ENSO Frequency Asymmetry and the Pacific Decadal Oscillation in Observations and 19 CMIP5 Models

Using observational data and the pre-industrial simulations of 19 models from the Coupled Model Intercomparison Project Phase 5（CMIP5）, the El Ni o（EN） and La Ni a（LN） events in positive and negative Pacific Decadal Oscillation（PDO） phases are examined. In the observational data, with EN（LN） events the positive（negative） SST anomaly in the equatorial eastern Pacific is much stronger in positive（negative） PDO phases than in negative（positive） phases. Meanwhile,the models cannot reasonably reproduce this difference. Besides, the modulation of ENSO frequency asymmetry by the PDO is explored. Results show that, in the observational data, EN is 300% more（58% less） frequent than LN in positive（negative）PDO phases, which is significant at the 99% confidence level using the Monte Carlo test. Most of the CMIP5 models exhibit results that are consistent with the observational data.

Phase Transition of the Pacific Decadal Oscillation and Decadal Variation of the East Asian Summer Monsoon in the 20th Century

This paper focuses on the relationship between the phase transition of the Pacific decadal oscillation （PDO） and decadal variation of the East Asian summer monsoon （EASM） in the twentieth century. The first transition occurred in the 1940s, with an enhanced SST in the North Pacific and reduced SST in the tropical eastern Pacific and South Indian Ocean. In agreement with these SST changes, a higher SLP was found in most parts of the Pacific, while a lower SLP was found in the North Pacific and most parts of the Indian Ocean. In this case, the EASM was largely enhanced with a southerly anomaly in the lower troposphere along the east coast of China. Correspondingly, there was less rainfall in the Yangtze River valley and more rainfall in northern and southern China. An opposite change was found when the PDO reversed its phase in the late 1970s. In the tropical Indian Ocean and western Pacific, however, the SST was enhanced in both the 1940s and 1970s. As a result, the western Pacific subtropical high （WPSH） tended to extend westward with a larger magnitude in the 1970s. The major features were reasonably reproduced by an atmospheric general circulation model （IAP AGCM4.0） prescribed with observed SST and sea ice. On the other hand, the westward extension of the WPSH was exaggerated in the 1970s, while it was underestimated in the 1940s. Besides, the spatial pattern of the simulated summer rainfall in eastern China tended to shift southward compared with the observation.

The Impact of Global Warming on the Pacific Decadal Oscillation and the Possible Mechanism

The response of the Pacific Decadal Oscillation （PDO） to global warming according to the Fast Ocean Atmosphere Model （FOAM） and global warming comparison experiments of 11 IPCC AR4 models is investigated. The results show that North Pacific ocean decadal variability, its dominant mode （i.e., PDO）, and atmospheric decadal variability, have become weaker under global warming, but with PDO shifting to a higher frequency. The SST decadal variability reduction maximum is shown to be in the subpolar North Pacific Ocean and western North Pacific （PDO center）. The atmospheric decadal variability reduction maximum is over the PDO center. It was also found that oceanic baroclinic Rossby waves play a key role in PDO dynamics, especially those in the subpolar ocean. As the frequency of ocean buoyancy increases under a warmer climate, oceanic baroclinic Rossby waves become faster, and the increase in their speed ratio in the high latitudes is much larger than in the low latitudes. The faster baroclinic Rossby waves can cause the PDO to shift to a higher frequency, and North Pacific decadal variability and PDO to become weaker.

Response of Growing Season Gross Primary Production to El Nino in Different Phases of the Pacific Decadal Oscillation over Eastern China Based on Bayesian Model Averaging

Gross primary production(GPP) plays a crucial part in the carbon cycle of terrestrial ecosystems.A set of validated monthly GPP data from 1957 to 2010 in 0.5°× 0.5° grids of China was weighted from the Multi-scale Terrestrial Model Intercomparison Project using Bayesian model averaging(BMA).The spatial anomalies of detrended BMA GPP during the growing seasons of typical El Nino years indicated that GPP response to El Nino varies with Pacific Decadal Oscillation(PDO) phases: when the PDO was in the cool phase,it was likely that GPP was greater in northern China(32°–38°N,111°–122°E) and less in the Yangtze River valley(28°–32°N,111°–122°E);in contrast,when PDO was in the warm phase,the GPP anomalies were usually reversed in these two regions.The consistent spatiotemporal pattern and high partial correlation revealed that rainfall dominated this phenomenon.The previously published findings on how El Nino during different phases of PDO affecting rainfall in eastern China make the statistical relationship between GPP and El Nino in this study theoretically credible.This paper not only introduces an effective way to use BMA in grids that have mixed plant function types,but also makes it possible to evaluate the carbon cycle in eastern China based on the prediction of El Nino and PDO.

