Tropical Pacific Decadal Oscillation in Subsurface Ocean Temperature

This study utilizes a new monthly-assimilated sea temperature and analyzes trend and decadal oscillations in tropical Pacific 100-200 m subsurface ocean temperature (SOT) from 1945 to 2005 on the basis of the harmonic analysis and Empirical Orthogonal Function (EOF) methods. Significant cooling trends in the SOT in the tropical western Pacific were found over this 60-year period. The first EOF of the SOT in tropical Pacific displays an ENSO-like zonal dipole pattern on decadal time scale, and we considered this pattern in subsurface ocean temperature the tropical Pacific decadal oscillation (TPDO). Our analysis suggests that TPDO is closely correlated with the Pacific decadal oscillation (PDO) in the surface sea temperature (SST). The correlation coefficient between the indices of TPDO and PDO is +0.81 and reaches a maximum of +0.84 when TPDO lags behind PDO by 2 months. Therefore, a change of TPDO is likely related to the variation of PDO. The long-term change in TPDO best explains decadal warming in the tropical eastern Pacific SST and implies potential impact on the weakening of East Asian summer monsoons after the late 1970s.     

North African climate variability. Part 1: Tropical thermocline coupling

Summary Tropical ocean thermocline variability is studied using gridded data assimilated by an ocean model in the period 1950–2000. The dominant patterns and variability are identified using EOF analysis applied to E–W depth slices of sea temperatures averaged over the tropics. After removing the annual cycle, an east–west ‘see-saw’ with an interannual to decadal rhythm is the leading mode in each of the tropical basins. In the case of the leading mode in the Pacific, the thermocline oscillation forms a dipole structure, but in the (east) Atlantic and (southwest) Indian Ocean there is a single center of action. The interaction of the ocean thermocline and atmospheric Walker circulations is studied through cross-modulus analysis of wavelet-filtered EOF time scores. Our study demonstrates how tropical ocean thermocline variability contributes to zonal circulation anomalies in the atmosphere. The equatorial Pacific thermocline oscillation explains 62 and 53% of the variability of the Pacific and Atlantic zonal overturning circulations, the latter driving convective polarity between North Africa and South America. The Pacific sea-saw leads the Atlantic zonal circulation by a few months.     

Meridional oscillations in an idealized ocean-atmosphere system,Part I: uncoupled modes

Meridional, linear, and free modes of global, primitive-equation, ocean-atmosphere models were analyzed to see if they contain multi-year, especially decadal ( 10–30 years), oscillation time scale modes. A two-layer model of the global ocean and a two-level model of the global atmosphere were formulated. Both models were linearized around axially-symmetric basic states containing mean meridional circulations. The linearized perturbation system was solved as an eigenvalue problem. The operator matrix was discretized in the north-south direction with centered finite differences. Uncoupled, meridional modes of oscillation of the ocean and the atmosphere models were calculated. Calculations were performed at three grid spacings (5°, 2.5° and 1.25°) and for two types of basic states (symmetric and asymmetric). Uncoupled, free oceanic modes in the presence of mean meridional circulations have oscillation time scales ranging from two years to several centuries. Such low frequency meridional modes do not exist in the ocean model if there are no mean meridional circulations. A large number of oceanic modes are grouped around decadal oscillation time scales. All the oceanic modes have neutral growth rates. The spatial structures of some of the oceanic modes are comparable to observed spatial structures of sea surface temperature variations in the Pacific Ocean. Most years to decades variability of meridional modes of the ocean model is contained in tropical and midlatitude modes. Some oceanic modes with years to decades periods have standing oscillations in the tropics and poleward propagation of zonal velocity and layer thickness outside the tropics. Uncoupled, free atmospheric modes in the presence of mean meridional circulations have oscillation time scales ranging from a week to several decades. Such low-frequency meridional modes do not exist in the atmospere model if there are no mean meridional circulations. A large number of modes are grouped around intraseasonal time scales. Unlike the oceanic modes, the atmospheric modes are weakly unstable. Most of the intraseasonal variability of atmospheric modes is contained in tropical, midlatitude, and polar modes. Atmospheric modes with oscillation periods longer than about one year have global extent. Meridional ocean-atmospheric modes exist in the models wherever there are mean meridional circulations, i.e., tropical, midlatitude, polar, and global. Oceanic and atmospheric eigenvectors have symmetric (assymetric) latitudinal structures if their basic states are symmetric (asymmetric) around the equator. For both models, models calculated at coarser than 2.5° grid spacing do not accurately represent low-frequency variability. Scale analysis shows taht advection by tge basic state meridional velocities is the primary cause of the meridional oscillations on time scales longer than two years in the ocean model and longer than a few weeks in the atmosphere model. Meridional modes of the coupled ocean-atmosphere models are the subject of a subsequent paper.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

