The linkage between two ENSO types/modes and the interdecadal changes of ENSO around the year 2000

This study focuses on the interdecadal changes in ENSO properties emerging around the year 2000. Compared to 1980-1999, after 2000, the ENSO amplitude weakened, the occurrence of the central Pacific （CP） Et Nino increased, and the eastern Pacific （EP） El Nino became suppressed. Meanwhile, the dominant period of ENSO shortened from quasi-quadrennial （QQ） to quasi-biennial （QB）. The authors show that these changes in ENSO properties are evidently consistent with the change in the stability of the ENSO mode through connecting the two ENSO types with the two coupled ENSO modes, i.e. the QQ and QB modes. It is suggested that the relative activity or stability of the two ENSO modes changed after the year 2000. The intensity of both the QQ and QB mode weakened. The QQ mode, which is linked to EP ENSO and was significantly strong during 1980-1999, became much weaker after 2000 in terms of the EP type almost disappearing. Compared with the weakness of the QQ mode, the QB mode, as manifested by the CP type, remained active and became dominant in the tropical Pacific after 2000. Analysis shows that the changes in mean states in the tropical Pacific were likely responsible for the interdecadal ENSO changes around the year 2000.

Model Evidence for Interdecadal Pathway Changes in the Subtropics and Tropics of the South Pacific Ocean

Numerical simulations using a version of the GFDL/NOAA Modular Ocean Model （MOM 3） are analyzed to demonstrate interdecadal pathway changes from the subtropics to the tropics in the South Pacific Ocean. After the 1976 -77 climate shift, the subtropical gyre of the South Pacific underwent significant changes, characterized by a slowing down in its circulation and a southward displacement of its center by about 5°- 10° latitude on the western side. The associated circulation altered its flow path in the northwestern part of the subtropical gyre, changing from a direct pathway connecting the subtropics to the tropics before the shift to a more zonal one after. This effectively prevented some subtropical waters from directly entering into the western equatorial Pacific. Since waters transported onto the equator around the subtropical gyre are saline and warm, such changes in the direct pathway and the associated reduction in equatorward exchange from the subtropics to the tropics affected water mass properties downstream in the western equatorial Pacific, causing persisted freshening and cooling of subsurface water as observed after the late 1970s. Previously, changes in gyre strength and advection of temperature anomalies have been invoked as mechanisms for linking the subtropics and tropics on interdecadal time scales. Here we present an additional hypothesis in which geographic shifts in the gyre structure and location （a pathway change） could play a similar role.

DIAGNOSTIC ANALYSIS OF INTERDECADAL CHANGES OF THE SUMMER MONSOON IN THE SOUTH CHINA SEA

The onset dates and intensities for the South China Sea (SCS) summer monsoon over the past 51 years are established using the reanalyzed gridpoint data of NCEP and SST data from 1950 to 2000.As is shown in the t test,the activities of the SCS monsoon (including the dates of onset and intensities) experienced a significant interdecadal change in the mid-1970s.The monsoon activity is closely related with the anomalies of the general circulation in the mid- and higher-latitudes but it is not related with those of the SST in tropical oceans before 1975.After 1975,the onset is earlier and the intensity is weaker and the SST anomalies in the tropical Indian Ocean and the Pacific Ocean have significant impacts on the activity of the SCS summer monsoon.These significant changes are thought to be associated with the interdecadal variation of SST over the oceans.

Changes in the Diurnal Cycles of Precipitation over Eastern China in the Past 40 Years

