Seasonal Footprinting Research Articles

The Interdecadal Changes in the Spatial Structure of the South Pacific Oscillation and Its Implication for ENSO Diversity

Abstract The South Pacific Oscillation (SPO), characterized by a north–south dipole-like pattern of sea level pressure anomalies, is one of the key factors in understanding tropical–extratropical interactions in the South Pacific. We show that in boreal summer (June–August), the center of the northern lobe sea level pressure anomalies in the SPO is shifted to the east gradually after the 1960–70s. This study focuses on the relationship between the boreal summer SPO and following winter El Niño–Southern Oscillation (ENSO) diversity before and after the eastward shift of the SPO’s subtropical lobe. The eastward shift of the SPO’s subtropical lobe altered both the seasonal footprint mechanism and the trade wind charging mechanism associated with the SPO and thus profoundly influenced the ENSO diversity. It is revealed that when the northern lobe of the SPO shifts to the west of its average location, it tends to strengthen the eastern Pacific (EP) El Niño mainly via the seasonal footprint mechanism. But after the SPO’s northern lobe shifts to the east of its average location, it tends to promote the development of central Pacific (CP) El Niño mainly via the trade wind charging mechanism. The changes in the spatial structure of convection over the tropical Pacific and Indian Oceans may be one of the possible causes for the eastward shift in the SPO’s northern lobe. The findings in the present study have implications for a better understanding of ENSO diversity. Significance Statement Previous studies have demonstrated that the South Pacific Oscillation (SPO), as an important El Niño–Southern Oscillation (ENSO) precursor in the South Pacific, has the potential to provide an enhancement of the prediction of specific ENSO flavor. However, the historical variation in the SPO’s spatial structure and related changes in the relationship with the diversity of ENSO are still unclear. In this paper, we show that the subtropical lobe of the boreal summer (June–August) SPO is shifted to the east gradually after the 1960–70s. The changes in the spatial structure have also altered both the seasonal footprint mechanism and the trade wind charging mechanism which play important roles in the developmental processes of different types of ENSO. Our work highlights the importance of the interdecadal changes in the spatial structure of the SPO in understanding the relationship between the SPO and ENSO diversity.

Strengthened impact of boreal winter North Pacific Oscillation on ENSO development in warming climate

The North Pacific Oscillation (NPO), an important mode of atmospheric variability, is a crucial trigger for the development of El Niño-Southern Oscillation (ENSO) via the seasonal footprinting mechanism. How the NPO effect on ENSO changes in response to greenhouse warming remains unclear, however. Here, using climate model simulations under high-emission scenarios, we show that greenhouse warming leads to an enhanced influence of NPO on ENSO as is manifested by enhanced responses of winter sea surface temperature (SST), precipitation and wind anomalies in the equatorial Pacific to the preceding winter NPO. The strengthened NPO impact is also reflected in an increased frequency of NPO events that are followed by ENSO events. Warmer background SST enhances the wind-evaporation-SST feedback over the subtropical North Pacific due to a nonlinear SST-evaporation relationship. This strengthens the NPO-generated surface zonal wind anomalies over the equatorial western-central Pacific, which trigger ENSO. Increased impact of winter NPO on ENSO could enable prediction of interannual variability at longer leads.