Dynamics of decadal variability in the Atlantic subpolar gyre: a stochastically forced oscillator

Internal variability of the Atlantic subpolar gyre is investigated in a 600 years control simulation of a comprehensive coupled climate model. The subpolar gyre shows irregular oscillations of decadal time scale with most spectral power between 15 and 20 years. Positive and negative feedback mechanisms act successively on the circulation leading to an internal oscillation. This involves periodically enhanced deep convection in the subpolar gyre center and intermittently enhanced air-sea thermal coupling. As a result, anomalies of the large-scale atmospheric circulation can be transferred to the ocean on the ocean’s intrinsic time scale, exciting the oscillator stochastically. A detailed understanding of oscillatory mechanisms of the ocean and their sensitivity to atmospheric forcing holds considerable potential for decadal predictions as well as for the interpretation of proxy data records.     

基于热含量探究PDO的三维结构及信号传播特征

基于1815—2013年SODA（Simple Ocean Data Assimilation）数据不同深度的温度数据资料,进行了热含量的计算,并通过EOF分解、功率谱分析等统计方法探究太平洋年代际振荡（Pacific decadal oscillation, PDO）的三维结构和周期性特征。结果表明,太平洋的年代际变化不仅仅存在于海洋表层,海洋300 m以浅均存在年代际变化特征,其中次表层（70 m左右）的年代际变化特征最为显著。功率谱分析的结果显示,北太平洋的年代际变化周期约为18 a。利用SODA数据的温度和盐度资料对北太平洋的Rossby（罗斯贝）波波速进行了计算,计算结果显示,Rossby波向西传播,其波速随着纬度的增大而减小。对300 m以浅水体的热含量时间序列与PDO指数做了超前滞后相关,在热含量序列滞后9 a时相关系数分布与同期相关反相。对不同层次的热含量与PDO指数进行了超前滞后相关,分析PDO的演变特征,结果表明,PDO在低纬度通过Rossby波向西传播,在传播过程中深度逐渐加深。     

东亚夏季风和中国东部夏季降水年代际变化的模拟

利用中国科学院大气物理研究所发展的第四代大气环流模式模拟了1970年代末东亚夏季风和相关的中国东部夏季降水年代际变化。结果表明,在给定的观测海温强迫下,模式能模拟出东亚夏季风的年代际减弱及 相关的环流场变化,包括东亚沿海的偏北风异常以及西太平洋副高的形态变化,模式还较好再现了中国东部夏季降水的雨型变化,即长江流域降水偏多,而华北和华南偏少,但位置略偏南。基于奇异值分解(SVD)的分析表明,热带海洋变暖是这次东亚夏季风的年代际减弱的主要因素,这与太平洋年代际振荡(PDO)在1970年代末期的位相转变有关。此外,模式还较好模拟了长江流域的变冷趋势,进而减弱了海陆温差,使东亚夏季风减弱。     

Simulated decadal oscillations of the Atlantic meridional overturning circulation in a cold climate state

The significance of the Atlantic meridional overturning circulation (MOC) for regional and hemispheric climate change requires a complete understanding using fully coupled climate models. Here we present a persistent, decadal oscillation in a coupled atmosphere–ocean general circulation model. While the present study is limited by the lack of comparisons with paleo-proxy records, the purpose is to reveal a new theoretically interesting solution found in the fully-coupled climate model. The model exhibits two multi-century-long stable states with one dominated by decadal MOC oscillations. The oscillations involve an interaction between anomalous advective transport of salt and surface density in the North Atlantic subpolar gyre. Their time scale is fundamentally determined by the advection. In addition, there is a link between the MOC oscillations and North Atlantic Oscillation (NAO)-like sea level pressure anomalies. The analysis suggests an interaction between the NAO and an anomalous subpolar gyre circulation in which sea ice near and south of the Labrador Sea plays an important role in generating a large local thermal anomaly and a meridional temperature gradient. The latter induces a positive feedback via synoptic eddy activity in the atmosphere. In addition, the oscillation only appears when the Nordic Sea is completely covered by sea ice in winter, and deep convection is active only near the Irminger Sea. Such conditions are provided by a substantially colder North Atlantic climate than today.     