This study analyzed the interdecadal changes in the diurnal variability of summer （June-August） precipitation over eastern China during the period 1966 2005 using hourly station rain gauge data. The results revealed that rainfall diurnal variations experienced significant interdecadal changes. Over the area to the south of the Yangtze River, as well as the area between the Yangtze and Yellow Rivers, the percentages of morning rainfall （0000 1200 LST） to total rainfall in terms of amount, frequency and intensity, all exhibited increasing interdecadal trends. On the contrary, over North China, decreasing trends were found. As a result, diurnal rainfall peaks also presented pronounced interdecadal variations. Over the area between the Yangtze and Yellow Rivers, there were 16 out of 46 stations with afternoon （1200-0000 LST） frequency peaks in the first 20 years of the 40-year period of study, while only eight remained in the latter 20 years. In North China, seven stations experienced the opposite changes, which accounted for about 21% of the total number of stations. The possible causes for the interdecadal changes in diurnal features were discussed. As the rainfall in the active monsoon period presents morning diurnal peaks, with afternoon peaks in the break period, the decrease （increase） of rainfall in the active monsoon period over North China （the area south of the Yangtze River and the area between the Yangtze and Yellow Rivers） may contribute to interdecadal changes in diurnal rainfall variability.

Interdecadal Change in Extreme Precipitation over South China and Its Mechanism

Based on the daily precipitation data taken from 17 stations over South China during the period of 1961 2003, a sudden change in summer extreme precipitation events over South China in the early 1990s along with the possible mechanism connected with the anomalies of the latent heat flux over the South China Sea and the sensible heat flux over the Indochina peninsula are examined. The results show that both the annual and summer extreme precipitation events have obvious interdecadal variations and have increased significantly since the early 1990s. Moreover, the latent heat flux over the South China Sea and the sensible heat flux over the Indochina peninsula also have obvious interdecadal variations consistent with that of the extreme precipitation, and influence different months＇ extreme precipitation, respectively. Their effects are achieved by the interdecadal increases of the strengthening convection over South China through the South China Sea Summer Monsoon.

Interdecadal Variability of the East Asian Summer Monsoon in an AGCM

It is well known that significant interdecadal variation of the East Asian summer monsoon （EASM） occurred around the end of the 1970s. Whether these variations can be attributed to the evolution of global sea surface temperature （SST） and sea ice concentration distribution is investigated with an atmospheric general circulation model （AGCM）. The model is forced with observed monthly global SST and sea ice evolution through 1958-1999. A total of four integrations starting from different initial conditions are carried out. It is found that only one of these reproduces the observed interdecadal changes of the EASM after the 1970s, including weakened low-level meridional wind, decreased surface air temperature and increased sea level pressure in central China, as well as the southwestward shift of the western Pacific subtropical high ridge and the strengthened 200-hPa westerlies. This discrepancy among these simulated results suggests that the interdecadal variation of the EASM cannot be accounted for by historical global SST and sea ice evolution. Thus, the possibility that the interdecadal timescale change of monsoon is a natural variability of the coupled climate system evolution cannot be excluded.

Interdecadal Change in the Connection Between Hadley Circulation and Winter Temperature in East Asia

Based on NCEP/NCAR reanalysis data, the interdecadal variability of Hadley circulation （HC） and its association with East Asian temperature in winter are investigated. Results indicate that the Northern Hemisphere winter HC underwent apparent change in the 1970s, with transition occurring around 1976/77. Along with interdecadal variability of HC, its linkage to surface air temperature （SAT） in East Asia also varied decadally, from weak relations to strong relations. Such a change may be related to the interaction between HC and the atmospheric circulation system over the Philippines, which is associated with the East Asian winter monsoon （EAWM）. Before the 1970s, the connection between HC and the anticyclonic circulation around the Philippines was insignificant, but after the late 1970s their linkage entered a strong regime. The intensification of this connection may therefore be responsible for the strong relations between HC and East Asian winter temperatures after the late 1970s.