Southeastward Extension of the Aleutian Low Favors Long Decaying El Niño Events

Abstract El Niño can be categorized into long and short decaying cases in terms of the lengths of decaying phases. We find that the difference between long and short decaying El Niño emerges as early as the decaying spring, rather than the decaying summer mentioned in previous studies. Observational evidence suggests that this difference corresponds to the different positions of the spring Aleutian low (AL). Further analysis using 15 Coupled Model Intercomparison Project phase 6 (CMIP6) model simulations suggests that the AL that extends southeastward into the northeastern subtropical Pacific contributes to the long decaying El Niño. The northeastern subtropical Pacific AL, with an anomalous southwesterly on the southeast edge, is associated with warming in situ, which further induces cyclonic circulation anomalies over the eastern tropical Pacific as Gill’s response. The anomalous westerly at the equator along the southern side of the cyclonic circulation anomalies is related to positive SST anomalies over the central and eastern tropical Pacific and therefore the persistence of El Niño in the decaying spring. On the other hand, the AL restricted to the North Pacific contributes to anticyclonic circulation anomalies over the central Pacific and favors the decline of El Niño through the seasonal footprint mechanism. Moreover, the models that reproduce many more long (short) decaying El Niño cases tend to simulate the AL southeastward extension (restriction over the North Pacific) during the El Niño decaying spring. The results imply that the AL position in the El Niño decaying spring could be a precursor to the length of El Niño decaying phase. Significance Statement El Niño events can be classified into long and short decaying cases with regard to the different lengths of decaying phases. We find that the difference between the two types of El Niño appears as early as the decaying spring, rather than the decaying summer noticed in previous studies. We highlight that this difference is due to the different positions of the spring Aleutian low (AL). Particularly, the AL that extends southeastward into the northeastern subtropical Pacific is associated with warming over the central and eastern tropical Pacific and contributes to the persistence of El Niño in the decaying spring. The results indicate that locations of the spring AL should be paid attention to when considering the decaying phase of El Niño.

The Pacific Meridional Mode Influences ENSO by Inducing Springtime Near-Equatorial TCs in the Western North Pacific

Abstract Previous research has shown that tropical cyclones in the near-equatorial western North Pacific (NE-WNP TCs) in boreal spring are associated with the onset of El Niño–Southern Oscillation (ENSO). However, the cause of these TCs is unclear. Here, we find that the interannual activity of the springtime NE-WNP TCs is insensitive to the simultaneous ENSO intensity but is closely related to the springtime Pacific meridional mode (PMM). The lead–lag correlation between PMM and NE-WNP TCs is high from January to April, and the evolution of the SST anomaly associated with the NE-WNP TCs strongly manifests the seasonal footprint of the PMM. Analysis using AGCM experiments confirms that the positive PMM tends to reduce the vertical shear of zonal wind and sea level pressure and to increase vertical velocity in the middle atmosphere and surface humidity in the WNP. All of these conditions are favorable for NE-WNP TC genesis. We conducted CGCM experiments to validate that the NE-WNP TCs significantly influence ENSO development. A statistical regression model with respect to the impact of PMM on NE-WNP TCs and further on ENSO intensity was also conducted. The results imply that modulating the springtime NE-WNP TCs is another effective way in which the PMM influences ENSO. Significance Statement Some studies have found that springtime near-equatorial western North Pacific (NE-WNP) tropical cyclones (TCs) heavily impact ENSO development. Using observations and AGCM experiments, we showed that the genesis of these TCs is closely associated with the Pacific meridional mode (PMM), presumably because the PMM provides favorable conditions for TC genesis. We further showed that including the PMM-induced NE-WNP TCs in a CGCM leads to an El Niño–like response. The linkage between the PMM and NE-WNP TCs not only supplements the existing theories about tropical–subtropical interactions but also provides a new way to improve ENSO simulation and prediction.

The Potential Influence of Maritime Continent Deforestation on El Niño‐Southern Oscillation: Insights From Idealized Modeling Experiments

AbstractDuring the past two decades, the Maritime Continent (MC) has experienced increased deforestation. Here we show, with ensemble idealized deforestation experiments, that the MC deforestation could potentially alter the complexity (i.e., event‐to‐event differences) of the El Niño‐Southern Oscillation (ENSO) in terms of its spatial pattern and temporal evolution. The deforestation model run increases the occurrences of the Central Pacific and multi‐year types of ENSO compared to the control experiments. This change in ENSO complexity can be attributed to MC's intensification of the subtropical ENSO dynamics, commonly known as the seasonal footprinting mechanism. The deforestation amplifies the mean state of the subtropical high over the northeastern Pacific, leading to an increased dominance of subtropical ENSO dynamics in determining the ENSO pattern and evolution. This idealized coupled climate modeling study suggests that MC deforestation has a potential to alter ENSO's complexity, making El Niño more complex and less predictable.