Evidence for the role of the Atlantic multidecadal oscillation and the ocean heat uptake in hiatus prediction

The recent hiatus in global temperature at the surface has been analysed by several studies, mainly using global climate models. The common accepted picture is that since the late 1990s, the increase in anthropogenic radiative forcings has been counterbalanced by other factors, e.g., a decrease in natural forcings, augmented ocean heat storage and negative phases of ocean–atmosphere-coupled oscillation patterns. Here, simple vector autoregressive models are used for forecasting the temperature hiatus in the period 2001–2014. This gives new insight into the problem of understanding the ocean contribution (in terms of heat uptake and atmosphere–ocean-coupled oscillations) to the appearance of this recent hiatus. In particular, considering data about the ocean heat content until a depth of 700 m and the Atlantic multidecadal oscillation is necessary for correctly forecasting the hiatus, so catching both trend and interannual variability. Our models also show that the ocean heat uptake is substantially driven by the natural component of the total radiative forcing at a decadal time scale, confining the importance of the anthropogenic influences to a longer range warming of the ocean.     

太平洋年代际振荡研究进展(英)

近10年来,太平洋年代际振荡(PDO)因其对全球气候系统的深远影响而得到广泛的研究。PDO指的足在太平洋的气候变率中具有类似ENSO空间结构但周期为10-30年的一种振荡,当北太平洋中部海面温度异常增暖(冷却)时,热带太平洋中部和东部以及北美沿岸常同时伴随有同等幅度的异常冷却(增暖)。总体而言,有两类观点分别认为PDO起源于确定的海气耦合过程或起源于大气的随机强迫。确定性起源论强调,一个海气耦合系统内部的物理过程可以提供一个正反馈机制以增强一初始扰动,及一个负反馈机制以促使振荡位相发生逆转;海洋环流的动力演变过程决定了振荡的时间尺度。随机性起源论则强调,因为大气活动没有一个特定的时间尺度,其时间尺度谱实际上对应于白噪音谱,所以大气对海洋的强迫是随机的;而海洋常在低频谱段有最大的响应振幅,其对应的周期约为十几年或几十年。作者试图系统性地理解PDO在观测、理论和数值方面的研究现状,从而为当前研究提供一个有用的背景性参考。     

Modulation of decadal ENSO-like variation by effective solar radiation

Prediction of the Pacific sea surface temperature (SST) anomaly in the coming decades is a challenge as the SST anomaly changes over time due to natural and anthropogenic climate forcing. The climate changes in the mid-1970s and late-1990s were related to the decadal Pacific SST variability. The changes in the mid-1970s were associated with the positive phase of decadal El Niño-Southern Oscillation (ENSO)-like SST variation, and the changes in the late-1990s were related to its negative phase. However, it is not clear whether this decadal SST variability is related to any external forcing. Here, we show that the effective solar radiation (ESR), which includes the net solar radiation and the effects of volcanic eruption, has modulated this decadal ENSO-like oscillation. The eastern Pacific warming (cooling) associated with this decadal ENSO-like oscillation over the past 139 years is significantly related to weak (strong) ESR. The weak ESR with strong volcanic eruption is found to strengthen the El Niño, resulting in an El Niño-like SST anomaly on the decadal time scale. The strong eruptions of the El Chicho’n (1982) and Pinatubo (1991) volcanoes reduced the ESR during the 1980s and 1990s, respectively. The radiation reduction weakened the Walker circulation due to the “ocean thermostat” mechanism that generates eastern Pacific warming associated with a decadal El Niño-like SST anomaly. This mechanism has been confirmed by the millennium run of ECHO-G model, in which the positive eastward gradient of SST over the equatorial Pacific was simulated under the weak ESR forcing on the decadal time scale. We now experience a reversal of the trend in the ESR. The strong solar radiation and lack of strong volcanic eruptions over the past 15 years have resulted in strong ESR, which should enhance the Walker circulation, leading to a La Niña-like SST anomaly.     