Interdecadal change in Western Pacific Subtropical High and climatic effects

Western North Pacific Subtropical High is a very important atmospheric circulation system influencing the summer climate over eastern China. Its interdecadal change is analyzed in this study. There is a significant decadal shift in about 1979/1980. Since 1980, the Western North Pacific Subtropical High has enlarged, intensified, and shifted southwestward. This change gives rise to an anti-cyclonic circulation anomaly over the region from the South China Sea to western Pacific and thus causes wet anomalies over the Yangtze River valley. During the summers of 1980-1999, the precipitation is 63.9 mm above normal, while during 1958-1979 it is 27.3 mm below normal. The difference is significant at the 99% confidence level as a t-test shown. The southwestward expanding of the Western North Pacific Subtropical High also leads to a significant warming in southern China, during 1980-1999 the summer mean temperature is 0.37篊 warmer than that of the period 1958-1979. The strong warming is primarily due to the clearer skies associated with the stronger downward air motion as the Western North Pacific Subtropical High expanding to the west and controlling southern China. It is also found that the relative percentage of tropical cyclones in the regions south of 20篘 is decreasing since the 1980s, but in the regions north of 20篘 that is increasing at the same time. The Western North Pacific Subtropical High responds significantly to sea surface temperature of the tropical eastern Pacific with a lag of one-two seasons and simultaneously to sea surface temperature of the tropical Indian Ocean. The changes in the sea surface temperatures are mainly responsible for the interdecadal variability of the Western North Pacific Subtropical High.

Interdecadal Enhancement of the Walker Circulation over the Tropical Pacific in the Late 1990s

The Walker circulation is one of the major components of the large-scale tropical atmospheric circulation and variations in its strength are critical to equatorial Pacific Ocean circulation. It has been argued in the literature that during the 20th century the Walker circulation weakened, and that this weakening was attributable to anthropogenic climate change. By using updated observations, we show that there has been a rapid interdecadal enhancement of the Walker circulation since the late 1990s. Associated with this enhancement is enhanced precipitation in the tropical western Pacific, anomalous westerlies in the upper troposphere, descent in the central and eastern tropical Pacific, and anomalous surface easterlies in the western and central tropical Pacific. The characteristics of associated oceanic changes are a strengthened thermocline slope and an enhanced zonal SST gradient across the tropical Pacific. Many characteristics of these changes are similar to those associated with the mid-1970s climate shift with an opposite sign. We also show that the interdecadal variability of the Walker circulation in the tropical Pacific is inversely correlated to the interdecadal variability of the zonal circulation in the tropical Atlantic. An enhancement of the Walker circulation in the tropical Pacific is associated with a weakening zonal circulation in the tropical Atlantic and vise versa, implying an inter-Atlantic-Pacific connection of the zonal overturning circulation variation. Whether these recent changes will be sustained is not yet clear, but our research highlights the importance of understanding the interdecadal variability, as well as the long-term trends, that influence tropical circulation.

Interdecadal change of winter SST variability in the Kuroshio Extension region and its linkage with Aleutian atmospheric low pressure system

By utilizing multiple datasets from various sources available for the last 100 years, the existence for the interdecadal change of the winter sea surface temperature（SST） variability in the Kuroshio Extension（KE） region is investigated. And its linkage with the Aleutian Low（AL） activity changes is also discussed. The results find that the KE SST variability exhibits the significant ~6 a and ~10 a oscillations with obvious interdecadal change. The ~6 a oscillation is mainly detected during 1930–1950, which is largely impacted by the anomalous surface heat flux forcing and Ekman heat transport associated with the AL intensity variation. The ~10 a oscillation is most evident after the 1980s, which is predominantly triggered by the AL north-south shift through the bridge of oceanic Rossby waves.

Interdecadal Change in the Interannual Variation of the Western Edge of the Western North Pacific Subtropical High During Early Summer and the Influence of Tropical Sea Surface Temperature

This study reveals that the interannual variability of the western edge of the western North Pacific(WNP)subtropical high(WNPSH)in early summer experienced an interdecadal decrease around 1990.Correspondingly,the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP(WNPHA)in abnormal years possess smaller ranges after 1990.The different influences of the tropical SSTAs are important for this interdecadal change,which exhibit slow El Nino decaying pattern before 1990 while rapid transformation from El Nino to La Nina after 1990.The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences.Before 1990,the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation,respectively.Meanwhile,the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response,which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific.After 1990,the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation,respectively.Overall,the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990.Consequently,the WNPHA features narrower zonal range and less eastward extension after 1990,corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH.On the other hand,the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990,contributing to the change of the El Nino SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.