Changes in El Niño characteristics and air–sea feedback mechanisms under progressive global warming

In this study, we investigate the potential changes of El Niño characteristics, including intensity, frequency and CP/EP El Niño ratio, under progressive global warming based on the 140-year CMIP6 model simulation outputs with the 1pctCO2 experiment. Major air-sea feedback mechanisms attributing to the changes are also examined. The CMIP6 ensemble means project a slight enhancement of El Niño intensity by about 2% and a modest increase of El Niño frequency by about 4% from the first to the second 70-year periods. It is found that these small changes result from the opposite response to global warming between CP and EP El Niño, i.e., the intensity of EP El Niño is projected to weaken by nearly 4.6% while the intensity of CP El Niño is projected to increase by about 4.5%. Since CP El Niño occurs more frequently than EP El Niño in CMIP6 simulations, this leads to a slight enhancement of the total El Niño intensity if these two types of El Niño were not separated. A similar situation occurs in projecting the future change of El Niño frequency, i.e., the frequency of EP El Niño is projected to decrease by about 1.4% while the frequency of CP El Niño is projected to increase by about 2%, thereby leading to a modest increase of the total El Niño frequency. By comparing the variance explained by key air-sea feedback mechanism between the two 70-year periods, we also note that the increased CP/EP ratio can be explained by the enhanced role played by the SF (seasonal footprinting) mechanism in a warmer atmosphere. Our study also points out that, as long as a climate model can correctly produce the intensity (variance) of major air-sea feedback mechanisms, the relationship between changes in El Niño characteristics and changes in feedback mechanisms can be physically robust.

Part II model support on a new mechanism for North Pacific Oscillation influence on ENSO

Owing to the significant influence of El Niño-Southern Oscillation (ENSO) on global climate, how ENSO events are initiated is an intriguing issue. The North Pacific Oscillation (NPO), a primary atmospheric variability over the midlatitude, is a well-known trigger for ENSO events, but the physical linkage is not yet fully understood. Based on observational analyses, in Part I, we proposed a new mechanism that the NPO-related wave activity flux (WAF) could directly induce the equatorial wind anomalies in both upper and lower levels. In this study, we substantiate the impacts of the WAF on tropical circulations using climate models participating in the Coupled Model Intercomparison Project phases 5 and 6 (CMIP5/6). We found that the intensity of the southward WAF over the central Pacific is a paramount factor resulting in intermodel diversity in simulating the NPO–ENSO linkage. By classifying the models into two groups of strong and weak meridional WAF (MWAF), we reveal that the strong MWAF models simulate stronger upper- and lower-level equatorial winds and precipitation anomalies that facilitate the ENSO in subsequent winter. We also reveal that the magnitude of the MWAF is closely related to the model’s climatological meridional wind and meridional shear of climatological zonal wind, emphasizing the role of systematic bias on the ENSO simulation. A comparison of the MWAF impact and seasonal footprinting mechanism demonstrates the dominant influence of the MWAF in determining the diversity of NPO–ENSO relationships.

Atmospheric CO2 and CH4 Fluctuations over the Continent-Sea Interface in the Yenisei River Sector of the Kara Sea

