Interdecadal Component Research Articles

New insights into the biennial-to-multidecadal variability of the water level fluctuation in Lake Titicaca in the 20th century

The water disponibility of Lake Titicaca is important for local ecosystems, domestic water, industry, fishing, agriculture, and tourism in Peru and Bolivia. However, the water level variability in Lake Titicaca (LTWL) still needs to be understood. The fluctuations of LTWL during the 1921–2018 period are investigated using continuous wavelet techniques on high- and low-pass filters of monthly time series, ERA-20C reanalysis, sea surface temperature (SST), and water level. We also built multiple linear regression (MLR) models based on SST indices to identify the main drivers of the LTWL variability. LTWL features annual (12 months), biennial (22–28 months), interannual (80–108 months), decadal (12.75–14.06 years), interdecadal (24.83–26.50 years), and multidecadal (30–65 years) signals. The high- and low-frequency components of the LTWL are triggered by the humidity transport from the lowland toward the Lake Titicaca basin, although different forcings could cause it. The biennial band is associated with SST anomalies over the southeastern tropical Atlantic Ocean that strengthen the Bolivian High-Nordeste Low system. The interannual band is associated with the southern South Atlantic SST anomalies, which modulate the position of the Bolivian High. According to the MLR models, the decadal and interdecadal components of the LTWL can be explained by the linear combination of the decadal and interdecadal variability of the Pacific and Atlantic SST anomalies (r > 0.83, p < 0.05). In contrast, the multidecadal component of the LTWL is driven by the multidecadal component of the North Atlantic SST anomalies (AMO) and the southern South Atlantic SST anomalies. Moreover, the monthly time series of LTWL exhibits four breakpoints. The signs of the first four trends follow the change of phases of the multidecadal component of LTWL, while the fifth trend is zero attributable to the diminished amplitude of the interdecadal component of LTWL.

Interdecadal variability of the austral summer precipitation over the Central Andes

The impacts of the interdecadal variability of the Pacific and the Atlantic Oceans on precipitation over the Central Andes during the austral summer (December-January-February, DJF) are investigated for the 1921–2010 period based on monthly gridded precipitation data and low-pass filtered time series of the Niño 4 index (IN4), the Niño 1 + 2 index with Niño 3.4 index removed (IN1+2*), Atlantic Multidecadal Oscillation (AMO), and Interdecadal Pacific Oscillation (IPO) indices, and the three first rotated principal components of the interdecadal component of the sea surface temperature (SST) anomalies over the Atlantic Ocean. A rotated empirical orthogonal function (REOF) analysis of precipitation in the Central Andes (10°S–30°S) yields two leading modes, RPC1 and RPC2, which represent 40.4% and 18.6% of the total variance, respectively. REOF1 features a precipitation dipole between the northern Bolivian and the Chilean Altiplano. REOF2 also features a precipitation dipole, with highest negative loading over the southern Peruvian Andes. The REOF1 positive phase is associated with moisture transport from the lowlands toward the Bolivian Altiplano, induced by upper-level easterly wind anomalies over the Central Andes. At the same time conditions tend to be dry over the southern Peruvian Andes. The positive phase of REOF2 is related to weakened moisture transport, induced by upper-level westerly wind anomalies over Peru. The IPO warm phase induces significant dry anomalies over the Bolivian Altiplano, albeit weaker than during the IN4 warm phase, via upper-level westerly wind anomalies over the Central Andes. No significant relationship was found between Central Andean precipitation and the AMO on interdecadal timescales.

