Stress Anomalies Research Articles

Oceanic patterns of thermal stress and coral community degradation on the island of Mauritius

Knowing the responses of high-latitude corals to thermal impacts will be critical to predicting the possibility for range expansion of reefs provoked by climate change. We, therefore, tested how oceanographic and island geography variation and subsequent interactions between chronic and acute environmental stresses would influence the temperate corals of Mauritius (~ 20°S). Specifically, we predicted higher impacts of thermal stress due to rare events on ocean-impacted windward than leeward reefs. To test this prediction, surveys of benthic cover and corals in the shallow lagoon’s perimeter reefs were repeated between 2004 and 2019—an interval with frequent warm thermal anomalies. Hard and soft coral cover declined 40% and 83%, respectively, and erect algae increased 78% over the 15-yr interval. Coral taxa were distributed along a Montipora-community axis dominant on the island’s leeward reefs and an Acropora-axis dominant on the windward reefs. Nine of the 30 originally encountered sub-genera were not observed in the second sampling, of which most losses were on the windward reefs and among taxa that were initially uncommon during the initial 2004 sampling. The largest declines occurred in the southeast where rare acute stress was higher and open-ocean conditions interacted strongly with the island. The north and western corals experienced less acute stress and greater persistence of taxa. Searching an additional 15 sites in 2019 found six of the missing coral taxa, often in deeper reef edges. Screening of potential environmental variables indicated that that skewness of the degree heating weeks and thermal stress anomalies were the strongest predictor of the changes. A chronic stress metric was more difficult to identify but water flow variability and chlorophyll-a concentrations were part of the oceanographic conditions associated with attenuated responses to acute stress. Frequent acute stress was associated with lower thermal acclimation rates over the 15-yr interval and more evident for the dominant than subdominant taxa. The extra-equatorial location of Mauritius will not ensure latitudinal sanctuary, apart from the leeward reefs.

Pacific variability reconciles observed and modelled global mean temperature increase since 1950

Global mean temperature change simulated by climate models deviates from the observed temperature increase during decadal-scale periods in the past. In particular, warming during the ‘global warming hiatus’ in the early twenty-first century appears overestimated in CMIP5 and CMIP6 multi-model means. We examine the role of equatorial Pacific variability in these divergences since 1950 by comparing 18 studies that quantify the Pacific contribution to the ‘hiatus’ and earlier periods and by investigating the reasons for differing results. During the ‘global warming hiatus’ from 1992 to 2012, the estimated contributions differ by a factor of five, with multiple linear regression approaches generally indicating a smaller contribution of Pacific variability to global temperature than climate model experiments where the simulated tropical Pacific sea surface temperature (SST) or wind stress anomalies are nudged towards observations. These so-called pacemaker experiments suggest that the ‘hiatus’ is fully explained and possibly over-explained by Pacific variability. Most of the spread across the studies can be attributed to two factors: neglecting the forced signal in tropical Pacific SST, which is often the case in multiple regression studies but not in pacemaker experiments, underestimates the Pacific contribution to global temperature change by a factor of two during the ‘hiatus’; the sensitivity with which the global temperature responds to Pacific variability varies by a factor of two between models on a decadal time scale, questioning the robustness of single model pacemaker experiments. Once we have accounted for these factors, the CMIP5 mean warming adjusted for Pacific variability reproduces the observed annual global mean temperature closely, with a correlation coefficient of 0.985 from 1950 to 2018. The CMIP6 ensemble performs less favourably but improves if the models with the highest transient climate response are omitted from the ensemble mean.

North–South Discrepancy of Interannual Sea Surface Temperature Anomalies over the South China Sea Associated with Eastern Pacific El Niño Events in the Spring