Observations of the atmospheric sources and sinks of carbon dioxide (CO2) and methane (CH4) in the pan-Arctic domain are extremely scarce, limiting our knowledge of carbon turnover in this climatically sensitive environment and the fate of the enormous carbon reservoirs conserved in the permafrost. Especially critical are the gaps in the high latitudes of Siberia, covered by the vast permafrost underlain tundra, where only several atmospheric monitoring sites are operational. This paper presents the first two years (September 2018–January 2021) of accurate continuous observations of atmospheric CO2 and CH4 dry mole fractions at the recently deployed tower-based measurement station “DIAMIS” (73.5068° N, 80.5198° E) located on the southwestern coast of the Taimyr Peninsula, Siberia, at the Gulf of the Yenisei River that opens to the Kara Sea (Arctic Ocean). In this paper, we summarized the scientific rationale of the site, examined the seasonal footprint of the station with an analysis of terrestrial vegetation and maritime sector contributing to the captured atmospheric signal, and illustrated temporal patterns of CO2 and CH4 for the daytime mixed atmospheric layer over the continent–sea interface. Along with the temporal variations reflecting a signal caused pan-Arctic and not very much influenced by the local processes, we analyzed the spatiotemporal distribution of the synoptic anomalies representing the atmospheric signatures of regional sources and sinks of CO2 and CH4 for the studied high-arctic Siberian domain of ~625 thousand km2, with nearly equal capturing the land surface (54%) and the ocean (46%) throughout the year. Both for CO2 and CH4, we have observed a sea–continent declining trend, presuming a larger depletion of trace gases in the maritime air masses compared to the continental domain. So far, over the Kara Sea, we have not detected any prominent signals of CH4 that might have indicated processes of subsea permafrost degradation and occurrence of cold seeps–still mainly observed in the eastern Arctic Seas—The Laptev Sea and the East-Siberian Sea.

Relative contributions to ENSO of the seasonal footprinting and trade wind charging mechanisms associated with the Victoria mode

Relative contributions to ENSO of the seasonal footprinting and trade wind charging mechanisms associated with the Victoria mode

Possible Impact of Boreal Winter Siberian High on ENSO Development in the Following Year

Siberian High (SH) is the dominant pressure system located in the mid-high latitudes of Eurasia during boreal wintertime. This study reveals a triggering impact of SH variation in preceding winter on the following ENSO events, and gives a possible explanation via diagnosing the SH-associated air-sea response over the tropical Pacific and North Pacific. When SH is anomalously enhanced (suppressed) during boreal winter, an Aleutian Low enhanced (suppressed) response will occur over the downstream North Pacific. The Aleutian Low response gradually evolves into a meridional dipole structure similar to the negative (positive) phase of the North Pacific Oscillation (NPO) during the following spring and early summer. Correspondingly, the oceanic response in the North Pacific features a pattern similar to the negative (positive) phase of the Victoria mode. These SH-associated air-sea responses over the subtropical North Pacific will be maintained and further delivered into the tropical Pacific through the so-called seasonal footprinting mechanism, which favors the Bjerknes feedback established around boreal summer and finally grows into a La Niña (El Niño).

Impact of tropical Atlantic SST anomaly on ENSO in the NUIST‐CFS1.0 Hindcasts

AbstractUsing both observations and ensemble hindcasts of Nanjing University of Information Science and Technology Climate Forecast System (NUIST‐CFS1.0) for the period of 1983–2020, the leading modes of sea surface temperature anomalies (SSTAs) over the tropical Atlantic in boreal spring and summer (March–April–May–June–July–August, MAM‐JJA) and their connections with El Niño‐Southern Oscillation (ENSO) are investigated with Empirical Orthogonal Function (EOF), partial‐correlation, and composite analyses. In both observations and hindcasts, the first EOF mode is characterized by a basin‐wide SSTA pattern, and the second mode features a meridional dipole pattern. The meridional dipole mode cannot trigger ENSO, while the basin‐wide SST warming mode contributes to a subsequent La Niña via both north‐tropical and equatorial pathways. The north‐tropical pathway involves a low‐level anomalous anticyclone over North Pacific as a Matsuno‐Gill‐type response to the basin‐wide SST warming, related to a North Pacific Meridional Mode characterized with a cold SSTA and north‐easterly wind coupling over the northeast tropical Pacific, and finally triggers La Niña via the seasonal footprinting mechanism. This pathway is less important in the hindcasts than in observations. The equatorial pathway is tightly linked to the Kelvin wave, characteristic of an eastward propagation of the coupled pattern of SST and low‐level winds and a wedge‐like 500‐hPa air temperature anomaly over the tropical Indian and western equatorial Pacific Oceans, which is more significant in the hindcasts than in observations. Further composite analyses confirm that, even without the effect of the preceding ENSO, a basin‐wide tropical Atlantic SST warming favours the development of La Niña.