Twentieth-century Pacific Decadal Oscillation simulated by CMIP5 coupled models

The authors examine the spatial and temporal characteristics of the simulated Pacific Decadal Oscillation （PDO） in 109 historical （i.e. all forcings） simulations derived from 25 coupled models within CMIPS. Compared with observations, most simulations successfully simulate the observed PDO pattern and its teleconnections to the SSTs in the tropical and southern Pacific. BNU-ESM, CanESM2, CCSM4, CESM 1 -FASTCHEM, FGOALS-g2, GFDL CM3, MIROCS, and NorESM 1 -M show better performance. Compared with the temporal phases of the observed PDO in the twentieth century, only five simulations -- from CNRM^CMS, CSIRO Mk3o6.0, HadCM3, and IPSL-CMSA-LR -- simulate an evolution of the PDO similar to that derived from observation, which suggests that current coupled models can barely reproduce the observed phase shifting of the PDO. To capture characteristics of the observed PDO in the twentieth century, a requirement is that all the relevant external forcings are included in the models. How to add realistic oceanic initial states into the model may be another key point.

Influence of the Pacific Decadal Oscillation on regional sea level rise in the Pacific Ocean from 1993 to 2012

The rate of regional sea level rise （SLR） provides important information about the impact of human activities on climate change. However, accurate estimation of regional SLR can be severely affected by sea surface height （SSH） change caused by the Pacific Decadal Oscillation （PDO-SSH）. Here, the PDO- SSH signal is extracted from satellite altimeter data by multi-variable linear regression, and regional SLR in the altimeter era is calculated, before and after removing that signal. The results show that PDO-SSH trends are rising in the western Pacific and falling in the eastern Pacific, with the strongest signal confined to the tropical and North Pacific. Over the past 20 years, the PDO-SSH accounts for about 30%/-400% of altimeter-observed SLR in the regions 8° 15°N, 130°-160°E and 30°-40°N, 170°-220°E. Along the coast ＆North America, the PDO-SSH signal dramatically offsets the coastal SLR, as the sea level trends change sign from falling to rising.

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.

Potential Correlation between the Decadal East Asian Summer Monsoon Variability and the Pacific Decadal Oscillation

This study discusses the potential contribution of the Pacific decadal oscillation(PDO)to the weakening of the East Asian summer monsoon(EASM)and the evident correlation between the positive PDO and"Southern flood and Northern drought(SFND)"summer rainfall pattern over East China.The mechanism behind this contribution is also discussed.

Response of Japanese anchovy(Engraulis japonicus)to the Pacific Decadal Oscillation in the Yellow Sea over the past 400 a

Quantitative identification of long-term changes in the abundance of Japanese anchovy(Engraulis japonicus)in the Yellow Sea is particularly important for understanding evolutionary processes of the Yellow Sea ecosystem.Unfortunately,the driving mechanisms of climate variability on the anchovy are still unclear due to the lack of long-term observational data.In this study,we used the fish scale deposition rate in the central Yellow Sea to reconstruct the time series of the anchovy stock over the past 400 a.On this basis,we further explored the impacts of the Pacific Decadal Oscillation(PDO)on the anchovy.Our results show that the anchovy stock is positively correlated with the PDO on a decadal time scale.In addition,anchovy abundance was relatively high during1620–1860 AD(the Little Ice Age,LIA),though in a state of constant fluctuation;anchovy abundance maintained at a relatively low level after~1860 AD.In particular,followed by overfishing since the 1980 s,the anchovy stock has declined sharply.Based on these findings,we infer that fluctuations of the anchovy stock may be regulated by basin-scale“atmosphere–ocean”interactions.Nevertheless,the role of overfishing should not be ignored.

Precipitation Regime Shift Associated with the Pacific Decadal Oscillation in the Maritime Continent

Recent changes in precipitation regime in South-East Asia are a subject of ongoing discussion. In this article, for the first time, evidence of a precipitation regime shift during the mid-1970s in the Northern Hemispheric part of South-East Asia is demonstrated. The detection of regime shifts is made possible by using a new comprehensive dataset of daily precipitation records (South-East Asian Climate Assessment and Dataset) and applying a novel Bayesian approach for regime shift detection. After the detected regime shift event in the mid-1970s, significant changes in precipitation distribution occurred in the Northern Hemispheric regions—Indochina Peninsula and the Philippines. More specifically, dry days became up to 10% more frequent in some regions. However, no precipitation regime shift is detected in Southern Hemisphere regions—Java and Northern Australia, were the number of observed dry days increased gradually.