This paper discovers a spatial feature of interannual sea surface temperature (SST) anomalies over the South China Sea (SCS) in the boreal spring, based on the Simple Ocean Data Assimilation (SODA) monthly data in the period from January 1958 to December 2010. The Empirical Orthogonal Function (EOF) analysis of interannual SST anomalies shows a north–south discrepant pattern of the first mode, which is characterized by higher (lower) anomalies in the northern (southern) SCS and possessing seasonal phase locking (in the boreal spring). Besides, the high correlation coefficient between the time series of the first EOF mode and the Nino 3 SST anomalies during winter reveals that this discrepant pattern is likely caused by El Niño events. The composites of SST anomalies show that this discrepant pattern appears in the eastern Pacific (EP) El Niño events, while it does not exist in the Central Pacific (CP) El Niño events. It is believed that the western North Pacific anticyclone (WNPA) plays a key role in conveying the El Niño impact on the interannual variabilities of SCS SST in the EP El Niño events. The anomalous anticyclone in the Philippine Sea weakens the northeasterly monsoon over the SCS by its southwest portion during the mature phases of the EP El Niño events. This anomalous atmospheric circulation contributes to the north–south discrepant pattern of the wind stress anomalies over the SCS in the EP El Niño mature winters, and then leads to the north–south dipole pattern of the contemporaneous latent heat flux anomalies. The latent heat flux is a major contributor to the surface net heat flux, and heat budget analysis shows that the net heat flux is the major contributor to the SCS SST anomalies during the spring for the EP El Niño events, and the north–south discrepancy of SCS SST anomalies in the succeeding spring is ultimately formed.

Elucidating Large‐Scale Atmospheric Controls on Bering Strait Throughflow Variability Using a Data‐Constrained Ocean Model and Its Adjoint

AbstractA regional data‐constrained coupled ocean‐sea ice general circulation model and its adjoint are used to investigate mechanisms controlling the volume transport variability through Bering Strait during 2002 to 2013. Comprehensive time‐resolved sensitivity maps of Bering Strait transport to atmospheric forcing can be accurately computed with the adjoint along the forward model trajectory to identify spatial and temporal scales most relevant to the strait's transport variability. The simulated Bering Strait transport anomaly is found to be controlled primarily by the wind stress on short time scales of order 1 month. Spatial decomposition indicates that on monthly time scales winds over the Bering and the combined Chukchi and East Siberian Seas are the most significant drivers. Continental shelf waves and coastally trapped waves are suggested as the dominant mechanisms for propagating information from the far field to the strait. In years with transport extrema, eastward wind stress anomalies in the Arctic sector are found to be the dominant control, with correlation coefficient of 0.94. This implies that atmospheric variability over the Arctic plays a substantial role in determining Bering Strait flow variability. The near‐linear response of the transport anomaly to wind stress allows for predictive skill at interannual time scales, thus potentially enabling skillful prediction of changes at this important Pacific‐Arctic gateway, provided that accurate measurements of surface winds in the Arctic can be obtained. The novelty of this work is the use of space and time‐resolved adjoint‐based sensitivity maps, which enable detailed dynamical, that is, causal attribution of the impacts of different forcings.

An extension of LDEO5 model for ENSO ensemble predictions

This study provided an extension to the latest version of Lamont-Doherty Earth Observation (LDEO5) prediction system. First, an ensemble coupled data assimilation (CDA) system, based on the Ensemble Kalman Filter, was established. Both the Kaplan sea surface temperature (SST) data from January 1856 to December 2018 and the ECMWF twentieth century reanalysis (ERA-20C) wind data from January 1900 to February 2010 were assimilated for prediction initialization. Second, an ensemble prediction (EP) system was established using stochastic optimal perturbation that represented the uncertainty in the physical process. The assimilation experiments showed that assimilating multi-source data yielded better results than assimilating single-source data. The analyses of Niño3.4 SST anomalies and zonal wind stress (ZWS) anomalies were in good agreement with the observed counterparts, respectively. The root mean square errors of both Niño3.4 SST anomalies and ZWS anomalies were found to be significantly reduced, compared to the values obtained before assimilation. The modeled upper layer depth anomalies along the equator, and subsurface temperature anomalies in the Niño3.4 region were also found to be similar to the observed counterparts. A long-term ensemble hindcast was conducted using the EP system for the past 163 years, from January 1856 to December 2018. Results showed that the predictions initialized by assimilating multi-source data yielded best deterministic skill, reaching the international advanced level. A comparative analysis revealed that the EP system predicted the warm events well, followed by cold and neutral events.

Effects of Semistochastic Westerly Wind Bursts on ENSO Predictability

AbstractWesterly wind bursts (WWBs) occurring over the tropical Pacific play an important role on El Niño‐Southern Oscillation (ENSO) dynamics. Currently, climate models have significant biases in their representation of WWBs, which may limit their ability to predict ENSO. In this study, we explore the possibility of improving ENSO prediction by introducing a semistochastic WWBs parameterization scheme into the Community Earth System Model (CESM). Three ensemble hindcasts, namely, the control run with the original CESM, the WWB run with parameterized WWBs, and the Non_WWB run with built‐in WWBs removed, are conducted for the period 1982–2016. We find that CESM with parameterized WWBs enables better ENSO prediction, especially for both amplitude and spatial distribution of eastern Pacific and central Pacific El Niño events. This improvement is related to more realistic representation of WWBs which leads to better prediction of surface wind stress anomalies and thermocline depth anomalies.