Salinity interdecadal variability in the western equatorial Pacific and its effects during 1950–2018

Ocean reanalysis products are used to examine salinity variability and its relationships with temperature in the western equatorial Pacific during 1950–2018. An ensemble empirical mode decomposition (EEMD) method is adopted to separate salinity and temperature signals at different time scales; a focus is placed on interdecadal component in this study. Pronounced interdecadal variations in salinity are seen in the region, which exhibits persistent and transitional phases in association with temperature. A surface freshening is accompanied by a surface warming during the 1980s and 1990s, but saltening and cooling in the 2000s, with interdecadal shifts occurring around in the late 1970s, late 1990s, and during 2016–2018, respectively. Determined by anomaly signs of temperature and salinity, their combined effects can be density-compensated or density-uncompensated, correspondingly acting to produce density variability that is suppressed or enhanced, respectively. The temperature and salinity effects are phase- and depth-dependent. In the subsurface layers at 200 m, where salinity and temperature anomalies tend to be nearly of the same sign during interdecadal evolution, their effects are mostly density-compensated. The situation is more complicated in the surface layer, where variations in sea surface salinity (SSS) and sea surface temperature (SST) exhibit different signs during interdecadal evolution. SST and SSS tend to be of opposite sign during the persistent phases with their effects being density-uncompensated; but they can be of the same sign during the transitional periods and the corresponding changes in SST and SSS undergo density-compensated relationships. Examples are given for the relationships among these fields which exhibit phase differences in sign transitions in the late 1990s; salinity effects are seen to cause a delay in phase transition of density anomalies. Furthermore, the relative contributions to interdecadal variabilities of density and stratification are quantified. The consequences of interdecadal salinity variability are also discussed in terms of their effects on local SST.

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.

Synchronized interdecadal variations behind regime shifts in the Pacific Decadal Oscillation

Interdecadal climate variability over the North Pacific is investigated using a reconstructed 298-year-long annual Pacific Decadal Oscillation Index during 1700–1997. Here, we show that its interdecadal component can be decomposed into the bi-decadal (~ 20 years), the tri-decadal (~ 30 years), and the multi-decadal variations (~ 60 years) through bandpass filtering. The phase reversals of the bi-decadal and the tri-decadal variations were synchronized throughout the data, while the synchronization of the bi-decadal and the multi-decadal variations was maintained during 1780–1880 and 1920–1997. Monte Carlo simulation reveals that this synchronous temporal structure is statistically significant and could not occur from random processes. We further demonstrate that these three phase-locked variations play an important role in the regime shifts in the North Pacific, both during the data period (1700–1997) and thereafter. The bi-decadal variation, which is self-sustained through the North Pacific mid-latitude air–sea interaction, is also found to be possibly linked to the lunar nodal cycle. Moreover, the tri-decadal and the multi-decadal variations, whose periods are approximately 1.5 and 3 times longer than that of the bi-decadal variation, may be linked to the bi-decadal variation through nonlinear process. This simple relationship could largely contribute to understanding and predicting the interdecadal climate variability.

Interdecadal and interannual evolution characteristics of the global surface precipitation anomaly shown by CMIP5 and CMIP6 models

AbstractCoupled Model Intercomparison Project Phase 5 (CMIP5) models have limited capacity for depicting the variability of precipitation at interannual and interdecadal timescales. This study analyses the relative magnitude of interannual to multidecadal variability in annual precipitation across the world in the recently released CMIP6 dataset by quantifying the discrepancy between observed and modelled CMIP5 and CMIP6 data, considering both the absolute and relative magnitude of precipitation variability. We found that: (a) similar to CMIP5, CMIP6 models were short of capacity to simulate the non‐homogeneity in the relative magnitude of interdecadal variability, which is linked to prolonged drought and pluvials in terms of the strength value. The relative value of the interdecadal variability ranged from 15% to more than 30% in observation. Compared with observational data, the relative magnitude, having spatial uniformity, mostly ranged from 10 to 20% in CMIP5 and CMIP6. This result suggests that future projections lack a sufficient decadal variability in CMIP6, indicating a limited capacity for the prediction of floods and droughts in regions like central Africa, North America, and Amazonia; (b) Most individual models in CMIP6 had a better performance in terms of the spatial distribution of the interdecadal precipitation as compared to CMIP5. However, the absolute variations of the overall, interannual, and interdecadal precipitation of multi‐model ensemble simulations (MME) in CMIP6 were larger than those in CMIP5; (c) the underestimation of the interdecadal component in different areas was markedly decreased in East Asia, South Asia but increased in North and South Africa in CMIP6, when compared with CMIP5 and (d) For future scenarios, the higher the range of future forcing pathway is, the larger interdecadal and interannual variabilities are in all regions in CMIP6. For future persistent droughts and floods predicting in a specific region, it is necessary to consider the discrepancy between historical simulations and observational data, and select the performance‐plus models.