Variability of the Atlantic thermohaline circulation described by three-dimensional empirical orthogonal functions

We describe the use of bivariate three-dimensional empirical orthogonal functions (EOFs) in characterising low frequency variability of the Atlantic thermohaline circulation (THC) in the Hadley Centre global climate model, HadCM3. We find that the leading two modes are well correlated with an index of the meridional overturning circulation (MOC) on decadal timescales, with the leading mode alone accounting for 54% of the decadal variance. Episodes of coherent oscillations in the sub-space of the leading EOFs are identified; these episodes are of great interest for the predictability of the THC, and could indicate the existence of different regimes of natural variability. The mechanism identified for the multi-decadal variability is an internal ocean mode, dominated by changes in convection in the Nordic Seas, which lead the changes in the MOC by a few years. Variations in salinity transports from the Arctic and from the North Atlantic are the main feedbacks which control the oscillation. This mode has a weak feedback onto the atmosphere and hence a surface climatic influence. Interestingly, some of these climate impacts lead the changes in the overturning. There are also similarities to observed multi-decadal climate variability.     

广西前汛期降水年代际变化与南半球印度洋海温的关系

利用NCEP/NCAR月平均再分析资料对广西前汛期降水年代际变化的环流差异及其与前期南半球印度洋海温的关系进行研究,结果表明:广西前汛期整体一致变化的降水分布型具有20年左右年代际振荡及3年左右的年际周期,桂南、桂北反相变化的降水空间型具有6年和准两年振荡.在前汛期降水偏多期,欧亚大陆地表温度偏高,热力作用增强,造成大陆热低压偏强,海陆差异加大,广西区域气柱不稳定性增强,上升气流显著增强,Hadley环流减弱,西太平洋副高及南亚高压减弱,南北半球越赤道气流增强,高原南侧南支槽气流加强,水汽输送增多,造成广西降水偏多;降水偏少期形势相反.相关分析表明前期2～3月南半球中纬度印度洋海温与广西前汛期降水年代际变化呈明显负相关,意味着南半球海温对广西前汛期降水年代际变化有调控作用,这种作用是通过海温异常影响越赤道气流从而影响亚洲季风的强弱而实现的.     

Mechanisms of the atmospheric response to North Atlantic multidecadal variability: a model study

The atmospheric circulation response to decadal fluctuations of the Atlantic meridional overturning circulation (MOC) in the IPSL climate model is investigated using the associated sea surface temperature signature. A SST anomaly is prescribed in sensitivity experiments with the atmospheric component of the IPSL model coupled to a slab ocean. The prescribed SST anomaly in the North Atlantic is the surface signature of the MOC influence on the atmosphere detected in the coupled simulation. It follows a maximum of the MOC by a few years and resembles the model Atlantic multidecadal oscillation. It is mainly characterized by a warming of the North Atlantic south of Iceland, and a cooling of the Nordic Seas. There are substantial seasonal variations in the geopotential height response to the prescribed SST anomaly, with an East Atlantic Pattern-like response in summer and a North Atlantic oscillation-like signal in winter. In summer, the response of the atmosphere is global in scale, resembling the climatic impact detected in the coupled simulation, albeit with a weaker amplitude. The zonally asymmetric or eddy part of the response is characterized by a trough over warm SST associated with changes in the stationary waves. A diagnostic analysis with daily data emphasizes the role of transient-eddy forcing in shaping and maintaining the equilibrium response. We show that in response to an intensified MOC, the North Atlantic storm tracks are enhanced and shifted northward during summer, consistent with a strengthening of the westerlies. However the anomalous response is weak, which suggests a statistically significant but rather modest influence of the extratropical SST on the atmosphere. The winter response to the MOC-induced North Atlantic warming is an intensification of the subtropical jet and a southward shift of the Atlantic storm track activity, resulting in an equatorward shift of the polar jet. Although the SST anomaly is only prescribed in the Atlantic ocean, significant impacts are found globally, indicating that the Atlantic ocean can drive a large scale atmospheric variability at decadal timescales. The atmospheric response is highly non-linear in both seasons and is consistent with the strong interaction between transient eddies and the mean flow. This study emphasizes that decadal fluctuations of the MOC can affect the storm tracks in both seasons and lead to weak but significant dynamical changes in the atmosphere.     