Wind Stress‐Induced Multiyear Predictability of Annual Extratropical North Atlantic Sea Surface Temperature Anomalies

AbstractLong‐term predictability of the North Atlantic sea surface temperature (SST) is commonly attributed to buoyancy‐forced changes of the Atlantic Meridional Overturning Circulation. Here we investigate the role of surface wind stress forcing in decadal hindcasts as another source of extratropical North Atlantic SST predictability. For this purpose, a global climate model is forced by reanalysis (ERA‐interim) wind stress anomalies over the period 1979–2017. The simulated climate states serve as initial conditions for decadal hindcasts. Significant skill in predicting detrended observed annual SST anomalies is observed over the extratropical central North Atlantic with anomaly correlation coefficients exceeding 0.6 at lead times of 4 to 7 yrs. The skill is insensitive to the calendar month of initialization and primarily linked to upper ocean heat content anomalies that lead anomalous SSTs by several years.

Determination of Global Density Inhomogeneities and Stresses Inside the Moon

The possible distribution of density and stress anomalies in the interior of the Moon is determined. For this purpose, we have developed a new solution of the incorrect inverse gravimetry problem developed by the authors, tested for the Earth and Mars. The idea of this solution is that the isostatic compensation depths are uniquely determined for the harmonics of the expansion of the relief heights in spherical functions based only on cosmic data on the gravitational field and the relief of the planet. We have shown that possible isostatic relief compensation is carried out in the depth range of 0–530 km. The most probable compensation depths in this range were determined and the distribution of compensation masses at these depths was found. For the remaining harmonics, two other compensation options were chosen: (1) compensation is carried out at two levels, the first of which is the mean depth of the crust of 2.7 km, corresponding to a layer of 0–10 km; the possible depths for the second level were determined from an analysis of the results obtained for isostatically compensated harmonics; (2) uncompensated harmonics of the relief lead to vertical stresses in the Moon’s lithosphere reaching the compressive strength (16 MPa) and tensile strength (15 MPa).

Interannual variability in the barrier layer and forcing mechanism in the eastern equatorial Indian Ocean and Bay of Bengal

Interannual variability (IAV) in the barrier layer thickness (BLT) and forcing mechanisms in the eastern equatorial Indian Ocean (EEIO) and Bay of Bengal (BoB) are examined using monthly Argo data sets during 2002–2017. The BLT during November–January (NDJ) in the EEIO shows strong IAV, which is associated with the Indian Ocean dipole mode (IOD), with the IOD leading the BLT by two months. During the negative IOD phase, the westerly wind anomalies driving the downwelling Kelvin waves increase the isothermal layer depth (ILD). Moreover, the variability in the mixed layer depth (MLD) is complex. Affected by the Wyrtki jet, the MLD presents negative anomalies west of 85°E and strong positive anomalies between 85°E and 93°E. Therefore, the BLT shows positive anomalies except between 86°E and 92°E in the EEIO. Additionally, the IAV in the BLT during December–February (DJF) in the BoB is also investigated. In the eastern and northeastern BoB, the IAV in the BLT is remotely forced by equatorial zonal wind stress anomalies associated with the El Nino-Southern Oscillation (ENSO). In the western BoB, the regional surface wind forcing-related ENSO modulates the BLT variations.

Role of Reemergence in the Central North Pacific Revealed by a Mixed Layer Heat Budget Analysis

AbstractWintertime sea surface temperature (SST) anomalies that disappear in summer and recur in the following winter are known as “reemergence.” The role of reemergence in recurrent SST anomalies in an area in the central North Pacific is investigated quantitatively for the first time by conducting an online mixed layer heat budget analysis with a realistic ocean model simulation. In contrast to past studies that have emphasized the importance of vertical entrainment of subsurface temperature anomalies, it is shown that several mechanisms are operating in recurrence of SST anomalies during 1977–1999. In particular, anomalous Ekman meridional advection of the mean meridional temperature gradient induced by zonal wind stress anomalies plays an important role in many years. Also, coincidence in the sign of SST anomalies in winter and surface heat flux anomalies in the following autumn leads to recurrence of SST anomalies.