Predicting wintertime windy days in Beijing with the preceding autumn sea surface temperature and large-scale circulation patterns

In this study, we investigate the role of sea surface temperature and large-scale circulation patterns in the preceding autumn for the prediction of wintertime windy days in Beijing during the period of 1979–2016, based on a time-scale decomposition approach. A windy day is defined according to the threshold of 2 m·s−1, which can effectively remove or reduce air pollution of Beijing. Wintertime windy days in Beijing, preceding autumn sea surface temperature and large-scale climate indices are separated into the interannual and interdecadal components. For the interannual component, the preceding autumn sea surface temperature in central western Pacific Ocean, Polar/Eurasia pattern, and Scandinavia pattern are identified as predictors of wintertime windy days in Beijing. For the interdecadal component, the identified predictors include the preceding autumn sea surface temperature in northeastern part of North Atlantic Ocean, Arctic Oscillation, and North Atlantic Oscillation. We develop statistical models for the interannual and interdecadal components based on these preceding autumn predictors, and take them together to predict wintertime windy days in Beijing. The leave-one-out cross-validation for 1979–2016 time series shows that 76% of the total variance of observed wintertime windy days in Beijing can be predicted. Hindcast experiments for the period of 1999–2016 also yield the high prediction skills. The physical mechanisms responsible for links between these predictors and wintertime windy days in Beijing are also discussed. The statistical models proposed herein are expected to be applied to and aid to improve seasonal forecasting of wintertime windy days in Beijing.

A statistical forecast model using the time-scale decomposition technique to predict rainfall during flood period over the middle and lower reaches of the Yangtze River Valley

In this paper, a statistical forecast model using the time-scale decomposition method is established to do the seasonal prediction of the rainfall during flood period (FPR) over the middle and lower reaches of the Yangtze River Valley (MLYRV). This method decomposites the rainfall over the MLYRV into three time-scale components, namely, the interannual component with the period less than 8 years, the interdecadal component with the period from 8 to 30 years, and the interdecadal component with the period larger than 30 years. Then, the predictors are selected for the three time-scale components of FPR through the correlation analysis. At last, a statistical forecast model is established using the multiple linear regression technique to predict the three time-scale components of the FPR, respectively. The results show that this forecast model can capture the interannual and interdecadal variation of FPR. The hindcast of FPR during 14 years from 2001 to 2014 shows that the FPR can be predicted successfully in 11 out of the 14 years. This forecast model performs better than the model using traditional scheme without time-scale decomposition. Therefore, the statistical forecast model using the time-scale decomposition technique has good skills and application value in the operational prediction of FPR over the MLYRV.

Interdecadal change in the Eurasia–Pacific anti-phase relation of atmospheric mass and its possible link with PDO

Based on the known climatic shift that occurred in 1976, we divide the present study period into two epochs: epoch-I, for 1958–1976; and epoch-II, for 1977–2002. Using ERA-40 and the 20th century reanalysis data, we investigate the interdecadal change in the Eurasia–Pacific anti-phase relation (EPAR) pattern of atmospheric mass (AM) during boreal winter before and after 1976. It is found that anomalous AM over lands is highly and negatively correlated with anomalous AM over oceans in the Northern Hemisphere during the winter season. This correlation does not change much from epoch-I to epoch-II. However, the correlation pattern of surface air pressure anomalies with variations of anomalous AM over lands changes remarkably from epoch-I to epoch-II; the EPAR pattern emerges evidently in the later period, whereas it is not significant in epoch-I. The occurrence of the EPAR pattern in epoch-II may be attributable to the Pacific Decadal Oscillation (PDO). The PDO may modulate the EPAR pattern in two ways. Firstly, the interdecadal component of the PDO as a background may modulate the intensities of the Aleutian low, East Asian trough, and westerly flow, acting as a waveguide during the warm phase (epoch-II) of the PDO. Secondly, the interannual variations of sea surface temperature anomalies in the North Pacific, in association with the PDO, may affect the interannual variations of AM, which facilitates the existence of the EPAR pattern in epoch-II only. With the teleconnection pattern having changed before and after 1976, winter climate anomalies, including rainfall and temperature, are found to be different in many regions in the Northern Hemisphere between epoch-I and epoch-II. All the results of the present work are meaningful for a better understanding of climate anomalies during boreal winter.