Ocean Dynamics are Key to Extratropical Forcing of El Niño

AbstractEl Niño–Southern Oscillation (ENSO) has been recently linked with extratropical Pacific Ocean atmospheric variability. The two key mechanisms connecting the atmospheric variability of the extratropical Pacific with ENSO are the heat flux–driven “seasonal footprinting mechanism” (SFM) and the ocean dynamics–driven “trade wind charging” (TWC) mechanism. However, their relative contributions to ENSO are still unknown. Here we present modeling evidence that the positive phase of the SFM generates a weaker, short-lived central Pacific El Niño–like warming pattern in the autumn, whereas the TWC positive phase leads to a wintertime eastern Pacific El Niño–like warming. When both mechanisms are active, a strong, persistent El Niño develops. While both mechanisms can trigger equatorial wind anomalies that generate an El Niño, the strength and persistence of the warming depends on the subsurface heat content buildup by the TWC mechanism. These results suggest that while dynamical coupling associated with extratropical forcing is crucial to maintain an El Niño, thermodynamical coupling is an extratropical source of El Niño diversity.

Continuous CO2 and CH4 Observations in the Coastal Arctic Atmosphere of the Western Taimyr Peninsula, Siberia: The First Results from a New Measurement Station in Dikson

Atmospheric observations of sources and sinks of carbon dioxide (CO2) and methane (CH4) in the pan-Arctic domain are highly sporadic, limiting our understanding of carbon turnover in this climatically sensitive environment and the fate of enormous carbon reservoirs buried in permafrost. Particular gaps apply to the Arctic latitudes of Siberia, covered by the vast tundra ecosystems underlain by permafrost, where only few atmospheric sites are available. The paper presents the first results of continuous observations of atmospheric CO2 and CH4 dry mole fractions at a newly operated station “DIAMIS” (73.506828° N, 80.519869° E) deployed on the edge of the Dikson settlement on the western coast of the Taimyr Peninsula. Atmospheric mole fractions of CO2, CH4, and H2O are measured by a CRDS analyzer Picarro G2301-f, which is regularly calibrated against WMO-traceable gases. Meteorological records permit screening of trace gas series. Here, we give the scientific rationale of the site, describe the instrumental setup, analyze the local environments, examine the seasonal footprint, and show CO2 and CH4 fluctuations for the daytime mixed atmospheric layer that is representative over a vast Arctic domain (~500–1000 km), capturing both terrestrial and oceanic signals.

Travelling to Bundesliga matches: the carbon footprint of football fans

ABSTRACT The purposes of this study were to estimate the carbon footprint caused by football fans travelling to Bundesliga matches (first division) in Germany in the 2018/19 season, to analyse determinants of seasonal carbon footprint, and to identify fan clusters based on travel behaviour. A nationwide online survey of football fans was conducted (n = 539) asking respondents to report their match-related travel behaviour. The average seasonal carbon footprint of a Bundesliga fan amounted 311.1 kg of carbon dioxide equivalent emissions (CO2-e), with car travel accounting for 70% of the emissions. The aggregate carbon footprint of all fans for the whole Bundesliga season was 369,765.2 t CO2-e. Buying out of these carbon emissions would cost over €9.2 million in total. The regression results revealed that club membership and commitment to favourite club significantly increased fans’ carbon footprint. The choice of favourite Bundesliga club also predicted carbon footprint, with fans of FC Bayern Munich and RB Leipzig producing a significantly higher carbon footprint than fans of Borussia Dortmund. The segmentation of Bundesliga fans by their travel behaviour identified three distinct clusters: Devoted travellers (19%), home fans (30%) and casual visitors (51%). These clusters differed significantly in terms of emissions per kilometre travelled, car use versus public transport, club membership, fan commitment, environmental consciousness, education and age. The findings have practical implications for policy makers, Bundesliga officials and club authorities, and can serve as a basis for initiatives to reduce carbon dioxide emissions in professional team sports.