An overview of decadal climate predictability in a multi-model ensemble by climate model MIROC

Decadal climate predictability is examined in hindcast experiments by a multi-model ensemble using three versions of the coupled atmosphere-ocean model MIROC. In these hindcast experiments, initial conditions are obtained from an anomaly assimilation procedure using the observed oceanic temperature and salinity with prescribed natural and anthropogenic forcings on the basis of the historical data and future emission scenarios in the Intergovernmental Panel of Climate Change. Results of the multi-model ensemble in our hindcast experiments show that predictability of surface air temperature (SAT) anomalies on decadal timescales mostly originates from externally forced variability. Although the predictable component of internally generated variability has considerably smaller SAT variance than that of externally forced variability, ocean subsurface temperature variability has predictive skills over almost a decade, particularly in the North Pacific and the North Atlantic where dominant signals associated with Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are observed. Initialization enhances the predictive skills of AMO and PDO indices and slightly improves those of global mean temperature anomalies. Improvement of these predictive skills in the multi-model ensemble is higher than that in a single-model ensemble.     

THE DECADAL CLIMATE VARIABILITY AND THE ANOMALOUS ENSO DEVELOPMENTS IN 1990S*

By using the wavelet transform method,the ENSO (2-7 a) signal and the decadal variability (8-20 a) are filtered out from the long-term SST data sets in order to investigate characteristics of the decadal variability and its impact on the ENSO.It is found that there are two different kinds of decadal SSTA modes-horseshoe and horse saddle patterns in the tropical Pacific.The horseshoe pattern represents that the decadal SSTA variability in the central Pacific is in phase with that in the eastern Pacific.The horse saddle pattern is named that they are out of phase.The former constituted the decadal variability before 1990s and the latter mainly prevailed during 1990s.As the response of atmosphere to the ocean,two decadal wind patterns appear in association with the SST decadal modes.One is characterized by anomalous development of the zonal wind,the other by anomalous development of the meridional wind.These two kinds of modes can also be regarded as different phases of the decadal oscillation.Further studies have shown that the influences of the two kinds of modes on the ENSO are different.The horse saddle mode has a stronger impact on the ENSO than the horseshoe mode.A possible mechanism for the influence of the decadal variability on the ENSO signal is presented.The central part of the thermocline along the equatorial Pacific moves up or down simultaneously with its eastern part while the decadal variability bears the horseshoe pattern.But the two segments of the thermocline in the central and eastern Pacific act oppositely while the decadal variability shows the horse saddle pattern.In this case it has an-influence on the individual ENSO'events by the superposition of the decadal variability.     

The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions

In this study, the effects of volcanic forcing on North Pacific climate variability, on interannual to decadal time scales, are examined using climate model simulations covering the last 600?years. The model used is the Bergen Climate Model, a fully coupled atmosphere–ocean general circulation model. It is found that natural external forcings, such as tropical strong volcanic eruptions (SVEs) and variations in total solar irradiance, play an important role in regulating North Pacific Decadal Variability (NPDV). In response to tropical SVEs the lower stratospheric pole–to–equator temperature gradient is enhanced. The North polar vortex is strengthened, which forces a significant positive Arctic Oscillation. At the same time, dipole zonal wind anomalies associated with strong polar vortex propagate downward from the lower stratosphere. Through positive feedbacks in the troposphere, the surface westerly winds across the central North Pacific are significantly weakened, and positive sea level pressure anomalies are formed in the North Pacific. This anomalous surface circulation results in changes in the net heat fluxes and the oceanic advection across the North Pacific. As a result of this, warm water converges in the subtropical western North Pacific, where the surface waters in addition are heated by significantly reduced latent and sensible heat fluxes from the ocean. In the eastern and high–latitude North Pacific the ocean loses more heat, and large–scale decreases in sea surface temperatures are found. The overall response of this chain of events is that the North Pacific enters a negative phase of the Pacific decadal oscillation (PDO), and this negative phase of the PDO is maintained for several years. It is thus concluded that the volcanic forcing plays a key role in the phasing of the PDO. The model results furthermore highlight the important role of troposphere–stratosphere coupling, tropical–extratropical teleconnections and extratropical ocean–atmosphere interactions for describing NPDV.     