On the Generation and Salinity Impacts of Intraseasonal Westward Jets in the Equatorial Indian Ocean

AbstractWhile westerly winds dominate the equatorial Indian Ocean and generate the well‐known eastward flowing Wyrtki Jets during boreal spring and fall, there is evidence of a strong westward surface jet during winter that is swifter than eastward currents during that season. A weaker westward jet is found in summer. In this study, we report the occurrence, characteristics, and intraseasonal variability of this westward jet and its impact on mixed layer salinity in the equatorial Indian Ocean using the HYbrid Coordinate Ocean Model (HYCOM) reanalysis with the Navy Coupled Ocean Data Assimilation (NCODA). The westward jet typically occurs in the upper 50 m, above an eastward flowing equatorial undercurrent, with peak westward volume transport of approximately −8 Sv. The westward jet builds up gradually, decays rapidly, and is primarily forced by local intraseasonal wind stress anomalies generated by atmospheric intraseasonal convection. Westward acceleration of the jet occurs when the dominant intraseasonal westward wind anomaly is not balanced by the zonal pressure gradient (ZPG) force. The intraseasonal westward jet generates strong horizontal advection and is the leading cause of mixed layer freshening in the western equatorial Indian Ocean. Without it, a saltier mixed layer would persist and weaken any barrier layers. Existing barrier layers are strengthened following the passage of freshwater‐laden westward jets. Deceleration of the westward jet occurs when the eastward ZPG becomes increasingly important and the westward intraseasonal wind anomalies weaken. A rapid reversal of atmospheric intraseasonal convection‐driven surface winds eventually terminates the westward jet.

Volcano morphology as an indicator of stress orientation in the Java Volcanic Arc, Indonesia

Stress fields in the Earth's crust are important indicators leading to better understanding of geologic processes. One way to acquire stress data is through morphometric analysis of volcanoes. Here we evaluate 141 Quaternary-age monogenetic and polygenetic volcanoes and volcanic fields in Java, Indonesia, using a 30-m SRTM DEM, an 8-m DEMNAS, aerial photos, published geological maps, and Landsat and Google Earth satellite imagery. We analyze parameters that imply the geometry of the shallow magma plumbing system and ascent pathways: crater ellipticity and elongation axis, and the orientation of crater-breaching bisectors. The dominant trend of all of these parameters should indicate the direction of the maximum horizontal compressive stress (σHmax) while the minimum horizontal stress (σHmin) should be perpendicular to it. The goal here is to estimate the principal stress axes at the time of dike emplacement, to recognize any spatial variations in stress orientations, and assess factors controlling the emplacement. The resulting orientation of σHmax in the western part of Java is NE-SW, and the trend gradually rotates to NW-SE moving eastward. Abrupt changes in stress orientations and anomalies are also observed in the vicinity of prominent geologic structures, highlighting the influence of preexisting structures on dike emplacement and eruption characteristics. Results of the morphometric analysis of volcanoes and volcanic fields conducted in this study help to estimate the past and present stress orientations in Java, and provide valuable understanding regarding the relationship between volcanic emplacement and crustal structures, complementing other types of analysis.

Can Tropical Pacific Winds Enhance the Footprint of the Interdecadal Pacific Oscillation on the Upper-Ocean Heat Content in the South China Sea?

AbstractIn this study, an enhanced footprint of the interdecadal Pacific oscillation (IPO) on the upper-ocean heat content (OHC) in the South China Sea (SCS) since the 1990s is revealed. The negative OHC–IPO correlation is significant (r= −0.71) during 1990–2010 [period 2 (P2)], whereas it is statistically insignificant during 1960–80 [period 1 (P1)]. Analyses show that the scope of the equatorial Pacific wind anomalies is wider during P2 compared with that during P1 due to a larger east–west SST gradient and enhanced tropical warming in the Indian Ocean. When the IPO is negative during P2, a wider scope of the wind stress anomalies associated with the IPO could lead to 1) the southward migration of the North Equatorial Current bifurcation latitude (NECBL) by affecting the wind stress curl over the key region where it is near the climatological NECBL and 2) an increase in the interbasin pressure gradient (sea surface height difference) between the western Pacific and the SCS; these two processes strengthen the Kuroshio and weaken the Luzon Strait transport (LST) or SCS throughflow into the SCS. Also, 3) the equatorial Pacific wind anomalies are wide enough to directly weaken the LST in the SCS through the “island rule.” These three pathways finally change the oceanic gyre in the SCS and increase the OHC. Our results suggest that the scope of the tropical wind stress is the crucial factor when we consider the relationship between the upper ocean thermal conditions in the SCS and the Pacific variability.

