific oscillation (IPO) is further investigated here to understand the nonstationary ENSO–IOBW relationship during ENSO decaying summer (July–September). During the positive IPO phase, significant warm sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean in El Niño decaying summers and vice versa for La Niña events, while these patterns are not well detected in the negative IPO phase. Different decaying speeds of ENSO associated with the IPO phase, largely controlled by both zonal advective and thermocline feedbacks, are suggested to be mainly responsible for these different ENSO–IOBW relationships. In contrast to ENSO events in the negative IPO phase, the ones in the positive IPO phase display a slower decaying speed and delay their transitions both from a warm to a cold state and a cold to a warm state. The slower decay of El Niño and La Niña thereby helps to sustain the teleconnection forcing over the equatorial Indian Ocean and corresponding SST anomalies there can persist into summer. This IPO modulation of the ENSO–IOBW relationship carries important implications for the seasonal prediction of the Indian Ocean SST anomalies and associated summer climate anomalies.

Interdecadal weakening of the cross-equatorial flows over the Maritime Continent during the boreal summer in the mid-1990s: drivers and physical processes

In this study, the cross-equatorial flows (CEF) on both high and low level (HCEF/LCEF) troposphere over the Maritime Continent (MC) in boreal summer are found to have experienced an interdecadal weakening in the mid-1990s based on both JRA55 and NCEP reanalyses. The outputs of 8 coupled models in CMIP6 are used to investigate drivers and the corresponding mechanisms. Model results show that the role of external forcing is weak in the interdecadal weakening of CEF. By contrast, the observed interdecadal weakening of both HCEF and LCEF can be largely explained by internal variability associated with a negative phase of the interdecadal Pacific Oscillation (IPO). Associated with negative IPO are anomalous divergence (convergence), enhanced precipitation over MC and anomalous cyclonic (anticyclonic) circulations, reduced precipitation over western North Pacific (WNP) in the upper (lower) troposphere. Sensitivity experiments based on MetUM-GA6 further manifest that this IPO phase transition can lead to the interdecadal weakening of CEF, in which the central tropical Pacific (CTP) sea surface temperature (SST) anomalies play a dominant role. The cold SST anomalies in CTP lead to reduced local convection and trigger enhanced convection over MC through changes in the Walker circulation. The enhanced convection over MC leads to a change in local Hadley circulation over the western Pacific sector. This change is characterized by anomalous ascents over MC, southerlies in the upper troposphere, descents and reduced precipitation over WNP and northerlies in the lower troposphere, leading to the weakening of CEF. Meanwhile, positive SST anomalies over MC associated with negative IPO also make a contribution to the weakening of CEF by inducing a change in the Hadley circulation in the western Pacific sector through similar processes.