The Role of Underlying Boundary Forcing in Shaping the Recent Decadal Change of Persistent Anomalous Activity over the Ural Mountains

Observational analyses demonstrate that the Ural persistent positive height anomaly event(PAE) experienced a decadal increase around the year 2000, exhibiting a southward displacement afterward. These decadal variations are related to a large-scale circulation shift over the Eurasian Continent. The effects of underlying sea ice and sea surface temperature(SST) anomalies on the Ural PAE and the related atmospheric circulation were explored by Atmospheric Model Intercomparison Project(AMIP) experiments from the Coupled Model Intercomparison Project Phase 6 and by sensitivity experiments using the Atmospheric General Circulation Model(AGCM). The AMIP experiment results suggest that the underlying sea ice and SST anomalies play important roles. The individual contributions of sea ice loss in the Barents-Kara Seas and the SST anomalies linked to the phase transition of the Pacific Decadal Oscillation(PDO) and Atlantic Multidecadal Oscillation(AMO) are further investigated by AGCM sensitivity experiments isolating the respective forcings.The sea ice decline in Barents-Kara Seas triggers an atmospheric wave train over the Eurasian mid-to-high latitudes with positive anomalies over the Urals, favoring the occurrence of Ural PAEs. The shift in the PDO to its negative phase triggers a wave train propagating downstream from the North Pacific. One positive anomaly lobe of the wave train is located over the Ural Mountains and increases the PAE there. The negative-to-positive transition of the AMO phase since the late-1990s causes positive 500-h Pa height anomalies south of the Ural Mountains, which promote a southward shift of Ural PAE.

Interdecadal Change of the Northward Jump Time of the Western Pacific Subtropical High in Association with the Pacific Decadal Oscillation

In this paper, the northward jump time of the western Pacific subtropical high（WPSH） is defined and analyzed on the interdecadal timescale. The results show that under global warming, significant interdecadal changes have occurred in the time of the WPSH northward jumps. From 1951 to 2012, the time of the first northward jump of WPSH has changed from ＂continuously early＂ to ＂continuously late＂, with the transition occurring in 1980. The time of the second northward jump of WPSH shows a similar change, with the transition occurring in 1978. In this study, we offer a new perspective by using the time of the northward jump of WPSH to explain the eastern China summer rainfall pattern change from ＂north-abundant-southbelow-average＂ to ＂south-abundant-north-below-average＂ at the end of the 1970 s. The interdecadal change in the time of the northward jump of WPSH corresponds not only with the summer rainfall pattern, but also with the Pacific decadal oscillation（PDO）. The WPSH northward jump time corresponding to the cold（warm） phase of the PDO is early（late）. Although the PDO and the El Nino–Southern Oscillation（ENSO）both greatly influence the time of the two northward jumps of WPSH, the PDO＇s effect is noticed before the ENSO＇s by approximately 1–2 months. After excluding the ENSO influence, we derive composite vertical atmospheric circulation for different phases of the PDO. The results show that during the cold（warm）phase of the PDO, the atmospheric circulations at 200, 500, and 850 h Pa all contribute to an earlier（later）northward jump of the WPSH.

Pacific decadal oscillation impact on East China precipitation and its imprint in new geological documents

This paper briefly introduces the history of the study of the Pacific decadal oscillation(PDO) and explores the relationship between the PDO and sediment grain size from two typical sediment cores from the lower Changjiang(Yangtze River) and Huanghe(Yellow River) estuaries. It is found that the median grain sizes of both cores exhibit relatively high correlation with the PDO. This is because the PDO causes interdecadal variability of precipitation in the East Asia Monsoon region, thus changing the hydrodynamics in both the Changjiang and Huanghe catchments, eventually resulting in variation of sediment grain size. Our analysis also revealed that during different phases of the PDO, the sediment grain size of the Changjiang and Huanghe estuaries showed different variations in cold and warm PDO phases. This is related to movement of the precipitation center driven by the shift in different PDO phases. Moreover, we compared more high resolution geological proxies with the PDO, including stalagmites and tree rings, in East China over the past century. The results indicate that variations of studied geological proxies are generally well correlated with the PDO but have some differences. Finally, longer variations of sediment grain sizes in the Changjiang and Huanghe estuaries are compared with a reconstructed PDO over the last 200 years;sediment grain sizes were still correlated with the PDO, implying that sediment grain size may be used as a new proxy for studying the long-term behavior of the PDO. This result supports previous knowledge of the PDO impact on East China climate evolution and offers a new proxy for further PDO study. Our study will improve paleoenvironment reconstruction in East China on a decadal time scale and benefit future climatic predictions.