Phase synchronisation in the Kuroshio Current System

Abstract. The Kuroshio Current System in the North Pacific displays path transitions on a decadal timescale. It is known that both internal variability involving barotropic and baroclinic instabilities and remote Rossby waves induced by North Pacific wind stress anomalies are involved in these path transitions. However, the precise coupling of both processes and its consequences for the dominant decadal transition timescale are still under discussion. Here, we analyse the output of a multi-centennial high-resolution global climate model simulation and study phase synchronisation between Pacific zonal wind stress anomalies and Kuroshio Current System path variability. We apply the Hilbert transform technique to determine the phase and find epochs where such phase synchronisation appears. The physics of this synchronisation are shown to occur through the effect of the vertical motion of isopycnals, as induced by the propagating Rossby waves, on the instabilities of the Kuroshio Current System.

Monitoring and modelling stress state near major geological structures in an underground coal mine for coal burst assessment

Stress states and geotechnical conditions often change significantly near major geological structures (e.g. faults, shear zones, dykes) which is the cause of most major mine instabilities and/or safety hazards including coal burst, roof falls, water inrush, gas outburst, etc. In order to understand and quantify the stress state near major geological structures, an integrated study had been conducted, which included a comprehensive field monitoring program in the vicinity of a dyke in an Australian underground coal mine; detailed analysis of monitoring data to identify the stress anomalies near the dyke; and three-dimensional numerical modelling to investigate the stress distribution and the strain energy concentrations for the purpose of coal burst risk assessment.The field monitoring program included installing microseismic geophones, stressmeters and extensometers in the roadway roofs and coal pillars, aiming to obtain seismic and stress change data during longwall mining. The monitoring results indicate that the stress state was clearly different on the inbye and outbye sides of the dyke. The inbye side had a much higher stress increase than the outbye side during longwall mining. It is apparent that the dyke had caused a stress redistribution in its vicinity, which led to a stress concentration in the “hanging wall” and stress release in the “footwall”.Three-dimensional numerical modelling was conducted with both a panel-scale large model and a roadway-scale small model to investigate the stress distribution and strain energy concentration in the vicinity of the dyke during longwall mining. The modelling results agreed well with the monitoring data, further clarifying the mechanisms of the dyke influenced stress distribution.

The influence of wintertime SST variability in the Western North Pacific on ENSO diversity

Sea surface temperature anomalies (SSTa) in the Western North Pacific (WNP) have been linked to the development of El Niño Southern Oscillation (ENSO) events a full year in advance. However, the contribution of the WNP precursor to the temporal evolution and spatial complexity of ENSO remains unclear. Using the preindustrial experiment of the Community Earth System Model as the control climate, a partially coupled experiment is conducted in which WNP SSTa are restored to the model climatology. By comparing the perturbed experiment to the control, we are able to clearly characterize ENSO’s response to WNP SST variability. We find that the WNP is predominantly linked to eastern Pacific (EP) ENSO events. Without SST variability in the WNP, central Pacific (CP) ENSO events are more likely to develop. This variation in ENSO flavor is controlled by how the WNP projects onto wind stress anomalies in the western equatorial Pacific, which in turn impacts the discharge and recharge of ocean heat during the ENSO cycle. Specifically, the removal of SSTa in the WNP weakens the buildup of ocean heat in the western equatorial Pacific, which then hinders the development of EP-type events.

Interannual Variability of the Global Meridional Overturning Circulation Dominated by Pacific Variability

AbstractThe most prominent feature of the time-mean global meridional overturning circulation (MOC) is the Atlantic MOC (AMOC). However, interannual variability of the global MOC is shown here to be dominated by Pacific MOC (PMOC) variability over the full depth of the ocean at most latitudes. This dominance of interannual PMOC variability is robust across modern climate models and an observational state estimate. PMOC interannual variability has large-scale organization, its most prominent feature being a cross-equatorial cell spanning the tropics. Idealized experiments show that this variability is almost entirely wind driven. Interannual anomalies of zonal mean zonal wind stress produce zonally integrated Ekman transport anomalies that are larger in the Pacific Ocean than in the Atlantic Ocean, simply because the Pacific is wider than the Atlantic at most latitudes. This contrast in Ekman transport variability implies greater variability in the near-surface branch of the PMOC when compared with the near-surface branch of the AMOC. These near-surface variations in turn drive compensating flow anomalies below the Ekman layer. Because the baroclinic adjustment time is longer than a year at most latitudes, these compensating flow anomalies have baroclinic structure spanning the full depth of the ocean. Additional analysis reveals that interannual PMOC variations are the dominant contribution to interannual variations of the global meridional heat transport. There is also evidence of interaction between interannual PMOC variability and El Niño–Southern Oscillation.