Interdecadal component variation characteristics in heavy winter snow intensity in North-Eastern China and its response to sea surface temperatures

Based on daily precipitation data from 208 weather stations, monthly NCEP/NCAR reanalysis data, and sea surface temperature data reconstructed by NOAA between 1961 and 2012, the heavy winter snow intensity in North-Eastern China was defined; its spatiotemporal variation characteristics were analyzed; the physical mechanisms of the relations between key sea surface temperature (SST) regions and heavy winter snow intensity were studied. Results showed that, in terms of temporal variation characteristics: heavy winter snow intensity in North-Eastern China has been rising, with obvious interdecadal variations during the study interval. In terms of spatial variation characteristics: the first empirical orthogonal function (EOF) mode of the interdecadal component in heavy winter snow intensity showed consistent anomaly characteristics throughout the region; the second mode exhibited opposite variation characteristics between the south and north; and the third mode exhibited opposite variation characteristics between the northwest and southeast. In terms of physical mechanisms underlying the effects of SST on heavy winter snow intensity, the key SST region of the first EOF mode was the Oyashio, the interdecadal component of the autumn Oyashio SST was abnormally high, which corresponded to a higher winter 500hPa height field in the northern section of the North Pacific. This condition led to a blocking situation and was associated with a weaker East Asian winter monsoon (EAMW), which resulted in a stronger interdecadal component of heavy winter snow intensity and vice versa. The second mode was closely related to the Pacific Decadal Oscillation (PDO): the interdecadal component of the autumn PDO was abnormal, which induced the teleconnection pattern of the winter Pacific-North America (PNA) pattern. The PNA pattern may have been related to the second EOF mode through its association with the Mongolian high.

Evaluation of RegCM4 in simulating the interannual and interdecadal variations of Meiyu rainfall in China

In this paper, the performance of the regional climate model (RegCM4) in simulating the interannual and interdecadal variation of Meiyu rainfall over lower and middle reaches of Yangtze River (LMRYR) in China is evaluated. The results show that the bias of the simulated Meiyu rainfall exhibits some uncertainties and obvious interdecadal variation similar with that of Meiyu rainfall itself. The rainfall bias and its interdecadal variation have little impact on the simulation of the interannual variation of the Meiyu rainfall. Not only the amplitude but also the phase of the interannual variation of the Meiyu rainfall can be reproduced by the RegCM4. However, the interdecadal variation of the rainfall bias has large impacts on the model’s performance in simulating the interdecadal component of the Meiyu rainfall, leading to lower skill of RegCM4 in simulating the interdecadal variation of the Meiyu rainfall than that in simulating the interannual variation. In addition, the interannual signals of the temperature, humidity, and vorticity in the lower troposphere in the large-scale NCEP lateral boundary force are stronger than the interdecadal signals, and the model can respond well to these interannual signals. Therefore, the RegCM4 has better performance in simulating the interannual variation of water vapor budget than the interdecadal variation, which further favors the simulation of the Meiyu rainfall at interannual time scale.