An Interdecadal Change in the Influence of the NAO on Atlantic-Induced Arctic Daily Warming around the Mid-1980s

After approaching 0℃owing to an Atlantic storm at the end of 2015,the Arctic temperature approached freezing again in 2022,indicating that Arctic daily warming events remain a concern.The NCEP/NCAR Reanalysis dataset was used to investigate the influence of the NAO on the Arctic winter daily warming events induced by Atlantic storms,known as the Atlantic pattern-Arctic Rapid Tropospheric Daily Warming(Atlantic-RTDW)event.Atlantic-RTDW events are triggered by Atlantic storms that transport warm and humid air masses moving into the Arctic.Furthermore,an interdecadal change in the influence of NAO on Atlantic-RTDW-event frequency was observed around the mid-1980s.Specifically,before the mid-1980s(pre-transition period),500-hPa southerly(northerly)wind anomalies occupied the North Atlantic(NA)in the positive(negative)phase of NAO,which increased(decreased)the Atlantic-RTDW events occurrence by steering Atlantic storms into(away from)the Arctic;thus,the NAO could potentially influence the Atlantic-RTDW-event frequency.However,the relationship between the NAO and the Atlantic-RTDW-event frequency has weakened since the mid-1980s(post-transition period).In the post-transition period,such 500-hPa southerly(northerly)wind anomalies over the NA hardly existed in the positive(negative)phase of NAO,which was attributed to a stronger Atlantic Storm Track(AST)activity intensity than that in the pre-transition period.During this period,the strong AST induced an enhanced NAOrelated cyclone via transient eddy-mean flow interactions,resulting in the disappearance of southerly and northerly wind anomalies over the NA.

Interdecadal Enhancement in the Relationship between the Western North Pacific Summer Monsoon and Sea Surface Temperature in the Tropical Central-Western Pacific after the Early 1990s

This study reveals the strengthened interdecadal relationship between the western North Pacific summer monsoon(WNPSM)and tropical central-western Pacific sea surface temperature anomaly(SSTA)in summer after the early 1990s.In the first period(1979–91,P1),the WNPSM-related precipitation anomaly and horizontal wind anomaly present themselves as an analogous Pacific-Japan(PJ)-like pattern,generally considered to be related to the Niño-3 index in the preceding winter.During the subsequent period(1994–2019,P2),the WNPSM-related precipitation anomaly presents a zonal dipole pattern,correlated significantly with the concurrent SSTA in the Niño-4 and tropical western Pacific regions.The negative(positive)SSTA in the tropical western Pacific and positive(negative)SSTA in the Niño-4 region,could work together to influence the WNPSM,noting that the two types of anomalous SSTA configurations enhance(weaken)the WNPSM by the positive(negative)phase PJ-like wave and Gill response,respectively,with an anomalous cyclone(anticyclone)located in the WNPSM,which shows obvious symmetry about the anomalous circulation.Specifically,the SSTA in Niño-4 impacts the WNPSM by an atmospheric Gill response,with a stronger(weaker)WNPSM along with a positive(negative)SSTA in the Niño-4 region.Furthermore,the SSTA in the tropical western Pacific exerts an influence on the WNPSM by a PJ-like wave,with a stronger(weaker)WNPSM along with a negative(positive)SSTA in the tropical western Pacific.In general,SSTAs in the tropical western Pacific and Niño-4 areas could work together to exert influence on the WNPSM,with the effect most likely to occur in the El Niño(La Niña)developing year in P2.However,the SSTAs in the tropical western Pacific worked alone to exert an influence on the WNPSM mainly in 2013,2014,2016,and 2017,and the SSTAs in the Niño-4 region worked alone to exert an influence on the WNPSM mainly in Central Pacific(CP)La Niña developing years.The sensitivity experiments also can reproduce the PJ-like wave/Gill response associated with SSTA in the tropical western Pacific/Niño-4 regions.Therefore,the respective and synergistic impacts from the Niño-4 region and the tropical western Pacific on the WNPSM have been revealed,which helps us to acquire a better understanding of the interdecadal variations of the WNPSM and its associated climate influences.