Influence of Internal Decadal Variability on the Summer Rainfall in Eastern China as Simulated by CCSM4

The combined impact of the Pacific Decadal Oscillation （PDO） and Atlantic Multidecadal Oscillation （AMO） on the summer rainfall in eastern China was investigated using CCSM4. The strongest signals occur with the combination of a positive PDO and a negative AMO （＋PDO- AMO）, as well as a negative PDO and a positive AMO （-PDO ＋ AMO）. For the ＋PDO- AMO set, significant positive rainfall anomalies occur over the lower reaches of the Yangtze River valley （YR）, when the East Asian summer monsoon becomes weaker, while the East Asian westerly jet stream becomes stronger, and ascending motion over the YR becomes enhanced due to the jet-related secondary circulation. Contrary anomalies occur over East Asia for the -PDO ＋ AMO set. The influence of these two combinations of PDO and AMO on the summer rainfall in eastern China can also be observed in the two interdecadal rainfall changes in eastern China in the late 1970s and late 1990s.

Possible Origins of the Western Pacific Warm Pool Decadal Variability

In this study, the impacts of the Pacific Decadal Oscillation （PDO） and the Atlantic Multidecadal Oscillation （AMO） on the western Pacific warm pool （WPWP） were investigated. Our results show that the WPWP is linked with the PDO and the AMO at multiple time scales. On the seasonal time scales, the WPWP and the PDO/AMO reinforce each other, while at decadal time scales the forcing roles of the PDO and the AMO dominate. Notably, a positive PDO tends to enlarge the WPWP at both seasonal and decadal time scales, while a positive AMO tends to reduce the WPWP at decadal time scales. Furthermore, the decadal variability of the WPWP can be well predicted based on the PDO and AMO.

CMIP 6 models simulation of the connection between North/South Pacific Meridional Mode and ENSO

The subtropical North and South Pacific Meridional Modes(NPMM and SPMM)are well known precursors of El Niño-Southern Oscillation(ENSO).However,relationship between them is not constant.In the early 1980,the relationship experienced an interdecadal transition.Changes in this connection can be attributed mainly to the phase change of the Pacific decadal oscillation(PDO).During the positive phase of PDO,a shallower thermocline in the central Pacific is responsible for the stronger trade wind charging(TWC)mechanism,which leads to a stronger equatorial subsurface temperature evolution.This dynamic process strengthens the connection between NPMM and ENSO.Associated with the negative phase of PDO,a shallower thermocline over southeastern Pacific allows an enhanced wind-evaporation-SST(WES)feedback,strengthening the connection between SPMM and ENSO.Using 35 Coupled Model Intercomparison Project Phase 6(CMIP6)models,we examined the NPMM/SPMM performance and its connection with ENSO in the historical runs.The great majority of CMIP6 models can reproduce the pattern of NPMM and SPMM well,but they reveal discrepant ENSO and NPMM/SPMM relationship.The intermodal uncertainty for the connection of NPMM-ENSO is due to different TWC mechanism.A stronger TWC mechanism will enhance NPMM forcing.For SPMM,few models can simulate a good relationship with ENSO.The intermodel spread in the relationship of SPMM and ENSO owing to SST bias in the southeastern Pacific,as WES feedback is stronger when the southeastern Pacific is warmer.

Interdecadal change in the South Asian summer monsoon rainfall in 2000 and contributions from regional tropical SST

The drying trend in the South Asian summer monsoon(SASM)area has been a focus of monsoon rainfall studies in the last two decades.However,this study reveals that a signi cant interdecadal change in the SASM rainfall occurred in approximately the year 2000.Obvious spatial inhomo-geneity was a feature of this change,with increased rainfall over the southern part of the India Pakistan border area that extends from the Arabian Sea,as well as in the western Bay of Bengal.Furthermore,there was decreased rainfall over the southern SASM and the western coast of the Indian Peninsula.Numerical experiments using CAM4 show that global SST changes can induce general changes in the SASM circulation consistent with observations.The tropical Pacific/Indian Ocean SST anomalies dominated the Walker and the regional Hadley circulation changes,respectively,while the descending motion anomalies over the southern SASM were further enhanced by the warmer tropical Atlantic SSTs.Moreover,the spatial inhomogeneity of this interdecadal change in the SASM rainfall needs further study.