中国冬季极端低温事件与海温和海冰的关系

本文利用站点资料和再分析资料分析中国冬季南、北方区域极端低温事件与海温和海冰的关系。研究发现，中国南、北方区域极端低温频数下降趋势与全球变暖有关，极端低温频数的线性趋势与印度洋和北大西洋海温异常呈负相关，还与前期边缘海海冰特别是格陵兰–巴伦支海海冰异常呈正相关。对于北方区域，在年代际变化尺度上，极端低温事件偏多时，冬季北太平洋海温表现为北负南正的异常分布，格陵兰–巴伦支海海冰呈正异常。在年际变化尺度上，冬季北太平洋中部与北美西岸海域海温出现西负东正的异常分布，北美东岸海温表现为负异常，楚科奇海和新瓦尔巴群岛以北海区海冰呈负异常。对南方区域，在年代际变化尺度上，极端低温事件偏多与PDO冷位相有关，并与格陵兰海和白令海海冰异常呈正相关，其他海区海冰异常呈负相关。在年际变化尺度上，前期阿拉斯加湾和白令海海区异常偏冷，对应着冬季南方区域极端低温事件偏多，另外还与冬季太平洋扇区海冰异常存在显著负相关关系。 Based on the dataset of daily minimum temperature in China during 1961-2010 and reanalysis data, the relationship between winter extreme minimum temperature (EMT) frequency and sea surface temperature (SST) and sea ice is analyzed. Research shows that, the decline of EMT fre-quency both in the north and south of China is associated with global warming. The linear trend has a negative correlation with the SST anomalies in the North Atlantic and India Ocean, and can be preceded by melting of the marginal sea ice, especially in the Greenland-Barents Sea ice during the preceding summer and autumn. For the north of China, its inter-decadal (ID) component may be affected by North Pacific SST anomalies which has a north-negative and south-positive distribution, and has a positive correlation with Greenland-Barents sea ice. While inter-annual (IA) component has a significant negative correlation with the SST anomalies along the east coast of North American and the anomalous SST between the subtropical North Pacific Ocean and the Gulf of Alaska represents a distribution of the dipole oscillation. And it is also affected by the sea ice melting in Chukchi Sea and Sea area in the north of Svalbard. On the other hand, for the south of China, its ID component is remarkable influenced by Pacific Decadal Oscillation (PDO). It also has a negative correlation with the marginal sea ice except the Greenland Sea and the Bering Sea. Beyond the IA time scale, the SST anomalies over the Bering Sea and the Gulf of Alaska prelude the anomalous winter EMT in the south of China. In addition, it also has a significant negative correlation with Pacific sector sea ice.

The Role of the Indonesian Throughflow on ENSO Dynamics in a Coupled Climate Model

Abstract The effects of the Indonesian Throughflow (ITF) on ENSO dynamics are studied in a coupled climate model by comparing two simulations, one with an open ITF and the other with a closed ITF. Closing the ITF results in an El Niño–like climate state in the Pacific, which is characterized by weakened trade winds, a flatter equatorial thermocline, and weaker equatorial upwelling. A weakened South Equatorial Current allows the western Pacific warm pool to extend eastward, thereby reducing the zonal temperature gradient along the equator. The interdecadal component of the ENSO-like variability collapses, although the interannual variability is maintained. The core region of the ENSO SST anomalies becomes confined farther east. This results from Bjerknes feedback processes that are shifted eastward. This study conducts an analysis utilizing the Bjerknes coupled stability index as formulated by Jin et al. and finds that the relative importance of the thermocline feedback is enhanced in the closed ITF experiment. This indicates a more prominent ENSO thermocline mode, thus explaining the existence of more prevalent eastward-propagating anomalies. A weaker zonal advective feedback due to the reduced mean zonal temperature gradient contributes to the dominance of the thermocline mode. The strength of the thermocline feedback itself is maintained by enhanced coupling between the zonal wind stress and the east–west thermocline slope. However, an increased thermal damping by air–sea heat flux dominates the overall ENSO feedback process.