Interdecadal change in the lagged relationship between the Victoria mode and ENSO

This paper investigates the interdecadal variability in the lagged relationship between the spring Victoria mode(VM)and the following-winter El Ni?o–Southern Oscillation(ENSO).It is found that the relationship is strong during high correlation(HC)periods,e.g.1957–1964 and 1981–2004,but weak during low correlation(LC)periods,e.g.1907–1924,1926–1956,1965–1980,and2005–2008.The surface air–sea coupling and the evolution of subsurface ocean temperature anomaliesh along the equatorial Paci?c associated with the VM are found to be strong during HC periods and weak during LC periods,which results in a stronger impact of the VM on the following-winter ENSO during HC periods.The interdecadal change in the relationship between the VM and ENSO is mainly due to the interdecadal change in the intensity of the VM,which is found to be in?uenced by the North Paci?c Oscillation.Our?ndings may improve the prediction skill for the onset of ENSO events.

Interdecadal Change in the Interannual Variability of South China Sea Summer Monsoon Intensity

The interdecadal change in the interannual variability of the South China Sea summer monsoon(SCSSM)intensity and its mechanism are investigated in this study.The interannual variability of the low-level circulation of the SCSSM has experienced a significant interdecadal enhancement around the end of the 1980s,which may be attributed to the interdecadal changes in the evolution of the tropical Indo-Pacific sea surface temperature(SST)anomalies and their impacts on the SCSSM.From 1961 to 1989,the low-level circulation over the South China Sea is primarily affected by the SST anomalies in the tropical Indian Ocean via the mechanism of Kelvin-wave-induced Ekman divergence.While in 1990 to 2020,the impacts of the summer SST anomalies in the Maritime Continent and the equatorial central to eastern Pacific on the SCSSM are enhanced,via anomalous meridional circulation and Mastuno-Gill type Rossby wave atmospheric response,respectively.The above interdecadal changes are closely associated with the interdecadal changes in the evolution of El Niño–Southern Oscillation(ENSO)events.The interdecadal variation of the summer SST anomalies in the developing and decaying phases of ENSO events enhances the influence of the tropical Indo-Pacific SST on the SCSSM,resulting in the interdecadal change in the interannual variability of the SCSSM.

An Interdecadal Change of Summer Atmospheric Circulation over Asian Mid-High Latitudes and Associated Effects

The atmospheric circulation over the mid-high latitudes in Asia has an important influence on regional climate,yet its long-term variation has not been fully explored.The main task of this study is to reveal the interdecadal variation features of summer atmospheric circulation over Asian mid-high latitudes in recent decades.The results show that the atmospheric circulation over mid-high latitudes of Asia has stronger interdecadal fluctuations than that over low latitudes and one significant change center appears near Lake Baikal.It is found that the atmospheric circulation near Lake Baikal has a significant interdecadal change around 1996 and a deep anomalous anticyclonic circulation has been controlling this region since then,which contributes to the significant increase in the surface temperature near Lake Baikal since 1997 and makes the region a remarkable warming center in Asia in recent 40 years.During 1997-2015,the pattern of less precipitation in the north and more precipitation in the south of east China is closely related to the anomalous anticyclonic circulation near Lake Baikal.Especially,this anomalous circulation near Lake Baikal has been found to contribute to the obvious interdecadal decrease of the precipitation in northeast China and north China near1997.The sea surface temperature(SST)of northwestern Atlantic is an important influence factor to the interdecadal change in the atmospheric circulation near Lake Baikal around 1996.

Interdecadal change of snow depth in winter over the Tibetan Plateau and its effect on summer precipitation in China

This paper obtained a set of consecutive and long-recorded observational snow depth data from 51 observation stations by choosing, removing and interpolating original observation data over the Tibetan Plateau for 1961-2006. We used monthly precipitation and temperature data from 160 stations in China for 1951-2006, which was collected by the National Climate Center. Through calculating and analyzing the correlation coefficient, significance test, polynomial trend fitting, composite analysis and abrupt change test, this paper studied the interdecadal change of winter snow over the Tibetan Plateau and its relationship to summer pre- cipitation and temperature in China, and to tropospheric atmospheric temperature. This paper also studied general circulation and East Asian summer monsoon under the background of global warming.