The North Pacific Blob acts to increase the predictability of the Atlantic warm pool

The Atlantic warm pool (AWP) has profound impacts on extreme weather events and climate variability. Factors influencing the AWP and its predictability are still not fully understood. Other than local ocean–atmosphere feedbacks and El Niño Southern Oscillation, we find an extratropical precursor from the Northeast Pacific (known as the Blob), which leads the AWP by 1 year with a robust correlation (r = 0.68). A suite of Northeast Pacific pacemaker experiments successfully reproduces the leading influence of the Blob on the AWP. The preceding summer Blob-related sea surface temperature (SST) warming signal can be transmitted towards the lower latitudes through the seasonal footprint mechanism, leading to the central Pacific warming in the winter and following spring. Such a strong tropical Pacific SST heating excites an anomalous atmospheric wave train that resembles the Pacific/North American (PNA) teleconnection pattern. At the downstream portion of the PNA, the low sea surface pressure anomalies can be found over the AWP region during the following spring. The anomalous low initiates the AWP SST warming, and the AWP warmer SST can persist into summer and is further amplified due to ocean–atmosphere feedbacks. Our results show that the North Pacific Blob may act as a useful predictor of the AWP 1 year in advance through trans-basin interactions. A Blob-based prediction model shows considerable hindcast skill for the observed AWP SST anomaly.

Role of the climatological intertropical convergence zone in the seasonal footprinting mechanism of the El Niño-Southern Oscillation

AbstractThe North Pacific Oscillation (NPO), a primary atmospheric mode over the North Pacific in boreal winter, is known to trigger the El Niño-Southern Oscillation (ENSO) in the following winter, the process of which is recognized as the seasonal footprinting mechanism (SFM). Based on the analysis of model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), we found that the SFM acts differently among models, and the correlation between the NPO and subsequent ENSO events, called the SFM efficiency, depends on the background mean state of the model. That is, SFM efficiency becomes stronger as the climatological position of the Pacific Intertropical Convergence Zone (ITCZ) moves poleward, representing an intensification of the northern branch of the ITCZ. When the Pacific ITCZ is located poleward, the wind-evaporation-sea surface temperature (SST) feedback becomes stronger as the precipitation response to the SST anomaly is stronger in higher latitudes compared to that of lower latitudes. In addition, such active ocean-atmosphere interactions enhance NPO variability, favoring the SFM to operate efficiently and trigger an ENSO event. Consistent with the model results, the observed SFM efficiency increased during the decades in which the northern branch of the climatological ITCZ was intensified, supporting the importance of the tropical mean state of precipitation around the Pacific ITCZ.

The seasonal footprinting mechanism in large ensemble simulations of the second generation Canadian earth system model: uncertainty due to internal climate variability

Previous studies indicated that the wintertime North Pacific Oscillation (NPO) could exert marked impacts on the following winter El Niño-Southern Oscillation (ENSO) via the seasonal footprinting mechanism (SFM). Here, we examine this winter NPO-ENSO relationship in a 50-member ensemble of historical simulations conducted with the Canadian Centre for Climate Modeling and Analysis second generation Canadian Earth System Model (CanESM2) over the period of 1950–2005. The observed NPO pattern, featured by a meridional dipole atmospheric anomaly over the North Pacific, can be well reproduced by all of the 50 ensemble members. The multi-member ensemble (MME) mean can well simulate the observed NPO-ENSO relationship, as well as the SFM process. However, there exists a large spread of the results among the 50 members due to internal climate variability. Internal climate variability influences the winter NPO-ENSO relationship through modulating the subtropical center of the NPO. Specifically, the ensemble members with high NPO-ENSO correlations tend to have strong atmospheric anomalies over the subtropical North Pacific in winter. The atmospheric circulation anomaly brings strong sea surface temperature and precipitation anomalies in the tropical central Pacific and westerly wind anomalies over the tropical western Pacific in the following spring. These anomalies sustain in the following seasons and eventually lead to ENSO events in the following winter.