A Distinctly Interdecadal Signal of Pacific Ocean–Atmosphere Interaction

Abstract A new and distinctly interdecadal signal in the climate of the Pacific Ocean has been uncovered by examining the coupled behavior of sea surface temperatures (SSTs) and Northern Hemisphere atmospheric circulation. This interdecadal Pacific signal (IPS) of ocean–atmosphere interaction exhibits a highly statistically significant interdecadal component yet contains little to no interannual (El Niño scale) variability common to other Pacific climate anomaly patterns. The IPS thus represents the only empirically derived, distinctly interdecadal signal of Pacific Ocean SST variability that likely also represents the true interdecadal behavior of the Pacific Ocean–atmosphere system. The residual variability of the Pacific’s leading SST pattern, after removal of the IPS, is highly correlated with El Niño anomalies. This indicates that by simply including an atmospheric component, the leading mode of Pacific SST variability has been decomposed into its interdecadal and interannual patterns. Although the interdecadal signal is unrelated to interannual El Niño variability, the interdecadal ocean–atmosphere variability still seems closely linked to tropical Pacific SSTs. Because prior abrupt changes in Pacific SSTs have been related to anomalies in a variety of physical and biotic parameters throughout the Northern Hemisphere, and because of the persistence of these changes over several decades, isolation of this interdecadal signal in the Pacific Ocean–atmosphere system has potentially important and widespread implications to climate forecasting and climate impact assessment.

Rainfall spectrum change in North China and its possible mechanism*

Abstract The seasonal distribution of the rainfall in North China has changed greatly since 1977, with more rainfall in spring and less in July, August, and September (JAS). Wavelet analysis showed that the JAS rainfall underwent an abrupt spectrum change in the mid-1960s. Its interannual variability has declined while the interdecadal component has become the dominant mode, associated with the dry climate. Correlation analysis found that the JAS rainfall is negatively correlated with the departures of the 500 hPa geopotential height significantly over the northwest and southwest of China, and positively correlated with the summer monsoon over eastern China. Therefore, the interdecadal ascending of the air pressure in northwestern China and the monsoon decaying over East Asia in the past 20 years may be the two major causes of North China drought.

Multiscale characteristics of the rainy season rainfall and interdecadal decaying of summer monsoon in North China

This paper focuses on the rainfall spectrum and its evolution of North China in rainy season with summer monsoon decaying in interdecadal time scale. The interannual component of the rainfall is the dominant part, accounting for 85% of the total variance, and has been changed significantly during the last 30 years. According to wavelet analysis its 5a periodic spectrum suddenly disappeared in the late 1960s, and its biennial oscillation gradually become weaker and weaker since 1970, accompanied by the summer monsoon decaying. Contrarily, the interdecadal component is principal in the summer monsoon over North China and is very similar to the counterpart of the rainfall. Their interdecadal parts are significantly correlated, and the correlation coefficient is nearly equal to the one of the original sequences. Besides, the dry and wet climate alternated with the monsoon abrupt changes in the 1960s and the 1970s over East Asia, apart from North China, climate drifted from a light drought to a severe drought during the past 30 years.

A wavelet characterization of the North Atlantic oscillation variation and its relationship to the North Atlantic sea surface temperature

In this study, Multiresolution Fourier Transform (MFT) spectral analysis is employed to resolve the temporal structure of the variation of the North Atlantic Oscillation (NAO) in terms of various frequency components. The NAO index displays fluctuations on multiple timescales, resulting in an MFT spectrum characterized by the occurrences of distinct frequency bands at certain times in the record. These frequency bands are examined within the context of annual to interdecadal spectral components. The relative contribution by each of these spectral components toward the overall NAO variability varies with time. Phase analysis of the spectrum demonstrates that, in addition to some apparent ‘random’ fluctuations, the phase of each of the spectral components on occasions exhibits relatively constant value over a period of time (from a few years to a decade). In addition, a significant coherent relationship between the NAO and the North Atlantic sea surface temperature (SST) is found at interannual and interdecadal timescales. Some evidence has also been found to suggest that the phase of the interdecadal component of the NAO is itself modulated by the North Atlantic SST at a timescale of about 60 years. Copyright © 1999 Royal Meteorological Society