Wind-Forced Variability of the Remote Meridional Overturning Circulation

AbstractThe mechanisms by which time-dependent wind stress anomalies at midlatitudes can force variability in the meridional overturning circulation at low latitudes are explored. It is shown that winds are effective at forcing remote variability in the overturning circulation when forcing periods are near the midlatitude baroclinic Rossby wave basin-crossing time. Remote overturning is required by an imbalance in the midlatitude mass storage and release resulting from the dependence of the Rossby wave phase speed on latitude. A heuristic theory is developed that predicts the strength and frequency dependence of the remote overturning well when compared to a two-layer numerical model. The theory indicates that the variable overturning strength, relative to the anomalous Ekman transport, depends primarily on the ratio of the meridional spatial scale of the anomalous wind stress curl to its latitude. For strongly forced systems, a mean deep western boundary current can also significantly enhance the overturning variability at all latitudes. For sufficiently large thermocline displacements, the deep western boundary current alternates between interior and near-boundary pathways in response to fluctuations in the wind, leading to large anomalies in the volume of North Atlantic Deep Water stored at midlatitudes and in the downstream deep western boundary current transport.

Amelioration of Repeated Restraint Stress-Induced Behavioral Deficits and Hippocampal Anomalies with Taurine Treatment in Mice.

Taurine, an essential neutraceutical, has been reported to exhibit antioxidant and anti-inflammatory properties. Substantial evidence indicates that prolonged stress is one of the leading causes of psychological and physiological anomalies. Restraint stress (RS) rat model is the most widely used experimental model for the induction of chronic psycho-emotional stress. In the present study, Swiss albino male mice were restrained for 6h/day for 28 consecutive days. Animals were divided into four groups: control, RS, RS + taurine, and taurine control group. Taurine, a potent antioxidant, was administered (200mg/kg) orally along with RS for 28days. The taurine intervention significantly restored the RS-induced neurobehavioral alterations evident by the elevated plus-maze, Morris water maze test, forced swim test, tail suspension test, and a sucrose preference test. Moreover, taurine significantly prevented hippocampal oxidative stress (lipid peroxidation, reduced glutathione, and nitrite) and other neurochemical (acetylcholinesterase, and IL-1β) anomalies. Using western blotting analyses, we demonstrate that taurine treatment significantly ameliorated the alterations in Brain-derived neurotrophic factor, caspase-3, and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) level in the hippocampus. Thus, Taurine effectively inhibited RS-induced oxidative stress, neuroinflammation, and apoptosis via a mechanism involving the inhibition of the NF-κB signaling pathway. In summary, our study is the first to demonstrate that NF-κB and caspase-3 inhibition, as well as BDNF augmentation, was involved in neuroprotective potential of taurine against RS-induced behavioural anomalies.

The role of the South Pacific in modulating Tropical Pacific variability

Tropical Pacific variability (TPV) heavily influences global climate, but much is still unknown about its drivers. We examine the impact of South Pacific variability on the modes of TPV: the El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO). We conduct idealised coupled experiments in which we suppress temperature and salinity variability at all oceanic levels in the South Pacific. This reduces decadal variability in the equatorial Pacific by ~30% and distorts the spatial pattern of the IPO. There is little change to overall interannual variability, however there is a decrease in the magnitude of the largest 5% of both El Niño and La Niña sea-surface temperature (SST) anomalies. Possible reasons for this include: (i) reduced decadal variability means that interannual SST variability is superposed onto a ‘flatter’ background signal, (ii) suppressing South Pacific variability leads to the alteration of coupled processes linking the South and equatorial Pacific. A small but significant mean state change arising from the imposed suppression may also contribute to the weakened extreme ENSO SST anomalies. The magnitude of both extreme El Niño and La Niña SST anomalies are reduced, and the associated spatial patterns of change of upper ocean heat content and wind stress anomalies are markedly different for both types of events.