Projection of winter NPO-following winter ENSO connection in a warming climate: uncertainty due to internal climate variability

Previous observational and modeling studies indicate that the wintertime North Pacific Oscillation (NPO) could significantly impact the following winter El Niño-Southern Oscillation (ENSO) variability via the seasonal footprinting mechanism (SFM). This study explores climate projections of this winter NPO-ENSO relation in a warming climate based on a 50-member large ensemble of climate simulations conducted with the second-generation Canadian Earth System Model (CanESM2). The ensemble mean of the 50 members can well reproduce the observed winter NPO pattern, the NPO-ENSO relationship, and the SFM process over the historical period 1950–2003. These 50 members are then employed to examine climate projections of the NPO-ENSO connection over the anthropogenic forced period 2020–2073. Results indicate that there exists a large spread of projected NPO-ENSO connections across these 50 ensemble members due to internal climate variability. Internal climate variability brings uncertainties in the projection of the winter NPO-ENSO connection originally seen in projected changes of the subtropical center of the winter NPO. The spread of projections of winter NPO-associated atmospheric anomalies over the subtropical North Pacific further results in various responses in the projections of winter and spring precipitation anomalies over the tropical North Pacific, as well as spring zonal wind anomalies over the tropical western Pacific, which eventually lead to uncertainties in the projection of the sea surface temperature anomalies in the tropical central-eastern Pacific from the following summer to winter.

Interdecadal Modulation of AMO on the Winter North Pacific Oscillation-Following Winter ENSO Relationship

It is known that the wintertime North Pacific Oscillation (NPO) is an important extratropical forcing for the occurrence of an El Nino-Southern Oscillation (ENSO) event in the subsequent winter via the “seasonal footprinting mechanism” (SFM). This study reveals that the Atlantic Multidecadal Oscillation (AMO) can notably modulate the relationship between the winter NPO and the following winter ENSO. During the negative AMO phase, the winter NPO has significant impacts on the following winter ENSO via the SFM. In contrast, the influence of the winter NPO on ENSO is not robust at all during the positive AMO phase. Winter NPO-generated westerly wind anomalies over the equatorial western Pacific during the following spring are much stronger during negative than positive AMO phases. It is suggested that the AMO impacts the winter NPO-induced equatorial westerly winds over the western Pacific via modulating the precipitation climatology over the tropical central Pacific and via modulating the connection of the winter NPO with spring sea surface temperature in the tropical North Atlantic.

Weakening of the El Niño amplitude since the late 1990s and its link to decadal change in the North Pacific climate

The amplitude of El Niño, measured by the Nino3.4 sea surface temperature anomaly (SSTA) index, has exhibited an interdecadal change with a weakening trend since the late 1990s, characterized by the distinct Bjerknes stability index between 1980–1998 and 1999–2014. Statistical results suggest that this was primarily induced by the attenuation of the zonal wind stress and low‐level wind anomalies in response to the zonal equatorial SSTA gradient. The weakened atmospheric responses to the zonal equatorial SSTA gradient were associated with the obvious westward extension of the negative sea level pressure anomalies (SLPA) over the tropics. This was mainly attributed to the transition of the dominant atmospheric circulation over the North Pacific, where the Aleutian Low (AL) mode was replaced by the North Pacific Oscillation (NPO) mode after the late 1990s. Numerical experiments from the long‐term historical simulations of the GFDL‐CM3 model indicate that both of the AL and NPO were dominant over the North Pacific and alternated on the interdecadal time‐scale. When the NPO mode was dominant, it became more effective at playing a role in triggering an El Niño in the central Pacific via the seasonal footprinting mechanism over the subtropical northeastern Pacific. Such a change might cause the westward shift of the tropical SLPA and the attenuated atmospheric responses to the zonal SSTA gradient, ultimately resulting in the weakening of the El Niño amplitude.