NAO–ocean circulation interactions in a coupled general circulation model

The interplay between the North Atlantic Oscillation (NAO) and the large scale ocean circulation is inspected in a twentieth century simulation conducted with a state-of-the-art coupled general circulation model. Significant lead–lag covariance between oceanic and tropospheric variables suggests that the system supports a damped oscillatory mode involving an active ocean–atmosphere coupling, with a typical NAO-like space structure and a 5 years timescale, qualitatively consistent with a mid-latitude delayed oscillator paradigm. The two essential processes governing the oscillation are (1) a negative feedback between ocean gyre circulation and the high latitude SST meridional gradient and (2) a positive feedback between SST and the NAO. The atmospheric NAO pattern appears to have a weaker projection on the ocean meridional overturning, compared to the gyre circulation, which leads to a secondary role for the thermohaline circulation in driving the meridional heat transport, and thus the oscillatory mode.     

Decadal climate predictions with a coupled OAGCM initialized with oceanic reanalyses

We investigate the effects of realistic oceanic initial conditions on a set of decadal climate predictions performed with a state-of-the-art coupled ocean-atmosphere general circulation model. The decadal predictions are performed in both retrospective (hindcast) and forecast modes. Specifically, the full set of prediction experiments consists of 3-member ensembles of 30-year simulations, starting at 5-year intervals from 1960 to 2005, using historical radiative forcing conditions for the 1960–2005 period, followed by RCP4.5 scenario settings for the 2006–2035 period. The ocean initial states are provided by ocean reanalyses differing by assimilation methods and assimilated data, but obtained with the same ocean model. The use of alternative ocean reanalyses yields the required perturbation of the full three-dimensional ocean state aimed at generating the ensemble members spread. A full-value initialization technique is adopted. The predictive skill of the system appears to be driven to large extent by trends in the radiative forcing. However, after detrending, a residual skill over specific regions of the ocean emerges in the near-term. Specifically, natural fluctuations in the North Atlantic sea-surface temperature (SST) associated with large-scale multi-decadal variability modes are predictable in the 2–5 year range. This is consistent with significant predictive skill found in the Atlantic meridional overturning circulation over a similar timescale. The dependency of forecast skill on ocean initialization is analysed, revealing a strong impact of details of ocean data assimilation products on the system predictive skill. This points to the need of reducing the large uncertainties that currently affect global ocean reanalyses, in the perspective of providing reliable near-term climate predictions.     

Pacific interdecadal variability driven by tropical–extratropical interactions

Interactions between the tropical and subtropical northern Pacific at decadal time scales are examined using uncoupled oceanic and atmospheric simulations. An atmospheric model is forced with observed Pacific sea surface temperatures (SST) decadal anomalies, computed as the difference between the 2000–2009 and the 1990–1999 period. The resulting pattern has negative SST anomalies at the equator, with a global pattern reminiscent of the Pacific decadal oscillation. The tropical SST anomalies are responsible for driving a weakening of the Hadley cell and atmospheric meridional heat transport. The atmosphere is then shown to produce a significant response in the subtropics, with wind-stress-curl anomalies having the opposite sign from the climatological mean, consistent with a weakening of the oceanic subtropical gyre (STG). A global ocean model is then forced with the decadal anomalies from the atmospheric model. In the North Pacific, the shallow subtropical cell (STC) spins down and the meridional heat transport is reduced, resulting in positive tropical SST anomalies. The final tropical response is reached after the first 10 years of the experiment, consistent with the Rossby-wave adjustment time for both the STG and the STC. The STC provides the connection between subtropical wind stress anomalies and tropical SSTs. In fact, targeted simulations show the importance of off-equatorial wind stress anomalies in driving the oceanic response, whereas anomalous tropical winds have no role in the SST signal reversal. We further explore the connection between STG, STC and tropical SST with the help of an idealized model. We argue that, in our models, tropical SST decadal variability stems from the forcing of the Pacific subtropical gyre through the atmospheric response to ENSO. The resulting Ekman pumping anomaly alters the STC and oceanic heat transport, providing a negative feedback on the SST. We thus suggest that extratropical atmospheric responses to tropical forcing have feedbacks onto the ocean dynamics that lead to a time-delayed response of the tropical oceans, giving rise to a possible mechanism for multidecadal ocean-atmosphere coupled variability.