A tripole winter precipitation change pattern around the Tibetan Plateau in the late 1990s

Classical monsoon dynamics considers the winter/spring snow amount on the Tibetan Plateau(TP)as a major factor driving the East Asian summer monsoon(EASM)for its direct influence on the land-sea thermal contrast.Actually,the TP snow increased and decreased after the late 1970s and 1990s,respectively,accompanying the two major interdecadal changes in the EASM.Although studies have explored the possible mechanisms of the EASM interdecadal variations,and change in TP snow is considered as one of the major drivers,few studies have illustrated the underlying mechanisms of the interdecadal changes in the winter TP snow.This study reveals a tripole pattern of change,with decreased winter precipitation over the TP and an increase to its north and south after the late 1990s.Further analyses through numerical experiments demonstrate that the tropical Pacific SST changes in the late 1990s can robustly affect the winter TP precipitation through regulating the Walker and regional Hadley circulation.The cooling over the tropical central-eastern Pacific can enhance the Walker circulation cell over the Pacific and induce ascending motion anomalies over the Indo-Pacific region.These anomalies further drive descending motion anomalies over the TP and ascending motion anomalies to the north through regulating the regional Hadley circulation.Therefore,the positive-negative-positive winter precipitation anomalies around the TP are formed.This study improves the previously poor understanding of TP climate variation at interdecadal timescales.

An enhanced influence of sea surface temperature in the tropical northern Atlantic on the following winter ENSO since the early 1980s

Previous studies have revealed a connection between springtime sea surface temperature （SST） in the tropical northern Atlantic （TNA） and the succeeding wintertime El Nino-Southern Oscillation （ENSO）. The present analysis demonstrates that the linkage between springtime TNA SST and the following ENSO experiences an obvious interdecadal change around the early 1980s, with the connection being weak before but significant after. After the early 1980s, springtime positive TNA SST anomalies induce an anomalous cyclone over the northeastern subtropical Pacific and an anomalous Walker circulation with a descending branch over the tropical central-eastern Pacific. This leads to anomalous cold SST in the northeastern Pacific and an anomalous anticyclone over the western-central tropical Pacific, with anomalous easterlies to the equatorward side. As such, springtime TNA SST anomalies are followed by wintertime ENSO after the early 1980s. In contrast, before the early 1980s, anomalous cold SST in the northeastern Pacific and related anomalous easterlies over the western-central tropical Pacific are weak, corresponding to springtime positive TNA SST anomalies and resulting in a weak linkage between springtimeTNA SST and the succeeding wintertime ENSO. Further investigation implies that the change in the TNA SST-ENSO relationship is probably due to a change in springtime mean precipitation over the tropical Atlantic and South America.

A Preliminary Study on the Global Land Annual Precipitation Associated with ENSO during 1948-2000

The global land monthly precipitation data (PREC / L) are used to investigate the relation between the global land annual precipitation and ENSO during 1948-2000, and the results of composite analysis are tested with Monte Carlo simulations. Results indicate that the global land annual precipitation was significantly reduced in large scale areas in warm event years; the areas were the equatorial West Pacific, North China; equatorial Central America; North Bengal Bay and Nepal; East Australia; West India and South Pakistan; the district east of the Lena River; West Europe; and Wilkes Land of Antarctica. In contrast to this, the areas where precipitation increased in warm event years were less, and mainly in Chile and Argentina of South America; Somali, Kenya, and Tanzania of East Africa; Turkey, Iraq, and Iran of the Middle East; Libya and Nigeria of North Africa; Namibia of Southwest Africa; and Botswana and Zimbabwe of southern Africa. Statistical tests show that in warm event years, the area where the land annual precipitation was reduced was larger than the area where the annual precipitation increased, and the reduction in precipitation was more significant than the increase. The results in this paper are compared with previous studies. It is also pointed out that the interdecadal change of ENSO had no significant effect on the interdecadal variation of precipitation in the above regions. However, the warm events of El Nino affected the droughts in East Australia and North China more after the 1980s than before.