Joint Propagating Patterns of SST and SLP Anomalies in the North Pacific on Bidecadal and Pentadecadal Timescales

Wavelet analyses are applied to the Pacific Decadal Oscillation index and North Pacific index for the period 1900-2000, which identifies two dominant interdecadal components, the bidecadal (15-25-yr) and pentadecadal (50 70-yr) modes. Joint propagating patterns of sea surface temperature (SST) and sea level pressure (SLP) anomalies in the North Pacific for the two modes are revealed by using the techniques of multi-channel singular spectrum analysis (MSSA) and linear regression analysis with the global sea surface temperature (GISST) data and the northern hemispheric SLP data for the common period 1903 1998. Significant differences in spatio-temporal structures are found between the two modes. For the bidecadal mode, SST anomalies originating from the Gulf of Alaska appear to slowly spread southwestward, inducing a reversal of early SST anomalies in the central North Pacific. Due to further westward spreading, the SST variation of the central North Pacific leads that of the Kuroshio-Oyashio Extension (KOE) region by approximately 4 to 5 years. Concomitantly, SLP anomalies spread over most parts of the North Pacific during the mature phase and then change into an NPO(North Pacific Oscillation)-like pattern during the transition phase. For the pentadecadal mode, SST anomalies develop in the southeast tropical Pacific and propagate along the North American coast to the mid-latitudes; meanwhile, SST anomalies with the same polarity in the western tropical Pacific expand northward to Kuroshio and its extension region; both merge into the central North Pacific reversing the sign of early SST anomalies there. Accompanying SLP anomalies are characterized by an NPO-like pattern during the mature phase while they are dominant over the North Pacific during the transitional phase. The bidecadal and pentadecadal modes have different propagating patterns, suggesting that the two interdecadal modes may arise from different physical mechanisms.

On the Decadal and Interdecadal Variability in the Pacific Ocean

The Pacific decadal and interdecadal oscillation (PDO) has been extensively explored in recent decades because of its profound impact on global climate systems. It is a long-lived ENSO-like pattern of Pacific climate variability with a period of 10-30 years. The general picture is that the anomalously warm (cool) SSTs in the central North Pacific are always accompanied by the anomalously cool (warm) SSTs along the west coast of America and in the central east tropical Pacific with comparable amplitude. In general, there are two classes of opinions on the origin of this low-frequency climate variability, one thinking that it results from deterministically coupled modes of the Pacific ocean-atmosphere system, and the other, from stochastic atmospheric forcing. The deterministic origin emphasizes that the internal physical processes in an air-sea system can provide a positive feedback mechanism to amplify an initial perturbation, and a negative feedback mechanism to reverse the phase of oscillation. The dynamic evolution of ocean circulation determines the timescale of the oscillation. The stochastic origin, however, emphasizes that because the atmospheric activities can be thought as having no preferred timescale and are associated with an essentially white noise spectrum, tne ocean response can manifest a red peak in a certain low frequency range with a decadal to interdecadal timescale. In this paper, the authors try to systematically understand the state of the art of observational, theoretical and numerical studies on the PDO and hope to provide a useful background reference for current research.

Decadal Variation of the Aleutian Low-Icelandic Low Seesaw Simulated by a Climate System Model(CAS-ESM-C)

Based on a simulation using a newly developed climate system model(Chinese Academy of Sciences-Earth System Model-Climate system component, CAS-ESM-C), the author investigated the Aleutian Low- Icelandic Low Seesaw(AIS) and its decadal variation. Results showed that the CAS-ESM-C can reasonably reproduce not only the spatial distribution of the climatology of sea level pressure(SLP) in winter, but also the AIS and its decadal variation. The period 496–535 of the integration by this model was divided into two sub-periods: 496–515(P1) and 516–535(P2) to further investigate the decadal weakening of the AIS. It was shown that this decadal variation of the AIS is mainly due to the phase transition of the Pacific Decadal Oscillation(PDO), from its positive phase to its negative phase. This transition of the PDO causes the sea surface temperature(SST) in the equatorial eastern(northern) Pacific to cool(warm), resulting in the decadal weakening of mid-latitude westerlies over the North Pacific and North Atlantic. This may be responsible for the weakening of the inverse relation between the Aleutian Low(AL) and the Icelandic Low(IL).